Paleo Profile: Kimbetopsalis simmonsae

by Brian Switek

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.

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