Articulated Remains

Skeletons of organisms in which the skeleton is composed of multiple elements weakly bound together by ligaments or musculature, such as trilobites, are only rarely preserved intact. Modern experimental studies indicate that the degradation of soft tissues in arthropods occurs within a period of a few hours after death, while destruction of ligaments ensues in weeks to months. As such, skeletons may be completely disarticulated within a period of a few months or less (Plotnick 1986, Allison 1988a).

A variety of articulated trilobite remains are found. Speyer and Brett (1986) recognized three basic categories: (1) partially articulated exoskeletons, (2) molt remains or exuviae, and (3) completely articulated skeletons representing carcasses (Figure 3.1). Partially articulated remains, such as groups of thoracic segments, are of indeterminate origin and may represent either partially decayed remains of carcasses or molt parts.

Exuviae in many trilobites are recognizable by the absence of specific structures. For most trilobites molting involves the shedding of free cheeks. Therefore, articulated exoskeletons with cranidia lacking free cheeks are almost certainly exuviae. Pha-copids had fused facial sutures and shed the entire cephalic shield in molting. Thus, articulated thoracopygidia ("headless trilo-bites") suggest molt parts. Molt ensembles are groups of closely associated molt parts, such as free cheeks lying in close proximity to articulated remains with cranidia, or cephalic shields closely associated with thoracopygidia in phacopids.

Carcasses are represented by completely articulated exoskele-tons, with the cephala intact and articulated (i.e., free cheeks are intact). This category may be subdivided further into outstretched or prone specimens and in some trilobite species, partially enrolled and fully enrolled individuals.

Organisms with multielement skeletons, including trilobites, are particularly sensitive indicators of rapid and permanent burial. Well-preserved, articulated trilobites typically occur on certain bedding planes within mudrocks that would not be recognizable as event-beds by other sedimentological means. Because such skeletons cannot be reworked, the occurrence of even a single intact specimen of a trilobite is an excellent indicator that the enclosing sediment accumulated rapidly and was not subsequently disturbed. The occurrence of large numbers of completely articulated trilobites provides dramatic evidence for a population of organisms that was abruptly wiped out. Conversely, the occurrence of well-preserved molt ensembles need not imply any mortality but rather indicates burial under relatively low-energy conditions that prevented scattering of parts. Certain widely traceable levels in the Middle Devonian Hamilton Group are characterized by abundant articulated molt ensembles but few, if any, complete trilobites (that would represent carcasses). Such findings may reflect rapid accumulation of thin layers of sediment that did not kill or smother living trilo-bites but were sufficient to preserve molts (Speyer 1987).

Many assemblages of well-preserved trilobites are also demon-strably in situ (buried in their living sites). A particularly sensitive indicator is the occurrence of trilobite molt ensembles, that is, associated, disarticulated molt parts. It is virtually impossible for different disarticulated portions of the skeleton to be transported any distance and still remain associated. The hydrody-namic properties of whole exuviae versus free cheeks would

FIGURE 3.1. Fossil assemblages reflecting various conditions and timing of burial. A. Low rates of sedimentation and slow burial along with reworking of the prefossilized hard parts. Sclerites are broken and scattered. B. Slow burial with disarticulation and mixing. C. Obrution: burial, under a thin blanket of sediment, with some disarticulation and infaunal scavenging. D. Obrution: sudden burial under a thick blanket of sediment, resulting in good preservation of articulated fossils and reduced infaunal scavenging. A-D from Brett and Baird (1993). E. Trilobites that exhibit some degree of disarticulation because they were not buried deeply enough or soon enough after mortality, PRI 49661. F. A group of well-preserved trilobites as a result of rapid, relatively deep burial, PRI 49662. G. A trilobite that is tightly coiled and well preserved, PRI 49663. Trilobites coiled under stress, probably associated with the burial event.

FIGURE 3.1. Fossil assemblages reflecting various conditions and timing of burial. A. Low rates of sedimentation and slow burial along with reworking of the prefossilized hard parts. Sclerites are broken and scattered. B. Slow burial with disarticulation and mixing. C. Obrution: burial, under a thin blanket of sediment, with some disarticulation and infaunal scavenging. D. Obrution: sudden burial under a thick blanket of sediment, resulting in good preservation of articulated fossils and reduced infaunal scavenging. A-D from Brett and Baird (1993). E. Trilobites that exhibit some degree of disarticulation because they were not buried deeply enough or soon enough after mortality, PRI 49661. F. A group of well-preserved trilobites as a result of rapid, relatively deep burial, PRI 49662. G. A trilobite that is tightly coiled and well preserved, PRI 49663. Trilobites coiled under stress, probably associated with the burial event.

be so different that it is extremely unlikely they could be transported and yet end up together. Molt ensembles thus constitute proof of life activity (i.e., molting) by trilobites in the precise site of burial.

Other dramatic examples of in situ trilobites are the very rare specimens of articulated skeletons directly associated with their trace fossils, such as famous specimens of Flexicalymene attached to Rusophycus (see Figure 2.14B). The occurrence of beds of enrolled trilobites also suggests a behavioral response to stressed conditions in which trilobites may have burrowed into the sediment and enrolled themselves, only to be buried in place and apparently unable to escape from mud blanketing.

Speyer and Brett (1985) also recognized species-segregated clusters of fully articulated skeletons ("body clusters"; Plates 59, 102, 147, and 152) and molt ensembles ("molt clusters") involving at least three species of trilobites on single bedding planes in the Devonian Hamilton Group. These trilobite clusters appear to represent preserved behavioral patterns. Using the argument that the molt ensembles could not have been transported any distance, Speyer and Brett inferred that these represented species-segregated aggregations of trilobites that molted en mass. Speyer and Brett further pointed out that synchronized molting typically serves as a prelude to mating in modern peracarid crustaceans (e.g., marine isopods) and that several species also may time their reproductive cycles to a common external stimuli, such as lunar cycles. This analog suggests that trilobite clusters may represent preserved mating "orgies."

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