Can you ear me now?

April 15, 2011 • 2:36 pm

by Greg Mayer

A chief difference between  mammals and reptiles is that mammals have a single bone in the lower jaw (the dentary), which articulates with the squamosal bone of the upper jaw to form the jaw joint (d/s for short); while in reptiles there are numerous bones in the lower jaw (the dentary plus several postdentary bones), and it is one of the postdentary bones (the articular) which forms the jaw joint, but not with the squamosal bone, but with the quadrate.

It might seem difficult to end one jaw joint and start another, but we do know how it happened, and a paper published yesterday in Nature by Jin Meng, Yuanqing Wang, and Chuankui Li adds interesting detail to the story.

Figure 1: Middle ears and eardrums in ventral view. a, In the transitional ear of Liaoconodon, as inferred by Meng et al.1, the eardrum (dotted line) would have been stretched between the ectotympanic bone (red) and the skull. Tension on the eardrum was maintained by the buttressing of the ectotympanic on the anterior process of the malleus (dark blue) and the ossified Meckel’s cartilage (orange). b, In the ear of an extant mammal, the opossum Didelphis, the ectotympanic is fixed to the skull and the eardrum is almost entirely attached to it. The other ossicles depicted are the incus (green) and the stapes (light blue). The bones depicted in light brown are the posterior of the jaw showing the joint with the skull. (From A. Weil. 2011 A jaw-dropping ear. Nature 472-474.)

As long ago as the 1870s Richard Owen and Edward Drinker Cope realized that reptiles were ancestral to mammals, and identified the particular group of reptiles from which the mammals descended; Cope called them the “theromorphous” (beast-shaped) reptiles. Owen and Cope did not have the fossils to show how it happened, but subsequently discovered fossils have amply born out their insight, and we now know how it did happen. Among those making major contributions were A.S. Romer, A.W. “Fuzz” Crompton, and Farish Jenkins, all of the Museum of Comparative Zoology.

The story is, in fact, even stranger than one might have imagined. Mammals don’t just lose the postdentary bones of their lower jaws: a number of these bones, along with one from the upper jaw,  become the bones of the mammalian middle ear, joining there the stapes, the single middle ear bone of reptiles. The reptilian angular, articular, and quadrate  become the mammalian tympanic, malleus, and incus, respectively.  The mammalian middle ear is composed (in part) by the reptilian jaw joint!

Many fossils document this transition (although creationists still don’t seem to know this). The postdentary bones gradually decrease in size, the dentary gradually enlarges, eventually making contact with the skull to form the d/s joint; and the postdentary bones lose their connection to the lower jaw, moving back into the middle ear. Some of the most advanced reptiles had both jaw joints, and this condition persisted into early mammals. The new genus and species described by Meng et al. comes form the exquisitely preserved Jehol biota of Liaoning, and shows the last tenuous connection between the mammalian ear ossicles and the lower jaw, via Meckel’s cartilage, an ancient part of the lower jaw lying along the medial surface of the dentary. They refer to it as a “transitional mammalian middle ear”, the transition being between the mandibular middle ear (i.e. attached to the lower jaw) of the earliest mammals (and most advanced reptiles), and the definitive mammalian middle ear present in the adults of all extant mammals, in which there is no persistent connection between the middle ear and the lower jaw.

As an aside, this paper should never have been published in Nature. It’s not that it’s no good, rather it’s too good. The authors have written a monograph on the new form, but only 5 pages of it were published. The other 73 pages, consisting of illustrations, methods, analysis, data, discussion, and references is in a supplementary file, that most readers will probably never see. It used to be that an important monographic work might be accompanied by a short paper highlighting key findings. Jared Diamond’s monumental Avifauna of the Eastern Highlands of New Guinea (1972, Pub. Nuttall Ornith. Club 12) was followed by a 10 page paper in Science on the “Distributional ecology of New Guinea birds” (1973, Science 179:759-769; a very long paper for Science). I fear that the present authors’ monographic labor will never see the light of day in a form which insures its availability and permanence. ______________________________________________________________

Cope, E. D. 1878. The theromorphous Reptilia. Am. Nat. 12: 829-830.

Crompton, A.W. & Jenkins, F. A. 1973. Mammals from reptiles: a review of mammalian origins. A. Rev. Earth Planet. Sci. 1: 131-155.

Crompton, A.W. & Jenkins, F. A. 1979. Origin of mammals. In Mesozoic mammals: the first two-thirds of mammalian history (ed. J. A. Lillegraven, Z. Kielan-Jaworowska & W. A. Clemens), pp. 59-73. Berkeley: University of California Press.

Meng, J., Y. Wang and C. Li. 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472: 181–185.

Owen, R. 1876. Description and illustrated catalogue of the fossil Reptilia of South Africa in the collection of the British Museum. London: British Museum.

Romer, A.S. 1969. Cynodont reptile with incipient mammalian jaw articulation. Science 166:881-882.

Weil, A. 2011 A jaw-dropping ear. Nature 472-474.

18 thoughts on “Can you ear me now?

  1. I would suggest giving the creationists an earful, but it seems like their middle ear bones are still being used to work their jaws.

    1. Monograph?! In Nature?! *bwa ha ha ha ha ha HAAAA* Don’t we wish. It’s five pages long, which Nature for some reason expects people to pay $32 for. I’d recommend emailing Jin for a copy.

  2. I know I’ve said this here before, but it bears repeating. Mammals did not evolve from reptiles, because Reptilia is defined as a group containing the most recent common ancestor of living reptiles and their descendants. Lizards’ (including snakes), Sphenodon’s, turtles’ and crocodilians’ most recent common ancestor evolved after the split with mammals. Synapsida (containing mammals) and Sauropsida (containing reptiles) are sister groups.

    Back in the 70’s people had a paraphyletic concept of Reptilia that was basically what we call Amniota today, but virtually no one working on mammal origins uses it anymore. For instance, you’ll note the Digimorph site you linked to for Probainognathus has reptiles sister to mammals in its cladogram, NOT including them. It’s as incorrect as saying birds aren’t dinosaurs, something else which started changing in the 70’s and 80’s. It does no good to perpetuate an outdated usage to the public. It will only result in the public being out of touch with the professional current consensus.

    1. Yeah, bullshit. The English vernacular word ‘reptile’ does not have to and is unlikely ever to correspond directly to the formal taxonomic Reptilia (sensu lato).

      To the vast majority of the public, ‘reptiles’ are amniotes other than mammals and birds; systematists have no power over the language and so this perception will change only slowly if at all, and so what?

      1. I suppose mammoths and pterosaurs are dinosaurs then, since the English vernacular word ‘dinosaur’ does not have to and is unlikely ever to correspond directly to the formal taxonomic Dinosauria (sensu lato)…

    2. As I’ve replied before, this is inside baseball of the sort that does not advance public understanding. It is an argument, as the late, great John Maynard Smith put it, about words, and not about the world. There are arguments in favor of making all taxa holophyletic (I don’t think they stand up under close scrutiny), but it’s not anything the public should much care about, and can even be misleading (the paraphyletic conception of Reptilia, dates not to the 1970s, but, as I note in the post, to the the 1870s).

      To use the example of dinosaurs and birds, surely the most interesting scientific development about them in the last 40 years, beginning with the work of John Ostrom and continuing with spectacular fossil finds (many from the same Jehol biota as Liaoconodon), is the demonstration that birds are descended from dinosaurs (a result also anticipated in Victorian times, this one by Owen’s nemesis, Huxley). To maintain a strictly holophyletic classification in light of this advance in knowledge requires changes in name and rank of various taxa (which changes must be chosen from a very large number of possible changes which would satisfy the criterion of holophyly); and there are real and interesting senses in which birds are dinosaurs (just as they also are reptiles and lobe-finned fish). But there’s nothing wrong with having a paraphyletic Saurischia, and nothing wrong with saying birds are descended from dinosaurs, as shown by the illustrations on the cover of WEIT.

      GCM

      1. I would have agreed with Maynard Smith in his generation, but with the spread of phylogenetic nomenclature, taxonomic issues have become about the world instead of just words. Now when someone says a species is a reptile, they’re asserting it can trace its lineage back to that common ancestor of living reptiles I mentioned earlier.

        The battle for retaining paraphyletic groups was lost long ago in vertebrate paleontology (I meant the 70’s was when almost everyone still used paraphyletic groups, not when the custom began). No one who uses paraphyletic groups without scare quotes is taken seriously anymore, and those who do are mostly fringe workers who have heterodox ideas on phylogeny and reject cladistics. It hasn’t resulted in changes of name and rank because the majority of workers see Linnaean ranks as artificial and useless scientifically.

        There would have been nothing wrong with thinking of Saurischia as paraphyletic and thus abandoning it, like has happened to Pelycosauria. But instead it was retained as monophyletic, so today any mainstream dinosaur paleontologist will correct you if you say birds aren’t saurischians. If this doesn’t make it wrong, I don’t know what would.

        1. Mickey– Don’t get me wrong: I think your views are entirely reasonable, but I disagree. The “reptiles” again provide a good example of the problem of mistaking words for the world, and thinking the names themselves have scientific (rather than nomenclatural) content. Your definition of reptiles– snakes, lizards, turtles, crocodilians and tuataras– is one way of making the Reptilia holophyletic. But there are people who aggressively publicize a definition of Reptilia that excludes crocodilians and turtles, which is a different way of making the name holophyletic. But the reason this is silly is not because it’s wrong (Reptilia, in either view, is a contiguous segment of the phylogenetic tree), but because it does not recognize the conventional aspect of nomenclature and its pragmatic communicative aspects.

          GCM

  3. Nature is not alone in jamming monographic-length (and critically important) data and analysis into the black hole of supplementary on-line information. The Ardipithecus issue of Science is another good example of this regrettable trend. The “paper” becomes an advertisement for a larger, more scientific product that is difficult for many readers to access.

    1. So do the authors of the monograph benefit from having it available behind a $32 pay wall? Do they see any of that money? If they do, I can understand doing it that way. If not, well– I’m with you.

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