Previously Unreported Historical Remains of an Elasmosaurus Recovered from Wyoming in 1927

TURCIOS, Leo A; University of Wisconsin-River Falls, River Falls, WI. HASTINGS, Alexander K;  Science Museum of Minnesota, Saint Paul, MN.


At its peak, the Cretaceous Western Interior Seaway reached 3,200 km long, with shores almost a kilometer apart, dividing North America in half from the Arctic Ocean to what is now the Gulf of Mexico. About 75 million years ago, this shallow inland sea was home to massive apex predators, larger than any other on Earth at the time, including mosasaurs and plesiosaurs, each capable of reaching lengths of 15 meters or more. However, these aquatic reptiles went extinct at the K/Pg boundary along with non-avian dinosaurs, leaving behind partial remains as the seaway retreated.

In 1927, a Macalester expedition to Mosasaur Hills led by Alexander Coll unearthed what appeared to be the remnants of an elasmosaurid in eastern Wyoming. Elasmosaurus is a genus of plesiosaur that lived in the Late Cretaceous Period with remains that have been found throughout marine and coastal deposits of North America from this time. In 1970, this specimen was accessioned at the Science Museum of Minnesota (SMM) as SMM P78.15.1, where it resides today. Not much is known about the SMM elasmosaurid, as it was transferred with very limited data. Accession notes did mention the bones were recovered from the Pierre Shale. Most of the skull is missing, making identification difficult; however, two fragments were recovered. The first fragment is a mid-line section of the frontal, due to the presence of the crista cranii frontalis. The second bone is a combination of the parietal bone, fused with prefrontal and postorbital bones on each side. The prefrontal and postorbital bones frame the posterolateral corner of the orbit, suggesting widely set eyes.

Postcranially, SMM P78.15.1 includes most of the pectoral and pelvic girdles, three limbs, 83 vertebrae, and partial ribs. Joints of the two humeri and one femur show poorly developed articular surfaces, suggesting the individual was a juvenile when it died. The potential cause of death may have been related to infections, found in several bones of SMM P78.15.1, characterized by heavily regrown bone surface. These features are very apparent on two of the proximal forelimbs, and may also be present in the preserved hind limb (the other hind limb was not preserved). Fragments of unidentified bones also appear to have scar tissue present. This elasmosaurid may have been attacked by another apex predator, and though it survived the initial barrage, its injuries likely became infected. The weakened state of the animal may then have led to its ultimate demise, sometime after the encounter.