| Allosaurids | |
|---|---|
 | |
| AnA. fragilis skeletal mount, at theCarnegie Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Allosauroidea |
| Family: | †Allosauridae Marsh, 1878 |
| Type species | |
| †Allosaurus fragilis Marsh, 1877 | |
| Genera[1] | |
| Synonyms | |
| |
Allosauridae is an extinct family of medium to large bipedal,carnivorousallosauroidtheropoddinosaurs from the LateJurassic.[2] Allosauridae is a fairly old taxonomic group, having been first named by the American paleontologistOthniel Charles Marsh in 1878.[3]
Allosaurids have a general anatomy typical of otherneotheropod dinosaurs, contributing to the difficulty in defining the family's membership. A typical 8m specimen ofAllosaurus fragilis had a skull of about 0.85m. Thepremaxilla has five teeth and themaxilla usually around 16. The dentary also typically has 16 teeth. All teeth are serrated and continuously replaced throughout the life of the animal. Allosaurid skulls are characterized by two sets of crests formed by the nasal and lacrimal bones respectively. These crests would have been covered by keratin sheathes.[4] The skull also exhibits features consistent with significant cranial kinesis: a synovial joint between the braincase and the frontals and a loose articulation between the dentary and the angular/surangular.[5] Thiscranial kinesis would have dampened forces on the bones of the skull and allowed allosaurids to open their mouths to very large angles.
Allosaurids have 28 precaudalvertebrae (9 cervical, 14 dorsal, 5 sacral) and an estimated 45–50 caudal vertebrae.[5] Gastralia and furculae are rarely preserved as fossils but are presumed to occur in all allosaurids.[6] The pubis is highly elongated and extends ventrally to form a pubic foot which like in other large dinosaurs is thought to have been used to support the weight of the body in a resting crouch position.[4]
Like most othertheropods, allosaurids have very short forelimbs relative to their hindlimbs with three digits on the hand and four on the foot. The first digit of the hand forms a semi-opposable thumb and digits 4 and 5 are absent. Digits 2–4 of the foot are robust but digit 1 is reduced and does not touch the ground and digit 5 is absent.[7] All distal phalanges were capped with large claws, those on the hand were especially long and were curved to facilitate raking and grasping of prey items.[4] Phalangeal formulae of the hand and foot are 2-3-4 and 2-3-4-5 respectively.[7]
In 2019, Rauhut and Pol describedAsfaltovenator vialidadi, a basal allosauroid displaying a mosaic of primitive and derived features seen withinTetanurae. Their phylogenetic analysis found traditionalMegalosauroidea to represent a basalgrade ofcarnosaurs,paraphyletic with respect toAllosauroidea. They also noted that the definitive taxonomic placement ofSaurophaganax withinAllosauroidea is unstable, being recovered as a sister taxon ofMetriacanthosauridae or Allosauria, or even as a basalmost carcharodontosaurian.[8]
In 2024, re-examinations of the specimens attributed toSaurophaganax suggested that it is a chimera of multiple dinosaur genera, since some specimens most likely belong to adiplodocidsauropod, while the other referred allosaurid specimens belong to a novel species ofAllosaurus, named asAllosaurus anax.[9]
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