posted 14. September 2003 14:20                   

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Thanks to Levi-Setti et al (The Eye: Palaeontology; Frontiers of Life. London: Academic Press, 2002) the evolution of the optics of these creatures is becoming calcite clear. These investigators suggest that the first trilobite probably had holochroal eyes with only a single calcite lens in the cone of the ommatidium, although there would be optical difficulties with these eyes including diffraction limitations. With room for improvement, the evolutionary tinkering that continued would prove advantageous—especially in a mesopelagic environment—to those species which could evolve improved optics.

A second step would be the appearance of an abathochroal eye with a larger round lens as these animals extended into a dimmer environment. As the lens enlarged the optical system would be less limited by diffraction but rather would be limited by spherical aberration producing a blurred focus.

The third, and truly revolutionary, step to be taken was the formation of a lens doublet with a perfected Huygensian profile. This dramatic but logical evolutionary step would allow for dramatic increased light gathering capabilities and would eliminate spherical aberration, as Descartes and Huygens would learn in the 1600s. Chromatic aberration would probably still exist in such a doublet but in a monochromatic world of mesopelagic light or perhaps even epipelagic light—this would matter little.

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What are ichnofossils? They are trace fossils: preserved tracks or other signs of the behaviors of animals in the substrate. Ichnofossils can provide some very intriguing insights on the behavior of an extinct animal such as a trilobite. It is very rare that the animal itself is found in direct association with the ichnofossil it created, but that kind of co-occurrence allows scientists to link ichnofossils with the species that created them. In fact, in the pre-Cambrian fossil record, there are no trilobites, but there are trace fossils which very closely resemble those made by trilobites, suggesting that before trilobites developed their hard calcite shells, their ancestors were crawling about leaving traces. While there are hundreds of named ichnofossil types, there are three main named categories of ichnofossils associated with trilobites: Rusophycus, Cruziana, and Diplichnites, described below.

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trilobite classification



[ 14. September 2003, 14:36: Message edited by: Pim van Meurs ]

posted 14. September 2003 15:00                   

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http://www.swcp.com/~diamond/trilobites.html

Certain creationists have pressed the allegation that the fossil record contains no evidence of predecessors or ancestors to trilobites and that the "abrupt" first appearance of trilobites is thus positive evidence for the instantaneous creation of the lineage. However, this assertion is plainly untenable if the fossil record is examined in more than a cursory manner. The Early Cambrian was a time when numerous taxa, including the arthropods, attained mineralized skeletons (Lipps and Signor, 1992); any soft-bodied ancestors of these taxa would have had little chance for preservation as fossils. (While soft-bodied organisms can be preserved under exceptional conditions, these conditions are very rare. Normally, it is only the hard-parts of organisms that are preserved as fossils.) Nonetheless, the fossil record does offer two lines of direct evidence that point to the existence of trilobite predecessors.

EVIDENCE FROM TRACE FOSSILS
The first line of evidence comes from trace fossils, the preserved tracks, trails, and burrows of living organisms. Marine sedimentary rocks of many different ages (Cambrian to Permian) have been found to contain a very distinctive suite of burrows, scratch-marks and furrowing traces that can confidently be ascribed to the activitites of trilobites (Seilacher, 1970; Crimes, 1973). Indeed, rare specimens have been found which contain trilobite body fossils within the burrows (Osgood, 1970). In sections of rock that span the boundary between the Cambrian and the preceding Vendian Period, these distinctive trace fossils are commonly found in strata which are lower stratigraphically (and thus older) than those containing the first preserved trilobite skeletons (Crimes, 1987, 1994). Non-existent organisms cannot produce burrows, and so these trace fossils demonstrate that trilobites, or their ancestors, did in fact exist prior to the first appearance of trilobite body fossils. Even more intriguing is the observed fact that these trace fossils become increasingly more complex with time during the period preceding the advent of mineralized skeletons (Crimes, 1992). This implies that the behaviors of these ancestral trilobites were evolving.

EVIDENCE FROM THE PRECAMBRIAN FOSSIL RECORD
The second line of evidence comes from the fortuitously preserved soft-bodied organisms of the latest Precambrian-aged (Vendian) Ediacaran Fauna. The Ediacaran Fauna were traditionally interpreted as ancestral representatives of and predecessors to taxonomic groups which occur more abundantly in the Cambrian Period (Glaessner, 1984; Fedonkin, 1985). This view was challenged by Seilacher (1984, 1989), who argued that the Ediacaran Fauna were not animals but instead were a group of structurally unique organisms belonging to a previously unrecognized kingdom, dubbed the Vendozoa. Seilacher's hypothesis has been vigourously debated (Gehling, 1991; Fedonkin, 1992; Conway Morris, 1993; Crimes et al., 1995) and Seilacher himself seems to have moved away from his earlier hypothesis (Buss and Seilacher, 1994). The present consensus is apparently that while the Vendozoan hypothesis may apply to some Ediacaran organisms, it does not apply to them all. Certainly, representatives of several extant phyla have been convincingly documented (Gehling, 1987, 1988, 1991; Fedonkin, 1992).

The organisms represented in the Ediacaran fauna were soft-bodied and are preserved as impressions, generally (but not exclusively) on the soles (bottoms) of sandstone beds. They were apparently preserved as the result of the fortuitous sediment-binding action of microbial mats (Gehling, 1991; Seilacher and Pfluger, 1994). In Gehling's (1991, p. 218) view, these microbial mats "acted both to erosion proof the substrates occupied by organisms buried under storm sands, and to initiate immediate mineral encrusting of organic surfaces and cementataion of the sand at the interface." (The interface referred to is the one between layers of sediment.) This mode of preservation seems to have been confined to the Precambrian--it was presumably curtailed by the activities of microbe-eating burrowing and grazing organisms which appeared in the latest Precambrian and early Cambrian (Gehling, 1991; Crimes, 1992, 1994).

The Ediacaran Fauna contains several species which may represent, if not ancestral trilobites, then at least ancestral arthropods. These include such taxa as Vendia, Vendomia, Onega, Praecambridium, Parvancorina, and Marywadea (Gehling, 1991; Fedonkin, 1992; Jenkins, 1992). All of these genera show well-developed head and tail differentiation and possess body outlines such as might be expected in a primitive arthropod. There is one additional taxon, yet to be formally described, which shows great promise to elucidate the early history of the trilobites. It has been described in the following terms by Gehling (1991, p. 206):

A new metameric form . . . features a head end with a symmetrically placed D- shaped ridge. The ridge parallels the front margin, and seems to contain a domed area within. The body has a raised axial zone, and segments that show evidence of articulation . . . This organism has more external resemblance to the trilobites than any form so far described from the Ediacara assemblage.

The photograph which Gehling provides (Plate 4, Fig. 4) definitely supports his last assertion. The same taxon has also been described in more detail by Jenkins (1992), who dubs it a "soft- bodied trilobite" and states that, "Its observed characteristics occur in a variety of Paleozoic trilobites, but it can probably be considered to differ from all known forms at the ordinal level" (p. 169). Jenkins (1992, Figure 15) figures sketches of several specimens of the organism; like Gehling's photograph, they show an organism strongly resembling a trilobite.

CONCLUSION
The fossil record provides a significant amount of evidence for the existence of trilobites prior to their aquisition of a mineralized skeleton. Trace fossils offer a record of the evolving behavior of the soft-bodied ancestral trilobites, while material in the Ediacaran Fauna indicates the presence of probable primitive arthropods and likely presence of ancestral trilobites in the latest Precambrian. Assertions that trilobites appear "instantaneously" in the rocks of the Cambrian System, without any sign of ancestral forms, are plainly incorrect.

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posted 14. September 2003 16:03                   

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The bilobed trace fossil Didymaulichnus cf. tirasensis Palij in the Lower Cambrian Voosi Formation of western Estonia adds to the small number of pre-trilobite Cambrian trace fossils identified from Estonia. A possible earliest Cambrian Baltica trace fossil province, including trace fossils with an undulating bilobed and three-lobed lower surface, of which D. tirasensis is an example, is proposed. In Baltica traces of this type occur also in the Ukraine and northern Scandinavia. In each occurrence they are associated with vertical spiral traces and precede the local first appearance of arthropod-type trace fossils. The earliest Estonian arthropod-type trace fossil, Monomorphichnus isp., described here from the Sõru Formation, belongs to the trilobite-bearing Lower Cambrian.

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posted 15. September 2003 09:51                    

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I propose that those who claim that the trilobite's origins are not a problem for Darwinian evolution each post to this thread, over the next day or so, a simple post of the following format:

transition 1(with reference and picture where possible), -> transition 2 -> ->->....trilobite
in the sequence they believe in, now the blind part, without looking at anyone elses post for support.
At the end, we'll see if the evidence is in for trilobite evolution from lower forms. This would of course be
on the honor system. Please no commentary in the first posts.

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Are you really going to tell us, Cre8, that the trilobites, helmetiids, tegopeltids, naraoiids, etc. were all independently created kinds?

What, by the way, is your definition of a kind? Trilobites are traditionally a class, like mammals and birds. If we show you the intermediates between mammals and reptiles, and birds and reptiles, will you accept evolution then?

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