>First off, they didn't stop changing. Horseshoe crabs do NOT look the same
>today as they did in the Permian. Go see Moore, Laliker and Fischer,
>Invertebrate Paleontology I forget the page. The permian horseshoe crab is
>a whole lot different from those you pick up on the east coast today. It has
>its shell swept outward today's is an ovoid form. The Jurassic was almost
>circular in shape. In fact, not a single "living fossil" that I have
>investigated is the same as what is alive today.
Living things are dynamic.
This includes ginkgoes,
>coelecanth and all the others. What the anti-evolutionists are doing is
>saying that the ginkgoes look like today. Well ginkgo is a genus, not a
>species. This is like saying that there were trees in the Pennsylvanian
>thus trees have remained unchanged over that time. They use a high
>hierachical category to make the claim.
>
Let's look a little more at the _Ginkgo_.
In _Paleobotany_ by Thomas N Taylor (TNT, famous paleobotanist, past
president of the International Organization of Paleobotany, now at Univ. of
Kansas) writes:
p.411 (with a description of the many unique features of Ginkgophytes)
"The Ginkgophyta is represented today by a single species of the maidenhair
tree, _Ginkgo biloba_. This taxon is sometimes referred to as a "living
fossil" because it was widespread throughout the world beginning in the
Mesozoic.... There is a considerable amount of variability to the leaf
margin, ranging from entire to deeply notched...."
p.412
"The genus appears to have persisted relatively unchanged morphologically
since the Mesozoic, with numerous reports of ginkgophyte foliage from many
stratigraphic levels and geographic regions of the Northern Hemisphere."
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.
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."
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.
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.
Karen