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Yanoconodon -- Chinese Eutriconodont With Transitional Middle Ear Bone Condition

Today's issue of the journal, _Nature_, features a new eutriconodont mammal
discovery from the Early Cretaceous Yixian Formation of the Hebei Province
of northeastern China: _Yanoconodon_.  The fossilized adult specimen
features "middle ear bones" that connect to its mandible via an ossified
Meckel's cartilage. 

Nature 446, 288-293 (15 March 2007) | doi:10.1038/nature05627; Received 4
August 2006; Accepted 29 January 2007

A new eutriconodont mammal and evolutionary development in early mammals
Zhe-Xi Luo1,2, Peiji Chen3, Gang Li3 and Meng Chen2

Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA 
Department of Earth Sciences, Nanjing University, Nanjing 210093, China 
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of
Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008,
Correspondence to: Zhe-Xi Luo1,2 Correspondence and requests for materials
should be addressed to Z.-X.L. (Email: luoz@carnegiemnh.org).


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.

Read the press release at 
http://www.carnegiemnh.org/news/07-jan-mar/yano.htm . 

Access images at
http://www.carnegiemnh.org/news/07-jan-mar/fossil/index.htm .

Ralph W. Miller III
Docent at the California Academy of Sciences
Dinosaur and Fossil Education
Member of the Society of Vertebrate Paleontology