Your coccyx is contemptible

April 8, 2008

Those of you who have been paying attention to my recent posts will have pretty much known this was coming. I’d hate to disappoint you, so here it is:

Apatosaurus ajax NSMT-PV 20375, first caudal

What you’re looking at here is the first caudal vertebra (i.e. the first tail bone) of Apatosaurus ajax, the newish specimen NSMT-PV 20375 described by Upchurch et al. (2005). The drawings are all from the plates at the end of that lavishly illustrated paper: all I’ve done is composite them. The top row, from left to right, shows the vertebra in anterior, left lateral, posterior and right lateral views. Below the left lateral view is a dorsal view, with the front pointing to the left (as in the left lateral view).

Oddly, the size of this vertebra doesn’t seem to be stated in the paper, but two lines of evidence suggest that it’s about 65 cm in total height. First, measuring the caudal on the skeletal reconstruction that is the frontispiece, and comparing with that figure’s 1m scale-bar, yields a height of 64 cm; second, the neural spine’s height (measured from the ventral margin of the poztzygapophyses) is given in Table 9 as 392 mm, and that extrapolates, using the posterior view figure, to a total height of 665 mm. So about 65 cm, then.

The caudal vertebrae of diplodocids such as Apatosaurus, Diplodocus and Barosaurus are unusually complex for sauropods, having been somewhat “dorsalised”, i.e. taking on some of the complex morphology of posterior dorsals rather than being the rather dull round-centrum-with-a-flat-spine-on-top affairs you get hanging off the rear end of brachiosauruids. You’ll notice that the lateral processes, or “caudal ribs”, take the form of tall, broad plates, so that the middle part of the vertebra is trapezoidal in anterior view. This is as different as can be from the boring, stick-like caudal ribs of Brachiosaurus. (What actually are caudal ribs? So far as I can tell, amazingly, no-one really knows. They might be homologous with the diapophysis of dorsal vertebrae, or with the parapophysis, or perhaps both of them fused, or one or both fused with an actual rib.)

Oh, yes: also in the picture is your coccyx, that is, the four or five bones that make up your vestigial tail. It is, needless to say, contemptible. It’s surprisingly hard to find a reference for how big it should be, but by cross-scaling from illustrations of whole human skeleton and sacra, I’ve come up with a figure of about 2.5 cm, and that’s what I’ve used here. If you want to compare your tail with Apatosaurus‘s, remember that Apato had about eighty caudals: they diminish in size posteriorly, of course, but they do stay about the same anteroposterior length for much of the tail. In fact, diplodocids have tremendous tails, something like half the entire length of the entire animal. One of my long-standing bugbears is that the biomechanics of sauropod tails gets almost no attention (except for speculations about whip-cracking) compared with the love and care lavished on their necks. One day, one of us might do something about that.

That concludes our short but humiliating series of abuse directed at your frail human body. I’ll have to come up with something else next time it’s my turn. Hope you’ve enjoyed the ride.

Finally, good news for everyone who was intererested in Matt’s Aegyptosaurus post: he’s made a PDF of Stromer 1932 so you can see that mystery vertebra for yourselves.

Bibliography

  • Upchurch, Paul, Yukimitsu Tomida, and Paul M. Barrett. 2005. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs No.26. Tokyo.
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36 Responses to “Your coccyx is contemptible”

  1. Vertebrat Says:

    I knew it was contemptible, but man, 2.5 cm? It’s… it’s like I have no tail at all!

    And it is so disappointing to read the Stromer PDF right after posting a poem on the subject. :(

  2. Amanda Says:

    I didn’t even see it at first…I had to go back and peek around. I’m so ashamed. We got nothin’.


  3. Curious — does “Your Ribs Are Ridiculous” come next? Followed by “Your Chevrons Are Shitty?” (Well, they are technically vertebral…


  4. I suppose about the only part of a human skeleton that isn’t ridiculously tiny compared to the corresponding feature of a largish sauropod is the braincase.

  5. Nathan Myers Says:

    Your neural spines are spurious.

  6. Sordes Says:

    Hallo everyone! I know this is not the really right topic to ask some questions about sauropod metabolisms, but I could not find any other older posts which would fit better. I asked already at Matt Wedel´s blog and was so to better ask them here (what makes a lot of sense as more readers can participate). Sauropods are not really my absolute favorourtis among dinosaurs, although I have to say that the discoveries during the last years about things like pneumatic bones or oddball like Amargasaurus or Nigersaurus really amazed me. Since a long time I asked me how the metabolism of sauropods, especially of the very large species actually worked, and as I don´t want to look in old papers or outdated internet articles (and as I am a bit lazy) and wanted to ask them to you;

    -What kind of metabolism was most probably common for sauropods?
    -Was there a change of their metabolism when they grew older and larger?
    -What effects had their masses on their metabolisms?

    Best wishes

    Sordes

    BTW, Darren if you read this, I´ll go today to Tübingen and will try to make some dicynodont-photos.

  7. Darren Naish Says:

    Hi Markus

    Sauropod metabolism, oh dear oh dear. We’ve discussed this a fair bit among ourselves and at the risk of being controversial I will say…. that all the stuff you’ve read about sauropods being ectothermic or bradymetabolic homeotherms or whatever is speculative and arm-wavy, NOT the good conservative science that many palaeontologists and biologists would like us to think.

    Given what we now know about dinosaurian growth rates it is difficult to continue to support ectothermy in non-avian dinosaurs, and the data we now have on bone histology, on pneumaticity and on soft-tissue anatomy and posture is more consistent with endothermy than with other physiological models. In fact – despite the fact that it seems to be default assumption among many palaeontologists and biologists – I am not aware of any good reason to think that Mesozoic dinosaurs were ectotherms given that all the usual reasons are erroneous, wrong or untested (e.g., that the Mesozoic world was a stable hothouse (not true), that dinosaurs lacked key structures required for endothermy (erroneous), that some dinosaurs were too big to be endotherms (untested)). I am planning to elaborate on all of this at Tet Zoo some time actually. Meanwhile, I expect others will give more informative answers. Sorry this only partially answers your questions.

    And yes please on the dicynodont photos, many thanks.

  8. Sordes Says:

    Thank you very much for this informative answer, although I really hoped that there is already a bit more known about this issue.
    BTW I made also some photos of sauropod fingers I will send you too, and just check your emails to see a preview of the dicynodont-photos.

  9. Sordes Says:

    I have still an additional question: Is it known how big the hatched sauropods were? I suppose still comparably small, as I can hardly imagine that they would top the eggs of Aepyornis in size, and it seems hard to imagine that they were viviparous or ovoviviparous.

  10. Nathan Myers Says:

    Following onto Sordes’s question… how big can an egg be and still allow enough oxygen to the developing li’l darling? While we’re at it, how can turtle eggs buried deep under the sand, and surrounded by other eggs leaking CO2, get enough oxygen to develop? (Of course I am asking about eggs of those kinds of turtle that happen to have sauropod vertebrae — nothing off-topic here!)

  11. Zach Miller Says:

    Bah! Stupid nonexistant tail! Curse you, coccyx!

  12. Vertebrat Says:

    On the question of turtle nests: the abstract of The respiratory gas exchange of sea turtle nests (Chelonia, Caretta) says:

    PO2 falls to 80-100 torr and PCO2 rises to 40-60 torr inside 100-egg man-made Chelonia and Caretta nests. The change in gas tensions in the nest during development is very similar to that seen in the air cell of the chicken egg. […] The rate of diffusion of gases in the sand is 30-50% of the rate found in the nest and 6-12% of the rate found in an equal volume of air.

    But some turtles don’t even seem to have that much hypoxia. Nest temperature and respiratory gases during natural incubation in the broad-shelled river turtle, Chelodina expansa (Testudinata : Chelidae) says:

    Nest respiratory gas tensions were not greatly different from those in the atmosphere above the ground except after periods of rain. After heavy rain during the last 3 months of incubation the nest became moderately hypoxic (PO2 ~ 100 Torr) and hypercapnic (PCO2 ~ 50 Torr) for several successive days. These short periods of hypoxia and hypercapnia were not lethal.

    I found and lost :( another abstract that said modest hypoxia retarded turtle development, though. But this alligator article found that 15% oxygen wasn’t a problem.

    It doesn’t look like anyone has done similar measurements on sauropod nests. There are a bunch on alligators, but I can’t get them.

  13. Mike Taylor Says:

    Nathan Miller asked: “how big can an egg be and still allow enough oxygen to the developing li’l darling?” The only treatment I’ve seen of this problem is in R. McNeill’s 1989 book, “Dynamics of dinosaurs and other extinct giants”. Excellent though this book is, it doesn’t provide references, so I don’t know what papers this section is working from, but I know Alexander’s book largely uses his own research, so you might try looking through his publications list. The relevant section is on pp. 156-157 of the book. It doesn’t include numeric calculations, but Alexander concludes that the 9 litre eggs of elephant birds were not close to an upper limit imposed by oxygen diffusion. But he thinks they were close to another limit: much larger eggs may have been too strong for their inhabitants to break out of.

  14. Mike Taylor Says:

    There doesn’t seem to be anything relevant on Alexander’s publications page at http://www.fbs.leeds.ac.uk/staff/showPubs.php?staff=Alexander but this only goes back as far as 1992, so that doesn’t mean much.

  15. Nathan Myers Says:

    Thanks, Vertebrat, and Mike.


  16. What’s the evidence suggesting sauropod “caudal ribs” were anything more than diapophyses? The chevron facets and chevrons seem to be a much more obvious homologous structure to parapophyses and ribs, though I have no idea what developmental studies have been done on the topic.

  17. Mike Taylor Says:

    Hi, Mickey. On the subject of caudal rib homologies, I had in mind the following passage from Wilson (1999:642):

    In adults, the sacral ribs and caudal transverse processes are firmly coossified with the centrum and neural arch, but in subadult specimens these elements are free, and the vertebrae are composed of four ossifications — two median and two paramedian elements (e.g. Haplocanthosaurus, Hatcher, 1903; Camarasaurus, Gilmore, 1825). The two median ossifications constitute the vertebral centrum and neural arch, and the two paramedian elements have uncertain serial homologies. In the sacrum, these lateral processes have been identified as either the serial homologues of the parapophyses (Hatcher, 1903) or dorsal ribs (Osborn, 1899), and in the caudal region, they have been referred to as representing fused diapophyses and parapophyses (Hatcher, 1903) or ribs (Huene, 1929:33; McIntosh et al., 1996:84).

    Reference:

    Wilson, Jeffrey A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19(4): 639-653. http://www-personal.umich.edu/~wilsonja/JAW/Publications_files/Wilson1999.pdf


  18. Unfused transverse processes? Crazy. I’ve never heard of such a thing in theropods, where the transverse processes are fused to the neural arch even in young specimens (e.g. Microvenator, Eustreptospondylus). Apparently Anchisaurus has unfused transverse processes though (Galton, 1976), showing it’s present early in massopod evolution. Wonder what the primitive condition is.

  19. Graham King Says:

    Way off topic strictly I guess but in its implications relevant still… what about relative atmospheric levels of O2 and CO2, and atmospheric pressure, in dinosaur days? I can’t help thinking that if they were substantially different from our own ‘norms’ for these, there might be big consequences for our understanding of extinct biology. From pterosaur and teratorn and giant dragonfly airworthiness, to respiratory and metabolic traits, to gigantism… (How come past arthropods so outdid today’s biggest ones… especially on land? I’ve read that spiracle-breathing puts an upper limit on insect size).

  20. Vertebrat Says:

    I’ve read that spiracle-breathing puts an upper limit on insect size

    Because of diffusion? This is mostly a myth. Large insects don’t rely on diffusion; they actively pump their air. (Air sacs: they’re not just for saurischians!) It is true that dependency on diffusion limits the size of insect legs, because they have no way to ventilate them. But if that were the main constraint on their size, they would have found a way around it by now. Big insects face a harder problem: molting. Being big is not so great when you have to shed your skeleton to grow.

    Spiders breathe through lungs like vertebrates, but they have the same size constraint, fortunately. Who wants to meet a spider big enough to eat sauropods?

  21. Bill Parker Says:

    Mickey Mortimer Said:

    “Unfused transverse processes? Crazy. I’ve never heard of such a thing in theropods, where the transverse processes are fused to the neural arch even in young specimens (e.g. Microvenator, Eustreptospondylus). Apparently Anchisaurus has unfused transverse processes though (Galton, 1976), showing it’s present early in massopod evolution. Wonder what the primitive condition is.”

    Happens in phytosaurs and in the archosauriform (?) Doswellia, so the condition is plesiomorphic for Archosauria.

  22. Mike Taylor Says:

    (Waaay off-topic by now, but still …)

    So what is it that prevents spiders and insects from getting anywhere near as big as other terrestrial arthropods such as land-crabs?

  23. Matt Wedel Says:

    So what is it that prevents spiders and insects from getting anywhere near as big as other terrestrial arthropods such as land-crabs?

    I’m pretty sure it is atmospheric oxygen and the limitations on arthropod gas exchangers. Back in the Carboniferous there were some godawful big terrestrial arthropods, like the awesome Arthropleura–not to mention immense insects like Meganeura, a “dragonfly” with a 75-cm wingspan, and Apthoroblattina, a 50-cm long cockroach. Seems to me that if a 2-meter terrestrial arthropod can molt, then molting is not what’s holding back insects today. It is true that insects can ventilate their tracheae (Westneat et al. 2003), but that doesn’t mean that higher oxygen levels didn’t facilitate arthropod gigantism in the Carboniferous, about which see Dudley (1998). According to Kirkton (2008), bigger insects have to give over more of their body mass to respiratory structures, which implies that atmospheric oxygen is potentially size-limiting for terrestrial arthropods. But I’d be interested in a compelling rebuttal.

    References

    Dudley, R. 1998. Atmospheric oxygen, giant Paleozoic insects, and the evolution of aerial locomotor performance. Journal of Experimental Biology 201:1043-1050.

    Kirkton, S.D. 2008. Effects of insect body size on tracheal structure and function; pp. 221-228 in Roach et al. (eds.), Hypoxia and the Circulation, Springer.

    Westneat MW, Betz O, Blob RW, Fezzaa K, Cooper WJ, Lee WK. 2003. Tracheal respiration in insects visualized with synchrotron x-ray imaging. Science 299:558–560.

  24. Mike Taylor Says:

    Dude, you didn’t read the question. I’m not asking why our feeble contemporary insects and spiders are so much smaller than their antecedents, but why they’re so much smaller than other modern terrestrial arthropods.

    http://www.stjohnbeachguide.com/land%20crab.htm

  25. Mike Taylor Says:

    See also http://en.wikipedia.org/wiki/Coconut_crab which cites typical reports of mass up to 4kg and body-length of 40 cm.

  26. Matt Wedel Says:

    No, dude, you didn’t read the response. I was responding to the suggestion that it is molting rather than respiration that puts a cap on terrestrial arthropod size. I was saving coconut crabs for…

    now!

    Insects breathe through tracheae, which are air-filled tubes–basically diverticula–that go all through their bodies (speculative zoology folks: we await your giant pneumatic terrestrial arthropods). Insects can pump air through their tracheae, but AFAIK they don’t have any dedicated mechanism for getting the oxygen from the tracheae to their tissues beyond simple diffusion. They have hearts and hemolymph, but they don’t seem to have any special interface between the respiratory system and the cardiovascular system (that I know of; please correct me, anyone, if you know otherwise).

    Land crabs breathe with gills that are rigid enough to perform gas exchange in air. They pump air over their gills on land the same way they pump water over their gills in the water (Burggen 1992). So far, no reason to prefer the crab setup. But the crab cardiovascular system pumps deoxygenated hemolymph through the gills to oxygenate it and then sends the oxygenated hemolymph out to the body from the heart. So the crabs seem to have the edge in gas exchange at the tissue level.

    I honestly don’t know if that’s enough to explain the size difference, though. Seems weird that insects would never have evolved a way to pipe their hemolymph past their tracheae, which is why I suspect I’m missing something. And the evolution of large size isn’t all about oxygen. There are also issues of feeding, predation, life history, and so on to consider.

    I know one thing for certain, though: coconut crabs are badass.

    Ref

    Burggen, W.W. 1992. Respiration and circulation in land crabs: novel variations on the marine design. American Zoologist 32:417-427.

  27. Mike Taylor Says:

    Dude, that’s great. I only have one thing to ask: the next time you decide to answer someone else’s completely different question, please don’t introduce your answer by quoting my question.

  28. Matt Wedel Says:

    I have to admit, you make a strong point.

    Looking back now, I can see that I set out to answer your question but got sidetracked. Funny how the mind works…or fails to.

    At least it’s not out in the open where everyone can see. Boy, that would be embarrassing!


  29. “Happens in phytosaurs and in the archosauriform (?) Doswellia, so the condition is plesiomorphic for Archosauria.”

    Thanks for that answer, Bill!
    While we’re on this crazy tangent, I wonder if anyone can answer a follow-up question. Juvenile archosaurs seem to have one ossification for the neural arch and one for the centrum, at least once they get to sizes that fossilize. But a juvenile seal skeleton I have has each vertebra formed of three main pieces- the centrum and two “hemiarches” that must fuse at the median during growth. Is this typically mammalian and/or primitive to tetrapods, and if so, where in the phylogeny did the hemiarches change to the archosaurian single arch?

  30. Matt Wedel Says:

    Juvenile archosaurs seem to have one ossification for the neural arch and one for the centrum, at least once they get to sizes that fossilize. But a juvenile seal skeleton I have has each vertebra formed of three main pieces- the centrum and two “hemiarches” that must fuse at the median during growth. Is this typically mammalian and/or primitive to tetrapods, and if so, where in the phylogeny did the hemiarches change to the archosaurian single arch?

    I think the operative clause here is ‘at least once they get to sizes that fossilize’. AFAIK, the neural arches of all vertebrates start out as paired elements, but they sometimes fuse very early during embryonic development.

    It is pretty darn interesting that some (all?) mammals only have midline ossifications in the centra, given that those started out as paired elements in evolutionary time.

    Too bad none of those little sauropod embryos have had any ossified vertebrae. Stingy little bastards.

  31. Mike from Ottawa Says:

    A somewhat off-topic question, while we’re at such things, but I can pull it back on-topic.

    This post was tagged with “stinkin’ mammals” which is not only figurative, but, for a lot of mammals literal, because so many use scent for things like marking territory. Now, I could ask if there is any evidence that dinosaurs did scent-marking, or even if sauropods did, but to bring it on topic, is there any evidence from sauropod vertebrae that sauropods used scent-marking? :-)

  32. David Marjanović Says:

    The chevron facets and chevrons seem to be a much more obvious homologous structure to parapophyses and ribs

    Almost a year too late, but there are tetrapods, even amniotes like Procolophon, which have proximal caudal vertebrae with both ribs (separate, unfused, mobile ribs) and chevrons.

    And yes, neural arches, pleurocentra and intercentra are all paired structures that later fuse — “later” being at very different points in ontogeny for different animals, however.


  33. […] Still, there’s no hesitation for me in picking my favourite series: it would have to be the four posts of axial-anatomy humiliation, Your neck is pathetic, Your torso is also pretty lame, Your sacrum is negligible and Your coccyx is contemptible. […]

  34. ech Says:

    Mr. Wedel: “and Apthoroblattina, a 50-cm long cockroach.” Why do you think Apthoroblattina was this big?


  35. […] which do not); those in the sacrum are called sacral vertebrae and those in the tail are called caudal vertebrae.  But you already know that if you read Tutorial […]

  36. steve culbreth Says:

    Heck, I think that 2.5cm is more than we need. Sure can wreck tour day falling on it. I was born with a tail, which was routinely snipt-off, what a pity. Saw a pic in the Enquirer years ago with a Chinese man and his 6yo boy in his arms to show off his 9″ tail. Don’t know how many vertebra would be present for a new born primate.


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