[Taxacom] Biogeographic age of New Caledonia

[John Grehan]

The following paper provides a nice illustration of confused or
inconsistent thinking about biogeography.

Garrouste et al 2021. New fossil discoveries illustrate the diversity of
past terrestrial ecosystems in New Caledonia.
www.nature.com/scientificreports

They assert:

“New Caledonia cannot be considered as a kind of Mesozoic Gondwanan
‘refuge’ and its flora thus appears to have undergone similar kinds of
changes as the broader biogeographic region in which it is situated, at
least in terms of a basic taxonomic turnover towards angiosperm-dominated
communities over long geological periods” citing Southerland et al 2020 as
the authority).

BUT

“Present fossil evidence does not rule out the possibility that some of the
characteristic deep-branched lineages endemic to New Caledonia (e.g.,
Amborella) are relics of earlier ecosystems (viz. those of Zealandia) that
potentially survived obduction on neighbouring now-drowned islands, as
suggested by geological studies (refs 58, 59) and biogeographic analyses
based on fossil and present-day taxa (refs 2,3,60–62.”

This is a great step forward as it agrees with the panbiogeographic
metapopulation model that was applied to New Caledonia (although amazingly
somehow the authors appear to be oblivious to the panbiogeographic
literature) – that taxa are able to persist even in regions suffering
extensive marine incursion through survival on those outcrops that at any
one time are terrestrial.

HOWEVER

“Nevertheless, assembly of the modern New Caledonian biota from the
Oligocene to early Miocene is most consistent with the ages inferred by
molecular dating for many extant clades (ref 6) and the idea that much of
the current biota was assembled via dispersal from elsewhere.”

Ref 6 is a study that is a compilation of divergence ages that are treated
as actual ages so that “provide strong evidence for refuting the hypothesis
of a Gondwanan refuge with a biota that originated by vicariance. Only a
few inclusive Pacific clades (6 out of 40) were older than the oldest
existing island”

But of course all these age estimates can only be minimums that do not
refute a 'Gondwanan' refuge for taxa with younger minim ages (Gondwanan in
quote marks since not all taxa of 'Gondwana' age in New Caledonia are
necessarily Gondwanan in origin)

LASTLY

“Our fossil evidence suggests that highly diverse angiosperm communities,
very similar to those found on New Caledonia today, were established on the
island by the early Miocene.”

This is a good reading of fossil ages – as minimums - so they correctly say
“by” the early Miocene rather than at or in the Miocene. But when applied
to molecules the ages somehow become something else.

John Grehan