friesen@psivax.UUCP (Stanley Friesen) (05/24/85)
>Paul Dubois quoting some reliable sounding source: >> >> "Although extinct for more than 300 million years, their fossil remains >> indicate that in one respect, the trilobites may have been superior to >> current living animals. They had, in principle, perfect vision: They >> possessed the most sophisticated eye lenses ever produced by nature." >> A major problem with this is that trilobites are far from the earliest known animals, thus no matter how early they were, there were still many millions of years to evolve these complex eyes. In fact there are even a number of more-or-less arthropod-like fossils in the *Pre*cambrian Ediacaran Faunas, which could be close to the ancestry of trilobites(maybe), and this fauna dates back as much as 60 million years before the earliest known trilobite, plenty of time for considerable evolution, especially under the P.E. model. As for the possible origin of complex eyes gradually, there is a fair bit of evidence how that occured. The first thing to realize is that the ancestral structure may not have been used for *image* formation. Among living invertebrates there is in fact a complete morphological series from scattered individual photosensitive cells to very complex eyes, *all* of which are entirely functional within the usage made of them by the organisms in question. The simplest forms are involved in phototaxy and perhaps in orientation. These forms include not only scattered cells, but also clusters of such cells, often near the anterior end of the animal. A transformation of the overlying ectoderm to a crude lens might produces the simplest organs which can be called "eyes". Thes probably had the same function as the original eyespots, with the lens being used to concentrate ambient light and increase the sensitivity to light at low intensities. From there it is simply a matter of progressively more efficient light collection and processing in the brain to produce fully functional eyes. And as I said, the entire series actually exist, even within a single phylum. -- Sarima (Stanley Friesen) {trwrb|allegra|cbosgd|hplabs|ihnp4|aero!uscvax!akgua}!sdcrdcf!psivax!friesen or {ttdica|quad1|bellcore|scgvaxd}!psivax!friesen