The most complete record of the earliest dinosaur lineages is from the Carnian from the higher latitudes of Pangea (e.g. present-day Brazil, Argentina), but dinosaurian assemblages from the upper stages of the Upper Triassic are better known from the low latitudes of Pangea (present day southwestern USA). How early carnivorous dinosaurian diversity matches or mismatches at various latitudes remains to be documented because of uncertainty around the spatio-temporal ranges and phylogenetic relationships of early dinosaur lineages. We examine low-latitude diversity patterns through the lens of the saurischian dinosaur Tawa hallae and close relatives, including a new species, Ptychotherates bucculentus gen. et sp. nov. The new taxon is known from an incomplete but well-preserved skull (CM 31368) from the uppermost Triassic Coelophysis Quarry in northern New Mexico. The new taxon clearly shares synapomorphies with Tawa hallae, such as distinctive fossae on the quadrate and otoccipital and a dorsoventrally tall and laterally flat jugal. However, the new taxon is distinguishable from all other coeval ornithodirans by a combination of many character states, including the proportionally dorsoventrally deepest jugal known for any Triassic-aged dinosaur. Higher-palaeolatitude ecosystems across Pangea show a complete turnover of carnivorous dinosaurs by neotheropods in the Norian and Rhaetian, but the ‘Chindesaurus–Tawa’ clade (Morphoraptora clade nov.) coexisted with neotheropods possibly until the End-Triassic Extinction Event. This suggests a low-latitude ‘museum’ where early-diverging lineages survived much longer than at higher latitudes, and that the End-Triassic Extinction Event affected dinosaur diversity more than previously hypothesized.
Keywords: biogeography, Saurischia, extinction, skull, phylogenetics, clade longevity
SYSTEMATIC PALAEONTOLOGY
DINOSAURIA Owen 1842 sensu Langer et al. 2020
SAURISCHIA Seeley 1888 sensu Gauthier et al. 2020
Clade MORPHORAPTORA nov.
Derivation of name: Morphe from the Greek for ‘form, shape’ and raptor from the Latin for ‘robber’, describing the morphological convergence between members of this clade and Theropoda, as if this clade is ‘stealing’ morphology. ‘Morphoraptor’ loosely translates to ‘bodysnatcher’, in honour of the 2007 song by Radiohead in the album In Rainbows, which author S. Srivastava listened to hundreds of times while preparing this manuscript.
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| The posterior portion of the skull Ptychotherates bucculentus (CM 31368) in posterior view. Abbreviations: ar, articular bo, basioccipital; bt, basituber; f, frontal; fm, foramen magnum; fno, fenestra ovalis; pa, parietal; par, paroccipital process of the otoccipital; mf, metotic foramen; n, nasal; pbs, parabasisphenoid; pf, prefrontal; po, postorbital; ptf, posttemporal fenestra; qj, quadratojugal; qu, quadrate; rap, retroarticular process; so, supraoccipital; sq, squamosal; st, stapes. Crossed circle indicates posterior direction out of page. Scale bar represents 1 cm. |
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| Digital reconstruction of the skull of Ptychotherates bucculentus (CM 31368) in left lateral view. Dashed lines indicate extrapolation and the coloured infill indicates known bone presence. Abbreviations: eaf, external antorbital fenestra; en, external naris; emf, external mandibular fenestra; itf, infratemporal fenestra; orb, orbit. Arrow indicates anterior direction. Scale bar represents 2 cm. |
Genus Ptychotherates nov.
Derivation of name: Ptycho from the Greek for ‘fold’ because of the numerous and challenging axes of reorientation on elements of the holotype. Therates from the Greek for ‘hunter’, for the carnivorous habits inferred from its teeth.
Ptychotherates bucculentus sp. nov.
Derivation of name: The species epithet is Latin for ‘with full cheeks’ in reference to the exceptionally tall jugal.
Diagnosis: Ptychotherates bucculentus bears the following combination of character states (local autapomorphies indicated with *): supratemporal fossa present on posterior portion of the frontal and dorsal surface of the parietal; tapering dorsal process of the maxilla; maxilla lacking antorbital fossa on its posterior portion ventral to the antorbital fenestra; prefrontal symmetrical in lateral view*; jugal body proportionally dorsoventrally deep* (i.e. more than three times as deep as the jugal posterior process, deeper than the length of postorbital ventral process, and more than half the height of the quadrate main body; Table 1); laterally extensive ventral process (= crista interfenestralis) of the otoccipital; ventral process of squamosal anteroposteriorly wide with a lateral fossa on the posterior part; retroarticular process upturned immediately posterior to glenoid; anterolateral portion of postorbital dorsally overhanging orbit; postorbital dorsolaterally overlapping the squamosal; posttemporal fenestra as wide as foramen magnum; nasal and frontals flat dorsally; serrated and recurved teeth with fine serrations (4–5 per 1 mm in the maxilla, 5–6 per 1 mm in the dentary) with pointed apices of the serrations.
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| Artistic rendition of Ptychotherates bucculentus. Artwork by Megan Sodano |
Simba Srivastava and Sterling J. Nesbitt. 2026. A New Taxon of saurischian Dinosaur from the Coelophysis Quarry of New Mexico, USA (Triassic: latest Norian or Rhaetian) highlights herrerasaurian diversity in the latest Triassic. Papers in Palaeontology. 12(2); e70069. DOI: doi.org/10.1002/spp2.70069 [14 April 2026]
