Did Triceratops Slouch or Stand Tall?

For decades, paleontologists have been debating how Triceratops stood. Did old “three-horned face” hold its forelimbs straight up and down like other dinosaurs, or did the horned dinosaur waddle along with its elbows out to the side? The dinosaur’s skeleton has not delivered an unambiguous answer. The critical articulation of the upper arm and shoulder can be reconstructed in a range of positions, and so it’s little wonder that different researchers have arrived at disparate conclusions.

According to paleontologist John Hutchinson of The Royal Veterinary College in London, reconstructing how dinosaurs like Triceratops walked from bones alone is very tricky. “Bones themselves tell you only a bit about locomotion or posture,” Hutchinson said. “Soft tissues and the nervous system have a huge role in such behaviors, so paleontology has long struggled to get past those unknown soft tissues to tackle the cool questions about behavior.” The few known ceratopsid footprints haven’t helped that much—the identities of the trackmakers are often ambiguous, and it can be difficult to relate the pattern in the tracks with the anatomy of an unknown species. “To me,” Hutchinson said, “biomechanics is the best way to integrate all those data and test questions about behavior.”

In a paper published last week in the Proceedings of the Royal Society B, Hutchinson and Shin-ichi Fujiwara of the University of Tokyo proposed a new biomechanical technique to test some of the previously proposed ideas about Triceratops posture. Instead of using skeletal articulation alone as a guide, Hutchinson said, “basically we estimated the moment arms (leverages) of key elbow muscles in three dimensions, using landmarks on the bones.” This method, he explained, allowed the researchers to “determine how the elbow is mechanically supported against gravity.” Fujiwara and Hutchinson then measured a variety of modern animals and determined that the moment arms reflected particular postures. This relationship, they conclude, could be used to study prehistoric creatures. “That gave us extra confidence that we could apply the method to extinct animals, so off we went to study some nicely preserved fossils that could illuminate controversial forelimb postures,” Hutchinson said.

Fujiwara and Hutchinson incorporated several different sorts of extinct creature in their study, including Triceratops. They found that the dinosaur probably had upright forelimbs that were held close to the body—a conclusion also supported by evidence from the dinosaur’s anatomy, scaling patterns and rare footprints attributed to horned dinosaurs. Nevertheless, Hutchinson explained that other evidence might indicate a semi-erect, sprawling forelimb posture. “I don’t think the controversy is over by any means,” he said, “but our method tips the scales closer to the upright end of the spectrum.”

Triceratops wasn’t the only dinosaur in the study. Fujiwara and Hutchinson also studied Protoceratops—a much smaller ceratopsian from Cretaceous Mongolia—to see how the forelimbs of horned dinosaurs might have changed with size. The results were ambiguous, Hutchinson says, but Protoceratops may have “had fairly upright forelimbs, albeit maybe no so much as Triceratops did.” This small ceratopsian, therefore, “would be a reasonable approximation of what the distant, smaller ancestor of Triceratops may have stood or moved like,” although Hutchinson stressed the need to obtain additional details from a wider range of horned dinosaurs.

Hutchinson also noted that the technique utilized in the study is “a new tool in the arsenal of techniques for reconstructing limb postures in land tetrapods.” The method can be extended to a variety of extinct animals with controversial limb postures. In addition to the dinosaurs, Hutchinson explained:

e applied our method to desmostylians (giant hippo/pig-like aquatic mammals), whose forelimb poses have been the subject of a controversy similar to that for ceratopsids. We found quite similar results for 2 genera of desmostylians as for Triceratops—they too seem to have been more upright on land. Similarly, the pterodactyloid Anhanguera emerged as having upright forelimbs, although our analysis cannot address the controversy over whether it was a biped or quadruped, so these results need to be taken with a grain of salt. As a reality check, we also applied the method to a recently extinct thylacine, which video and photos tell us was upright, and obtained that result, which was reassuring.

Perhaps, by combing this technique with other lines of evidence, paleontologists will eventually solve the mystery of the Triceratops slouch.

References:

Fujiwara, S., & Hutchinson, J. (2012). Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2012.0190

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.