A Brief Introduction to the Osteology of Viavenator exxoni

Viavenator exxoni, Museo Municipal Argentino Urquiza

The Abelisauridae is the best-known carnivorous dinosaur group from Gondwana. Their fossil remains have been recovered in Argentina, Brazil, Morocco, Niger, Libya, Madagascar, India, and France. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes and strongly reduced forelimbs and hands. The group was erected by Jose Bonaparte with the description of  Abelisaurus comahuensis, and includes: Carnotaurus sastrei, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Eoabelisaurus and Viavenator exxoni

The holotype of Viavenator exxoni (MAU-Pv-LI-530) was found in the outcrops of the Bajo de la Carpa Formation (Santonian, Upper Cretaceous), northwestern Patagonia, Argentina. Viavenator series of autapomorphies are: transversely compressed parietal depressions on both sides of the supraoccipital crest; ventral edges of the paraoccipital processes located above the level of the dorsal edge of the occipital condyle; basioccipital-opisthotic complex about two and a half times the width and almost twice the height of the occipital condyle, in posterior view; well-developed crest below the occipital condyle; deeply excavated and sub-circular basisphenoidal recess; basipterygoid processes horizontally placed with respect to the cranial roof and located slightly dorsally to the basal tubera; mid and posterior cervical centra with slightly convex lateral and ventral surfaces; presence of an interspinous accessory articular system in middle and posterior dorsal vertebrae; presence of a pair of pneumatic foramina within the prespinal fossa in anterior caudal vertebrae; distal end of the scapular blade posteriorly curved.

Figure 1. Rendering of the type braincase of Viavenator exxoni (MAU-Pv-LI-530) in dorsal (A,B), and right lateral (C,D) view. Adapted from Carabajal y Filippi, 2017.

Viavenator presents highly-derived postcranial characters, and a relatively plesiomorphic skull in comparison with Carnotaurus and Aucasaurus. Cranial elements of this specimen include the complete neurocranium: frontals, parietals, sphenethmoids, orbitosphenoids, laterosphenoids, prootics, opisthotics, supraoccipital, exoccipitals, basioccipital, parasphenoids and basisphenoids. The plesiomorphic traits of the skull of Viavenator are mainly related with the anatomy of frontals, wich lack osseous prominences such as domes or horns. The dorsal surface of the frontals exhibits an ornamentation that consists of pits and sinuous furrows and ridges, although it is not well-preserved. The  exoccipitals form the lateral and possibly the laterodorsal margins of the foramen magnum, as apparently occurs in Carnotaurus. 

Vertebrae of Viavenator exxoni. Scale bar: 5 cm. From Filippi et al., 2017),

The postcranial skeleton of Viavenator is represented by eight cervical vertebrae (the atlas; seven dorsal vertebrate, four of them articulated; twelve caudal vertebrae); ribs; gastralias; one chevron; scapulocoracoid; ischium foot; and fibulae. The atlas is similar to that of Carnotaurus, though less robust and anteroposteriorly shorter; and there  are not observed prezygapophyseal facets in the neurapophyses, so it is inferred that the proatlas was absent, as also occurs in Carnotaurus and Majungasaurus. The shape of the epipophyses of the cervical region, which are
characterized by anterior and posterior projections, is shared by Viavenator and Carnotaurus, but it is not present in pre-Santonian forms such as Ilokelesia and Skorpiovenator. The derived vertebral characters of Viavenator are linked with an increase in the structural rigidity of the vertebral column, and with an increase in the cursorial abilities of these abelisaurids. This combination of plesiomorphic and derived traits suggests that Viavenator is a transitional form.

 

References:

Filippi, L.S., Méndez, A.H., Gianechini, F.A., Juárez Valieri, Rubé.D., Garrido, A.C., Osteology of Viavenator exxoni (Abelisauridae; Furileusauria) from the Bajo de la Carpa Formation, NW Patagonia, Argentina, Cretaceous Research (2017), doi: 10.1016/j.cretres.2017.07.019.

Leonardo S. Filippi, Ariel H. Méndez, Rubén D. Juárez Valieri and Alberto C. Garrido (2016). «A new brachyrostran with hypertrophied axial structures reveals an unexpected radiation of latest Cretaceous abelisaurids». Cretaceous Research 61: 209-219. doi:10.1016/j.cretres.2015.12.018

Paulina-Carabajal, A., Filippi, L., Neuroanatomy of the abelisaurid theropod Viavenator: The most complete reconstruction of a cranial endocast and inner ear for a South American representative of the clade, Cretaceous Research (2017), doi: 10.1016/j.cretres.2017.06.013

 

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From Argentina with Love: Top Fossils of 2016

Geographic provenance and speculative reconstruction of the gigantic titanosaurian sauropod dinosaur Notocolossus gonzalezparejasi gen. et sp. nov. (From González Riga  et al., 2016; Credit: Scientific Reports)

Since the discovery of dinosaur remains in the Neuquen basin in 1882, Argentina has gained the title of Land of the Giants. And 2016 has brought us amazing fossil discoveries. From Notocolossus to Gualicho, my fossil pick for this year are:

  • Notocolossus

Notocolossus gonzalezparejasi gen. et sp. nov. from the Upper Cretaceous of Mendoza Province, Argentina is one of the largest known dinosaurs. The name derived from the Greek notos (southern) and the Latin colossus, in reference to the gigantic size and Gondwanan provenance of the new taxon. The species name honours Dr. Jorge González Parejas, who provided legal guidance on the research, protection, and preservation of dinosaur fossils from Mendoza Province. The holotype of Notocolossus (UNCUYO-LD 301) consists of a partial skeleton lacking the skull. It contains an anterior dorsal vertebra, an anterior caudal vertebra, the right humerus (with 1.76 m in length), and the proximal end of the left pubis. The pes of  Notocolossus is comparatively shorter and more mediolaterally symmetrical than those of other titanosaurs, and indeed, most other sauropods. Notocolossus also presents truncated unguals, characteristics otherwise unknown in the Sauropoda.

Cranium of Sarmientosaurus musacchioi in right lateral view. Scale bar = 10 cm. (From Martínez et al., 2016)

Cranium of Sarmientosaurus musacchioi in right lateral view. Scale bar = 10 cm. (From Martínez et al., 2016)

  • Sarmientosaurus

Another remarkable new species of titanosaurian sauropod was Sarmientosaurus musacchioi. The holotypic and only known specimen consists of an articulated, virtually complete skull and part of the cranial and middle cervical series. The new titanosaur comes from the Lower Member of the Upper Cretaceous Bajo Barreal Formation on the Estancia Laguna Palacios near the village of Buen Pasto in south-central Chubut Province, central Patagonia, Argentina. It is the most basal known titanosaur to be represented by a well-preserved skull. Furthermore, the cranial endocast preserves some of the most complete information about the brain and sensory system for any sauropod.

Scapulocoracoid of Viavenator exxoni gen. et sp. nov. MAU-Pv-LI-530. in lateral view. Scale bar: 10 cm

Scapulocoracoid of Viavenator exxoni in lateral view. Scale bar: 10 cm (a, acromion; cf, coracoid foramen; gc, glenoid cavity; pvp, posteroventral process. From Filippi et al., 2016)

  • Viavenator

The holotype of Viavenator exxoni was found in the outcrops of the Bajo de la Carpa Formation (Santonian, Upper Cretaceous), northwestern Patagonia, Argentina. The new taxon belongs to the South American clade of abelisaurid and possesses, among other characteristics, hypertrophied structures in the presacral axial skeleton. The name derives from the latin word ‘Via’ (road) and ‘venator’ (hunter), meaning the hunter of the road; ‘exxoni’ is in recognition of Exxonmobil’s commitment to the preservation of paleontological heritage of the La Invernada area, Rincón de los Sauces, Neuquen, Patagonia Argentina.

Right postorbital (holotype) of Taurovenator violentei gen. et sp. nov. A, lateral view

Right postorbital (holotype) of Taurovenator violentei gen. et sp. nov. A, lateral view. Scale bar: 3 cm (From Motta et al., 2016)

  • Taurovenator.

Taurovenator violantei gen. et sp. nov. was is a medium-sized carcharodontosaurid theropod from the Huincul Formation (Upper Cretaceous) in northwestern Río Negro province, Patagonia, Argentina. The generic name derives from the Latin words “tauro” (Bull) and “venator” (Hunter). The specific name honours Enzo Violante, owner of the farm where the specimen was discovered. Taurovenator is similar in gross morphology to Giganotosaurus, Carcharodontosaurus, and Mapusaurus, but shows two unique features: the presence of a horn-like structure in the orbital brow and the presence of an excavation housed at the posterodorsal surface of the eye socket.

Different appendicular elements of Murusraptor in their original burial positions (From Coria et al., 2016)

Different appendicular elements of Murusraptor in their original burial positions (From Coria et al., 2016)

  • Murusraptor

Murusraptor barrosaensis, from the Upper Cretaceous of Neuquén Province, Argentina, belongs to a Patagonian radiation of megaraptorids together with Aerosteon, Megaraptor and Orkoraptor. Murusraptor, meaning “Wall Raptor”, was discovered in a canyon wall in 2001 during an expedition to Sierra Barrosa in northwestern Patagonia. The holotype specimen includes much of the skull, axial skeleton, pelvis and tibia. The braincase is intact and most of the sutures are still visible, indicating that this was not a fully mature animal. Murusraptor barrosaensis is unique in having anterodorsal process of lacrimal longer than height of preorbital process; sacral ribs hollow and tubelike; short ischia distally flattened and slightly expanded dorsoventrally. Murusraptor also exhibits some characters that are interpreted as convergencies of this taxon with non-tyrannosauroid theropods, including lacrimal with a small pneumatic recess; and a highly pneumatic braincase.

Gualicho shinyae, at the Centro Cultural de la Ciencia.

Gualicho shinyae, at the Centro Cultural de la Ciencia.

  • Gualicho

Gualicho was discovered on a paleontological expedition led by Sebastian Apesteguía in 2007. The name derived from the Gennaken (Northern Tehuelche languaje) watsiltsüm, an old goddess now considered a source of misfortune. The name was chosen to reflect the difficult circumstances surrounding the discovery and study of the specimen. The specific name honors Ms. Akiko Shinya, Chief Fossil Preparator at the Field Museum. The specimen exhibits a new and unusual combination of characters observed in various remotely related clades including ceratosaurs, tyrannosaurids, and megaraptorans. The didactyl manus with a semilunate distal carpal are indicative of derived tetanuran affinities, while the expanded posterior margin of the metatarsal III proximal articulation, are shared with ceratosaurs. The reduced forelimbs with didactyl manus are similar to those of the tyrannosaurids. However, in tyrannosaurids, the carpal elements are reduced and proximodistally flattened, whereas in Gualicho the semilunate and scapholunare carpals retain a more complex shape typical of the carpal elements of most non-coelurosaurian tetanurans. In addition, the manus of Gualicho differs from tyrannosaurids in having a proportionately more robust metacarpal I with a rectangular, rather than triangular, proximal articulation in end view.

 

References:

Bernardo J. González Riga et al. A gigantic new dinosaur from Argentina and the evolution of the sauropod hind foot, Scientific Reports (2016). DOI: 10.1038/srep19165

Martínez R.D.F. et al. 2016. A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria. PLoS ONE 11 (4): e0151661; doi: 10.1371/journal.pone.0151661

Leonardo S. Filippi, Ariel H. Méndez, Rubén D. Juárez Valieri and Alberto C. Garrido (2016). «A new brachyrostran with hypertrophied axial structures reveals an unexpected radiation of latest Cretaceous abelisaurids». Cretaceous Research 61: 209-219. doi:10.1016/j.cretres.2015.12.018

Matías J. Motta, Alexis M. Aranciaga Rolando, Sebastián Rozadilla, Federico E. Agnolín, Nicolás R. Chimento, Federico Brissón Egli, and Fernando E. Novas (2016). «New theropod fauna from the Upper Cretaceous (Huincul Formation) of northwestern Patagonia, Argentina». New Mexico Museum of Natural History and Science Bulletin 71: 231-253

Rodolfo A. Coria, Philip J. Currie. A New Megaraptoran Dinosaur (Dinosauria, Theropoda, Megaraptoridae) from the Late Cretaceous of Patagonia. PLOS ONE, 2016; 11 (7): e0157973 DOI: 10.1371/journal.pone.0157973

Apesteguía S, Smith ND, Juárez Valieri R, Makovicky PJ (2016) An Unusual New Theropod with a Didactyl Manus from the Upper Cretaceous of Patagonia, Argentina. PLoS ONE 11(7): e0157793. doi: 10.1371/journal.pone.0157793