Bioirrigation in Alph n. igen., arthropod cubichnia from the Mississippian Hartselle Sandstone of Alabama (USA)

Abstract

Cubichnia from the Hartselle Sandstone, described as Rusophycus hartselleanus and herein renamed Alph hartselleanus n. igen., includes specimens with well-defined, terminal cylindrical structures. We interpret these oblique shaft-like structures as having a respiratory function based on: (1) exact placement of cylindrical structures on the cubichnial axis; (2) unique sculpture with both transverse and longitudinal striation, absent elsewhere in the assemblage despite excellent preservation of trace fossils; (3) associated features in the trace such as bubbly structure that also could be caused by bioirrigation. Alph hartselleanus is much larger than known Carboniferous trilobites. While similar to Rusophycus, the Hartselle cubichnia has a complex morphology suggesting a crustacean tracemaker. The trace is quadrilobate, with distinct anterior and posterior regions. The posterior region has 5 or 6 pairs of coarse, obliquely transverse striae corresponding to digging appendages, crosscut by fine, obliquely longitudinal striae that may record bioirrigation. The anterior region has a bubbly structure suggestive of thixotropic movement of sand, again, possible evidence for bioirrigation. The axial shafts do not match known trilobite appendages, but could have been made by crustaceans. Were the cubichnia merely resting traces, or did they have additional functions such as nesting or protection during moulting? In a broader context, the physical evidence for bioirrigation represents a nearly untapped opportunity for understanding the Cambrian Substrate Revolution as well as Phanerozoic ecosystems. Where preserved, the apertures of burrows should be investigated for evidence of bioirrigation.