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Dallasaurus ("Dallas lizard") is a basal from the Upper Cretaceous of North America. Along with Russellosaurus, Dallasaurus is one of the two oldest mosasauroid taxa currently known from North America. It is also one of the smallest known Mosasaurinae, measuring approximately in length.
The holotype specimen (TMM 43209-1, Texas Memorial Museum, University of Texas at Austin) consists of an incomplete and disarticulated skull, along with considerable portions of the postcranial skeleton, making up about 80 percent of the animal. "Dino-Era Lizard Is Missing Link to Swimming Reptiles, Experts Say", National Geographic News, 21 November 2005. The second referred specimen (DMNH 8121-8125, 8143-8149, and 8161-8180, Dallas Museum of Natural History) lacks any skull material and consists entirely of disarticulated postcranial remains. The stratum containing these fossils were temporarily exposed during excavations for a housing development, and both sites have now been reburied by construction. The two specimens were discovered about 100 meters from one another; the first was found by an amateur collector, Van Turner, for whom the type species ( Dallasaurus turneri) was named. "SMU and Dallas Museum of Natural History Announce Missing Fossil Link Dallasaurus" , Southern Methodist University, 16 November 2005. The genus is named for Dallas County, where both specimens were found.
Bell and Polcyn use the term "plesiopedal" to indicate a "conservative ecologically adaptive grade" characterized by "small size, slightly modified swimming tail and a relatively plesiomorphic limb condition" compared to more derived mosasauroids. Polcyn and Bell note that plesiopedal mosasauroids tend to be relatively small lizards possessing limbs in which the "propodial elements equals remain elongated, generally constituting one-half or more of the full length of the osseus limb",Bell & Polcyn, Dallasaurus turneri, p. 178. as compared to more derived "hydropedal" mosasaurs in which the propodial elements are stout and have been substantially shortened, constituting less than one-half of the full length of the ossueus limb. While hydropedal mosasaurs were probably entirely aquatic, plesiopedal mosasauroids were still capable of terrestrial locomotion and so likely lived an amphibious lifestyle.
In the popular press, Dallasaurus has been hailed as a "missing link" uniting fully aquatic mosasaurs with their terrestrial ancestors. "Missing fossil link 'Dallasaurus' found", Science Daily, 16 November 2005.
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