Fossils of Future Past

Dinosaurs were prehistoric wonders, separated from us by an unfathomable amount of geologic time. But, albeit briefly, the Victorian geologist Charles Lyell thought that the ruling reptiles might someday return.

Years before his friend Charles Darwin would turn Lyell’s head toward evolutionary views, the Scottish champion of uniformitarianism tried to find a balancing point between the order and flux of life through time. In his scientific epic Principles of Geology, published in three volumes between 1830 and 1833, Lyell rightly recognized that different sets of organisms inhabited the world at different times. But, he cautioned, there was no evidence that evolution was behind the stately march of species.

What controlled the life and death of species? Lyell couldn’t say. Creatures simply popped into existence, ideally-suited to their surrounding conditions. Those conditions, Lyell believed, were symptoms of geologic change – the constant, consistent reconstitution of the planet. As lava poured out, mountains eroded away. Earth was always shifting, but in a completely balanced way. In Lyell’s mind, there could be no doubt that climate and habitats were affected as seas rose or receded, as hills were thrust up or ground down.

If biology was at the mercy of geology, then accumulated geologic conditions might eventually return the continents and climate to a state not seen since what Lyell’s generation called the Secondary era – the time when reptiles ruled the Triassic, Jurassic, and Cretaceous. As Lyell explained in Principles of Geology:

We might expect, therefore, in the summer of the “great year,” which we are now considering, that there would be a great predominance of tree-ferns and plants allied to palms and arborescent grasses in the isles of the wide ocean, while the dicotyledonous plants and other forms now most common in temperate regions would almost disappear from the earth. Then might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree-ferns.

Lyell’s artistically-talented colleague Henry De la Beche lampooned the notion of future ichthyosaurs in a cartoon titled “Awful Changes—Man Found Only In A Fossil State—Reappearance of Ichthyosauri”, and, a few decades later, Darwin and Alfred Russel Wallace tied the succession of prehistoric life to an evolutionary idea with a plausible mechanism. The idea that Earth will continually cycle through Primitive, Secondary, and Tertiary periods at the whim of geologic change never took hold. Natural selection ruled out such a possibility. The wonder of evolution is that the form of future life is contingent upon what has come before, yet organisms are not locked into predetermined pathways. What Nature will look like a million, ten million, or a hundred million years from now rests in the realm of speculation.

Yet, strange as it may seem, paleontologists, climate scientists, and conservationists have been plumbing the depths of the fossil record for clues about the kind of “awful changes” our own species is responsible for. Although we’re still supposed to be in the shuffle of Ice Age oscillations, our thirst for fossil fuels has dumped so many climate-altering compounds into the air that we may be creating an artificial greenhouse world. No one expects that Triceratops and Allosaurus will return – as in Mark Schultz’s fantastic Xenozoic Tales series – but the way species have reacted to past climates might give us a broad outline of what’s to come. Included in the swath of species paleontologists have been interrogating for clues are mollusks that thrived in a warm, shallow sea that once splitNorth America in two.

About 100 million years ago, Earth was a greenhouse world. There was no polar ice, the global climate was more consistent between the equator and the poles, and sea level was about 300 meters higher than it is today. And it was about this time that arms of two separate oceans connected to create an aquatic divide over North America that teemed with seagoing reptiles, toothed birds, and a variety of archaic mollusks. The lost marine highway is technically known as the Western Interior Seaway, and holds some unexpected clues about what might happen as we approach a warmer future.

To understand how species invaded new habitats and went extinct during the Late Cretaceous, University of Kansas paleontologist Corinne Myers and coauthors used Geographic Information Systems analyses to estimate the ranges of marine mollusks based upon where their fossils had been found. From stage to stage, the researchers tracked the expansion and contraction of ranges during the last 35 million years of the Cretaceous. What they found ran contrary to conventional wisdom about how species might react to an evenly-warmed world.

In times of rapid ecological change, generalist species found over a wide range are expected to have a better chance at survival than others. Studies of mass extinctions and modern ecology support this rule. Yet Myers and coauthors didn’t detect this trend. Among the Cretaceous clams, ammonites, ans snails they studied, there was no connection between range size and resistance to going extinct.

Even stranger, species that appeared in relatively small ranges were the most successful at later invading other parts of the seaway. Between 86 and 83 million years ago, a straight-shelled ammonite cephalopod called Baculites thomi lived in a small section of what is now Montana. In the following 83-72 million year bracket, though, the range of Baculites thomi expanded to cover the seas of Cretaceous Wyoming and Utah, as well. Other species showed similar patterns. Initially rare organisms seemed to have the best chance at later proliferating.

Exactly what accounts for these trends isn’t yet clear. In the case of species invasions, Myers  and colleagues note that the pattern might be species gradually filling out a geographic range rather than an “invasion” of habitats already occupied by other species. Why range size was decoupled from extinction risk is also a mystery. Perhaps, the paleontologists propose, the warming trend in the Late Cretaceous wiped out many specialist species, leaving a major community of generalists. In this case, specific details of life history and niche might make the difference in terms of who survived and who perished rather than geographic span alone.

If the Late Cretaceous seas offer a window to the future, the vision they present initially seems counterintuitive. Rather than species retreating to refuges as climate change bumps their preferred habitats to new geography, Myers and colleagues suggest, we might see an extinction of specialist species in the tropics. From there, generalist species from higher latitudes might actually head towards the equator and create new niches. Long-term survival won’t hinge on species that are best able to cling to their present habitats, but to flexible organisms that can carve out a new place for themselves in a warmer, more equatable world.

The lesson of the Late Cretaceous is that the life and death of species is not regulated by geographic spread alone. Extinction and survival hinge on the natural history of individual species. Life will undoubtedly survive and continue to evolve in the greenhouse world that we’re creating. The record of Deep Ecology can give us a tantalizing glimpse at how species might react to a future heavily impressed with our fingerprint, whether we survive to see it or not.

Reference:

Myers, C., MacKenzie, R., Lieberman, B. 2013. Greenhouse biogeography: the relationship of range to invasion and extinction in the Cretaceous Western Interior Seaway. Paleobiology. 39,1 : 135-148