Gondwana distributions of 'young taxa'

Steven Dessein Steven.dessein at BIO.KULEUVEN.AC.BE
Fri Feb 21 15:02:39 CST 2003

Dear all,

Since a first survey of literature did not provide me the answer, I ask
the question to all of you:

How to explain a Gondwana distribution of a genus of which the origin
is situated in the Miocene (ca. 26-7 million years ago)? The fruits are
dry capsules with only 1 seed per capsule, the seeds are relatively
large (ca. 1-5 mm), and they are presumably dispersed by ants.

Do you know similar genera that show such a distribution? Which
hypotheses have been formulated in literature to explain this kind of
distributions? Or is the way we date the origin of taxa by the aid of
fossils and molecular clock hypotheses unreliable???

Some more information:

The genus mentioned is Spermacoce, a herbaceous member of the
Rubiaceae. It is deeply nested within the  subfamily Rubioideae (origin
suspected to be no earlier than 65 million years ago), and is a derived
taxon (pollen records of Spermacoce (often called Borreria in the New
World) are recorded from the Miocene, both in the paleo- and
neotropics). Spermacoce belongs to the tribe Spermacoceae which has in
total ca. 20 genera. Most of these genera are endemic in the Neotropics.
The highest number of Spermacoce species is found in America (ca. 125),
a lesser amount in Australia and Asia (each ca. 70), and the least in
Asia (ca. 20 species). It is remarkable that very similar species (with
very characteristic pollen grains) are present at all these continents.



Steven Dessein
Laboratory of Plant Systematics
Kasteelpark Arenberg 31
3001 Leuven

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