Re: eggs with thin shells

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

> I've been wondering about tails, anyway.
>Among the dinosaurs, they are universal, and nearly all are large, heavy
>and muscular.  Yet outside of dinosaurs and lizards, large tails are
>almost unknown - and certainly not very popular in mammals.  Compare an
>elephant's tail to an apatosaur's!  In the case of bipedal dinosaurs, this
>is not such a mystery, they were counterweights.  But for sauropods and
>others, there must have been a very good reason why they stayed so large.

The reason dinosaurs (and crurotarsians) retained their large tails has to
do with the musculature associated with walking.  Whereas in the Synapsida
(mammals included) and in primitve reptiles, the major "thrust" muscles for
the femur attach to the hip, in the Diapsida (lepidosauromorphs and
archosauromorphs) the major "thrust" muscle is the M. caudofemoralis.  As
the name implies, it attaches on the femur and the tail (or at least the
proximal part of the tail).

Or, as I tell my students, mammals haul ass, but reptiles haul tail.

In a few dinosaurs (i.e., dromaeosaurids and birds), the was significant
transformation of the hip and proximal tail region, and it is believed this
is associated with a transformation of the locomotory muscles.

Steve Gatsey (whose switched institutions so many times in the last few
years that I can't remember where he is now) has done some major reseach in
the M. caudofemoralis, both in living and in fossil groups.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.