Dolichosauridae

Dolichosauridae (from Latin, ''dolichos'' = "long" and Greek ''sauros''= lizard) is a family of Cretaceous aquatic ophidiomorphan lizards closely related to the snakes and mosasaurs.

Description

''Dolichosaurus'' was a small marine squamate at about 0.5 to 1 meter in total length. ''Coniasaurus'' was similarly sized at about 0.5 meters in length. They were elongated (especially apparent in the neck) marine lizards with reduced limbs and small, thin heads. Dolichosaurs may have occupied a niche similar to the earlier nothosaurs and modern sea snakes, in using their thin heads to feed in crevices and narrow spaces along coral reefs and rocky shores. One of the earliest dolichosaurs, '' Kaganaias'' from Barremian, probably lived in freshwater environment unlike other members in the family.

The degree to which the limbs were reduced suggest that the dolichosaurs would have been unable to generate any significant movement on land, and they thus likely spent most of their time underwater. Though the reduction of the limbs have no obvious evolutionary advantage, it is an adaptation that might be shared with early snakes.

History of discovery

Both of the genera routinely referred to the Dolichosauridae, ''Dolichosaurus'' and ''Coniasaurus'', were described by Richard Owen in 1850 based on specimens of Cenomanian age collected from the Lower Chalk of southeastern England.Owen, R. 1850. Description of the Fossil Reptiles of the Chalk Formation; pp. 378–404 in F. Dixon (ed.), ''The Geology and Fossils of the Tertiary and Cretaceous Formations''. The unusually large number of dorsal and cervical vertebrae present in ''Dolichosaurus'' prompted it to be named as a new genus and ''Coniasaurus'' could be distinguished from known Cretaceous lizards by dental characteristics. Nopcsa (1908)Nopcsa, F. 1908. Zur kenntnis der fossilen Eidechsen. Beitra¨ge zur Pala¨ontologie und Geologie O¨sterreich-Ungarns und des Orients 21: 33–62. restudied the specimens discovered by Owen and described several new specimens of ''Coniasaurus'' and ''Dolichosaurus''.

Classification

Both known species of ''Coniasaurus'', ''C. gracilodens'' and ''C. crassidens'', are known from disarticulated skulls with isolated vertebral elements. In contrast, ''Dolichosaurus longicollis'', the only species of ''Dolichosaurus'', is known from postcranial skeletons with only very fragmentary skulls and no teeth. This has made the exact nature of their relation difficult to determine.

''Coniasaurus'' and ''Dolichosaurus'' are generally considered to be either nested within basal mosasauroids as a third group alongside mosasaurs and aigialosaurs, or as the sister group to the mosasauroids. The difficulties in establishing more certain relationships not only between the Dolichosauridae and the Mosasauroidea, but also between ''Dolichosaurus'' and ''Coniasaurus'', arise from the nature of the fossil remains referred to the genera. With ''Coniasaurus'' largely lacking postcranial material and ''Dolichosaurus'' largely lacking cranial material, they are difficult to compare with each other.

Studies that propose a close relation between snakes and mosasauroids in a group dubbed Pythonomorpha demonstrate the importance of ''Dolichosaurus'', ''Coniasaurus'' and other Late Cretaceous marine squamates in squamate phylogeny and evolutionary history. The hypothesis suggesting that snakes, mosasaurs, aigialosaurs and dolichosaurs share a common aquatic ancestor is not universally accepted and stands in stark contrast with the previously widespread hypothesis of snakes having a fossorial origin.

The cladogram below is based on Palci and Caldwell (2010a), Caldwell and Palci (2010b), placing the Dolichosauridae as a sister group to modern snakes and their closest ancestors.

References

{{Taxonbar, from=Q3712637

Cretaceous lizards
Prehistoric reptile families
Prehistoric squamates
Fossil taxa described in 1852