Evolution and Cladistics

Fortey and Owens (1997) reviewed the evolutionary history of trilobites. This review should be considered the latest in what will be an ongoing series of arguments.

The phylogeny or evolution of trilobites is known from the first calcified remains in the Lower Cambrian rocks to the last trilobites in the Late Permian. It is generally believed that trilo-bites as a class are monophyletic; that is they are from a common ancestor (Ramskold and Edgecombe 1991). The earliest known trilobites are from the suborder Olenellina found exclusively in the Lower Cambrian. These trilobites lack facial sutures, which is considered a primitive characteristic (Fortey and Whittington 1989). In New York Elliptocephala asaphoides is a member of this early group. Facial sutures first appear in the suborder Redlichiina, also in the Lower Cambrian. It is probable that there are ancestral trilobites that did not have mineralized exoskeletons, perhaps back into the late Precambrian. There is no compelling reason to believe that all trilobites sprang from olenellin ancestral stock.

From these ancestors evolved the large number of trilobite orders, families, and genera that are recognized today. Evolution, however, is not something that proceeds smoothly with a calendar-like precision. For evolutionary change to occur, at least two major factors must be in place. First, any genetic change or muta tion must offer a competitive advantage to the species. Noncom-petitive genetic changes will soon disappear, as the animal is unable to compete for the opportunity to pass the changes along to its offspring. Next, the new trait should occur in an isolated small population so that it will not be "lost" in a much, much larger gene pool. Since mutation occurs relatively frequently, a beneficial mutation will happen at somewhat regular intervals, statistically. In a stable environment, however, with many others of the same species, this change may not become fixed. A better opportunity for a new mutation to be passed along is when a group, for some reason, is occupying an environment where there is little outbreeding competition or there are new environmental niches to occupy. Essentially populations are stable until a change takes place in an isolated environment, and after this change (or changes), the new species becomes competitive with the former stable species and displaces it or in some cases occupies a different available niche. Thus, evolution is not a process of continual small changes but one of relative stability followed by abrupt step changes. Eldredge and Gould (1972) used the term punctuated equilibria to reflect their observation that evolution is not a smooth transition from one species to the other but rather is composed of periods of relative stability punctuated by periods of rapid change.

The observation that some fossil communities show remarkable stability, sometimes over millions of years, has led to some in-depth investigations. In New York some Middle Devonian fossil communities, particularly in dysaerobic environments, show this stability. As a result of these and other studies, Brett and Baird (1992,1995) coined the term coordinated stasis for this form of evolutionary stability (see also Kammer et al. 1986, Morris et al. 1995, Brett et al. 1996). Similarly in the Cambrian, for example, there is evidence that populations of trilobites in the shallower water environments underwent a number of extinctions, and the area where they once were was repopulated with trilobites from deeper water. These episodic extinctions and repopulations, called biomeres (Palmer 1965, 1984), are recorded from the Cambrian of North America (see also Edgecomb 1992).

Relationships of trilobites at all taxonomic levels are currently being revised using cladistic methodology. Cladistics as used in paleontology is the grouping of taxa by their shared physical characteristics. Close relationships are based on synapomorphies or "shared derived" characteristics. Traits may be divided into primitive (plesiomorphic) or derived (apomorphic). For trilobites the biramous appendages are plesiomorphic or "shared primitive" characteristics, meaning they are primitive to the group (i.e., they represent a common ancestral trait of arthropods not exclusive to trilobites) and may be used only to compare trilobites with other arthropods. Schizochroal eyes are only found in the suborder Phacopina, and thus this characteristic forms a derived characteristic or apomorphy. Another example involves the trilobite hypostome. Fortey (1990a) developed the argument that hypos-

tome attachment, natent versus conterminant versus impendent, is an indicator of trilobite phylogeny. Using these and other relationships, Fortey (1990a, Figure 19) constructed a phylogeny/hypostome attachment chart for trilobites, which summarizes the current state of knowledge.

By selecting a significant number of derived characteristics and evaluating their presence or absence, one can group related trilobites by the timing of the appearance of the selected derived characteristics. With a large number of characteristics, the groupings are best done with computer programs designed for cladis-tics that search for the closest or most parsimonious fit. Cladistics is clarifying many relationships among trilobites. For a more in-depth review, see the works by Fortey (1990b, 2001) and Novacek and Wheeler (1992).

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