| Saniwa | |
|---|---|
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| Skeleton ofSaniwa ensidens in theField Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Family: | Varanidae |
| Genus: | †Saniwa Leidy,1870 |
| Species | |
| Synonyms | |
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Saniwa is an extinctgenus ofvaranidlizard that lived during theEocene epoch. It is known from well-preservedfossils found in theBridger andGreen River Formations ofWyoming, United States. Thetype speciesS. ensidens was described in 1870 as the first fossil lizard known fromNorth America. A second species,S.orsmaelensis, is recognised from remains found in Europe. It is a close relative ofVaranus, 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 toextant monitor lizards,Saniwa had many primitive traits, including teeth on itspalate, ajugal bone beneath the eye that extended farther forward, and a suture between thefrontal andparietal bones that was straight rather than curved.[3]
A study in 2018 by scientists from theSenckenberg Research Institute andYale University foundSaniwa had twoparietal eyes, one that developed from thepineal gland and the other from the parapineal gland. The parietal eye is a light-sensitive structure present in thetuatara, most lizards,frogs,salamanders, certainbony fish,sharks andlampreys, a group ofjawless fish.[4]It plays an important role in geographical orientation and regulatingcircadian and annual rhythms.Saniwa is the only knownjawed 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 thatSaniwa reevolved the pineal eye.[5]
In 1870, American geologistFerdinand Vandeveer Hayden found the first fossils ofSaniwa near the town ofGranger, Wyoming, and gave them to paleontologistJoseph Leidy.[2] Later that year, Leidy described the type speciesSaniwa ensidens on the basis of these fossils.Saniwa was the first extinct lizard to be named from North America.[6] The first remains ofS. ensidens were preserved as black bones inmarl that was part of theBridger Formation. Hayden suggested the nameSaniwa 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 betweenSaniwa and the livingNile monitor.
Although his first description was brief, Leidy studied the genus thoroughly and provided illustrations in an 1873 paper. In this paper, Leidy called itSaniwa. He also named a second species,Saniwa [sic]major, on the basis of a brokenhumerus and some isolated dorsal vertebrae.[7] In 1918, Baron G. J. de Fejérváry suggested thatS. major was not a species of lizard, noting that the humerus was "undoubtedly" nonreptilian.[8] Leidy even pointed out similarities between the bone and those of birds in 1873.
Soon after Leidy namedSaniwa, American paleontologistOthniel Charles Marsh erected the genusThinosaurus in 1872 for several species of extinct lizards in the western United States. He never published a full description of these lizards, andThinosaurus was later considered ajunior synonym ofSaniwa. The speciesT. leptodus was synonymized withS. ensidens, but all other species have remained distinct, includingT. agilis,T. crassa,T. grandis, andT. paucidens.[2]
In the 1920s, much of the holotype specimen ofS. ensidens was prepared by removing marl from around the bones. This revealed many new features ofSaniwa, including the underside of the skull and parts of the vertebrae. American paleontologistCharles W. Gilmore restudied the holotype and described new features in 1922.[6] 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 axenosaurid lizard, notSaniwa.[9]
Fossils from many other parts of the world have been assigned toSaniwa, although all are fragmentary. In 1899, Argentine paleontologistFlorentino Ameghino named another species ofSaniwa,S. australis, from lowerMiocene rocks in Argentina. It is now considered adubious name, because the material cannot be assigned with confidence toSaniwa.S. orsmaelensis was described fromBelgium in 1923, but because its naming was informal, it was designated anaked name.S. orsmaelensis was later suggested to be either synonymous withS. ensidens or a different, indeterminate species ofSaniwa. Unlike the Argentine fossils, the Belgian remains represent a definite occurrence ofSaniwa outside North America.[2] A 2022 study foundS. orsmaelensis to be a distinct and valid species ofSaniwa, with remains of the species also reported from France.[10] "S." feisti was named from the EoceneMessel Pit in Germany in 1983.[11] "S." feisti is no longer considered to be a species ofSaniwa, but is placed in the separate genusParanecrosaurus within the familyPalaeovaranidae, which is more distantly related toVaranus thanSaniwa.[3][12]
A complete and articulated skeleton ofS. ensidens was described from the Green River Formation of Wyoming in 2007. It preserves soft tissues like scales, cartilage between bones and in thesternum, and even thetrachea. The individual is thought to have been a juvenile.[2]
Since its first description,Saniwa has been recognized as a close relative of living monitor lizards in the genusVaranus. It is a member of the family Varanidae.Saniwa ensidens is often placed as thesister taxon ofVaranus inphylogenetic analyses, meaning it is more closely related toVaranus than any other varanid. Below is acladogram from Conradet al. (2008) that shows a sister-group relationship betweenSaniwa ensidens andVaranus:[3]
Below is acladogram from Dong et al. 2022.[13]
| Varanidae | |
