SCOTLYN FALLS EGGS

Ornithischian eggshell fragments and theropod egg clutches from near Moab, Utah represent the first eggs from the Yellow Cat Member of the Cedar Mountain Formation.

While dinosaur eggshells and eggs have been reported from the Cedar Mountain Fm, these occurrences have been limited to the Albian to Cenomanian-aged Mussentuchit Mbr. Here we report the first occurrence of dinosaur eggs in the Barremian-aged Yellow Cat Member of the same formation near Moab, Utah. The find consists of abundant eggshell fragments and two clutches of eggs. All are preserved in a dull green diamictite that consists of sparse, rounded, coarse sand to small pebble-sized chert clasts in a matrix of mm to cm-scale rip-up clasts of silty mudstone. Root casts, indicative of a soil horizon, are located below the shell horizon. The horizon represents floodplain deposits, possibly splays, lateral to sandstone fluvial channels.

Two egg clutches were found at the same stratigraphic position 325 m apart. Each clutch consists of at least four eggs. The eggs were intact or nearly so, at the time of burial as indicated by a lenticular outline in vertical section, consisting of a bowl-shaped base roofed by a dome of shell fragments, indicating the eggs were crushed by sediment loading. The eggs remain to be prepared but are estimated to be 5-7 cm in diameter. The shell is thin, ~800 µms, with a rugose external texture. In thin section, the shell resembles the “ratite” morphology, bearing a distinct, abbreviated spherolithic layer overlain by a thick tabular zone, typical of Elongatoolithae, common to oviraptor theropods.

Another egg type is represented by an isolated shell fragment 1035-1050 µm thick. It is tentatively assigned to the Spheroolithidae based on its prolatospherolithic morphotype with a prolatocanaliculate pore system in a tabular crystallite structure. The outer ornamentation is sagenotuberculate. This morphotype pertains to the Ornithischia.

The egg clutches indicate the favored (or most likely to be preserved) environment for two clades of Early Cretaceous dinosaurs, namely a floodplain adjacent to stream channels. The presence of egg clutches at the same horizon but hundreds of meters apart does not necessarily indicate the nests are part of the same nesting site, but that the egg-laying environment persisted through time as the faces migrated across a fluvially dominated landscape.