Transport and Reorientation

Trilobite skeletons can be sensitive indicators of hydrody-namic conditions in the depositional environment. Under low-energy environments, typical of mudrocks, organism remains may be buried in situ. One might use the argument that well-articulated fossils such as trilobites must not have been transported and that their occurrence therefore indicates quiet water environments, but this inference must be made cautiously. Aside from obvious cases where this is not so (e.g., in which these skeletons are found at the bases of turbidites or even within skeletal grainstone deposits), there is experimental evidence that if organisms are transported within the first few hours following death, their remains may stay articulated. Allison (1986) demonstrated in tumbling barrel experiments that arthropods such as shrimp, which provide possible analogs to trilobites, may be potentially moved even tens of kilometers without disarticulating, provided that this happens within the first few hours following their demise.

Trilobite fossils also may provide evidence for reworking or transport of skeletons that would otherwise remain unsuspected. It is not unusual in marine mudrocks to observe concentrations of shells in localized pockets, typically associated with evidence for current scour. Such accumulations commonly display convex-

downward or lateral-oblique positions along linear features that may be interpreted as indicators of storm-generated scour and fill.

Many skeletal elements, including articulated trilobites, as well as their cranidia and pygidia, have approximately concavo-convex dish-shapes. Random or preferred convex-up or convex-down orientations may be observed in different trilobite assemblages, and each has distinct implications for burial conditions. Instances of random or nearly one-to-one ratios of convex-up and -down orientations occur primarily in heavily bioturbated sediments, where, in addition, skeletal elements may display lateral or edgewise orientations. The mixing of skeletal components into sediment may account for the randomizing of orientation. However, such orientations are unstable under hydrodynamic currents and hence, are probably one of the better indicators of bioturbation or some type of protective concentration trap on the substrate.

Concavo-convex trilobite skeletal elements and whole bodies, like many shells, commonly display preferred convex-up or convex-down orientations. For example, cephala and pygidial shields in a preferred convex-up position are probably most typical of concentrated trilobite beds. Flume studies (e.g., Hesselbo 1987, Lask 1993) showed that even gentle currents will affect trilobite remains resting on the seafloor in such a way that they flip to a hydrodynamically stable orientation, at which point currents will glide over their streamlined, convex-up surfaces. The effect is particularly evident in areas of muddy substrate because of the impedance (i.e., frictional drag).

Hence, the occurrence of abundant convex-up trilobite cephala or pygidia on bedding planes may provide evidence for reworking of skeletal material under slightly current-agitated conditions. Beds of this sort represent skeletal material that was processed by one or more storm-generated current events. Excellent examples are found in large assemblages of Flexicalymene and Isotelus pygidial and cephalic shields on the bases of hummocky laminated calcisiltite beds, and pavements of Pseudogygites from the Ordovician Collingwood Shale (Brett, in prep.).

Beds of preferentially concave-upward trilobite tagma are not as common. In settings where the molt parts are suspended briefly and allowed to resettle from suspension, they will almost invariably settle concave-upward (Lask 1993). Such reorientation would occur in areas very close to storm wave-base in which the rather gentle storm-generated waves would lift skeletal materials temporarily off the bottom and allow them to free-fall back to the substrate. Rapid burial following this stirring would incorporate such shells as a basal pavement of a possibly graded mudstone layer. Excellent examples of this mode of burial are seen in pavements of Triarthrus cranidia in Ordovician dark shales. Counts of many bedding planes from the Ordovician Collingwood Shale of Ontario show a predominance of concave-upward orientations of the cranidia and of tiny ostracode valves, even on bedding planes in which larger skeletons are random or convex-upward (C. Brett, unpublished observation, 1998). We suggest that under certain conditions only the smallest, lightweight skeletons were suspended and then resettled —typically concave-upward.

Stacks of nested or shingled fossils apparently occur where densely packed skeletal remains were affected by oscillatory, storm-generated waves or currents, and provide an indication of deposition well within storm wave-base. A similar mode of preservation in shell accumulations involves concavo-convex shells bundled together in nested groupings, either convex-upward or -downward and often both in the same layers. These, too, seem to reflect the effects of settling and concentration of skeletal remains during turbulence events, and they are commonly associated with minor sediment grading. An intriguing example consists of masses of nested cephalic and pygidial shields of bumastid trilobites from Silurian bioherms, such as those in the Silurian Irondequoit Limestone. These may represent molt parts that were concentrated in sheltered "pockets" or scours on bioherm surfaces.

A particularly intriguing and still enigmatic aspect of orientation in trilobites is the predominantly concave-upward (dorsal shield down) orientation of articulated trilobites. Many occurrences of clusters of trilobites display this phenomenon, including mass occurrences of Ceraurus pleurexanthemus from the Ordovician Trenton Limestone of New York (Brett et al. 1999), beds of Dalmanites limulurus in the Silurian Rochester Shale (Taylor and Brett 1996), and aggregations of Eldredgeops rana from the famous trilobite beds of the Lake Erie region (Speyer and Brett 1985). Several explanations for such orientations have been put forth. Raymond (1920a) believed that these represented examples of trilobites buried in a life position, postulating that these animals swam upside down, like horseshoe crabs, and perished in this position. However, most later authors have considered this unlikely and argued for a postmortem reorientation of carcasses. Once again, if the concavo-convex carcasses of trilo-bites were suspended and resettled freely, they would very likely assume this position. Several features associated with the dorsal-downward trilobites may bear on this issue. For example, Speyer and Brett (1985) noted that some of the "inverted" trilobites were headless and thus, likely molted thoracopygidia. Such specimens obviously were not "swimming," and yet they are found alongside complete specimens that are also predominantly convex-downward. Even among these latter specimens there may be signs of incipient disarticulation (e.g., cephalon or pygidium is rotated slightly away from thorax; thoracic segments are very slightly pulled apart). Such observations suggest that the trilobites represent carcasses that had undergone a very slight amount of decay prior to burial. In all of the above-mentioned cases, there is also a vaguely to strongly preferred long-axis orientation (e.g., see Brett et al. 1999). This would also seem to imply that the trilo-bites reflect carcasses or molts that were transported, if only slightly, by currents after death. As noted already, however, currents usually have the effect of flipping concavo-convex elements to a preferred convex-upward position. If trilobite remains were lifted into a current slightly and then resettled, they might still assume this position. Also, the generation of decay gasses within the body cavities of trilobites might give them slight buoyancy and make them more subject to the lifting and settling required to invert the carcasses. All such inferences seem to point to a brief interlude between mortality and final burial.

A few occurrences of predominantly convex-upward outstretched trilobites are also known; for example, groupings of Eldredgeops milleri from the Silica Shale typically show this orientation. These are commonly associated with perfectly enrolled trilobites and they may represent examples of more nearly instantaneous burial with little disturbance.

Speyer (1987) and Brandt (1985) also considered the possibility of preferential orientation of enrolled trilobites but came to different conclusions. Brandt reported essentially random orientations in some mass occurrences of enrolled Flexicalymene meeki from Upper Ordovician shales of the Cincinnati, Ohio, region. In contrast, Speyer recognized preferred and species-specific orientations in Devonian enrolled trilobites from New York. Greenops specimens were most commonly found in a cephalon-up position, while Eldredgeops rana more commonly showed cephalon-lateral or -downward positions. Speyer suggested that these findings recorded different modes of pre-enroll-ment behavior by the trilobites.

The preferred azimuthal (compass bearing) orientation of elongate skeletons has been the subject of numerous studies, including both observational and experimental studies (see Kidwell and Bosence 1991, for summary). From such studies, it has become common knowledge that elongate particles typically orient themselves selectively within a current. Elongate objects that do not roll, such as the trilobite carcasses mentioned earlier, commonly will be aligned parallel to the direction of the current. Such may be the case with consistently aligned specimens of C. pleurexanthemus reported by Brett et al. (1999). The occurrence of some aligned trilobites in elongated "windrows," such as in the Silurian Rochester Shale, may represent accumulation in very minor scours or gutters (Whiteley and Smith 2001).

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