THE ROBISON BONEBED: A PRELIMINARY REPORT ON THE MOST DIVERSE VERTEBRATE FOSSIL SITE KNOWN FROM THE MID-CRETACEOUS WAYAN FORMATION OF IDAHO

In 2013 construction in the Caribou-Targhee National Forest of eastern Idaho revealed the most diverse and prolific fossil site known in the mid-Cretaceous (latest Albian to Cenomanian) Wayan Formation. These activities exposed and removed an isolated lens of fine-grained sandy conglomerate consisting of chert and quartz grains, rounded chert pebbles, and sub-rounded caliche and green mudstone clasts. Scattered within this matrix are fossil bones and teeth of numerous taxa which hint at a greater diversity than has been recognized so far.

Theropods are represented by enigmatic teeth of a large, possibly piscivorous theropod, and teeth of 2 smaller uncertain forms; as well as isolated vertebrae, one of which suggest a possible neovenatorid allosauroid. Armor scutes and teeth indicate the presence of an ankylosaur. The ornithopod Oryctodromeus, which dominates the fossil assemblage of the Wayan, is well-represented by a tooth and numerous bones. A hadrosaurid is known from one tooth. Eggshell of the oogenus Macroelongatoolithus (probably representing a large oviraptorid) is common. Mammals include a non-cimolodontan multituberculate as well as a possible triconodontid. Turtles, fish, crocodilians, and possible lizards are represented by isolated remains.

Though the nature of the construction and heavy soil cover did not allow detailed observations, the lithology suggests the deposit is a fluvial lag. Varying degrees of taphonomic modification are indicated by a mixture of delicate elements such as an Oryctodromeus dentary retaining fully erupted teeth, broken limb shafts missing ends, and unidentifiable rounded and abraded bone fragments. Except for a few articulated Oryctodromeus caudal vertebrae with ossified tendons, all fossil bones are isolated and unassociated. These observations suggest that the deposit is a significantly time-averaged accumulation partially sourced from upstream floodplains proximal to the fluvial system. Having produced remains from at least 4 kinds of theropods, 3 types of ornithischians, 1 or more types of mammals, and aquatic vertebrates, this locality easily represents the most significant vertebrate fossil locality in the Wayan Formation. Many fossils await preparation and collection of fossils from stockpiled matrix in the field will continue in 2014.