July 20, 2009
Everybody knows that dinosaurs like Tyrannosaurus and Velociraptor were meat eaters, but what might come as a shock is that some of their close relatives often ate plants.
When I was a kid things were simple. Theropod dinosaurs were meat-eaters and all the rest were plant-eaters. Since the 1980s, though, numerous discoveries have shed light on a group of coelurosaurs (the group of theropods to which Tyrannosaurus and Velociraptor also belong) called therizinosaurs. These dinosaurs had beaks, small heads, long necks, barrel-shaped bodies, and long arms tipped with huge claws, yet some of them had feathers and they were close relatives of the dinosaurs that gave rise to the first birds. One such therizinosaur was Nothronychus, and a nearly complete skeleton of this dinosaur was just announced in the journal Proceedings of the Royal Society B.
The approximately 90-million-year-old Nothronychus was originally described from bones found in the American southwest in 2001. There was enough of it left to tell that it was a therizinosaur, but this new skeleton, found in southern Utah, is much more extensive. As such it provides for better evolutionary comparisons not only to other therizinosaurs, but to other coelurosaurs as well.
What the researchers found was that the therizinosaurs were rooted near the base of the maniraptoran family tree (the maniraptorans being that group of coelurosaurs that contains the ornithomimosaurs, dromaeosaurs, birds, and a few others). In other words, the earliest members of the therizinosaur lineage split off before the first members of other maniraptoran dinosaurs did. What makes this especially interesting is that this placement seems to reveal some important shifts in coelurosaur evolution.
It appears that the earliest coelurosaurs (including the ancestors of the tyrannosaurs) were hypercarnivorous, or that they only ate meat. Interestingly, though, several groups of dinosaurs near the base of the maniraptoran family tree show adaptations for plant eating; the dromaeosaurs (or “raptors”) are the only members of this larger group that appear to have exclusively eaten meat. Rather than being an abnormality, herbivory might have been rather common among the maniraptorans.
What this suggests is that the last common ancestor of the maniraptoran dinosaurs might have been herbivorous or omnivorous. This hypothesis will have to be tested and re-tested as more fossil evidence comes to light, but if the researchers are correct then sometime around 160 million years ago there lived an omnivorous or herbivorous dinosaur ancestral to all maniraptorans. Rather than being the rule, predators like Velociraptor might have been oddballs compared to the rest of their close kin.
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