Gilwern Trilobite

The two parts of the specimen in plate 49 are now assembled to show how the trilobite was originally found. In no way could the trilobite bring the cephalic margin to match the border of the long pygidium. Even so, enrollment provided effective protection, also in view of the presence of a robust telson, not preserved here, protruding from the tip of the pygidium.

A perfectly enrolled specimen of the Ordovici an trilobite

Flexicalymene meeki

(Foerste) from Waynesville, Ohio (x9). (RLS coll.; now at FMNH.)

figure 15

Probable life posture of the illaenid trilobite Panderia megalophtalma Linnarson. (Adapted from Bergstrom 1973a.)

figure 15

Probable life posture of the illaenid trilobite Panderia megalophtalma Linnarson. (Adapted from Bergstrom 1973a.)

surface to the ventral side, as seen in section 3.3 for Pricycbpyge and others, is telltale evidence of swimming life. If we should learn anything from the horseshoe crab, we might infer that some trilobites liked to swim upside-down. With their carapace functioning as a glider in this attitude, their dorsal eyes could scan the sea floor for prey more efficiently than in the upright position. This, however, is only speculation, after having seen the horseshoe crab, particularly the young, frolic and swim upside-down in a marine tank at Hinds Laboratory for the Geophysical Sciences at the University of Chicago. One can also speculate that the turretlike eyes of many trilobites, protruding as they did from the cephalic surface, could have been very profitably used as watchtowers above the seafloor level and still enable complete concealment of the hunter below a layer of sand. The appearance of a hard carapace itself, the enrollment capabilities, and various forms of spinosity are response to a need for protection against an environment that may have become hostile in many ways. On the whole, we must conclude that 350 million years of survival are good evidence of successful trilobite adaptation.

3.6 Trilobite Classification

The systematization of the fossil record of trilobites into a scheme that reflects the phylogeny and evolution of different stocks in a consistent manner has been the constant preoccupation of paleontologists since the early nineteenth century. Even the most basic subdivision of the material into major groups has been and still is the subject of great controversy. The problem of properly assigning the parenthood and relationships of the fifteen hundred or more genera known today, encompassing some ten thousand different species, is a monumental task indeed.

The discovery of particular anatomical or functional features in trilobites has led, in the past, to attempts at classification based on such features. The presence or absence of eyes (Dalman 1827), the eye structure (Emmrich 1839), and the enrollment ability (Milne-Edwards 1840) are only a few examples of the criteria which played a role in the earliest attempts of classification. The list is long indeed and is discussed in detail in the Treatise (Moore 1959). In the search for a "natural classification," Beecher (1897) believed that the natural sequence of evolutionary events could be unraveled by studying the ontogenetic development of the species (Haeckel's law of ontogenetic recapitulation). This led Beecher to subdivide the trilobites into three main groups on the basis of the pattern of their cephalic sutures. The Proparia and Opisthoparia are two of them, already mentioned in section 3.1; in addition he included a third group, the Hypoparia, which have ventral cephalic sutures. The first two groups have formed the basis of numerous subsequent classifications, though not without objection from other paleontologists. The third group has been rejected altogether. In the more recent attempts at classification, it has been recognized that no individual feature, no matter how significant it may be, is a sufficient guide for classification and that affinities based on collective characteristics must be weighed. The cephalic sutures have always been considered a significant guideline, up to the classification adopted in the Treatise, together with the cephalic axial characters. Further discussion along this line, however, threatens to become too technical and beyond the scope of this work. For this we refer the reader to the Treatise (Moore 1959) and to some of the literature that will be referred to in the following discussion.

At the Oslo Trilobite Conference of July 1973, I met Dr. Jan Bergstrom, then at the Department of Historical Geology and Paleontology, University of Lund, Sweden. In a comprehensive survey and innovative study (Bergstrom 1973a), Dr. Bergstrom had just reexamined critically the problem of trilobite classification. In Bergstrom's work, the two main types of trilobite enrollment, spheroidal versus spiral, have been adopted as an additional phylogenetical index that has helped to sort out natural groups when integrated with previously used criteria (Richter 1933; Henningsmoen 1951; Hupe 1953, 1955). As a result, a more consistent picture had emerged—one which consolidated much of the previous proliferation of orders and suborders, often based on characters of no phylogenetic significance. In many respects, Bergstrom's classification departed radically from the Treatise.

Having been brought up to date by Dr. Bergstrom, and still preparing the first edition of this book at that time, I did decide to organize the atlas material according to the newly proposed classification, which recognized nine orders of trilobites. A pictorial illustration of these groupings, adapted from Bergstrom (1973a), was given in figure 14 of my book (Levi-Setti 1975). Modern science is a communal enterprise. In both experimental and theoretical work, novel results and their interpretation must await confirmation and acceptance by a broad community of scholars before becoming established. Unless a consensus is reached, based on uncontroversial evidence, even innovative ideas remain relegated to the realm of speculation. It is now almost twenty years since Bergstrom's revision of trilobite classification first appeared. Alas, a consensus is still wanting. As much as several of its implications have been praised, others have raised objections. For example, recourse to enrollment type as a criterion to sort out one group from another, the Ptychopariida in particular, has not been universally accepted. Morphological characters are still carrying different weight, depending on the school of thought.

Dr. Bergstrom has in the meantime updated and refined his original revision and has kindly provided me with a still unpublished sketch representing his current thinking about the phylogeny of trilobite groups and consequent classification at the order and suborder level. This updated classification attempt is schematized in figure 16, where sketches of the trilobites that are representative of each grouping are also shown. It should be noted that the Agnostida are included in this scheme with a question mark: they may not be related to the trilobites in view of their affinities with the Crustacea. This novel scheme has many attractive features and may well form the basis for a classification that may be universally accepted at some future time. Confronted, however, with a still unsettled outcome of many controversies, I had to decide how to reorganize the presentation of the material in the atlas, in a manner that would survive the test of time and avoid rejection on part of any of the contending and expert factions. Of some help in this dilemma have been the objections to recently advanced trilobite classification schemes that have been raised by my colleague and trilobite scholar Franco Rasetti. In a critical discussion of the problems of trilobite taxonomy (Rasetti 1972), he conceded that, with Jaekel (1909) and Hupe (1953, 1955), only two groups of trilobites, the Miomera (two or three segments) and Polymera (more than three segments) may deserve order ranking. This view inspired me to seek a simple solution.

In a brainstorm, I visioned standing in front of an exposure of fossiliferous strata, asking myself what, if any, could be regarded as uncontroversial evidence concerning trilobite phylogeny and evolution. I realized that the revised version of my atlas presentation was layed out for me page by page, layer by layer, forever embedded in the rocks I was staring at. What is uncontroversial in the fossil record is the stratigraphic location of a particular group of life forms in the so-called geological column. Moderately objective is also the recognition of families, encompassing types that are viewed as related at the genus and species level. As a result of the above considerations, I have decided to adopt a chronological sequence in this presentation of the atlas material. The trilobites that populate each geological period will be listed at the family level, without concern about higher taxonomic groupings. Unlike the sequence of strata that we find in sedimentary rocks, however, where the youngest layers are the first to be encountered, I prefer to start at the bottom of the column, showing first the oldest trilobites. The geologic column is not evenly populated. Out of 140 families listed in the Treatise, 92 appear in the Cambrian, while only two remain during the Permian period. A graphic representation of the frequency of occurrence (diversity profile) of trilobite families over the various geologic periods is given in figure 17. It is clear that, even if the catastrophic extinction that is known to have taken place at the Permian-Triassic boundary (Sepkoski 1990) had not occurred, trilobites were well on their way toward a natural disappearance by the end of the Permian.

A possible phylogenetic assignment of trilobites to orders and suborders. Arrows point to sketches of the typical representatives of the various groups. Interpretation by the author of recenr views expressed by J. Bergstrom (private communication).

Frequency of occurrence (diversity profile) of trilobite families over the geologic column.

3.6 Trilobite Classification 85

Appendix a

A Case History:

0 0

Post a comment