| Tralkasaurus | |
|---|---|
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Abelisauridae |
| Genus: | †Tralkasaurus Cerroniet al.,2020 |
| Type species | |
| †Tralkasaurus cuyi Cerroniet al., 2020 | |
Tralkasaurus (meaning "thunder lizard") is agenus ofabelisauriddinosaur from theHuincul Formation fromRío Negro Province inArgentina. Thetype and only species isTralkasaurus 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-divergingabelisauroids with others that characterize the highly specialized cladeBrachyrostra, and thus its position within the clade is poorly-resolved.[1]
The Violante Farm fossil site inRío Negro Province,Argentina, near the village ofEl Cuy, was discovered in 1999 by Sebastián Apesteguía on the north shore of theEzequiel Ramos Mexía Reservoir.[2] 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. Thestrata exposed are yellowish and greenishsandstones that are occasionallytuffaceous (i.e. bearing volcanic ash), which belong to theCenomanian-TuronianHuincul Formation.[1][3]
The specimens discovered included an incomplete skeleton of anabelisaurid, catalogued asMPCA-Pv 815, which comprises amaxilla;dorsal,sacral, andcaudal vertebrae;cervical ribs; and a badly-preservedpubis.[1] Motta and colleagues recognized that this abelisaurid, along with acoelurosaur andcarcharodontosaurid, present distinct traits (autapomorphies) from other Huincul Formationtheropods.[3] The abelisaurid was formally named in 2020 by Mauricio Cerroni and colleagues asTralkasaurus cuyi; thegeneric name is formed from "tralka", theMapudungun word for "thunder", and the suffix-saurus, meaning lizard, while thespecific name refers to the village of El Cuy.[1]
Tralkasaurus was described by Cerroni and colleagues as a "medium-sized abelisaurid" based on the available material. Its maxilla has a length of 22 centimetres (8.7 in), smaller than that of the similarly "medium-sized"Skorpiovenator (at 32 centimetres (13 in), with the whole animal measuring 6 metres (20 ft) long).[4] It is most comparable to the smallest-known abelisaurid, MMCh-PV 69, from theCandeleros Formation, withTralkasaurus' pubis measuring 35 centimetres (14 in) long and MMCh-PV 69's pubis measuring 41 centimetres (16 in) long.[1][5]
The maxilla ofTralkasaurus is a subtriangular bone covered in neurovascularforamina (pits formed by innervation) and rugosities (roughened areas). It encloses theantorbital fenestra 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 createsfossae on both the upward-projecting process of the maxilla and (atypically among abelisaurids) the inner surface of the bone; this condition is more similar tobasalabelisauroids likeNoasaurus andMasiakasaurus, as well as theAverostra. Deep neurovascular grooves extend downward from the bottom rim of the antorbital fenestra, which is unlikeCarnotaurus,Ekrixinatosaurus,Majungasaurus, andSkorpiovenator but similar toRugops. More typically, the ascending process tapers and becomes laminar (sheet-like) at the top, likeSkorpiovenator,Carnotaurus, andMajungasaurus. The maxilla also bears eleven sockets with four preserved teeth, which are typical of abelisaurids. LikeMajungasaurus but unlikeSkorpiovenator, theserrations on both edges of the teeth are the same size.[1]
Based on comparisons withCarnotaurus andMajungasaurus, the dorsal vertebrae ofTralkasaurus originated from the mid-to-rear back. Thetransverse processes that extend out and upward are subtriangular like those ofDahalokely,Viavenator,Majungasaurus, andMasiakasaurus, but unlike the subrectangular processes ofCarnotaurus. theparapophyses that project out and downward are distinctive among abelisaurids in their unusual narrowness and shortness; even in abelisaurids likeMajungasaurus and MCF-PVPH-237, where the parapophyses are relatively narrow, they are not as rod-like as inTralkasaurus. A ridge between them, thedorsal paradiapophyseal lamina, is also shallow and relatively low, unlike the strongly-developed ridges inCarnotaurus,Majungasaurus, andIlokelesia. Another ridge, the posterior paradiapophyseal lamina, is robust and cross-cuts a fossa below the transverse process (the infradiapophyseal fossa), likeDahalokely.[1]
The large transverse processes of the caudal vertebrae, which originated from the front of the tail, are typically abelisaurid, projecting strongly out and upward likeCarnotaurus,Skorpiovenator,Aucasaurus, andViavenator, but are more inclined thanMajungasaurus. Processes projecting forwards and backwards at their tips would have been overlapping between caudal vertebrae; this is a distinguishing characteristic of theBrachyrostra. Meanwhile, although the pubis is poorly preserved, thepubic apron at its tip appears to have been expanded, likeRahiolisaurus but unlikeMasiakasaurus orCarnotaurus.[1]
Cerroni and colleagues conducted aphylogenetic analysis to determine the affinities ofTralkasaurus. They found that it possessessynapomorphies of the Abelisauridae: a maxilla with a deep body, low ascending process, and reduced maxillary fossa, covered by foramina and rugosities; fusedinterdental plates 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 ofTralkasaurus.[1]
Within the Abelisauridae, the position ofTralkasaurus 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 isplesiomorphic (i.e. characteristic of basal abelisauroids, and unlike other abelisaurids). The phylogenetic analysis accordingly placedTralkasaurus among basal abelisaurids, but was unable to resolve its affinities beyond apolytomy (collapsed tree). The resultingphylogenetic tree is partially reproduced below.[1]
Numerous theropods are known from the Violante Farm fossil site of theHuincul Formation, includingGualicho[2] andAoniraptor (which may or may not represent the samemegaraptoran), thecarcharodontosauridTaurovenator,[6] acoelurosaur (identified by Cerroni and colleagues as anunenlagiine) and another carcharodontosaurid that remain unnamed, but are likely distinct species, and another indeterminate megaraptoran.[1] Other dinosaurs also include atitanosauriansauropod and anornithopod. Additional vertebrates from the Violante Farm site include the eilenodontidrhynchocephalianPatagosphenos;[7] acrocodyliform, possibly belonging to theNeosuchia; asquamate; achelid turtle; and a fish referred toLepidotes.[3]
Dinosaurs named from other localities in the Huincul Formation include titanosaurs (Argentinosaurus,Choconsaurus, andChucarosaurus),[8] rebbachisaurids (Cathartesaura andLimaysaurus),[9] carcharodontosaurids (Mapusaurus andMeraxes),[10] abelisaurids (Skorpiovenator andIlokelesia), an elaphrosaurine (Huinculsaurus),[11] and a paravian (Overoraptor).[12]
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