Chindesaurus

Chindesaurus is known from six incomplete specimens (seven if Caseosaurus is included). Of these, the type specimen PEFO 10395 is the most complete. This was a partial skeleton, discovered in the Petrified Forest National Park member of the Chinle Formation in Apache County, Arizona. Its remains were discovered by Bryan Small in 1984, in blue mudstone which was deposited during the Norian stage of the Triassic, approximately 228-208 million years ago.[5][6] It consists of a single tooth, a fragmentary cervical (neck) vertebra, fragmentary dorsal (back) vertebrae, several rib fragments, two complete sacral (hip) vertebrae, fragmentary caudal (tail) vertebrae, a chevron, several fragmentary hip bones, a complete left femur and a fragmentary right femur, a fragmentary right tibia, and a right astragalus bone of the ankle.[1]

Several more incomplete specimens have been referred to the genus. Specimen PEFO 4849 consists of a single dorsal vertebra collected in Norian-aged sediments in a different part of the Upper Petrified Forest member. Specimen PEFO 33982, which consists of nine vertebrae, an ilium fragment, a proximal femur, and other bone fragments was also collected in Norian-aged sediments of Upper Petrified Forest member. These two specimens are housed in the collection of the University of California Museum of Paleontology in Berkeley, California.

Specimens NMMNH P16656 and NMMNH P17325 were discovered in the Bull Canyon Formation of New Mexico, sediments deposited during the Norian stage of the Late Triassic. The assignment of these two specimens to Chindesaurus has now come under question.[7] These specimens are housed in the collection of the New Mexico Museum of Natural History and Science in Albuquerque, New Mexico.

Additional material referable to Chindesaurus was discovered in 2006, in gray/green siltstone at the Hayden Quarry of the Petrified Forest Member of the Chinle Formation in New Mexico. Another specimen, TMM 31100-523 which consists of a proximal femur, was discovered in the Colorado City Formation in Texas, in mudstone considered to be from the Carnian stage of the Triassic, approximately 235-228 million years ago. It is currently housed in the collection of the Texas Memorial Museum in Austin, Texas.

The genus name Chindesaurus is derived from the Navajo word chindi meaning "ghost" or "evil spirit" and the Greek word "sauros" (σαυρος) meaning "lizard";[8] thus, "ghost lizard" or "Lizard from Chinde Point". The specific name, bryansmalli, honors the discoverer, Bryan Small. Chindesaurus was described and named by R.A. Long and P.A. Murry in 1995; the type species is Chindesaurus bryansmalli. When this specimen was first discovered it was nicknamed "Gertie" after Gertie the Dinosaur and received much publicity.[1]

The size of Chindesaurus (interpreted as a herrerasaurid) compared to a human

The type and paratype specimens were approximately 2 to 2.3 metres (6.6 to 7.5 ft) in length.[9] Some estimates suggest that Chindesaurus weighed 50 kilograms (110.2 pounds) at most.[10]

Restoration as a theropod, from Petrified Forest National Park

Chindesaurus has been difficult to classify, and has been recovered in several different positions at the base of the saurischian family tree. When it was first discovered in 1984, the fossil specimen which would eventually be named Chindesaurus was thought to be a prosauropod.[11] When it was finally described and named a decade after its discovery by Long and Murry, they regarded it as a herrerasaurid, an opinion that has been followed by most paleontologists since.[1] Most phylogenetic analyses published through 2007 continued to find it to fall among the herrerasaurids,[2] though a few studies cast doubt on this, including one in 2007 by Irmis, Nesbitt and colleagues which found Chindesaurus to be a probable basal saurischian dinosaur, and noted that it shares a wide range of characteristics with several lineages of basal saurischians, making any classification problematic.[12] Rauhut (2003) noted that the medially expanded brevis shelf of Chindesaurus resembles that of crurotarsans, and not that of most dinosaurs, which is usually laterally expanded.[13] Benson and Brussatte (2012) found that Chindesaurus was a basal saurischian related to Herrerasaurus.[9]

One specimen originally assigned to Chindesaurus, from the Tecovas Formation of Texas, was later placed in its own genus and species, Caseosaurus crosbyensis.[14] Subsequent research has shown that this separation was probably in error, and that the two forms represent the same species.[15] Nesbitt, Irmis and Parker agreed in a 2007 paper that there is little reason to separate Caseosaurus from Chindesaurus, and the two even share some unique characteristics not found in similar species. Nesbitt and colleagues suggested that any differences between the two were probably related to differences in size. However, since both species are so fragmentary, they decided not to formally make them synonyms.[7]

The following is a cladogram based on the phylogenetic analysis by Hans-Dieter Sues, Sterling J. Nesbitt, David S Berman and Amy C. Henrici, in 2011, which indicated that Chindesaurus is a herrerasaurid.[16] A similar position for Chindesaurus was recovered by the analyses by Baron, Norman & Barrett and Baron & Williams.[17][3]

In 2019, the holotype specimen of Chindesaurus was redescribed by Adam D. Marsh, William G. Parker, Max C. Langer, and Sterling J. Nesbitt. They included a phylogenetic analysis which placed Chindesaurus as a basal theropod. It was specifically found to be the sister taxon of Tawa hallae, a theropod known from Ghost Ranch in New Mexico.[4]

The Upper Petrified Forest National Park member of the Chinle Formation was an ancient floodplain where phytosaurs, rauisuchids, archosaurs, pseudosuchians, and other tetrapods lived and competed with the dinosaur Chindesaurus and its relative Coelophysis for resources. This paleoenvironment also had abundant lungfish and clams.