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Turtle shoulder girdle and ribcage evolution

From: Ben Creisler

A new online paper:

Tyler R. Lyson and Walter G. Joyce (2012)
Evolution of the turtle bauplan: the topological relationship of the
scapula relative to the ribcage.
Biology Letters (advance online publication)
doi: 10.1098/rsbl.2012.0462

The turtle shell and the relationship of the shoulder girdle inside or
‘deep’ to the ribcage have puzzled neontologists and developmental
biologists for more than a century. Recent developmental and fossil
data indicate that the shoulder girdle indeed lies inside the shell,
but anterior to the ribcage. Developmental biologists compare this
orientation to that found in the model organisms mice and chickens,
whose scapula lies laterally on top of the ribcage. We analyse the
topological relationship of the shoulder girdle relative to the
ribcage within a broader phylogenetic context and determine that the
condition found in turtles is also found in amphibians, monotreme
mammals and lepidosaurs. A vertical scapula anterior to the thoracic
ribcage is therefore inferred to be the basal amniote condition and
indicates that the condition found in therian mammals and archosaurs
(which includes both developmental model organisms: chickens and mice)
is derived and not appropriate for studying the developmental origin
of the turtle shell. Instead, among amniotes, either monotreme mammals
or lepidosaurs should be used.