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Turtles as sister group of archosaurs

From: Ben Creisler

A new online paper about turtles:

Nicholas G. Crawford, Brant C. Faircloth, John E. McCormack, Robb T.
Brumfield, Kevin Winker, and Travis C. Glenn (2011)
More than 1000 ultraconserved elements provide evidence that turtles
are the sister group of archosaurs.
Biology Letters (advance online publication)

We present the first genomic-scale analysis addressing the
phylogenetic position of turtles, using over 1000 loci from
representatives of all major reptile lineages including tuatara.
Previously, studies of morphological traits positioned turtles either
at the base of the reptile tree or with lizards, snakes and tuatara
(lepidosaurs), whereas molecular analyses typically allied turtles
with crocodiles and birds (archosaurs). A recent analysis of shared
microRNA families found that turtles are more closely related to
lepidosaurs. To test this hypothesis with data from many single-copy
nuclear loci dispersed throughout the genome, we used sequence
capture, high-throughput sequencing and published genomes to obtain
sequences from 1145 ultraconserved elements (UCEs) and their variable
flanking DNA. The resulting phylogeny provides overwhelming support
for the hypothesis that turtles evolved from a common ancestor of
birds and crocodilians, rejecting the hypothesized relationship
between turtles and lepidosaurs.


Another paper, about fossil turtles:

Nicholas G. Crawford, Brant C. Faircloth, John E. McCormack, Robb T.
Brumfield, Kevin Winker, and Travis C. Glenn (2011)
Variation in osteocytes morphology vs bone type in turtle shell and
their exceptional preservation from the Jurassic to the present.
Bone (advance online publication)

Here we describe variations in osteocytes derived from each of the
three bone layers that comprise the turtle shell. We examine
osteocytes in bone from four extant turtle species to form a
morphological ‘baseline’, and then compare these with morphologies of
osteocytes preserved in Cenozoic and Mesozoic fossils. Two different
morphotypes of osteocytes are recognized: flattened-oblate osteocytes
(FO osteocytes), which are particularly abundant in the internal
cortex and lamellae of secondary osteons in cancellous bone, and
stellate osteocytes (SO osteocytes), principally present in the
interstitial lamellae between secondary osteons and external cortex.
We show that the morphology of osteocytes in each of the three bone
layers is conserved through ontogeny. We also demonstrate that these
morphological variations are phylogenetically independent, as well as
independent of the bone origin (intramembranous or endochondral).
Preservation of microstructures consistent with osteocytes in
morphology in Cenozoic and Mesozoic fossil turtle bones appears to be
common, and occurs in diverse diagenetic environments including
marine, freshwater, and terrestrial deposits. These data have
potential to illuminate aspects of turtle biology and evolution
previously unapproachable, such as estimates of genome size of extinct
species, differences in metabolic rates among different bones from a
single individual, and potential function of osteocytes as capsules
for preservation of ancient biomolecules.