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.
Saniwa measured 1.3 to 2.1 m (4.3 to 6.9 ft).[1][2] 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.[2] 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.[3]
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.[4]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.[5]
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.[2] 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.[6] 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."[7] 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 [sic] major, on the basis of a broken humerus and some isolated dorsal vertebrae.[7] In 1918, Baron G. J. de Fejérváry suggested that S. major was not a species of lizard, noting that the humerus was "undoubtably" nonreptilian.[8] 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.[2]