The Aerosteon saga, Part 1: Introduction and background

October 4, 2008

Pneumatic dorsal vertebrae of Aerosteon (Sereno et al. 2008:fig 7)

Big news this week: Sereno et al. (2008) described a new theropod, aptly named Aerosteon (literally, “air bone”), with pneumaticity out the wazoo: all through the vertebral column, even into the distal tail; in the cervical and dorsal ribs; in the gastralia; in the furcula; and in the ilium. This is huge news, and it’s free to the world at PLoS ONE. Pneumatic vertebrae and ribs are the norm in theropods and most sauropods (hence our interest here), but the axial elements of Aerosteon are extremely pneumatic. A pneumatic furcula was reported in the dromaeosaur Buitreraptor (Makovicky et al. 2005), but Aerosteon appears to be a basal tetanuran so it pushes furcular pneumaticity a good distance down the tree. Most exciting are the pneumatic ilium and gastralia. Ilial pneumaticity has been suspected in some sauropods and non-avian theropods but the evidence has been lacking until now; either the ilial chambers could not be traced to pneumatic foramina, or the suspected pneumatic foramina could not be shown to lead to internal cavities. Pneumatic gastralia are really wacky–according to the paper, it is the first discovery of pneumatized postcranial dermal bone, and I certainly don’t know of any other examples.

Why is this important? In extant birds, the furcula is only pneumatized by diverticula of the interclavicular air sac, and the ilia are only pneumatized by the abdominal air sacs, so the presence of big pneumatic foramina leading to big internal chambers in both the furcula and ilia of Aerosteon is evidence not just for bird-like air sacs, but specifically air sacs from both the cranial and caudal groups within the thorax that are responsible for the flow-through lung ventilation of birds. It’s pretty dynamite stuff.



We-ell . . . There is no question that the fossil material is pretty stunning and shows all the morphological features that Sereno et al. claim (and even some that they don’t–stay tuned for Part 2). But there are parts of the paper that I disagree with, and to understand why, I have to tell you a little about recent research on pneumaticity in sauropods and theropods. In this post and the next I’ll be discussing papers by Pat O’Connor and Leon Claessens, as well as my own; all of these are freely available at the links just provided. So, please, if you have a beef with anything I say below, go read all the relevant literature for yourself, weigh the evidence, and make up your own mind.

First, a brief sketch of what we’ve been up to. Except for the occasional weirdo (surveyed on the third page here), the only extant tetrapods with postcranial pneumaticity are birds. In birds, postcranial pneumaticity is the skeletal footprint of the lung/air sac system. So if we find postcranial pneumaticity in dinosaurs–say, sauropods, or Aerosteon–we can use the ‘rules’ from birds to make inferences about the morphology of the respiratory system. We can’t tell which way the air was blowing in the lungs, but we can tell the minimum extent of the pneumatic diverticula, and we can make some inferences about lung structure. All of the logic of this is really nicely and concisely laid out in O’Connor and Claessens (2005), which is only three pages of text, so if you want to know more, just go read it.

The hypothesis that sauropods and theropods had air sacs like that of birds has been opposed in a couple of ways: pneumaticity doesn’t tell us anything, and vertebral pneumaticity only indicates cervical air sacs. Neither of these counterarguments has gotten much traction, probably because they’re so easily falsified. Let’s have a look.

Historical Misconception #1: Pneumaticity Is Completely Uninformative

“Without integrating functional data into the study, the most that can be inferred from post-cranial pneumaticity in extinct animals is that, as pointed out by Owen (1856), the pneumatized bones received parts of the lung in the living animal… Because pneumaticity has no known functional role in ventilation or thermoregulation or metabolic rates, its usefulness as a hard-part correlate for lung structure and metabolism is, unfortunately, questionable.” (Farmer 2006, pp. 91-92)

Farmer does not distinguish here between inferences based on the presence of postcranial pneumaticity and inferences based on the distribution of postcranial pneumaticity. If all we know about a bone is that it is pneumatic, then she is correct in stating that the most we can conclude is that it was connected to the respiratory system in some way. (The thermoregulatory function of pneumaticity discussed by Seeley [1870] has been demonstrated for cranial pneumaticity [Warncke and Stork 1977] but not for postcranial pneumaticity [Witmer 1997, O'Connor 2006]). But the inference of cervical and abdominal air sacs in non-avian dinosaurs does not depend simply on the existence of postcranial pneumaticity. Rather, these inferences are based on patterns of postcranial pneumaticity that are diagnostic for specific air sacs.

Verdict: Fail.

Historical Misconception #2: Vertebral Pneumaticity Only Comes From Cervical Air Sacs

“Pneumatization of the vertebrae and ribs is invariably accomplished by diverticuli [sic] of the cervical air sacs (McLelland 1989a), which are located outside the trunk and contribute little, if anything, to the respiratory air flow (Scheid and Piiper 1989). Presence of pneumatized vertebrae in non-avian dinosaurs therefore only speaks of the possible presence of such nonrespiratory diverticuli [sic], and cannot be regarded as indicative of an extensive, avian-style abdominal air-sac system.” (Ruben et al. 2003, p. 153)

This remarkable statement is repeated pretty much verbatim by Chinsamy and Hillenius (2004) and Hillenius and Ruben (2004). What’s remarkable about it is that is so thoroughly inaccurate. People have known for more than 100 years that the posterior parts of the vertebral column of birds are pneumatized by diverticula of the abdominal air sacs, and said as much in many papers–for example, Muller (1908), Cover (1953), King (1966, 1975), Duncker (1971), Hogg (1984a, b), and Bezuidenhout et al. (1999). Still, if McLelland said that the vertebrae and ribs are “invariably” pneumatized by diverticula of the cervical air sacs, it’s not their bad, right?

Okay, first, McLelland (1989) is a review paper and presents no new data (this will become really important later on, when we get back to Aerosteon). Second, here’s what McLelland actually said:

“What can be stated with certainty is that in birds generally the cervical air sac aerates the cervical and thoracic vertebrae (Fig. 5. 22) and the vertebral ribs; the clavicular air sac aerates the sternum, sternal ribs, pectoral girdle and humerus (Fig. 5. 23); and the abdominal air sac aerates the synsacrum, pelvis and femur.” (pp. 271-272)

By listing the synsacrum and pelvis separately, McLelland clearly meant that the synsacral vertebrae are pneumatized by the abdominal air sac, and this is confirmed by the sources he cited elsewhere: Hogg (1984a, b).

So Ruben et al. (2003)–and those who recycled that text–were relying not on any of their own research, or any primary research at all, but on a single review paper that actually says exactly the opposite of what they claim it does, based on other primary research papers (those by Hogg) that themselves say the same (opposite) thing.

Verdict: EPIC FAIL.

The Brave New Post-2005 World

He said, she said, yadda yadda. There are lots of inaccuracies in the literature, and it’s not like birds are extrasolar planets. If we want to know what is going on inside them, we can just look. That’s what O’Connor and Claessens (2005) did, by injecting and dissecting 200+ birds representing 19 avian orders. Know what they found? The cervical diverticula do not EVER go farther down the vertebral column than the middle of the thorax. NEVER EVER. So if you find pneumatic vertebrae in the posterior dorsals, sacrum, or tail, it’s pretty likely that they were pneumatized by diverticula of the abdominal air sacs.

I say “pretty likely” because it’s always possible that dinosaurian diverticula worked differently, and that the air that got into the posterior part of the vertebral column actually came from the cervical air sacs, or the lungs directly, or from arse gills, or possibly magic rocks. We can imagine lots of ways for air to get into the back half of the vertebral column, but the only one that we’ve ever seen work in a tetrapod* is diverticula of the abdominal air sacs. Dinosaurs may have worked differently, and had wacky cervical diverticula or arse gills or whatever. But those are not the obvious choices, and we don’t have any evidence for them; all the available evidence points to abdominal air sacs.

*Some osteoglossomorph fishes pneumatize the vertebral column from the swimbladder–strange but true!

So, great. The old confusion has been swept away by a blood-dimmed tide of bird carcasses and good science. Pneumatization of the posterior vertebral column implies abdominal air sacs. The combination of pneumaticity in the neck, trunk, sacrum, and even tail of many theropods and sauropods shows that both cervical and abdominal air sacs were present (as in Apatosaurus, above), which means air sacs both anterior and posterior to the lungs, which means that most (maybe all) saurischians had at least some of the gear they would need for flow-through breathing like that of birds (O’Connor and Claessens 2005, O’Connor 2006, Wedel 2007).

And yea, verily, anatomical accuracy and scientific clarity reigned throughout the land . . .

. . . until now.



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13 Responses to “The Aerosteon saga, Part 1: Introduction and background”

  1. William Miller Says:

    This is really, really interesting!

    And I didn’t know about the pneumatic hyoid in Alouatta. How many vertebrates have not had their bones checked for postcranial skeletal pneumaticity? Could there be other pneumatic bones hiding?

  2. [...] 5, 2008 In the last post I introduced Aerosteon, which has been touted as providing the first solid evidence for bird-like [...]

  3. Allen Hazen Says:

    I note that, in the paper you link to for referrences of pneumaticization in other tetrapods, you mention “post cranial” pn. Is that because cranial pneumatizxation is too common & widespread to be worth commenting on? Are the frontal and maxillary sinuses flu sufferers know about diverticula of roughly the same sort as the ones you are interested in? And all the spaces in the upper regions of an elephant’s skull?

  4. [...] pneumaticity and raises some criticisms of Sereno et al.’s paper describing Aerosteon in two posts over on [...]

  5. Nathan Myers Says:

    Interesting use of “historical” here: 2003 and 2005. It’s as if the forces of error and misconception have only lately succeeded in confusing the record in this area after curiously neglecting it for too long. The events must indicate the topic’s new importance in The Deceiver’s plans.

  6. Matt Wedel Says:

    How many vertebrates have not had their bones checked for postcranial skeletal pneumaticity? Could there be other pneumatic bones hiding?

    It’s possible. A surprising amount of what we know about animal anatomy comes from British and especially German anatomists of the 19th century, who were tearing down critters like nobody’s business. Still, they didn’t hit everything, and surprising new anatomical features are being found all the time, even in very familiar animals. My favorite recent example is the big blood-filled space in the lower jaw of the flamingo.

    I note that, in the paper you link to for referrences of pneumaticization in other tetrapods, you mention “post cranial” pn. Is that because cranial pneumatizxation is too common & widespread to be worth commenting on?

    Pretty much. Even us stinkin’ mammals have cranial pneumaticity.

    Seriously, there is something interesting in the fact that the only groups that have really gone in for cranial pneumaticity are archosaurs (including both crocs and birds) and mammals. But since the demise of the vertebral theory of the skull . . . (moment of silence) . . . I can only get so excited about heads.

    Are the frontal and maxillary sinuses flu sufferers know about diverticula of roughly the same sort as the ones you are interested in?

    Yeah, roughly, and maybe more than roughly. There is a concise and very readable summary of what’s known about the possible function of skeletal pneumaticity, cranial and post-, on pages 53-59 of the Witmer (1997) paper linked above (warning, it’s 20+ megs, so you might want to shut down Resident Metal Halostrike III first).

    I say “more than roughly” because Cranial Pneumaticity Came First. It will be really interesting to see how much of the control of postcranial pneumatization at the level of genes, hormones, and cell signaling is evolutionarily borrowed from pre-existing cranial developmental pathways. Anyone want to give me a million bucks?

    And all the spaces in the upper regions of an elephant’s skull?

    Oh yeah. At the museum in Munich there is a skull of a steppe mammoth with almost all of the cortical bone blown off, so all you can see is pneumatic cavities. It’s downright awe-inspiring.

    You know, for a head.

  7. In a word: Brilliant

  8. [...] 12, 2008 What with all the fuss over Aerostron, it seems that we missed SV-POW!’s first birthday. Yes, it’s been just over a year [...]

  9. [...] The Aerosteon saga, Part 1: Introduction and background Ottobre 4, 2008 [...]

  10. [...] is a two-part critical analysis of the Aerosteon that I found [...]

  11. [...] that we see in birds. This is what Pat O’Connor and Leon Claessens established so firmly with their work on mapping parts of the respiratory system to skeletal domains in [...]

  12. [...] The world of dinosaur palaeontology is probably just as aware of Matt’s series of Aerosteon response articles here on SV-POW! as it would be if he’d put those together into a paper that [...]

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