Allosauridae

Allosauridae is one of the oldest named theropod families. Marsh erected it in 1878 to contain Allosaurus fragilis, then a newly described "different lizard" from the Morrison Formation. The family today is restricted to Allosaurus and a small number of close relatives, such as Saurophaganax (currently disputed), Allosaurus jimmadseni, and possibly Epanterias.

These dinosaurs are defined by a triangular, deep skull with prominent lacrimal "horns" or ridges above the eyes. Large antorbital fenestrae reduced skull weight, while blade-like teeth, flattened side-to-side and finely serrated, allowed for efficient slicing. Their open jaw architecture enabled them to swallow large pieces of meat. The three-fingered hand featured a massive thumb claw used in prey handling. For their body size, the hindlimbs were long and gracile, suggesting they were fast walkers rather than sprint pursuit predators.

Allosaurids were the apex predators of the Late Jurassic across North America, Europe, and Africa. They occupied this niche before the Cretaceous rise of carcharodontosaurids and tyrannosaurids. Allosauridae belongs to the broader Allosauroidea, which also includes Carcharodontosauridae and Metriacanthosauridae.

The Cleveland-Lloyd Dinosaur Quarry in Utah is a Morrison-aged predator trap containing at least 46 Allosaurus individuals, the highest concentration of large theropods anywhere. While the bonebed has been interpreted as a long-term mire-trap rather than evidence of pack-hunting, the assemblage allows for detailed growth-series and population studies. Allosaurus skulls were likely used as "hatchets" to deliver open-mouth strikes to flesh. Finite-element modelling shows the skull was strong in vertical compression but weak in twisting, which is consistent with a slashing rather than crushing bite.

• Big Al (MOR 693) — sub-adult Allosaurus, ~95% complete with 19 healed pathologies, Museum of the Rockies / Natural History Museum of Utah.
• Big Al II (SMA 0005) — even more complete than Big Al, Sauriermuseum Aathal, Switzerland.
• Cleveland-Lloyd Quarry assemblage — multi-individual bonebed, multiple institutions.
• Saurophaganax maximus (OMNH specimens) — Sam Noble Oklahoma Museum of Natural History; size and validity contested.

The validity of Saurophaganax as a separate genus is the longest-running allosaurid debate — some consider it a large Allosaurus species (A. maximus), others a distinct genus. Whether Allosauridae is a tightly defined family or a paraphyletic grade leading to Carcharodontosauridae has shifted with new analyses. The functional ecology of the Cleveland-Lloyd assemblage (predator trap vs aggregation behaviour) is unresolved.

Allosaurus has long played second fiddle to Tyrannosaurus in popular media but is the headline predator in Walking with Dinosaurs: The Ballad of Big Al (2000), and appears in Prehistoric Planet, Fantasia (1940, the Stegosaurus fight), and many museum mounts. Charles Knight's classic Allosaurus and Brontosaurus paintings are foundational paleoart.

• allosaurus
• theropoda
• stegosaurus
• diplodocus

• Carrano, M. T., Benson, R. B. J., & Sampson, S. D. (2012). The phylogeny of Tetanurae (Dinosauria: Theropoda). Journal of Systematic Palaeontology, 10, 211–300.
• Foster, J. (2007). Jurassic West: The Dinosaurs of the Morrison Formation. Indiana University Press.
• Bakker, R. T. (1998). Brontosaur killers: Late Jurassic allosaurids. Gaia, 15, 145–158.
• Rayfield, E. J., et al. (2001). Cranial design and function in a large theropod dinosaur. Nature, 409, 1033–1037.