REOPENING THE MOTHER'S DAY QUARRY (JURASSIC MORRISON FORMATION, MONTANA) IS YIELDING NEW INFORMATION

The Mother’s Day Quarry (MDQ) is a dinosaur-bearing bone bed in the Late Jurassic (Kimmeridgian) Morrison Formation located in Carbon County, Montana. First discovered in 1994, the quarry has yielded well over 2,500 elements after having been excavated for two seasons by crews from the Museum of the Rockies, and subsequently from 2000 to 2012 by teams from the Cincinnati Museum Center. Aside from approximately 12 theropod teeth and numerous skin impressions, all of the vertebrate remains are exclusively from the sauropod dinosaur Diplodocus sp. The fossil-bearing unit has been interpreted to be the result of a debris flow following a drought-induced mass mortality event.

The size of these Diplodocus elements is notable, as they range from 38% to 75% of the length of the same elements in the smallest adult Diplodocus specimen at the Carnegie Museum (CM-94). Initially, the consistently small sizes were interpreted as representing an age-segregated herd of juvenile to subadult individuals. More recently, however, histological evidence suggests that there were both subadult and adult animals comprising a herd of exclusively dwarfed individuals, caused by a decrease in growth rates.

The Bighorn Basin Paleontological Institute (BBPI) has assumed responsibility for excavating the MDQ each summer since 2017. Four theropod teeth and two skin impressions have been collected, as well as approximately 50 isolated or partially articulated Diplodocus elements, the modest sizes of which are consistent with the previously reported remains. Additionally, numerous pathologies have been observed on these elements, including a well excavated furrow on the mediodistal portion of an ulna, as well as several rib fractures showing differing levels of reactionary bone growth.

The prevalence of pathologies observed on the these BBPI-collected elements suggests that hundreds may be present on the previously collected remains. A systematic study of those pathologies may yield important information about the life histories of these iconic sauropods, and contribute to the subadult vs. dwarf population discussion.