Re: [dinosaur] Ichthyornis skull reveals origin of beak

[Tim Williams <tijawi@gmail.com>]

Great work based on amazing specimens.  As noted by the authors, one
of the more striking features of _Ichthyornis_ is the impressive
mobility and flexibility of its jaws - producing a wide gape,
excellent for swallowing fish - aided by the recurved, very theropodan
teeth.

Another quote from the paper:

"All parts of the Ichthyornis skull provide insights into the form and
function of the ancestral ornithuran head and the transition from an
early avialan to an avian condition. The pincer-like action of a
sharp-tipped, toothless beak would have facilitated fine manipulation
and preening - essentially performing the role of a surrogate hand as
the hands themselves became bound up into wings"

I've no doubt this is true for _Ichthyornis_.  But I'm skeptical that
this pincer-like ability of the beak was replacing a pre-existing
function of the forelimbs.  I doubt that any theropod was capable of
fine manipulation using its hands.  The theropod hand was primitively
capable of grasping, but it's debatable whether it was capable of much
(if any) manipulation.  I would argue that this favored the
transformation of the forelimbs into wings.  The forelimbs weren't
much use in prey capture, notably for theropods that targeted small
prey or were not carnivorous.  So the forelimbs were 'free' to take on
other functions.  As a relatively 'advanced' avialan, the hand of
_Ichthyornis_ (as part of the carpometacarpus) was fully incorporated
into the wing.  But the manus probably had very limited grasping
ability long before the _Ichthyornis_ stage of avialan evolution -
especially if two fingers were bound together in the maniraptoran
ancestors of birds.



David Marjanovic <david.marjanovic@gmx.at> wrote:

> And indeed, 5 of 7 data matrices support *Ichthyornis* as the sister-group of 
> Hesperornithes + Neornithes. (One finds the traditional position closer to 
> Neornithes, another is inconclusive.)

This fits with the plesiomorphic 'thecodont' dentition of mature
_Ichthyornis_, with teeth set in individual sockets separated by
ossifications - not implanted along a continuous groove as in
_Hesperornis_ (is 'aulacodont' the right word?).  This was first noted
by Marsh in 1880, and supported by Martin and Stewart (1977 - although
this study incorrectly interpreted the teeth as crocodilian-like) and
Clarke (2004).


> "evolutionary elaboration of the bird brain may have been in service to the 
> exigencies of avian flight, the most sophisticated and demanding form of 
> locomotion in the history of vertebrate life."
>
> Except for bat flight, right?

The flight abilities of _Ichthyornis_ would have been quite good, but
I doubt if they were anywhere near as sophisticated as those of crown
Aves.  Although _Ichthyornis_ had a huge sternal keel, the
supracoracoideus pulley doesn't appear to have been fully developed,
and wing elevation was most likely predominantly driven by the
deltoideus.