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RE: Belly Ribs and air sacs

Stephen V. Cole wrote-

Ten years ago (the last time I bought a dinosaur book and went to Dinofest) Bakker was talking about finding belly ribs on a theropod (Trex?). Was this ever confirmed?

I'm an engineer, not a paleontologist, but as I understand it....

Mammals and reptiles have elastic lungs. Air sucked in, air spewed out via the same plumbing. Two way street, less efficient.

Birds have rigid lungs. Air is pulled into air sacks in the belly, then blow n out through different plumbing. I'm not sure if the air goes through the lungs on the inbound or outbound stroke, but the point is that air only goes one way through the lungs, which is more efficient and needed to fly.

Belly ribs are needed to make air sacs work. If T-rex (or raptors or other theropods) had belly ribs, they had air sacs, and if they have air sacs, they are warm blooded. Belly ribs are mostly cartillage (except on Sauropods due to size) and dont fossilize well, so the idea that theopods used bird-type lungs is controversial and not accepted by eveyone until proven.

In the ten years i was one, did anybody ever prove if theropods had air sacs?

Usually, "belly ribs" refers to gastralia, which are primitive for dinosaurs and known in all theropods except modern birds and their closest relatives (e.g. Gansus). Basal sauropodomorphs also have gastralia, but these are lost in taxa more derived than Gongxianosaurus. No ornithischians had gastralia. The tuatara and crocodilians also have gastralia, as did many extinct "amphibians" and reptiles. Living amphibians, mammals, turtles and squamates have lost them. Obviously they have little to do with airsacs or endothermy. A good source on gastralia is Claessens (2004).

However, your statements imply you're speaking about sternal ribs. Sternal ribs are usually cartilaginous, but they are ossified in some dinosaurs. These include Dilong, Pelecanimimus and most maniraptorans (except Jinfengopteryx, archaeopterygids and Sapeornis). Though I'm less familiar with non-theropods, I know Apatosaurus, Eobrontosaurus and some "hypsilophodonts" have ossified sternal ribs as well. These were first reported in non-bird theropods in the mid-1980's by Barsbold (in Velociraptor), but not very popularly known until 1999. They aren't known in tyrannosaurids (and were probably cartilaginous), but were thought to be preserved in Gorgosaurus (Lambe, 1917) until a Tyrannosaurus specimen (FMNH PR 2081) showed them to be fused gastralia. Perhaps you heard an early report on this specimen? Sternal ribs attach the dorsal ribs to the sternum and are thus useful for ventilating airsacs, but they don't necessarily serve this purpose. For example, some mammals have ossified sternal ribs, but lack airsacs of course.

The presence of airsacs is inferred from fossae and pneumatophores on bones which resemble their counterparts in modern birds. These are known for almost all theropods, Thecodontosaurus and most sauropods. They are absent in ornithischians and most basal sauropodomorphs. Although recently claimed to be present in some basal archosauromorphs, these instances have been questioned. However, they are known from pterosaurs and apparently the theropod-like crurotarsan Effigia. Some living reptiles have pulmonary extensions that don't pneumatize bones and are more poorly and differently developed than those of birds. The only paleontologists who doubt theropods had airsacs are Ruben et al., whose evidence has been firmly debunked by Paul, O'Connor and others.

As most theropods and sauropods had airsacs, but lacked ossified sternal ribs, the latter are not necessary for the former to work. Similarly, the "hypsilophodonts" with ossified sternal ribs show no sign of airsacs, so the former does not indicate the latter. O'Connor shows there is no evidence airsacs assist endothermy in any case, so none of these features indicate dinosaurs were warm-blooded.

Mickey Mortimer