The Taphonomy of the Trilobite Elrathina cordillerae from the Greater Phyllopod Bed, Burgess Shale  

To deduce how closely fossil assemblages represent original faunal composition, one must quantify the amount of alteration caused by transport and decay.  Trilobites, with their multiple parts, provide a sensitive index of post-mortem disturbances in fossil lagerstätten, and are thought to be excellent biostratinomic indicators.  Here we examine the taphonomy of the most common polymerid trilobite from the Burgess Shale biota, Elrathina cordillerae. In particular, we consider what constraints its taphonomy provides on the transportation of a vagrant benthic element of the Burgess Shale fauna.  Our studies focus on specimens collected by the Royal Ontario Museum on oriented slabs within homogeneous mudstone layers, those at about 245 cm (−245), and 420 cm (−420) below the base of the Phyllopod Bed respectively.  These two fossil assemblages were selected for their abundant specimens, making it possible to examine patterns of clustering as well as patterns of disarticulation. 

The −420 layer contains a spatially variable, but relatively low density, concentration of E. cordillerae. Six slabs, yielding 124 specimens, were examined.  90% of specimens were articulated dorsal exoskeletons, commonly associated with dark staining thought to represent decay fluids.  Notable dorsal flexure occurred in more than half of these specimens.  Some rare specimens are preserved with limbs.  Other material includes an articulated axial shield, conjoined thoracic segments, and isolated cranidia.  The articulated dorsal exoskeletons show a wide array of postures, and are almost evenly split between specimens inverted or inclined toward bedding (52%) and those preserved dorsal side up (48%). These exoskeletons do not show a preferred orientation in azimuth.  Their size distribution is approximately normal, with a mean glabellar length of 5.45 mm.  Preservation of soft tissues or decay fluids (limbs and dark stain) suggests the rapid burial of live animals.  The patchy distribution, lack of preferred orientation and clear size sorting, favors preservation in situ after swamping and failed extraction. 

Specimens from the −245 layer occur at a higher density than in −420, and appear to be distributed in a patchier pattern, with some particularly dense concentrations.  Of the 116 total trilobite specimens found on three slabs from this layer, only 18% are articulated dorsal exoskeletons, with a mean glabellar length of 3.96 mm.  The postures of these specimens are varied, and include both enrolled and folded specimens.  The remaining specimens from −245 show varying degrees of disarticulation, ranging from axial shields with associated but detached free cheeks that are plausibly molt configurations, to isolated, conjoined thoracic segments.  The presence of abundant “ptychopariid” protaspids associated with much larger skeletal debris suggests little or no size sorting.  The −245 level apparently represents a longer period of accumulation than −420, but evidence of transportation is lacking.  Despite the range of preservational styles – suggesting different degrees of time averaging between the beds studied – neither indicates any significant transportation of E. cordillerae.