At 12:31 PM 11/11/97 -0600, Karen G. Jensen wrote:
>p.414 (after descriptions and drawings of many fossil genera and species)
>"There are numerous reports of foliage types that are indistinguishable
>from the leaves of the modern _Ginkgo biloba_. Some of there have been
>referred to the extant taxon, while others have been designated as
>_Ginkgoites_....."
>
>
>In other words,
>Ginkgophytes were very widespread in the past, and one form has persisted.
>
>Extant _Ginkgo_ leaves are highly variable, and there was a lot more
>variability of ginkgophytes in the past.
>
>Some of the fossil leaves look very much like leaves on extant Ginkgo trees.
I knew someone would pull Taylor's report out. Here is what Andrews says,
"As mentioned above, many of the fossil leaves assigned to the ginkgo group
from the Mesozoic rocks are rather deeply divided. In a general way this
dissection becomes more prominent the further back we go in the past." henry
N. Andrews, Ancient Plants, Comstock Publishing, 1947, p. 166
This was the difference I was referring to. I would not deny that Ginkgo is
probably the slowest evolving of the living fossils. But I would deny that
there are no differences. I would refer you to the rest of the paragraph
that you cited above which outlines some of the differences between the
various species.
>
>
>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. I would also note
that the fossil reproductive structures of these plants have not been
described in the literature very often. (p. 412)
>
>Ginkgophytes seem adaptable, with tolerate for a wide variety of
>environmental conditions, past and present. But they were not very
>adaptable morphologically, since the extant species hasn't changed much,
>and since much of the wider morphological diversity of the past became
>extinct as Tertiary conditions changed, leaving only this one species
>surviving today.
>
>The "problem" of finding fossils which are morphologically similar to
>living species is especially challenging for paleobotany since the various
>plant organs are usually separated (leaves, fruits, trunks, flowers,
>pollen, etc. are found in different deposits, usually not attached to one
>another, because of taphonomic and hydrodynamic conditions of deposition).
>Usually a ginkgo-like fossil leaf is not listed as "Ginkgo", but as a
>fossil in the form-genus "Ginkgoites". But it is hard to define the edges
>of such a form-genus.
>
>Taylor also notes (p.414)
>"Harris and Millington (1974) have recently suggested the abandonment of
>_Ginkgoites_ because the genus is ill-defined and cannot be applied with
>consistency to all the morphologic forms known to have existed within a
>single species." T.M.Harris (who spent much time on Ginkgophytes) used
>cuticular characteristics to differentiate species (summary in Henry N
>Andrews, in Studies in Paleobotany, 1961, p.335-347: see also
>Stewart&Rothwell noted below, p.391), but there are problems with that,
>too, and I (personally) heard Harris say in 1980 that it seemed to be one
>big continuum with various characters varying variously!
>
>What does all this mean? What I see is:
>
> - _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.
>An interesting end note:
>
>Andrews mentions (p.337):
>"The ginkgo is known to have been cultivated for many centuries in Chinese
>gardens and it is very probable that man's interest in the plant saved it
>from extinction -- somewhat the reverse of the usual fate of living things
>as human occupation has spread over the earth." Stewart & Rothwell
>(Cambridge Univ. Press, 1993), in their chapter 27 titled "The record of a
>living fossil: Ginkgo", start with a similar comment.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm