Yanoconodon - Nature

[Tim Williams <twilliams_alpha@hotmail.com>]

Nice stuff...

Zhe-Xi Luo, Peiji Chen, Gang Li and Meng Chen (2007).  A new eutriconodont 
mammal and evolutionary development in early mammals.  Nature 446: 288-293.

Abstract: "Detachment of the three tiny middle ear bones from the reptilian 
mandible is an important innovation of modern mammals. Here we describe a 
Mesozoic eutriconodont nested within crown mammals that clearly illustrates 
this transition: the middle ear bones are connected to the mandible via an 
ossified Meckel's cartilage. The connected ear and jaw structure is similar 
to the embryonic pattern in modern monotremes (egg-laying mammals) and 
placental mammals, but is a paedomorphic feature retained in the adult, 
unlike in monotreme and placental adults. This suggests that reversal to (or 
retention of) this premammalian ancestral condition is correlated with 
different developmental timing (heterochrony) in eutriconodonts. This new 
eutriconodont adds to the evidence of homoplasy of vertebral characters in 
the thoraco-lumbar transition and unfused lumbar ribs among early mammals. 
This is similar to the effect of homeobox gene patterning of vertebrae in 
modern mammals, making it plausible to extrapolate the effects of Hox gene 
patterning to account for homoplastic evolution of vertebral characters in 
early mammals."

Introduces _Yanoconodon allini_ (gen. et sp. nov.), family Jeholodentidae 
(nov.).

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