Happy Mother’s Day! To honour the greatest women in our lives, today’s article will discuss the Mother’s Day Quarry in southern Montana. Discovered on Mother’s Day in 1994, this quarry has given paleontologists insights into the biology and behaviours of sauropod dinosaurs in their youth.
Located an hour south of Billings, Montana, the Mother’s Day Quarry (or MDQ) is part of the Morrison Formation, which spans most of the continental United States. The Morrison is the most famous formation for Jurassic Fauna, as its rocks span a 10-million-year period throughout the late Jurassic. The MDQ is part of the Salt Wash member of the Morrison, which dates to the Kimmeridgian age of the late Jurassic (~154-150 million years old). At this time, dozens of giant sauropods roamed across the dry landscape of the Morrison, while famous taxa such as Allosaurus and Stegosaurus thrived alongside them.
The rock composition at the MDQ indicates that the area would have been a river channel during the Jurassic. The presence of rip-up clasts, or muddy sediments found within rock conglomerates, suggest that the animals of MDQ were transported after death by high-velocity water currents[i]. It’s likely that the animals at MDQ died during a drought and were later washed downstream during a flooding event, which occurred in the Morrison with some frequency[ii].
The majority of fossils at the MDQ belong to juvenile sauropods. Of the thousands of fossils recovered from the sight, over 99% have been attributed to the genus Diplodocus. The only other elements known from the quarry are a handful of theropod teeth (likely belonging to Allosaurus) and an isolated Stegosaur limb, making the abundance of young Diplodocus (over 15 individuals) overwhelming.
Why were there so many juveniles? Well, young sauropods likely lived away from their parents, a concept known as age segregation. Numerous fossil sites around the world contain juvenile sauropods with no adults. This behaviour occurred throughout sauropod evolution, as both older (the Triassic Mussaurus) and younger genera (the Late Cretaceous Alamosaurus) exhibited this behaviour[iii][iv]. For reference, over 130 million years separates the evolution of these species, indicating that age segregation was beneficial for sauropods. Why they practised such behaviours remains unclear, though it may be due to adults attracting larger predators, differing diets, and the potential for multi-tonne adults to step on their duck-sized babies!
I should note that not all Diplodocus found at MDQ were juveniles. Bone histology performed on MDQ fossils has found that a few individuals had reached maturity[v]. This has led some to posture that the mature individuals at MDQ are potentially a dwarf species or an exceptionally small population of Diplodocus. The second hypothesis has more support, as it wouldn’t be hard to imagine that a population encountering a severe drought would displays signs of malnutrition. Having said this, imagining a tiny species of Diplodocus in the heart of Jurassic North America is cute!
The preservation of Diplodocus at MDQ varies tremendously. While most bones are in poor condition, some notable exceptions have emerged. In 2018, Woodruff et al. described a baby Diplodocus skull from the quarry, rare for both its tiny size (~24 centimetres) and the absence of sauropod skulls in the fossil record. The Diplodocus, known as Andrew, has shown that juveniles had narrower heads and different teeth configurations than their adult counterparts. This supports the notion that adults and juveniles lived separately due to differing diets, thus enabling both to maximize the food resources of the Morrison.
Andrew isn’t the only well-preserved fossil found at MDQ. Numerous Diplodocus fossils exhibit skin impressions, which has given paleontologists a rare and detailed glimpse at their outward appearance. Six different scale types were observed on the skin impressions, all having different sizes, patterns, and elevations on their body[vi]. Scale segmentation was present in each main dinosaur lineage, with the theropod Carnotaurus and some hadrosaurs exhibiting similar scale patterns[vii].
Detailed analysis of the scales by paleontologist Tess Gallagher has revealed something fascinating: they were porous. Instead of being filled with bone, the scales had an interconnected series of pores running throughout. Such a structure had a unique function, but what was it?
The answer lies in the complex world of thermodynamics.
Large animals, like sauropods, have small surface-volume ratios. Explaining what this means would take a whole article, so for the sake of time, the crucial thing to understand is that large animals have more difficulties losing heat. To survive, animals must evolve to combat this constraint. More large animals live at the poles, such as Polar Bears and Emperor Penguins. Others, like African Elephants, have developed large ears that act as thermoregulators to diffuse heat into their environment. While being big does have its benefits, it makes animals feel the heat. As a taller person, I can attest to this!
Paleontologists have long wondered how sauropods – animals that were (in some cases) 10x larger than elephants – dispersed heat. Gallagher’s research on sauropod scales may have finally found the answer. Porous scales would have increased the surface-volume ratio of sauropod skin exponentially, allowing for much higher rates of heat diffusion. Greater diffusion may have been a crucial adaptation for the sauropod lineage, allowing them to reach the titanic sizes seen by species like Diplodocus and the 50-tonne Argentinosaurus. Future research will be required to determine this possibility, though it may be difficult due to the rarity of sauropod skin in the fossil record.
The Mother’s Day Quarry has given paleontologists a fascinating glimpse at juvenile sauropods. While the circumstances of their preservation may have been tragic, they have provided paleontologists with important information about the biology and ecology of Diplodocus and other titanic dinosaurs. Work on the quarry is ongoing, so who knows what future secrets are still being held by the MDQ…
Thank you for reading today’s article! If you want to know more about Morrison sauropods, I suggest you read complimentary articles here at Max’s Blogosaurus!
I would also like to give a special thank you to my mother and all the other mothers out there. While we may not express it enough, we love and appreciate you and everything you’ve done for us. So, enjoy! Today is for you!
I do not take credit for any images found in this article. Header image courtesy of Andrey Atuchin, found here.
[i] MYERS, T. S., and G. W. STORRS. “Taphonomy of the Mother’s Day Quarry, Upper Jurassic Morrison Formation, South-Central Montana, USA.” PALAIOS, vol. 22, no. 6, 2007, pp. 651–666, https://doi.org/10.2110/palo.2005.p05-123r.
[ii] Foster, John Russell. Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press, 2020.
[iii] Pol, Diego, et al. “Earliest Evidence of Herd-Living and Age Segregation amongst Dinosaurs.” Scientific Reports, vol. 11, no. 1, 2021, https://doi.org/10.1038/s41598-021-99176-1.
[iv] Myers, Timothy S., and Anthony R. Fiorillo. “Evidence for Gregarious Behavior and Age Segregation in Sauropod Dinosaurs.” Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 274, no. 1–2, 2009, pp. 96–104, https://doi.org/10.1016/j.palaeo.2009.01.002.
[v] Woodruff, C, et al. “The Smallest Diplodocid Skull Reveals Cranial Ontogeny and Growth-Related Dietary Changes in the Largest Dinosaurs (Project).” MorphoBank Datasets, 2018, https://doi.org/10.7934/p3281.
[vi] Jeanne, Timmons. “Diplodocus May Have Been One Cool Dinosaur-Thanks to Its Skin.” Ars Technica, 14 Dec. 2022, arstechnica.com/science/2022/12/diplodocus-may-have-been-one-cool-dinosaur-thanks-to-its-skin/.
[vii] Hendrickx, Christophe, and Phil R. Bell. “The Scaly Skin of the Abelisaurid Carnotaurus Sastrei (Theropoda: Ceratosauria) from the Upper Cretaceous of Patagonia.” Cretaceous Research, vol. 128, 2021, p. 104994, https://doi.org/10.1016/j.cretres.2021.104994.