THE TAPHONOMY AND FAUNA OF THE EARLIEST LATE CRETACEOUS (CENOMANIAN) ROBISON BONEBED, WAYAN FORMATION, EASTERN IDAHO

The latest Albian-Cenomanian Wayan Formation of Idaho, coeval with the Vaughn Member of the Blackleaf Formation of Montana and representing the same depositional basin, contains a vertebrate assemblage dominated by the burrowing dinosaur Oryctodromeus cubicularis and the theropod ootaxon Macroelongatoolithus carleylei. Remains from other taxa are rare and fragmentary. The majority of these other remains consist of over two hundred specimens sourced from a deposit known as the Robison Bonebed.

The Wayan Fm. is dominated by pedogenically overprinted fine sediments representing xeric environments with sediment accumulation in a rapidly subsiding basin. Coarser grained sediments are uncommon, but conglomeratic lenses occur. The removal of the original deposit by construction activities has precluded in-situ studies, but some observations can be made via ex-situ boulders. The deposit appears to have been laterally restricted, with an estimated lateral extent of a few tens of meters, and an estimated thickness <3 m.

The bonebed is a conglomerate with a cherty sand matrix. Clasts consist of cm-scale caliche nodules, green mudstone clasts, and bones (complete to fragmentary), teeth, and eggshell. Sands interfinger within this conglomerate, and the deposit appears to have coarsened upward.

Taxa recovered include fish, turtles, crocodyliforms, mammals, theropod/ornithischian dinosaurs, and a possible pterosaur. Oryctodromeus dominates the assemblage (21% of identified specimens), while theropod bones/theropod teeth (15%), and taxonomically unidentified specimens (28%) form dominant components also. Bones are usually isolated, broken, and some are rounded to pebbles. The few segments of articulated vertebrae, 2 articulated crocodyliform osteoderms, and pristine jaws with teeth, indicate they were transported with adherent soft tissues.

The deposit is posited to have formed as a stacked set of basal lag deposits in a fluvial channel fill, possibly indicating the stream was ephemeral or anastomosed. Large clay clasts represent minimally transported collapsed cut banks while the caliche nodules and fossils were sourced from reworked overbank areas and deposits, with the bone pebbles transported a greater distance. The broad range of fossil preservation indicates the assemblage is time averaged.