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.

A creationist objects to some fossil evidence for evolution

April 21, 2009 • 7:26 am

Over on his blog, Skeptic Dave takes on a woman who criticized the “supposed fossil evidence” for evolution given in my book:

“total BUNK! There are PLENTY of instances of mammals, invertebrates and insects mixed in the same layer of rocks. In one case, an entire tree was found UPSIDE DOWN through layered strata. So… did it just somehow stay upright for “billions” of years while rock slowly accumulated around it? Highly improbable. We also still have NO IDEA how something in between a wing and a leg is somehow better than a plain ol leg. It isn’t better unless it’s ALL there, and evolution tells us that it can’t all be there at once. When you go to look at something determined to see evidence to support your claim, that’s not scientific inductive reasoning anymore. That’s deductive. And that’s what most of today’s scientists set out to do. They want to “prove” what they already deeply, desperately believe.”

Go have a look at how Skeptic Dave and his friends have mustered the evidence against this “upside down tree” objection.   All supporters of evolution owe it to themselves to be able to address this very common objection.

Don Prothero’s superb book on the fossil record

February 22, 2009 • 9:30 am

In WEIT I mention, in the recommended reading at the end, Donald Prothero’s new book, Evolution: What the Fossils Say and Why It Matters, published by Columbia University Press. Prothero, a professor of geology at Occidental College in California, is a prolific writer, having produced or co-produced 26 books, most of them on paleobiology.

I don’t think that What the Fossils Say has gotten nearly the attention it deserves, so I wanted to give it a shout-out here.  It is simply the best existing book on the evidence for evolution from fossils; there is no competitor. It lays out, for the educated layperson, not only the process of fossilization and the biases it imposes on our knowledge of ancient life, but also discusses systematics, how fossils are used to infer genealogy, and (my favorite part), the evidence.  All the favorite transitional forms are there, in nice detail, and many that are not as well known.  Prothero does not shy away from engaging creationists–he uses the evidence against their wrongheaded claims at every turn.  The writing is superb, the illustrations (many by the great Carl Buell) are a joy.  Everybody with an interest in biology and evolution–whether you be a professional or a layperson–should buy and read this book!  If you have any doubts, check all the positive reviews on Amazon.