Theropoda: Ceratosauria: Abelisauridae

A remarkably succesful clade of large theropods, known from South America, India, Madagascar, and Northern Africa. No doubt future discoveries will reveal and even greater distribution. During the Middle Cretaceous they not only co-existed with giant charcharodontosaur and spinosaur theropods, but actually outlived them, becoming the apex predators of every continet except Asia-America. These were large preditors that filled the same ecological role in middle and late Cretaceous Gondwana as Allosaurs and Tyrannosaurs did during the Cretaceous of the northern supercontinent (Laurasia). An original division of abelisaurids into abelisaurines, with rather long, flat skulls that resemble in some ways the skulls of tyrannosaurids, and carnotaurines, with short, deep, ornamented skulls, has not been confoirmed by cladistic analysis and so is rejected here. It seems instead that the development of horn-like structures and differential cranial thickening appear to be convergently acquired within Abelisauridae. These short thin crests or horns are very different to the dilophosaur crests. All abelisaurs appear to have had side-to-side expanded "cheeks," like those of tyrannosaurids. Whilst Abelisaurus and Carnotaurus are the two best known genera, there are a number of other forms that have recently been placed here, and more are being discovered all the time. Abelisaurids had the greatest reduction in forelimbs among the theropods, even tyrannosaurs had proprtionally larger arms. The vestigal arms (reminiscent of the tiny hind limbs of early whales) would have been useless in grappling or any other purpose. As with tyrannosaurs, the powerful skulls and jaws were used to subdue prey.

Known from a maxilla (part of upper jaw) and postcranial elements, this is the oldest known and most primitive Abelisaurid. Features of the maxilla such as vascular grooves and a narrow antorbital fossa place Kryptops among the abelisaurids (Sereno & Brusatte 2008). This is one of those instances, useful in vertebrate paleontology when dealing with incomplete specimens, where a sintgle bone can identify a species. As many distinct lineages of abelisaurids co-existing in this area is perhaps not likely, it is possible that Kryptops ("hidden face") was the ancestor, or at least a very close cousin, of Rugops ("rugged face"). Both lived in the same geographic region, and the latter only ten million or so years later. This animal was a contemporary of theprimitive carcharodontosaur Eocarcharia dinops, both abelisaurids and carcharodontosaurs appeared at the same time and would evolve in parallel in Gondwana during the middle Cretaceous

Known from a partial skull lacking the palate and most of the bones along the side of the face behind the eyes, this is one of the earliest abelisaurids with textured external skull surfaces. It consistently resolves as a basal form in cladistic analyses

All analyses agree that Abelisaurus and Carnotaurines share a common ancestor above the level of Rugops, regardles sof whether or not Carnotaurinae are a natural group. The Cenomanian appears to have been an important time in abelisaurid evolution.

Known only from a large, nearly complete skull, Abelisaurus was unusual when first discovered in view of its late occurance and primitive megalosaur-like features. The eye socket's orbital bar and simple lower cheek bones indicate it is closely related to Carnotaurus. Genyodectes serus Woodward, 1901, also from the latest Cretaceous of Argentina, and based on part of the front end of some theropod jaws, may be closely related, if not synonymous.

The basal position of Abelisaurus assumes that the rugged horned and crested form only evolved once (Carnotaurinae) (Sereno et al 2004, see also cladogram in Wilson et al 2003). Current cladistic analaysis indicates there was a lot of parallelism in abelisaurid evolution (Carrano & Sampson 2008, Canale et al 2008), since both horned and hornless forms evolved among higher taxa, there is no reason why Abelisaurus should be discriminated against. Therefore we have moved this taxon to the crown group Abelisaurids

analysis by Carrano & Sampson 2008 recovered a clade of large, powerfully-built abelisaurids, that populated Central Gondwana (India and Madagascar ). As it doesn't seem to have a name we here informally refer to this lineage as Majungasaurinae, after the best known species, Majungasaurus crenatissimus

The Majungasaurines represent a distinct lineage to the short-headed, highly derived West Gondwanan forms. The total tibia-femur length is notably short compared to other similarly sized theropods, indicating a relatively slow moving animal. They also lack the unique caudal morphology and powerful leg muscles of the advanced West Gondwanan forms (Persons & Currie 2011 pp.1-2). These differences may reflect different faunal compositions. For example, ornithopods are unknown from Central Gondwana. These powerfully built abelisaurs therefore would have preyed on large and slow titanosaurs (sauropods) rather than smaller and faster ornithopods, All members of this unnamed clade are currently known only from the Maastrichtian, implying a ghost lineage of thirty or forty million years.

This theropod was for a long time considered to be a large pachycephalosaurid, called Majungatholus atopus. It is distinguished by thickened nasal bones, a horn on the frontal bone, and a parietal (skull bone near the rear) prominence. The earlier named Majungasaurus crenatissimus, originally based on an incomplete dentary from the same time and place, has a curved mandible is very like that of Carnotaurus. Recent excavations undertaken by the Mahajanga Basin Project resulted in abundant and spectacular remains of this theropod, including multiple specimens of various size and presumably age ranges, and covering the great majority of the skull and skeleton, make Majungasaurus among the best known of Gondwanan theropods, and conclusively placing it within Abelisauridae . The confused taxonomy is also resolved, with Majungasaurus crenatissimus as the valid name for this theropod (Krause et al 2007). Previously considered closely related to Carnotaurus (Wilson et al 2003), new analysis places it separate from the South American forms (Carrano & Sampson 2008, Canale et al 2008)

Matching tooth marks on bones show that this species sometimes fed on its own kind (Rogers et al 2007) (Cannibalistic behaviour had previously been attributed to Coelophysis bauri, but this is now known not to have been the case)

The face of Majungasaurus crenatissimus Artwork by Deviant Paleoart, Wikipedia Creative Commons Attribution

The skull of Majungasaurus is exceptionally well-known compared to most theropods. Like other abelisaurid skulls, its length was proportionally short for its height, although not as short as in Carnotaurus. The skulls of large individuals measured 60-70 centimeters (24-28 in) long. The tall premaxilla (frontmost upper jaw bone), which made the tip of the snout very blunt, was also typical of the family. The nasal bones were extremely thick and fused together, with a low central ridge running along the half of the bone closest to the nostrils. A distinctive dome-like horn protruded from the fused frontal bones on top of the skull as well. In life, these structures would have been covered with some sort of integument, possibly made of keratin. Computed tomography (CT scanning) of the skull shows that both the nasal structure and the frontal horn contained hollow sinus cavities, perhaps to reduce weight (Sampson & Witmer 2007). The hollow cavity inside the frontal horn of Majungasaurus would have weakened the structure and probably precluded its use in direct physical combat, although the horn may have served a display purpose (Tykoski & Rowe 2004). While there is variation in the ornamentation of Majungasaurus individuals, there is no evidence for sexual dimorphism.(Sampson & Witmer 2007). The teeth were typical of abelisaurids in having short crowns, although Majungasaurus bore 17 teeth in both the maxilla of the upper jaw and the dentary of the lower jaw, more than in any other abelisaurid except Rugops (Smith2007).

Abelisaurids may instead have been adapted for a feeding strategy more similar to modern felids, with short and broad snouts, that bite once and hold on until the prey is subdued. Majungasaurus had an even broader snout than other abelisaurids, and other aspects of its anatomy may also support the bite-and-hold hypothesis. The neck was strengthened, with robust vertebrae, interlocking ribs and ossified tendons, as well as reinforced muscle attachment sites on the vertebrae and the back of the skull. These muscles would have been able to hold the head steady despite the struggles of its prey. Abelisaurid skulls were also strengthened in many areas by bone mineralized out of the skin, creating the characteristic rough texture of the bones. This is particularly true of Majungasaurus, where the nasal bones were fused and thickened for strength. On the other hand, the lower jaw of Majungasaurus sported a large fenestra (opening) on each side, as seen in other ceratosaurs, as well as synovial joints between certain bones that allowed a high degree of flexibility in the lower jaw, although not to the extent seen in snakes. This may have been an adaptation to prevent the fracture of the lower jaw when holding onto a struggling prey animal. The front teeth of the upper jaw were more robust than the rest, to provide an anchor point for the bite, while the low crown height of Majungasaurus teeth prevented them from breaking off during a struggle. Finally, unlike the teeth of Allosaurus and most other theropods, which were curved on both the front and back, abelisaurids like Majungasaurus had teeth curved on the front edge but straighter on the back (cutting) edge. This structure may have served to prevent slicing, and instead holding the teeth in place when biting.(Sampson & Witmer 2007)

Majungasaurus was the largest predator in its environment, while the only known large herbivores at the time were sauropods like Rapetosaurus. Scientists have suggested that Majungasaurus, and perhaps other abelisaurids, specialized on hunting sauropods. Adaptations to strengthen the head and neck for a bite-and-hold type of attack might have been very useful against sauropods, which would have been tremendously powerful animals. This hypothesis may also be supported by the hindlegs of Majungasaurus, which were short and stocky, as opposed to the longer and more slender legs of most other theropods. While Majungasaurus would not have moved as fast as other similar-sized theropods, it would have had no trouble keeping up with slow-moving sauropods. The robust hindlimb bones suggest very powerful legs, and their shorter length would have lowered the animal's center of gravity. The tibia (lower leg bone) of Majungasaurus was even stockier than that of its relative, the sprinter Carnotaurus, with a prominent crest on the knee (Carrano 2007). Thus Majungasaurus may have sacrificed speed for power (Sampson & Witmer 2007) Majungasaurus tooth marks on Rapetosaurus bones confirm that it at least fed on these sauropods, whether or not it actually killed them. (Rogers et al 2007)

the first Indian theropod preserving associated cranial and postcranial remains; known from an almost complete skull and partial skeleton . The skull was short, measuring 60 cm (23.6 in) in length, and bore a distinctive low rounded horn or crest, made of outgrowths from the nasal and frontal bones. The configuration of its nasal bones and possession of a growth ("excrescence") on its frontal bone places it among the carnotaurine abelisaurids (Wikipedia), although the presence of these features developed independently among the abelisauruids (hence Abelisaurus itslef may not be as basal as usually portrayed (Carrano & Sampson 2008)). Rajasaurus seems to have been a fairly heavily built animal. Indosaurus matleyi from the Lower Lameta Group, Madhya Pradesh, India (mid-late Maastrichtian ) may be a synonym or at least a very closely related species (which would mean renaming Rajasaurus), although the type material is lost. It was known from a skull roof and other cranial and post-cranial elements, with frontal bones massively thickened into what appear to be the bases of horn cores above the eyes. In any case, proper identification of these Indian forms is hampered by poor preservation Carrano & Sampson 2008. The contemporary Indosuchus, known from very scrappy remains, was a smaller, longer-legged animal (Paul 2010).

The Brachyrostra ("short snouts") are a postulated clade of abelisaurs geographically limited to South America, showing that abelisaurids divided early on along geographical lines, which fits with the timing of the break up of Gondwana. Distinct from both the Abelisaurines and the Majungasaurines, these advanced abelisaurs were characterised by a shortening of the skull and hyperossification (thickening of the bone) of the skull roof seem to be correlated with a progressive enclosure of the orbit. This may be related to shock-absorbing, implying head-butting intra-specific behaviour. Canale et al 2008. Phylogenetic analysis of Canale et al 2008, recovered three species ina stratigraphic sequence, the newly discovered Skorpiovenator, the Carnotaurus-like Ekrixinatosaurus, and the poorly known Ilokelesia as part of a newly discovered clade that is the sister group to the Carnotaurini (Aucasaurus + Carnotaurus). This contradicts Sereno et al 2004, for whom Ilokelesia is a basal form, and Carrano & Sampson 2008, who place Ekrixinatosaurus intermediate between Rugops and higher Abelisaurs, although the latter authors did recover a connection between Ilokelesia and Carnotaurus. Persons & Currie 2011 in their study of abelisaurid tail morphology note that these five taxa form both a morphological and a stratigraphic series, especially in view of their close geographic proximity, and we have followed this hypothesis here. Although Canale et al 2008 find that in Skorpiovenator and Ekrixinatosaurus horns consist of dorsally inflated postorbital bones, distinct from the frontal horns of Carnotaurus, such reversals would not be unusual in any evolutionary (anagenetic) sequence. Advanced members of this clade became progressively more cursorial, culminating in Carnotaurus, the fastest known large theropod (Persons & Currie 2011)

Ekrixinatosaurus (the name means "explosion-born lizard," as it was discovered during a blasting operation (ref Thescelosaurus)) is based on a partial skeleton and skull elements. It shares a mixture of characters with several other abelisaurids, highlighting the homoplastic, probably ontogenetic, nature of many of these traits (Carrano & Sampson 2008). Either a fairly large abelisaur with an oversized head, or the postcrania from one indivdual along with skull fragments of a much lartger one. A particularly robust animal, a contemporary of the even larger carcharodontosaurid Giganotosaurus. As a result of the post-Cenomanian Carcharodontosaurs, both animals being the largest known representatives of their respective clades. (Juarez Valieri et al 2011). Stratigraphically this is earliest of the Brachyrostra, and as its stocky build and lacks of advanced cursorial adaptations means it was not a fast moving animal (Persons & Currie 2011 p.2). Variously considered a derived (Canale et al 2008) or a primitive (Carrano & Sampson 2008, Persons & Currie 2011) species, the combination of brachyrostran head and "Majungasaurine" body and locomotary traits, along with early stratigraphic position (the earliest well known abelisaurid) support its more basal position.

Known from fragmentary material, and initially thought to be a basal abelisauroid due to the retention of basal characteristics, it now seems to a more advanced animal, representing an early stage in the brachyrostral lineage Canale et al 2008 and/or related to Carnotaurus (Carrano & Sampson 2008) or may be an intermediate form between Ekrixinatosaurus and Skorpiovenator (Persons & Currie 2011) The base of the tail is exceptionally broad (Paul 2010)

Known from a complete skull and most of the skeleton, an dnamed after the large number of scorpions at the fossil locality, this long legged form represents the beginning of the tendency towards advanced cursorial ability rapid sprinting and - by implication, as these were the largest predators of their environment - pursuit that reached its culmination in the avanced carnotaurini (Persons & Currie 2011)

Based on similarities between Skorpiovenator and carcharodontosaurid tooth morphology, it is quite likely that stratigraphically younger isolated teeth originally referred as Carcharodontosauridae belong to abelisaurids Canale et al 2008. Since no post-Cenomanian Carcharodontosaurs are otherwise known, this implies that that group became extinct at the end of the Cenomanian, their ecological role being replaced by large abelisaurs, although their smaller (average 6 meters) cousins or descendents the neovenatorids continued to flourish in the medium megapredator role.

Canale et al 2008 place Carnotaurus and Aucasaurus as sister taxa in the tribe Carnotaurini. This was the only West Gondwanan abelisaurid lineage to survive to the end of the Cretaceous. In comparison to other Abelisaurids, the legs are long and gracile. This lineage of large theropods specialise din fast pursuit, unlike the more heavily built central Gondwanan clade. As Persons & Currie 2011 explain in the abstract:

In the South American abelisaurids Carnotaurus sastrei, Aucasaurus garridoi, and, to a lesser extent Skorpiovenator bustingorryi, the anterior caudal ribs project at a high dorsolateral inclination and have interlocking lateral tips. This unique morphology facilitated the expansion of the caudal hypaxial musculature at the expense of the epaxial musculature. Distinct ridges on the ventrolateral surfaces of the caudal ribs of Aucasaurus garridoi are interpreted as attachment scars from the intra caudofemoralis/ilio-ischiocaudalis septa, and confirm that the M. caudofemoralis of advanced South American abelisaurids originated from a portion of the caudal ribs Digital muscle models indicate that, relative to its overall body size, Carnotaurus sastrei had a substantially larger M. caudofemoralis than any other theropod yet studied. In most non-avian theropods, as in many extant sauropsids, the M. caudofemoralis served as the primary femoral retractor muscle during the locomotive power stroke. This large investment in the M. caudofemoralis suggests that Carnotaurus sastrei had the potential for great cursorial abilities, particularly short-burst sprinting. However, the tightly interlocking morphology of the anterior caudal vertebrae implies a reduced ability to make tight turns.

Related to but smaller than Carnotaurus, and lacking the head crests, with a longer, lower skull with bumps instead of horns. The arms were also somewhat longer. Based on an almost complete skeleton, including soft tissue impressions about the hips. (Ref - Thescelosaurus). Despite the complete fossil, this animal remains poorly known.

Known remains consist of a skull and and almost complete skeleton with skin impressions.

This theropod is known from a virtually complete skeleton (lacking only the hind feet and end of the tail), a skull with extensive skin impressions. The skull is very high and short with two stout horn-cores, which in life were the basis of extended horns. this has inspired the strange name "flesh-eating bull". The horns were probably quite blunt and used for intraspecific combat (head butting etc). The eyes are quite small (much of the orbit cut off by a bar of bone) although Greg Paul raises the possibility the deep snout may have contained nasal organs. Perhaps unlike most other theropods this dinosaur hunted by smell rather than sight. The skin empressions show this animal to have had widely-spaced rows of nonbony scutes. The hands and arms are so small as to be practically useless.

Carnotaurus seems to have had a highly kinetic skull and could gape widely to swallow large objects. A prokinetic hinge rostral to the orbits would have allowed (as in birds) elevation of the rostrum while the eyes could remain fixed on prey. The vaulted skull gave a fast, rather than a strong, bite. Carnotaurus was envisioned as an active predator of fast, small prey.. These large speialised animals were among the last of the South American abelisaurids

It is possible the distinctive shape of the type skull of Carnotaurus is due to distortion. Mark Goodwin et al., in their initial presentation on Majungatholus (=Majungasaurus), suggested that the deep and narrow skull shape of Carnotaurus owed itself to compaction and might not have been natural. (SVPCA - Darren Naish)

This was the fastest large theropod, with the largest M. caudofemoralis in proportion to its size (Persons & Currie 2011). It represented the culmination of the West Gondwanan brachyrostran lineage.