Selmasaurus

What is Selmasaurus? Basic Information and Physical Characteristics

Currently, two species of Selmasaurus are known: “Selmasaurus russelli” (S. russelli) and “Selmasaurus johnsoni” (S. johnsoni).

A Small Predator with Few Teeth

With a total length of 3 to 5 meters, it was a small predator among the Mosasauridae.
Additionally, it had a relatively small number of teeth for a mosasaur, and when first discovered, it was considered the “genus with the fewest teeth.”

Unique Anatomical Features of the Skull

Classified in the subfamily Plioplatecarpinae, Selmasaurus possesses unique characteristics in its skull.
Based on the research by Polcyn and Everhart (2008), the following features have been proposed to distinguish it from other genera.

Structure Around the Stapes (Quadrate bone)

The suprastapedial process projects to roughly half the height of the quadrate and contacts, but does not fuse with, a peg-like infrastapedial process.

Shape of the Parietal

The medially crushed semi-rectangular parietal diverges anteriorly and converges posteriorly to form “parasagittal crests” that bifurcate at the terminus.

Bone Articulation

The posteroventral median process of the parietal makes a long, narrow contact with the supraoccipital.

Supratemporal and Parietal Bones

The anteromedial process of the supratemporal is crushed against the parietal ramus, extending its entire length.

History of Fossil Discovery and Research: Excavation Drama in the United States

Research on Selmasaurus began in the United States in the 1970s.

Description of S. russelli and Identification of the Stratum

The name Selmasaurus was first described in geologist Samuel Wayne Shannon’s master’s thesis in 1975, but it remained an unofficial name (nomen nudum) for a long time.
Later, it was officially described by Wright in 1988.
The type specimen is housed at the Geological Survey of Alabama (later transferred to the Alabama Museum of Natural History) and includes an incomplete but well-preserved skull, cervical vertebrae, and other bones.
The specific name “russelli” was named in honor of paleontologist Dale Russell.

Although the exact stratum was unknown at the time of discovery, Caitlin R. Kiernan extracted microplankton from the matrix in 1998, revealing that it originated from the lower Campanian deposits of the Mooreville Chalk Formation in the Selma Group.
In this fauna, this specimen was an extremely rare element, comprising only 0.3% of the assemblage.

Discovery of the New Species S. johnsoni

In 1996, an extremely well-preserved skull and postcranial skeleton were discovered by Steve Johnson and others in the Niobrara Formation (Santonian or Campanian) in western Kansas.
Housed at the Sternberg Museum of Natural History, this fossil was identified as a new species, “Selmasaurus johnsoni,” in 2008 after more than 10 years of research.
This discovery deepened the understanding of Selmasaurus’s anatomical information and greatly expanded its geographical and temporal habitat range.

A Major Discovery in Japan (Kagoshima Prefecture)! Updating the Spatiotemporal Distribution

Although Selmasaurus was thought to be endemic to North America, there was a dramatic discovery across the Pacific in Japan.

In July 2022, a quadrate bone of Selmasaurus was discovered from the Upper Cretaceous system distributed on Shimokoshiki Island in Satsumasendai City, Kagoshima Prefecture.
This fossil was announced to the general media by Dr. Takuya Konishi in July 2025.

The reasons this discovery is extremely important are the following two points:

• It is the first Selmasaurus fossil discovered outside of the North American continent.
• It was yielded from a stratum 3 million years newer than previously known specimens.

As a result, the data on the spatiotemporal distribution of the genus Selmasaurus has been significantly updated.

Classification and Close Relatives of Selmasaurus

Wright and Shannon classified Selmasaurus into the “Plioplatecarpinae” subfamily of Mosasauridae based on the morphology of the circulatory system passing through the basicranium.

This group includes genera such as Platecarpus, Plioplatecarpus, and Ectenosaurus.
While Selmasaurus may be most closely related to Ectenosaurus, there is a clear difference in that its skull is “far shorter and more robust” than that of Ectenosaurus.

It is expected that the discovery of new specimens in the future will further deepen our understanding of Selmasaurus.