We Still Don’t Know What Killed the Biggest Shark of All Time

We just can’t let Carcharocles megalodon rest. From Peter Benchley’s JAWS to the dreck that regularly bobs up to the surface of basic cable “science” channels, we can’t seem to resist invoking the specter of a shark so large that it could easily engulf a person without a drop of blood spilled into the sea.

Art by Fernando G. Baptista; Research by Ryan T. Willians, Fanna Gebreyesus; Source: STEPHEN J. GODFREY, CALVERT MARINE MUSEUM
Art by Fernando G. Baptista; Research by Ryan T. Willians, Fanna Gebreyesus; Source: STEPHEN J. GODFREY, CALVERT MARINE MUSEUM

Despite our fascination with this enormous, extinct relative of today’s great white shark, there’s still a great deal we don’t know about the life and death of the biggest shark that ever lived. For starters, we still don’t know why the last of the megatooths died over 2.5 million years ago.

In the entire history of cartilaginous fish, Carcharocles megalodon was a huge success story. And that’s not just because of the predator’s size and inferred ferocity. This species patrolled the coasts of the Atlantic, Pacific, and Indian Oceans for about 20 million years. Few creatures can claim such a record. And that only makes the disappearance of the shark all the more puzzling.

Changes brought on by a cooling climate have been the focus of the traditional explanation for the monstrous shark’s demise. C. megalodon has often been thought of as a warm-water hunter, and so, the argument goes, as sea temperatures dipped at the end of the Pliocene the whales, seals, and other fatty mammals the shark relied upon migrated to chilled seas where the shark couldn’t follow. The pitiful selachian was simply left behind as cetaceans spouted off for the poles.

But was the great shark so restricted by temperature? To find out, paleontologist Catalina Pimiento and colleagues drew from the Paleobiology Database to analyze occurrences of C. megalodon over time in relation to climate.  Contrary to what had previously been thought, temperature probably didn’t freeze the shark into extinction.

Curator Jeff Seigel stands in the five–-foot mouth of a fossil shark jaw. The shark is called Carcharoles Megalodon and was large enough to swallow a small car. Photograph by Rick Meyer, Los Angeles Times, Getty
Curator Jeff Seigel stands in the five–-foot mouth of a fossil shark jaw. The shark is called Carcharoles Megalodon and was large enough to swallow a small car. Photograph by Rick Meyer, Los Angeles Times, Getty

The big picture looks something like this. During the shark’s early years, around 20 million years ago, C. megalodon primarily swam through waters of the northern hemisphere. Populations expanded around 15 million years ago to include every major ocean basin on the planet, the researchers write, but from there the sharks populations steadily declined.

All of this happened irrespective of climate. During times of major temperature spikes and dips, Pimiento and coauthors note, C. megalodon occurrences didn’t seem to show any direct response. Not to mention that the shark seemed fully capable of coping with a range of temperatures from 53 to 80 degrees Fahrenheit, and there have been waters in this range from the shark’s time until today. As Pimiento and coauthors write, “C. megalodon would have not been affected significantly by the temperature changes during the Pleistocene, Holocene and Recent.”

Populations of C. megalodon over time. From Pimiento et al., 2016.
Populations of C. megalodon over time. From Pimiento et al., 2016.

So if it wasn’t cooler waters, what drove the shark to extinction? There’s still no definitive answer. Even today, when we can witness species disappear, it’s often difficult to precisely retrace the road from the vanishing point back to the first signs of trouble. In the case of C. megalodon, though, Pimiento and coauthors have some ideas about possible killswitches.

Through hindsight, we can see that the road to extinction for the megatooth shark started in the middle of the Miocene. This coincided with two major events, as previously pointed out by paleontologist Dana Ehret as well as the authors of the new study. Against a background of crashing whale diversity during this time, the world saw the evolution of some stiff competition for C. megalodon: large sharks close to the ancestry of the great white and sperm whales that behaved and hunted more like today’s orcas. This trend continued only through the Pliocene, with fewer big baleen whales and an increasing array of predators that young megatooth sharks would have struggled against to get enough food down their throats. There was less food to go around for an expanding guild of predators who relied upon warm, blubbery prey.

The case isn’t closed yet, though. So much of what’s known about C. megalodon comes from teeth, the occasional vertebra, and some bite marks. Those pieces only reach so far in revealing the massive shark’s biology, including how much the fish actually relied on filter-feeding whales for food or the other predators it was striving against to survive.

We can be sure the megatooth shark is dead. The fish’s fossil record taps out by 2.5 million years ago, and we surely wouldn’t miss populations of fifty-foot-long sharks patrolling the global coastlines. But why the shark vanished is a secret still waiting to be dredged from the fossil record.

Reference:

Pimiento, C., MacFadden, B., Clements, C., Varela, S., Jaramillo, C., Velez-Juarbe, J., Silliman, B. 2016. Geographical distribution patterns of Carcharocles megalodon over time reveal clues about extinction mechanisms. Journal of Biogeography. doi: 10.1111/jbi.12754

Paleo Profile: Alcide d’Orbigny’s Dawn Beast

From de Muizon et al., 2015.
The nearly-complete skeleton of Alcidedorbignya inopinata. From de Muizon et al., 2015.

Vertebrate paleontology has a sample size problem. Only a fraction of all the creatures that ever lived became preserved in the fossil record, and an even tinier sliver of that array has been discovered, cleaned, and studied. Even the most famous animals, like the fearsome Tyrannosaurus, are known from a (figurative) handful of individuals scattered through swaths of rock spanning a million years or more. Finding enough fossils to even start to pick at the biology of an extinct species is a tall order.

But every now and then paleontologists strike just the right spot. One such locality, near Tiupampa, Bolivia, is simply known as “The Quarry”, but don’t be fooled by the lackluster name. This is one of the best places in the world to find the skeletons of the mammals that thrived just a million years after a wayward asteroid closed the Age of Reptiles for good, and, as paleontologist Christian de Muizon and colleagues report in a massive monograph, this place has supplied an exquisite record of one of the earliest placental mammals to skitter around in the end-Cretaceous aftermath.

de Muizon and Larrry Marshall named the mammal Alcidedorbignya inopinata in 1992. Back then, it was mostly known from teeth and pieces of jaw. But in the years since the initial finds paleontologists have uncovered a nearly-complete skeleton, several partial skulls, and hundreds of other scattered elements. With all this new material, de Muizon and colleagues set about piecing together this dawn beast in greater detail than possible before.

There’s not an exact modern equivalent for what Alcidedorbignya was. It was a tiny member of a totally-extinct group of mammals called pantodonts that thrived in the earliest days of the Paleocene. And unlike its larger relatives, Alcidedorbignya was a nimble little beast that was probably adept at running through the trees as well as scampering around on the ground, able to stand up on its hind legs to have a look around or grab a morsel when necessary. The restoration of the mammal by Justine Jacquot-Haméon makes me think “cat squirrel” isn’t too far off, although the beast wasn’t closely related to either.

What’s still unclear is what led so many Alcidedorbignya to become buried in the same place. Especially strange is that The Quarry has yielded 33 jaws of juvenile animals and 35 jaws from adults, raising the prospect that these were gregarious mammals or that the youngsters hung around with their parents. Were these mammals social, or did the a local flooding event hit at just the wrong time of the year, taking out the next generation as well as their parents? The case remains open. With so many bones of Alcidedorbignya to compare and scrutinize, though, this little mammal offers one of the best chances we have to envision the world just after it was freed from the claws of the “terrible lizards.”

Caption
Alcidedorbignya in its Paleocene environment. Art by Justine Jacquot-Haméon.

Fossil Facts

Name: Alcidedorbignya inopinata

Meaning: The genus name honors French naturalist Alcide d’Orbigny.

Age: About 65 million years old.

Where in the world?: Tiupampa, Bolivia.

What sort of critter?: A pantodont, one of the mammals that proliferated after the demise of the non-avian dinosaurs.

Size: About the size of a large squirrel.

How much of the creature’s body is known?: Multiple fossils including a nearly-complete skeleton, at least three juvenile skulls, several hundred dental specimens, and more.

Reference:

de Muizon, C., Billet, G., Argot, C., Ladevèze, S., Goussard, F. 2015. Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny, and palaeobiology. Geodiversitas. doi: 10.5252/g2015n4a1

Previous Paleo Profiles:

The Unfortunate Dragon
The Cross Lizard
The South China Lizard
Zhenyuan Sun’s dragon
The Fascinating Scrap
The Sloth Claw
The Hefty Kangaroo
Mathison’s Fox
Scar Face
The Rain-Maker Lizard
“Lightning Claw”
The Ancient Agama
The Hell-Hound
The Cutting Shears of Kimbeto Wash
The False Moose
“Miss Piggy” the Prehistoric Turtle
Mexico’s “Bird Mimic”
The Greatest Auk
Catalonia’s Little Ape
Pakistan’s Butterfly-Faced Beast
The Head of the Devil
Spain’s Megatoothed Croc
The Smoke Hill Bird
The Vereda Hilarco Beast
The North’s Sailback
Amidala’s Strange Horn
The Northern Mantis Shrimp
Spain’s High-Spined Herbviore
Wucaiwan’s Ornamented Horned Face

How Jaguars Survived the Ice Age

A jaguar at the St. Louis Zoo. Photo by Brian Switek.
A jaguar at the St. Louis Zoo. Photo by Brian Switek.

The mastodons, ground sloths, and sabercats are all gone. They all slipped into extinction around 10,000 or so years ago, along with an even wider variety of fantastic beasts and birds that fall under the category “megafauna.” But not all the Ice Age megamammals died out. We spend so much time mourning the losses that we often forget the survivors that carry whispers of the Pleistocene world. Among these resilient beasts is the jaguar.

Jaguars are old cats. They first evolved in Eurasia sometime around three million years ago before spreading both west and east, eventually inhabiting a range from southern England to Nebraska and down into South America. Today’s range of southern Arizona to Argentina—over 3.4 million square miles—is only a sliver of their Ice Age expansion. And it wasn’t just the jaguar’s range that shrunk. Today the spotted cats are about fifteen percent smaller than their Pleistocene predecessors.

Nevertheless, jaguars survived while the American lion, the sabercats, and other predators vanished. How? In order to investigate this question, biologist Matt Hayward and colleagues looked at the jaguar diet and how the cat’s prey preferences changed over time.

Drawing from 25 published studies documenting 3,214 jaguar kills, Hayward and coauthors found that jaguars are pretty finicky for apex predators. The big cat’s menu spans 111 species—ranging from cattle to rodents to monkeys to turtles—but, contrary to what has often been written about the cat, the jaguar is not really a generalist that hunts anything and everything.

The most common parts of the jaguar diet, Hayward and colleagues found, are capybara, wild pig, caiman, collared peccary, nine-banded armadillo, giant anteater, and white-nosed coati. These species account for 16-21% of the jaguar diet. The stats also showed that prey including peccaries, brocket deer, giant anteaters, and coatis which were hunted 85% of the time when they were present in the jaguar’s range. Crunching the numbers a bit further, the zoologists found that jaguars seemed to especially target capybara and giant anteater. On the other hand, jaguars never preyed upon tapirs and almost never touched primates.

Jaguars come out of all this as a paradox. They are burlier than leopards, yet they prefer to hunt a narrow range of prey that falls in the shallow end of what jaguars should be able to tackle. This might have something to do with why the cats have shrunk. Jaguars aren’t large enough to take on tapirs alone, yet human hunting on mid-range prey—such as deer—has made such herbivores too rare to rely upon. So despite their size, jaguars responded by picking out smaller prey which Hayward and coauthors dub “suboptimal” for what the cats initially evolved to do.

The jaguar’s not alone in this. Coyotes have gone through similar changes. The scrappy canids are Ice Age survivors, too, and they were significantly larger during the Ice Age. When all their competition disappeared, coyotes became smaller and ended up living on the fringes in a world heavily influenced by humans.

Flexibility made all the difference for these carnivores. Even though jaguars no longer prowl as much of the world as they once did, and are currently listed as “near threatened” on the IUCN Red List, they were able to persist where so many other carnivores perished by shifting their diets. “It may be that jaguars survived this mass extinction event by preferentially preying on relatively small species,” Hayward and coauthors write. The fossil record of cougars tells a similar story: By eating parts of carcasses other cats didn’t want, mountain lions were able to survive the tough times. And even though the cause of the loss of many Ice Age celebrities remains debated, the survivors are truly the animals we should be looking at in greater detail. How they succeeded may hold the secrets to why so many other species failed.

Reference:

Hayward, M., Kamler, J., Montgomery, R., Newlove, A., Rostro-García, S., Sales, L., Van Valkenburgh, B. 2016. Prey preferences of the jaguar Panthera onca reflect the Post-Pleistocene demise of large prey. Frontiers in Ecology and Evolution. doi: 10.3389/fevo.2015.00148

Meet Marshosaurus, the Jurassic’s Forgotten Predator

Marshosaurus considers at Stegosaurus snack at the Natural History Museum of Utah. Photo by Brian Switek.
Marshosaurus considers a Stegosaurus snack at the Natural History Museum of Utah. Photo by Brian Switek.

I’ve spilled more than a little digital ink over the top carnivores of the Jurassic west. Allosaurus, Ceratosaurus, and Torvosaurus are all very dinosaur-y dinosaurs, checking the boxes for big, scary, and strange. But as I’ve poked around the Morrison Formation bones held at the Natural History Museum of Utah over the past few weeks, I realized I’ve done a disservice to ancient ecology by focusing on the flesh-rippers of the most imposing stature. There was an entire guild of Jurassic carnivores running around North America around 150 million years ago, and one of the least-known – at least to the public – is a mid-sized carnivore named Marshosaurus bicentesimus.

We know Marshosaurus was part of an wide array of predators, from tiny to giant, because of where it was found. The theropod’s bones were scattered through the jumble that is Utah’s Cleveland-Lloyd Dinosaur Quarry. Allosaurus totally dominates this site, but the bonebed has also yielded Ceratosaurus, Torvosaurus, a small tyrannosaur named Stokesosaurus, and, as Madsen, Jr. announced in 1976, Marshosaurus.

Madsen, Jr. made his case on a collection of hip bones and jaw elements that represented at least two individuals of 15-foot-long carnivorous dinosaur. That might not seem like much to go on, but the size and anatomy of Marshosaurus in comparison to its neighbors confirmed that the predator was something never before seen. This was a dinosaur that was larger than the sleek little Stokesosaurus but smaller than the biggest predators, more in the size range of a young subadult Allosaurus.

The trouble with Marshosaurus was that it was either a very rare dinosaur or it more regularly lived in environments away from the floodplains where so many of its neighbors became buried. Only a few pieces have been found at other sites, such as Dinosaur National Monument, and so most of the skeleton remains unknown. The features paleontologists have been able to spot on the assembled bones have narrowed the dinosaur’s identity down to some sort of megalosaur – a cousin of its larger competitor Torvosaurus – but, for the most part, Marshosaurus remains as much of a mystery now as it was in 1976.

In order to solve the puzzle, we need more puzzle pieces. And I’m hopeful that future fieldwork might do just that. This past summer, while I was scratching away at another site near the main bonebed, paleontologists and volunteers from the University of Wisconsin, Oshkosh moved tons of limestone to expose a fresh surface of the Cleveland-Lloyd Dinosaur Quarry. Every bone, every fragment is going to be carefully uncovered and documented when next summer raises the eastern Utah temperature to a setting fit for paleontology. If we’re lucky, more Marshosaurus might be waiting just below the surface.

Reference:

Madsen, J. 1976. A second new theropod dinosaur from the Late Jurassic of east central Utah. Utah Geology. 3 (1): 51-60

Death of Ice Age Giants Shaped Today’s Landscapes

The extinct Bison antiquus at the Page Museum. Photo by Brian Switek.
The extinct Bison antiquus at the Page Museum. Photo by Brian Switek.

I can’t help the impulse. Whenever I’m hiking through the western forests, prairies, and deserts, my mind conjures up images of Ice Age beasts. I guess I keep engaging in the fantasy because I know I just missed them. Ten thousand years is nothing from a geologic perspective. Shasta ground sloths wouldn’t seem out of place shuffling through groves of Joshua trees, mastodons should still be cracking conifer boughs in the woods, and the grasslands were so recently the place where deer, antelope, and camels played.

But the places I scuff my boots aren’t the same as they were back in the heyday of North America’s enormous herbivores. While there have only been a few clicks of the Cosmic Clock since the end of the Pleistocene, the world hasn’t remained in stasis as if waiting for the return of our missing megafauna.

In fact, as Elisabeth Bakker, Jacquelyn Gill, and others argue in a recent review, the extinction of the great Ice Age beasts has created a world dramatically different than the one roamed by shaggy proboscideans and enormous sloths.

Herbivores are plant predators. It might be strange to think of them that way. A horse plucking up grass or an elephant chawing a mouthful of leaves isn’t as violent or gory a spectacle as a pack of wolves taking down an elk. Yet ecologists call the feeding habits of herbivores “predation” for good reason. The interactions may be slower and harder to see, and there is cooperation in addition to competition, but there are still arms races between herbivores and the plants they rely on. And this constant shuffle is what helps create the landscapes we see all around us.

Consider the American mastodon. This “bubby toothed” elephant was a browser, preferring tree branches and other woody vegetation to the diet of grass enjoyed by its distant cousin, the woolly mammoth. But the mastodon didn’t just eat trees and shrubs. The beast would have trampled down paths through the woods and scraped its tusks against tree trunks, further altering the landscape around it by stamping out some young plants and hindering the growth of others. Mammut americanum wasn’t just a big vegetarian.

The beast, Bakker and coauthors write, was an ecosystem engineer.

The same was true of many other herbivores that lived around the world until very recently. Up until about 600 years ago, for example, giant, flightless birds called moas browsed in the forests of New Zealand, helping to create openings in the woodlands that allowed light-sensitive plants to thrive in the patches they opened up. Since their disappearance, however, the forest has closed, with wire plants—whose springy anatomy made them resistant to moa depredations—spreading in greater numbers.

Imagine the damage the claws of Harlan's ground sloth could have done to Pleistocene trees. Photo by Brian Switek.
Imagine the damage the claws of Harlan’s ground sloth could have done to Pleistocene trees. Photo by Brian Switek.

Modern, short-term experiments have shown the same pattern. When big herbivores or either eliminated or excluded from an area, Bakker and colleagues write, the woodlands start to close up. Not as many young plants are being pulled up and fewer adult plants are having to struggle to put out leaves against the onslaught of big browsers.

For example, experimental studies have shown that areas where African elephants have been excluded can host 42% more trees than when the mammals are stripping them and pushing them over. Through extinction and the restriction of our last few megaherbivores to parks and game reserves, we’re playing out a miniature version of what happened when the Pleistocene’s major plant predators went extinct.

So when I set out from my Salt Lake City apartment and climb up above the level of the Lake Bonneville shoreline, to where my Ice Age inspirations used to roam, I need to do more than picture Harlan’s ground sloth or a mastodon trundling through the stands of oak and aspen. I need to think about what those herbivores did. Of branches stripped bare, toppled trunks, and wide paths through the woods pressed down by the hungry herbivores.

The lost megafauna weren’t just charismatic characters from a closed act in Earth’s history. They helped make the world what it was. I am only sorry that I can’t see what they created.

Reference:
Bakker, E., Gill, J., Johnson, C., Vera, F., Sandom, C., Asner, G., Svenning, J. 2015. Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation. PNAS. doi: 10.1073/pnas.1502545112

Paleo Profile: Kimbetopsalis simmonsae

Name: Kimbetopsalis simmonsae

Meaning: “Simmon’s cutting shears of Kimbeto Wash” in recognition of mammal paleontologist Nancy Simmons, the place where the fossils were found, and the snipping front teeth of the beast.

Age: Around 64.5 million years old.

Where in the world?: The San Juan Basin of northern New Mexico.

What sort of critter?: One of the multituberculates, a superficially rodent-like and long-lived group of extinct mammals.

Size: Estimated at around three feet long and over 22 pounds.

How much of the creature’s body is known?: A partial skull and elements of the upper jaws, including teeth still in their sockets.

A restoration of Kimbetopsalis simmonsae. Art by Sarah Shelley.
A restoration of Kimbetopsalis simmonsae. Art by Sarah Shelley.

Claim to fame: Multituberculates were one of evolution’s greatest success stories. That may seem odd to say now, being that they’ve been extinct for over 30 million years, but that’s why a Deep Time perspective is essential to comprehending Life. As New Mexico Museum of Natural History and Science paleontologist Thomas Williamson and colleagues write at the top of their latest paper on the beasts, multituberculates originated and thrived while the dinosaurs still gripped the world in their claws, survived the mass extinction at the end of the Cretaceous, and again proliferated during the “Age of Mammals” for tens of millions of years before finally expiring. And thanks to some pieces of skull found in northern New Mexico, Williamson and colleagues have identified one of the pioneering “multis” that evolved soon after the dinosaurs had global dominance wrested from them.

From its incisors and size, Kimbetopsalis simmonsae would have looked something like a fully-terrestrial beaver. You’d have to look into its mouth and see the ludicrous number of cusps on its cheek teeth to immediately spot it as a multituberculate. And if you really knew your anatomy, as Williamson and coauthors do, you’d eventually work out that Kimbetopsalis is a taeniolabidoid – a subset of particularly large multis whose bones and teeth have been found in Palaeocene rocks through western North America and Asia.

Those teeth may have been what made Kimbetopsalis and its relatives so successful in the wake of the Cretaceous mass-extinction. The anatomy of taeniolabidoid jaws, Williamson and coauthors write, gave them “a grinding-focused chewing stroke”, which, with their snipping incisor teeth, allowed them to tackle a variety of vegetation in the lush world of the Palaeocene. And Kimetopsalis lived large. While not the biggest of the taeniolabidoids, the mammal was significantly more massive than its Cretaceous forebears. The end-Cretaceous mass extinction was devastating to the world’s biodiversity, that’s without question, but the existence of Kimbetopsalis so soon after the catastrophe is a testament to life’s resilience.

Reference:

Williamson, T., Brusatte, S., Secord, R., Shelley, S. 2015. A new taeniolabidoid multituberculate (Mammalia) from the middle Puercan of the Nacimiento Formation, New Mexico, and a revision of taeniolabidoid systematics and phylogeny. Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12336

Previous Paleo Profiles:

Atychodracon megacephalus
Sefapanosaurus zastronensis
Huanansaurus ganzhouensis
Zhenyuanlong suni
Lepidus praecisio
Nothronychus graffami

Ganguroo robustiter
Vulpes mathisoni

Ichibengops munyamadziensis
Pulanesaura eocollum
“Lightning Claw”
Gueragama sulamericana
Kerberos langebadreae

The Rise and Fall of America’s Fossil Dogs

When it came time to adopt a dog, I knew I wanted a dog. An angry, yappy gremlin wouldn’t do. I was on the lookout for a companion that at least somewhat resembled the gray wolf stock from which our domesticated canine companions descended. A real canid. Jet, a black German shepherd I took in earlier this summer, met that description perfectly.

But while I appreciate that Jet has a bit of ancient wolfishness about him, he’s not anything like the earliest dogs. Those dawn canids, which split from their cat cousins around 50 million years ago, were more weasel-like than any modern pooch. These were little ambush predators with dexterous forelimbs that let them grapple with prey. Wolves and other canids as we know them today – pursuit predators that use teeth, rather than paws, to catch and dispatch their victims – are relatively new creatures, and the last remaining sliver of what was once a wider array of prehistoric dogs.

A pair of papers published this month by two different teams of researchers tell the tale. The latest, published today by Universidad de Málaga‘s Borja Figueirido and colleagues, tracks how prehistoric dogs changed as forests gave way to grasslands.

Prior to 40 million years ago, much of North America would have be unrecognizable. Places that host prairies and deserts today were clothed in thick, humid forests where lemur-like primates and little, multi-toed horses browsed on soft leaves. By about 37 million years ago, though, these forests were ceding ground to grasslands.

Herbivores track the change. Some, like the forest-bound primates, went extinct, while the horses, rhinos, and other plant-chewers evolved higher-crowned molars better suited to grinding, grinding, grinding the tougher grasses. And while it was previously thought that this major ecological shift had relatively little effect on North America’s large carnivores, the new study by Figueirido and coauthors concludes that dogs actually went through some dramatic changes as grasslands carpeted the continent.

There were three major lineages of North American dogs during the past 40 million years. There were the early, weaselish hesperocyonines, the bone-crushing borophagines, and the more familiar, wolfish canines. And by looking at the elbow joints of species in all three groups, Figueirido and colleagues were able to reconstruct when canids evolved distinct hunting methods over time.

The hesperocyonines, for the most part, were ambush predators. They could easily turn their paws upwards and had arm flexibility similar to cats. But by around 27 million years ago, when grasslands had become widespread, the borophagines started doing something different. Their elbow joints, Figueirido and co-authors report, were more similar to “pounce-pursuit” predators – similar to modern foxes. Being able to run out in the open was becoming more important as the receding cover gave ambush predators less space to hide. And, speaking of the vulpine canids, the same pounce-pursuit method apparently evolved again around 7 million years ago around the last common ancestor of foxes and wolves. It wasn’t until the Pleistocene, around 2 million years ago, that the first full-on pursuit canids evolved. These dogs, including the predecessors of wolves, were endurance runners with elbows that primarily permitted a forward-back range of motion. As prey wandered out into the open, canids evolved into predators that could go the distance.

The skull of the small dog Archaeocyon pavidus compared to the large Epicyon haydeni. © AMNH/J. Tseng.
The skull of the small dog Archaeocyon pavidus compared to the large Epicyon haydeni. © AMNH/J. Tseng.

But dogs couldn’t stay on top forever. A second study, published earlier this month by University of Gothenburg paleontologist Daniele Silvestro and colleagues, found that competition between carnivores eventually drove two of the three dog lineages extinct.

Why some groups of organisms prosper and others are totally obliterated is the sort of mystery that keeps paleontologists up at night. Why, for example, were dinosaurs winnowed down to only the birds while mammals were better-able to resist the mass extinction of 66 million years ago and proliferate? This is still unknown. Yet, even if we can’t always get a handle on why balances tip this way or that, researchers can still track how rates of speciation and extinction have altered the course of life. The record of North America’s dogs offered one such opportunity.

In addition to the hesperocyonines, borophagines, and canines, Silvestro and coauthors investigated the speciation and extinction records for cats, nimravids (false sabercats), barbourofelids (another false sabercat lineage), amphicyonids (bear dogs), and bears over the last 40 million years. All of these included large carnivores that jostled for space and prey with the canids. By looking at how these different groups fared, the paleontologists picked up patterns that can be explained by groups outdoing each other.

Some of the competition was between the dogs. Between 20 and 10 million years ago, for example, the archaic hesperocyonines suffered a high extinction rate just as the borophagine dogs were undergoing an expansion in their range of body sizes. They were infiltrating the predatory niche the hesperocyonines had controlled for millions of years. This, Silvestro and coauthors write, is a sign of what’s called active displacement – one lineage shouldering out another. The telltale clue is that once the more archaic dogs were gone, the borophagines didn’t undergo a major radiation (as would be expected if they were somehow being suppressed under “passive displacement” by their competition).

But the borophagines eventually went extinct, too, and competition with a wider array of predators may be the reason why. The borophagines had an intermediate, pounce-pursuit joint structure and many were hypercarnivorous, subsisting almost wholly on flesh and bone. This put them in direct competition with the more dexterous cats who jumped from the tall grass as well as the marathon-running canines. Borogphagines may have wound up in the middle of two guilds of predators that had specialized on the ambush and pursuit modes of taking down prey, respectively, leaving the borophagines in the generalist valley between them. Their extinction rate outpaced the rate at which they spun off new species, leaving the running to the canines and the stalking to the felids. Think about that the next time you throw a ball for your puppy or dangle a toy mouse in front of your housecat. The way they play – which is itself predatory practice – is a faint, compact glimmer of 40 million years of extinction and evolution.

References:

Figueirido, B., Martin-Serra, A., Tseng, Z., Janis, C. 2015. Habitat changes and changing predatory habits in North American fossil canids. Nature Communications. doi: 10.1038/ncomms8976

Silvestro, D., Antonelli, A., Salamin, N., Quental, T. 2015. The role of clade competition in the diversification of North American canids. PNAS. doi: 10.1073/pnas.1502803112

Hothouse Climate Slowed Dinosaurs’ Rise

It gets hot at the Hayden Quarry. Hot enough to keep away the biting gnats – eternal foe of the field paleontologist – and to require a brief siesta every afternoon in the cool of a nearby streambed. The August monsoons do little to help. The New Mexico desert greedily slurps every drop of moisture and within a few hours you forget that water ever falls from the sky. But that’s as it should be. It helps get you in the mindset of creatures that lived and died in the spot over 211 million years before, back when ancient aridity kept early dinosaurs down.

We often think of the Mesozoic as an endless summer when reptilian monsters stalked jungles and swamps choked with vegetation. Some dinosaurs really did live that large. But the Hayden Quarry tells a very different story. It’s from a time when the “Age of Dinosaurs” hadn’t truly begun. When tropical heat created wildly-fluctuating habitats repeatedly scorched by wildfire.

Paleontologist Adam Pritchard excavating part of the Hayden Quarry in August 2011. Photo by Brian Switek.
Paleontologist Adam Pritchard excavating part of the Hayden Quarry in August 2011. Photo by Brian Switek.

The signs of the Triassic blazes are easy to spot. Chunks of charred wood are scattered amongst the black Hayden Quarry bones, sometimes leading to moments of nervous excavation when it’s not clear whether you’re uncovering part of a little phytosaur jaw or the burnt remnants of a prehistoric conifer. (When in doubt, treat a fossil like it’s bone until you’ve proven otherwise.) And while they seem rather mundane next to the skulls, limb bones, and other vertebrate fossils that pack the quarry, the crispy remnants of prehistoric plants are what truly set the stage for this slice of time.

University of Southampton geologist Jessica Whiteside and colleagues tell the tale in a new paper in PNAS. By turning to the burnt plants, fossil pollen, and carbon isotopes of the Hayden Quarry, the researchers were able to piece together what this spot in northern New Mexico was like 211 million years ago. The site, which was then within the tropics, was a hot, arid place continuously tossed between wet and dry seasons (roughly similar to the seasonal shifts that bake the quarry today). It was so persistently dry that the local foliage often turned to tinder. Dessicated and dead plants provided the fuel for frequent wildfires that raged between 320 and 680 degrees Celsius.

Those fires altered the plant communities from season to season. Even though the forests hosted an increasing number of conifers alongside the more archaic seed ferns, Whiteside and coauthors found, the plant species present kept changing as wildfires reshuffled the ecological deck. Later the rains returned to batter that blackened ground, washing loose soil, singed wood, and bones together into the stream channels that now preserve this snapshot of Triassic life.

Dinosaur distribution in the Late Triassic. Only small carnivores managed to get a toehold in the tropics. From Whiteside et al., 2015.
Dinosaur distribution in the Late Triassic. Only small carnivores managed to get a toehold in the tropics. From Whiteside et al., 2015.

Yet, despite all this ecological chaos, the animals of the age lived in stable communities. They were resilient creatures that were able to carve out a living in the shifting landscape. The most diverse and disparate creatures of the tropical Triassic were pseudosuchians – crocodile cousins that included bipedal “dinosaur mimics”, huge carnivores, heavily-armored omnivores, and more. Dinosaurs, meanwhile, were only represented by a few small, sleek carnivores. There were no giant herbivores, like the long-necked sauropodomorphs found at higher latitudes, or, in fact, plant-eating dinosaurs of any kind. The plant communities in the low latitudes were too changeable to support dinosaurs that required a great deal of forage to keep their metabolisms running hot. Only little hunters could eke out a living here.

What Whiteside and colleagues found at the Hayden Quarry holds across the planet. Dinosaurs didn’t dominate the Triassic tropics. The increased carbon dioxide in the atmosphere created a hothouse climate where the sharp swings between the seasons prevented dinosaurs from getting any more than toehold at low latitudes. It was only in the wetter, lush regions away from the equator that dinosaurs started to get big and diversified. These were the centers of dinosaur evolution that produced the diversity which later took over the planet when a mass extinction decimated the protocrocs at the end of the Triassic. The true “Dawn of the Dinosaurs” didn’t start until the world had turned in their favor.

[Note – I’ve previously volunteered on Hayden Quarry excavations with several of the study authors, and will be returning there this coming August.]

Reference:

Whiteside, J., Lindström, S., Irmis, R., Glasspool, I., Schaller, M., Dunlavey, M., Nesbitt, S., Smith, N., Turner, A. 2015. Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years. PNAS. doi: 10.1073/pnas.1505252112

The Making of California’s Mini-Mammoths

Over 80,000 years ago, somewhere on a southern California beach, a mammoth wanders across the sand. The beast doesn’t stop at the water’s edge. Step by step, its trunk held high, the towering proboscidean walks into the waves until its feet no longer touch the bottom. One, two, three, kick, the elephant keeps paddling, the air-filled sinuses of its great skull helping to keep the mammoth’s head near the surface as it moves further and further out from the coastline. It has a long swim ahead. In the distance, rising from the surface 12 miles to the west, are a small set of islands. That’s where the mammoth will again emerge onto dry land.

We know that such an event must have happened, and likely happened more than once. The evidence doesn’t come from swim tracks. Those would be nice, but haven’t been found yet (if they were ever preserved in the first place). The clues are in skeletons. On California’s Channel Islands, in sediments left from the last Ice Age, paleontologists have found the remains of mammoths that could have only gotten there thanks to some intrepid pachyderms that made the journey from the mainland.

The species that braved the surf were Columbian mammoths – a species only found in North America that ranged over much of the continent, and was presumably less shaggy than its famous woolly cousin. These mammoths weren’t just visitors to the Channel Islands, though. They stayed, and, thanks to a phenomenon called insular dwarfism, their population eventually evolved into a smaller species – the pygmy mammoth Mammuthus exilis. These little mammoths stood only about five and a half feet at the shoulder, less than half the stature of their ancestors.

Swimming was the only way the mammoths could have arrived. There was no landbridge between the continent and the Channel Islands for them to cross. That’s why other parts of California’s Ice Age fauna – like sloths and sabercats – haven’t been found along the chain. Only aquatically-adept species could make it, and, if the skills of today’s Asian elephants are any indication, Columbian mammoths were likely strong swimmers. And they got a little help from ice. When the world’s glaciers crept over the land, sucking up water to expand their reach, the sea level off the California coast fell and vastly reduced the distance to the Channel Islands. During these times, when the Ice Age was in full effect, the mammoths could swim out to the islands.

An excavation of the Channel Islands pygmy mammoth. Photo by Bill Faulkner, NPS.
An excavation of the Channel Islands pygmy mammoth. Photo by Bill Faulkner, NPS.

It was thought that the Columbian mammoths made this trek during the Last Glacial Maximum, sometime around 26,000 years ago or so. Most mammoth remains on the Channel Islands fall within the window of 22,000 to 12,000 years ago. But now there’s an even older date. Thanks to a tusk found along a Channel Islands beach, U.S. Geological Survey researcher Daniel Muhs and colleagues have now pushed back the oldest known mammoth on the islands by tens of thousands of years.

Which species the tusk belonged to isn’t clear. It’s small enough that it could either by a pygmy mammoth or a juvenile Columbian mammoth. But dates derived from prehistoric corals from levels beneath the tusk indicate that this mammoth died around 80,000 years ago.

This newly-discovered mammoth probably wasn’t the first to stomp across the archipelago. At the time the beast inhabited the Channel Islands, Muhs and coauthors report, the world was in an interglacial. The local sea level was high, and, even if mammoths were accomplished swimmers, the distance was likely too far for them to cross. Therefore, the paleontologists hypothesize, the mammoths must have arrived during even earlier times when glaciers created a shallower sea.

The two earlier periods when mammoths could have swum across the channel were 150,000 and 250,000 years ago. With luck, the bones of the earlier arrivals will help refine the date and help fill in this new gap in the mammoth story. It could be that the pygmy mammoths evolved from their Columbian ancestors earlier than thought. Then again, perhaps there were multiple waves of mammoth habitation, flourishing and declining as the seas rose and fell. This isn’t just a story of evolution, though. It’s also one of extinction.

Changes in sea level allowed mammoths to swim to the Channel Islands. From Muhs et al., 2015.
Changes in sea level allowed mammoths to swim to the Channel Islands. From Muhs et al., 2015.

No one knows what wiped out California’s mini-mammoths. The traditional Pleistocene culprits have been invoked, and both have their problems.

While hunting by humans is often cited as an extinction trigger, especially on islands, there was relatively little overlap in time between the Channel Islands mammoths and humans. Not to mention that no one has yet found unequivocal evidence of hunting or butchery. Extinction by overkill can’t be taken as a default explanation.

Swift ecological change is the other popular option. Rising sea levels could have put the squeeze on mammoth populations, and changing vegetation may have limited the beasts’ food supply. But if mammoths arrived on the islands over 80,000 years ago and maintained stable populations from that time on, Muhs and colleagues argue, then they would have previously experienced even more extreme shifts in sea level, climate, and vegetation. If they survived these changes in the past, why not at the end of the last Ice Age?

Not that these explanations are mutually exclusive. Perhaps both changing habitat and humans had roles to play. We often confuse the reason the last members of a species died for the singular extinction trigger. The truth may be that multiple causes made populations vulnerable to a particular stress which eventually eradicated any hope of recovery. Extinction is a process, not a singular event. How that process played out amongst California’s lost mammoths, however, is a secret still held tight in tusk and bone.

Reference:

Muhs, D., Simmons, K., Groves, L., McGeehin, J., Schumann, R., Agenbroad, L. 2015. Late Quaternary sea-level history and the antiquity of mammoths (Mammuthus exilis and Mammuthus columbi), Channel Islands National Park, California, USA. Quaternary Research. 83: 502-521. doi: 10.1016/j.yqres.2015.03.001

You Just Missed the Last Ground Sloths

When did the last of the ground sloths disappear? The standard answer is “about 10,000 years ago”. That’s the oft-repeated cutoff date for when much of the world’s Ice Age megafauna – from mastodons to Megatherium – faded away. It’s nice and neat, falling just after the close of the last Ice Age and during a time when humans were spreading to new continents. In fact, it’s too clean a cutoff. The shaggy, ground-dwelling sloths that inhabited almost the entire span of the New World didn’t all topple over at once. They very last of their kind, both protected and made vulnerable by life on islands, were still shuffling 4,200 years ago.

Calling the time of death for any species or lineage is always complicated by definitions and details. Should a species be considered extinct when its very last member perishes, or when the population sinks below a level from which they can recover? And in these fading families, should the explanation for extinction be the cause of death of the last individual, or do we assemble a more complex picture that considers factors that made the population vulnerable in the first place? Both science and storytelling influence our answers to these questions, but one thing is abundantly clear. Extinction is a process, not a single fell swoop.

Consider the times when the giant ground sloths disappeared. They were one of the great success stories of the Ice Age – with 19 genera ranging through South, Central, and North America, as well as Caribbean islands at the end of the Pleistocene – but, as reported by paleontologist David Steadman and colleagues in a 2005 study, 90% of the existing Ice Age sloths disappeared within the last 11,000 years.

Megalonyx and other giants from North America were some of the first to go. While Steadman and colleagues stressed that the dates represent “last appearance dates” rather than actual time of species death, the youngest known sloth remains from North America date to about 11,000 years ago. South America’s ground sloths, such the enormous Eremotherium, soon followed – the youngest dung and tissue samples found on the continent date between 10,600 and 10,200 years ago.

But for another 5,000 years, ground sloths survived. They weren’t on the continents, but scattered through the islands of the Caribbean. I had not even heard about these sloths until paleo geneticist Ross Barnett told me about them in a Twitter exchange long ago, and, as reviewed in the paper by Steadman and colleagues, there were at least five genera and thirteen species of large ground sloths that were unique to these islands.

Cuba's extinct ground sloth Megalocnus rodens. Photo by Ghedoghedo.
Cuba’s extinct ground sloth Megalocnus rodens. Photo by Ghedoghedo.

The largest of all was Megalocnus. This sloth hasn’t received nearly as much attention as the other “mega”-prefixed sloths, but, as you can see from the bones on display at the American Museum of Natural History’s fossil mammal hall, this 200-pound sloth was still an impressive beast. Based on remains found in a limestone cave on Cuba, Steadman and colleagues determined that Megalocnus lived until at least 6,250 years ago.

Other smaller sloths persisted even longer. Parocnus, also found on Cuba, lived until about 4,960 years ago, and the small ground sloth Neocnus trundled over Hispaniola until about 4,500 years ago. There’s no direct evidence that people were hunting or eating the sloths, but, based on tentative evidence for human occupation of Caribbean islands around 5,000 years ago, Steadman suggest that the arrival of Homo sapiens tipped the sloth into extinction.

Of course, last appearance dates are often revised with new finds and updated techniques. Two years after the Steadman study, Ross MacPhee and coauthors published a new, youngest date for Cuba’s Megalocnus. From a tooth found on the island, the researchers estimated that the ground sloth survived to at least 4,200 years ago.

Through the lens of geologic time – wherein millions of years are thrown around because the numbers are too big to truly comprehend – extending the lifetime of a ground sloth another 2,000 years might not sound like much. But MacPhee and colleagues underscore the importance of getting good dates for when Ice Age creatures vanished. If people really showed up on Cuba and other sloth-bearing islands around 5,500 years ago, then humans and ground sloths coexisted for over a thousand years and the “blitzkrieg” model of extinction starts to crumble. Humans may have still been responsible for the extinction of the sloths and other species, but the record doesn’t show the pattern of rapid die-off that has sometimes been used to pin our species as the chief cause of megafaunal extinctions.

In time, we may get a clearer picture of why such a diverse and widespread ground of mammals disappeared. Assuming that humans, climate change, or any of the other traditional suspects without more detailed evidence masks the complexity of how extinction happens. But even if paleontologists eventually puzzle together what happened to these great beasts, I’ll still be saddened by the fact that I just missed the ground sloths. Especially because there are habitats – such as vast stretches of desert in the basin and range I call home – that could still host them. Sometimes, when hours of rolling over the interstate starts to addle my brain, I start to imagine them out among the Joshua trees – reminders that we still live in the shadow of the Ice Age world.

References:

MacPhee, R., Iturralde-Vinent, M., Vázquez, O. 2007. Prehistoric sloth extinctions in Cuba: Implications of a new “last” appearance date. Caribbean Journal of Science. 43, 1: 94-98.

Martin, F., San Román, M., Morello, F., Todisco, D., Prevosti, F., Borrero, L. 2013. Land of the ground sloths: Recent research at Cueva Chica, Ultima Esperanza, Chile. Quaternary International. 305: 56-66. doi: 10.1016/j.quaint.2012.11.003

Steadman, D., Martin, P., MacPhee, R., Jull, A., McDonald, H., Woods, C., Iturralde-Vinent, M., Hodgins, G. 2005. Asynchronous extinction of late Quaternary sloths on continents and islands. PNAS. 102, 33: 11763-11768. doi: 10.1073/pnas.0502777102

Book in Brief: How to Clone a Mammoth

“Will there ever be a real Jurassic Park?” I’ve heard this question more times than I can count. The answer is always “No“. Aside from the problem of getting a viable clone to develop inside a bird egg – one that scientists haven’t cracked yet – DNA’s postmortem decay happens too fast to give us any hope of saying “Bingo! Dino DNA!” someday. But just because it won’t work for Tyrannosaurus doesn’t mean that it’s impossible for other forms of life. In How to Clone a Mammoth, ancient DNA expert Beth Shapiro offers a thrilling tour of the science that might – might – recreate lost worlds from the not-too-distant past.

how-to-clone-a-mammothThe book’s title is a bit of a bait-and-switch. On the very first page, Shapiro explains that for long-extinct organisms such as “the passenger pigeon, the dodo, the mammoth – cloning is not a viable option.” If at all, these organisms are going to come back to us piecemeal as revived genetic material expressed in hybrid creatures that may, or may not, look like the lost species. And this cuts to the core of what de-extinction is really all about.

From a purist’s perspective, extinction really is forever. It’s impossible to recreate lost species exactly as they were, down to every last gene and quirk of behavior. But with a broader definition of de-extinction – creating organisms that can fill vacant ecological roles – an elephant with a touch of mammoth trundling around the Arctic steppe would count as what Shapiro dubs an unextinct species. This is the goal of de-extinction efforts – not to recreate extinct species down to the finest detail, but to generate organisms that rehabilitate ecosystems. Not so much resurrection as carefully-crafted reinvention focused on ecosystem-scale repair.

As a researcher who is shaping this field, Shapiro is the perfect guide to the ongoing discussion about de-extinction. While many news items and conference presentations have focused on the technology required to recreate extinct life, Shapiro carefully considers every step along the journey to de-extinction, from choosing a species to revive to making sure they don’t become extinct all over again. As Shapiro says herself, she’s a realist rather than a cynic, and her finely-honed prose cuts through the hype that has clouded the debate around whether or not we should be striving to recreate lost species when so many living species are hanging on by the barest thread.

In fact, Shapiro uses the tension between those advocating for the return of extinct species and critics who argue that the effort would be better spent saving today’s imperiled organisms to propose a third option that has barely been discussed. Whether or not proxy mammoths, dodos, or sabercats come back, exploring such possibilities may give conservationists new tools to manage and assist threatened species and ecosystems. We’re already carrying out conservation triage on the weak and wounded, so why not use every tool at our disposal to sustain – and perhaps even improve – what we’re already managing by hand? Or, as Shapiro writes near the end of the book, “De-extinction is a process that allows us to actively create a future that is really better than today, not just one that is less bad than what we anticipate.”

Will genetically-modified pseudo-mammoths or passenger-ish pigeons be the first symbols of a new age in conservation? That’s still unclear. But even if we never see shaggy elephants or the shade cast by immense pigeon flocks, de-extinction research already underway has the potential to both tell us about the past and provide us with new tools to decide the future shape of nature. Whether you’re all for de-extinction or against it, Shapiro’s sharp, witty, and impeccably-argued book is essential for informing those who will decide what life will become.

The Mediterranean’s Missing Sawfishes

In 1959, off the southern coast of France, a tuna boat hauled up a largetooth sawfish. The catch wasn’t particularly large. The razor-snouted fish only stretched about four feet long; still a baby by the standards of its species. But it was one of the last to be seen in those waters. Within a decade, the largetooth sawfish entirely disappeared from the Mediterranean Sea.

Such accidental catches and sightings in the Mediterranean have often been regarded as signs of largetooth and smalltooth sawfish migrating into the sea from the coast of Africa. When sawfish experts gathered in London in May 2012 under the auspices of the IUCN, they concluded that the Mediterranean gets too cold in winter to have hosted resident populations of the warm-water fish. Therefore all the historic accounts must have referred to “vagrant” animals, and sawfish blades in museums were brought to coastal museums by trade routes that have been in place for centuries. But after trawling through bibliographic records and museum displays, Hopkins Marine Station biologist Francesco Ferretti and colleagues have suggested a different interpretation. The Mediterranean is missing its native sawfishes.

Ferretti and coauthors cast a wide net. They searched everything from records on pre-dynastic Egypt through modern ocean biodiversity databases to find any sign of sawfishes in the Mediterranean. Being that the science of ichthyology didn’t get going until the 16th century, it’s not surprising that the earlier part of their timerange came up empty. But between the 18th and 20th century – when naturalists often kept track of who was landing what at the local docks – the researchers turned up 48 original accounts of sawfish in the Mediterranean, 24 of which could be verified in the literature or in museums. These verified records were split between largetooth and smalltooth sawfish, and the size of many of these fish hint that they were not migrants from the African coast.

Map of Mediterranean sawfish records, with smalltooth in orange and largetooth in green. From Ferretti et al., 2014.
Map of Mediterranean sawfish records, with smalltooth in orange and largetooth in green. From Ferretti et al., 2014.

Largetooth and smalltooth sawfish grow slowly. Largetooth sawfish, in particular, take between eight and ten years to reach maturity, at which time they’re about 10 feet long and start reproducing. But in their youth, sawfish typically stick very close to the place they’re born. Smalltooth sawfish, for example, have a home range of less than a half a mile. Ferretti and coauthors counted 15 sawfishes in their sample that fell within the juvenile category.

If those juvenile sawfishes were swimming from the nearest population centers to the Mediterranean, they must have journeyed more than 2,000 miles – over ten times the distance an adult sawfish has ever been observed to travel. Unless the historic populations of large and smalltooth sawfish undertook truly exceptional journeys, Ferretti and colleagues argue, it’s more likely that they had a resident population in the Mediterranean.

While lacking in as much detail as modern biologists wish for, the 18th and 19th century naturalist accounts of sawfishes also throw some support to the idea that sawfish had a home in the Mediterranean. Some accounts list them as relatively rare, and others as common, but there doesn’t seem to be any hint that it was strange to see sawfishes along Europe’s southern coastlines. It was only in the 20th century – when sawfish populations plummeted and totally disappeared – that sawfishes were regarded as especially rare and the idea of migration started to take hold.

Not all marine biologists are convinced by the historic evidence. In a paper published around the same time as the paper by Ferretti and coauthors, Nicholas Dulvy and colleagues argue that the Mediterranean gets too cold for sawfishes and that the past occurrences really do represent piscine vagrants. More than that, Dulvy and colleagues write, “Whether or not sawfishes were previously extant in the Mediterranean Sea has little bearing on current conservation priorities as any activities benefitting West African sawfishes can only restore migration and improve the likelihood of vagrancy to the Mediterranean Sea once again.”

Ferretti and colleagues disagree on both counts. Regarding temperature, the researchers suggest, past Mediterranean sawfish populations may have been better-adapted to cooler waters than others. If not that, then young sawfishes could have taken refuge in deeper water that maintains warmer, more constant temperatures than those at the surface during winter. There may not be a way to know for sure – the Mediterranean sawfishes are all gone – but temperature alone can’t be used to rule out the previous presence of resident populations.

And this question does have relevance for the future of the largetooth and smalltooth sawfish. If sawfishes previously had a home in the Mediterranean, perhaps they could live there again. Ferretti and colleagues even have a spot in mind – a national park in southern France near where the last recorded sawfishes were seen. What hopes sawfishes might have for survival in such a place are murky, but if restoration attempts are to be considered at all, the Mediterranean may be a place where these awkwardly charismatic fish may find a refuge.

How long the longtooth and smalltooth sawfish will survive is unknown, and their fate largely rests on the decisions we make. And in making those decisions we must be aware of our own history. The only good records of Mediterranean sawfishes we have come from a timespan when these vulnerable fish had already been coping with centuries of human disturbance to their nearshore haunts. Our species only started keeping track of what was “natural” when the sawfishes were already in decline. Marine biologists know this as “shifting baselines“, and it’s the same reason why many don’t feel the absence of ground sloths and mastodons in North America’s forests. The megamammals were already gone by the time naturalists started paying attention to the woods, and we don’t consider how empty the landscape is. We just don’t know what we’re missing.

References:

Dulvy, N., Davidson, L., Kyne, P., Simpfendorfer, C., Harrison, L., Carlson, J., Fordham, S. 2014. Ghosts of the coast: global extinction risk and conservation of sawfishes. Aquatic Conservation. doi: 10.1002/aqc.2525

Feretti, F., Verd, G., Seret, B., Šprem, J., Micheli, F. 2014. Falling through the cracks: the fading history of a large iconic predator. Fish and Fisheries. doi: 10.1111/faf.12108

Guilhaumon, F., Albout, C., Claudet, J., Velez, L., Lasram, F., Tomasini, J., Douzery, E., Meynard, C., Moupuet, N., Troussellier, M., Araújo, M., Mouillot, D. 2014. Representing taxonomic, phylogenetic, and functional diversity: new challenges for Mediterranean marine-protected areas. Diversity and Distributions. doi: 10.1111/ddi.12280

The Opossum’s Tale

In the continental United States, there’s only one native marsupial – the Virginia opossum. The title’s a bit of a misnomer. Opossums aren’t just restricted to the “Mother of Presidents.” They raid garbage cans and scurry across streets up and down the entirety of the east coast and the west, really only becoming scarce in deserts and badlands. This resourcefulness truly makes them the American opossum, and they’re an echo of a prehistoric time when their kin used to be quite common around here.

North America used to be a hotspot for archaic marsupials and their pouched relatives. Over 66 million years ago, when the likes of Tyrannosaurus and Triceratops were still stomping around, ancestral marsupials and their closest relatives – called metatherians – were both diverse and abundant. Our own ancestors and close cousins – the eutherians – were marginal mammals by comparison. The mass extinction that wiped out the non-avian dinosaurs is what decimated the metatherians and changed the course of mammalian history.

A map of metatherian localities circa 75 million years ago. From Williamson et al., 2014.
A map of metatherian localities circa 75 million years ago. From Williamson et al., 2014.

Thomas Williamson, Steve Brusatte, and Gregory Wilson track the upset in a new ZooKeys paper. The first part is tracking the rise of the protomarsupials. While the first metatherians evolved late in the Jurassic, by around 160 million years ago, they didn’t really get going until the later part of the Cretaceous.

From teeth, bits of jaw, isolated postranial bones, and extremely rare partial skeletons, paleontologists have counted 68 metatherian species from Europe, Asia, and North America during the entirety of the Cretaceous. Of all these, 29 species were present around 75 million years ago – the height of Cretaceous metatherian diversity. And even though their global diversity dipped to 25 recognized species during the last part of the Cretaceous, they were still common mammals that came in a variety of shapes and sizes.

Figuring out how these metatherians lived is hindered by the fact that almost everything we know about them comes from teeth and scraps of jaw. Still, Williamson and coauthors write, Cretaceous metatherians ranged from shrew-sized insectivores to carnivores and omnivores the size of a small Virginia opossum, some of them with specialized crushing teeth to bust up insect carapaces or tough seeds. Along with a totally extinct lineage of mammals called multituberculates, the metatherians were doing quite well during the Late Cretaceous.

This is what a mass extinction looks like. The global record of metatherians from the Cretaceous to the earliest part of the Paleocene. From Williamson et al., 2014.
This is what a mass extinction looks like. The global record of metatherians from the Cretaceous to the earliest part of the Paleocene. From Williamson et al., 2014.

But the protomarsupial heyday couldn’t last forever. The fossils found in the 68-66 million year old rocks of western North America document the metatherian downfall.

In the famous Hell Creek Formation, Williamson and colleagues note, 12 of the 31 mammal species present were metatherians, and, depending on the locality, they make up 35-60% of the mammal species found. (I was even lucky to stumble across one a few years ago – part of the upper jaw and two teeth from a metatherian called Pediomys.) In the rock just above the Hell Creek, though, there’s just a single metatherian – Thylacodon montanensis. Whether this was a true survivor or a species that migrated to the area isn’t clear, but, despite the fact that the mammal was abundant during life’s recovery from the great ecological shock of extinction, North America’s metatherians were not able to reclaim their former diversity or numbers. The scales had tipped in favor of our own eutherian relatives.

The metatherians didn’t fade away into nothing, of course. During the latest part of the Cretaceous, some metatherians started to invade South America. This emigration continued after the mass extinction, with almost twice as many metatherian species in South America as in the whole of North America. This left metatherians – including early marsupials – with an entire isolated continent to undergo a new radiation upon and start spreading elsewhere around the planet.

And even after the great extinction, North America remained an important place for metatherian evolution. It may have even been the place where the very first opossums started to snuffle through the lush forests that once covered the American west. One of the earliest opossums, named Mimoperadectes houdei by Inés Horovitz and colleagues in 2009, was found in the 55 million year old rock of Wyoming. Along with other finds, Mimoperadectes hints that opossums, at least, originated in North America before spreading south. The American opossum continues the legacy of Mimoperadectes today, both an echo from ancient time and a testament to the metatherian ability to survive.

References:

Horovitz, I., Martin, T., Bloch, J., Ladevèze, S., Kurz, C., Sánchez-Villagra, M. 2009. Cranial anatomy of the earliest marsupials and the origin of opossums. PLOS One. 4 (12): e8278. doi: 10.1371/journal.pone.0008278

Williamson, T., Brusatte, S., Wilson, G. 2014. The origin and early evolution of metatherian mammals: the Cretaceous record. ZooKeys. 465: 1-76. doi: 10.3897/zookeys.465.8178

Missing the Mastodon

When I look out my front window to the Wasatch Front, I can’t help but feel that there’s something missing. It’s not something seen, but something that comes from knowing what used to be here. The absence takes the shape of big, shaggy elephants called American mastodon. This was their home until not so very long ago.

From a collection of bones described in 1981 – the first such find reported in all of Utah – paleontologists know that the “bubby toothed” proboscideans lived in the Salt Lake Valley during the last Ice Age. That’s practically yesterday. When I write about non-avian dinosaurs or other ancient creatures, I can’t really get my head around just how long a span of time separates me from them. It’s easy to rattle off dates in millions of years. But American mastodon may have trod through the ground that makes up my front yard close to the boundary of when prehistory became history. It seems close enough to almost touch them, but extinction keeps them just as far away from me as any other vanished species.

Why Mammut americanum and many of its Ice Age neighbors – the giant ground sloths, sabercats, and others – died out is a mystery, and a contentious one at that. A slew of possible culprits have been implicated, including climate change, the impact of a comet, disease, and hungry, hungry humans. Some – such as the comet and hypervirulent disease – have been discarded, but even left with climate change and hunting as frontrunners, uncovering the truth about the disappearance of North America’s great megafauna is a fraught undertaking. Tracking the comings and goings of species around us is difficult enough. Replaying prehistory is even more challenging, especially when extinction cannot be boiled down to a single phenomenon such as warming temperatures or the invention of the atlatl.

If we’re going to understand what happened to the American mastodon and its megafaunal ilk, we need a more refined view of when they lived, where they lived, and how their habitats changed through time. Paleontologists are still piecing together this essential picture of Pleistocene life, and one of the latest attempts to do so was published last week in PNAS. Focusing on American mastodon bones from the Arctic, the scientists found that the great beasts had already vanished from the chilly north by the time humans arrived.

American mastodon followed the expansion and contraction of Ice Age forests, only inhabiting the Arctic during warmer, wetter periods. From Zazula et al., 2014.
American mastodon followed the expansion and contraction of Ice Age forests, only inhabiting the Arctic during warmer, wetter periods. From Zazula et al., 2014.

The study was spurred by an anomaly. Despite living during the Ice Age, American mastodons were not cold-weather elephants. They preferred warmer, wetter climes – usually forests dense with conifers and lowland swamps. But radiocarbon dates from rare American mastodon bones found in Alaska and the Yukon suggested that the animals lived there during the last gasp of glaciation between 18,000 and 10,000 years ago. Were the mastodons really living among the open, cold, dry steppe that covered the Yukon then, or were the dates wrong?

Grant Zazula of the Yukon Paleontology Progam joined 14 other researchers to come up with new 53 new radiocarbon dates for 36 American mastodon fossils, including those dated previously. It turned out that the earlier dates were wrong. Contamination – either naturally-occurring or from museum preservation practices – had given erroneously young dates. The mastodons weren’t trundling across the frosty steppe, after all.

The dates Zazula and his colleagues came up with are all over and around 50,000 years ago. This is the limit of radiocarbon dating. What that means is that most, if not all, the American mastodons in their sample perished before 50,000 years ago. And while the difference between 18,000 years ago and more than 50,000 years ago might not seem like much, it made all the difference to the American mastodons.

The Ice Age was not consistently icy. The great glaciers of the north waxes and waned through time, and the last time ice started to take over North America was around 75,000 years ago. The relatively warm forests fell away to make way for chilly, open steppe. And when this happened, Zazula and coauthors write, the American mastodons were extirpated from the Arctic. There was no longer anywhere for them to live, and so their range contracted to those populations living in more southern forests. The new radiocarbon dates – indicating ages over 50,000 years – are in accord with the shifting forests. And it wasn’t just the mastodons that were affected this way. The bones of Jefferson’s giant ground sloth and the giant beaver Castoroides show the same pattern.

This means that the American mastodons had already vanished from the Arctic by the time humans arrived in prehistoric Alaska around 12,500 years ago. “Over-chill” had already made the Arctic inhospitable to the beasts. Humans likely played some role in the ultimate extinction of the mastodon, but humans had to spread southward before meeting the mastodon.

Whatever happened, though, we now live in a world that could still be home to American mastodon. The world may have changed too much for the woolly mammoth, but it’s not very difficult to picture mastodon among the forests and swamps of North America today. In fact, they were making a comeback just before they were snuffed out. In the eastern part of the continent, Zazula and colleagues write, American mastodons were starting to follow forests north as the last great ice sheet receded. And that’s when their time on Earth closed, leaving me to only imagine how wonderful they would have been in life.

References:

Miller, W. 1987. Mammut americanum, Utah’s first record of the American mastodon. Journal of Paleontology. 61 (1): 168-183

Zazula, G., MacPhee, R., Metcalfe, J., Reyes, A., Brock, F., Druckenmiller, P., Groves, P., Harington, C., Hodgins, G., Kunz, M., Longstaffe, F., Mann, D., McDonald, H., Nalawade-Chavan, S., Southon, J. 2014. American mastodon extirpation in the Arctic and Subarctic predates human colonization and terminal Pleistocene climate change. PNAS. doi: 10.1073/pnas.1416072111

Big-Headed Carnivore a Sign of Triassic Recovery

I’ve spent much of my weekend writing about Jurassic World. I won’t rehash the details here – you can read those over at VICE – but it struck me how easy it is to talk about paleontology when everyone knows the animals you’re discussing. I don’t have to explain who Tyrannosaurus or Velociraptor were, and, from museums and movies, most everyone has some idea of what a dinosaur is.

But if Colin Trevorrow were directing Triassic World, my job would be a lot more difficult. With the exception of the first dinosaurs, and maybe the “armadillodiles“, most of the strange creatures that thrived between 252 and 200 million years ago don’t have common names or much presence at all in the public consciousness. So you’re going to have to bear with me for a second while I introduce you to Garjainia madiba.

Discovered in the 247 million year old rock of South Africa, and described by Natural History Museum, London paleontologist David Gower and colleagues in PLoS One, Garjainia madiba belonged to a group of carnivores called erythrosuchid archosauriforms. Let’s unpack that.

You know birds and crocodiles? They’re the two living lineages of a group of animals called archosaurs – the “ruling reptiles” – that, in turn, were part of a larger radiation of critters called archosauriforms. So lower down on the tree, close to the roots, there was a lineage of predatory archosauriforms called erythrosuchids to which Garjainia belonged. To give it a little more context, Garjainia madiba was archaic enough that, in hindsight, we can say it’s equally-closely-related to birds and crocodiles. Garjainia and its carnivorous kin evolved before that great split in the archosauriform family tree.

The animal that Gower and coauthors describe is not the first of its kind. The first species of Garjainia was described in 1958 from fossils uncovered in Russia. What makes the new species special is that it’s a little older and living in a different region, and, as long as you’re looking at the skull, it’s easy to tell the two species apart. The South African species, Garjainia madiba, has bulbous bosses of bone behind its eye and on its cheek that are lacking in the other species. Why this animal had these bumps isn’t yet clear – perhaps they were sign of maturity, differences between the sexes, or something else – but they’re among the traits that mark Garjainia madiba as a new species.

And in terms of size, Garjainia madiba was large enough to take on a variety of prey. Gower and colleagues estimate that the animal grew to over eight feet long, with a significant portion of that being a big, narrow-snouted skull. But what makes Garjainia madiba remarkable is not its fearsome appearance. The real story is in its bones.

Thin sections of Garjainia madiba bones, showing signs of rapid growth. From Gower et al., 2014.
Thin sections of Garjainia madiba bones, showing signs of rapid growth. From Gower et al., 2014.

Gower and colleagues examined thin sections of seven Garjainia madiba limb bones from individuals of different sizes. Inside, they found signs of rapid growth – relatively messy organization riddled with vascular canals and newly-made bone structures called primary osteons. Even in Garjainia that had periodic stopping points in their growth, likely in response to dry seasons or other times of stress, the bone in between those lines show quick growth spurts.

These starts and stops might explain why the archosauriforms, and not the surviving protomammals, came to rule the Triassic. Garjainia madiba and its relatives may have outpaced our own ancestors and cousins in terms of their life cycle, growing faster and reaching sexual maturity earlier. Simply put, the archosauriforms may have simply out-reproduced the protomammals, letting them evolve more quickly and limiting niches the protomammals could then create.

This archosauriform takeover happened quickly. Garjainia madiba lived a scant five million years after the worst mass extinction of all time – the end-Permian catastrophe that eliminated over 90% of species in the seas and over 75% of species on land. It’s a sign of a rapid burst of evolutionary novelty that paleontologists are truly just beginning to track. In the earliest days of the Triassic, life was bouncing back, with the archosauriforms leading the way.

[For more, read Mark Witton’s account of illustrating Garjainia.]

Reference: Gower, D., Hancox, P., Botha-Brink, J., Sennikov, A., Burlet, R. 2014. A new species of Garjainia Ochev, 1958 (Diapsida: Archosauriformes: Erythrosuchidae) from the Early Triassic of South Africa. PLoS One. 9, 11: e111154. doi: 10.1371/journal.pone.0111154