The Mother’s Day Quarry Keeps Giving

Story by: Greg Liggett, Paleontologist, Montana/Dakotas State Office

Discovered on Mother’s Day in 1994, the Mother’s Day Quarry (MDQ) has been excavated almost continuously since, producing

The hypothetical body size of the juvenile

Mother’s Day Quarry (MDQ) Diplodocus

individual compared to an adult human

and an adult Diplodocus (partially shown).

The smallest known skull from the MDQ

on left and illustrated version on right in

right (B) and left (C) profile.

over 2,500 dinosaur bones and skin fragments. The site is in south-central Montana and is within the Morrison Formation, making it Late Jurassic (~150 million years ago) in age.

Almost all the bones from the site belong to 15 or more different individuals of a single sauropod species, identified as Diplodocus sp. The sauropods are the iconic group of long-necked dinosaurs, with small heads, long necks and tails, and massive bodies held up on pillar-like legs. The bones from MDQ represent juvenile and subadult individuals which are thought to have died together prior to their burial.

Diplodocus is well represented in the general fossil record with over 100 individuals known since its first discovery in 1878. However, skull material is exceedingly rare, and juvenile material even more so, with only three known examples. One of the best preserved, and smallest, skulls found to date came from the MDQ, and was recently described.

Life reconstruction of juvenile Diplodocus

from the Mother’s Day Quarry with adults.

Note how the shape of the snout in the

juvenile is pointed compared to the flat

shape of the adult. Evidence that they

were feeding differently.

This new juvenile skull is only about 9.5 inches in total length, compared to an adult skull of about 40 inches. One of the most important implications of this juvenile skull is showing how much the skull and tooth shapes changed over an individual’s lifetime. The juvenile skull has more teeth than the adult, and the juvenile teeth are shaped differently, for example. This strongly suggests that over its lifetime Diplodocus modified its diet, focusing on certain vegetation when young, and switching to different vegetation as an adult. Clues like these help paleontologists reconstruct the life history of long-extinct species.

Illustrations from: Woodruff, D. C., Carr, T. D., Storrs, G. W., Waskow, K., Scannella, J. B., Nordén, K. K., and Wilson, J. P., 2018, The Smallest Diplodocid Skull Reveals Cranial Ontogeny and Growth-Related Dietary Changes in the Largest Dinosaurs: Scientific Reports, v. 8, no. 1, p. 14341. MSO PL 2576.

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