Mammalian middle ear evolved twice???
kinman2 at YAHOO.COM
Mon Feb 14 15:17:28 CST 2005
In a recent issue of Science, new fossil evidence purportedly shows the definitive mammalian middle ear evolved twice. Hmmmm??? Extremely interesting, *IF* it is really true. However, it is based on the assumption that Teinolophos is cladistically a member of the monotreme lineage. Move it down just a bit, prior to where monotremes and therians split apart, and you'd have a whole different news story (a retraction): Teinolophos is the most advanced known mammal that still retains the mandible/middle-ear bone connection, but it is neither a monotreme nor a therian, and the definitive mammalian middle ear probably did evolve just once afterall.
Therefore, before we start printing this major new hypothesis in the textbooks, it would be best to look at it more closely. Isn't it perhaps more likely (parsimonious) that the tooth of Teinolophos is similar to other fossil monotreme teeth due to homoplasy or plesiomorphy, rather than jumping to this conclusion that the mammalian middle ear evolved twice? Best to think these things through before splashing them across the newspapers of the world.
Either way this finding is interesting, but I cringe to think that the wrong conclusion was drawn and nobody raised these concerns before the paper was published. Hopefully my concerns are ill-founded, and their hypothesis is correct, but homoplasy (or plesiomorphy) in this tooth seems a lot more likely than homoplasy in something as complex as mammalian middle ear bones.
----- Ken Kinman
P.S. As I wrote this, it also just occurred to me that this trough in the mandible could well have been retained in forms where the mandibular/middle-ear bone connection had already been detached. No reason to believe the trough would disappear all that quickly. Maybe yet another reason to take this new hypothesis with a very large grain of salt.
Thomas H. Rich, James A. Hopson, Anne M. Musser, Timothy F. Flannery, Patricia Vickers-Rich (2005). Independent Origins of Middle Ear Bones in Monotremes and Therians. Science 307: 910-914.
ABSTRACT: A dentary of the oldest known monotreme, the Early Cretaceous _Teinolophos trusleri_, has an internal mandibular trough, which in outgroups to mammals houses accessory jaw bones, and probable contact facets for angular, coronoid, and splenial bones. Certain of these accessory bones were detached from the mandible to become middle ear bones in mammals. Evidence that the angular (homologous with the mammalian ectotympanic) and the articular and prearticular (homologous with the mammalian malleus) bones retained attachment to the lower jaw in a basal monotreme indicates that the definitive mammalian middle ear evolved independently in living monotremes and therians (marsupials and placentals).
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