First dinosaurs may have been omnivores in the north hemisphere

Hips really can lie. In 1888, H. G. Seeley split the dinosaur family tree into two branches based on pelvic bones, but a new analysis suggests a complete rejig of early dinosaur types and challenges assumptions about where the first dinosaurs lived and what they ate.

“Maybe we shouldn’t just blindly accept this 130-year-old idea,” says Matthew Baron at the University of Cambridge. “Seeley’s idea, while it was brilliant for his time, it’s arguably archaic. It’s based on very few specimens.”

Seeley divided dinosaurs into “bird-hipped” animals, like the herbivorous Stegosaurus and Triceratops, and “reptile-hipped” ones, including carnivores like Tyrannosaurus rex and long-necked herbivores like Apatosaurus.

Instead of focusing on the pelvic bone, Baron and his team analysed 457 characteristics of 74 species
and found that 21 other anatomical features divide the dinosaurs differently.  Some of the common features shared between dinosaurs that were previously thought unrelated include straight thigh bones instead of the S-shaped ones found in some later dinosaurs, shoulder bones three times the length of the forelimb, and the first metatarsal – a long foot bone – not reaching the ankle joint.

“It sounds like trivial little features, very picayune things, but when you get that big a pile of bits of information just accumulating, you really do come up with a picture, a rearrangement,” says Kevin Padian, a palaeontologist at the University of California, Berkeley.

Based on these inherited features, the new tree puts T. rex and other theropods on one side with the old “bird-hipped” creatures, and leaves the sauropods like Apatosaurus grouped with those related to Herrerasaurus, a bipedal carnivore found in South America.

This tree structure hasn’t been suggested before, says Susannah Maidment, a palaeontologist at the University of Brighton, UK. However, people have long recognised that there are a striking number of similarities in how the different dinosaur lines evolved. “Those have always been considered to be convergent evolution. Perhaps they’re not,” she says.

Omnivorous ancestor

Because both new branches include carnivores and herbivores, Baron’s team concludes that the common ancestor of all dinosaurs may have been omnivorous.

The results also suggest that the cradle of dinosaur evolution may not have been South America, as has long been accepted. It could instead have been in the northern hemisphere since fossils of the oldest members of the new branches are found there.

“It’s as if somebody suddenly said, ‘Actually chickens are mammals, not birds’,” says Mike Benton, a palaeontologist at the University of Bristol, UK. “But we have a lot of knowledge of chickens, so we can say that’s ridiculous. In this case, we don’t have complete specimens for them all, so we can’t say with certainty it’s ridiculous.”

A few of the 21 features that link the new branches could be questioned, but if they hold up under scrutiny, the team’s case is “undeniable”, says Benton.

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