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Huge morphological analysis reconfirms deep branch relationships among Squamates.
Okay, so it turns out that this was a month off from what I was initially told,
but the magnum opus on squamate phylogeny has finally been released by the
Peabody Museum. This is the culmination of seven years worth of data collection
and analysis on 192 taxa with just under 1000 characters. This is, in many
ways, a response to the molecular phylogeny of Townsend et al. 2004 that
rewrote the squamate family tree and nested iguanians deep within scleroglossa.
As the authors point out in their analysis, if the molecular phylogenies
proposed by Townsend, Vidal, Hedges, and/or Wiens are correct then it would
require between 51 and 147 evolutionary reversals. Reversals that, to date,
have not shown any evidence in the fossil record.
Sadly the paper does nothing to resolve the dichotomy between molecular and
morphological systematics, as it's results verify many previous morphological
analyses, but it is the first to really point out the inherent evolutionary
problems with the current molecular topology; a view that seems to have been
either missed, or ignored by much of the current literature.
One other key aspect of this paper (and one that will likely leave it as a
constant reference for years to come) is the illustration of all the different
characters the authors used. In many ways this makes this paper a bit of a
"definitive" atlas for squamate morphological analysis. I'd say this is a must
read for anyone doing squamate systematics.
Abstract and link to the paper are below.
Long live Scleroglossa and Iguania!
"I am impressed by the fact that we know less about many modern [reptile] types
than we do of many fossil groups." - Alfred S. Romer
Assembling the Squamate Tree of Life: Perspectives from the Phenotype and the
Jacques A. Gauthier, Maureen Kearney, Jessica Anderson Maisano, Olivier Rieppel
and Adam D.B. Behlke
lected species—51 extinct and 141 extant—and 976 apomorphies distributed among
610 phenotypic characters to investigate the phylogeny of Squamata (“lizards,”
including snakes and amphisbaenians). These data enabled us to infer a tree
much like those derived from previous morphological analyses, but with better
support for some key clades. There are also several novel elements, some of
which pose striking departures from traditional ideas about lizard evolution
(e.g., that mosasaurs and polyglyphanodontians are on the scleroglossan stem,
rather than parts of the crown, and related to varanoids and teiids,
respectively). Long-bodied, limb-reduced, “snake-like” fossorial lizards—most
notably dibamids, amphisbaenians and snakes—have been and continue to be the
chief source of character conflict in squamate morphological phylogenetics.
Carnivorous lizards (especially snakes, mosasaurs and varanoids) have proven a
close second. Genetic data, presumably less burdened by the potential for
adaptive convergence related to fossoriality, were expected to resolve these
conflicts. Although recent gene phylogenies
seem to do so, they also differ radically from any phylogeny based on the
phenotype, especially for the most ancient crown squamate divergences that
occured during the latter half of the Mesozoic. Our study relied on
traditionally prepared specimens as well as
high-resolution computed tomography scans that afforded unprecendented access
to the cranial anatomy of Squamata. This, along with the inclusion of stem
fossils, provided an unparalleled sample of the
phenotype enabling us to more fully explore the extreme incongruences between
molecular and morphological topologies for the squamate tree of life. Despite
this extensive new database, we were unable to find morphological support for
the major rearrangement of the deep divergences in Squamata proposed by recent
molecular studies. Instead, our data strongly support the same fundamental
topology suggested by most previous morphological studies—an
Iguania-Scleroglossa basal split, a sister-group relationship between Gekkota
and Autarchoglossa, and the divergence between Anguimorpha and Scincomorpha—and
documents the extreme degree of morphological homoplasy required by those