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Nanchangosaurus redescribed, marine reptile from Lower Triassic of China
New in PLoS ONE:
Xiao-hong Chen, Ryosuke Motani, Long Cheng, Da-yong Jiang & Olivier
The Enigmatic Marine Reptile Nanchangosaurus from the Lower Triassic
of Hubei, China and the Phylogenetic Affinities of Hupehsuchia.
PLoS ONE 9(7): e102361.
The study of the holotype and of a new specimen of Nanchangosaurus
suni (Reptilia; Diapsida; Hupehsuchia) revealed a suite of hitherto
unrecognized characters. For example, Nanchangosaurus has bipartite
neural spines and its vertebral count is nearly identical to that of
Hupehsuchus. It differs from the latter in having poorly developed
forelimbs despite the advanced ossification in the rest of the
skeleton. Other differences all pertain to hupehsuchian plesiomorphies
retained in Nanchangosaurus, such as low neural spines. The
relationship of Hupehsuchia within Diapsida was analyzed based on a
data matrix containing 41 taxa coded for 213 characters, of which 18
were identified as aquatic adaptations from functional inferences.
These aquatic adaptations may be vulnerable to the argumentation of
character homology because expectation for homoplasy is high. There is
an apparent incongruence between phylogenetic signals from aquatic
adaptations and the rest of the data, with aquatic adaptations
favoring all marine reptiles but Helveticosaurus to form a
super-clade. However, this super-clade does not obtain when aquatic
adaptations were deleted, whereas individual marine reptile clades are
all derived without them. We examined all possible combinations of the
18 aquatic adaptations (n = 262143) and found that four lineages of
marine reptiles are recognized almost regardless of which of these
features were included in the analysis: Hupehsuchia-Ichthyopterygia
clade, Sauropterygia-Saurosphargidae clade, Thalattosauria, and
Helveticosaurus. The interrelationships among these four depended on
the combination of aquatic adaptations to be included, i.e., assumed
to be homologous a priori by bypassing character argumentation.
Hupehsuchia always appeared as the sister taxon of Ichthyopterygia.