New research by Dr. Craig Scott, Royal Tyrrell Museum of Palaeontology, Dr. Anne Weil, Oklahoma State University, and Dr. Jessica Theodor, University of Calgary, introduces a new species of multituberculate, Catopsalis kakwa, which challenges previous research proposing the evolution of larger body mass. This new research suggests a more complex evolutionary history of Taeniolabidoid multituberculates than previously thought.
Multituberculates were successful rodent-like mammals, named for their teeth that have many cusps, or tubercles, arranged in rows. Over 200 species are known from their 165-million-year existence. They were among the most diverse mammals of the early Palaeocene, the first epoch of the Palaeogene Period, having survived the catastrophic Cretaceous-Palaeogene mass extinction 66 million years ago that caused the dinosaurs’ demise.
During the early Palaeocene, multituberculates had increasingly specialized arrangement of teeth; however, multituberculates generally remained small and rarely exceeded body sizes greater than today’s rabbits. One exception is the Taeniolabidoidea, a group of multituberculates primarily found in North America that includes the largest Catopsalis is a genus of Taeniolabidoidea that is geographically wide ranging.
Until the discovery of C. kakwa, the evolution of Catopsalis appeared to document increasing body size. C. kakwa is the smallest species of Catopsalis and the smallest Taeniolabidoid ever discovered. With an estimated body mass between 400 and 600 g, it is comparable in size to the eastern grey squirrel.
In contrast to previous studies of the evolution of Taeniolabidoidea body mass, Dr. Scott and his colleagues used methodologies from recent research on calculating the body mass of modern rodents. Using the biting surface area of the entire cheek tooth row as a proxy for size, instead of the surface area of the first lower molar, Dr. Scott and his colleagues calculated body mass estimates for C. kakwa and other Taeniolabidoidea that are notably smaller than those from previous studies.
The size and occurrence of C. kakwa in the late early Palaeocene complicates the evolutionary history of Taeniolabidoidea, and implies either a ghost lineage, or an evolutionary reversal of characteristics, going from large to small body size. (A ghost lineage is when there is an extensive part of the evolutionary record of an animal that is not currently recognized in the fossil record; in this case, the fossil history of small-sized Catopsalis has not yet been found.) “Because the trend in these multituberculates seems to be getting bigger and bigger, this animal is so unexpected in that it’s quite small and temporally it’s quite late in the game,” stated Dr. Scott.
Further study of C. kakwa is needed to determine which alternative, a ghost lineage or a reversal of characteristics, is more likely and to better understand the evolutionary history of C. kakwa and Taeniolabidoid multituberculates.