129-75 Preservation of Dermal Soft Tissue in the Marine Reptile Clidastes propython (Squamata, Mosasaurinae)

Abstract:

Mosasaurs are a group of secondarily aquatic lizards that diversified into a variety of marine environments during the Late Cretaceous (98–66 Ma). Numerous taxa are known from the late Coniacian to early Campanian aged strata of the Smoky Hill Chalk Member of the Niobrara Chalk Formation, in western Kansas, USA. Mosasaurs comprise the bulk of vertebrate remains from the Smoky Hill Chalk, with a few specimens containing remnants of soft tissue. Here we report a moderately complete and articulated specimen (UWGM PL-011218) of the early-diverging mosasaurine Clidastes cf. propython with preserved scale impressions along the anterior surface of the trunk and potential gut content near the anteriormost ribs. UWGM PL-011218 was prepared in oblique left lateral to ventrolateral view.

The specimen is assigned to Clidastes cf. propython based on its forelimb morphology: (1) presence of a large, pronounced radiale that borders on the antebrachial foramen; (2) first metacarpal is significantly shorter proximodistally and wider anteroposteriorly than second metacarpal and possesses an anterodistal flange; (3) presence of a robust, posteriorly-oriented pisiform similar in size to the ulnare; (4) proximal ends of metacarpals greatly longitudinally expanded.

Instances of soft tissue preservation are known from numerous mosasaur specimens. Rhomboid, overlapping, non-keeled scales are present in varying dimensions across the body of Platecarpus (LACM 128319). Distinctly keeled axial scales are known from the russellosaurinans Tylosaurus (KUVP 1075) and Ectenosaurus (FHSM VP-401). The mosasaurine Plotosaurus (UCMP 152664) has been hypothesized to have potentially possessed both keeled and non-keeled scales, with the smoother scales inferred to cover the ventral surface. On UWGM PL-011218, a large section (~15 x 7 cm) of scale impressions has been prepared on the abdomen near the first few posterior dorsals. The scales are proportionally small (1.5-2 mm anteroposteriorly), rhomboid, imbricated, and variably textured but lacking an obvious keel. The scales found in Clidastes are morphologically most similar to those of Platecarpus, which may have implications for the ancestral conditions of mosasaur scales and the correlation between scale structure and lifestyle.  

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.