Trilobite Taphofacies

Various aspects of fossil preservation can be combined into the recognition and description of taphonomic facies or tapho-facies (Speyer and Brett 1986, 1991). Together with lithofacies, biofacies, and ichnofacies, taphofacies tend to vary predictably with sedimentary environments, as shown by studies in modern marine settings (Parsons and Brett 1991). The modes of preservation of fossils can provide important insights into a number of features of mudrock deposition, including (1) the sedimentary environment (depth, temperature, salinity, oxygen level, substrate consistency); (2) the dynamics of sediment accumulation, average rates, as well as evidence for episodicity of sedimentation and erosion; (3) the temporal scope of individual mudrock units; and (4) the sediment geochemistry and early diagenetic environments.

The overall condition of trilobite skeletons can be assessed qualitatively or quantitatively but certainly should be noted in the field. Semiquantitative indices can be formulated by determining the proportion of skeletal parts in different, arbitrarily defined preservational states. In such cases, it is commonly useful to determine a set of standards with which particular shells can be compared and assigned to a category, in much the same way that grain shapes and roundness indices have long been assessed on the basis of standardized profiles by sedimentologists. Skeletal condition is particularly valuable for recognizing qualitatively the differing relative extents of sedimentary time-averaging and therefore, may be related to burial rates.

In their original formulation of the notion of taphofacies, Speyer and Brett (1986) used Middle Devonian trilobites and their modes of preservation to exemplify the general model. They also subsequently emphasized the fact that fossils in a particular taphofacies may show both background and event taphonomic signatures. That is, there may be distinct aspects of preservation of fossils under day-to-day conditions or under episodic catastrophic burial conditions in a given environment (Speyer and Brett 1988). The well-known Devonian trilobite species E. rana and Greenops species occur in many distinct associations representing different environments. Yet, these trilobites show very distinctive modes of common preservation that are unique to particular onshore-offshore positions and sedimentation conditions. For example, in high-energy shallow water, most trilobite remains are disarticulated, abraded fragments, although articulated, typically outstretched individuals may occur occasionally, owing to pulses of burial. In low-energy, fully oxic settings, mainly unbroken, though typically disarticulated trilobite material is preserved; under episodes of higher sedimentation, com plete typically outstretched, inverted individuals may occur, even in clusters. Under lower sedimentation the event signature comprises primarily intact molt ensembles, with few if any fully articulated carcasses. Finally, under low-energy, dysoxic (background) conditions, intact molt parts and tagma are the rule; the event signature of this setting is distinctive in showing an abundance of enrolled, commonly pyritized, specimens of trilobites.

Each taphofacies records different types of information. The distinctive modes of preservation provide information on sedimentary environments that cannot be determined otherwise. For example, the enrollment of trilobites may reflect a response to toxic stimuli triggered by a stirring up of anoxic, sulfide-rich muds. Similarly, pyritization suggests burial in anoxic, low-organic muds.

In this way, the study of trilobite preservation has helped to provide a new tool in paleoenvironmental analysis. Development of predictive models relating preservation to depositional environments and positions in sedimentary sequences, in turn, may aid paleontologists in prospecting for new fossil bonanzas, including spectacular trilobite beds.

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