what makes a dino-sore? – calling him a birdbrain harry jerison (ucla)

What Makes a Dino-sore? – Calling Him a Birdbrain

Harry Jerison (UCLA)

<hjerison@ucla.edu>

In the first graph you’ll see why dinosaurs should feel flattered to be called

birdbrains.

Minimum convex polygons containing presently available data on living amniotes (mammals, birds, and reptiles). Adding data on dinosaur points suggests an extended reptilian polygon (dashed lines). Earliest bird (Archaeopteryx) is at lower edge of living birds. Data on dinosaur genera (estimated body weights and brain weights): Protoceratops (200kg/28g); Camptosaurus (400kg/23g), Kentrosaurus (780kg/24g), Euoplocephalus (1,900kg/41g), Iguanodon (2,100kg/125g), Allosaurus (2,300kg/168g), Stegosaurus 3,100kg/22.5g, Edmontosaurus (6,200kg/200g), Tyrannosaurus (7,400kg/202g), Triceratops (9,000kg/72g); Diplodocus (19,000kg/57g), Brachiosaurus (78,300kg/186g).

Why is brain size a useful measure? In mammals, it estimates information-processing capacity. Is brain size similarly useful in birds and reptiles? We don’t know, but it’s a reasonable guess.

Edmontosaurus, the first dinosaur “brain” in the show.

Next, a bad drawing of Edmontosaurus, a six ton duck-bill Cretaceous dinosaur from Montana. It lived there about 70 million years ago.

Next will be Maiasaura,a dinosaur that left fossil eggs in Montana, and was about the same size and shape as Edmontosaurus. Jack Horner (Museum of the Rockies) dug up both dinosaurs. He figured that if Maia laid eggs it had to be a she, so he gave her a feminine name.

Maiasaura

3D scan of a Maiasaura model, which was scaled up to estimate Edmontosaurus body weight for the graph.

Brain-Body Space in Amniotes

The next dinosaur, Troodon, weighed about 45 kg and also acted for Spielberg and Lucas in Jurassic Park III.

Its “brain” is on the next slide.

Troodon's brain - an endocast of course - compared to an ostrich and an albatross endocast.

When I showed Bill Hodos the Troodon endocast he thought he saw a suggestion of a Wulst. Here is a view of the albatross endocast with its prominent Wulst. Troodon is next.

Snapshot of 3D image of Troodon endocast.

The Field Museum in Chicago, talked the Boeing company into doing a CT scan of their Tyrannosaurus rex (“Sue”) skull. Here

is what turned up.

Endocast and skull of Tyrannosaurus rex. A. Dorsal view of endocast. B. CT section: olb, olfactory bulb area; se, sphenethmoid bone; orb, orbit; I, olfactory tract; fm, foramen magnum. C. Reconstruction of skull from CT scans, showing level of CT section of B. (From Brochu, C. Journal of Vertebrate Paleontology, 20:1-6, 2000. By permission.)

How should we interpret the very large T rex "olfactory bulbs" on the endocast? They are at least four times the size of the "brain" itself. In living birds, the kiwi has the biggest olfactory bulbs relative to its brain’ size, and they are about 1/5th the brain volume. The turkey vulture's is about 1/10th brain volume.

The next slide shows the situation in living mammals. The biggest olfactory bulbs (relative size) in mammals are no more than about 10 % of the whole brain. Best guess is T rex's olfactory bulbs didn't fill the space for them in its head, and its olfaction is interpretable as in living vertebrates.

We need numbers involving brain size in more species. These help us analyze the fossil evidence for brain evolution. For many major structures in the brain we can estimate sizes from gross brain size. The brain hangs together.

(Adapted from Braitenberg, V. & Schüz, A. (1998 1991. Anatomy of the Cortex: Statistics and Geometry. Berlin, Heidelberg: Springer.)