Awful Changes

Every once in a while scientists get thrown a curve ball by the popular press, a kind of question that requires a short and carefully-devised answer as the query, in any other setting, what make the researcher say "What the hell?" One such question was recently posed to paleontologist Steve Brusatte about the new species Carcharodontosaurus iguidensis (which I blogged about here) and its connection to a warmer climate in Cretaceous Africa. Julia, the Ethical Palaeontologist, has been kind enough to share the exchange with us;

Host: And you say this was a period when the Earth was particularly warm?

Steve: Yeah, it was very very warm at this time. This was the middle part of the Cretaceous period.

Host: There's, I suppose, an inevitable temptation with the Bali conference beginning today to ask whether we might see this kind of thing arriving on the planet again?

Steve: Yeah, you know, I think it's one of the more important implications of this sort of work. I mean, I'm not going to sit here this morning and pretend like we can say something, you know, overarching about global warming, but the point is that these sorts of ecosystems from the deep past are the sorts of time periods, the sorts of species that we need to study in order to start to get a picture of how our world may change as it warms and as sea levels rise.

Host: Steve Brusatte, thank you very much indeed.

If posed such a question I probably would have had a more Morbo-like response (i.e. "Evolution does not work that way! Goodnight!"), but the idea that dinosaurs might someday come back isn't a new one. In terms of science fiction, Mark Schultz's Xenozoic Tales is set in the not-too-distant future where humans polluted the planet to the point where they had to go underground for 600 years, dinosaurs and other ancient forms of life having retaken the planet when the mammals came out of their subterranean burrows. If we go back even further, however, it turns out that a similar idea was once proposed by the famed "father" of uniformitarianism Charles Lyell. In his Principles of Geology Lyell wrote;

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.... [Hat-tip to Neil, again, for help with the full quote]

Indeed, it seemed that if the present was the key to the past, the past could just as well be the key to the future, and under the right conditions life that had gone extinct could re-emerge after a prolonged hiatus. (The cartoon "Awful Changes" enclosed above by Henry De la Beche is a caricature of Lyell's view.) I'm fairly certain that the radio host during the interview wasn't aware of Lyell's view, but it operates from a similar premise; if conditions are the same, why not expect the same forms of life? Our current understanding of the fossil record, however, shows that this isn't so and once a creature is extinct there is no way for it to suddenly reappear as if they had simply been sleeping in the earth for the intervening time. There can be some forms of convergence based upon natural history, but while something like a dinosaur might again walk the land in the distant future it could only potentially arise from animals living today and would be much more derived than anything living during the Mesozoic.

The difficulty here is that contingency makes nearly impossible to predict what different habitats might favor in the future and what groups will diversify as others collapse (be it in an geologic instant or over millions of years), our own imagination and our desire to detect patterns further limiting what we might expect to live in some future time. Who could have predicted that dinoceratids like Uintatherium, huge pliosaurs like Kronosaurus, or even the dinosaurs as a whole once lived before their remains were discovered? After our own species is gone, what creatures will crawl, scamper, scuttle, and trample over the vestiges of our hollow edifices and technology? As tempting as it might be to try and dream up some future inhabitants of the earth millions of years after our demise, we should be careful not to fail the geological Rorschach test and start invoking "time's cycle" when we're really being carried along in the trail of "time's arrow."