Gondwana distributions of 'young taxa'

Ken Kinman kinman at HOTMAIL.COM
Sat Feb 22 00:04:37 CST 2003

      My experience is that taxa are usually older than we might think they
are.  An Oligocene (or even Eocene?) origin for Spermacoce would not
surprise me.
      As for dispersal, their salt-tolerance and ability to hop to islands
might indicate these plants or their seeds could be easily spread by ocean
currents.  Although a 1.0 mm seed might also be light enough to be blown by
strong storms over smaller ocean gaps.
      And if Antarctica was still connected to Australia at the beginning of
the Miocene, there would have been little (if any) ocean gap to traverse in
that direction.  Even today the gap between Antarctica and South America is
relatively small.  So to me Spermacoce looks like another genus in which
Antarctica probably played a central role.  I therefore wouldn't be at all
surprised if their pollen were to be found in the Oligocene of Antarctica.
           ----- Cheers,
                    Ken Kinman
>From: Steven Dessein <Steven.dessein at BIO.KULEUVEN.AC.BE>
>Reply-To: Steven Dessein <Steven.dessein at BIO.KULEUVEN.AC.BE>
>Subject: Gondwana distributions of 'young taxa'
>Date: Fri, 21 Feb 2003 15:02:39 +0100
>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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