189-11 When San Antonio was an ocean: the first record of mosasaurs from Late Cretaceous (late Early-early Middle Campanian) chalk deposits in San Antonio, Bexar County, Texas

Session: Paleontology, Paleoecology/Taphonomy (Posters)

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Abstract:

We report on the discovery of three mosasaurs from Upper Cretaceous Campanian deposits in San Antonio, Bexar County, Texas. The first specimen (WM 2025.04.001) is conservatively referred to as a large-bodied mosasaurine, aff. Prognathodon sp. (Mosasaurinae, Globidensini) and consists of a distal left quadrate, nearly complete right dentary, a partial left dentary with associated teeth, and 8 cervical, thoracic, and post-thoracic vertebrae. The preserved teeth are strongly bicarinate with no serration and with fluting on both the lingual and labial surfaces. The teeth are longer than wide, giving it an estimated 14 or 15 dentary tooth positions. All elements were found in close association as part of a surface collection. The second mosasaur (WM 2025.17) is a smaller individual that can be identified as a plioplatecarpine, aff. Latoplatecarpus sp. (Plioplatecarpinae, Plioplatecarpini). It consists of a nearly complete skull and right mandible, as well as cervical vertebra 3. The skull roof and partial suspensorium are in articulation, while additional cranial elements remain in direct association. Both quadrates are nearly complete, including most of the delicate tympanic ala. Tooth crowns are slender, with well-developed carinae without serrations. They are posteromedially recurved from mid-height and medially finely striated. These first two mosasaur specimens were collected 4.5 kilometers apart along the same northeast-southwest trending exposure of chalky, limestone. A third occurrence of mosasaur, represented by single post-thoracic vertebra (WM 2024-12G), was found approximately 28 kilometers away on the northeast edge of the city. The vertebra is similar in size and shape as those preserved with WM 2025.04.001. Fossils from all three sites were recovered from a fine-grained and slightly carbonaceous, off white to gray chalk. The molluscan assemblage includes Cataceramus balticus, Exogyra (E.) ponderosa erraticostata, Pycnodonte (Phygraea) mutabilis, Radiolites austinensis, and the nautiloid Eutrephoceras cf. E. campbelli. All three localities are mapped as Pecan Gap Chalk (Middle Campanian) on the Geologic Map of Texas. However, the presence of the inoceramid C. balticus from all three localities suggests an age no younger than late Early-early Middle Campanian. These mosasaur discoveries are significant, as they represent the first Cretaceous vertebrate fossils collected from San Antonio and Bexar County, and potentially one of the most southerly occurrences of the genus Latoplatecarpus globally.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5168

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