Abstract
A left mandible of a small crocodyliform found in the Upper Jurassic Morrsion Formation of northeastern Wyoming represents the first occurrence of the atoposaurid Theriosuchus in North America. The specimen demonstrates lower jaw morphology, including heterodonty (as indicated by alveolus shape), similar to Theriosuchus and Knoetschkesuchus, but autapomorphies and a unique combination of characters among these taxa indicate that it is a distinct, new species of Theriosuchus.
Highlights
Fossil crocodylomorphs are diverse in the Upper Jurassic Morrison Formation of western North America, with seven species of terrestrial and semi-aquatic forms occurring, with collective widespread distribution and high abundance (e.g., Clark, 2011; Pritchard and others, 2013; Foster and McMullen, 2017)
A left mandible of a small crocodyliform found in the Upper Jurassic Morrsion Formation of northeastern Wyoming represents the first occurrence of the atoposaurid Theriosuchus in North America
Specimen MWC 5625 was originally described as a juvenile goniopholidid that would have undergone dramatic allometric growth in its lower jaw through ontogeny (Foster, 2006)
Summary
Fossil crocodylomorphs are diverse in the Upper Jurassic Morrison Formation of western North America, with seven species of terrestrial and semi-aquatic forms occurring, with collective widespread distribution and high abundance (e.g., Clark, 2011; Pritchard and others, 2013; Foster and McMullen, 2017). Theriosuchus species with the following unique combination of characters (*denote autapomorphic for species relative to Theriosuchus and Knoetschkesuchus): greatly enlarged D2 and D3 alveoli*; dramatic reduction in mesiodistal diameter of alveoli from D3 to D4*; overall structure of mandible similar to T. pusillus in depth:length ratio, lack of external mandibular fenestra, retroarticular process angle, and dentary dorsal profile in two “waves” in lateral view; dentary nutrient foramina row and symphysis orientation relative to tooth row both more similar to K. guimarotae.
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