Tom Lehman's Dinosaur Fossil
Classified as New Genus, Species
Written 7.17.19 by Glenys Young
Back in 1985, Tom Lehman was a geology master's student at the University of Texas. He was out in the field studying rock layers in southwestern Big Bend National Park, working on the west side of Rattlesnake Mountain, when he discovered some badly weathered bones. He and his fellow graduate students collected them and took them back to Austin, but the fossils couldn't immediately be studied because they were stuck together.
Research in the 1990s revealed two arched nasal crests, so scientists believed the fossilized animal might have been a member of the Gryposaurus genus, which includes three different species of duck-billed dinosaurs, more formally known as hadrosaurids. But new research has shown Lehman's specimen was more primitive than Gryposaurus. On July 12, the fossil was announced in the Journal of Systematic Paleontology as representing a new genus and species, Aquilarhinus palimentus.
"New dinosaur species are being discovered and named all the time," said Lehman, now a professor in the Department of Geosciences at Texas Tech University. "This new species is particularly interesting because it is among the earlier representatives of the duck-billed dinosaurs, and it shows us what the narial crest, or nose, of these animals looked like before the wide variety of distinctive noses evolved in later species of duck-billed dinosaurs.
"In addition, instead of a duck bill, this animal had a shovel-shaped beak that suggests it had a unique way of feeding. Most of what we know about duck-billed dinosaurs is based on the wide variety of species found in Alberta, Canada, Montana and Utah. So, this new species also is interesting because it shows us that duck-billed dinosaurs that lived in Texas were different from those that lived farther north."
Hadrosaurids were the most common herbivorous dinosaurs at the end of the Mesozoic Era, and all had a similar-looking snout. The front of the jaws meet in a U-shape to support a cupped beak used for cropping plants. The beak of some species is broader than in others, but there was no evidence of a significantly different shape until Aquilarhinus was discovered. The lower jaws of Aquilarhinus meet in a peculiar W-shape, creating a wide, flattened scoop.
Around 80 million years ago, this particular dinosaur would have been shoveling through loose, wet sediment to scoop loosely rooted aquatic plants from the tidal marshes of an ancient delta, what is today the Chihuahuan Desert. When the dinosaur died, some of its bones were transported downstream by the tide and became lodged in vegetation. The twice-daily flow of the tide dropped silt that built up the bank of the channel around its body, fossilizing the bones in thick ironstone.
The jaw and other characteristics of the specimen show that it does not fit with the main group of duck-billed dinosaurs, known as Saurolophidae. It is more primitive than this group, suggesting there might have been a greater number of lineages than previously recognized that evolved before the great radiation that gave rise to the bewildering array of unadorned, solid and hollow-crested forms.
Most saurolophids had bony cranial crests of many different shapes and sizes. Aquilarhinus also sported a bony crest, albeit a simple one shaped like a humped nose. The discovery of a solid crest outside the major radiation of hadrosaurids supports the hypothesis that all crests derived from a common ancestor that had a simple, humped nose.