MIDDLE CAMBRIAN COPROLITES FROM NORTHWESTERN CANADA: THE POOP ON BENTHIC PALEOBIOLOGY AND PALEOECOLOGY

The Rockslide Formation (middle Cambrian, Series 3, Drumian, Bolaspidella Zone) of the Mackenzie Mountains, northwestern Canada, was deposited in a slope setting flanking the Selwyn Basin. An interval of greenish, finely laminated, slightly calcareous mudstone, 1 m thick, hosts a fairly low-diversity, low-abundance biota of several bivalved arthropod taxa, ptychoparoid and agnostoid trilobites, hyoliths, and various ‘worms.’ Compressed horizontal and U-shaped burrows are locally common. Also present are dark-colored, flattened circular objects averaging 15 mm in diameter, interpreted as coprolites preserved in either dorsal or ventral view. Many consist of aggregates of ovate carbonaceous flakes 0.5–2 mm long, which are probably compacted fecal pellets. About two-thirds contain a variably disarticulated pair of valves, likely belonging to a single arthropod taxon, and many also contain coiled to fragmented, corrugated ‘worm’ cuticle, either alone or together with valves. In rare cases an enrolled agnostoid, ptychoparioid cranidium, bradoriid valve, or hyolith conch or operculum is present; these are taken to be due to unintended capture or ingestion of bioclasts. Semi-circular spreiten of the same diameter as the coprolite indicate that the predator occupied a shallow vertical burrow and shifted laterally before depositing its feces at the end of a single feeding episode and swimming away. Its identity is unknown but it was clearly an apex predator exhibiting prey selectivity. Many coprolites contain one or more articulated hyoliths—without helens—oriented dorsal side up and positioned side-by-side or overlapping. A few also contain complete ptychoparioid or outstretched agnostoid exoskeletons also oriented dorsal side up. These are interpreted as opportunistic coprovores drawn to the organic-rich feces. This suggests that agnostoids were mainly benthic rather than pelagic, and hyoliths were mobile detritivores rather than sessile suspension feeders. Fecal matter probably played a more important role in the Cambrian food web than hitherto recognized.