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stapes (reply to reply to CYNODONT)
> From: NJPharris@aol.com
> Date: Tue, 17 Jun 1997 18:44:39 -0400 (EDT)
> To: Pieter.Depuydt@rug.ac.be, email@example.com,
> Subject: Re: LATE SURVIVING CYNODONT (lenghty and boring)
> In a message dated 97-06-16 05:53:20 EDT, Pieter.Depuydt@rug.ac.be writes:
> > Very right. The stapes we mammals inherited from our pre-Synapsid amniote
> > ancestor since this small bone is also present in extant [reptiles],
> > lizards and birds.
> Do I remember correctly that the stapes is derived from the hyomandibular of
> fishes, the upper half of the ancestral second gill arch (the lower half of
> which has become the hyoid apparatus)?
You are right. In fact, well preserved braincases of the
stem-tetrapod Acanthostega already show the presence of a stout,
massive stapes, firmly attached to the fenestra ovalis of the
braincase (Acanthostega also possessed already a fenestra ovalis!,
probably derived from the vestibular fontanelle of osteolepiform
fishes) (See Clack Nature 342 1989 and 369 1994).
The stapes in early tetrapods probably had a mechanical,
supportive, function, linking the otic capsule (braincase) to the palate.
Later on, it became lighter and rod-like, and looser attached to the
braincase, together with the evolution of a tympanum (Acanthostega
probably lacked a tympanum), with the increasing acquisition of
high-frequency hearing in terrestrial tetrapods.