Arcovenator

Arcovenator ("Arc hunter") is a genus of abelisaurid theropod dinosaur hailing from the Late Cretaceous of France.[1] The type and only described species is Arcovenator escotae.

Description
Though shallower, the nearly complete braincase of Arcovenator is otherwise similar in size to those of Majungasaurus and Carnotaurus, implying an animal about 5–6 metres (16–20 ft) long.[1] The skull roof exhibits as a unique diagnostic character a midline foramen, possibly housing the pineal gland, situated on the posterior surface of a slight dome formed by frontal bones as moderately thick as in Aucasaurus, thus less so than for Rajasaurus, though more than those of Rugops.[1] Less characteristically there is above the orbit a low fossa with a small fenestra bordered by the lacrimal, frontal and postorbital.[1] The parietal bordering the supratemporal fenestrae forms ridges medially on the latter's respective anteromedial margins which, as they approach the parietal eminence, fuse into a sagital crest.[1] The postorbital is intermediate between the plesiomorphic T-shaped condition of Eoabelisaurus and the derived inverted L-shaped one of Carnotaurus due to the unique feature of having a sheet of bone linking its ventral and posterior processes.[1] It has, in a similar autapomorphic fashion, a thick, rough-surfaced process dorsal to the eye socket that extends to the lacrimal, forming a bony brow ridge, and, in a less notable way, a lateral rugose tuberosity on the extremity of its ventral process.[1] The paroccipital processes have remarkable accessory dorsal and ventral bony bars, that thus bound depressions lateral to the foramen magnum.[1] The ear region closely resembles that of Majungasaurus, though differing most substantially on a laterally directed basipterygoid process, with the shorter crista prootica and the smaller extent of a groove anterior to the 2nd and 3rd cranial nerve foramina being minor deviances from Majungasaurinae's type.[1] The squamosal is similar to that of the latter except for a less prominent parietal process.[1] Generally the external bone ornamentation is more subdued than that of Majungasaurus.[1] The tall teeth (3-5.5 cm) have denticles on the apical portion of the mesial carina and along the length of the distal one, with varying density.[1]

The caudal vertebrae of A. escotae are remarkably similar to those of Majungasaurus, though more dorso-ventrally compressed.[1] The centra possess amphicoelous articulations with the pertinent facets of an intermediate nature between the circular ones of Ilokelesia and those of the elliptical persuasion in Rajasaurus and have neither pneumatic recesses nor accessory hyposphene-hypantrum articulations.[1] The transverse processes of the neural arches aren't as inclined as in Brachyrostra.[1]

The cnemial crest of Arcovenator's the slender 51-cm tibia is well developed as is characteristic of abelisauroids.[1] It has a proximal lateral condyle more prominent than the medial one, a slight anterodorsal curve on the proximal aspect of the fibular crest, a noticeable distal longitudinal ridge and tapered malleoli.[1] The nearly half-meter-long fibula possesses the typical anatomical characters of ceratosaurs.

Classification and systematics
Arcovenator is a theropod genus nested within the clade Abelisauridae,[1] which in Linnaean taxonomy has the rank of family.[2] This taxonomical group has as close relatives noasaurids within Abelisauroidea.[1][3] The latter in turn along with Limusaurus and Ceratosaurus nests within Ceratosauria.[1][4]

Distinguishing characters of abelisaurids are their short, tall skulls with extensively sculptured external surfaces, the drastically reduced forelimbs, and the stout hindlimbs.[5]

As with many dinosaur clades, the structure of the phylogenetic tree of Abelisauridae and which genus pertains to which subgroup are in a state of flux as more data is obtained from both new fossils and applying new analytic techniques.[3][5][6] Arcovenator escotae, being the most complete and informative find since Genusaurus as far as French abelisaurids are concerned, suscitates readily the parallel interests of attempting to determine its position in relation to other genera, and ascertaining what its suite of characters resolves further of the relatedness between them.[1]

Thus Thierry Tortosa and colleagues conducted a phylogenetic analysis, which is summarized in the cladogram to the right and is based, in part, on previously published works including both the newly discovered fossil remains and other described but unnamed French abelisaurs.[1]

The study generally agrees with previous results, namely a relatively recent one obtained both by Matthew Carrano & Scott Sampson (2008)[7] and Diego Pol & Oliver W. M. Rauhut (2012)[4] of a clade that includes at least Majungasaurus, Indosaurus and Rajasaurus, which in the more recent analysis includes Arcovenator.[1] Tortosa et al. name this well-supported clade Majungasaurinae, ranking it as subfamily and defining it to contain all abelisaurids more closely related to Majungasaurus than to Carnotaurus.[1] The members of this taxonomical group have various cranial characters in common including an elongated antorbital fenestra, and a parietal with a sagittal crest that widens anteriorly into a triangular surface.[1] Also of note is that, in partial agreement with some analyses, the more fragmentary French ceratosaur remains are placed within Abelisauridae, and contrary to others, Abelisaurus is recovered as a carnotaurin.[1]

There are also insights into the paleobiogeography of abelisauroids: just presence of them in the so-called European Archipelago[8] confounds hypotheses that only consider the continents derived from the breakup of Mesozoic Gondwana.[1] Two lineages of European abelisaurs are discerned: a basal one, the small Albian Genusaurus with the African Kryptops as a likely close relative, and a derived one, the larger Campanian Arcovenator allied with the Madagascan Majungasaurus and the Indian Rajasaurus in Majungasaurinae.[1] As the inferred character distributions obtained through the phylogenetic analysis make it unlikely that these lineages are more closely related to each other than to other abelisaurids, this suggests a more complicated series of events regarding their biogeography with vicariance applicable to the older one and oceanic dispersal being likelier for the more recent one.[1] These results lend support to the proposed role of Africa as a hub for faunal movements between Europe and India or Madagascar[9] and the isolation of South American abelisaurids.[