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New papers for Archosauria and Ichthyosauria

From: Ben Creisler

A couple of new papers of interest:

Nesbitt, Sterling J. (2011)
The early evolution of archosaurs : relationships and the origin of major
Bulletin of the American Museum of Natural History 352: 292 pp.
doi: 10.1206/352.1 
Free download at: http://digitallibrary.amnh.org/dspace/handle/2246/6112

Abstract - 
Archosaurs have a nearly 250 million year record that originated shortly
after the Permian-Triassic extinction event and is continued today by two
extant clades, the crocodylians and the avians. The two extant lineages
exemplify two bauplan extremes among a diverse and complex evolutionary
history, but little is known about the common ancestor of these lineages.
Renewed interest in early archosaurs has led to nearly a doubling of the
known taxa in the last 20 years.

This study presents a thorough phylogenetic analysis of 80 species-level
taxa ranging from the latest Permian to the early part of the Jurassic
using a dataset of 412 characters. Each terminal taxon is explicitly
described and all specimens used in the analysis are clearly stated.
Additionally, each character is discussed in detail and nearly all of the
character states are illustrated in either a drawing or highlighted on a
specimen photograph. A combination of novel characters and comprehensive
character sampling has bridged previously published analyses that focus on
particular archosauriform subclades.

A well-resolved, robustly supported consensus tree (MPTs  = 
360) found a monophyletic Archosauria consisting of two major branches, the
crocodylian-line and avian-line lineages. The monophyly of clades such as
Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria
is supported in this analysis. However, phytosaurs are recovered as the
closest sister taxon to Archosauria, rather than basal crocodylian-line
archosaurs, for the first time. Among taxa classically termed as
?rauisuchians,? a monophyletic poposauroid clade was found as the sister
taxon to a group of paraphyletic ?rauisuchians? and monophyletic
crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of
?rauisuchians,? and are not more closely related to aetosaurs than to taxa
such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and
similar forms (?Sphenosuchia?) were found as a paraphyletic grade leading
to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is
well supported. A monophyletic clade containing Silesaurus and similar
forms is well supported as the sister taxon to Dinosauria. Pterosaurs are
robustly supported at the base of the avian line.

A time-calibrated phylogeny of Archosauriformes indicates that the origin
and initial diversification of Archosauria occurred during the Early
Triassic following the Permian-Triassic extinction. Furthermore, all major
basal archosaur lineages except Crocodylomorpha were established by the end
of the Anisian. Early archosaur evolution is characterized by high rates of
homoplasy, long ghost lineages, and high rates of character evolution.
These data imply that much of the early history of Archosauria has not been
recovered from the fossil record. Not only were archosaurs diverse by the
Middle Triassic, but they had nearly a cosmopolitan biogeographic
distribution by the end of the Anisian. 

Philippa M. Thorne, Marcello Ruta, and Michael J. Benton (2011)
Resetting the evolution of marine reptiles at the Triassic-Jurassic
Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1018959108

Ichthyosaurs were important marine predators in the Early Jurassic, and an
abundant and diverse component of Mesozoic marine ecosystems. Despite their
ecological importance, however, the Early Jurassic species represent a
reduced remnant of their former significance in the Triassic. Ichthyosaurs
passed through an evolutionary bottleneck at, or close to, the
Triassic-Jurassic boundary, which reduced their diversity to as few as
three or four lineages. Diversity bounced back to some extent in the
aftermath of the end-Triassic mass extinction, but disparity remained at
less than one-tenth of pre-extinction levels, and never recovered. The
group remained at low diversity and disparity for its final 100 Myr. The
end-Triassic mass extinction had a previously unsuspected profound effect
in resetting the evolution of apex marine predators of the Mesozoic. 

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