Ghost Ranch Coelophysis Quarry: The 1947 Triassic Bonebed

The Ghost Ranch Coelophysis Quarry in northern New Mexico is one of the most famous dinosaur sites in North America. In June 1947 a field party from the American Museum of Natural History, led by Edwin H. Colbert, found a dense layer of skeletons of the small Triassic dinosaur Coelophysis. The bonebed has since yielded hundreds of individuals and turned a poorly known animal into one of the best-studied early dinosaurs.

Background: a dinosaur known from scraps

Coelophysis had been named decades earlier by Edward Drinker Cope, the combative palaeontologist of the American Bone Wars. In 1881 the collector David Baldwin sent Cope fragmentary bones from northern New Mexico, and Cope named the genus Coelophysis (“hollow form”) in 1889 after its hollow limb bones.

Cope’s material was too incomplete to show what the animal looked like. For more than half a century Coelophysis remained an obscure name attached to a few broken bones.

What happened in 1947

In the summer of 1947 Colbert led a small American Museum party to the Southwest to look for Triassic fossils. At Ghost Ranch, a property in the red-rock country near Abiquiú, his field assistant George Whitaker found bone fragments eroding from a hillside in June. Digging into the slope, the team uncovered not one skeleton but a layer packed with dozens of articulated Coelophysis.

The bones lay in the Chinle Formation, a sequence of Late Triassic river and floodplain deposits. The quarry is usually dated to roughly 210 million years ago, although estimates for this part of the Chinle vary by a few million years.

Because the skeletons were tangled together, the excavators did not try to separate them in the field. They cut large blocks of rock, wrapped them in plaster jackets and shipped them to museums. Over the following decades blocks went to the American Museum, the Carnegie Museum of Natural History, the Yale Peabody Museum, the New Mexico Museum of Natural History and Science and other institutions, as well as the Ruth Hall Museum of Paleontology at Ghost Ranch itself.

What the bonebed reveals

Coelophysis was a lightly built, two-legged predator about 3 metres long, with a long neck and tail, grasping hands and a narrow skull lined with serrated teeth. The Ghost Ranch sample contains animals of many sizes, which has allowed scientists to study growth and variation within a single early dinosaur species.

Researchers have identified two body types, often called “robust” and “gracile”, which some interpret as males and females. Others argue the differences reflect individual variation or growth, and the question remains open.

The concentration of so many animals of one species has also prompted suggestions that Coelophysis lived or gathered in groups. Because the bodies may have been brought together by water after death, however, the bonebed does not prove that they lived in herds or packs.

The cannibalism question

Colbert noted small bones inside the body cavities of two adult skeletons and interpreted them as juvenile Coelophysis, suggesting cannibalism. The idea became a staple of popular dinosaur books. In 2006 Sterling Nesbitt and colleagues re-examined the specimens and concluded that the bones belonged to small crocodylomorphs, distant relatives of crocodiles. Coelophysis ate small reptiles, but the evidence for cannibalism has largely collapsed.

Why they died together

The skeletons lie in the deposits of an ancient stream channel. Proposed explanations include a flash flood that drowned and buried a group of animals, or a drought that concentrated animals around shrinking water before a later flood swept their bodies together. The fine sediment and quick burial explain the exceptional preservation.

Consequences and legacy

The Ghost Ranch skeletons made Coelophysis a standard reference for early theropods, the group that later included Tyrannosaurus and gave rise to birds. Because Cope’s original bones were not diagnostic, scientists asked the International Commission on Zoological Nomenclature to fix the name to a Ghost Ranch skeleton instead. In 1996 the Commission designated a Ghost Ranch specimen as the neotype, the new reference specimen for the species.

Coelophysis became the state fossil of New Mexico in 1981. In 1998 a Coelophysis skull from the Carnegie Museum was carried into space on the Space Shuttle Endeavour, one of the few dinosaur fossils ever taken into orbit.

The quarry lived in the period after the Permian–Triassic extinction, when dinosaurs were still a minor part of ecosystems dominated by other reptiles. Roughly ten million years later the Triassic–Jurassic extinction would remove many of those competitors and open the way for dinosaurs to dominate.

Timeline

• 1881: David Baldwin collects fragmentary bones in northern New Mexico for Edward Drinker Cope.
• 1889: Cope names the genus Coelophysis.
• 1947, June: George Whitaker finds bones at Ghost Ranch; Colbert’s team uncovers a mass of skeletons.
• 1947–1948: Large plaster-jacketed blocks are removed and shipped to museums.
• 1981: Coelophysis is adopted as the state fossil of New Mexico.
• 1996: The ICZN designates a Ghost Ranch specimen as the neotype of Coelophysis bauri.
• 1998: A Coelophysis skull travels into space aboard the Space Shuttle Endeavour.
• 2006: Nesbitt and colleagues show that the supposed cannibalised juveniles are crocodylomorphs.

Why it matters

The Ghost Ranch quarry gave science an entire population of one of the earliest dinosaurs, preserved in exceptional detail. It shows how small predatory dinosaurs were built, how they grew and what they ate in a world where dinosaurs were only beginning their rise. Its history also shows how long-standing ideas, such as cannibalism, can be overturned by careful re-examination.