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Placoderm sexual claspers formed as extra limblike organs

From: Ben Creisler

A recent non-dino paper that may be of interest:

Kate Trinajstic, Catherine Boisvert, John Long, Anton Maksimenko, and
Zerina Johanson (2014)
Pelvic and reproductive structures in placoderms (stem gnathostomes).
Biological Reviews (advance online publication)
DOI: 10.1111/brv.12118

Newly discovered pelvic and reproductive structures within placoderms,
representing some of the most crownward members of the gnathostome
stem group and the most basal jawed vertebrates, challenge established
ideas on the origin of the pelvic girdle and reproductive complexity.
Here we critically review previous descriptions of the pelvic
structures in placoderms and reinterpret the morphology of the pelvic
region within the arthrodires and ptyctodonts, in particular the
position of the pelvic fin and the relationship of the male clasper to
the pelvic girdle. Absence of clear articular surfaces on the clasper
and girdle in the Arthrodira, along with evidence from the
Ptyctodontida, suggest that these are separate structures along the
body. We describe similarities between the pectoral and pelvic girdles
and claspers, for example, all these have both dermal and perichondral
(cartilaginous) components. Claspers in placoderms and chondrichthyans
develop in very different ways; in sharks, claspers develop from the
pelvic fin while the claspers in placoderms develop separately,
suggesting that their independent development involved a posterior
extension of the 'competent stripes' for fin development previously
limited to the region between the paired pectoral and pelvic fins.
Within this expanded zone, we suggest that clasper position relative
to the pelvic fins was determined by genes responsible for limb
position. Information on early gnathostome reproductive processes is
preserved in both the Ptyctodontida and Arthrodira, including the
presence of multiple embryos in pregnant females, embryos of differing
sizes and of different sexes (e.g. male claspers preserved in some
embyros). By comparison with chondrichthyans, these observations
suggest more complex reproductive strategies in placoderms than
previously appreciated.


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