Skull Study Sheds Light On Dinosaur Diversity

The largest animals known to have walked theearth, sauropods were common in North America during the middle of thedinosaur era but were thought to have been pushed to extinction by morespecialized plant-eaters at the end of that era. New discoveries,however, are showing that one lineage of sauropods diversified at theend of the dinosaur era, University of Michigan paleontologist JeffreyWilson says.

Wilson's recent restudy and reconstruction of theskull of a Mongolian sauropod adds to a growing body of evidence forsauropod diversity at the end of the dinosaur era. Wilson described thereconstruction and the conclusions he drew from it in a paper publishedAug. 24 in the Journal of Systematic Palaeontology.

He based thereconstruction on two nearly complete skulls that were found in theGobi Desert in the 1950s and 1960s but whose evolutionary relationshipshave remained enigmatic. In the 1990s and early 2000s, Wilson restudiedthe skulls and found characteristics identifying them as skulls oftitanosaurs, a late surviving sauropod lineage.

"Titanosaurs,which were surprisingly common at the end of the dinosaur era, werereally the only sauropod lineage that flourished. All the others wentextinct," said Wilson, an assistant professor of geological sciencesand an assistant curator at the University of Michigan Museum ofPaleontology. But as prevalent as titanosaurs were, they left behindsurprisingly few skulls. Paleontologists have found plenty of othertitanosaur bones, providing a picture of a group of sauropods withspecialized limb bones.

Wilson began to appreciate the finerpoints of titanosaurs as a graduate student, when he and anotherstudent studied fossilized sauropod tracks and titanosaur limb anatomy.From those studies, Wilson concluded that unlike other sauropods,titanosaurs walked with their feet planted far from the middles oftheir bodies, an unusual style of "wide gauge" locomotion.

"Mostanimals walk with a narrow gauge, with their feet close to the midline,because it's energetically more efficient to walk that way. But somesauropod trackways tell us that a group of sauropods were walking witha new wide-gauge stance. We can identify characteristics of titanosaursthat would have allowed that stance, and we can tie the appearance ofthose features with the proliferation of wide gauge tracks everywherein the fossil record at the end of the dinosaur era." Wilson wonders ifthe change in locomotion—from typical sauropod narrow-gauge walking totitanosaur wide-gauge walking—corresponded to lifestyle changes, suchas different feeding habits. But without skulls to study, it has beenhard to draw conclusions about how and what titanosaurs ate.

Withhis work and that of researchers at the State University of New York,Stony Brook who announced the discovery of a complete titanosaurskeleton in 2001, sauropod specialists finally can start piecingtogether a clearer picture of the dinosaurs' lives.

One featureof the skulls is particularly intriguing. "They have elongate, sort ofhorse-like skulls with many openings and grooves on the outer surfaceof their snouts," said Wilson, who worked closely with U-M Museum ofPaleontology artist Bonnie Miljour over the course of a year preparingthe paper's many illustrations of the skull reconstruction. "Bloodvessels and nerves passed through these holes and may suggest anespecially sensitive snout. This probably had some role in feeding, butwe haven't investigated it at all."

Oddly, a group of distantlyrelated sauropods evolved a similarly grooved snout. "Apparently, thesetwo different branches of sauropods gravitated toward similaranatomical structures, perhaps because they were specialized for eatingcertain types of vegetation."