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Crocodylian pelvis: no "prepubis"
From: Ben Creisler
A new online paper:
Leon P.A.M. Claessens and Matthew K. Vickaryous (2012)
The evolution, development and skeletal identity of the crocodylian
pelvis: Revisiting a forgotten scientific debate.
Journal of Morphology (advance online publication)
Unlike most tetrapods, in extant crocodylians the acetabulum is formed
by only two of the three skeletal elements that constitute the pelvis,
the ilium, and ischium. This peculiar arrangement is further confused
by various observations that suggest the crocodylian pelvis initially
develops from four skeletal elements: the ilium, ischium, pubis, and a
novel element, the prepubis. According to one popular historical
hypothesis, in crocodylians (and many extinct archosaurs), the pubis
fuses with the ischium during skeletogenesis, leaving the prepubis as
a distinct element, albeit one which is excluded from the acetabulum.
Whereas the notion of a distinct prepubic element was once a topic of
considerable interest, it has never been properly resolved. Here, we
combine data gleaned from a developmental series of Alligator
mississippiensis embryos, with a revised interpretation of fossil
evidence from numerous outgroups to Crocodylia. We demonstrate that
the modern crocodylian pelvis is composed of only three elements: the
ilium, ischium, and pubis. The reported fourth pelvic element is an
unossified portion of the ischium. Interpretations of pelvic skeletal
homology have featured prominently in sauropsid systematics, and the
unambiguous identification of the crocodylian pubis provides an
important contribution to address larger scale evolutionary questions
associated with locomotion and respiration.