Diggin’ dinos

Three enigmatic structures embedded in 106-million-year-old Australian rocks may be the remains of dinosaur burrows that were filled in by an ancient flood, a new study suggests. If that’s true, these fossils represent the oldest known dinosaur burrows and the first found outside North America.

The purported burrows sit within three meters of each other in now-hardened material that was laid down as sandy sediment along a stream bank near what is now Australia’s southern coast, says Anthony Martin, a paleontologist at Emory University in Atlanta who described the fossils online and in an upcoming Cretaceous Research. Recent erosion has erased much of two of the structures, but what remains of the largest one is remarkably similar to the remains of dinosaur burrows discovered in North America a few years ago (SN: 10/27/07, p. 259).

The largest of the remaining structures is a 2.1-meter-long, stomach-shaped clump of rock with a narrow, slightly curved protrusion and appears to represent a complete burrow. The broad end of the clump is 1 meter long and 40 centimeters wide, and its lowermost 30 centimeters is composed of large pebbles, Martin says. The rest of the structure, including the protrusion, is made of coarse-grained sandstone. Because the clump is embedded in a different material, a fine-grained sandstone, Martin suggests that it represents a chamber that once was open but was then filled with material washed in by two separate floods.

Of the other two structures, only the S-shaped necks of fine-grained sandstone remain. Martin suggests these necks are the filled-in remnants of the burrows’ entrance tunnels. Because the now-clogged burrows sit at the same level and are of similar size and orientation, they’re unlikely to be random rock formations, he contends. And because all three rock clumps are made of the same material, the tunnels were likely filled in by the same flood.

Other possible explanations for the structures exist, “but I don’t know if any of those alternate explanations are better,” says David J. Varricchio, a paleontologist at Montana State University in Bozeman. Characteristics of the clumps suggest that the once-open chambers certainly weren’t carved by erosion, he notes.

Unlike similar burrows found in North America, the Australian burrows don’t contain remains of the creatures that supposedly dug them. Evidence from rocks nearby, however, hints that small bipedal dinosaurs of the right size (1 meter long and 20 kilograms) lived in the area at the time. These dinosaurs are probably closely related to the North American burrow makers, Martin says. All other animals inhabiting the area at that time, including some mammals, were either too small to have dug the burrows or too large to have fit inside.

Besides hiding from predators in the chambers, the Australian dinosaurs may also have used their lairs to escape harsh conditions, says Martin. At the time, the site was at 77Ë S latitude — well within the Antarctic Circle — and wintertime was dark around the clock and cold despite a global climate warmer than today’s.

Because dinosaur fossils found in Australia are typically fragmentary and poorly preserved, the new find “has made us very excited,” says Tom Rich, a paleontologist at Museum Victoria in Melbourne, Australia. The skeleton of any dinosaur that happened to be buried in its burrow by floodborne sediment stands a better chance than usual of being complete and fairly well preserved, he notes. For this reason, filled-in burrows provide paleontologists with a ready-made place to look for fossils, he adds.