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.

Stephen Jay Gould and the origin of jaws

March 4, 2009 • 5:34 pm

by Greg Mayer

There’s been some interesting discussion in the comments on the post on Change we can believe in concerning gradual and punctuational evolution, including the question of whether Stephen Jay Gould ever advocated macromutation. (Among his many accomplishments, Gould joined Niles Eldredge in the explication and elaboration of Eldredge’s initial suggestion of the idea of punctuated equilibria.) Over a very productive 30+ year career, Gould’s views were of course not static, but in a 1980 paper (Is a new and general theory of evolution emerging? Paleobiology 6:119-130) he does advocate macromutation:

Instead, I envisage a potential saltational origin for the essential features of key adaptations. Why may we not imagine that gill arch bones of an ancestral agnathan moved forward in one step to surround the mouth and form proto-jaws?

Gould refers here to the serial homology of vertebrate jaws to the gill skeleton, the discovery of which is one of the triumphs of classic comparative anatomy. He proposes jaws to have arisen from the gill skeleton in a single mutation (i.e. a macromutation). There is always the problem that what’s a big mutation to one person is not big to another, but I think most or all vertebrate morphologists would consider the conversion of a gill arch into a jaw in one step a macromutation. In context, Gould is not arguing that all the features of jawed vertebrates would have arisen at once, but that a very major feature would have. Two arches are involved in the jaws: the mandibular arch, forming the jaws themselves, and the hyoid arch, which supports and suspends the mandibular.  Since there are extinct fishes (the acanthodians) whose hyoid arch is little modified from a gill arch, both of the arches involved in jaws did not change in one step. The exact way in which jaws arose is not known, and is the subject of continuing  anatomical, developmental, molecular genetic, and paleontological research.  New fossils from the Chengjiang Lagerstatte in China are beginning to throw more light on early vertebrate evolution, but are a bit early for the origin of jaws; we may hope, and predict, that further discoveries will shed more light on the origin of jaws.