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New skeletons from the Age of Dinosaurs answer century-old questions

TypothoraxFlipped_t.jpgBOONE—Appalachian State University geology students helped retrieve a nearly complete fossilized skeleton of a reptile-like dinosaur called Typothorax while participating in a field study in New Mexico in 2006. The fossil is now helping scientists fill in the gaps about the ancient reptile.

More than 100 years ago, paleontologist E.D. Cope of “Dinosaur Wars” fame found a few fragmentary bones of a reptile in the deserts of New Mexico. He named the reptile Typothorax, which belongs to a group of reptiles called aetosaurs. A century later, it remained something of a mystery, known mainly from pieces of armor, a few limb bones, and some sections of tail.

Typothorax coccinarum artwork.jpgReconstruction of the aetosaur, Typothorax coccinarum based on skeletons from the Bull Canyon Formation of eastern New Mexico. (artwork by Matt Celeskey)

Typothorax coccinarum_t2.jpgA team collects a specimen of Typothorax coccinarum in 2006.

The most recent Typothorax specimens were found by volunteers at the New Mexico Museum of Natural History and Science: the first, by Paul Sealey in the late 1980s, and the second by retired US Air Force major Scott Sucher on the so-called Badlands Ranch in 2005.

The Appalachian students helped excavate the second specimen in 2006 while on a field study led by Dr. Andy Heckert, an assistant professor of geology at Appalachian.

In the latest issue of the Journal of Vertebrate Paleontology, paleontologists are finally revealing what Typothorax really looked like, how large it was, how it walked, and myriad other questions. Typothorax is also one of the last large herbivores to evolve in the Late Triassic period, before dinosaurs would come to dominate the planet.

Heckert, lead author of the study, regards the Typothorax as an “animal designed by a committee combining a crocodile with a cow and armadillo.”

Heckert was a new professor at Appalachian when he led the student trip to work with volunteers at the New Mexico Museum of Natural History and Science (NMMNHS). Heckert worked at the museum for three years after completing his Ph.D. The students excavated and retrieved a 700-pound block  of material containing the articulated partial skeleton of Typothorax that was discovered in 2005 but not excavated. The specimen was one of the best fossils of Typothorax ever recovered, Heckert said at the time.

The fossil was cleaned and assembled by another NMMNHS volunteer, Bill Ortman, who spent years cleaning and gluing the second specimen back together to make this research possible.

“We now know that some previously established ideas about these animals were mistaken,” Heckert said. “For the first time we can get a realistic estimate of the size of these animals, and at only 2.5m (~7 feet) and about 100kg (225 lb). they are not as large as previously thought.”

The new specimens show that the body was completely enclosed in bands of bony armor, similar to that of an armadillo, even to the extent of having a series of tiny overlapping plates extending down each leg, and onto the hands and feet. The front limbs apparently sprawled, but the hind limbs were much larger and upright.

“I doubt Professor Cope would have ever imagined this animal quite this way,” said Heckert. “One really interesting feature is that the front half of the skeleton is so slender we probably would have thought it belonged to a juvenile if it weren’t articulated to the rest of the skeleton.”

Aetosaurs were widespread during Late Triassic times (230 – 200 million years ago). The largest species of aetosaur grew up to five meters long, although the two new specimens, representing a species called Typothorax coccinarum, were smaller – growing up to 2.5 meters long. All were covered by a protective armor of overlapping bony plates, but some species sported massive spikes protecting the neck region – an additional deterrent to any hungry predator. Fragments of the characteristic bony armor are well known to paleontologists, but complete specimens of any aetosaur are very rare and none were known for Typothorax prior to the discovery of these specimens. The ornamentation on the plates varies from species to species and paleontologists have long recognized them as a diverse and important group of plant eaters living alongside some of the earliest dinosaurs.

The new specimens are also providing new information about the way these animals moved. Fossil skeletons with complete hands and feet are so rarely preserved that it is very difficult to confidently match a skeleton to the maker of any particular trackway. However, the preserved feet in the new specimens demonstrate for the first time that trackways known as Brachychirotherium were almost certainly made by aetosaurs.

Brachychirotherium tracks are known from various localities around the world, and they are an almost perfect match to the arrangement of bones in the aetosaur foot,” said Dr. Spencer Lucas, curator at the New Mexico Museum of Natural History, where the specimens are now on display, and another member of the research team. “We now know that the front legs of aetosaurs sprawled to the sides, but their back legs were more robust and pillar-like.” With their short and stubby necks, blunt-nosed skulls, and small leaf-shaped teeth, these distant relatives of crocodiles, may also have grubbed around in the soil looking for succulent roots.

“The important contribution of amateurs to our science cannot be underestimated,” Lucas said of the work of the Appalachian students and others. “As the Badlands erode, we look forward to many more exciting new finds that will contribute to our understanding of the world at this important time in its history.”