Tralkasaurus

''Tralkasaurus'' (meaning "thunder lizard") is a genus of abelisaurid dinosaur from the Huincul Formation from Río Negro Province in Argentina. The type and only species is ''Tralkasaurus cuyi'', named in 2020 by Mauricio Cerroni and colleagues based on an incomplete skeleton. A medium-sized abelisaurid, ''Tralkasaurus'' exhibits a conflicting blend of characteristics found among the early-diverging abelisauroids with others that characterize the highly specialized clade Brachyrostra, and thus its position within the clade is poorly-resolved.

Discovery and naming

The Violante Farm fossil site in Río Negro Province, Argentina, near the village of El Cuy, was discovered in 1999 by Sebastián Apesteguía on the north shore of the Ezequiel Ramos Mexía Reservoir. In a 2018 conference abstract, Matías Motta and colleagues reported new fossil discoveries, including a diverse fauna of dinosaurs, made at the Violante Farm site. The strata exposed are yellowish and greenish sandstones that are occasionally tuffaceous (i.e. bearing volcanic ash), which belong to the Cenomanian-Turonian Huincul Formation.

The specimens discovered included an incomplete skeleton of an abelisaurid, catalogued as MPCA-Pv 815, which comprises a ; , , and vertebrae; ; and a badly-preserved . Motta and colleagues recognized that this abelisaurid, along with a coelurosaur and carcharodontosaurid, present distinct traits (autapomorphies) from other Huincul Formation theropods. The abelisaurid was formally named in 2020 by Mauricio Cerroni and colleagues as ''Tralkasaurus cuyi''; the generic name is formed from "tralka", the Mapudungun word for "thunder", and the suffix ''-saurus'', meaning lizard, while the specific name refers to the village of El Cuy.

Description

''Tralkasaurus'' was described by Cerroni and colleagues as a "medium-sized abelisaurid" based on the available material. Its maxilla has a length of , smaller than that of the similarly "medium-sized" ''Skorpiovenator'' (at , with the whole animal measuring long). It is most comparable to the smallest-known abelisaurid, MMCh-PV 69, from the Candeleros Formation, with ''Tralkasaurus'' pubis measuring long and MMCh-PV 69's pubis measuring long.

The maxilla of ''Tralkasaurus'' is a subtriangular bone covered in neurovascular (pits formed by innervation) and rugosities (roughened areas). It encloses the from the front; ''Tralkasaurus'' is unique among abelisaurids in that front margin of the antorbital fenestra slopes forwards and upwards, instead of being vertical or sloping backwards. The antorbital fenestra creates on both the upward-projecting process of the maxilla and (atypically among abelisaurids) the inner surface of the bone; this condition is more similar to basal abelisauroids like ''Noasaurus'' and ''Masiakasaurus'', as well as the Averostra. Deep neurovascular grooves extend downward from the bottom rim of the antorbital fenestra, which is unlike ''Carnotaurus'', ''Ekrixinatosaurus'', ''Majungasaurus'', and ''Skorpiovenator'' but similar to ''Rugops''. More typically, the ascending process tapers and becomes laminar (sheet-like) at the top, like ''Skorpiovenator'', ''Carnotaurus'', and ''Majungasaurus''. The maxilla also bears eleven sockets with four preserved teeth, which are typical of abelisaurids. Like ''Majungasaurus'' but unlike ''Skorpiovenator'', the on both edges of the teeth are the same size.

Based on comparisons with ''Carnotaurus'' and ''Majungasaurus'', the dorsal vertebrae of ''Tralkasaurus'' originated from the mid-to-rear back. The that extend out and upward are subtriangular like those of ''Dahalokely'', '' Viavenator'', ''Majungasaurus'', and ''Masiakasaurus'', but unlike the subrectangular processes of ''Carnotaurus''. the that project out and downward are distinctive among abelisaurids in their unusual narrowness and shortness; even in abelisaurids like ''Majungasaurus'' and MCF-PVPH-237, where the parapophyses are relatively narrow, they are not as rod-like as in ''Tralkasaurus''. A ridge between them, the , is also shallow and relatively low, unlike the strongly-developed ridges in ''Carnotaurus'', ''Majungasaurus'', and ''Ilokelesia''. Another ridge, the posterior paradiapophyseal lamina, is robust and cross-cuts a fossa below the transverse process (the infradiapophyseal fossa), like ''Dahalokely''.

The large transverse processes of the caudal vertebrae, which originated from the front of the tail, are typically abelisaurid, projecting strongly out and upward like ''Carnotaurus'', ''Skorpiovenator'', ''Aucasaurus'', and ''Viavenator'', but are more inclined than ''Majungasaurus''. Processes projecting forwards and backwards at their tips would have been overlapping between caudal vertebrae; this is a distinguishing characteristic of the Brachyrostra. Meanwhile, although the pubis is poorly preserved, the at its tip appears to have been expanded, like '' Rahiolisaurus'' but unlike ''Masiakasaurus'' or ''Carnotaurus''.

Classification

Cerroni and colleagues conducted a phylogenetic analysis to determine the affinities of ''Tralkasaurus''. They found that it possesses synapomorphies of the Abelisauridae: a maxilla with a deep body, low ascending process, and reduced maxillary fossa, covered by foramina and rugosities; fused lining the inside of the maxillary tooth row bearing strong vertical ridges; the subdivision of the infradiapophyseal fossae by the posterior paradiapophyseal laminae; a connection between the transverse processes and parapophyses by the dorsal paradiapophyseal laminae; large transverse processes strongly inclined upwards on the caudal vertebrae; and a thin pubic shaft. They also identified the forward-inclined front margin of the antorbital fenestra and its excavation of the body of the maxilla, the rod-like parapophyses, and the low paradiapophyseal laminae as autapomorphies of ''Tralkasaurus.

Within the Abelisauridae, the position of ''Tralkasaurus'' was more poorly resolved. In particular, it exhibits a conflicting blend of characteristics: the projections on the caudal transverse processes are typical of the Brachyrostra, but the presence of extensive antorbital fossae on the maxilla is plesiomorphic (i.e. characteristic of basal abelisauroids, and unlike other abelisaurids). The phylogenetic analysis accordingly placed ''Tralkasaurus'' among basal abelisaurids, but was unable to resolve its affinities beyond a polytomy (collapsed tree). The resulting phylogenetic tree is partially reproduced below.

Palaeoecology

Numerous theropods are known from the Violante Farm fossil site, including ''Gualicho'' and ''Aoniraptor'' (which may or may not represent the same megaraptoran); the carcharodontosaurid ''Taurovenator''; a coelurosaur (identified by Cerroni and colleagues as an unenlagiine) and another carcharodontosaurid that remain unnamed, but are likely distinct species; and another indeterminate megaraptoran. Other dinosaurs also include a titanosaurian sauropod and an ornithopod. Additional vertebrates from the Violante Farm site include the eilenodontid rhynchocephalian ''Patagosphenos''; a crocodyliform, possibly belonging to the Neosuchia; a squamate; a chelid turtle; and a fish referred to ''Lepidotes''.

See also

* Timeline of ceratosaur research

References

{{Taxonbar, from=Q77035223

Abelisaurs
Cenomanian life
Turonian
Late Cretaceous dinosaurs of South America
Cretaceous Argentina
Huincul Formation
Fossil taxa described in 2020