Bulletin 23 - Abstract

Systematics and Morphology of American Mosasaurs

By Dale A. Russell

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Mosasaurs were large, marine platynotan lizards which became abundant and diversified during the latter half of Cretaceous time, but disappeared at the close of the period. Three subfamilies are recognized within the Family Mosasauridae: the Mosasaurinae (including the new tribes, Mosasaurini, Globidensini and Plotosaurini), the Plioplatecarpinae (including the new tribes, Plioplatecarpini and Prognathodontini), and the Tylosaurinae. At present thirteen genera and thirty species are diagnosed from the North American Cretaceous, with one new genus (Ectenosaurus) and one new species (Plioplatecarpus primaevus). Of the older names available, nineteen generic and twenty-nine specific names have been placed into synonymy, two generic and eighteen specific names are based on indeterminate material, and an additional generic and six specific names are only of dubious validity.

The osteolow of mosasaurs is described in detail and with reference to the soft anatomy. Mosasaurs possessed a good sense of sight and a poor sense of smell. A calcified tympanum, present in all three subfamilies, was probably useful in transmitting waterborne sound to the middle ear and is not indicative of deep-diving habits. Streptostylic quadrates permitted anteroposterior movement of the mandibles, which in turn facilitated the underwater swallowing of prey. Mosasaurs swam by lateral undulations of the body, the flippers and relatively long neck serving as ovgans of equilibration. They fed on smaller mosasaurs, chelonians, fish, ammonites, belemnites, echinoderms and pelecypods, and or the most part were highly active aquatic carnivores.

Mosasaurs inhabited subtropical epicontinental seas of less than 100 fathoms (about 180 meters) depth and of variable salinity. Individual forms had a wide geographic distribution that was little affected by changes in depositional environments. The range of many species of mosasaurs inhabiting the western edge of the North Atlantic basin probably extended into western European waters. Those from the eastern edge of the Pacific basin appear to have belonged to a distinct zoogeographic province.

Mosasaurs descended from primitive middle Cretaceous varanoids (aigialosaurs) possessing many cranial characteristics of mosasaurs but with a postcranial morphology similar to that of the modern Varanus. Ancestral mosasaurs seem to have been of two basic types; forms with long bodies and short dilated tails giving rise to the mosasaurines, and forms with short bodies and long pointed tails giving rise to the plioplatecarpines and tylosaurines. During their relatively brief geologic existence mosasaurs exhibited a few clearly progressive trends, such as a tendency to increase in overall size, to telescope the frontals over the anterior edge of the parietals (consequently suppressing kinesis), to increase the number of pygal vertebrae, and to alter the primitive webbed paddIes into long, hyperphalangic flippers.