A few weeks ago there was a
post here about the controversy over exactly what position turtles have in
vertebrate evolution. Turtles have long
been considered anapsida
reptiles (Romer 1966). Anapsids
lack a temporal foramen and all of them, except the turtles (if in fact they
are anapsids), were extinct by the Triassic. But, even Romer seemed to question
whether or not turtles were really anapsids, he wrote:
“The turtles, forming the
order Chelonia, may technically be included as members of the Anapsida, since
the skull roof, although often emarginated and incomplete, has never developed
true temporal openings. In almost every other respect, however, the turtles
have departed widely from the structural pattern of their cotylosaur ancestor.”
Other
hypotheses about turtle relationships were forth coming: Rieppel and deBraga (1996)
suggested turtles belong to a sister-group to extant lepidosaurian diapsids (tuatars,
lizards and snakes). But, molecular studies (most recently Hugall
et al. 2007) have
consistently suggested turtles have a close affinity to archosaurian diapsids
(crocodilians and birds); a hypothesis also supported by some similarities of the
karyotypes and genomes of sauropsidian lineages and turtles (Chapus and Edwards,
2009) and a view held by classical embryologists such as Ernst Haeckel and Gavin
de Beer. Turtles have changed the pattern of their axial skeleton, placing
their scapula inside the rib cage and fusing their ribs to the carapace (upper
shell) making them a novelty. Attempts to understand this has created the variety
of hypotheses.
In a recent paper, Shigeru Kuratani of the RIKEN
Center for Developmental Biology, in Kobe, Japan and colleagues propose the key to understanding the turtle’s shell patterning
lies in the modification of the ribs. Turtle embryos have a unique structure,
the carapacial ridge (CR). It is assumed to be responsible for the changes in
the growth pattern that result in the turtle’s carapace. In turtle embryos
growth of the ribs is arrested in the axial part of the body, this allows dorsal
and lateral oriented growth to encapsulate the scapula. Although the CR does
not appear to induce this axial arrest, it has been shown to support the
fan-shaped patterning of the ribs, which occurs at the same time as the marginal
growth of the carapace along the line of the turtle-specific folding that takes
place in the lateral body wall. During the process of folding, some trunk
muscles maintain their ancestral connections, while the limb muscles establish
new attachments specific to the turtle. Kuratani et al. suggest the turtle body
plan can thus be explained with our knowledge of vertebrate anatomy and
developmental biology, and that it is consistent with the evolutionary origin
of the turtle suggested by the recently discovered fossil species, Odontochelys – the very unique turtle with only the lower shell (plastron) - lower art work.
Literature
Chapus, C., and
Edwards, S. V. 2009. Genome evolution in Reptilia: in silico chicken mapping of
12,000 BAC-end sequences from two reptiles and a basal bird. BMC Genomics 10 (suppl 2): S8.
Kuratani, S., S. Kuraku, H. Nagashima. 2011. Evolutionary developmental perspective for the
origin of turtles: the folding theory for the shell based on the developmental
nature of the carapacial ridge. Evolution
& Development 13(1):1-14.
Rieppel, O., and
deBraga, M. 1996. Turtles as diapsid reptiles. Nature 384:453–455.
Romer, A. S.
1966. Vertebrate Paleontology. 3rd
Ed. University of Chicago Press, Chicago.
Labels: carpacial ridge, Chelonia evolution, hypotheses