Published online 29 October 2008 | Nature | doi:10.1038/news.2008.1192


How does your dinosaur smell?

Quite well, on the whole.

T. rexTyrannosaurus rex had the nose of a bloodhound.Royal Tyrrell Museum

An extensive survey of dinosaur skulls adds to the evidence that Tyrannosaurus rex had a killer sense of smell, and finds that the earliest known birds were much better at sniffing out prey than their descendents.

Using fossil evidence to figure out how extinct animals looked and moved is difficult enough — assessing their senses is harder still, and questions about dinosaurs' sense of smell have been particularly difficult to answer. Comparisons between extinct and modern counterparts that fill similar ecological niches can yield some information, but to really know what an extinct animal could smell, palaeontologists need to look at the creature's olfactory bulb — the area of the brain dedicated to scent.

Although computed tomography (CT) X-ray scans of T. rex skulls, for example, have previously revealed substantial olfactory bulbs, the scientists behind this latest survey say that it is the first time that trends in smelling ability have been evaluated across a range of meat-eating dinosaurs.

Darla Zelenitsky at the University of Calgary in Alberta, Canada, François Therrie, curator of dinosaur palaeoecology at the Royal Tyrrell Museum in Drumheller, Alberta, and Yoshitsugu Kobayashi of Hokkaido University in Japan, assessed the size and shape of the olfactory bulb in 21 species of theropod — bipedal dinosaurs that were the ancestors of today's birds.

They took CT scans, along with casts and measurements of brain cases, to calculate an 'olfactory ratio' that compares the diameter of the olfactory bulb with the diameter of the brain itself. When adjusted for the body mass of the animal, this ratio gauges the creature's sense of smell.

Bloodhound senses

The team reports in the Proceedings of the Royal Society B1 that the theropods had a wide range of smelling abilities. Although direct comparisons with modern species are tricky at best, the findings suggest that Velociraptor and T. rex rivalled bloodhounds in their ability to smell prey, says Zelenitsky.

But that won't settle arguments about whether T. rex was a predator or a scavenger. "While this makes it clear that that these species had a better sense of smell than other theropods of the time, it does not support any arguments that Tyrannosaurus was a scavenger, as there are both predatory and scavenging birds today with large olfactory bulbs," says Zelenitsky.

Ornithomimid skullThis computed tomography (CT) X-ray scan of an ornithomimid skull suggests that the creature may not have hunted much meat.Yoshitsugu Kobayashi

The relatively small olfactory bulbs found in other theropods, such as oviraptorids (Citipati osmolskae) and the ostrich-like Ornithomimus edmontonensis — both toothless, beaked dinosaurs — supports theories that the two species were not particularly carnivorous and that plants may have comprised a big part of their diet.

Bird brain

Remarkably, Archaeopteryx, the earliest known flying bird, had a sense of smell that was quite average among the 21 theropods. "This was surprising because modern birds are thought, with few exceptions, to have poor senses of smell, yet clearly Archaeopteryx had an olfactory-bulb size typical of that of a small theropod dinosaur," comments Zelenitsky. This suggests that birds may have lost much of their sense of smell later in their evolution.

"This is a major step towards understanding dinosaur sensory function," says anatomist Lawrence Witmer at Ohio University in Athens. "With more work like this we should be able to have a very good idea of how these animals, saw, smelled, and felt the world around them." However, he points out that there are limits to how much CT scanning technique can reveal: "We shouldn't try to paint with too fine a brush — this is neurology without the neurons."

As a next step, "it would be fascinating to compare these data to similar measurements collected from a range of animals, especially lizards and ground-dwelling birds like the kiwi, which is both nocturnal and known to have an excellent sense of smell," says palaeontologist Philip Currie at the University of Alberta.

"Seeing all of these data in one place is pretty cool," he adds. "I can't wait to go through all of it with a fine-toothed comb and see if it confirms our thinking on who is related to who, and which lineages led to birds." 

  • References

    1. Zelenitsky D. K.[/author], [author]Therrie, F. & Kobayashi, Y. Proc. R. Soc. B doi:10.1098/rspb.2008.1075 (2008).
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