Trilobite systematics: The last 75 years

Journal of Paleontology, Nov 2001 by Fortey, Richard

ABSTRACT-The progress achieved in trilobite systematics over the last 75 years is briefly reviewed. Different approaches to phylogenetics have influenced the way trilobites have been classified. Classical evolutionary taxonomy, the stratigraphical approach, and cladistics have all contributed in different ways to the current classification, which has evolved piecemeal, and is still unsatisfactory is some ways. Nonetheless, progress towards a phylogenetic classification has been made, especially as the result of information from ontogenies provided by well-preserved silificified material. Trilobites are a well-defined clade within a larger arachnomorph group. Agnostida have been excluded from Trilobita, but are perhaps best considered as specialised trilobites, at least until limbs of eodiscids are described. The outstanding problems in classification of each trilobite order are reviewed. Most are concerned with the recognition of the appropriate Cambrian sister taxa, and the discovery of the relevant ontogenies. It is very likely that post-Cambrian Glades "root" deeply into the Cambrian. The coherence, or otherwise, of Proetida, Asaphida, Corynexochida and the lichid/odontopleurid groups will be resolved by such studies. The problems of paraphyly in Ptychopariida and Redlichiina may prove more obdurate. The temporal brevity of certain Cambrian family ranges may be partly a taxonomic artefact. The possibility of a late Cambrian gap in the record on some Glades should be considered.

INTRODUCTION

TRILOBITES HAVE COMPLEX morphology and a long geological history with an apparently good fossil record but their systematics is still in an unsatisfactory state. Many important monographs on trilobites have been published in the Journal of Paleontology, and several of these have proved important in the debates about how trilobites should be classified. In the current intellectual climate obsessed by "impact factors" it might be as well to recall that these papers, many published in the 1940's for example by Franco Rasetti (Rasetti, 1945). are still regularly referred to half an century later; they comprise the raw material upon which hypotheses of trilobite relationships are founded. It is also pertinent that trilobites have played an important part in in theoretical discussions about classification and how it ought to be be done. One of the first cladograms ever published featuring fossils was used to illustrate inter-relationships between species of the Devonian genus Phacops (Eldridge, 1972). Possibly the first application to a wholly fossil group of the computer-based parsimony analysis known as PAUP-now used routinely on almost any laptop--was by Fortey and Chatterton (1988) to analysis relationship in Asaphida. Ramskold and Werdelin (1991) has been cited as an exemplary phylogenetic analysis by Smith (1994), and so on. In this paper I shall examine some of the history of progress in trilobite systematics that has been accomplished since the foundation of the journal, list some of the problems that remain, and speculate briefly how they may be tackled. When I say that trilobite systematics is in an unsatisfactory state this is not to say that considerable advances towards a scheme that most workers can accept have not been made. New insights have arisen, and consensus is emerging, albeit slowly. The Journal of Paleontology had had a part to play in the new systematics.

TRILOBITA: PLACE IN ARTHROPODA

That trilobites were arthropods has been accepted for nearly two hundred years. What has been at issue is the place that the group occupies within the Arthropoda, sensu lato. Inevitably this is entangled with the phylogenetics of this greater group, itself a perennial problem. As far as the trilobites alone are concerned two questions predominate: 1) Are the trilobites a natural group, or paraphyletic? and 2) If they are a clade how do they relate to the other great groups of arthropods: Hexapoda (insects) Myriopoda (centipede and millipeds) Crustacea and Chelicerata (horseshoe crabs and arachnids)? Of the taxonomic status of the Trilobita not much need be said. A group comprising several thousand genera would seem to merit high level recognition. This has usually been as Class Trilobite. This assignment would be disputed by those who maintain that wholly fossil groups should not have formal status, or those who conclude that status can only be decided by an exhaustive cladistic analysis including all potential sister taxa. The former view is defensible, but overly authoritarian, the latter view depends on having a long-term stable and comprehensive cladogram which has scarcely been achieved, and anyway it is acknowledged that not all Classes are commensurate across phyla. Class Trilobite it shall remain for the time being.

Constitution of Trilobite as a group.-The coherence of Trilobita as a clade has been challenged by Lauterbach (1980, 1983), who noted a number of characters that allegedly linked primitive olenelloids with limuloids in preference to the rest of the Trilobite. For example he identified a posterior spine on the middle of the axis in Olenellus thompsoni with the telson spine of limuloids. He stressed the importance of the macropleural segment in olenelloids in his interpretation of segmental homology. Fortey and Whittington (1989) countered this assertion by listing a greater number of characters which were synapomorphic between Trilobita (sensu Lauterbach) and olenelloids than between olenelloids and limuloids. These synapomorphies effectively defined Trilobita, at least in the sense of calcified exemplars. Characters cited included: the calcified structure of the eye, and its circumocular suture; the hypostomal wings and their connection with anterior pits in the axial furrow; the presence of a fused pygidium having more than one segment. Ramskold and Edgecombe (1991) challenged a number of these characters but concurred with the view of trilobite monophyly (see also Edgecombe and Ramskold, 1999). Even if Lauterbach were mistaken it was interesting that he emphasised similarities between primitive chelicerates and the Trilobite.