Osteology of Pseudochampsa ischigualastensis gen. et comb. nov. (Archosauriformes: Proterochampsidae) from the early late triassic ischigualasto formation of Northwestern Argentina.

Proterochampsids are crocodile-like, probably semi-aquatic, quadrupedal archosauriforms characterized by an elongated and dorsoventrally low skull. The group is endemic from the Middle-Late Triassic of South America. The most recently erected proterochampsid species is "Chanaresuchus ischigualastensis", based on a single, fairly complete skeleton from the early Late Triassic Ischigualasto Formation of northwestern Argentina. We describe here in detail the non-braincase cranial and postcranial anatomy of this species and revisit its taxonomy and phylogenetic relationships. The phylogenetic analysis recovered 'Chanaresuchus ischigualastensis' as part of a trichotomy together with Gualosuchus reigi and Chanaresuchus bonapartei. Accordingly, "Chanaresuchus ischigualastensis" can be potentially more closely related to Gualosuchus reigi, or even Rhadinosuchus gracilis, than to Chanaresuchus bonapartei. In addition, after discussing previously claimed synapomorphies of Chanaresuchus, we could not find unambiguous support for the monophyly of the genus. As a result, we propose here the erection of the new genus Pseudochampsa for 'Chanaresuchus ischigualastensis', which results in the new combination Pseudochampsa ischigualastensis. The information provided here about the anatomy and taxonomy of Pseudochampsa ischiguaslastensis will be useful for future quantitative analyses focused on the biogeography and macroevolutionary history of proterochampsids

CT-scan slices and 3D reconstruction of the skull of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in (A) dorsal view, (B) slice at level of the palate, (C) ventral, (D) dorsolateral, (E) and anterodorsal view, (F) slice at level of the dentary alveoli, and (G) occipital view.

<p>Abbreviations: anfe, antorbital fenestra; bo, basioccipital; cho, choanae; ctg, centre of growth; da, dentary alveoli; dt, dentary; en, external naris; iptv, interpterygoid vacuity; itf, infratemporal fenestra; fm, foramen magnum; mx, maxilla; nf, narial fossa; or, orbit; pbs, parabasisphenoid; pmx, premaxilla; plt, palate teeth; ri, ridge; stf, supratemporal fenestra, stfo, supratemporal fossa. Scale bar equals 5 cm.</p

Cervical and dorsal vertebrae, and scapulacoracoids of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in ventral view.

<p>Abbreviations: ax, axis; cr, cervical ribs; dlc, deltopectoral crest; dr, dorsal ribs; hh, humeral head; k, ventral keel; lcl, left clavicle; lhu, left humerus; lsc, left scapulacoracoid; rcl, right clavicle; rhu, right humerus; rsc, right scapulacoracoid. Scale bar equals 5 cm.</p

Caudal vertebrae of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in (A, B) ventral/left lateral, and (C, D) dorsal/right lateral views.

<p>Close-up of two middle caudal vertebrae in dorsal view in (C). Abbreviations: bne, base of neural spine; ce, centrum; ch, chevron; hc, hemal canal; ne, neural spine; tpr, transverse process. Scale bar equals 5 cm and 1 cm in close-up of (C).</p

Left hindlimb of <i>Chanaresuchus ischigualastensis</i> (PVSJ 567) in anterior view.

<p>Abbreviations: as-ca, astragalus-calcaneum; lfi, left fibula; lfe, left femur; lg, lateral groove; lti, left tibia; mttII–V, metatarsals II–V. Scale bar equals 5 cm.</p

Right hindlimb of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in anterior view.

<p>Abbreviations: as-ca, astragalus-calcaneum; ft, fourth trochanter; mttII–V, metatarsals II–V; rfi, right fibula; rfe, right femur; ril, right ilium; rti, right tibia; un, ungueal. Scale bar equals 5 cm.</p

Phylogenetic relationships of <i>Pseudochampsa ischigualastensis</i> and other proterochampsians among basal archosauriforms.

<p>Numbers above nodes are Bremer support, absolute and GC bootstrap frequencies, respectively.</p

Pelvic girdle and hindlimbs of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in dorsal view.

<p>Abbreviations: as-ca, astragalus-calcaneum; ce, centrum; mttII–V, metatarsals II–V; lfe, left femur; lfi, left fibula; lti, left tibia; ne, neural spine; os, osteoderm; ppil, postacetabular process of ilium; pr, prezygapophysis; rfe, right femur; rfi, right fibula; rpu, right pubis; rti, right tibia; tp, transverse process. Scale bar equals 5 cm.</p

Posterior dorsal vertebrae of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in dorsolateral view.

<p>Abbreviations: ce, centrum; ne, neural spine; os, osteoderm; po, postzygapophysis; pr, prezygapophysis; tpr, transverse process. Scale bar equals 3 cm.</p

Line drawings showing sutures in the skull of <i>Pseudochampsa ischigualastensis</i> (PVSJ 567) in (A) left lateral, (B) dorsal, and (C) ventral views.

<p>Abbreviations: bo, basioccipital; bt, basal tubera; dt, dentary; ecpt, ectopterygoid; fr, frontal; ju, jugal; la, lacrimal; lja, lower jaw; mx, maxilla; na, nasal; op, opisthotic; pa, parietal; pbs, parabasisphenoid; pmx, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; qj, quadratojugal; q, quadrate; san, surangular; sof, suborbital fenestra; sq, squamosal; spl, splenial; t, teeth; T2, palate tooth row 2; T3, palate tooth row 3; v?, vomer?. Scale bar equals 5 cm.</p