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The Plantigrade Segnosaurians: Sloth Dinosaurs or Bear Dinosaurs?

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Abstract

This article considers the unusual Macropodosaurus gravis tracks from the Early Cretaceous (Albian) locality of Shirkent-1 (Tajikistan). The trackmaker was bipedal and plantigrade. Through a comparison of Macropodosaurus tracks with those of other dinosaurs and by a juxtaposition of track morphology and pes skeletal morphology, it is argued that they belong to an aberrant group of theropods, the segnosaurs. Data on the stratigraphic and geographical distribution of segnosaur tracks are presented. Aspects of functional morphology of the hind limbs are analyzed, as well as the locomotory mode and the possible lifestyle of this group of dinosaurs. Segnosaurs were most likely herbivorous; their possible ecological counterparts are giant ground sloths. The ancestors of segnosaurs were probably facultatively digitigrade and underwent a short-term stage as arboreal, climbing forms, and then reversed to a purely terrestrial lifestyle. Return to plantigrade locomotion in segnosaurs apparently required increased area of support during the slow obligate bipedal locomotion of these heavy theropods.

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ACKNOWLEDGMENTS

The author thanks the geologists V.P. Novikov and V.V. Kurbatov for providing photographs of Macropodosaurus tracks and valuable consultations on dinosaur tracks in Central Asia. The author thanks B.P. Hedrick (Department of Cell Biology and Anatomy, School of Medicine, Louisiana State University, Health Sciences Center, New Orleans, United States) for providing photographs of the pelvis of the segnosaur Nothronychus graffami, and A.A. Karhu (Borissiak Paleontological Institute, Russia) for sharing his hypothesis about the arboreal life habits and locomotion of Deinonychus and providing a drawing illustrating this hypothesis, and A.N. Kuznetsov (Borissiak Paleontological Institute) for suggesting to study the orientation of the acetabulum in segnosaurs, and for valuable comments.

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This work was supported by the Russian Foundation for Basic Research grant number 20-04-00070.

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    A. G. Sennikov

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Sennikov, A.G. The Plantigrade Segnosaurians: Sloth Dinosaurs or Bear Dinosaurs?. Paleontol. J. 55, 1158–1185 (2021). https://doi.org/10.1134/S0031030121100087

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