Trilobite systematics: The last 75 years

Journal of Paleontology, Nov 2001 by Fortey, Richard

Trilobita related to other arthropods.-Stormer (1933) followed earlier workers such as Garstang in considering that trilobites were related to Chelicerata, especially by comparison with Limulus. Garstang had noted such features as the so-called "Trilobite larva" of Limulus in support of his claim. The structure of the trilobite limb had been established in essentials by Beecher (1894). However, redescription of the Cambrian Burgess Shale trilobites by Whittington (1975), and the pyritised Triarthrus from the Utica Shale first by Cisne (1975) and then in detail by Whittington and Almond (1987) served to establish the structure of the trilobite's biramous limbs in some detail. Trilobites are generally believed to have had three pairs of cephalic limbs plus the antennae, although four pairs of cephalic limbs are claimed by Hou and Bergstrom (1997). Discussion on the nature and function of the limb exite continues, with some authors (following Bergstrom, 1972) claiming that the setae are too rigid to function as a gill branch, a function which is accepted by most other workers. This does make a difference to the interpretation of relationships. It was recognized that many of the uncalcified athropods from the Burgess Shale had similar biramous limbs to trilobites, at least on the trunk: this led to the proposition of a wider group-Trilobitomorpha-comprising those animals sharing this character, the implication being that this was a primitive design for an arthropod. This group was recognized in the 1959 Treatise on Invertebrate Paleontology, Part 0 (Moore, ed., 1959). Following Manton's (1977) review of arthropod classification there was a period in the 1970s when arthropod polyphyly was an acceptable idea. Under this view it was considered possible that `arthropodisation' occurred more than once from a soft-bodied ancestor. Manton claimed this for the main groups of arthropods she recognised, trilobites being one of them. This had an influence in the subsequent discussions of the Cambrian evolutionary `explosion.' It was implicit that the base of the Cambrian marked the origination of the group from the unknown ancestor.

In 1975 Hessler and Newman suggested that the crustaceans (one of Manton's major groups) might have had an 'origin' among the trilobitomorphs; it is possible to make a plausible scenario to derive the specialised limbs of crustaceans from that of the generalised trilobitomorph limb, and differences in segmentation of the cephalic region being a subsequent specialization.

Resolution of these apparently contradictory views depended on the development of cladistic analyses of the fossils (including a selection of Burgess Shale animals) in relation to their possible living relatives. The first of such analyses was that of Briggs and Fortey (1989), which included Trilobita sensu lato as part of an arachnomorph clade, and a group including Limulus was a sister taxon. Many subsequent versions have improved upon the character codings (e.g., Wills et al., 1998 and references therein), and further improvements are doubtless possible, but one common feature of them all is the inclusion of Trilobita within a larger clade of "arachnomorphs." It seems most improbable that this will change. In all cladograms trilobites are several steps removed from the basal node of the Arthropoda. The phylum as a whole is supported by several characters which strongly suggest a monophyletic origin for the group, and support for arthropod polyphyly is muted today (but for a contrary view see Fryer, 1998). The position of the trilobites in cladograms further suggests to some that the view of the origin of the group as coincident with the time of the Cambrian 'explosion' will require modification, not least because when they first appear in the fossil record trilobites are already biogeographically differentiated.