USING INVERTEBRATE TRACES TO ASSESS SEDIMENTATION AT A RICH TERRESTRIAL VERTEBRATE LOCALITY FROM THE CRETACEOUS OF MONTANA

The Egg Mountain locality occurs in the Upper Cretaceous (Campanian) Two Medicine Formation of western Montana. Discovered in 1979, the site most famously produced the first dinosaur eggs from North America and multiple clutches for the small dinosaur Troodon formosus. Recent work has exhumed well-preserved lizards and mammals, including associated skulls and postcrania. The massive calcareous mudstone comprising the quarry contains an abundance of invertebrate trace fossils. Here we examine the diversity and abundance of apocritan (bee and wasp) pupation chambers and cocoons, as well as unidentified small burrows, as a measure of deposition rates and paleoenvironments. Our sample area was a recent 7 by 11 m quarry on Egg Mountain. In the absence of recognizable beds, the quarry, beginning in 2010, was lowered in a succession of 15-cm deep jack-hammer passes. Sampling techniques were not established until the third jack-hammer pass, but surface-collected traces and qualitative observations of the first two passes indicated pupation chambers and cocoons in the uppermost portion of the quarry. Pupation chambers or cocoons were placed in five different sizes (small, medium, large, extra large, and extra-wide) based on length and diameter; burrows and a more spherical type of cocoon were assigned separate categories. Trace abundance varied markedly throughout the remaining passes, ranging from 26 to 744, with jack-hammer pass 7 yielding the most diversity. Large pupae (16.2-27.4 by 9.1-16.8 mm) and small pupae (5.1-14.7 mm by 3.3-7.5 mm) are common throughout all sampled passes. However, large pupae exhibit great variance in frequency. Extra-wide and extra-large pupae are generally uncommon, but had high numbers in passes 6-7 and 7-9, respectively. Cocoons and other traces through the sequence suggest persistent soil conditions suitable for apocritan nesting and pupation. Additionally, it implies an absence of major sediment pulses sufficiently large to prohibit thorough colonization. Peaks in pupation-chamber abundance may reflect sedimentation hiatuses as well as varying degrees of induration affecting specimen collection. Well-drained soil conditions favorable for apocritan nesting also may help explain the abundance of vertebrate nesting and presence of small terrestrial forms.