Mosasaur Moment

About Mosasaurs and Dallasaurus

Onion Creek Mosasaur on exhibit in the Texas Memorial Museum.

Mosasaurs are lizards that evolved and adapted to life in the oceans during the final phase of the age of dinosaurs, the Late Cretaceous. Later forms evolved flippers and streamlined bodies, optimized for life at sea. The earliest mosasaur is known from near the village of ‘Ein Yabrud in the West Bank and is dated at 98 million years old. It was described by the SMU research team in 1999.

The oldest mosasaurs retain complete limbs including clawed fingers and toes and were able to navigate on land as well as the in the water. These early forms evolved against a backdrop of global warm temperatures and rising sea levels. The most highly evolved forms, those that developed fin-like limbs, are known from rocks between about 90 and about 65 million years old. They became extinct at the end of the Cretaceous period, a fate shared with the land-dwelling dinosaurs.

During their evolutionary history, mosasaurs evolved and diversified to occupy various ecological niches, feeding styles and different ways of life. Some of those changes are preserved in the fossils of these magnificent animals and give us clues about the genetic relationships and evolution of the Mosasaurs.

Historically, mosasaurs were classified into three subfamilies. These highly evolved forms, possessed fin-like limbs, and were theorized to have evolved from the primitive limbed forms. However, recent work by SMU researcher Mike Polcyn suggests that fin-like limbs and other marine adaptations evolved multiple time within the group.

Earth as it looked 94 million years ago.
(courtesy of the Paleomap Project)

Dallasaurus turneri is a primitive limbed mosasaur and was pivotal in the new study. It possesses many of the characteristics defining the sub-family mosasaurinae, and is related to later forms like Clidastes and Mosasaurus, but retains primitive limbs. Another primitive-limbed form, Haasiasaurus gittelmani, also described by the SMU researchers, occupies a basal position in another lineage. A more advanced form, Russellosaurus coheni was found at the same locality near Cedar Hill, Texas, as Dallasaurus and also provided vital information for the study.

Along with the new descriptions, two other studies headed by SMU paleontologist Louis Jacobs, a professor and president and director of the Institute for the Study of Earth and Man, were published this month (November 2005) in the same volume of the Netherlands Journal of Geosciences.

These studies provide a clearer view of both the age of the fossils and the ecological setting in which they lived. The new Texas fossils and other recently described fossils from the West Bank, Columbia and Morocco paint a rich and complex picture of the evolution of mosasaurs and will allow us to better understand the patterns of evolution and potentially the influence of changing environments on the evolution of the group.

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