Trilobite Taphonomy and Taxonomy: A Problem and Some Implications

Abstract

Recent advances in trilobite taphonomy have highlighted the importance of sclerite preservation for understanding sedimentary processes and aspects of trilobite paleoecology (Speyer and Brett, 1985, 1986; Westrop, 1986; Babcock and Speyer, 1987; Speyer, 1987, 1988). The effect of taphonomy on trilobite taxonomy has received less attention. Consequently, it is not possible to evaluate the broader effects of taphonomy on our understanding of evolution and biogeography, and some recent studies of evolutionary patterns within the group (e.g., Foote, 1990, 1991, 1993) have been restricted to forms occurring within particular lithofacies. Numerous systematic papers discuss how preservation affects particular characters or character sets. Examples include the effects of compaction on sclerite morphology in specimens preserved in fine grain clastic sediments (e.g., Kiaer, 1917; Fortey, 1974), differences between carbonate and clastic preservation (e.g., Taylor, 1978), the differences in the morphology of internal and external molds (e.g., Jell, 1985), and how taphonomic artifacts have been mistaken for biological features (Hughes, 1993). However, such examples refer to specific instances, and are of limited use in evaluating the broader implications of taphonomy on trilobite taxonomy. As taphonomic conditions vary markedly between sedimentary environments, the effects of taphonomy on trilobite systematics are likely to be greatest on those taxa which occur in a wide range of lithofacies. Such taxa are often the most useful for providing inter-regional correlations and hence have critical significance for biogeography and biostratigraphy. Here I present a brief illustrative example of the potential effects of taphonomy on the taxonomy of one trilobite, the middle Cambrian conocoryphid Bailiella, and a discussion of the implications of this example for other areas of paleontology. Bailiella is an appropriate trilobite to use as an example for several reasons. Firstly, as an eyeless conocoryphid trilobite its morphology is distinct from that of many generalized ptychopariid trilobites, with which it is co-occurrent. Its distinctiveness allows us to be relatively confident of its identification, despite the taphonomic differences detailed below. Secondly, the genus is widespread and occurs in a variety of paleoenvironments. A revision of the systematics of Bailiella species from China, Vietnam, India and Siberia, indicates that the position of the facial suture (cutting the border) provides a consistent characteristic that can be used to recognize the species B. lantenoisi (see Zhang and Jell, 1987, pl. 35, figs. 1-7). This species is found both in shelfal carbonate settings (China and Siberia), where it is usually found as disarticulated sclerites, and in finegrain clastic slope deposits (Vietnam and India) where it occurs both as articulated dorsal shields and as disarticulated sclerites. These different preservation styles present alternative character suites to the systematist. For example, cranidia preserved in carbonates possess original relief and fine details of surface ornament. The preservation potential of these types of features is reduced in specimens preserved in shales, because the skeletal material is often absent and the specimens are compressed composite molds. The presence of articulated dorsal exoskeletons in this type of setting, however, provides a suite of characteristics that are not available for evaluation in specimens from shelf carbonates. Such characteristics include the number of thoracic segments, and the overall body proportions. Peter Jell and I are preparing a redescription of B. lantenoisi that lists some 38 major characters (Table 1). Parentheses are used here because the question of what constitutes a character is subjective in any systematic analysis (see Adrain and Chatterton, 1990), and character choice is especially difficult when the morphology includes both biological and preservational components. Another systematist would not describe an identical set of characters, but the magnitude of preservational factors documented below far outweighs slight differences of character interpretation. The greater number of characters listed for the cranidium (23) than for the remainder of the dorsal exoskeleton (15) (Table 1) reflects the concentration of distinctive morphological features on that sclerite. Of the 38 characters, only 14 appear to have resisted modification by preservational biases, and can be compared with confidence in specimens from all lithofacies settings (Table 1). Variation in any of these 14 characters could potentially be used to define different taxa, but apparently does not do so in this case. Of the remaining 24 characters we can be less confident. 18 characteristics can only be coded with confidence in specimens from carbonates preserving relief and fine detail, 4 are codable only in articulated dorsal shields, presently known only from slope settings, and 2 cannot be coded with confidence in either setting. Each of these 24 characteristics could potentially indicate taxonomic differences between B. lantenoisi collections preserved in different taphonomic settings. However, as these