Question on bird flight and skeletons for paleontologists, zoologists, or experts

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Discussion by: rckiser

I am curious how one decides, based on an individual skeleton, where a given bird skeleton should be classified in a lineage as regards flight. For instance, if a given skeleton looks like an ostrich, how does one decide that this bird had ancestors that could fly, versus "this is a bird whose ancestors that could not fly but this species is developing characteristics that could lead to flight." I ask because ostrich skeletons seem so therapod like that I wonder why it is believed that they had ancestors that could fly, rather than postulating that they are a transitional species between non-flying therapods and flight capable birds–or to put it a different way, whether they are "proto-flight" birds rather than "post-flight" birds. If there are specific osseos markers looked for by paleontologists I would be most interesting in knowing of them. Thanks to anyone willing to help.

 

Rob

20 COMMENTS

  1. like an ostrich, how does one decide that this bird had ancestors that could fly, versus “this is a bird whose ancestors that could not fly but this species is developing characteristics that could lead to flight.”

    Feathered dinosaurs roamed the Earth for millions of years before any of them evolved flight.

    I ask because ostrich skeletons seem so therapod like that I wonder why it is believed that they had ancestors that could fly, rather than postulating that they are a transitional species between non-flying therapods and flight capable birds

    They are too big to fly. Also rapid adaptations are more likely in small short lived species which have many generations in a short time.

    –or to put it a different way, whether they are “proto-flight” birds rather than “post-flight” birds.

    There are evolutionary pressures which can lead to an increase in size once birds become flightless – basically big equals better defended on the ground. A kick from an ostrich can do a predator serious damage.

    It would be difficult for large birds like this to become adapted to flight because of the nature of surface to mass ratios. Even flight-capable large birds like swans find take-off difficult.

  2. Ostriches and their relatives belong to a group of birds called “Ratites” referring skeletally to the absence of a ridge on the sternum which is used, in flighted birds, for the attachment of the powerful flight (wing) muscles. Whether they are “post” or “proto” flight birds depends on whose study you read. The previously held view was “proto” and had them as Gondwana land residents whose descendants remained in Australia, New Guinea, South America and New Zealand as well as Africa and Madagascar (extinct) as the super continent broke up, isolating the flightless communities.

    New Zealand is particularly interesting as harbouring, on islands without predators, until human contact about 900 years ago, diminutive forms (Kiwis) and the recently extinct very large Moa.

    But, recent research shows a more complex evolutionary history that questions the simple “Proto / Gondwanaland” scenario, and indicates possible multiple instances of loss of flight ability.

    Back to the original question however, the lack of a sternum ridge is the distinguishing skeletal feature.

    • In reply to #2 by Sheepdog:

      Ostriches and their relatives belong to a group of birds called “Ratites” referring skeletally to the absence of a ridge on the sternum which is used, in flighted birds, for the attachment of the powerful flight (wing) muscles. Whether they are “post” or “proto” flight birds depends on whose study y…

      Thank you, this was the sort of information for which I was looking. Very helpful and concise explanation. I would assume, based on your answer, that in a flightless species recently speciated from flying ancestors, we would expect to see at least a vestigial sternal ridge, and in species that have never developed the ability to fly we would expect to see an absence of such a ridge. However, I would also infer from your statement regarding “recent research” that suggests “a more complex evolutionary history,” that an osseos marker such as a sternal ridge might not be sufficient evidence by itself. I am speculating that additional evidence, such as finding intermediate species in the fossil record that show transition from flight to flightless and can be reasonably thought to be ancestors of, say, the ostrich, would be necessary to show that how the species evolved into its present form. I hope that last sentence is a cogent and accurate statement. I’m basically trying to gain a better understanding of the evolution of ostriches in particular and large flightless birds in general, from theropods. I’m particularly interested in the forelimbs (and hence proto or post flight evolution) because the undersized forelimbs of theropods look very much to me like the forelimbs (wings) of ostriches.

      • In reply to #4 by rckiser:
        >

        I’m particularly interested in the forelimbs (and hence proto or post flight evolution) because the undersized forelimbs of theropods look very much to me like the forelimbs (wings) of ostriches.

        You may be interested in this earlier discussion and linked article and diagrams on the old RDFS site:-

        http://old.richarddawkins.net/discussions/645960-the-common-hand-by-carl-zimmer-illustration-by-bryan-christie-national-geographic

        You may also find this link helpful:-

        EVOLUTION OF THE DINOSAURS – http://palaeo.gly.bris.ac.uk/communication/boulton/evolution.html

        In looking at forelimbs, it can be helpful to go all the way to fins on ancestral fish and modern adaptations on flying fish.

        • In reply to #5 by Alan4discussion:

          In reply to #4 by rckiser:

          I’m particularly interested in the forelimbs (and hence proto or post flight evolution) because the undersized forelimbs of theropods look very much to me like the forelimbs (wings) of ostriches.

          You may be interested in this earlier discussion and linked article and dia…

          Thanks will look these up.

      • In reply to #4 by rckiser:

        In reply to #2 by Sheepdog:

        I would assume, based on your answer, that in a flightless species recently speciated from flying ancestors, we would expect to see at least a vestigial sternal ridge, and in species that have never developed the ability to fly we would expect to see an absence of such a ridge. However, I would also infer from your statement regarding “recent research” that suggests “a more complex evolutionary history,” that an osseos marker such as a sternal ridge might not be sufficient evidence by itself.

        I made those limited references not just in the interest of brevity, but because I was reaching the limit of my own (amateur) knowledge. The sternum ridge is one of these things that we, the terrestrial animals, all have. Feel under your jugular notch, an inch or two down, and you will find it. It would have been more accurate for me to have referred to a “massively developed sternal ridge” as being the osseous marker of flighted birds.

        While flight is an effective strategy for both avoiding predators and accessing food sources, it is an energy expensive one, and in an environment with no predators and easily accessible food (New Zealand) it is easy to envisage evolution moving musculature to the walking limbs and diminishing the flight dependent forelimbs, and their muscle attachment.

        This is very much supposition on my part, but it is the sort of complexity that I understand to be clouding the evolutionary understanding of this particular group.

      • In reply to #4 by rckiser:

        In reply to #2 by Sheepdog:

        I just realized that I wrote in complete contradiction to my own earlier post, which was at best clumsy of me, my apologies. My earlier references to “absent” or “present” sternum ridge should really have been “massively enlarged” and “vestigial.” Again, apologies for the misinformation.

        • In reply to #8 by Sheepdog:

          In reply to #4 by rckiser:

          In reply to #2 by Sheepdog:

          I just realized that I wrote in complete contradiction to my own earlier post, which was at best clumsy of me, my apologies. My earlier references to “absent” or “present” sternum ridge should really have been “massively enlarged” and “vestigi…

          No problem, I expect an incremental improvement of my understanding. I would argue, however, that vestigial is usually reserved for structures previously developed and later atrophied in subsequent generations–so to be even more accurate we probably need a different word when we aren’t sure if the characteristic is vestigial or evolving toward a different utility. For instance, if a flightless bird lineage starts to develop bigger wings, more powerful muscles, etc, generations would start to develop some increased attachments for those muscles. If, for whatever reason, a flght-capable species begins to evolve toward non-flight, transitional generations would have true vestigial osseos attachments. I’m still wondering if these would look the same or have tell-tale differences.

      • In reply to #4 by rckiser:

        You should find this article useful.

        http://jerrycoyne.uchicago.edu/excerpt.html
        >

        VESTIGES

        The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co-opted for new uses. Like all flightless birds, ostriches are descended from flying ancestors. We know this from both fossil evidence and from the pattern of ancestry that flightless birds carry in their DNA. But the wings, though still present, can no longer help the birds take flight to forage or escape predators and bothersome graduate students. Yet the wings are not useless—they’ve evolved new functions. They help the bird maintain balance, mate, and threaten its enemies.

        There is another article on the vulnerability of flightless birds which have evolved on remote islands, to extinction.
        http://www.livescience.com/28153-pacific-island-bird-extinction.html

        • In reply to #9 by Alan4discussion:

          In reply to #4 by rckiser:

          You should find this article useful.

          http://jerrycoyne.uchicago.edu/excerpt.html

          VESTIGES

          The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefu…

          Thanks Alan, this looks like what I am searching for. I wonder why no pre-flight avian creatures would have made it to modern times but post-flight birds are running around. I also wonder if we have DNA of pre-flight avian or proto-avian species to verify whether they had entirely divergent DNA. It seems like the lineage “leading up to” flighted birds would have similar DNA structure to the generations that could fly. I mean, the DNA would incrementally change by the pressures of natural selection, so well before generations began to fly, I would expect that the DNA would be quite similar to those generations that could fly. But this is a good article to start digging in, thanks again.

          • In reply to #13 by rckiser:

            In reply to #9 by Alan4discussion:

            I like the excerpt, but am still left wanting to know how Coyne “knows” that ostriches wings are vestigial rather than not-yet-fully developed-for-flight.

            Ostriches are simply too big to be evolving into flight-worthy birds. Scale is everything when relating mass to the ability of air to support it. There are limits and birds which approach the size limits can only glide, not power themselves by flapping. – It is physics.

            I wonder why no pre-flight avian creatures would have made it to modern times but post-flight birds are running around.

            It is probable that all non-flight worthy dinosaurs perished in the mass extinction 65 million years ago simply because of the rapid climate changes, where mobility to new distant habitats, was the key to survival..

            I also wonder if we have DNA of pre-flight avian or proto-avian species to verify whether they had entirely divergent DNA. It seems like the lineage “leading up to” flighted birds would have similar DNA structure to the generations that could fly. I mean, the DNA would incrementally change by the pressures of natural selection, so well before generations began to fly, I would expect that the DNA would be quite similar to those generations that could fly.

            Once gliding and flight began there would be strong selection pressure for things like weight reduction.

            I’m guessing their DNA does not have an embedded message that says, “Wing-flight assembly used to go here, but we discontinued it.”

            It probably does. It is probably as simple as switching off genes for growth after a shorter period of development, producing smaller wings. The shared genes for arm/wing development go right back to fish and the first tetrapods. (see links @5 and @6)

            So bringing it to the DNA level only moves the question from bones to base-pairs, how do we know that the DNA is similar to flighted birds because they evolved from flighted birds–rather than because they are of a different but related lineage that never evolved the ability to fly?

            All birds are descended from a common ancestor. Those which lose the ability to fly are commonly found in niches they have expanded into on remote islands, where competition from mammals etc is absent. If wings are of little benefit in a particular habitat, selection benefits those individuals which do not waste energy growing them. – A similar situation arises with eyes in blind cave fish which live in perpetual darkness.

            I grew up in the generation that, during childhood was told that we “knew” dinosaurs were slow, cold-blooded, reptiles, and by adulthood we “knew” that the theropods at least were fast, probably homeothermic, proto-birds.

            Dinosaurs were a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, 231.4 million years ago, and the non-avian ones became extinct 65 million years ago.

            Early generalisations and simple children’s encyclopaedia articles, could not possibly cover this range of species or their evolution over that length of time scale. Many of them were cold-blooded reptiles. The widespread covering of dinosaur feathers is a more recent discovery.

            But this is a good article to start digging in, thanks again.

        • In reply to #9 by Alan4discussion:

          In reply to #4 by rckiser:

          You should find this article useful.

          http://jerrycoyne.uchicago.edu/excerpt.html

          VESTIGES

          The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefu…

          I like the excerpt, but am still left wanting to know how Coyne “knows” that ostriches wings are vestigial rather than not-yet-fully developed-for-flight. It’s a nice lay reference, but requires I trust his authority without adequate source citation. I mean, I grew up in the generation that, during childhood was told that we “knew” dinosaurs were slow, cold-blooded, reptiles, and by adulthood we “knew” that the theropods at least were fast, probably homeothermic, proto-birds. So I’m a bit skeptical when I hear statements like, “their DNA tells us so.” I mean, I’m guessing their DNA does not have an embedded message that says, “Wing-flight assembly used to go here, but we discontinued it.” So bringing it to the DNA level only moves the question from bones to base-pairs, how do we know that the DNA is similar to flighted birds because they evolved from flighted birds–rather than because they are of a different but related lineage that never evolved the ability to fly? Perhaps some transitional fossils would solve the issue… some flying proto-ostriches. But at this point I’m concerned that the belief in ostriches and perhaps other ratites, being post-flight might be a hold-over from the days before a theropod-bird lineage was accepted. I mean–why would no lineage of pre-flight birds be extant when it appears to be a reasonably winning strategy for modern ostriches. Of course, it has also been a losing strategy for Dodo, Moa, etc. so perhaps there were selection pressures that forced all birds into flight for many generations, but I’d like to hear the evidence.

  3. Cladistics is used to identify phylogenetic lineages – that is, how taxonomic characters are ordered and compared. This is unique for each species, and doesn’t rely on general descriptions such as “looks like an ostrich” which is too general to be of use. So while there may be a clear answer to your question you would need to be more specific about what particular species you are referring to and also you would need to learn some phylogenetic terminology in order to understand the explanation. Do a search in Google.

    • In reply to #10 by Reckless Monkey:

      Hollow Bones? Definitely needed in the air not so on the ground.

      Yes, but would non-hollow bones indicate whether the bird was of a lineage that had not yet developed flight vs. a lineage that had lost the ability to fly. I mean, hollow bones would have likely developed in a lineage some generations before flight could occur. In a species that was moving away from flight toward a terrestrial lifestyle, one would expect hollow bones to persist for some generations until sufficient incremental changes driven by natural selection selected for heavy boned fast runners.

    • In reply to #17 by Roddy:

      I am curious how one decides, based on an individual skeleton, where a given bird skeleton should be classified in a lineage as regards flight.

      Essentially, you shouldn’t be so essentialist around here.

      Well, no, you seem to have missed the point, which likely means I haven’t communicated it well. In simplified form I’m asking, “How would one know if an ostrich is a species of bird that was a direct descendant of non-flying dinosaurs and thence non-flying birds, as opposed to a species that evolved from flying ancestors.” This is not a question about whether ostriches are ideal forms in a taxonomic classification or any other type of Essentialst question, rather it is about how we know where a given animal fits terms of its lineage as regards a particular trait (in this case flight).

      • In reply to #18 by rckiser:

        … therapod… proto-flight… osseos markers… taxonomic…

        It sounds like you are the expert on the question and all that you are doing is encouraging other people to be essentialist at the same time that articles about Dawkins being anti-essentialist appeared. For a laugh. And it is funny!

      • In reply to #18 by rckiser:
        >

        “How would one know if an ostrich is a species of bird that was a direct descendant of non-flying dinosaurs

        The non-flying feathered Theropod dinosaurs, pre-dated the first bird for millions of years.

        and thence non-flying birds,

        They were not yet birds at that stage of evolution.

        as opposed to a species that evolved from flying ancestors.”

        All birds are descended from a common flying ancestor, prior to their diversification into numerous species and genera.

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