Tue, 11 Nov 1997 08:44:44 -0600 you wrote:
Karen G. Jensen wrote:
>>It was a thrill to me to dig in the Yorkshire Jurassic (on the coast at
>>Scarborough, at low tide) and come up with fossil ginkgophyte leaves that
>>look much like extant _Ginkgo_ leaves. I enjoy living _Ginkgo_ trees
>>because they represent one of the many tree species which has "persisted
>>relatively unchanged morphologically since the Mesozoic."
>I would point you to Taylor, Paleobotany, p. 413. The ginkgophyte leaves
>shown there are quite distinct from the modern variety.
The leaves I found are indeed deeply dissected Jurassic examples, like many
juvenile leaves on modern Ginkgo trees, often found on sprouts at the base
of the tree, not like most adult leaves on the rest of the tree. Stewart &
Rothwell's Fig 27:2 shows silhouettes of 5 leaves from modern Ginkgo which
illustrate this variability. Found separately as fossils, such different
morphologies would likely be placed in different species, except that there
are so many "intermediates" that the "species" distinctions become
difficult, as you see in Andrews. We don't know how many biological
species there were.
The Jurassic examples I found are mostly like the one often called Ginkgo
digitata (as in Andrews '61 Fig 11-13 A, p.339), not linear like the ones
in Taylor '81 p.413 Fig. 15-3 (Baiera furcata). The closest Taylor example
is in his Fig 15-1 B (Ginkgoites acosmia) p.412.
I would also note
>that the fossil reproductive structures of these plants have not been
>described in the literature very often. (p. 412)
There have been very few reproductive structures found. Stewart & Rothwell
'93 note (p391) "In 1976 Harris was able to report paired ovules joined by
a pad of tissue with Ginkgo-like stomata". There in the Yorkshire Jurassic
the pollen organs have also been identified, with monoscolpate pollen (as
in modern Ginkgo).
>> - _Ginkgo biloba_ is generally recognized as a "living fossil"
>> - fossil ginkgophytes were highly variable, quite dynamic, not static
>> - ginkgophytes nevertheless had their limits; only one species survives
>>So the fact that the species doesn't stop changing doesn't mean that it is
>>not a "living fossil", but rather it means that the taxon is dynamic,
>>within its limits.
>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?