FIGURE 3. Evolution of the mammalian cranio-mandibular joint and the definitive mammalian middle ear through the cynodont–mammal transition.

From the following article:

Transformation and diversification in early mammal evolution

Zhe-Xi Luo

Nature 450, 1011-1019(13 December 2007)

doi:10.1038/nature06277

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Homoplasies occurred for the simplification of the incus articulation, the stapedial process of the incus and the detachment of the ectotympanic from mandible. a, The cynodont Probainognathus51, 55; ventral view of left basicranium and posterior view of the incus (quadrate). b, The mammaliamorph Pachygenelus51, 55. c, The mammaliamorph Brasilitherium (modified from ref. 57 by personal observation). d, The mammaliaform Sinoconodon. e, Morganucodon (redrawn from refs 53 and 54): left panel, left basicranium, ventral view; middle panel, left incus, posteromedial view; and right panel, the mandible and 'mandibular' middle ear in ventral (below) and medial (above) views. ae, Homoplastic loss of the quadratojugal for a more mobile incus occurs in Pachygenelus (b) and mammaliaforms (d, e), but not in Probainognathus (a), tritylodontids (not shown) and Brasilitherium (c). The stapedial process of the incus, the out-lever for the middle ear, is present in tritylodontids (not shown), Brasilitherium (c) and Morganucodon (e), but not in other taxa (a, b, d). f, The monotreme Ornithorhynchus lower jaw (ventral view): the middle ear attached anteriorly to the mandible by Meckel's cartilage in the embryonic stage59, but detached from the mandible after re-absorption of Meckel's cartilage in the adult. g, The eutriconodont Yanoconodon lower jaw (lower panel, ventral view; upper panel, medial view): the middle ear is medio-laterally (M-L) separated from, but anteriorly connected to, the mandible by the prematurely ossified Meckel's cartilage, similar to the embryonic condition of monotremes of medio-lateral (M-L) separation of the ear from the mandible, and to the monotreme configuration of the ectotympanic and malleus. h, The medial view of the mandible and middle ear of the marsupial Monodelphis: the middle ear is attached to the mandible by Meckel's cartilage in the embryonic stage60, 61, but detached from the mandible after the re-absorption of Meckel's cartilage in the adult. Because Yanoconodon (g) is nested between extant monotremes (f) and therians (h), both of which have have separation of the middle ear from the mandible, the Meckel's connection of the ectotympanic to the mandible in Yanoconodon shows that some Mesozoic mammals had homoplastic evolution of the definitive mammalian middle ear, defined by full detachment of the ectotympanic from the dentary. The ossified Meckel's cartilage of Yanoconodon is very similar to the embryonic Meckel's cartilage of extant monotremes, and has paedomorphic resemblance to the embryonic condition of extant mammals. The homoplastic attachment of the mandible and the middle ear in Yanoconodon is correlated with changes in the timing and rate of development. D, dentary; SQ, squamosal; D/SQ, the dentary–squamosal contact or joint.

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