Giraffes have taught generations of students how evolution works. Not directly, of course. Communicating through nocturnal humming is a barrier to classroom instruction. But the modern giraffe – Giraffa camelopardalis – is often used as the textbook example of why Darwin and Wallace were right and Lamarck was wrong.
The setup goes something like this. Think of a little protogiraffe gazing hungrily at some tasty leaves high up on a tree. Someone from the Lamarckian school of evolution, the argument goes, might assume that the little giraffoid would stretch its neck to grab the lowest of those high leaves and, through exertion, develop a longer neck that it would then pass on to its offspring. Repeat for best results. A Darwinian, on the other hand, would expect the protogiraffes to vary in neck length and those that just happened to have slightly longer necks would be able to reach more food, survive longer, and mate often enough to pass on that variation to the next generation, who would play out the scenario over again.
While the scenario is a bit of a caricature of what Lamarck actually thought, it’s still useful in getting at the basic evolutionary equation that Darwin and Wallace independently distilled. Yet, despite the thought experiment’s popularity, we’ve known little of how the giraffe actually got its neck. Today’s tall browsers definitely evolved from shorter-necked ancestors, but how? A new study by New York Institute of Technology’s College of Osteopathic Medicine anatomist Melinda Danowitz and colleagues now provides an answer.
Giraffes aren’t the only animals to have evolved impressively-long necks. The sauropod dinosaurs and aquatic plesiosaurs, for example, stretched out to ludicrous lengths both by adding additional vertebrae to the column and elongating those individual bones. But giraffes have the standard number of neck vertebrae shared by most mammals – seven – with the first element in the thoracic part of the spine being modified as a possible eighth “neck” bone. But that’s it. Evolution, constrained by mammalian anatomy, molded giraffes in a different way than the long-necked saurians.
Danowitz and coauthors looked at anatomical landmarks on 71 giraffe vertebrae spanning 11 species from over 16 million years ago to the present, focusing on the second and third vertebrae in the neck. As it turns out, a proportionally-long neck isn’t new for these mammals.
The best candidate for a real protogiraffe, Prodremotherium, and an early giraffe named Canthumeryx already had neck bones that were long compared to their width. “[N]ot only did the giraffid lineage begin with a relatively elongated neck,” Danowitz and coauthors write, “but that this cervical lengthening precedes Giraffidae” – the giraffe subgroup typically thought of as encompassing all the long-necked forms.
But even though the earliest giraffes already had slightly-elongated neck bones, there was no “March of Progress” towards towering heights. At least one – and possibly more – giraffe lineages reverted to abbreviated necks hung around stout vertebrae. Giraffokeryx was among the earliest of the short-necked giraffes, browsing low-lying foliage around 12 million years ago, and within the last three million years Sivatherium, Bramatherium, and the okapi followed suit. The short-necks proliferated alongside their lankier relatives, which is why we still have both short- and long-necked giraffes today.
Truly long-necked giraffes didn’t evolve until about 7.5 million years ago. Samotherium, Palaeotragus, Bohlinia, the extinct Giraffa sivalensis and the living Giraffa camelopardalis preserve enough transitional features to let Danowitz and colleagues reconstruct how this stretching occurred. It wasn’t simply a matter of drawing out their vertebrae as if they were in some sort of anatomical taffy pull. The front half of the neck vertebrae became elongated in Samotherium and Palaeotragus, generating forms intermediate between today’s Giraffa and their foreshortened predecessors. Then, within the last two millions years or so, the lineage leading up to the modern Giraffa elongated the back half of their neck vertebrae, giving them even more reach and making them literally at the top of their class.
If you could assemble all these fossil bits and pieces into a short film replaying giraffe evolution, you wouldn’t end up with the smooth transformation of a small-statured herbivore into a towering, checkered browser. There’d be starts and stops and side stories, the ending not being a goal but a happenstance. In short, it’s time again to update those textbooks.
We’ve all had moments of sympathy pain. That little twinge when we see or hear about a fracture, burn, kick in the groin, or other familiar trauma that hits a little too close to home. And this phenomenon crosses species boundaries. I know because I just read a paleontology paper with an injury that made me clench my jaw and suck the air through my teeth.
The study, written by Adam Hartstone-Rose and colleagues, is titled “The Bacula of Rancho La Brea.” It’s all about Ice Age penis bones. While we lack the genital bones of our ancestors – the baculum in males and baubellum in females — they’re pretty common in other groups of mammals, and paleontologists working at Los Angeles’ famous asphalt seep have pulled hundreds of os penis from the ancient mire.
The bones include specimens from coyotes, weasels, badger, and fox, but dire wolves far outstrip the competition with about 400 hundred bacula, 159 of which are complete. But not all of these bones were in good health. Eight of the dire wolf bacula in the La Brea sample had some kind of pathology. “Most of these demonstrate some degree of twisting along the long axis that may have been either congenital or the result of trauma”, Hartstone-Rose and coauthors write. And of this smaller sample, a few bacula were fractured.
LACMHC 8291 had an especially rough time of it. The caption accompanying the image reads “A broken, displaced, and healed C. dirus baculum”, meaning that the tip broke and healed in a different position than it started. That was enough to make me cross my legs while reading the paper.
Without what would be one of the stranger sets of time travel coordinates ever, we’ll never know exactly what happened to the poor wolf. The leading hypothesis for such pathologies is that a mating male gets accosted by a rival in the middle of the act, which could “cause the mating male to jump suddenly and snap the bone.”
As cringe-inducing as such injuries may be, though, they were relatively rare among La Brea’s dire wolves. The number of broken bacula is actually lower than expected for the sample size compared to the incidence the same injury in their modern relatives. The size of their bacula, Hartstone-Rose and colleagues suggest, might explain why.
The os penis of adult dire wolves were about 44% longer than those of gray wolves and were significantly more massive. This made it a tougher bone to break. Paired with the unexpected rarity of baculum injuries, the paleontologists propose, this might mean that the modified members of dire wolves were an evolutionary response to competition for mates. So in evolutionary terms, the solution for dire wolves was to walk softly and carry a big stick.
Hartstone-Rose, A., Dundas, R., Boyde, B., Long, R., Farrell, A., Shaw, C. 2015. The bacula of Rancho La Brea. Natural History Museum of Los Angeles County Science Series, 42. pp 53-64
Dinosaurs lived all over the world during their Mesozoic heyday. Maps in books and documentaries drove the point home to me during my childhood dinomania, dots scattered across the globe showing all the places where the great reptiles had been found. But, no doubt influenced by Fantasia and the “monkey puzzles and parking lots” style of paleo art, I envisioned all those little specks as steaming swamps or lush floodplains in an endless dinosaurian summer. The thought of dinosaurs striding through the snow didn’t occur to me at all.
Bones excavated from Alaska’s extension into the Arctic Circle have changed that. Fossils from the 71-68 million year old stone of the Price Creek Formation have revealed a dinosaurian fauna that lived year-round in a place where it occasionally got cold enough to snow. The image of tyrannosaurs, horned dinosaurs, and hadrosaurs walking through the cool forests of ancient Alaska has run so counter to the classic Mesozoic imagery that it’s not surprising that this environment has been the subject of several recentdocumentaries and even a feature film. But in each cinematic reprise, the cast of dinosaurs keeps changing.
Dinosaurs don’t usually come down to us as complete or even articulated skeletons. Jumbles of bones, piles of fragments, and isolated teeth are far more common. These are usually enough to narrow down the type of dinosaur present, and from there paleontologists sometimes turn to more completely-known dinosaurs from the same time, but different places, as hypothetical stand-ins for the more fragmentary species. In the southwestern United States, for example, tyrannosaur bones from New Mexico were categorized as those of the northern Daspletosaurus before paleontologists realized they represented a hitherto unknown genus subsequently rechristened Bistahieversor.
Excavations since the 1990s have turned up numerous hadrosaur bones from the Price Creek Formation, and, given their age and anatomy, those remains have generally been categorized as those of Edmontosaurus – a famous hadrosaur found further south through Canada and the United States. But no one had made a strong case for this assignment. When Mori and colleagues went back to the fossils, they found that the Arctic hadrosaur differed in a variety of anatomical landmarks and bone shapes on the skull. In short, the Price Creek Formation hadrosaur was different enough to merit establishing it as a new dinosaur. The dinosaur is now Ugrunaaluk kuukpikensis – a title based on the Iñupiaq language meaning “ancient grazer of the Colville River”. And if you have a little trouble with the name, the authors included a little pronunciation guide. Say it “oo-GREW-nah-luk”.
Ugrunaaluk is far from the last new dinosaur hiding in the Alaskan Arctic. So far paleontologists have recognized 13 distinct dinosaurs from the Price Creek Formation, but Ugrunaaluk is only the fourth – along with Nanuqsaurus, Pachyrhinosaurus, and Alaskacephale – to be confidently identified down to species. For paleontologists, there’s still much to discover in the Arctic’s lost world.
Name: There is no official name yet, but the specimen has been given the informal name “Lightning Claw”.
Meaning: The nickname comes from Lightning Ridge, where the bones were found, and the large claws that help distinguish the lineage this dinosaur belonged to.
Age: Around 110 million years ago.
Where in the world?: New South Wales, Australia.
What sort of critter?: One of the theropod dinosaurs known as megaraptorids, famous for their enlarged hand claws.
Size: Estimated at over 20 feet long.
How much of the creature’s body is known?: Natural casts of a fragmentary skeleton including elements of the lower arm, claws, lower leg, part of the hip, and pieces of ribs.
Claim to fame: No two dinosaurs look alike. This is as true in death as in life. Aside from biological differences – in species, in age, in size, and so on – the way bone becomes preserved in stone gives each skeleton its own character. In the case of a skeleton recently uncovered in the Cretaceous rock of Australia’s Lightning Ridge, the bones of a very sharp dinosaur has come down to us as a set of natural casts of opal.
The dinosaur, described by paleontologist Phil Bell and colleagues, doesn’t have a scientific name. There’s too little of the skeleton to raise the banner of a new genus or species just yet. But there’s enough of the fossil to tell that the dinosaur was one of the megaraptorids – large, predatory dinosaurs that bore extra-long claws on their hands.
Megaraptors are still mystery dinosaurs. No one’s quite sure what group of theropod dinosaur they group most closely to. But, as far as Australia goes, the “Lightning Claw” was found in rock about 12 million years older than the next megaraptor found in the country. It likely represents something new, making the fact that some of its skeleton might have been lost during opal-mining operations all the more frustrating.
Still, Bell and coauthors write, the fact that a megaraptor was found earlier in the Cretaceous than its next closest neighbor suggests that Australia may not have been the evolutionary backwater for dinosaurs that paleontologists have often thought. Cretaceous Australia has often been characterized as a place where members of different dinosaur lineages, which originated elsewhere, eventually wound up. The “Lightning Claw” might indicate that the megaraptors, at least, originated in Australia, or that the area was a critical place for their evolution before they spread elsewhere throughout the southern group of continents called Gondwana. With any luck, future finds will help explain what these dinosaurs were and how they terrorized the southern hemisphere.
Every bone has stories to tell. Some, such as size or the animal’s place in the evolutionary tree, you scrape off the surface. But others remain hidden until you cut inside.
Thanks to more than 30 years of fieldwork, Montana’s Museum of the Rockies has amassed a huge collection of Maiasaura bones. From the number of right-side tibiae – the larger of the two bones that make up the lower leg – they’ve been able to count at least 32 individual dinosaurs at the one site. (The sample probably contains a higher number, but 32 is the absolute minimum.) Most of these limb bones appear to be healthy, but, when MOR researchers cut into them, two of the tibiae showed signs of ancient injury.
The two bones – one from a one-year-old dinosaur, the other from a four-year-old – don’t show anything as simple as a fracture or bone infection. Instead, paleontologists Jorge Cubo, Holly Woodward, Ewan Wolff, and Jack Horner report, the pair of elements show rinds of extraneous bone that quickly grew over the outer surfaces. Not only might this be a sign of bone forming in response to biomechanical stresses and strains, but it could throw additional support to the idea that Maiasaura changed how they trotted around as they aged.
Thanks to studies of modern animals, such as sheep, pigs, and humans, osteologists know that a break in one bone can cause another bone to change. If you were to break your fibula but didn’t have the luxury of resting up until it was healed, the failure of the one bone would change the stresses on the neighboring tibia. This is what Cubo and colleagues propose for the Maiasaura. These young dinosaurs probably broke their respective right fibulae, and the extra bone formed in response to the altered strains on the tibia.
But the bone bulges aren’t in the same place in the two dinosaurs. The extra bone formed towards the outer side in the younger animal and on the back in the older one. Cubo and coauthors propose that this pattern is consistent with how such extraneous bone forms in bipeds versus quadrupeds. The bone growth in the one-year-old Maiasaura follows the pattern seen among bipeds, such as ourselves, while the growth in the four-year-old is more consistent with what’s seen in quadrupeds such as pigs and sheep.
Back in 2001 paleontologist David Dilkes suggested that Maiasaura changed postures as they aged. Muscle scars on the Cretaceous bones seemed to show that the young ran around on two feet and went down to walk on all fours as they got larger. With any luck, additional cases will help test this idea, but the pathologic bones seem to support this idea. Maiasaura was a dinosaur that walked on two legs in the morning and on four legs from the afternoon through the evening.
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.
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.
Some of our bones are easy to see. The zygomatic bones that give our cheeks shape, the delicate phalanges of our fingers, and the bony bulbs that are our kneecaps all stand out from beneath the flesh. But the hyoid bone is hidden.
Even if you’re looking at a study skeleton in a museum or doctor’s office, the hyoid is easy to miss. On the replica skeleton standing next to my desk, the U-shaped bone is hiding behind the lower jaw and is anchored to the fourth neck vertebra by wire. It needs the artificial connection because, in our bodies, the hyoid has no bony bridge to the rest of the skeleton. Instead, the hyoid is an anchor among scaffolds of flesh, supported by and providing support for the enveloping soft tissues.
And while this bone might not come up in day-to-day conversation, the hyoid has been critical to an argument about speech. The osteological horseshoe is an attachment point for muscles of the tongue, larynx, and pharynx. Without this bone, we wouldn’t be capable of making the symphony of sounds we use to help put our ideas into other people’s heads. That’s why the hyoid has played such a prominent role in the ongoing debate over whether our close Neanderthal relatives could talk and sing.
The argument breaks down like this. Even though we belong to the same species – we interbred, after all – anthropologists can still differentiate Neanderthals on the basis of their bony anatomy. Among the ways they differed from modern people was a thinner hyoid bone that may have rested a little higher in the throat. According to anthropologists Philip Lieberman and Edmund Crelin, this would have caused the larynx to open into the pharynx higher along its path, limiting the ability of Neanderthals to make the “i”, “a”, and “u” sounds which are nearly ubiquitous in modern languages. Therefore, Liberman and Crelin argue, Neanderthals could not articulate “human speech”, and, given that everyone and their mother has a pet hypothesis for the extinction of our closest relative, this linguistic deficiency was the deciding factor in the Neanderthal downfall.
Needles to say, their hypothesis is not the final word. In a 2007 letter to the Journal of Phonetics, Louis-Jean Böe and other researchers offered an alternate interpretation of where the Neanderthal hyoid fit and what sounds the prehistoric people were capable of. The Neanderthal hyoid was not restrictively high, and even then, the researchers countered, the tongue, jaw, and lips are more important in making “i”, “a”, and “u” sounds. Neanderthals didn’t have any physical impediment to speaking like we do, although whatever they may have said to each other, and to us, is lost in time.
Humans aren’t the only creatures to have hyoids, though. The bone has a very ancient origin, modified from gill arches of fish that lived over 375 million years ago, and has been inherited by amphibians, reptiles, birds, and mammals. And in some of these lineages, different species have been adapted to use their hyoid for a very specialized function – to suck.
A few weeks ago, while I was walking through the National Museum of Natural History’s osteology hall, I stopped in my tracks to stare at the skeleton of a matamata. Its large hyoid is what grabbed my attention. The bony stirrup is what allows the matamata to feed without biting, and is part of what herpetologist Patrick Lemell and coauthors have dubbed “adaptation perfected”.
Imagine a twitchy little fish wriggling through the water in front of a matamata. The morsel is just out of range for the turtle to snatch with its jaws, but the reptile has another method. Muscles anchored to the turtle’s large hyoid bone throw down the lower jaw as skin at the side of the mouth create reptilian cheeks. The turtle makes such an effective vacuum that it doesn’t even need to bite. The fish is drawn in and swallowed whole. While I can imagine some situations in which such an ability might be advantageous – like when when presented with a pizza – I’m happier not looking like the fringed turtle. I will have to live with being imperfectly adapted.
Lemell, P., Beisser, C. Gumpenberger, M. Snelderwaard, P., Gemel, R. Weisgram, J. 2010. The feeding apparatus of Chelus fimbriatus (Pleurodira; Chelidae) – adaptation perfected? Amphibia-Reptilia. 31: 97-107.
Flippers, the need to breath air, the ability to give milk, and a streamlined shape. There are plenty of signs that let us immediately distinguish whales from fish and the other creatures of the sea. Yet, alone or in combination, the traits I just mentioned aren’t unique to whales. Seals and sea lions, for example, are also flippered, air-breathing, milk-giving, streamlined mammals. If you truly wish to draw out leviathan, you have to look beyond the blubber to a clue made of bone.
The secret is in the skull. Being mammals, whales have domes of bone on their skulls that enclose the middle ear. These are called tympanic bullae. But whales have a peculiar modification to these bony bulges. The inner edges of these bulbs are so thick and dense that they get their own name – the involucrum.
It’s not just modern whales that display this structure. An involucrum is a very, very ancient cetacean trait. When Philip Gingerich and Donald Russell first described the 55 million year old Pakicetus in 1981, for example, they were able to immediately recognize it as an early whale because of a partial skull preserving the thickened tympanic bullae on the bottom. Living and fossil, whales are united by a thick wall of bone over their ears.
But here’s where things get a little complicated. In 2007 Hans Thewissen and colleagues announced that Indohyus – a small, hoofed, 48 million year old mammal found in India – had an involucrum, too. A happy accident had broken open the beast’s middle ear and revealed a dense, thick margin just like that in whales. And while Indohyus wasn’t technically a whale – it belonged to a group called raoellids – the mammal was nevertheless an extremely close relative of the very first whales, providing paleontologists a proxy for what the ancestors of whales were like. Whales had inherited the involucrum from their deer-like ancestors and have kept it throughout their history, a rind of bone that whispers of the distant past.
Earlier this year, in the journal Gondwana Research, paleontologists Gerald Grellet-Tinner and Vlad Codrea announced an unexpected pterosaur. Consisting solely of a triangular hunk of bone found in the 70 million year old rock of Romania, the fossil was presented as a snout piece of Thalassodromeus sebesensis, a new species of a previously-named genus. This was very strange.
The original Thalassodromeus species – T. sethi – lived 42 million years earlier in Early Cretaceous Brazil. If this new animal really belonged to the same genus, as Grellet-Tinner and Codrea proposed, then paleontologists were missing an extended period of this pterosaur’s history. Grellet-Tinner and Codrea tentatively filled in the gap with hypotheses about dispersal routes, the co-evolution of Thalassodromeus with flowering plants, and the odd “island effect” that alters species bound to geographic dots in the ocean, but there is a much simpler explanation for the out-of-place pterosaur. This “pterosaur” was really a turtle.
In a comment to the same journal, Gareth Dyke and 19 other paleontologists affirm that the peculiar Thalassodromeus bone from Romania is actually a piece of a turtle’s belly shell. The single piece of bone lacks any characteristic that conclusively identifies it as belonging to a pterosaur. “[I]t is not a pterosaur head crest, or a pterosaur bone of any kind,” the researchers write. Rather, the bone is a perfect fit for part of the bottom shell of a Cretaceous turtle named Kallokibotion that has been known from the same area for about 90 years. Sadly, Dyke and co-critics write, “the misidentification of one fragmentary fossil [led] to a cascade of elaborate ideas with increasingly far-reaching implications.”
Grellet-Tinner and Codrea had entertained the turtle possibility in the supplementary information of their study, but rejected it in favor of a pterosaur interpretation. And they still uphold their original conclusion.
In their own reply, Codrea and Grellet-Tinner say their critics’ comments are “most welcome” but then pile on personal jabs and conclude with a cryptic line that the “conspicuous persistence, hastiness, and zeal of [the critical comment], may indeed reflect of deeper, perhaps irritating, issues in Transylvania.” Just as strange, Codrea and Grellet-Tinner complain that their critics have not studied the fossil in person, yet principally defend their own position by citing figures from other studies.
Codrea and Grellet-Tinner promise that planned 3D scans of the fossil “will concomitantly eliminate any doubt regarding [the specimen], if there were any.” I’m betting the results come up turtle. The shape and anatomy of the bone are a much better fit for a familiar chelonian than a pterosaur unstuck in time. Such is the nature of paleontology. The true identity of puzzle pieces isn’t always what we first expect them to be.
The pterosaur-turtle mix-up isn’t the only case of mistaken identity, though. A different team of researchers has just revised the identity of another European pterosaur.
In 1990 paleontologists found a pair of weird bones in England’s Cromhall Quarry. They seemed to be hand bones – metacarpals – from a pterosaur, and this was especially interesting because the site was Late Triassic in age. If the identification was correct, then the bones would be from the early days of these leathery-winged reptiles and the only evidence of pterosaurs in Triassic England.
But upon re-examination, paleontologists Fabio Marco Dalla Vecchia and Andrea Cau found that the bones more closely match the fingers of weird “monkey lizards” called drepanosaurs. Other bones from these weird, clasp-footed reptiles have been found in the same quarry, but were not known in 1990, and so it was easy to mistake the unusual fossils for pterosaur hand bones. The upshot, Dalla Vecchia and Cau write, is “there is no unequivocal evidence of pterosaurs in the Triassic of the UK.” Sometimes that’s the way the pterosaur crumbles.
[For more on the Thalassodromeus debacle, read Mark Witton’s post breaking down the details.]
I’m missing a bone. You are, too, although which bone that is depends on your anatomical sex. For me and male readers of this post, it’s the baculum – the enigmatic “penis bone” found in the members of many mammals and not us. There is nothing bony about a human boner. But, through the winding path of evolution, female readers are lacking their own genital ossification that’s just as mysterious and has been rarely discussed – the os clitoridis.
The os penis and os clitoridis are osteological correlates of each other. They are the same bone, but in different form in each sex. And while the os clitoridis – sometimes called by the more elegant-sounding name “baubellum” – isn’t a feature of all mammal lineages, the bone has been found in a variety of species among distantly-related beasts. In a 1954 paper in which he lamented that the peculiar bone has “been only sporadically studied”, zoologist James Layne documented that the os clitoridis has been found in a variety of rodents, carnivorans, and primates – marmots, seals, cats, bats, bears, galagos, gibbons, and more had some sort of bone beneath the clitoris.
For his part, Layne focused on the curious genital structures of squirrels. The bones were quite small, often between one and three millimeters long, but many were just as oddly ornamented as the penis bones of males. Figured in a gallery of equivalent bones from other species in Layne’s paper, the os clitoridis of the Tropical ground squirrel (Notocitellus adocetus) is a flared, spurred spoon. The same bone in other squirrel species twist, taper, and flare in their own distinctive ways, and, often, closely resemble the os penis in male conspecifics. Experiments in the lab, and not just anatomy, have underscored that baubellem and baculum are different versions of the same bone.
Rats and mice, those icons of laboratory studies, are our guides here. Female mice have a clitoral bone, and female rats of some study strains have a small bone that corresponds to the tip of the baculum in male rats. Few seemed to pay much attention to this fact until, in their efforts to understand how different hormones affected tumor formation in the genital tracts of rats during the late 1960s, researchers Alfred Glucksmann and Cora Cherry found that injections of testosterone caused the female rats to form new genital bone. Following that thread, Glucksman and Cherry discovered that testosterone injections during the early days of life caused the female rats to grow a larger os clitoridis which closeled resembles the male counterpart. Working with other researchers, Glucksmann later found the same to be true for mice. “With prolonged androgen treatment”, osteologist Brian Hall has written about this find in scientific deadpan, “growth of these induced bones can be promoted and the distal element extended.”
Yet the connection between hormone and baubellem isn’t always so clear. For one thing, testosterone and other androgens seem to have nothing to do with why female fossa have disappearing clitoral bones. Fossa (Cryptoprocta ferox), made famous by the exceptionally annoying kid’s film Madagascar, are lithe, tawny carnivores closely related to civets and genets. And, apparently, they are the first recognized case of what zoologist Clare Hawkins and colleagues term “transient masculinization.”
So far as zoologists have documented, about nine mammal species have females that take on male characteristics during the course of their lives. The most famous example is the spotted hyena. Females of Crocuta crocuta are masculinized to the point where they develop a prominent pseudo-penis that, in one of the most startling examples of evolutionary jury-rigging, hyena mothers give birth through.
Young female fossa don’t have to cope with changes quite so extreme, but, as Hawkins and colleagues noted in a 2002 study, they produce a “a mildly pungent orange secretion” also found in mature males and have “an enlarged, spinescent clitoris supported by an os clitoridis” similar to the corresponding genital equipment of the opposite sex. Unlike the female hyenas, though, female fossa don’t retain these traits. They’re only masculinized for a short part of their lives, starting around seven months of age until the fossa’s second or third year, before the traits start to fade. Only a smaller form of the clitoral bone is left in adult female fossa. Why?
Sex hormones don’t appear to be the answer. From anatomical, genetic, and hormonal examinations of wild and captive fossa, Hawkins and coauthors couldn’t find any difference in androgen levels between juvenile and adult females. Nor was there any connection between androgen levels and the masculine traits – such as “os clitoridis length” and “secretion score” – of young female fossa. It could be that young female fossa have “target tissues” where the relatively low levels of androgens have more influence over anatomy, but, Hawkins and colleagues noted, no one knows whether this is the case.
Exactly how the temporary change transpires is unknown. Despite the black box of the carnivoran’s transient masculinization, though, Hawkins and collaborators offered two hypotheses as to why secreting orangish fluid and having a spiny clitoris supported by bone might be advantageous to young female fossa.
The male traits start to become prominent about the time female fossa are aggressively run off by their mothers and, even though they are not yet mature enough to breed, they may encounter the unwanted attention of a wandering male seeking mates during the very brief mating season. Such an encounter can be dire. A male could maul or even kill a young female fossa, so, Hawkins and coauthors propose, looking like a male during the vulnerable period in their lives “could allow them to escape detection or could signal to males that they are not a potential mate.” Then again, territorial adult females can be just as much as a threat as roving males, so perhaps young female fossa have a better chance of finding their own patch of forest if they disperse from home in disguise.
Female fossa changes could be attributable to both scenarios. The immature civets face dangers from males and females alike. But, beyond the enigmatic mechanics of the anatomical change, we still don’t know why young female fossa evolved such mimicry. Nor is this the only mystery the os clitoridis embodies. Why some of our primate relatives have genital bones, while we lack them, is an evolutionary conundrum just as vexing.
For my part, I’m glad I don’t have a baculum. I can’t imagine that I would have been keen on playing soccer or sparring in Taekwondo classes if there was a possibility that I’d be carted off to the emergency room with a shattered penis bone. (Although, speaking of such injury, there are pathological cases of penis bones forming in humans, including after trauma such a kick or gunshot wound to the area. Perhaps I should have reconsidered my childhood activities, after all.) But I still want to know why the genital bone – in both female and male humans – disappeared without a trace.
We’re not unique among primates in lacking osteological genital curiosities. In a review of human reproduction from a primatological perspective, anthropologist Robert Martin noted that tarsiers don’t have penis or clitoris bones, nor do several genera of New World monkeys. And even in primates that have them, such as our close ape relatives, the genital bones have shrunk to a negligible, almost unnoticeable size. While os penis and os clitoridis bones are prominent in ring-tailed lemurs, for example, they’ve almost entirely disappeared in gibbons and chimpanzees.
In primates, at least, a baculum is hypothesized to help support the penis during long bouts of sex between individuals that rarely encounter each other during the mating season. Males that are often around their mates, the following argument goes, can copulate more frequently for shorter amounts of time and therefore don’t require osteological assistance for their erections. Perhaps. There are other hypotheses out there, not all of which are mutually exclusive. But what, then, are we to make of the baubellem? The baculum and baubellum are old bones that have been retained in some forms of placental mammal and lost in others over millions and millions of years. And while a great deal of ink has been spilled over the function of the baculum and our lack, I haven’t been able to find so much as even a minor degree of interest into why the baubellum evolved and exists in many mammal species.
Even in reduced form, a clitoral bone is present in an array of female primates. And when we look beyond our primate kin – among Layne’s squirrels, for one – female mammals have os clitoridis of prominent size and intricately complex morphology. So much so that zoologists have occasionally pondered using the bones to tell species apart. But beyond such utility, the clitoral bone is often ignored.
There’s an assumption that the clitoral bone is functionally unimportant and just a case of females developing a masculine trait – a reverse of males having nipples. But I believe the bone is more unstudied than insignificant. The importance of the clitoral bone in the life of female fossa, for one, is a clue that there is more to the curious bone than researchers have appreciated. Given how little we know about this varied skeletal feature, we’d be terribly foolish to relegate the os clitoridis as a masculine leftover of only passing interest. All the same, the os clitoridis, so inscrutable, is a totem of our shared history with our therian relatives. Tracing back what we’re missing, we walk along the pathway of our deep history – a mysterious absence guides us back through evolutionary enigmas barely considered.