Paleontologists have identified a new species of softshell turtle from a partial carapace found in North Dakota, the United States.
Hutchemys walkerorum lived 66.5 million years ago, during a period when giant dinosaurs — including Tyrannosaurus rex and Triceratops — roamed Earth.
The ancient turtle was less than 30 cm (one foot) long and inhabited ponds, streams, and slow-moving rivers.
It belongs to Plastomeninae, a group of fossil softshell turtles currently known from the Late Cretaceous to the Eocene, with some genera known to survive the end-Cretaceous mass extinction.
“Plastomenines lived during the Cretaceous and Paleogene periods, around 80 to 50 million years ago,” said Dr. Steven Jasinski, a paleontologist at Harrisburg University, the University of Pennsylvania, and the Don Sundquist Center of Excellence in Paleontology, and his colleagues.
“These turtles are similar to the softshell turtles that exist today, although the plastron of plastomenine turtles — the bones covering their stomach and abdominal area — are more strongly sutured together and often larger and more robust than in other softshell turtles.”
“Until recently we didn’t understand these softshell turtles very well. However, we are starting to get more information on this extinct group of turtles and further understanding their evolution, including how they dealt with the mass extinction.”
Hutchemys walkerorum’s fossilized carapace — the bones that cover the turtle’s back — was found in the Hell Creek Formation of North Dakota.
There are several key features of the shell, including how it is shaped and its curvature, that distinguish it from other known turtles.
“Hutchemys walkerorum represents one of the rare occurrences of Hutchemys turtles prior to the mass extinction event that brought the Age of Dinosaurs to an end,” the researchers said.
“It also represents the easternmost occurrence of the genus during the Cretaceous period.”
“With this study we gain further insight into winners and losers during the cataclysm that ended the Age of Dinosaurs. The mighty dinosaurs fell, and the lowly turtle survived,” added Dr. Peter Dodson, a paleontologist at the University of Pennsylvania.
The phylogenetic analysis placed Hutchemys walkerorum with other known species of Hutchemys and several other turtles in a distinct group of derived plastomenines, which they named Plastomenini.
In addition, the scientists found a group of early trionychids, placing them in a newly-established subfamily, Kuhnemydinae.
“Kuhnemydines are fossil species from Asia, and our analysis suggests the family Trionychidae originated in Asia before migrating to North America sometime in the Late Cretaceous,” they said.
“Our investigations also led us to another new classification in the Trionychidae family, a subfamily we named Chitrainae.”
“This group encompasses modern softshell turtles, including the narrow-headed and giant softshell turtles found in southern Asia.”
The discovery of Hutchemys walkerorum is described in a paper in the journal Cretaceous Research.
Steven E. Jasinski et al. 2022. A softshell turtle (Testudines: Trionychidae: Plastomeninae) from the uppermost Cretaceous (Maastrichtian) Hell Creek Formation, North Dakota, USA, with implications for the evolutionary relationships of plastomenines and other trionychids. Cretaceous Research 135: 105172; doi: 10.1016/j.cretres.2022.105172
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