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.