>Stephen Jones wrote:
>
>|Indeed, the trilobite, one of the earliest animals, had a highly
>|developed eye:
>
>
> The type of eye being discussed is called a schizochroal eye.
>They do not occur until the Ordovician, considerably after the initial
>occurrence of trilobites near the base of the Cambrian. The earliest
>trilobites had holochroal eyes, which were not as sophisticated in terms
>of optics.
The earliest Trilobites had complex compound eyes that were highly
developed, regardless of whether they were holochroal or schizochral. It's
just that the schizochral eyes employ extremely sophisticated optical
principles not present in the eyes of any other known forms.
> The first occurrence of trilobites is not as simple as a general
>examination of the fossil record would indicate.
The Cambrian in many areas is practically defined by the first occurrence of
trilobites because there are no compelling earlier metazoan forms. In these
cases by definition, trilobites appear suddenly.
> There are many trace
>fossils (trails, burrows, etc.) of trilobite-like animals prior to the
>first occurrence of trilobite skeletal material. There are also examples
>of entirely soft-bodied trilobites and trilobite-like animals from
>localities with exceptional preservation (e.g., the Burgess Shale). These
>observations suggest the first appearance of common skeletal fossils of
>trilobites near the base of the Cambrian may be a taphonomic artifact of
>the development of mineralized skeletons rather than their actual time of
>origin.
Cowen says: "If Cambrian animals lived in Ediacaran times, we would surely
have found them by now" (R. Cowen, 1995, "History of Life" (2nd ed.).
(Blackwell Scientific Publications, Boston) p. 76. A few pages later he
concedes
"The 'Cambrian Problem' remains a puzzle. The waves of evolutionary
novelty that appeared in the seas in the 20 m.y. of the Tommotian and
Atdabanian stages have few parallels in the history of life. Tommotian
fossils appeared quite suddenly in abundance, thanks to the evolution
of skeletons. Atdabanian forms [including trilobites] also appeared
suddenly as they evolved skeletons at comparatively large body size.
The fact that skeletonization happened in two waves does not allow us
to pretend that this was some ordinary event in the history of life."
Cowen, p. 78-9
He states further that this event "...was driven by worldwide ecological
factors--but we do not know what they were." p. 80.
>It is likely the exact origin of the oldest trilobite eyes will
>be unknown until soft-bodied preservation of at least Burgess Shale
>quality is found in even older rocks than is currently known (e.g., older
>than the Chenjiang fauna).
For now we should probably concentrate on the origin of the oldest
trilobite, rather than worrying about whether it had eyes or not. The oldest
described trilobites do. Precambrian sediments appear capable of preserving
soft bodied fossils, just as Cambrian sediments do. Yet after many years of
intensive searching, the best candidate we have for an ancestor is an
undescribed cephalized metamerized Ediacaran blob. As complex as the eyes
are, they are trivial compared with the complexity of the arthropod itself.
Art
http://chadwicka.swac.edu