Hi everyone. Apologies for my enforced absence recently. I'm hoping to get some more time to devote to the board soon.
I would like to make a quick response to some-guy, however. The major problem with the entire "living fossil" concept is that it ignores the fact that the living representatives are almost invariably the last remnants or relicts of highly diverse groups of organisms. Although making a great sound-bite, the term "living fossil" is misleading. Almost invariably, these few survivors are 1) different from their fossil ancestors, showing that they've continued to evolve; 2) are found in highly isolated, often extremely environmentally stable locales; and 3) usually have very small populations - some of which (like the
Sphenodon spp. or
Wollemia nobilis) are hovering at the margins of extinction.
Taking your list to illustrate my point (although I don't know anything about molluscs, unfortunately - lack of interest on my part rather than lack of data):
horseshoe "crab" (actually closer to spiders and other arthropods, not crustaceans): only four living species in three genera still extant from a hugely successful group that extends back to the Upper Cambrian (520 mya). Last remnants of a family containing 12 genera and over 25 identified species. The most common representative,
Limulus polyphemus - the one found often on the Atlantic coast of Mexico, the US and Canada, for instance - has NO fossil record indicating its ancestry, and the genus itself only extends back ~20 mya. The other five members of the genus are all extinct, as are most other members of the family. The horseshoe crab we see today did NOT live at the same time as the dinos.
coelacanths: Again, another highly diverse group of organisms (order Coelacanthiformes) which only left two species (and possibly two additional species or subspecies). In this case, we have only two species out of an entire order still living - meaning three complete families comprising dozens of species have all gone extinct. In addition, the two living species are greatly different in morphology to their nearest fossil relative, indicating that evolution has proceeded apace. In fact, they represent a fairly extreme example of "true gradualism" - tiny incremental changes over vast periods of time. Since the two living representatives are both deep-sea species living in the open ocean in fairly out-of-the-way locales, it's not surprising that they were essentially unknown to Western science until recently.
ginkgo: The ginkgo is probably one of the most extreme examples of what I'm referring to. One single species (
Ginkgo balboa) living from what was a phenomenally diverse clade with 6 families and 19 genera, all of which are extinct. Moreover,
G. balboa is the last remaining member of its genus, which is credited with over 12 species all by itself. There's a fascinating (well, to me, anyway) article in the journal Paleobiology (Royer DL, Hickey LJ, Wing SL 2003
"Ecological conservatism in the 'living fossil' Ginkgo", Paleobiology 29:84-101) that goes fair to illustrating how such a relictual species could come into existence and persist.
From the abstract:
The living species Ginkgo biloba is phylogenetically isolated, has a relictual distribution, and is morphologically very similar to Mesozoic and Cenozoic congenerics. To investigate what adaptations may have allowed this lineage to persist with little or no morphological change for over 100 Myr, we analyzed both sedimentological and floral data from 51 latest Cretaceous to middle Miocene Ginkgo-bearing fossil plant sites in North America and northern Europe. The resulting data indicate that throughout the late Cretaceous and Cenozoic Ginkgo was largely confined to disturbed streamside and levee environments, where it occurred with a consistent set of other plants. These inferred habitats are surprising because the life-history traits of Ginkgo (e.g., slow growth rate, late reproductive maturity, extended reproductive cycle, large and complex seeds, large and slowly developing embryos) are counter to those considered advantageous in modern disturbed habitats. Many flowering plant lineages first appeared or became common in disturbed riparian habitats, and are inferred to have had reproductive and growth traits (e.g., rapid reproduction, small easily dispersed seeds, rapid growth) suited to such habitats. Paleoecological inferences based on both morphology and sedimentary environments thus support the idea that Ginkgo was displaced in riparian habitats by angiosperms with better adaptations to frequent disturbance.
tuatara: Once again we're looking at a once-highly-diverse order (Rhynchocephalia) of which only two closely-related, highly isolated species in a single genus exist (
Sphenodon spp.). All other members of the entire order are extinct. From an ecology standpoint, it’s quite clear that the only reason they were able to survive when all of their relatives perished was their presence on an isolated oceanic island (New Zealand) — where they’ve been all alone since the breakup of Gondwanaland. And again, the living species are morphologically distinct from their nearest fossil ancestor.
Wollemi pine: A very relictual population - a prime example of the adage that nature always holds surprises. In the case of
Wollemia nobilis, we not only are looking at the last remaining species of a diverse group of plants, but may be looking at the last three
individuals (because of the way the species propagates) of a once thriving and extremely widespread population. Isolated in their box canyon, these three individuals have survived not only the extinction of their family, but all of the other members of their own species. A true relict in every sense of the word. Although the exact phylogeny of
Wollemia hasn't been completely worked out, it appears that its nearest fossil relatives - not identical - were alive during the Cretaceous. The genus
Wollemia itself seems to be a sister genus to the genus
Dilwynites from the Late Cretaceous (reference: Chambers, T. Carrick, Andrew N. Drinnan and Stephen McLoughlin. 1998. Some morphological features of wollemi pine (Wollemia nobilis: Araucariaceae) and their comparison to Cretaceous plant fossils. International Journal of Plant Sciences 159: 160-171). However, there is no indication that it existed at the time of the dinosaurs alongside its nearest Araucarician relatives.
My biggest problem with the "living fossil" designation is that it ignores the incredible diversity of life which has lived, flourished, and died out - leaving only these impoverished remnants of once-great lineages. Changed, reduced, and isolated, these species are wonderful reminders of the glorious heritage we've inherited from the past. To be misused through misunderstanding as some kind of "refutation" of evolution - rather than a perfect example of the same - is saddening.
. Any evidence?
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