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Sphenodon anatomy: genital swelling and knee caps

Ben Creisler

Some recent (and not so recent) papers on the Sphenodon.

Patricia L. R. Brennan (2016)
Evolution: One Penis After All.
Current Biology 26(1): pR29–R31
DOI: http://dx.doi.org/10.1016/j.cub.2015.11.024 |

Amniote penises come in many shapes but are missing from the basal
tuatara. This has been taken as evidence for multiple evolutionary
origins of the penis. Now, genital swellings have been found in
tuatara embryos, arguing for a single origin.


Thomas J. Sanger, Marissa L. Gredler & Martin J. Cohn (2015)
Resurrecting embryos of the tuatara, Sphenodon punctatus, to resolve
vertebrate phallus evolution.
Biology Letters 11: 20150694.

The breadth of anatomical and functional diversity among amniote
external genitalia has led to uncertainty about the evolutionary
origins of the phallus. In several lineages, including the tuatara,
Sphenodon punctatus, adults lack an intromittent phallus, raising the
possibility that the amniote ancestor lacked external genitalia and
reproduced using cloacal apposition. Accordingly, a phallus may have
evolved multiple times in amniotes. However, similarities in
development across amniote external genitalia suggest that the phallus
may have a single evolutionary origin. To resolve the evolutionary
history of amniote genitalia, we performed three-dimensional
reconstruction of Victorian era tuatara embryos to look for
embryological evidence of external genital initiation. Despite the
absence of an intromittent phallus in adult tuataras, our observations
show that tuatara embryos develop genital anlagen. This illustrates
that there is a conserved developmental stage of external genital
development among all amniotes and suggests a single evolutionary
origin of amniote external genitalia.


Sophie Regnault, Marc E. H. Jones, Andrew A. Pitsillides and John R.
Hutchinson (2015)
Anatomy, morphology and evolution of the patella in squamate lizards
and tuatara (Sphenodon punctatus).
Journal of Anatomy (advance online publication)
DOI: 10.1111/joa.12435

The patella (kneecap) is the largest and best-known of the sesamoid
bones, postulated to confer biomechanical advantages including
increasing joint leverage and reinforcing the tendon against
compression. It has evolved several times independently in amniotes,
but despite apparently widespread occurrence in lizards, the patella
remains poorly characterised in this group and is, as yet, completely
undescribed in their nearest extant relative Sphenodon
(Rhynchocephalia). Through radiography, osteological and fossil
studies we examined patellar presence in diverse lizard and
lepidosauromorph taxa, and using computed tomography, dissection and
histology we investigated in greater depth the anatomy and morphology
of the patella in 16 lizard species and 19 Sphenodon specimens. We
have found the first unambiguous evidence of a mineralised patella in
Sphenodon, which appears similar to the patella of lizards and shares
several gross and microscopic anatomical features. Although there may
be a common mature morphology, the squamate patella exhibits a great
deal of variability in development (whether from a cartilage anlage or
not, and in the number of mineralised centres) and composition (bone,
mineralised cartilage or fibrotendinous tissue). Unlike in mammals and
birds, the patella in certain lizards and Sphenodon appears to be a
polymorphic trait. We have also explored the evolution of the patella
through ancestral state reconstruction, finding that the patella is
ancestral for lizards and possibly Lepidosauria as a whole. Clear
evidence of the patella in rhynchocephalian or stem lepidosaurian
fossil taxa would clarify the evolutionary origin(s) of the patella,
but due to the small size of this bone and the opportunity for
degradation or loss we could not definitively conclude presence or
absence in the fossils examined. The pattern of evolution in
lepidosaurs is unclear but our data suggest that the emergence of this
sesamoid may be related to the evolution of secondary ossification
centres and/or changes in knee joint conformation, where enhancement
of extensor muscle leverage would be more beneficial.