| Brachyrostrans | |
|---|---|
 | |
| Mounted cast of aCarnotaurus sastrei skeleton, Chlupáč Museum,Prague | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Abelisauridae |
| Clade: | †Brachyrostra Canaleet al., 2008 |
| Subgroups | |
Brachyrostra (meaning "short snouts") is a clade within thetheropoddinosaur familyAbelisauridae. It includes the famous generaCarnotaurus,Aucasaurus, potentiallyAbelisaurus as well as their close relatives from theCretaceous Period ofArgentina andBrazil plusCaletodraco fromFrance.[1] The group was first proposed in an analysis conducted by Juan Canale and colleagues in 2008. They found that all South American abelisaurids described up to that point grouped together as a sub-clade of Abelisauridae, which they named based on the relatively unusual shape of their skulls (in comparison with other theropods). They defined theclade Brachyrostra as "all the abelisaurids more closely related toCarnotaurus sastrei than toMajungasaurus crenatissimus."[2]
Brachyrostrans were relatively lightly built compared to other large theropods, ranging in size from 6.1–7.8 m (20–26 ft)[3] and 1400–2000 kg (1.6–2.3 short tons) in weight.[3][4] They are considered the most derived abelisaurids, with traits like very short, narrow skulls and extremely reduced forearms, even more so than other abelisaurids.[5][6] Many brachyrostrans had horns or rugosities on thefrontal andnasal bones, which have been interpreted as bearing cornified structures or dermal armor.[7]
Studies of the skull anatomy of the most well-known species,Carnotaurus sastrei, lead to debate over what type of prey these animals hunted. Studies by Mazzettaet al. in 1998, 2004, and 2009 suggest that the jaw structure inCarnotaurus was built for swift, rather than strong, bites, with adaptations for mandibularkinesis to assist in swallowing small prey items whole.[8] Surprisingly, it exhibits a form of paracraniokinesis in which thedentary bone articulates against thesurangular bone, further jointing the lower jaw and hypothetically allowing this animal a wider array of hunting strategies.[9]
However, in 1998 and 2009,Robert Bakker and Francois Therrien and colleagues contested this finding, stating thatCarnotaurus had the exact same skull adaptations (short snout, small teeth, and a fortifiedocciput) as theJurassic theropodAllosaurus, which presumably preyed upon large animals by gradual jaw slashing.[10]
Mazzettaet al. 1998–1999 andPhil Currieet al. 2011 foundCarnotaurus to be a swift-running predator with semicursorial adaptations such as femoral resistance againstbending moments[11] and a hypertrophiedcaudofemoralis muscle, the primary locomotory muscle in theropods which was located in the tail and pulled the femur backwards.[12] This enlarged caudofemoralis, giving them a speed estimate of 48–56 km/h (30–35 mph), allowed brachyrostrans to be one of the fastest-running large theropod groups yet known.[13][12]
Within Brachyrostra, there is a slightly more restrictive clade, calledFurileusauria ("stiff back lizards").[14] They represent some of the larger brachyrostrans, with an average length of 7.1 ± 2.1 m (23.3 ± 6.9 ft).[15] The taxon is a stem-based clade and is defined as the most inclusive clade containingCarnotaurus sastrei but notIlokelesia aguadagrandensis,Skorpiovenator bustingorryi, orMajungasaurus crenatissimus.[14]
Synapomorphies of Furileusauria recovered by Filippi and colleagues include: the presence of a tip in the middle area of the posterior surface of the ventral process of thepostorbital, the presence of a knob followed by a deep notch in the postorbital-squamosal contact, the absence offenestra between thefrontal andlacrimal bones, an anterior projection of thedistal end of the cervicalepiphophyses, a posterior margin of thepostzygapophyses which is level with the intervertebral articulation indorsal vertebrae, a crescent-shaped morphology of the distal tip of the transverse processes in anterior and middle caudal vertebrae, transverse processes of anteriorcaudal vertebrae that is distally expanded and projected anteriorly, a convex external margin of the transverse processes in anterior caudal vertebrae, and a downturned process on thecnemial crest of thetibia.[14]
A more restrictive clade within Furileusauria is thetribeCarnotaurini. This group is a node-based clade and was first proposed by paleontologistsRodolfo Coria,Luis Chiappe, and Lowell Dingus in 2002, being defined as a clade containing "Carnotaurus sastrei,Aucasaurus garridoi, their most recent common ancestor, and all of its descendants." The tribe Carnotaurini was named in 2002 byRodolfo Coriaet al. in 2002 after their discovery ofAucasaurus garridoi.[6] Their morphological definition of it is by severalsynapomorphies of the clade, with two ambiguous ones: "the presence of hyposphene–hypantrum articulations in the proximal and middle sections of the caudal series, and cranial processes in the epipophyses of the cervical vertebrae." They defined more ambiguous synapomorphies due to the homologous materials not yet found in all other abelisaurids being: "a very broad coracoid (coracoid maximum width three times the distance across the scapular glenoid area), a humerus with a large and hemispherical head, an extremely short ulna and radius (ulna to humerus ratio 1:3 or less), and frontal prominences (swells or horns) that are located laterally on the skull roof."[6]
In their description of theabelisauridLlukalkan, Federico Gianechini and colleagues performed a phylogenetic analysis to test the affinities of the new taxon. The simplified strict consensus tree of the analysis is shown below.[16] Similar results have been recovered by other analyses including Coria and colleagues (2002),[6] Canale and colleagues (2008),[2] and Cerroni and colleagues (2020).[17]
Brachyrostrans were initially known exclusively from South America. Members of the group have been unearthed from theAnacleto Formation, theBajo de la Carpa Formation, theCandeleros Formation, theHuincul Formation, and possibly the Sir Fernandez field of theAllen Formation to the southeast.[18][5][19] A single named taxon,Pycnonemosaurus is also known from the Cachoeira do Bom Jardim Formation inMato Grosso, Brazil.[20] The description of the French taxonCaletodraco, was the first definitive evidence of the clade from outside South America. The Albian taxonGenusaurus, which also hails from France, may also represent a European member of this clade. If it is truly a brachyrostran, it would represent the oldest member of the clade.[21] The enigmatic taxonDahalokely, fromMadagascar may also belong to Brachyrostra, although this remains uncertain.
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