Enigmatic origin of massive Late Cretaceous-to-Neogene coprolite-like deposits in North America: a novel palaeobiological alternative to inorganic morphogenesis

Innumerable coprolite specimens, alternatively pseudo-coprolites, are distributed in several Late Cretaceous-to-Neogene continental deposits hosted by the Whitemud Formation (Maastrichtian) in southern Saskatchewan, Canada, Golden Valley Formation (Paleocene) in western North Dakota and the Wilkes Formation (Miocene) in Washington State, USA. These massive accumulations of excrement-shaped specimens also include multi-decimetre-long specimens previously interpreted as casts of intestinal organs by an unknown zoology, possibly reptilian. The possible vertebrate zoology origin of these has been widely disputed because of the lack of any skeletal remains or any other definitive evidence of palaeobiology. Alternative originations suggest non-zoological processes, such as sideritic muds extruded by biogenic gas-driven flows during diagenesis. Multi-decimetre-long intestine-like casts recovered from the three deposits differ from the more widely distributed faeces-like specimens because their elongated forms have bilateral symmetry in cross-section and surface textures consisting of sub-mm-wide longitudinal parallel striations extending without interruption along the looped and coiled surfaces. These possible impressions of musculature are not present on smaller specimens having coprolite-like shapes. A novel interpretation proposes that these multi-decimetre-long specimens may be gut casts of a previously unrecognized giant terrestrial earthworm (Oligochaeta) that existed from the Late Cretaceous to the Neogene. The possibility of this Annelida zoology provides palaeobiological resolution for the lack of any evidence of vertebrate skeletal remains or other evidence of origination and further suggests that the innumerable excrement-like specimens may also have palaeobiological affinity.