Saniwa Ensidens Head

''Saniwa'' is an extinct genus of varanid lizard that lived about 48 million years ago during the Eocene epoch. It is known from well-preserved fossils found in the Bridger and Green River Formations of Wyoming. The type species ''S. ensidens'' was described in 1870 as the first fossil lizard known from North America. Several other species have since been added, but their validity is uncertain. It is a close relative of '' Varanus'', the genus that includes monitor lizards.

Description

''Saniwa'' measured . Like other varanid lizards, ''Saniwa'' had a long, pointed snout and nostrils placed farther back in the skull than most lizards and a tail that was almost twice as long as the body. Although similar in appearance to extant monitor lizards, ''Saniwa'' had many primitive traits, including teeth on its palate, a jugal bone beneath the eye that extended farther forward, and a suture between the frontal and parietal bones that was straight rather than curved.

A study in 2018 by scientists from the Senckenberg Research Institute and Yale University found ''Saniwa'' had two parietal eyes, one that developed from the pineal gland and the other from the parapineal gland. The parietal eye is a light-sensitive structure present in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks and lampreys, a group of jawless fish.
It plays an important role in geographical orientation and regulating circadian and annual rhythms. ''Saniwa'' is the only known jawed vertebrate to have both a pineal and a parapineal eye, as the only other vertebrates that have both are the jawless lampreys. In most vertebrates, the pineal gland forms the parietal eye, however, in lepidosaurs, it is formed from the parapineal gland. This implies that ''Saniwa'' reevolved the pineal eye.

History and species

In 1870, American geologist Ferdinand Vandeveer Hayden found the first fossils of ''Saniwa'' near the town of Granger, Wyoming, and gave them to paleontologist Joseph Leidy. Later that year, Leidy described the type species ''Saniwa ensidens'' on the basis of these fossils. ''Saniwa'' was the first extinct lizard to be named from North America. The first remains of ''S. ensidens'' were preserved as black bones in marl that was part of the Bridger Formation. Hayden suggested the name ''Saniwa'' to Leidy because it was "used by one of the Indian tribes of the Upper Missouri for a rock-lizard." Leidy saw a close similarity between ''Saniwa'' and the living Nile monitor.

Although his first description was brief, Leidy studied the genus thoroughly and provided illustrations in an 1873 paper. In this paper, Leidy called it ''Saniwa''. He also named a second species, ''Saniwa'' ic''major'', on the basis of a broken humerus and some isolated dorsal vertebrae. In 1918, Baron G. J. de Fejérváry suggested that ''S. major'' was not a species of lizard, noting that the humerus was "undoubtably" nonreptilian. Leidy even pointed out similarities between the bone and those of birds in 1873.

Soon after Leidy named ''Saniwa'', American paleontologist Othniel Charles Marsh erected the genus ''Thinosaurus'' in 1872 for several species of extinct lizards in the western United States. He never published a full description of these lizards, and ''Thinosaurus'' was later considered a junior synonym of ''Saniwa''. The species ''T. leptodus'' was synonymized with ''S. ensidens'', but all other species have remained distinct, including ''T. agilis'', ''T. crassa'', ''T. grandis'', and ''T. paucidens''.

In the 1920s, much of the holotype specimen of ''S. ensidens'' was prepared by removing marl from around the bones. This revealed many new features of ''Saniwa'', including the underside of the skull and parts of the vertebrae. American paleontologist Charles W. Gilmore restudied the holotype and described new features in 1922. He described many of these features from a fragment of the snout and lower jaw. Although this fossil was well preserved, it was not found in the same block of marl as other parts of the specimen. This fossil was reexamined in 2003 and was found to belong to a xenosaurid lizard, not ''Saniwa''.

Fossils from many other parts of the world have been assigned to ''Saniwa'', although all are fragmentary. In 1899, Argentine paleontologist Florentino Ameghino named another species of ''Saniwa'', ''S. australis'', from lower Miocene rocks in Argentina. It is now considered a dubious name, because the material cannot be assigned with confidence to ''Saniwa''. ''S. orsmaelensis'' was described from Belgium in 1923, but because its naming was informal, it was designated a naked name. ''S. orsmaelensis'' was later suggested to be either synonymous with ''S. ensidens'' or a different, indeterminate species of ''Saniwa''. Unlike the Argentine fossils, the Belgian remains represent a definite occurrence of ''Saniwa'' outside North America. "''S." feisti'' was named from the Eocene Messel Pit in Germany in 1983. "''S." feisti'' is no longer considered to be a species of ''Saniwa'', but is placed in the separate genus '' Paranecrosaurus'' within the family Palaeovaranidae, which is more distantly related to ''Varanus'' than ''Saniwa.''

A complete and articulated skeleton of ''S. ensidens'' was described from the Green River Formation of Wyoming in 2007. It preserves soft tissues like scales, cartilage between bones and in the sternum, and even the trachea. The individual is thought to have been a juvenile.

Classification

Since its first description, ''Saniwa'' has been recognized as a close relative of living monitor lizards in the genus ''Varanus''. It is a member of the family Varanidae. ''Saniwa ensidens'' is often placed as the sister taxon of ''Varanus'' in phylogenetic analyses, meaning it is more closely related to ''Varanus'' than any other varanid. Below is a cladogram from Conrad ''et al.'' (2008) that shows a sister-group relationship between ''Saniwa ensidens'' and ''Varanus'':

References

{{Taxonbar, from=Q3948438

Eocene reptiles of North America
Eocene lizards
Monitor lizards
Lizard genera
Prehistoric reptile genera
Taxa named by Joseph Leidy
Fossil taxa described in 1870