Abelisauria

Abelisaurus, artwork courtesy of Frank DeNota

The abelisaurs are an important Cretaceous group known mainly from Gondwana. Most were large and a few had strange horns and crests. There was a tendency in one lineage for the hands and arms to be reduced so they became even smaller than the tiny forelimbs of the Tyrannosaurs. The abelisaur phylogenetic placement is still very questionable. They have some tyrannosaurid characters, as well as ceratosaur and carcharodontosaur characters. Their position is still far from being resolved. The general consensus is that they are late surviving ceratosaurs. Another interesting possibility may be an abelisaur-spinosaur relationship. Abelisaurs, spinosaurs and carcharodontosaurs are all Gondwanan theropods for which relationships have been proposed in various ways, and which are all difficult to place in a phylogeny.

Here is a suggested Linnean/Evolutionary Systematic classification of the Abelisaurs (note - the following is unofficial and should not be taken as authoritative!). The Linnean system - so central to many older books - is no longer held by many biologists and paleontologists, and most now advocate the Cladistic stance. The following is therefore suggested to complement (but not replace) the current paradigm.

The following cladogram presents a suggested overview of Abelisaurian phylogenetic relationships. A number of fragmentary and poorly known taxa are not shown.

<==-o ABELISAURIA
           |?- Ligabueino andesi
           |?-o Velocisauridae
           |  `-- Velocisaurus unicus
           |--o Noasauridae
           |  |-- Noasauridae genera et species indet. [Madagascar]
           |  |-- Noasaurus leali 
           |  `-- Laevisuchus indicus
           `--o-Abelisauridae
               `--+-o-Abelisaurinae
                  |   |?- Tarascosaurus salluvicus
                  |   |-- Indosuchus raptorius
                  |   `-- Abelisaurus comahuensis 
                  | ?--- Xenotarsosaurus bonapartei 
                  `--o Carnotaurinae
                     |?--o Indosaurus 
                     |  |- I. madhyamensis
                     |  `-- I. matleyi 
                     |-- Majungatholus atopus [Majungasaurus crenatissimus) 
                     `-- Carnotaurus sastrei 

family Abelisauridae

The Abelisauridae includes the typical representatives of this group. 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). There is an important division among the abelisaurids, between the abelisaurines and the carnotaurines. The abelisaurines (image) have rather long, flat skulls that resemble in some ways the skulls of tyrannosaurids, while the carnotaurines (image) have short, deep, ornamented skulls, equipped with short thin crests rather like horns (very different to the dilophosaur crests). Both groups 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.

Guild/Ecological niche: Giant terrestrial predator
Modern equivalent: none
Time: to Albian to Maastrichtian
Distribution: Chiefly Gondwanaland, but also known from Europe
Evolved from: basal Abelisaur? Megalosaur? Ceratosaur?
Replaced: Jurassic Megalosaurs
Replaced by: none at first, later large phorushucid birds
Extinction because of:  terminal Cretaceous extinction (K/T event)
Descendents: none
Linnean status: Family
Cladistic status: Monophyletic clade
Parent clade: Abelisauria
Adult length: 6 to 8 meters
Adult weight: 700 kg to 1500 kg
Preferred environment: floodplain forests, coastal environments
Activity period: mostly diurnal
Metabolism: endothermic? gigantotherms
Senses: eyesight poor for a theropod but with good depth perception, hearing good, sense of smell superb
Intelligence: probably equivalent to modern crocodilian
Preferred food: other dinosaurs, mainly ornithopods, small fast prey, juvenile sauropods
Possible hunting and attack strategy: located distant prey by smell, would approach and wait in ambush, judge distance, then rush in sudden burst of speed. Attack using slashing bite, with most force exerted by the upper jaw. Smaller animals swallowed whole
Food Processing: Large chunks of meat swallowed whole
Social behavior: Intraspecific combat through head-butting in competition for mates and/or territory
Possible voice: alligator-like hiss-chirp-roar?
Possible Life Cycle: Parent(s) guards nest, young very active, lively, possibly endothermic?, able to hunt for themselves, protected by parent(s) but may be eaten by other adults, grow rapidly. Metabolism slows to homeothermic gigantotherm as becomes subadult. As adults may have hunted singularily or in small groups
Movement: active erect terrestrial biped, also good swimmer, using strong kicks of hind feet and sweeping tail
Walking speed: about 7 kph
Running speed: short bursts of maybe 40 kph or more
Predators: big Cacharodontosaurs, otherwise none (top of foodchain)
Defense against Predators: slashing bite
Weaknesses: relatively poor eyesight; bite and teeth not very strong

Subfamily Abelisaurinae

These are "typical" Abelisaurs - large preditory theropods that share a similar skull structure and also (through covergent evolution) resemble in some ways the skulls of tyrannosaurids.

Abelisaurus comahuensis Bonaparte and Novas, 1985

Comments: Known only from a 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.  Reconstruction

Indosuchus raptorius Huene and Matley, 1933

Comments: A contemporary of the carnotaurine Indosaurus, this animal appears to be closer to Abelisaurus. New material, making up most of the skeleton, shows it to have had a relatively long arm, unlike Carnotaurus.
 Indosuchus raptorius

Tarascosaurus salluvicus Le Loeuff and Buffetaut, 1990

Comments: the only non-Gondwana abelisaurid.

Subfamily Carnotaurinae

This group includes a number of stange-looking theropods, characterised by horns above the eyes, thickening of the skull roof, deep skulls, very lightly built lower jaws, and vestigal, apparently useless arms and handsare things seen in some or all of these creatures.

Carnotaurus sastrei Bonaparte, 1985

Comments: 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.  Reconstruction

Carnotaurus 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.

It is possible the distinctive shape of the type skull of Carnotaurus is due to distortion. Goodwin et al., in their initial presentation on Majungatholus, suggested that the deep and narrow skull shape of Carnotaurus owed itself to compaction and might not have been natural. Darren Naish has expressed scepticism regarding the cranial kineticism

Indosaurus matleyi Huene and Matley, 1933

Comments: Known from a skull roof and other cranial and post-cranial elements, this theropod has frontal bones massively thickened into what appear to be the bases of horn cores above the eyes. It seems to have been a fairly heavily built animal, possibly related to Carnotaurus if the horn-cores are any indication, although it must have seperated from the Abelisaur stem very early on, as India was an Island-continent throughout the Cretaceous.

Majungatholus atopus Sues and Taquet, 1979

Comments: This theropod was for a long time considered to be a large pachycephalosaurid. 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 (but non-diagnostic) Majungasaurus crenatissimus, based on an incomplete dentary from the same time and place, may be a synonym. It has a curved mandible is very like that of Carnotaurus.

Subfamily Indeterminate

Xenotarsosaurus bonapartei Martinez, Gimenez, Rodriguez, and Bochatey, 1987

Comments: all that can be said with confidence about this large but poorly known animal is that it belongs in the Abelisauridae. It lived in the same area but a little later than Carnotaurus.

Family Noasauridae

These astonishing animals are known from throughout Gondwana. The family is composed of small abelisaurs, included a new, unnamed form from the late Cretaceous of Madagascar, unusual by its forward-jutting teeth (indicating a piscivous diet). The similarity between Noasaurs and the sickle-clawed maniraptors of the northern continents is reminiscent of that between the sabre toothed cats and the South American sabre-toothed marsupials that lived during the same time but in isolation from the rest of the land-masses. In both cases we see parallel evolution between unrelated lineages of predators, in which a medium-sized animal is equipped with a powerful weapon to bring down animals much larger than itself.

Guild/Ecological niche: medium-sized terrestrial predator
Modern equivalent: none?
Time: currently known from the Maastrichtian only
Distribution: Chiefly Gondwanaland
Evolved from: unknown Abelisaur ancestors
Replaced: earlier small theropods
Replaced by: none at first, later early mammalian carnivores and smaller phorushucid birds
Extinction because of:  terminal Cretaceous extinction (K/T event)
Descendents: none
Linnean status: Family
Cladistic status: Monophyletic clade
Parent clade: Abelisauria
adult length: 1.5 to 2 meters
weight: 15 kg
Preferred environment: floodplain forests, coastal environments
Activity period: mostly diurnal
Metabolism: partial endotherm?
Intelligence: perhaps equivalent or superior to modern crocodilian
Preferred food: small dinosaurs, ornithopods especially, other tetrapods in general; some types fish-eaters.
Possible hunting and attack strategy: Noasaurus: located distant prey by smell, would approach and wait in ambush, judge distance, then rush in sudden burst of speed. Attack using kicks and slash with huge pedal claw, also slashing bite, but jaw not very strong. It is not clear whether Noasaurs hunted singularily or in packs.
Movement: active, bipedal, erect gait, efficient terrestrial movement, also good swimmer
Running speed: short bursts of maybe 40 or 50 kph
Swimming ability: good, using strong kicks of hind feet and sweeping tail
Predators: Abelisauridae
Defense against predators: Speed, also big pedal claw and bite

Laevisuchus indicus Huene and Matley, 1933

Comments: This theropod has previously been considered a coelurid, but has more recently been shown to probably be a small abelisaurian.

Noasaurus leali Bonaparte and Powell, 1980

Comments: This small theropod is unusual in that it independently has come up with a sickle-claw system similar to that of the dromaeosaurids and troodontids. That it does not belong one of those two groups is shown by the primitive skull and different claw mechanism. The upper jaw's maxilla is tall and simple, like Abelisaurus, indicating a deep but lightly built skull. The claw's retractor tendon is anchored in a depression at the claw's base, rather than in the standard Dromaeosaur and Troodontid large tuber-like heel process. It is not known whether the arms for short (like Carnotaurus) or relatively large (like Laurasian maniraptors). The drawing presented here therefore is speculative in that regard. Reconstruction

Family Velocisauridae

The family Velocisauridae is listed in the Fossil Record II Fossil reptile families (table). Although based on only a singly fragmentary specimen, it hints at the existence of a rich Cretaceous Gondwanan fauna. As the name indicates, tehse were fast running animals; the equivalent perhaps of the Ornithomimids of the northern continents. Tantalising scraps of gracile running theropods are also known from the mid-Cretaceous of Australia (south-east Gondwana) - Timimus and Kakura. These may or may not be related.

Velocisaurus unicus Bonaparte, 1991

Comments: A small swift-running animal, the relationships of which are not certain. The middle metatarsal bone in the foot is greatly enlargened, while the outer ones were reduced. Although probably an adaptation for running, it is the opposite to that of many running coelurosaurs (whose outer metatarsals became stronger, greatly reducing the inner one). The toe proportions suggest it fed like a chicken, through scratching at the ground.
VELOCISAURUS UNICUS

Family Indeterminate

Ligabueino andesi Bonaparte, 1995

Comments: On of the earliest known abelisaurs

Links

Indosuchus raptorius - not only about Indosuchus, but also the best general introduction (at time of writing) to the Abelisauridae anywhere on the Web

Abelisauridae - Thescelosaurus!

Dinosauricon

Ceratosauria Mikko's phylogeny site

Jurassic Gallery - fantastic artwork by M. Shiraishi. By Japanese and English

Gregory S. Paul, Predatory Dinosaurs of the World Simon and Schuster, 1988

Molnar, R., E., Problematic Theropoda: "Carnosaurs". 306-317, in Weishampel, D. B., Dodson, P., & Osmólska, H. (eds.), The Dinosauria University of California Press, Berkley, Los Angeles, Oxford, 1990

Norman, D., B., Problematic Theropoda: "Coelurosaurs". 280-305 in Weishampel, D. B., Dodson, P., & Osmólska, H. (eds.), The Dinosauria University of California Press, Berkley, Los Angeles, Oxford, 1990

Gerardo Mazzetta: "Some functional aspects of the skull of the South American horned theropod Carnotaurus sastrei (Saurischia: Theropoda)." - summarised by Darren Naish in Theropods

cautionary note (please read before using this page as reference material!)

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