Why Dinosaur Skulls Are So Rare — and Why Utah’s Carnegie Quarry Has So Many

Of all the fossils paleontologists uncover, skulls are among the rarest. At Dinosaur National Monument’s Carnegie Quarry in Utah, though, they turned up again and again.

A nearly complete skull of the predatory dinosaur Allosaurus fragilis from the Carnegie Quarry; note how thin the connecting bones are. NPS photo.

Built to break

Unlike leg and arm bones, skulls are hollow and full of openings. In life those spaces held brains, eyes, tongues and sinuses, so the bones joining the skull had to be thin. That makes skulls fragile — easily crushed during fossilization, like a soda can underfoot — while a solid femur, built to carry the body, survives far more often.

Preservation bias

Paleontologists call it preservation bias: some things are simply more likely to become fossils than others.

• Hard parts like bones decay more slowly than soft parts like organs, so bones are the most common fossils.
• Sturdy bones outlast delicate ones — a femur over a skull.
• Big animals’ bones resist decay better, there is more of them to preserve, and they are easier to spot in a landscape.

Why the Carnegie Quarry is different

In the Late Jurassic, northeastern Utah was a wide, flat floodplain crossed by a river. Fossil freshwater mollusks in the Morrison sandstone suggest the quarry’s dinosaurs died during droughts. Their bones show they lay decaying along the riverbank when the rains returned; the swollen river swept their remains downstream until they caught on a bend or a sandbar and were quickly buried.

That fast burial slowed decay, and the sand grains on the riverbed were just the right size to fill even the most delicate spaces — from large dinosaur skulls to the bones of tiny animals such as Glirodon grandis and Hoplosuchus kayi.

The result:

• 14 dinosaur skulls found in the first excavations, 1909–1924
• more skulls found when the quarry’s remains — today’s “Wall of Bones” — were excavated from the 1950s to the 1990s
• two Camarasaurus skulls still on the wall, and a nearly complete Allosaurus fragilis skull in a glass case in the Quarry Exhibit Hall

What skulls tell us

Extra holes. Skull openings other than the eye sockets and nostrils are called fenestrae. Scientists group animals into lineages, or clades, partly by how many they have. Most reptiles belong to the Sauropsida; within it, crocodiles, dinosaurs, birds and lepidosaurs sit in the Diapsida, which usually have two sets of fenestrae — behind the eye and above it — where jaw-closing muscles attach. Dinosaurs had a third set, in front of the eye, thought to make room for air sacs in their breathing system. Some diapsids, like modern birds, have since lost fenestrae but keep the group name from their ancestors.

Teeth that don’t fit the mammal rules. In mammals, sharp teeth usually mean meat and flat teeth mean plants. But a reptile’s teeth are usually all the same shape, because reptiles swallow food whole rather than chewing — and dinosaurs seem to have done the same.

Endless teeth. Plant eaters wore their teeth down tearing and swallowing plants all day, and meat eaters used theirs to slash and hold struggling prey. But dinosaurs never ran out: whenever one lost a tooth, a new one grew in its place, as often as needed throughout life.

Sources

Based on "Why Dinosaur Skulls Are Rare," National Park Service (Dinosaur National Monument); a work of the United States government in the public domain. The source’s aside that lepidosaurs include “all modern reptiles” conflicts with its own listing of crocodiles as a separate group, so it is left out.