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ORNITHISCHIAN FOURTH TROCHANTER ANATOMY



Peter Frick wrote:
<<Is there anybody out there who can answer the question
why in Hypsilophodon the 4th trochanter is long and
free-standing and in hadrosaurs robust and entirely
connected with the shaft?>>

Well, simply put, it's because _Hypsilophodon_ is little, fast, and bipedal,
and hadrosaurs weren't.

The femoral fourth trochanter in ornithischians is a sort of cone-shaped
process that points caudoventrally in basal forms.  

The dorsal portion of the process is the insertion of the M caudi-femoralis
brevis, whose origin is on the ventral post-acetabular flange of the illium.  

The M caudi-femoralis longus has an insertion  in a depression just dorsal
and medial to the base of the fourth trochanter, and is not actually
connected to the trochanter.  Its origin is the proximal caudal vertebrae and
haemal arches.  

The medial portion of the trochanter is the origin for the M femoro-tibialis
3, whose insertion is the proximal tibia.

The tip of the trochanter apparently was a connection point for a tendon or
ligament that ran to the proximal fibula.

Most small ornithischians exhibit the same fourth trochanter morphology as
_Hypsilophodon_ (_Gapirinisaura_, _Othnielia_, Thescelosaurus_ etc).  As for
large ornithischians... well the trochanter is less like a prong and more
like an asymmetrical triangle with the point ventral to the midline.  All the
same muscles attach in all the same spots as with the smaller ornithischians,
just to a less pronounced process.

The most likely explanation for the differing shape is that because they were
larger and less cursorial than their ancestors, they had reduced leg flexing
muscles (M caudi-femoralis brevis, M femoro-tibialis 3), and as a result, had
a smaller fourth trochanter.  A shift from obligate bipedal locomotion, to
functional quadrupedal locomotion might have some influence on the reduction
as well.

Pete Buchholz
Tetanurae@aol.com

"Don't fight it boy, confess early.. if you hold out, it could effect your
credit rating."