stapes (reply to reply to CYNODONT)

[Pieter.Depuydt@rug.ac.be]

> From:          NJPharris@aol.com
> Date:          Tue, 17 Jun 1997 18:44:39 -0400 (EDT)
> To:            Pieter.Depuydt@rug.ac.be, radass@orion.neca.com, 
> dinosaur@usc.edu
> 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.

Pieter Depuydt