Paleoenvironments and paleoecology of the vertebrate fauna from a late cretaceous marine bonebed, Canada

Bonebeds – concentrations of bioclastic debris of vertebrates in geological strata – can accumulate under a variety of conditions. They are common in marine deposits of the Late Cretaceous Western Interior Seaway of North America, and are often characterized as lag deposits. In general, these deposits represent unknown periods of accumulation and contain a mélange of taxa, possibly transported from a variety of habitats. As such, the contents of these marine bonebeds are often considered less useful for studies of paleoecology and paleoenvironments than are fossils recovered from rock units that represent continuous sedimentary deposition within one or more contiguous paleoenvironments. We examined the fossil contents of one particularly rich marine bonebed of “middle” Cenomanian age to determine if useful conclusions can be drawn with respect to the habitats and probable interactions of its pre-depositional fauna. This bonebed occurs as discontinuous lenses in shales of the upper part of the Belle Fourche Member of the Ashville Formation in the Pasquia Hills of Saskatchewan, Canada. Acid preparation of these lenses revealed an assemblage containing: 20 chondrichthyan taxa: a chimaeriform, hybodontiforms, diverse lamniforms and rare rajiforms; 15 actinopterygian taxa: a caturid, pycnodonts, an aspidorhynchid, a pachycormid, a plethodid, ichthyodectids, pachyrhizodontids, an albulid, a putative salmoniform, enchodontids, and an acanthomorph; and nine tetrapods including turtles, pliosaurs and elasmosaurs, four marine bird taxa, a terrestrial bird, and a lizard. Evaluation of probable habitats of the fish fossils reveals many pelagic forms, but a sparse nectobenthic fauna. Only one or two genera of those identified are considered to be euryhaline, and there are no obligate freshwater forms. The overwhelming majority of taxa identified are fully marine. Inferred feeding strategies for fish taxa include several durophagous forms including Ptychodus, a pycnodont and an albulid, with most other taxa, including the most abundant shark species (lamniforms) and osteichthyans (Enchodus) appearing to have been active predators with piercing dentitions. There are also predators/scavengers with cutting dentitions (anacoracid sharks) and a probable planktivore (Cretomanta). Interpretation of the taphonomy and faunal content indicates that the bonebed accumulated near shore over a period up to tens of thousands of years and was largely composed of bioclastic detritus from shallow, contiguous habitats, with some input from a deeper, anoxic shelf assemblage