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Eardrum evolved independently in mammals and diapsids (free pdf)

Ben Creisler

A new paper (open access) that may be of interest:

Taro Kitazawa, Masaki Takechi, Tatsuya Hirasawa, Noritaka Adachi,
Nicolas Narboux-Nême, Hideaki Kume, Kazuhiro Maeda, Tamami Hirai,
Sachiko Miyagawa-Tomita, Yukiko Kurihara, Jiro Hitomi, Giovanni Levi,
Shigeru Kuratani & Hiroki Kurihara (2015)
Developmental genetic bases behind the independent origin of the
tympanic membrane in mammals and diapsids.
Nature Communications 6, Article number: 6853
doi: 10.1038/ncomms7853 Terms

The amniote middle ear is a classical example of the evolutionary
novelty. Although paleontological evidence supports the view that
mammals and diapsids (modern reptiles and birds) independently
acquired the middle ear after divergence from their common ancestor,
the developmental bases of these transformations remain unknown. Here
we show that lower-to-upper jaw transformation induced by inactivation
of the Endothelin1-Dlx5/6 cascade involving Goosecoid results in loss
of the tympanic membrane in mouse, but causes duplication of the
tympanic membrane in chicken. Detailed anatomical analysis indicates
that the relative positions of the primary jaw joint and first
pharyngeal pouch led to the coupling of tympanic membrane formation
with the lower jaw in mammals, but with the upper jaw in diapsids. We
propose that differences in connection and release by various
pharyngeal skeletal elements resulted in structural diversity, leading
to the acquisition of the tympanic membrane in two distinct manners
during amniote evolution.