Abstract
The physical properties of water and the environment it presents to its inhabitants provide stringent constraints and selection pressures affecting aquatic adaptation and evolution. Mosasaurs (a group of secondarily aquatic reptiles that occupied a broad array of predatory niches in the Cretaceous marine ecosystems about 98–65 million years ago) have traditionally been considered as anguilliform locomotors capable only of generating short bursts of speed during brief ambush pursuits. Here we report on an exceptionally preserved, long-snouted mosasaur (Ectenosaurus clidastoides) from the Santonian (Upper Cretaceous) part of the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas, USA, that contains phosphatized remains of the integument displaying both depth and structure. The small, ovoid neck and/or anterior trunk scales exhibit a longitudinal central keel, and are obliquely arrayed into an alternating pattern where neighboring scales overlap one another. Supportive sculpturing in the form of two parallel, longitudinal ridges on the inner scale surface and a complex system of multiple, superimposed layers of straight, cross-woven helical fiber bundles in the underlying dermis, may have served to minimize surface deformation and frictional drag during locomotion. Additional parallel fiber bundles oriented at acute angles to the long axis of the animal presumably provided stiffness in the lateral plane. These features suggest that the anterior torso of Ectenosaurus was held somewhat rigid during swimming, thereby limiting propulsive movements to the posterior body and tail.
Highlights
Mosasaurs are a group of marine reptiles that occupied a wide array of predatory niches in epicontinental seas and shallow oceans worldwide during the latter half of the Cretaceous [1]
Because the skeletal anatomy of Ectenosaurus is reasonably well known, the exceptionally preserved dermal covering of FHSM VP-401 is the focus of this report
The vertebrate integument serves a number of important biological and mechanical roles, including e.g., protection against predation and parasites, support of the enclosed body contents and control of water loss [21,22,23]. It provides a strong yet flexible covering that allows changes in body shape occurring during locomotion, or as a means of resisting changes to body shape resulting from muscular activity and movement [18,19,21]
Summary
Mosasaurs are a group of marine reptiles that occupied a wide array of predatory niches in epicontinental seas and shallow oceans worldwide during the latter half of the Cretaceous [1]. Crucial to our understanding of mosasaur evolution, and the degree of aquatic adaptations they achieved, are the preservation of soft-tissue structures, which for a long time were limited to small patches of scales in a few forms [4,9,12,13,14,15]. FHSM VP-401 provides information on the deeper structure and mechanical properties of the mosasaur integument. These new data are central to expand our understanding of the degree of aquatic specializations convergently achieved by multiple organ systems in distantly related mosasaurs, in what we know was a short period of geological time (i.e., less than 10 million years [10])
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