Systematic palaeontology

Diapsida Osborn, 1903 sensu Laurin, 1991

Sauria McCartney, 1802 sensu Gauthier et al. 1988

Archosauromorpha Huene, 1946 sensu Gauthier et al. 1988

Teyujagua paradoxa gen. et sp. nov.

Etymology

Genus named after Teyú Yaguá, one of the seven legendary beasts in the mythology of the Guarani ethnic group, who occupied a large territory of central east South America, including the type locality of the new species. Teyú Yaguá, literally meaning ‘fierce lizard’, is commonly represented as a dog-headed lizard. Species name derived from paradoxa, Greek meaning ‘paradoxical’, ‘unexpected’, owing to its unusual combination of plesiomorphic and derived characters.

Holotype

UNIPAMPA 653, an almost complete, well-preserved skull with associated cervical vertebrae.

Type locality and age

Exposure of the Sanga do Cabral Formation13, Paraná Basin, São Francisco de Assis municipality, Rio Grande do Sul State, Southern Brazil (29°36′56″S, 55°03′10″W) (Fig. 1). An Induan to early Olenekian age (Lower Triassic) is inferred for the Sanga do Cabral Formation based on the presence of the parareptile Procolophon trigoniceps and comparisons with the coeval Lystrosaurus Assemblage Zone of the South African Karoo13,14,15. Teyujagua was found in close association with archosauromorph vertebrae, cranial and postcranial remains of P. trigoniceps, temnospondyl amphibians and as-yet-unidentified tetrapod bones.

Figure 1 Type locality of Teyujagua paradoxa. (A), geographic map with the location of the Paraná Basin within Brazil and Teyujagua type location; (B), simplified stratigraphic profile of the outcrop, showing the level where Teyujagua was found. Area map was modified from Strapasson et al.30; stratigraphic profile modified from Da Rosa, et al.13. Full size image

Diagnosis

Archosauromorph with the following unique character combination: confluent, dorsally positioned external nares; maxilla participating in orbital margin; antorbital fenestra absent; trapezoidal infratemporal fenestra with incomplete lower temporal bar; teeth serrated on distal margins; surangular bearing a lateral shelf; external mandibular fenestrae present and positioned beneath the orbits when the lower jaw is in occlusion (autapomorphic for Teyujagua).

Description

The 115 mm long skull is well preserved and almost complete and is associated with four cervical vertebrae (Figs 2 and 3; Supplementary Fig. 1). The occipital and palatal regions and parts of the left side of the skull are still covered by the enclosing matrix, but were partially examined using computed tomography (CT) (Supplementary Fig. 2).

Figure 2 Teyujagua paradoxa holotype (UNIPAMPA 653). Photographs and interpretative drawings in right lateral (A,B) and dorsal (C,D) views. Abbreviations: an, angular; dt, dentary; emf, external mandibular fenestra; fr, frontal; ju, jugal; la, lacrimal; mx, maxilla; na, nasal; pa, parietal; pmx, premaxilla; po, postorbital; pofr, postfrontal; prf, prefrontal; q, quadrate; q j, quadratojugal; rap, retroarticular process; sa, surangular; sq, squamosal; st, supratemporal. Artwork by J. Anderson. Full size image

Figure 3 Close ups of the skull of Teyujagua paradoxa (UNIPAMPA 653). (A), posterior left maxillary teeth, showing serrations; (B), posterior process of the left jugal; (C), rostrum. Full size image

The snout is relatively broad and flattened, with dorsally positioned, confluent external nares. Dorsal confluent nares are an unusual condition that is often linked to aquatic or semi-aquatic habits, being present in many crocodyliforms, although they also occur in the terrestrial rhynchosaurs16, which are near relatives of early archosauriforms. The nasals contribute substantially to the skull table, followed by short and broad frontals and parietals that bear a small pineal foramen. Although the loss of the pineal foramen has been identified as a synapomorphy of archosauriforms8, this structure is variably absent or present in the early archosauriform Proterosuchus fergusi12 and the close archosauriform relative Prolacerta broomi17. The prefrontals and sculptured postfrontals almost exclude the frontals from the dorsal orbital margin, whereas the maxilla participates in the anteroventral orbital margin. The orbits face anterolaterally and were probably capable of at least limited binocular vision. The slender supratemporals are visible in dorsal view.

The premaxillae have well-developed, slender posterodorsal processes but lack anterodorsal processes, as a result of the confluent nares. An antorbital fossa/fenestra is absent from the maxillae (Fig. 3C). The jugals are triradiate, with main bodies ornamented with longitudinal ridges. The posterior jugal process tapers distally and does not reach the quadratojugal (Fig. 3B). The trapezoidal infratemporal openings were ventrally bordered by incomplete lower temporal bars, similar to the condition in non-archosauriform archosauromorphs such as Prolacerta, Protorosaurus and Mesosuchus16,17. By contrast, the lower temporal bar is complete in nearly all archosauriforms, although this character is variable in Proterosuchus fergusi12.

On the lower jaw, the surangulars bear lateral shelves that match closely with the ventral margins of the posterior processes of the jugals. The external mandibular fenestra is present, unusually anteriorly positioned and ventrally bordered by a slender ascending process of the angular. The posterior contacts of the dentaries with the post-dentary bones cannot be identified. However, CT scans reveal that the dentary tooth row ends slightly anterior to the maxillary one (Supplementary Fig. 2).

Each premaxilla possesses four teeth and the maxilla had a maximum of 15. The teeth bear serrations on their distal margins only, as in proterosuchid archosauriforms, but differing from the condition in more derived archosauriforms in which serrations are usually present on both mesial and distal margins18 (Fig. 3A). Pronounced heterodonty is evident, with small premaxillary teeth followed by considerably larger anterior maxillary teeth. The teeth are labiolingually compressed, held in well-defined sockets and not firmly associated with surrounding alveolar bone. Implementation therefore appears to be thecodont, rather than ankylothecodont as in many of the earliest archosaurifoms19.

Phylogeny

Our novel cladistic analysis recovered two most parsimonious trees with 872 steps (Supplementary Fig. 3). The strict consensus of these topologies (Fig. 4) positions Teyujagua as the sister taxon of Archosauriformes, a position previously occupied by the Lower Triassic Prolacerta8,11,17. The clade Teyujagua plus Archosauriformes is supported by five synapomorphies: (i) Serrations on tooth crowns; (ii) trapezoidal shape of the infratemporal fenestrae; (iii) frontal-parietal suture at right angle to parasagittal plane; (iv) mandible bearing an external fenestra; (v) lateral shelf on surangular.

Figure 4 Archosauromorph phylogeny showing the recovered position of Teyujagua. Top: strict consensus tree summarising phylogenetic results. Bottom: sequence of acquisition of archosauriform features among the archosauromorphs Prolacerta, Teyujagua and the basal archosauriform Proterosuchus. (i) serrated teeth; (ii) external mandibular fenestra; (iii) closed lower temporal bar; (iv) antorbital fenestra. Prolacerta and Proterosuchus skulls redrawn from an artwork by M. Ezcurra. Not to scale. Full size image

Archosauriformes includes the traditional basal groups, such as Proterosuchidae and Erythrosuchidae, together with the crown group Archosauria. Proterosuchidae consists of Proterosuchus, Archosaurus and Sarmatosuchus, although the relationships within this clade are unresolved. Fugusuchus, Koilamasuchus and the clade Erythrosuchus + Archosauria also have unresolved positions relative to one another. The clade Chanaresuchus + (Doswellia + Vancleavea) is recovered as the sister group of Archosauria and Euparkeriidae is the sister group of this less inclusive clade. As such, four recognised groups compose non-archosaurian Archosauriformes: Proterosuchidae, Erythrosuchidae, Euparkeriidae and the clade including Chanaresuchus + (Doswellia + Vancleavea).

Another analysis was performed including the poorly known Eorasaurus, which may be the oldest known archosauriform. The analysis recovered 14 most parsimonious trees with 873 steps. Most of the recovered topologies are similar to those recovered in the first phylogenetic analysis. The consensus tree differs in positioning Eorasaurus in an unresolved polytomy together with Koilamasuchus, Fugusuchus, erythrosuchid taxa and a clade composed of Euparkeriidae + Proterochampsia + Archosauria (Supplementary Fig. 5). This provides additional support for the archosauriform affinities of Eorasaurus and the existence of archosauriform ghost lineages extending into at least the middle Wuchiapingian11.