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Thalattosuchia sister group to Crocodyliformes + youngest tapinocephalid dinocephalian



Ben Creisler
bcreisler@gmail.com


Two recent non-dino papers that may be of interest:

Eric W. Wilberg (2015)
What's in an Outgroup? The Impact of Outgroup Choice on the
Phylogenetic Position of Thalattosuchia (Crocodylomorpha) and the
Origin of Crocodyliformes.
Systematic Biology (advance online publication)
doi: 10.1093/sysbio/syv020
http://sysbio.oxfordjournals.org/content/early/2015/04/02/sysbio.syv020.short?rss=1

Outgroup sampling is a central issue in phylogenetic analysis.
However, good justification is rarely given for outgroup selection in
published analyses. Recent advances in our understanding of archosaur
phylogeny suggest that many previous studies of crocodylomorph and
crocodyliform relationships have rooted trees on outgroup taxa that
are only very distantly related to the ingroup (e.g., Gracilisuchus
stipanicicorum), or might actually belong within the ingroup.
Thalattosuchia, a group of Mesozoic marine crocodylomorphs, has a
controversial phylogenetic position - they are recovered as either the
sister group to Crocodyliformes, in a basal position within
Crocodyliformes, or nested high in the crocodyliform tree.
Thalattosuchians lack several crocodyliform apomorphies, but share
several character states with derived long-snouted forms with a
similar ecological habit, suggesting their derived position may be a
result of convergent evolution. Several of these "shared" characters
may result from ambiguously worded character state definitions -
structures that are superficially similar but anatomically different
in detail are identically coded. A new analysis of crocodylomorphs
with increased outgroup sampling recovers Thalattosuchia as the sister
group to Crocodyliformes, distantly related to long-snouted
crocodyliforms. I also demonstrate that expanding the outgroup
sampling of previously published matrices results in the recovery of
thalattosuchians as sister to Crocodyliformes. The exclusion of
thalattosuchians from Crocodyliformes has numerous implications for
large-scale evolutionary trends within the group, including extensive
convergence in the evolution of the secondary palate characteristic of
the group. These results demonstrate the importance of careful
outgroup sampling and character construction, and their profound
effect on the position of labile clades.

===

Free pdf:


Michael O. Day, Saniye Güven, Fernando Abdala, Sifelani Jirah, Bruce
S. Rubidge & John Almond (2015)
Youngest dinocephalian fossils extend the Tapinocephalus Zone, Karoo
Basin, South Africa.
South African Journal of Science 111, No 3/4 (2015) (5 Pages)
http://www.sajs.co.za/sites/default/files/publications/pdf/Day_Research%20Letter_0.pdf


The dinocephalians (Synapsida, Therapsida) were one of the dominant
tetrapod groups of the Middle Permian (Guadalupian Epoch, ~270–260
million years ago) and are most abundantly recorded in the
Tapinocephalus Assemblage Zone (AZ) of the Main Karoo Basin, South
Africa. Dinocephalians are thought to have become extinct near the top
of the Abrahamskraal Formation of the Beaufort Group and their
disappearance is one criterion used to define the base of the
overlying Pristerognathus AZ. Because of the abundance of fossils in
the Karoo, the Beaufort Group biozones form the biostratigraphic
standard for later Permian terrestrial tetrapod ecosystems, so their
stratigraphic delineation is of great importance to Permian
palaeobiology. We report two new specimens of the rare tapinocephalid
dinocephalian Criocephalosaurus from the lowermost Poortjie Member,
which makes them the youngest dinocephalians known from the Main Karoo
Basin and extends the Tapinocephalus AZ from the Abrahamskraal
Formation up into the Teekloof Formation. The extension of the
Tapinocephalus AZ relative to the lithostratigraphy potentially
affects the biozone or biozones to which a fossil species can be
attributed; this extension has implications for biostratigraphic
correlations within the Main Karoo Basin as well as with other basins
across Gondwana. These discoveries also indicate that a population of
herbivorous tapinocephalids survived as rare constituents of the
tetrapod fauna after most generic richness within the clade had
already been lost.