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RESPIRATORY TURBINATES (70)
Respiratory turbinates (RT) are folded structures of bone or cartilage in the sinus cavities, considered by some to be essential in conserving water and heat loss and thus obligatory for warm-blooded creatures. John Ruben presented data in late 1995 at the SVP meeting suggesting that dinosaurs did not have these structures. His work was based on a reconstructed Nanotyrannus skull. Not only did he not find turbinates but suggested that the nasal passage was too small for any to exist. Others suggest that the skull is actually a juvenile T. rex with a somewhat compressed nasal passage, and Horner presented counter evidence that a Lambeosaurus skull CT scan shows RT-like structures, but the scans were indistinct and the 'turbinates' incomplete at best and possibly incorrectly orientated. Greg Paul has strongly opposed the conclusions of Ruben, claiming that the only way to prove the lack of respiratory turbinates is to show that there is not enough room in the dinosaur nasal passages and that this has not been done; Ruben's conclusions being based on an illusion created by not drawing both dinosaur and bird to the same scale. Most theropods have very large heads, making the nasal passages appear small by comparison, whereas their actual volume equals or exceeds that of birds. Ruben's evidence has since been published in Science (Vol 273, pp 1204-7, August, 1996), and includes both Horner and Currie as co-authors. The authors have used modern medical imaging technology to examine the size and structure of the nasal cavities of the theropod dinosaurs Nanotyrannus and Ornithomimus and the hadrosaur Hypacrosaurus and compared these with 8 modern extant mammals, 3 birds and 4 reptiles. The results show that while all of the mammals and birds have RTs, none of the reptiles or dinosaurs did, and for a given body weight, reptiles (and dinosaurs) had smaller ( by a factor of 4) nasal cavity cross sections than birds and mammals - too small, in Ruben's opinion, to have included RTs at all, and also too small, again in his conclusion, to have allowed a high enough lung ventilation rate to support the higher metabolic rate required for an endothermic animal.
The paper concludes that a variety of Cretaceous dinosaurs possess crocodile or lizard-like nasal passages, too small to have accomodated RTs and endothermic lung ventilation rates, although conceding that their observations do not rule out the possibility that they may still have had routine metabolic rates somewhat greater than modern ectotherms .
Not surprisingly, the paper has come under attack from some sources for a number of reasons including sample size and selection. These objections may or may not be valid, but there are reasons for querying the conclusions drawn by Ruben and his co-authors. Initially it appears odd that there is no attempt to include any fossilized mammals or birds in the study, but apparently results published elsewhere indicate that RTs have been found in fossilized bird skulls back to 70 million years ago (but not earlier). This means, by the logic used to exclude endothermy in dinosaurs without RTs, that Archaeopteryx and many other early birds were also not endothermic - a very difficult option to accept given that they were feathered and could most likely fly!
Greg Paul has noted that Ruben measured only the narrow, horizontal front section of the nasal passage which is enclosed in bone and therefore fairly easy to measure. He points out that birds have a rear section as well that is not enclosed by bone and thus difficult to measure, but, importantly, also in a much wider part of the skull so that the nasal passage in this area may be up to 3 times wider. In birds this region contains a larger RT, and it is possible that in dinosaurs the same may be true. Paul is also critical of the use of a heavily reconstructed Dromaeosaur skull to picture the nasal passage, when fossil material from the nasal passage was not found. Closely related Velociraptor complete skulls do indeed show the second, larger part of the nasal passage, and it may have contained cartilaginous RTs that would be unlikely to fossilize.
Criticism can also be levelled at the conclusions regarding lung ventilation rates. As the paper itself notes, nasal passages must be significantly enlarged to make way for the presence of RTs. However, much of the increase in space is taken up by the turbinates themselves. In the absence of detailed measurement of the actual nasal passage free space it is impossible to determine whether the larger nasal cavity size in endotherms is related to lung ventilation rates at all, or is simply a reflection of the presence of RTs. The lung ventilation argument against endothermy must at this stage be considered unproven, so that the anti-endothermy evidence from the RT investigation rests solely on the absence of such structures in dinosaurs and modern ectotherms , and is based on the water conservation function of RTs. At least 2 modern groups of endotherms are known that also lack RTs - the Pelecaniforme diving birds, and whales. In both cases there are compensatory mechanisms to balance the additional water loss and heat transfer ability of the missing RTs.
The argument then reduces to this; RTs appear to be necessary in modern endotherms unless for some reason water loss during respiration can be counterbalanced by some other mechanism. In order for this argument to infer non-endothermy in dinosaurs it would be necessary to demonstrate that such mechanisms were not present. The lack of a detailed model of dinosaur water handling makes this impossible.
This subject has plainly not yet been decided one way or the other, and, like so many other aspects of extinct animals where information on soft tissue remains is unavailable, may never be definitively decided. The apparent lack of RTs in dinosaurs is evidence for lack of endothermy , but hardly strong enough to discard the body of evidence supporting it. The debate promises to continue for some
considerable time.
NOTE: Respiratory turbinates should not be confused with olfactory turbinates which some dinosaurs almost certainly did have but which were quite different in function.

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