Carpal Morphology and Function in the Earliest Cetaceans

ABSTRACT

During the land-to-water transition in the Eocene epoch, the cetacean skeleton underwent modifications to accommodate life in the seas. These changes are well-documented in the fossil record. The forelimb transformed from a weight-bearing limb with mobile joints to a flipper with an immobile carpus. We used micro-CT imaging to assess evolutionary changes in carpal size, orientation, and articulation within Eocene cetacean taxa associated with the transition from a terrestrial to amphibious niche. We compared Ambulocetus natans, a well-preserved amphibious archaeocete, with other archaeocetes, and with Eocene terrestrial artiodactyls, the sister group to Cetacea. A cylindrical carpus in terrestrial taxa evolved into a mediolaterally flattened, cambered carpus in the semi-aquatic and fully aquatic cetaceans. Specifically, the pisiform bone shifted from a ventral orientation in terrestrial taxa to a lateral orientation, in plane with the carpus, within semi-aquatic and fully aquatic taxa. Flattening of the carpus, including lateral rotation of the pisiform, likely relates to functional shifts from weight-bearing terrestrial locomotion to aquatic locomotion. This laterally projecting pisiform morphology is retained in all extant cetaceans. Our results suggest this shift, along with other modifications to the carpus, predominantly occurred during the middle Eocene and facilitated an obligatorily aquatic lifestyle in late Eocene cetaceans.

ACKNOWLEDGMENTS

We thank the University of Texas Austin CT Imaging Core for their short course on data segmentation and visualization, as well as for the guidance of their staff. We are grateful to S. Usip and C. Mayerl for assistance with data processing and software troubleshooting. Special thanks to C. Fish for assistance with figure design. Funding for travel related to this project comes from the Musculoskeletal Research Group at NEOMED, and we are appreciative of their financial backing. Thanks to D. Waugh and K. Kjosness for discussion. We thank our two reviewers, R. Bebej and M. Uhen, for their highly beneficial comments during the revision process that greatly improved this text. We also thank O. Lambert for his comments and recommendations on a later draft of the manuscript.