SJ>Indeed, the trilobite, one of the earliest animals, had a highly
>developed eye:
|I am re-instating my Macbeth quote for clarity:
|"One example of this is the little animal called the trilobite. There
|are a great many fossils of the trilobite right there at the beginning
|with no build-up to it. And, if you examine them closely, you will
|find that they are not simple animals. They are small, but they have
|an eye that has been discussed a great deal in recent years-an eye
|that is simply incredible. It is made up of dozens of little tubes
|which are all at slightly different angles so that it covers the
|entire field of vision, with a different tube pointing at each spot on
|the horizon. But these tubes are all more complicated than that, by
|far. They have a lens on them that is optically arranged in a very
|complicated way, and it is bound into another layer that has to be
|just exactly right for them to see anything....But the more
|complicated it is, the less likely it is simply to have grown up out
|of nothing. And this situation has troubled everybody from the
|beginning-to have everything at the very opening of the drama. The
|curtain goes up and you have the players on the stage already,
|entirely in modern costumes. The creationists say, 'That is abrupt
|appearance,' and they hammer away at that. Instead of building up bit
|by bit, it appears suddenly, and that to them signifies creation. I
|don't want to argue that, but I admit it is very strange that there is
|no slow build-up. The evolutionists have strained very hard to find
|earlier fossils and have had very meager results" (Sunderland L.D.,
|"Darwin's Enigma: Fossils and Other Problems", Master Book
|Publishers: El Cajon, CA, Revised Edition, 1988, p150-151)
AM>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.
|It is noteworthy that this statement by MacBeth was made during a
|debate at Harvard in 1983 with Dr Kenneth Miller, Professor of Biology
|at Brown University. Sunderland records that :
|"Macbeth's debate opponent, Dr. Miller, made no attempt to address
|these specific points. Instead he talked about what he would have
|said if he had been debating a creationist, i.e., attack a
|straightforward interpretation of the Bible." (Sunderland L.D.,
|"Darwin's Enigma: Fossils and Other Problems", Master Book
|Publishers: El Cajon CA, Revised Edition, 1988, p152)
Well, I did not and would not attempt this, so I am not sure what
the relevance is.
|Macbeth was no slouch.
I did not imply that either Macbeth or Sunderland were slouches.
|He was not a creationist
I do not consider this a factor.
|and was a regular
|attender of Systematics meetings at the American Museum of Natural
|History. He may be in the heat of debate conflating features of the
|earlier trilobite's holochroal eyes with later trilobite's
|schizochroal eye?
Why not? It is difficult to say how familiar he was with the
chronology of features found in trilobite eyes, and difficult to know
whether he omitted subtlties for the sake of time in a debate format if he
was.
|Whatever type of trilobite eye Macbeth is describing, it is
|acknowledged that at or soon after the first appearance of trilobites
|had an eye that was highly complex.
Yes. I said trilobites were sophisticated, and I said their eyes
were too. However, some of the features mentioned in the quote (of
Macbeth) from Sunderland are clearly present in later trilobites, not the
earliest ones. They are not present from the start.
|Gould says that the "eyes of early trilobites" were already highly
|complex and acute:
...
|David Attenborough (not just a TV journalist but a qualified
|zoologist), says that the later trilobite eyes were "even more
|sophisticated", implying that the earlier trilobites' eyes were
|themselves "sophisticated":
Attenborough confirms my point -- that the acme of trilobite eye
sophistication, which is represented by the features discussed in the
quote above, came later.
|"Trilobites were the first creatures on earth to develop
|high-definition eyes. They are mosaics, a cluster of separate
|components, each with its own lens of crystalline calcite orientated
|in the precise position in which it transmits light most efficiently.
The "c" crystallographic axis. In other orientations, there are
severe refraction and interference problems.
|One eye may contain 15,000 elements, providing images that together
|constitute an almost hemispherical field of view.
But in most of the earlier ones the field of view was much more
restricted than later ones, and facet numbers this high were also not
reached until later. The highest facet numbers (are you sure it was 15000
and not 1500?) were also in holochroal eyes, so this was not combined with
the sophisticated optics of schizochroal eyes. Schizochroal eyes had a
maximum number of lenses in the several hundreds.
|Late in the dynasty,
Again, later.
|some species developed an even more sophisticated kind of eye
|and one that has never been paralleled by any other animal. Here the
|components are fewer but larger. Their lenses are much thicker and it
|is thought that these species lived where there was little light and
|needed thick lenses to collect and concentrate what light there was.
|However, the optical properties of a simple calcite lens in contact
|with water are such that it transmits light in a diffused way and
|cannot bring it to a sharply focused point.
But are calcite lenses a "feature" or a "bug" that trilobites took
millions of years to figure out a workaround for?
Why not choose an isotropic mineral or organic material -- i.e.
one without birefringence, which is a problem with calcite? Birefringence
is caused by different refractive indices depending upon the orientation
of the light ray and the crystal structure. This causes "double
refraction" if you look through a clear calcite crystal. If an isotropic
mineral or organic material were used, crystal orientation could be
anything and it would still work.
It is also important to realize that even with misaligned
crystallographic axes, a lens composed of calcite would *not* be useless
(contrary to the claims of some of your quotes), just not as effective as
it could be.
|To do this, a two-part
|lens is needed which has a waved surface at the junction between its
|two elements. And this is exactly what these trilobites have
|developed. The lower element of the double lens was formed by chitin
|and the surface between the two conforms to a mathematical principle
|that man discovered only three hundred years ago when trying to
|correct the spherical aberration of lenses in his telescopes."
|(Attenborough D., "Life on Earth: A Natural History", Collins-British
|Broadcasting Corporation: London, 1979, p54),
I am not denying that these amazing corrective optics were present
in trilobites, only that they were *not* present at the start. They occur
later. The "doublet" structure is not known in the earliest trilobites.
I suppose it is possible they were down there (the record for Cambrian
trilobite eyes is not great because of the way the skeleton was shed by
many at that time), but there is no sign of them.
AM>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
|Raup, speaking particularly of later trilobite eyes, says they were
|"nearly identical to designs published independently by Descartes and
|Huygens in the seventeenth century" and "used an optimal design which
|would require a well trained and imaginative optical engineer to
|develop today":
Yes. The later schizochroal eyes.
...
|"[T]he lens systems were very different from what we now have.
|Riccardo Levi-Setti (a Field Museum research associate in geology and
|professor of physics at the University of Chicago) has recently done
|some spectacular work on the optics of these lens systems. Figure 7
...
|trilobites lived, the lower lens was necessary. Thus, the trilobites
|450 million years ago used an optimal design which would require a
|well trained and imaginative optical engineer to develop today-or one
I highly recommend Levi-Setti and Clarkson's papers (cited in my
first comment). They provide far more detail than I could ever
communicate here. The point I tried to make is not that schizochroal eyes
are "simple". They are impressive. I was trying to make the point that
they did not appear at the point trilobites first appeared. The "450
million year" time frame mentioned in this quote refers to the Ordovician,
when schizochroal eyes are first known, not the Cambrian, when trilobites
are first known.
The quote from Sunderland also says, while talking about the
doublet structure, "... and it is bound into another layer that has to be
just exactly right for them to see anything..." This is an exaggeration.
For them to collect light and see *optimally*, this is correct, but if the
arrangement were otherwise, they would still see. The eye would still
function as a light detector at least. Not all features would have to be
present at the same time in order for the eyes to be effective to some
degree. In fact, this is empirically demonstrated by trilobite eyes
without these features.
...
AM>Many trilobites were also blind. The visual system of
>trilobites is unusual because it stands a reasonable change of
>preservation. This is quite different from the preservation potential of
>eyes in most organisms. The difference in preservational potential
>occurs because the lenses consist of calcite -- CaCO3 -- the mineral
>which is the main ingredient of limestone. The lenses are also closely
>integrated with the rest of the skeleton in many trilobites.
|Indeed, Taylor writes of this "calcite" eye:
|"Kenneth Towe of the Smithsonian Institute reported that the lenses in
|the eyes of fossil trilobites consisted of precisely aligned crystals
|of calcite.
Aligned along the "c" optical axis to minimize (but not eliminate)
the effect of birefringence and maximize the refractive index.
...
|with the transparency of glass only if they are exactly aligned with
|the beam of light entering them. At any other angle, the light
|bounces off the walls and splits into various colours. Some modern
This is a reference to the birefringence. It does not "split" the
light into "various colours", but colours can be an effect when the rays
emerge and interfere at the surface of the lens.
|arthropods have calcite crystals in their eyes, but these- and Towe
|finds this 'an altogether surprising thing' - are arranged randomly,
|and do not correspond with the ommatidia or optical units of which the
|compound eye is composed.
Incidentally, from what can be determined about holochroal
trilobite eyes from the lenses, they appear to have had similar structure
and function to the ommatidia of other "arthropods".
|By what mechanism did these 'primitive'
|creatures discover how to incorporate calcite crystals, align them
|precisely and protect them with a cornea? Answer comes there none."
|(Taylor G.R., "The Great Evolution Mystery", Abacus: London, 1983,
|p96)
Hard to say. Alignment of crystallographic axes in calcite is
common in other organisms for structural reasons, because one of the other
problems with calcite is its three perfect cleavages -- i.e. it has 3
orientations of parallel planes of weakness that are related to the
crystal structure. For example, crinoids and other echinoderms, whose
platey skeletons are composed of single crystals of calcite, do this. It
is completely unrelated to optical alignment, but many organisms do have
ways to align calcite crystal growth, mainly by establishing organic
templates upon which the crystals can grow. I am not sure of the details,
but if you look up the subject of biomineralization, you could probably
find them. As to how trilobites aquired this ability specifically for
optics, it is impossible to tell from the fossil record, just as it would
be for determining crystal alignment in the rest of the skeleton without
having the corresponding soft tissues.
[reiteration of what is contained in the earlier quote, based upon
Levi-Setti and Clarkson's work]
|Thus the trilobites evolved a lens shaped to correct for optical
|aberration identical to that proposed (quite independently of any
|knowledge of trilobites) by Descartes and Huyghens half a billion
|years later.
|Why was such perfection needed? Dr Clarkson suggests that trilobites
|may have lived in very muddy, turbid water. Or perhaps they only came
|out at night or at dusk. The thick lenses, thanks to the optical
|correction, would be more efficient light-collectors.
It is interesting that this develops later, as more and nastier
predators start appearing (e.g., cephalopods).
|But to make the
|matter more puzzling still there is the fact that some trilobites were
|blind. How did the earliest trilobites collect together the intricate
|genetic information needed to construct this semi-miraculous
|structure?"
|(Taylor G.R., "The Great Evolution Mystery", Abacus: London,
|1983, p96-97)
Without at least skeletal remains going back much further, it is
impossible to tell.
...
AM>The first occurrence of trilobites is not as simple as a general
>examination of the fossil record would indicate. 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. 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).
|Whatever,
"Whatever"? These issues are crucial. There is evidence that the
first appearance of *skeletal* remains of trilobites does not represent
their first appearance in any form. There are trace fossils and at least
possibly-related, older, soft-bodied organisms. There are also limits on
preservation because of size. The first trilobites found in the Cambrian
are already quite small, and if earlier ones did not possess a skeleton
(as evidenced by the trace fossils), their chances of preservation are
nearly nil. The "poof" of creation could as easily be the "poof" of
development of skeletons and a taphonomic artifact, at least in part. It
is at least as plausible as the interpretation you have proposed, which
depends upon the failure to find Burgess-Shale quality preservation
further back into the Precambrian. If it turns up, what then?
Trace fossils and soft-bodied fossils of uncertain interpretation
may not seem like much in terms of evidence, but we know "soft-bodied"
trilobites existed later, and *something* was making those trilobite-like
trace fossils earlier.
|when all the facts are in, trilobites will no doubt fit the
|basically Creation pattern of fully formed sudden appearance followed
|by stasis, as do "most" fossil species:
[Quote from Gould regarding the "punctuated equilibrium" pattern in the
fossil record.]
This is a different issue. Why do you bring it up?
When preservation is adequate, examples of tiny incremental change
still occur even while matching the pattern of "stasis" and "punctuation"
originally described by Eldredge and Gould. Geography conspires against
paleontologists finding the time and location where species originated,
not to mention the "punctuated" nature depositional processes themselves.
Appropriate conditions are sometimes found, though.
I am not sure how these examples could be distinguished from the
creation model. I suppose any degree of incremental change could still be
the result of the actions of a creator. I can not eliminate that
possibility, and I can not forsee a way to test it.
...
AM>This is probably a reference to the problems which result from
>using a mineral like calcite for optics. It is highly anisotropic.
>Refraction effects are severe in calcite unless the crystals are suitably
>oriented and compensated for. This may be why so many other animals use
>less anisotropic or more versitile organic materials for lenses. It is a
>relatively simple material for many organisms to produce, but can be
>pretty problematic to use.
|Before we skip over this point, one of the major artefact theories for
|lack of hard-shells before the Cambrian era, was the alleged lack of
|calcium.
Again, this is not really relevant to my original point.
Lack of calcium? Who proposed that? The chemistry of the oceans
may have been inappropriate for the deposition of structurally-continuous
calcite skeletons, but it would not be due to "lack of calcium". Calcium
would be available in the oceans from erosion, unless some sort of very
effective non-biological process were removing it.
Lack of skeletons before the Cambrian (which is approximately
defined on the first appearance of skeletons) could be as simple as
organisms not having an advantage if they possessed them (e.g., if
predation was lower), or that developing the capability of a mineralized
skeleton took a long time, or both.
In many modern organisms, there is a continuum of skeleton
development, from organisms possessing partially-mineralized body walls
(e.g., containing calcite spicules) to those with mineralized and organic
skeletons, to continuous mineralized skeletons with little organic
material. Unless structurally-competent mineralized skeletons occur, the
chances of preservation are quite low. Organisms with only a limited
number of mineralized spicules in their otherwise soft body wall stand a
poor chance of preservation complete, although the spicules might
individually preserve in the right conditions. It would be a difficult to
determine what they were from, though.
This was the case for some of the "small shellies", which are a
diverse group of tiny shells found in the time immediately before the
first appearance of trilobites and for a while after in the Cambrian.
Paleontologists had no clue what most of them represented. Were they
complete shells of some sort or pieces of a larger organism with many
skeletal elements? Some "small shelly" fossils have recently been found
in early Cambrian (post-trilobite, unfortunately) localities with
soft-bodied preservation. Some clearly represent the "body armor" of much
larger metazoan animals that are otherwise soft-bodied. This is more or
less what one would expect for the earliest skeletal metazoan animals -- a
skeleton consisting of many separate mineralized elements, making up a
"scleritome". To say the mystery of the "small shellies" has been solved
would be an exaggeration, but some of them clearly represent
partially-skeletonized organisms, and they "coincidentally" are common
prior to the first trilobites. They are intermediate between being
entirely soft-bodied and having a complete, strong skeleton.
|Yet here we have "the blind watchmaker", which:
|"has no purpose in mind. It has no mind and no mind's eye. It does
|not plan for the future. It has no vision, no foresight, no sight at
|all." (Dawkins R., "The Blind Watchmaker", Penguin: London, 1991,
|p5)
Exactly. Why would trilobites use calcite for lenses when they
were going to run into all sorts of optical problems the moment the lenses
thickened or became appreciably more convex to gather more light? If
they were thin and used in copious light, okay, but what a poor
foundation. Why not choose an isotropic material where birefringence and
optical alignment would not be an issue? Bad choice. Poor planning.
Perhaps what one would expect from *no* planning. They made the best of
it with schizochroal eyes and their complex corrective optics, but they
still became extinct.
|just happening to find by chance, what Taylor claims is "the one
|material in the universe- namely calcite" (a form of the
|supposedly rare CaCO3)
CaCO3 in the form of calcite is not rare on the surface of the
Earth. It even forms by non-biological processes.
|"which had the required optical properties":
|"One could perhaps accept the idea that a happy accident caused the
|sensitive spots to become more numerous, and that another happy
|accident placed them at the bottom of tubes. It is a little harder to
|accept that by chance the tubes were not parallel, which would have
|been the obvious pattern of repetition, but were slightly divergent.
|But by what conceivable chance could the trilobite have accumulated
|the one material in the universe- namely calcite - which had the
|required optical properties and then imposed on it the one type of
|curved surface which would achieve the required result? There are
|innumerable possible shapes, none of which offer the unique
|advantage of spherical correction, except the one I have
|described." (Taylor G.R., "The Great Evolution Mystery", Abacus:
|London, 1983, p98)
The spherical correction has *nothing* to do with the use of
calcite. The shape of the lens discovered in the 1700s works as well for
glass as calcite. I do not know why Taylor thinks calcite is so special.
All that is needed is relatively high refractive index for an effective
lens, and an isotropic material could work even better because it could be
oriented in any direction. There are also many minerals with higher
indices of refraction (e.g., dolomite, aragonite, magnesite, siderite). I
am not sure how appropriate these are for biologic systems, with the
exception of aragonite (another phase of CaCO3), which is used by some
organisms.
To produce the spherical abberation correction requires little
more than an appropriately-sized dimple on the inner side of the lens.
Even an approximation of the exact shape would have a positive effect.
AM>This is an exaggeration. Cambrian trilobites had simpler eyes.
>They were only "half clothed" or dressed in "19th century" vintage
>attire. 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.
|See above. While Macbeth may be exaggerating during the heat of a
|debate, it agrees with what Gould and Attenborough say above, even of
|the earliest trilobite eye.
It also agrees with what I said -- that Cambrian trilobites were
sophisticated creatures, but that the intial ones did not possess the eye
capabilities explained at length by Attenborough and others.
...
>typical invertebrate larvae today). More than what is preserved/known as
>body fossils was present, judging by the burrows and trails. There are
>also some fossils which might be trilobite ancestors, but they are not
>well enough preserved to recognize eye details. Most of the soft-bodied
>preservation so far found in Precambrian rocks is just not detailed
>enough.
|Or they were just not there? :-)
Sure. Always a possibility.
But something was making trilobite-like burrows prior to the first
appearance of trilobite skeletal fossils. Evolutionary theory predicts
these were soft-bodied trilobites that had not yet developed mineralized
skeletons, and it predicts that if Burgess-Shale quality fossil localities
are found in that interval, soft-bodied trilobites should be amongst the
fauna. This is discounting the occurrence of potentially trilobite-like
soft-bodied organisms in the Ediacaran fauna. What does your theory
predict should be found in this interval?
...
>Zhang Xi-Guang and Clarkson, E.N.K., 1990. The eyes of Lower Cambrian
>eodiscid trilobites. Palaeontology, v.33, part 4, p.911-932.
>
>Unfortunately, there is more to eyes than just the lenses, and
>compared to the large number of trilobites known, the eyes of relatively
>few have been examined in detail.
|Maybe knowing more about the trilobite's eye, will only make matters
|worse for evolution?
Entirely possible. Then again, appropriate preservation in the
Late Precambrian or earliest part of the Cambrian might make things
"worse" for creationist models that depend upon the Precambrian-Cambrian
interval representing an "abrupt" appearance with no ancestors for the
organisms found later [I am not suggesting your theory would experience
such a problem.]
In fact, looking back on the history of paleontology, the
Precambrian-Cambrian interval has become, paradoxically, both more
"abrupt" and more "gradual". Back in the early 20th century, the first
appearance of trilobites defined the base of the Cambrian (it is lower
now), and virtually nothing was known of earlier metazoans. That was a
truely "abrupt" appearance. Now a succession of organisms are known, even
if the changes in them are dramatic and poorly understood.
|As in the origin of life, the evolutionist's
|problem is "not what we do not know, but what we do know" (Thaxton
|C.B., Bradley W.L. & Olsen R.L., "The Mystery of Life's Origin:
|Reassessing Current Theories, Lewis & Stanley: Dallas TX, 1992,
|p185). Has anyone and idea why or how the trilobite managed to
|stumble on calcite to make an eye,
It was not much of a "stumble". The chemistry of calcite makes it
one of the simplest minerals to crystallize from seawater. Your question
is like asking, "How did people stumble on rocks to make a tool?" Calcite
is often a byproduct of other biological processes, particularly if the
local ocean chemistry brings calcite nearly to saturation, as is sometimes
the case.
|and how it managed by trial and
|error, to develop an already sophisticated eye, up to a degree of
|sophistication that even a modern optical engineer would have
|difficulty emulating?
I could compose a "just so" story -- I have little problem
imagining how the spherical-abberation-correcting optics could develop,
and Clarkson briefly discusses how doublets could have developed -- but I
see little point, because...
|And if it was by trial and error, where is the
|fossil evidence of those myriads of failed trials, before it got it
|right?
Much of the evidence to test such a hypothesis stands very little
chance of preservation. I know, that hoary old chestnut -- poor
preservation potential -- but go ahead and tell me everything about
trilobite eyes should commonly be preserved, or that if they were
soft-bodied at any point, they should be popping out of the rock
everywhere. Unfortunately the fossil record limits us in terms of the
questions that can be resolved. The changes in trilobite eyes in the
Cambrian are particularly problematic because the shedding of the skeleton
often involved the separate loss of the visual surface. This makes
finding Cambrian trilobite eyes an exercise in finding needles in a
haystack. They are known, but are fairly rare. I have several trilobite
specimens from the Cambrian, but none of them possess eyes. They only
have an open hole in the skeleton at that point.
Practicalities also make it difficult to examine the morphology of
trilobite eyes through time. There are *alot* of trilobite species, and
they had a great deal of provincialism. Tracing lineages and cutting up
specimens for their eyes would take a great deal of work. So far, only a
limited number of paleontologists who work on trilobites have chosen to
investigate the optics in any detail. Just the work of Clarkson and
Levi-Setti covers most of the recent literature. However, there is still
much more to this issue than what I have described here. If people are
interested in more detail, I recommend the literature I mentioned
previously.
-Andrew
macrae@geo.ucalgary.ca
home page: http://www.geo.ucalgary.ca/~macrae