Who Were the Mosasaurs?

I – Main characteristics of the family

The Mosasauridae (or mosasauridae in French) were not dinosaurs but their contemporaries. The first fossils of this family of marine reptiles, belonging to the genus Mosasauruswere found in 1764 in a quarry in Maastricht (Netherlands) near the Meuse. Hence their name meaning “lizards of the Meuse”. These members of the order of squamates, in which we find snakes and lizards in particular, returned to the aquatic environment which they dominated during the Upper Cretaceous (from approximately -98 to -66 million years ago). The first mosasaurids had a body morphology similar to that of modern monitor lizards. Later forms are represented by long, smooth bodies with an enlarged tail tapering to its end.

II – Its main representatives

Mosasaurus : The original genus of Mosasauridae… but initially poorly represented! Mosasaurus is first taken for a semi-aquatic reptile with webbed feet. It was only in 1854 that the German herpetologist Hermann Schlegel (1804-1884) saw in its members swimming palettes typical of a purely marine way of life. But his vision was not shared until many years later, after more complete skeletons had been unearthed by the famous American paleontologists Othniel Charles Marsh (1831-1899) and Edward Drinker Cope (1840-1897).

© Dotted Yeti – SHUTTERSTOCK 3D illustration of a specimen of the genus Mosasaurus.

Prognathodon : The genre Prognathodonmeaning “tooth of the anterior jaw”, was described in 1889 by the Belgian paleontologist Louis Dollo (1857-1931). But you have to wait until 2013 to see the description in Nature Communications of a specimen of Prognathodon a little special. “Only” 1.5 meters long compared to more than 10 meters for some adults, the juvenile of the Mosasauridae family was collected in 2008 in central Jordan and presents soft tissues. These show a hypocercal caudal fin, in other words with a lower lobe larger than the upper lobe. A novelty since until now we imagined a straight tail like a large eel. It would therefore seem that Prognathodon was closer to pelagic sharks in terms of swimming performance.

© Arctium Lappa – SHUTTERSTOCK Skull drawings of three mosasaurs, including species of the genus Mosasaurus (hoffmanni) and Prognathodon (solvayi).

Clidasts : Animals of the genus Clidasts measured some 4 meters long, including 60 centimeters for their heads alone! Living around 90 million years ago, they must have fed mainly on ammonites – cephalopods reminiscent of today's nautilus – and fish. Fossilized remains of Clidasts were notably discovered in the chalk rocks of Kansas (United States). Pieces of skull from two newborn specimens found in North America were described in 2015 in Palaeontology. Very young, these animals were probably born in the open sea. Previously, the absence of juvenile fossils suggested that the adults traveled inland via rivers to lay their eggs on the banks. But this find indeed demonstrates the viviparity of Clidasts.

© Encyclopedia of Alabama. Clidastes jaw fragments found in 1975.