Allosaurus

The skull as a chopping tool

The skull is large relative to the body and lightly built, with large openings that reduce its weight. The upper jaw joins the rest of the skull at a flexible suture, which lets the whole snout rock upward as part of the bite. Combined with the short, recurved teeth, the motion is a downward chop rather than the crushing bite of a tyrannosaurid.

The Cleveland-Lloyd Quarry

Excavated from the 1960s, the quarry has produced over 10,000 bones and at least 46 individual Allosaurus against far fewer remains of Stegosaurus, Camarasaurus and Diplodocus. The bones are disarticulated and mixed, which means the animals died elsewhere and were washed in, or died over a period at the same place. Both readings have been argued from the sedimentology.

Reading	Argument for it	Problem
Predator trap	many predators, few prey	no obvious trap structure
Drought mortality	anoxic mudstone, seasonality	predators should be a minority
Poisoned spring	high bone concentration	hard to test

Three explanations for the most Allosaurus-rich site in the world.

A catalogue of injuries

Study of the quarry's Allosaurus bones found a high frequency of injuries and infections: broken ribs, osteomyelitis in limb bones, a dislocated and healed toe, and bite marks on skulls consistent with other Allosaurus. The picture is of an animal that fought often, including with its own species.

Growth

Histology indicates Allosaurus grew quickly for the first decade and slowed after. Juvenile specimens have relatively longer hind limbs and larger feet, so young animals were probably faster than adults -- the opposite of what a simple scaling argument would predict.

Sources

Reference works and further reading for the material in this entry.

• USGS
• International Commission on Stratigraphy
• Paleontological Research Institution

Where the argument is

What the Cleveland-Lloyd Quarry represents. It contains the remains of at least 46 individual Allosaurus, far outnumbering every other animal there. Predator trap, drought mortality around a shrinking waterhole, and a poisoned spring have all been proposed. The extreme over-representation of one species is the fact any explanation has to handle.

Frequently misread

• ClaimAllosaurus is just a smaller Tyrannosaurus.

  What the record showsThe proportions differ. Allosaurus has a larger, lighter skull with three fingers and prominent brow horns rather than two fingers and a deep, boxy skull. It also lived about 80 million years earlier and on a different continent.

• ClaimIt was a sluggish scavenger because its arms were small.

  What the record showsThe arms are short but the forelimb joints are robust and the three fingers end in large curved claws, capable of a strong pull. Healed and unhealed bite marks from Allosaurus occur on Stegosaurus and on sauropod bones, which is direct evidence of attack.