BEAK EVOLUTION IN TETRAPODS

[darren.naish@port.ac.uk]

Yet again the thread has died and I never had the time to send this 
message in when it was alive. Too much work, too little time.

Those of you wondering about the evolution of beaks in reptiles might 
want to consult:

LEE, M.S.Y. 1997. The evolution of beaks in reptiles: a proposed 
evolutionary constraint. _Evolutionary Theory_  August 1997: 249-254 
(I don't seem to have the volume number, sorry).

Mike notes that tetrapod beaks only occur in taxa that exhibit a
caruncle (egg tooth) when hatching. Apparently, the caruncle is an 
amniote synapomorphy that has been lost in therian mammals and 
squamates and it's suggested that beaks evolve via modification of 
this structure. This explains the evolution of beaks in all 
groups bar therians, squamates, lissamphibians and basal tetrapods. 
Little known is that monotremes have a caruncle (with a bony core, 
the os carunculae) as do marsupials. Maybe it's possible, therefore, 
for marsupials to evolve beaks (!). 

Interesting in another context is Mike's contention that 
huphesuchians are ichthyosaurs AND that they are beaked. He cites 'M. 
Caldwell, unpublished data 1996' for this. Has Dr. Caldwell published 
this does anyone know? I haven't seen any huphesuchian fossils, but I 
am familiar with Carroll and Dong's monograph on _Huphesuchus_. No 
hint of a beak there: I would really like to see confirmation.

Other stuff...

Are turtles neodiapsids, or parareptiles? I have an opinion on this, 
but regardless, one new piece of data should be mentioned: recently 
identified procolophonoid scutes have sculpturing and morphology more 
like that of turtles than of any other group of armoured tetrapods. 
This is consistent with the placement of turtles within parareptilia. 

Lingham-Soliare has a new paper in _Lethaia_ on a _Mosasaurus_ brain 
endocast that supposedly reveals a fatal injury. Shards of bone 
preserved within the endocast indicate that the mosasaur was rammed, 
its braincase was invaded, and it died of a cerebral trauma! Other 
mosasaur specimens reveal injuries related to ramming behaviour (Dale 
Russell has mentioned a tylosaur with a squashed rostral prow).

Tom Rich and colleagues assert in the same issue that 
_Ausktribosphenos_ is a tribosphenid, and not a symmetrodont or 
monotreme, as a number of other early mammals workers have argued in 
recent months. 

"In the words of a famous rabbit, this means war: total and 
unforgiving"

DARREN NAISH
darren.naish@port.ac.uk