Added Later: The Hairy Museum of Natural History has more. Including links to pics of the fossil, the paper it was described in and more!
The mosasaurs are species of aquatic reptiles that are related to lizards (to be more precise the varanoid lizards - of which the Komodo dragon is a good example). During the upper cretaceous they reached their peak. Almost 20 genera are recognized for this period with the largest approaching 30 feet. They were ocean going carnivores that ate almost anything that swam in the sea. Below are some representitive species.
And here are some fossils.
One with a person in it for scale.
However, we are not concerned with one of the larger species. We are interested in a little three foot long specimen discovered in Texas. Most Mosasaurs have flippers, Dallasaurus turneri has limbs similar to other land lizards. From Science Daily:
Until the discovery of Dallasaurus, however, only five primitive forms with land-capable limbs were known, all of them found in the Middle East and the eastern Adriatic.
The advanced fin-bearing mosasaurs have been grouped into three major lineages. Although a small number of primitive mosasaur have been known to retain land-capable limbs, they were thought to be an ancestral group separate from the later fin-bearing forms. Dallasaurus represents a clear link to one lineage of the later forms and the first time researchers can clearly show mosasaurs evolved fins from limbs within the different lineages of mosasaurs.
I was unable to find any pictures of the fossil, but here is a reconstruction of what it is believed to look like.
For a quick overview of Mosasaurs you can do no better than Oceans of Kansas