Your ear bones came from your jaws

October 15, 2009 • 6:22 am

by Greg Mayer

Although the mammals and reptiles most people know are quite distinct– mammals are hairy, warm-blooded, live-bearers, that suckle their young, while reptiles are scaly, cold-blooded, egg-layers– a wider knowledge of the modern forms reveals that the differences are less absolute. There are many live-bearing reptiles, for example, and platypuses and echidnas lay eggs and are nipple-less. And it has long been known that mammals are descended from a particular group of fossil reptiles:  both the great British anatomist Richard Owen and the American paleontologist and zoologist Edward Drinker Cope noted this in the 1800s (Cope doing so in a paper with the wonderful title “The theromorphous Reptilia”, “theromorphous” meaning, roughly, “beast-shaped”).

Because the vertebrate fossil record consists mainly of bones, paleontologists need an osteological distinction between mammals and reptiles, and the definition of mammals is that our jaw joint is between the squamosal bone of the skull and the dentary bone of the lower jaw, while in reptiles the joint is between the quadrate and the articular.

Mammal and reptile jaw joints
Mammal and reptile jaw joints, from Wikipedia by Philcha

The stages in the picture above were about all that were known to Cope and Owen, but they could still see the connection between the groups. (The lower picture is of a pelycosaur, an early type of synapsid reptile, the synapsids being the group of reptiles from which mammals eventually evolved; Dimetrodon was a pelycosaur). Cope’s identification of early synapsids as the ancestors of mammals could be considered a prediction that intermediate forms would be found (I leave out Owen, because his views on evolution were equivocal). Later work has abundantly confirmed this, and the reptile-mammal transition is now probably the best documented of all higher level transitions in the vertebrates. A classic paper by A.W. ‘Fuzz’ Crompton and Farish Jenkins, teachers of mine from grad school, summarized the first 100 years of work on the subject.

Here’s a diagram of one of the intermediate forms. Note that it has a double jaw joint, and the bones in the lower jaw have become much smaller. If you look above to the mammal, you will see that these bones have become even smaller still, and detached from the lower jaw.

Double jaw joint
Double jaw joint from Wikipedia by Philcha. This figure is not quite right. The dentary/squamosal contact is actually much nearer to the quadrate/articular contact. The two joints are lateral and medial to one another, not anterior-posterior.

What has happened is that two bones of the lower jaw (the angular and the articular), and the quadrate of the upper jaw, of reptiles have become (some of) the ear bones of mammals– the tympanic, malleus, and incus, respectively (mammals have another ear bone, the stapes, which is the only ear bone in reptiles). This reduction in size and detachment from the jaw occurred in many gradual steps over many millions of years, all documented in the fossil record. Clifford Cuffey has a nice set of figures of some of these, and Karen Peterson of the University of Washington has posted class notes with some very nice figures. What makes this even neater is that the jaws themselves are derivatives of the anteriormost parts of the branchial (gill) arch skeleton, a subject I’ve mentioned before, and thus we can trace the history of these bones from the branchial apparatus to the ear by way of the mouth.

Just as Matthew was inspired to post about sponges after lecturing about them to one of his classes, I bring up the ear bones because I was lecturing to my vertebrate zoology class about the branchial skeleton and its derivatives this past Tuesday. It was also the very day that the New York Times had an article by Natalie Angier on the evolution of the mammalian ear bones inspired by a recent paper in Science (subscription required for full article) by Qiang Ji and collaborators. They describe the jaw of an early Cretaceous mammal that had a persistent reptile-like connection of the ear bones to the jaw.  The authors propose, quite reasonably, that this is a paedomorphic condition, that is, that it is the retention into the adult of an embryonic condition: mammalian embryos pass through a stage in which their jaw/ear bones resemble those of reptiles.

The working out of the history of these bones is one of the great triumphs of vertebrate comparative anatomy. Neil Shubin (sorry Jerry!) summarizes the highlights nicely in chap. 10 of Your Inner Fish.

17 thoughts on “Your ear bones came from your jaws

  1. Good summary!

    There is something else even more interesting about this, in my opinion. A number of groups of animals have eardrum-based ears, including mammals, reptiles (including birds), and frogs and toads. However, this structure evolved independently in all of these groups. The eardrum-based ear evolved at lest 3 times and maybe as many as twice that. For instance the common ancestor of lizards, snakes, birds, dinosaurs, and crocodilians probably evolved the eardrum separately from the common ancestor of turtles and tortoises. Further, it is possible that the common ancestor of dinosaurs, birds, and crocodilians evolved it separately from the common ancestor of snakes and lizards. Further, they appear to all have evolved at about the same time, during the triassic.

    1. It’s not like being the director of the NIH is a full-time job or anything. There will still be plenty of time of Collins to work part-time at the Vatican, write more faith-based books, and evangelise some hell-bound Catlicks.

  2. I think it’s interesting how creationists denied that the intermediate stages were possible, and fossils demonstrated that they existed.

    The thing is that their “evidence” is never what IDists and creationists care about, so they just move on to other “impossibilities.” You’d think even they’d notice sometime that the data only come after their conclusions.

    As I understand it, the malleus and the incus (at least one, but I think both) are still derived in part from the same tissues from which our jaws form, while the stapes is not, thus reflecting their evolutionary origins imperfectly during development.

    Glen Davidson

  3. Minor pedantry- Synapsids such as Dimetrodon are no longer considered reptiles. Instead, if Reptilia is used it is seen as only including animals more closely related to modern reptiles than to mammals. Thus reptiles (including birds) and synapsids (including mammals) both developed from a non-reptilian amniote.

    1. No, Dimetrodon is a reptile. A mammal-like reptile to be sure, but a reptile. The dispute is between those who want a cladistic classification, versus those who don’t. Eugenie Scott took Jerry to task for not being a cladist in her review of WEIT, and Jerry and I responded here on the blog, so I won’t rehash those responses here. If birds are to be reptiles (which in a certain and interesting sense they are), then there’s no reason mammals can’t be reptiles too (which, again, in a certain interesting sense they are), so some criteria must be elaborated to determine which of the descendants of reptiles are to be called reptiles, and which aren’t.

      1. Birds are a lot more reptile than mammals are. Birds are more closely related to crocodiles than they are to lizards, and more related to lizards than they are to turtles. If you call lizards, turtles, and crocodiles reptiles it is hard to not do the same with birds. All modern reptiles are more closely related to each other than any are to mammals, though. So it is quite reasonable to say that birds are reptiles while mammals are not. That is not to say that you cannot say mammals are also part of the reptile clade, but saying that birds are reptiles does not automatically imply mammals are as well.

        If you say “reptiles are all modern animals currently identified as reptiles, their last common ancestor, and the descendants of that last common ancestor”, then clearly birds are reptiles while mammals are not. On the other hand if you say reptiles are all animals that would ever have been identified as reptiles, their last common ancestor, and all of its descendants, then both birds and mammals are both reptiles.

      2. Well, you COULD define Reptilia in a way that includes mammals, but nobody has done so to my knowledge. Every recent technical paper I know of that uses the term Reptilia would exclude synapsids from it. As TheBlackCat says, since mammals are outside the lizard+turtle+bird clade, you don’t need to include mammals if you’re including birds. The criterion to determine what is a reptile and what isn’t is the phylogenetic definition, which is arbitrary sure, but all taxonomy is. I agree with everything that Nick Matzke said in the link you provided and think that by insisting on a non-cladistic classification, you’re at odds with the scientific literature and the present state of biology.

  4. Geoff North, editor of Current Biology, has reminded me that his journal had an excellent “Primer” on the evolution of sound localisation in vertebrates, back in August. It’s an open access article and can be found here:

  5. Just wondering – how did reptiles eat whilst their bones were migrating to the ear? As I understand it is the hinged bone that was later to be used by mammals..

    What evidence to we have for these evolutionary ‘facts’?

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