Tetrapod Zoology

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At a vertebrate palaeontology workshop held in Maastricht in 1998, some colleagues and I sat in a bar, lamenting the fact that nobody cared about anatomy any more, and that funding bodies and academia in general were only interested in genetics. Given the poor to non-existent coverage that anatomy gets in many biology courses and textbooks, you might think that anatomy has had its day and that – as some molecular biologists told us in the 1980s and 90s – all the anatomical work worth doing had been published in the days of Owen and Huxley. Nothing could be further from the truth, and if you don’t believe this you only have to consider the fact that a thousand basic questions of everyday anatomy have never really been studied, let alone answered. As demonstrated at the Centre for Ecology and Evolution workshop ‘Modern Approaches to Functional Anatomy’ (held at the Natural History Museum in London on April 23rd), we are in the midst of a new anatomical revolution, and these are very, very exciting times…

Organised by John Hutchinson of the Royal Veterinary College (with help from the NHM’s Adrian Lister), the meeting combined new work on primates, dinosaurs, elephants and other tetrapods with contributions on fishes and insects. The application of new computational approaches was at the fore as a major theme. I was honoured and surprised when, during the opening welcome, John mentioned the bloggers who were in attendance: specifically Darren Naish of Tetrapod Zoology and Mike P. Taylor of SV-POW! Wow, what an accolade.

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The talks opened with Robin Crompton’s plenary lecture on locomotor ecology and locomotor evolution. Primates can be distinguished from other mammals by way of their elongate hindlimbs and divergent big toes (still evident in humans, as demonstrated by a video of a person able to play the guitar with their feet), and the main locomotor dichotomy that we see in anthropoid primates – that between pronogrades and orthogrades – occurred round about 20 million years ago during the Early Miocene. But even many of the ‘key’ adaptations linked to locomotion are controlled by the life history of the individual: an excellent example being the bicondylar angle of the knee joint, the development of which is governed by the animal’s post-natal development (Shefelbine et al. 2002). Chimps forced for whatever reason to adopt a bipedal gait during growth have ended up being obligate bipeds: Oliver is the example most people have heard of, but there’s also Poko, a bipedal chimp who spent the first ten years of his life in a tall and narrow parrot cage and was never able to develop normal quadrupedal walking. The bent-hip, bent-knee walking (BHBK) practised occasionally by normal gorillas and chimps is less efficient (in terms of wattage per kilo) than the striding bipedality of modern humans, but orangutan bipedality is highly efficient and more so than that of humans. Orangutans can and do walk bipedally on the ground, but it’s mostly practised arboreally, yet rather than practising their bipedal gait on strong, robust branches, orangutans use bipedality to access small, flexible branches. They use the compliancy of such supports to their advantage, walking on them with a stiff-legged gait that is very reminiscent of what we do on the ground (Thorpe & Crompton 2006, Thorpe et al. 2007a, b: for data on arboreal bipedality in chimps see Stanford 2006). In view of the fact that pongines are basal relative to African hominids (which include us hominins), it is tempting to infer from this that hominid bipedality evolved in an arboreal context, and was later exapted for terrestriality [bipedally walking orangutan shown above; reconstructed bipedal Australopithecus below].

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Why did primates evolve long hindlimbs and a grasping hallux (big toe) in the first place? Basal primates were small arboreal animals, well within the predation range of raptors and other predators (raptors might take as much as 20% of the annual birth population of some small extant primate species), so the impressive leaping allowed by long powerful hindlimbs and grasping feet probably had an important role in predator avoidance. Among Madagascan primates, Prof. Crompton suggested that the evolution of fossas (Cryptoprocta) might have driven the evolution of both large body size and diurnality in lemurs.

Modelling of australopithecines like Lucy now indicates that these primates were capable bipedal walkers (at least over short distances), and similar in their abilities to modern human children. This view matches new work on the famous Laetoli tracks, which were produced by australopithecines walking at a speed similar to that practised by modern humans. The Laetoli trackmakers had a clear medial arch, a hallux more divergent than that of modern humans, and also produced high-pressure areas of impact both at the heel at in the area of the metatarsal heads. That last point is significant because this can only be produced by erect walking, and not by BHBK walking. Even further down the hominid tree, frequent terrestrial bipedality was very probably practised by Orrorin and Australopithecus anamensis, both of which were denizens of fully wooded habitats (an observation indicating that hominid bipedality did not evolve as a result of the opening of woodland). As you might have gathered from this long write-up, this was a very data-heavy talk, and a great overview of primate locomotor evolution and ecology. Mary Blanchard, who you might remember from my brief congratulatory article from December 2007, is part of Prof. Crompton’s research group. The subject of primate anatomy, its evolution and diversity, and how we can study it, was to be revisited by several other speakers during the meeting.

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Moving on, John Hutchinson discussed his work on dinosaur biomechanics, urging the importance of an integrated approach that incorporates work on living animals. A nice photo of a researcher about to vomit over a toilet bowl accompanied the message that there are so many unknowns that we’re likely to be wrong about most or all of our assumptions. But what we’re now learning about extant animals means that we’re approaching an ‘interpretative asymptote’ and are soon to have enough data to make more confident assertions about palaeobiomechanics. Sexy work on gigantic bipeds like Tyrannosaurus has gone hand in hand with an effort to better understand form and function in large ground-birds like ratites: two really important components of bipedal walking and running – the role of anti-gravity muscles and of limb posture – have been poorly studied and are poorly understood, so John and colleagues have been doing pioneering work on ostriches and chickens. For more on this sort of thing see Carrano & Hutchinson (2002), Hutchinson (2001a, b, 2002), Hutchinson & Garcia (2002) and Hutchinson & Gatesy (2000, 2006) [many available for free here]. New modelling on Cretaceous theropods like Microraptor and Velociraptor is underway, and eventually we should have enough models to reliably extrapolate biomechanical behaviour up and down the archosaur cladogram: John suggested that this technique be termed Quantitative Anatomical Phylogenetic Optimisation, or QAPO. Nice acronym [image below shows range of possible hindlimb postures in a model tyrannosaur (from Hutchinson & Gatesy 2006); image above is Luis Rey's painting of running tyrannosaur and giant chicken.... it's a long story].

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Renate Weller spoke about the different three-dimensional imaging tools that are now available for anatomical work (and hence not so much about animals), but I was particularly impressed with and/or shocked by the fact that MRI scanners have such strong magnetic attractivity that they can literally pull big metal objects, like radiators and hospital trolleys, across the room, the results being both spectacular and expensive. Renate also spoke about her work in producing a 3D reconstruction of Eclipse (1764-1789), the famous British thoroughbred racehorse (he won every race he took part in, fathered over 320 foals at least, and is thought to have made a genetic contribution to over 80% of all modern thoroughbreds). Kept at the Natural History Museum for about a century, his mounted skeleton is now back at the Royal Veterinary College.

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Going back to fossil taxa, Jenny Clack reviewed the new ideas and interpretations that she and her colleagues have developed about the famous Devonian tetrapod Ichthyostega, traditionally imagined as a rather nondescript salamander-like animal but now known to have been more bizarre than anyone ever thought [new reconstruction shown here, with Acanthostega too]. The polydactyly and paddle-like hindlimbs of Ichthyostega suggest both that pentadactyly was not primitive for tetrapods, and that basal forms like Ichthyostega were still predominantly aquatic. Less well known is that Ichthyostega has a somewhat differentiated vertebral column, with a short neck, weird tall neural spines in the pelvic region, and a tail which is proportionally shorter than long imagined (Ahlberg et al. 2005). The orientation of the zygapophyses in the ‘lumbar’ region suggest that some dorsoventral motion was possible, and that lateral flexion wasn’t really possible. Perhaps Ichthyostega worked something like a pinniped. Whatever, the new picture of this iconic animal is very different from the conventional one developed by Jarvik. Mike P. Taylor asked afterwards if the possibility of secondary adaptation to aquatic life has been considered for Ichthyostega: in fact it has (Henderson 1999), the main problem being that there are no known terrestrial ancestors for aquatic Ichthyostega.

More to come in part II, to be published tomorrow.

Refs – –

Ahlberg, P. E., Clack, J. A. & Blom, H. 2005. The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437, 137-140.

Henderson, D. M. 1999. Late Devonian amphibians as secondarily aquatic tetrapods. In Hoch, E. & Brantsen, A. K. (eds) Secondary Adaptation to Life in Water. University of Copenhagen, p. 18.

Hutchinson, J. R. 2001a. The evolution of pelvic osteology and soft tissues on the line to extant birds (Neornithes). Zoological Journal of the Linnean Society 131, 123-168.

– . 2001b. The evolution of femoral osteology and soft tissues on the line to extant birds (Neornithes). Zoological Journal of the Linnean Society 131, 169-197.

– . 2002. The evolution of hindlimb tendons and muscles on the line to crown-group birds. Comparative Biochemistry and Physiology Part A 133, 1051-1086.

– . & Garcia, M. 2002. Tyrannosaurus was not a fast runner. Nature 415, 1018-1021.

– . & Gatesy, S. M. 2000. Adductors, abductors, and the evolution of archosaur locomotion. Paleobiology 26, 734-751.

– . & Gatesy, S. M. 2006. Beyond the bones. Nature 440, 292-294.

Shefelbine, S. J., Tardieu, C. & Carter, D. R. 2002. Development of the femoral bicondylar angle in hominid bipedalism. Bone 30, 765-770.

Stanford, C. B. 2006. Arboreal bipedalism in wild chimpanzees: implications for the evolution of hominid posture and locomotion. American Journal of Physical Anthropology 129, 225-231.

Thorpe, S. K. S. & Crompton, R. H. 2006. Orangutan positional behaviour and the nature of arboreal locomotion in Hominoidea. American Journal of Physical Anthropology 131, 384-401.

– ., Crompton, R. H., Alexander, R. McN. 2007a. Orangutans use compliant branches to lower the energetic cost of locomotion. Biology Letters doi:10.1098/rsbl.2007.0049 [free pdf, and others, here].

– ., Holder, R. L. & Crompton, R. H. 2007b. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science 316, 1328-1331.

Comments

  1. #1 Laelaps
    April 28, 2008

    Interesting stuff about hominid bipedalism, although as far as I can tell most workers have ruled out orangs as good models for shedding light on the origins of bipedalism. There has been something of a resurgence of interest in gibbons, but regardless of what living ape is the preferred model there seems to be a movement towards considering bipedal walking the exaptation of features associated with an arboreal way of life.

    I had the chance to see Brian Richmond talk about his recent Orrorin research last week and there definitely seemed to be a shift in considering the effects of an arboreal ancestry on our evolution. He did point out, however, that there is essentially no fossil record of chimpanzees, and filling out that side of the evolutionary tree could definitely shed more light on how much chimpanzees have evolved and what the LCA might have been like.

  2. #2 shiva_dan
    April 28, 2008

    Re the bipedal chimps and orangs: 1) Very interesting that the chimps forced to walk bipedally when young actually became obligate bipeds when adult. I knew about Oliver, but thought it had been psychological conditioning (reward and punishment – just as cruel, but in a slightly more subtle way) that had been used to make him into a habitual biped, as I thought I had seen footage of him in his old age (he might even be still alive, actually) at a chimp sanctuary, in which he was moving quadrupedally like a normal chimp.

    This of course makes me think both of the various feral human children supposedly raised by canines, and their apparently fast and efficient quadrupedal gait (at least relative to anything most humans can do), and of all the kids in those horrific Eastern European orphanages who ended up with physical disabilities, even if they didn’t have them when they went in there, because they were kept confined to their cots all day and in some cases had things like their legs tied together (although, of course, things like muscle wastage clearly played a role there too)…

    (Wasn’t there a feral kid recently-ish who was raised by chimps? I wonder what his locomotion ended up like…)

    2) On a few cryptozoology blogs and forums, i’ve encountered people arguing passionately and stubbornly that Australopithecus was not a biped but a knuckle-walker, that the foot anatomy of australopithecine skeletons does not match that of the Laetoli footprints, and that therefore there must have been a species of Homo that produced those prints, and Australopithecus was a member of the chimp lineage rather than the hominid one. I haven’t encountered any strong evidence for this theory, but its proponents seem insistent that it has been widely accepted and no respectable scientist regards Australopithecus as a hominid any more.

    I’m kind of suspicious about this because a few terms used set off warning bells that Christian creationism might be hiding behind this position somehow. Can you shed any light on the matter?

  3. #3 Cameron
    April 28, 2008

    Is orangutan bipedality really more efficient than that of humans? Greater orangutan efficiency at slow speeds is certainly plausible, they are more maneuverable, but I’m betting humans are much more efficient at running. Can orangutans even run bipedally (I think gibbons can)? Still, this is all making me wonder about the locomotion of a certain very large pongid…

    I’d love to know why Ichthyostega abandoned lateral flexing
    in exchange for, uh, limb movement and limited vertical movement? It could look pinniped-ish on land, but since that group is incredibly flexible in any plane and phocids undulate laterally, I’m not so sure about that analogy.

  4. #4 Jerzy
    April 28, 2008

    Lots of interesting stuff!

    I wonder if Homo floresiensis developed so small brain because of constrain of birth canal opening? Somebody with access to data could measure birth canal in hominids. I’m sure he/she will debunk the theory that Homo floresiensis sized brain should be on a hominid the size of meerkat.

  5. #5 David Marjanovi?
    April 28, 2008

    orangutan bipedality is highly efficient and more so than that of humans.

    There is an Intelligent Designer, and it’s laughing at us. Constantly.

    (Has someone mentioned the Giant Squid Theory yet?)

    the main problem being that there are no known terrestrial ancestors for aquatic Ichthyostega.

    …and that I. retains not only fin rays, but also internal gills, judging from the postbranchial lamina on the cleithrum.

    Did Clack say anything that wasn’t mentioned in the Nature paper on the vertebral column or earlier? Or are we supposed to W4tP?

    their apparently fast and efficient quadrupedal gait (at least relative to anything most humans can do)

    I should add, though, that most humans have simply never tried. I can walk on all fours pretty fine, and quite fast; I can also gallop, but not far because my wrists are much too unstable, and the speed I reach is pathetic.

    Wasn’t there a feral kid recently-ish who was raised by chimps?

    What!

    I’m kind of suspicious about this because a few terms used set off warning bells that Christian creationism might be hiding behind this position somehow. Can you shed any light on the matter?

    That probably is it. Pulling “widely accepted facts” out of their asses about what “respectable scientists” believe, and then fervently believing in those “facts”, looks… very similar to what cdesign proponentsists do all day long.

  6. #6 shiva
    April 28, 2008
  7. #7 Neil
    April 28, 2008

    Interesting stuff about Ichthyostega, not to mention bipedal orangutans :)

  8. #9 Alan Kellogg
    April 28, 2008

    On Oliver

    As I recall, Oliver was rejected by his young mother and raised by humans. There were no restraints placed on his movements, he just preferred toddling about on his legs. in addition, Oliver had a head grossly different in size and conformation from those of common chimpanzees. There is also the fact Oliver and common chimps simply did not get along, and Oliver would only respond sexually to humans and never to common chimp females. Behaviors not usually seen in human raised chimps.

    Then you have the odd thing about Oliver’s genome. Or, rather, about the genome of Oliver’s mitochondria. While Oliver’s genome very strongly resembled that of a common chimpanzee, that of his mitochondria did not. Indicating a separate evolutionary path for Oliver’s ancestors from those of the common chimpanzee.

    As far as I know the differences between Oliver and common chimps consisted of…

    Shape and size of skull.

    Shape of hip and knee.

    Sexual response to common chimp and human females.

    The mutual animosity between Oliver and common chimps.

    The differences in mitochondrial genomes between Oliver and common chimps.

    The adoption of knuckle walking in his later years was due to the development of osteoarthritis.

    Oliver died because of advanced cancer a number of years ago.

  9. #10 Alan Kellogg
    April 28, 2008

    Update:

    Got the mtDNA data wrong. Over on this page the authors reveal that Oliver’s mitochondria had the same DNA has specimens of P. t. troglodytes.

    However, the differences in skull, facial, hip and knee morphology are not explained by this, nor the mutual animosity between Oliver and common chimps.

  10. #11 dragonet2
    April 28, 2008

    Apparently Oliver is still with the living, though he’s elderly and frail.

    He’s still at Primarily Primates in San Antonio, TX

    http://www.primarilyprimates.org/newsletter/index.html

  11. #12 Nathan Myers
    April 29, 2008

    My daughter (age 8) achieves a very creditable gallop, and can leap over obstacles without breaking stride. She has received compliments on her (“graceful”) bipedal running gait, but it’s the gallop she is proud of.

  12. #13 David Marjanovi?
    April 29, 2008

    My daughter (age 8) achieves a very creditable gallop, and can leap over obstacles without breaking stride.

    :-o

    Is there a way you could put this on YouTube?

  13. #14 Nathan Myers
    April 29, 2008

    David: Watch this space next week.

  14. #15 phil poburka
    September 18, 2008

    The line drawing of the Tyrannosaur shows the ‘usual’ view, which could not have been a normal ‘Walking’ position…it has him leaning too far forward.

    Observe any ‘Chicken’…granted, their mass is distributed to compliment where their Hip Socket is, given the articulation and extention of their Legs, for walking in a position intermediate to ‘leaning’ over, or, standing tall.

    I must assume the same would apply to all bipedal Theropods.

    Hence, the general history of their being represented in art or drawings, tend too often to show them ‘leaning’ far too forward for ‘Walking’.

    The ‘forward’ pose in Birds is one of ‘lunging’ either to catch prey, or to repulse a tresspasser or other tansient ‘moment’…and none ‘Walk’ in that pose.

    Phil
    l v

  15. #16 David Marjanovi?
    September 18, 2008

    Phil, you have overlooked the fact that birds have a very, very short tail, and that tails are quite massive, consisting of bone and muscle. This put the center of gravity very close to the hip joints. The recent illustrations are correct, and the 50-year-olds are almost all dead wrong — many are even anatomically impossible.

  16. #17 DDeden
    February 12, 2009

    Orangutans wading with stick tool
    link

  17. #18 DDeden
    August 28, 2009

    Reviewing this thread, please disregard my reference to a swimming orang, it was more likely wading, clinging to a fallen tree branch.

    With placoderm-like forbears in brackish shallows, perhaps Tiktaalik was a primitive proto-salamander/reptile, Acanthostega a primitive pelagic proto-ray-finned fish (with its duplicated digits for better hydrodynamic propulsion), and Ichthyostega a primitive proto-frog, with its lack of abdominal ribs allowing the gradually lengthening rear limbs with broad paddled feet to come far forward to launch or lunge (and later to leap), “differentiated vertebral column, with a short neck, weird tall neural spines in the pelvic region, and a tail which is proportionally shorter” sounding a lot like a short-legged frog, to eventually lose a few digits and fuse the coccyx, but retain the primitive skin breathing ability and need to reproduce in shallow water.

  18. #19 Graham King
    September 29, 2009

    DDeden said[blockquote]Ichthyostega a primitive proto-frog.. to eventually lose a few digits and fuse the coccyx, but retain the primitive skin breathing ability[/blockquote]

    That’s an interesting speculation on Ichthyostega’s locomotion (to thrust frog-like with its hindlegs and lunge).. but wouldn’t skin-breathing have been far less effective for a creature of its size, given that area scales as square and volume as cube of change in linear dimension (for similar body shape and skin texture)? Frogs being pretty small, skin-breathing works fine for them. There’s enough skin gas-exchange area for each unit volume of flesh to be supplied.
    Also (if terrestrial not aquatic) I would imagine Ichthyostega had to have had a tougher thicker skin than frogs, being more massive and so subjecting its skin to larger absolute forces of wear and tear; which also might lessen suitability of skin for gas exchange.
    Just thinking..
    I would love to know how these BIG amphibians did live!

  19. #20 M. O. Erickson
    September 29, 2009

    Although I have great respect for John Hutchinson and his contrubutions to paleontology have been of great value, I will never forgive him for restoring Tyrannosaurus with straight knees (just TRY to straighten the knee out without dislocating it – really, just TRY), and forcing it to be a “slow runner, at best” – a statement that comepletely contradicts the great beast’s anatomy.

  20. #21 David Marjanović
    September 30, 2009

    perhaps Tiktaalik was a primitive proto-salamander/reptile, Acanthostega a primitive pelagic proto-ray-finned fish (with its duplicated digits for better hydrodynamic propulsion), and Ichthyostega a primitive proto-frog

    You are pulling this out of your ass.

    No, I can’t be more polite about this. You blissfully ignore everything that has ever been published about these animals, look at two or three characters, and proclaim grand new schemes that would — unbeknownst to you — turn the work of about a century on its head, based on no evidence. In short…

    You are pulling this out of your ass.

    That’s an interesting speculation on Ichthyostega’s locomotion (to thrust frog-like with its hindlegs and lunge)…

    Not possible. Part of the “new look” is that Ichthyostega was not capable of putting the soles of its feet on the ground. Like the palms of theropod hands, the soles of Ichthyostega feet faced each other, and no rotation was possible without ripping joints apart or breaking bones.

    Besides, the knee isn’t really a joint in these animals, and neither is the ankle. Both are more like a mosaic of plates fitting together.

    if terrestrial not aquatic

    For what it’s worth, Ichthyostega had functional gills (their blood supply has left traces on certain bones).

    Why do you call it an amphibian? We are more closely related to frogs, salamanders, and caecilians than Ichthyostega is.

  21. #22 Monado, FCD
    December 15, 2009

    Late to the party, as usual, but I thought I’d mention roadrunners (the bird). The greater roadrunner would make a much better comparison for dinosaur gait than a fat, domestic chicken. The roadrunner is an active and agile hunter, without hypertrophied wing muscles to supply breast meat. There’s probably less adjustment to any calculations made except for scaling because of the square-cube law, which is needed for both chickens and roadrunners.

    Of course, research chickens are easier to get… but perhaps a video of roadrunner motion?

  22. #23 Michael O. Erickson
    December 24, 2009

    @ #20 – I’m not so sure about that now. Re-reading Hutchinson’s evidence is making me reconsider things just a wee bit.

  23. #24 David Marjanović
    December 25, 2009

    I forgot if Hutchinson included the caudofemoralis in his calculations…

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