Tainrakuasuchus

Tainrakuasuchus is an extinct genus of poposauroid pseudosuchians known from the Middle Triassic Dinodontosaurus Assemblage Zone, part of the Santa Maria Supersequence of Brazil. The genus contains a single species, Tainrakuasuchus bellator, known from part of the lower jaw, five vertebrae, and a partial pelvic girdle.

Tainrakuasuchus Temporal range: Middle Triassic, Ladinian PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	†Poposauroidea
Genus:	†Tainrakuasuchus Müller et al., 2025
Species:	†T. bellator
Binomial name
†Tainrakuasuchus bellator Müller et al., 2025

History and naming

Tainrakuasuchus was described based on fossil material discovered at the Posto site of the Ladinian Pinheiros-Chiniquá Sequence, Santa Maria Supersequence, in Rio Grande do Sul, Brazil. The holotype and only known specimen, CAPPA/UFSM 0506, consists of a partial lower jaw, five vertebrae and a partial hip.^[1]

The first part of the name Tainrakuasuchus derives from a combination of the Guarani words "tain" and "rakua", meaning "tooth" and "pointed" respectively, meant to invoke the pointed teeth of the lower jaw. The second part of the name derives from the Greek "suchus" meaning "crocodile". The species name "bellator" is of Latin origin and translates to "warrior" or "fighter". Müller and colleagues explain the species name as a tribute to the people of Rio Grande do Sul, citing their "historical resilience and enduring spirit" and highlighting the at the time recent struggles with floodings in the state.^[1]

Description

The dentary bone of Tainrakuasuchus is described as slender, with a height to length ratio of 0.1, which resembles the closely related Mandasuchus from Africa while setting it appart from the tall dentaries seen in the ornithosuchid Dynamosuchus. The lateral face of the dentary prominently features a groove that begins about two thirds down the bones length and runs just below the toothrow until the elevated posterodorsal process. Similar structures have been identified in other early poposauroids, namely Mambawakale, while a series of foramina occupies the same position in Mandasuchus. Towards the back of the element the dentary forms a posterodorsal process that rises above the toothrow and is split into a distinct upper and lower surface by a notch. The dentary is the only bone to participate in forming the mandibular symphysis, the part of the mandible where the two halves are sutured to each other, excluding the splenial as in most other poposauroids. The symphysis itself is short and only occupies the front-most part of the dentary, which contrasts with Mambawakale and ornithosuchids which have a more substantially elongated symphysis.^[1]

In contrast to the lateral surface of the dentary, the medial face of the splenial is generally described as plain, lacking foramina or grooves. The splenial begins anteriorly as a slender bone, but grows taller further back in the lower jaw. Like the dentary the splenial forms a posterodorsal and ventrolateral process, the latter of which extending further back than the former. Here the splenial does bear a ridge and accompanying groove.^[1]

While only four actual teeth are preserved in the holotype specimen of Tainrakuasuchus, the dentary bone bears 17 dental alveoli, a count that is comparable to Mambawakale. Some more derived poposauroids bear fewer teeth or lost them altogether in the case of shuvosaurids. The preserved teeth show a blade-like morphology, being lateromedially compressed and recurved. While the teeth bear some features indicative of ziphodonty, the actual presence of serrations that defines this tooth morphology cannot be determined due to the holotypes poor preservation.^[1]

Vertebrae

The anteriormost known neck vertebra of Tainrakuasuchus is amphicoelous, paralellogram-shaped and notably longer than the mid-dorsal vertebrae, with the latter being a feature shared by other poposauroids and avemetatarsalians. In contrast, some other pseudosuchians like the contemporary Prestosuchus have neck vertebrae shorter than the mid-dorsals. The neural spine is also elongated, stretching almost the entire anteroposterior length of the neural arch. The neural spine is roughly fan-shaped, expanding both in width and lenght along its height. A similar neural spine shape can be found across poposauroids, having been observed in both basal forms like Mandasuchus and Schultzsuchus as well as members of Ctenosauriscidae. Tainrakuasuchus resembles Schultzsuchus in preserving both epipohyses and postzygopophyses, with the former being a rarity among pseudosuchians. While this sets both of them apart from the larger Prestosuchus, the two differ in the details of this morphology. In Schultzsuchus the two structures are separated by a prominent gap, while in Tainrakuasuchus the two are only weakly separated.^[1]

The known mid-cervical vertebra is also elongated relative to dorsal vertebrae, though not as long as its predecessors. The centrum of this vertebrae further differs as it is not paralellogram- but spool-shaped, bearing a strong constriction at the midpoint. The neural canal is rectangular on the front face of the vertebrae with narrowing walls, resulting in the posterior opening being triangular in shape. Notably, the canal bears a notch in its dorsolateral corner, situated between the walls of the arch and the medial ridge, which gives the dorsal margin of the canal the appearance of an M when seen from the front. The neural spine that sits atop the arch is shorter in anteroposterior length than that of the predecessor, but retains the fan-shaped morphology caused by the lengthening and thickening with height. Looking at the vertebra from above shows that the neural spine is thicker towards the front and narrower towards the back. The postzygapophyses of this neck vertebra are notably connected to each other by a intrapostzygapophyseal lamina at their base, which helps distinguish it from Schultzsuchus in which such a lamina is missing. Like in the previous vertebra the epipophyses remains only poorly separated from the postzygapophyses.^[1]

Classification

In their 2025 phylogenetic analysis, Müller and colleagues recovered Tainrakuasuchus as a member of the paracrocodylomorph clade Poposauroidea, as the sister taxon to Mandasuchus, known from rocks of approximately the same age in Tanzania. These results are displayed in the cladogram below:^[1]

Paracrocodylomorpha	Loricata Poposauroidea Mambawakale ruhuhu Benggwigwishingasuchus eremicarminis Mandasuchus tanyauchen Tainrakuasuchus bellator Schultzsuchus loricatus Xilousuchus sapingensis Arizonasaurus babbitti Qianosuchus mixtus Poposaurus gracilis Lotosaurus adentus Sillosuchus longicervix Shuvosaurus inexpectatus Effigia okeeffeae

Paleobiology

Prestosuchus and Parvosuchus, the largest and smallest pseudosuchians fo the Dinodontosaurus AZ

The sediments of the Dinodontosaurus Assemblage Zone are known to preserve a both taxonomically and ecologically diverse pseudosuchian fauna with between 6 and 8 recognized species. The smallest among them was Parvosuchus, which with a length of only around 1 m (3 ft 3 in) was likely restricted to small prey items. On the opposite side of the spectrum sits Prestosuchus, likely the apex predator of the fauna and the largest of the local pseudosuchians. With a range between 2–4 m (6 ft 7 in – 13 ft 1 in) erpetosuchids like Archeopelta and Pagosvenator and poposauroids like Schultzsuchus and Tainrakuasuchus represent medium-sized predators. While falling into a similar size range, early poposauroids like Tainrakuasuchus would have likely been distinct in their ecology from erpetosuchids due to differences in their skulls and dentition. Basal poposauroids possess teeth throughout the maxilla, whereas in erpetosuchids the teeth of the upper jaw are concentrated closer to the tip, suggesting that these animals captured prey in different manners or fed on different prey altogether.^[1]

While the holotype of Tainrakuasuchus suggests a moderate size of up to 3 m (9.8 ft), isolated remains form the Posto locality could indicate the presence of even larger individuals. However, even at a greater size Tainrakuasuchus would have been ecologically distinct from the other large predator of the fauna, as poposauroid neck vertebrae are longer and shallower and therefore not built to deal with the same stresses as those likely dealt with by "rauisuchians" such as Prestosuchus.^[1]