>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.
Art Chadwick (chadwicka@swac.edu) said:
|The earliest Trilobites had complex compound eyes that were highly
|developed,
Hi Art:
To the extent that they are preserved, yes.
|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.
Yes. But the features of the schizochroal eyes are what the
article was discussing, and seemed to be implying the acme of trilobite
eye sophistication was was present from the beginning. This is not
correct.
> 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.
The Cambrian in many areas is also defined by a major unconformity
(i.e., depositional gap), there are skeletal remains of many other
organisms which occur prior to the first occurrence of trilobites, and in
a few areas, there are older (Ediacaran) entirely soft-bodied metazoans,
but localities for them are relatively rare compared to the slightly
younger skeletal remains, which are found almost everywhere someone has
looked below the first appearance of trilobites. There are also many
trace fossils (burrows, trails, etc.). At least some of these earlier
metazoans have been interpreted by some as possible trilobite ancestors,
although there is plenty of controversy about their interpretation because
the preservation consists only of impressions.
> 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.
There is no question that the interval between the late
Precambrian and Cambrian was an exceptional time, and that if there are
Precambrian ancestors of many Cambrian organisms, the relationships are
uncertain or largely unrecognized. However, that was not the issue I was
addressing. The possibility of a taphonomic (i.e. preservation) effect on
the expression of the first appearance of trilobites is unquestionable --
if they did not have skeletons earlier, and soft-bodied organisms which
produce trace fossils similar to those produced by trilobites extend
older, then it seems reasonable to hypothesize that the first appearance
of trilobite skeletal fossils may not represent the time they were first
present on Earth. It is not an appeal to the unknown. There is evidence
for something burrowing around in the sediment with plenty of legs, prior
to the first appearance of trilobites. Were they trilobites or just some
other arthropod-grade creature? Hard to say. But presenting the first
appearance of trilobites as a sudden "poof" without mention of these
caveats is, as I stated, an oversimplification. Cowen mentions two of
these caveats above -- the taphonomic effect of size and skeletons.
We could discuss the Precambrian/Cambrian interval in general(to
the limits of my familiarity, anyway :-)), but it was not my point.
...
>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.
I agree.
|The oldest described trilobites do. Precambrian sediments appear capable
|of preserving soft bodied fossils, just as Cambrian sediments do.
The taphonomy of these occurrences is quite different from the
preservation observed in faunas like the Burgess Shale and the Chengjiang
fauna (middle and early Cambrian, respectively). The soft-bodied
specimens from the Precambrian consist almost entirely of impressions in
siltstones or sandstones, usually on the base of beds. The amount of
detail preserved is limited compared to localities of exceptional
preservation known from the Cambrian, and the environment of deposition is
also different.
There *are* some Ediacaran fossils which have been interpreted as
trilobite or other arthropod ancestors, but some of the same fossils have
been interpreted as something completely different (e.g., by Seilacher)
and completely unrelated to later metazoans. Personally, I think reality
is somewhere in between, and this is a sentiment I hear expressed by many
paleontologists. Some organisms are clearly different, some are clearly
similar (e.g., the pennatulaceans -- sea pens), but there is a large
proportion where it is a toss-up, and conclusively demonstrating which
interpretation is correct is severely limited by the amount of detail
preserved in most Ediacaran faunas. More information is needed. If we
had Burgess-Shale type preservation from that interval, it would be
possible to conclusively say whether these creatures were completely
different or not.
Another issue is size. If arthropod ancestors started out as
organisms only a few millimetres long, there would be almost zero chance
of their preservation in Ediacaran type faunas, or even Burgess Shale type
(although they might be found in orsten-like deposits like those found in
Scandinavia in the Cambrian).
|Yet after many years of intensive searching, the best candidate we have
|for an ancestor is an undescribed cephalized metamerized Ediacaran blob.
Well, a cephalized, metamerized, "blob" (e.g., similar to a
cephalized, metamerized turbellarian flatworm) *is* what invertebrate
zoologists have suggested the ancestor of arthropods would look like based
on comparative anatomy of modern invertebrates :-)
|As complex as the eyes are, they are trivial compared with the complexity
|of the arthropod itself.
I agree completely. I said:
"Trilobites of the Cambrian are sophisticated creatures, but lack many
features found in later trilobites. To a first approximation, what
Sunderland is saying is correct, but it is an oversimplification."
I am not saying that a problem does not exist, or that the first
trilobites to appear were not sophisticated creatures (even turbellarian
flatworms are sophisticated creatures!), just that Sunderland appears to
have overstated the case a bit by implying that the most sophisticated
optics in trilobite eyes appear "poof", right from the very start; and
that there is not a single trace of a trilobite ancestor. At the very
least, there are possible ancestors. People are welcome to dispute their
significance and regard them as attempts of some scientists to find the
evidence they are hoping for, but people should at least be aware of them.
Goodness knows there is plenty of controversy over the interpretation of
the Ediacaran fauna and the "small shelly" fauna found immediately before
trilobites.
I hope everyone had a good Easter.
-Andrew
macrae@geo.ucalgary.ca
home page: http://www.geo.ucalgary.ca/~macrae