Shuvosaurus

Not to be confused with Shunosaurus.

Shuvosaurus Temporal range: Late Triassic, early to middle
Shuvosaurus head restoration, after the skull reconstruction of Lehane (2005, 2023)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	Paracrocodylomorpha
Superfamily:	†Poposauroidea
Family:	†Shuvosauridae
Genus:	†Shuvosaurus Chatterjee, 1993
Species:	†S. inexpectatus
Binomial name
†Shuvosaurus inexpectatus Chatterjee, 1993
Synonyms
Chatterjeea Long & Murry, 1995 Chatterjeea elegans Long & Murry, 1995

Shuvosaurus (meaning "Shuvo [Chatterjee]'s lizard") is a genus of beaked, bipedal poposauroid pseudosuchian from the Late Triassic (early to middle Norian) of western Texas. Despite superficially resembling a theropod dinosaur, especially the ostrich-like ornithomimids, it is instead more closely related to living crocodilians than to dinosaurs. Shuvosaurus is known by the type and only species S. inexpectatus, and is closely related to the very similar Effigia within the clade Shuvosauridae. Shuvosaurus was originally described from a restored skull and very few fragmentary postcranial bones as a probable ornithomimosaur, or at least a very ornithomimosaur-like early theropod. The true pseudosuchian affinities of Shuvosaurus were only recognised after the discovery of Effigia linked the skull of Shuvosaurus with similar poposauroid skeletal remains found in the same quarry.

History of discovery

Inaccurate life restoration of Shuvosaurus as a coelophysoid theropod

Shuvosaurus was described by Sankar Chatterjee in 1993 after its fossils were first discovered in 1980s during preparation by his son Shuvo Chatterjee, for whom he named it after (combining "Shuvo" with the Ancient Greek σαῦρος (sauros), meaning "lizard"). These fossils consisted of the partially disarticulated remains of three skulls and a partial lower jaw (the holotype TTU (Texas Tech University)-P9280 and paratypes TTU-P9281 and TTU-P9282), as well as a fragmentary vertebra, scapula and tibia.

The fossils were collected from the Post Quarry of the lower Cooper Canyon Formation (Dockum Group) near Post, Garza County, Texas, US, and was one of many new discoveries made at this quarry in the 1980s by Chatterjee and his team from the Texas Tech University (such as Technosaurus and Postosuchus). Although precise dating is lacking for much of the Dockum Group, including the Post Quarry, it has been correlated to the Adamanian teilzone, a local biostratigraphic unit in the southwestern United States—that has elsewhere been dated to the early to middle Norian stage of the Late Triassic, between 224–215 million years old. The fossils of Shuvosaurus were preserved in a bonebed containing the remains of at least nine partially articulated and associated skeletons alongside the skeleton of a Postosuchus, with a minimum of 14 total individuals indicated by the number of right partial femurs collected.

Upon its description, Chatterjee tentatively interpreted Shuvosaurus as a Triassic member of Ornithomimosauria, a group of theropod dinosaurs otherwise known only from the Cretaceous, due to similar construction of the skull, including toothless jaws and large eye sockets. This is reflected in the species name, inexpectatus, for the unexpected nature of finding a toothless, ornithomimid-like skull in Late Triassic deposits. As with the contemporary purported avian Protoavis and Postosuchus, Chatterjee's proposed Cretaceous coelurosaurian affinities invoked a long ghost lineage and was consequently greeted with scepticism by other researchers (such as Halszka Osmólska in 1997).

Prior to the discovery of the Shuvosaurus skulls, Chatterjee had also described the numerous small pseudosuchian postcranial remains from the Post Quarry—all lacking heads—as juveniles of the large predatory rauisuchid Postosuchus in 1985. However, in a 1995 monograph on Late Triassic tetrapods from the American Southwest, Robert Long and Philip Murry regarded this material as so "radically different" from Postosuchus that they identified it as a new taxon of gracile "rauisuchian" allied to poposaurids (i.e. Poposaurus) which they named Chatterjeea elegans—named after Sankar Chatterjee and from Latin elegans for "very fine" or "beautiful". At the same time, they raised the possibility that Shuvosaurus and Chatterjeea were the same animal, noting the close association of their remains, lack of any postcrania otherwise referrable to Shuvosaurus in the quarry, and that the available material for Shuvosaurus and the Chatterjeea did not overlap (one known by heads, the other skeletons). Following Long and Murry (1995), opinions were divided on the identity of Shuvosaurus and its proposed synonymy with Chatterjeea. Notable among them, Oliver Rauhut (1997, 2000, 2003) argued that Shuvosaurus was indeed a theropod and distinct from Chatterjeea, but that it was instead a specialised basal member convergent with ornithomimosaurs.

In the early 2000s, Sterling Nesbitt and Mark Norell prepared previously unopened jackets of an unknown archosaur from the Whitaker Quarry at Ghost Ranch combining a Shuvosaurus-like skull with Chatterjeea-like postcrania, which they named Effigia in 2006. This discovery showed that Shuvosaurus is more closely related to crocodilians and other pseudosuchians, and that similarities between it and ornithomimids are result of convergent evolution, while also demonstrating that the bodies of Chatterjeea almost certainly belong to Shuvosaurus and that the two are synonymous. Indeed, the two are so similar that in 2007 Spencer Lucas and colleagues proposed that the two genera were synonymous, tentatively referring Effigia to Shuvosaurus as S. okeeffeae, a proposal that has not been followed in subsequent research.

Shuvosaurus itself would not be thoroughly redescribed for many years until late 2023 and early 2024 when two separate redescriptions were published. The skull was first redescribed by Lehane (2023), following on from the work of his previously unpublished 2005 master's thesis, and was then followed shortly after by a complete skeletal osteology from Nesbitt and Chatterjee (2024). Although both were published closely together, the work in each was conducted independently and published in parallel. Notably, Nesbitt and Chatterjee (2024) provide differing and novel interpretations of some of the cranial material compared to previous interpretations, including that of Lehane (2023).

Many isolated shuvosaurid remains found in rocks of the southwestern US from throughout the Late Triassic have been referred to Shuvosaurus (including to Chatterjeea), namely from elsewhere in the Dockum Group and the Chinle Formation to the west. However, these referrals have been questioned after the discovery of Effigia, as it shows that many of the traits used to assign material to Shuvosaurus are only diagnostic of Shuvosauridae as a whole. Following their osteological description, Nesbitt and Chatterjee revised the taxonomic diagnosis of Shuvosaurus in 2024 and in doing so restricted Shuvosaurus to the type and associated material of the Post Quarry bonebed alone, as isolated bones cannot be reliably differentiated between the two genera.

Classification

Shuvosaurus was at first tentatively classified as a member of the otherwise Cretaceous coelurosaurian theropod group Ornithomimosauria by Chatterjee, based on the superficial resemblance shared by their skulls. In an early report at the annual Society of Vertebrate Paleontology meeting in 1991, Chatterjee even explicitly referred Shuvosaurus to the family Ornithomimidae. However, in its formal description in 1993 he instead more cautiously referred it to the broader group Ornithomimosauria and therein erected the monotypic family Shuvosauridae. Notably, despite its age and classifying it individually from other ornithimosaurs, Chatterjee regarded Shuvosaurus as very derived and especially similar to Dromiceiomimus. Chatterjee nonetheless offerred an alternate possibility that it represented a basal theropod extensively convergent with ornithomimosaurs, but considered this less likely. Its affinity to Ornithomimosauria was questioned by later researchers (e.g. Osmólska, 1998), and was taken even further by Hunt et al. (1998) and Heckert & Lucas (1998) who argued that Chatterjee (1993) did not demonstrate that the skull even had any diagnostic traits of dinosaurs.

Theropod affinities for Shuvosaurus were nonetheless still supported by some researchers in the late 1990s and early 2000s, most notably by Oliver Rauhut. In 1997, Rauhut rejected Long and Murry's proposal Shuvosaurus was synonymous with Chatterjeea (and therefore a pseudosuchian) on the basis of theropod-like features of its skull absent in any known pseudosuchians at the time, but did not identify it as an ornithomimosaur. Instead, he believed Shuvosaurus to be an early-diverging theropod but could not confidently determine its relationships further due to its derivity. Rauhut later included Shuvosaurus in a phylogenetic analysis of theropods in 2003, in which it was recovered as a coelophysoid. However, its inclusion led to a polytomy amongst coelophysoids, while their relationships were fully resolved when Shuvosaurus was excluded. A similar relationship was argued for in the master's thesis of James Lehane in 2005, who specifically identified it as a close relative of "Syntarsus" (now known as Megapnosaurus). Lehane later revised this classification when formally publishing his description of the skull in 2023, subsequent to the discovery of Effigia. Additionally, in 2005 Thomas Lehman and Chatterjee briefly eluded to purported additional material claimed to suggest that Shuvosaurus was a theropod more derived than ceratosaurs. However, this claim was never followed up on in literature, and in the 2024 redescription Chatterjee accepted Shuvosaurus was a poposauroid pseudosuchian related to Effigia.

Pseudosuchian affinities were made correctly for the postcrania representing Chatterjeea by Long and Murry (1995), who regarded it as belonging to a highly derived "rauisuchian" derived from poposaurids that they classified under the new family Chatterjeeidae. With the discovery of Effigia in 2006 the association between the Shuvosaurus skull and "Chatterjeea" postcrania was confirmed and Shuvosaurus was conclusively reidentified as a "rauisuchian" pseudosuchian related to Poposaurus. This grouping of Poposaurus-like taxa was later defined as the clade Poposauroidea. Phylogenetic analyses since then consistently find Shuvosaurus and Effigia as sister taxa, and together with Sillosuchus make up the clade Shuvosauridae deeply nested within Poposauroidea.

Below is a simplified cladogram modified from Nesbitt (2011), highlighting the relationship of Shuvosaurus to other poposauroids:

Poposauroidea	Qianosuchus Ctenosauriscidae Arizonasaurus Xilousuchus Hypselorhachis Ctenosauriscus Bromsgroveia Waldhaus Taxon Poposaurus gracilis H Poposaurus gracilis Y Lotosaurus Shuvosauridae Sillosuchus Shuvosaurus Effigia