Mass extinctions can’t be planned for. The triggers for these catastrophic events – the impact of an asteroid, rapid changes in the makeup of the earth’s atmosphere, and the like – are contingent events in the history of life on earth that no species can forsee. (Not even our own.) The devastating global extinction which occurred about 65.5 million years ago, for example, may have been the result of a really bad day in which asteroid impact, intense volcanic activity, and other causes played a part. The rapid ecological changes which resulted rapidly winnowed down the diversity of life on earth.
Of course, life was not entirely extinguished by the end-Cretaceous extinction. Many species survived, and, although the reason why certain species perished most immediately draws our attention, it can be profitable to look at the survivors and ask why they survived. One such survivor was Boremys – a turtle that previously seemed to have disappeared from the fossil record millions of years before the Cretaceous catastrophe struck.
The extended legacy of Boremys is described by paleontologists Tyler Lyson, Walter Joyce, Georgia Knauss, and Dean Pearson in the latest issue of the Journal of Vertebrate Paleontology. Previously Boremys was thought to have been restricted to a slice of Cretaceous time called the Campanian, about 83 to 70 million years ago. It was just one of many turtle species found in the Late Cretaceous of North America – the sort of fossils that are often found but are overshadowed by the dinosaurs found in the same levels – but the geologic provenance of several new specimens have given the prehistoric turtle reason to shuffle however briefly into the spotlight.
Lyson and co-authors report that Boremys fossils have been found in both the Hell Creek Formation and the Fort Union Formation. The former is the last portion of Cretaceous time in North America – the part leading up to the mass extinction – and the latter presents a narrow view of the world after the extinction of the non-avian dinosaurs, pterosaurs, mosasaurs, ammonites, and the other creatures which perished by 65.5 million years ago. Boremys was not just a Campanian turtle. The shell-bearing reptiles made it up to the end of the Cretaceous and crossed the boundary into the earliest part of the next chapter in earth’s history, the Paleocene.
So what does the survival – or, to put it another way, the delayed extinction – of Boremys mean? That’s difficult to say. The persistence of Boremys through the extinction boundary brings the tally of turtles which survived the catastrophe to eight, and the find fits the general pattern of extinction which vertebrate paleontologists have found in North America. While active animals which maintained high body temperatures – including many mammal lineages and non-avian dinosaurs – suffered dearly, creatures which lived in aquatic habitats and had body temperatures which fluctuated with their environments were not as badly hit. There may have been something about their ecology, physiology, or both which created a kind of buffer to extinction – some quirk of natural history which made them resistant to unpredictable events which would forever change the world.
[PS: During the next two weeks I’m going to be out doing some fossil fieldwork – starting at Dinosaur National Monument, moving on to the Bighorn Basin, and ending in Montana – but I’ve scheduled new posts for this blog and Dinosaur Tracking while I’m gone. I’ll still be updating Twitter when I can, too, so stay tuned and don’t wreck the place while I’m out.]
Top Image: The upper part of the shell of Boremys found in the Paleocene – after the end-Cretaceous extinction – in North Dakota. From Lyson et al., 2011.
Lyson, T., Joyce, W., Knauss, G., & Pearson, D. (2011). Boremys (Testudines, Baenidae) from the latest Cretaceous and early Paleocene of North Dakota: an 11-million-year range extension and an additional K/T survivor Journal of Vertebrate Paleontology, 31 (4), 729-737 DOI: 10.1080/02724634.2011.576731