Re: Limits of Kinds - Isoetes

R. Joel Duff (
Thu, 13 Nov 1997 15:13:35 -0600 (CST)

Glenn Morton responding to Karen Jensen:
<most of message chopped>
>>I think we could agree on this. And I will re-iterate, the Ginkgo is the
>>most similar of ancient to modern examples of living fossils.
>Good. It may well be.
>P.S. What do you think about conifers such as Metasequoia, the cycads, and
>ferns, lycopods, and equisitum, which have long fossil records?

I would suggest that rather than Ginkgo being the most similar of ancient
to modern examples that Isoetes (Lycophyta) might make a run for that
record. Members ascribed to the MODERN genus have been found in the
earliest Triassic shales of Australia (Retallack, 1997). The familily is
thought to go back to the early Carboniferous.

This may not be all that surprising though. Isoetes has a highly reduced
vegatative morphology which such that any two species of the 150 species
world wide would be difficult to tell apart from one another based on
leaves, roots, or corm structure alone. The few definitive charaters
involve the sporangia and spores.

All of this talk of rates of change and ginkgo leaves reminds me that
plants consist of many parts which evolve at different rates. Isoetes is a
good example of this mosaic pattern. Some characters remained virtually
unchanged over 200 million years while other exhibit a wide degree of
change over time. Although leaves are usually considered quite pliable
features such as the floral morphology of plants are much more apt to be
conserved through time. Thus in the fossil record we might see little
change in one character while other portions of the plant might have
exhibited quite a bit of change. Unfortunately this makes the designation
of species status for fossil plants very difficult as if it weren't
difficult enough for extant taxa.

Note also that lack of morphology change does not mean there is a lack of
genetic change over time. Morphology and underlying genetic change are not
always tightly correlated.

Joel Duff

Retalack, R. 1997. Earliest Triassic origin of Isoetes and quillwort
evolutionary radiation. Journal Paleontology 7: 500-521

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