New papers on Cretaceous divergence dates - Neornithes and Solenodon

[Christopher Taylor <ck.taylor@auckland.ac.nz>]

Hi all,

    A couple of new papers that have appeared over the last couple of days:

Dyke, G. J., & M. van Tuinen. 2004. The evolutionary radiation of modern
birds (Neornithes): reconciling molecules, morphology and the fossil record.
Zoological Journal of the Linnean Society 141(2): 153-177.

    "The pattern, timing and extent of the evolutionary radiation of
anatomically modern birds (Neornithes) remains contentious: dramatically
different timescales for this major event in vertebrate evolution have been
recovered by the 'clock-like' modelling of molecular sequence data and from
evidence extracted from the known fossil record. Because current synthesis
would lead us to believe that fossil and nonfossil evidence conflict with
regard to the neornithine timescale, especially at its base, it is high time
that available data are reconciled to determine more exactly the
evolutionary radiation of modern birds. In this review we highlight current
understanding of the early fossil history of Neornithes in conjunction with
available phylogenetic resolution for the major extant clades, as well as
recent advancements in genetic methods that have constrained time estimates
for major evolutionary divergences. Although the use of molecular approaches
for timing the radiation of Neornithes is emphasized, the tenet of this
review remains the fossil record of the major neornithine subdivisions and
better-preserved taxa. Fossils allowing clear phylogenetic constraint of
taxa are central to future work in the production of accurate molecular
calibrations of the neornithine evolutionary timescale."

    Not much new here - it's a 'state of the art' review. 166 references in
the bibliography, though, so might be a good reference source. Does include
a section going, at least briefly, over each of the modern orders and
reviewing fossil evidence for minimum ages for each - and despite the
authors probably leaning towards supporting Cretaceous divergences for all
the modern orders (van Tuinen has done a lot of work on avian mlecular
clocks), they take a fairly sceptical view of all the supposed Mesozoic
representatives of modern orders, with most of them being put in the 'too
scrappy' pile (data on such is drawn mostly from Hope's chapter in 'Mesozoic
birds'. The only one which is looked at in detail is _Polarornis_, and they
suggest that it can't be confirmed as a loon because Chatterjee didn't
compare it to enough taxa (no comparison to grebes, for instance).


Roca, A. L., G. K. Bar-gal et al. 2004. Mesozoic origin for West Indian
insectivores.

    "The highly endangered solenodons, endemic to Cuba (Solenodon cubanus)
and Hispaniola (S. paradoxus), comprise the only two surviving species of
West Indian insectivores. Combined gene sequences (13.9 kilobases) from S.
paradoxus established that solenodons diverged from other eulipotyphlan
insectivores 76 million years ago in the Cretaceous period, which is
consistent with vicariance, though also compatible with dispersal. A
sequence of 1.6 kilobases of mitochondrial DNA from S. cubanus indicated a
deep divergence of 25 million years versus the congeneric S. paradoxus,
which is consistent with vicariant origins as tectonic forces separated Cuba
and Hispaniola. Efforts to prevent extinction of the two surviving solenodon
species would conserve an entire lineage as old or older than many mammalian
orders."

    Most of the interesting stuff for this one is hidden in the
supplementary data. I'm not convinced. The calibration dates are all
divergences of clades in other parts of the mammalian tree - caniforms vs.
feliforms, hippomorph vs. ceratomorph, armadillo vs. anteater, etc. I'm not
a fan of molecular clocks at the best of times, but especially not when
dates are used to estimate points older than themselves, and in a separate
clade. There's no control here for the divergence of any of the major clades
(Laurasiatheria vs. Euarchontoglires, for instance). And offhand, the clock
model and data used give an age of around 105 Ma for the placental crown.
Any comments?
    One more thing - the calibration dates include maximum ages for some
divergences. How would you calculate a maximum age for a clade?

    Cheers,

        Christopher Taylor