This study provides the first focused investigation of rudist bivalves from the Upper Cretaceous of the Gulf Coastal Plain (GCP) in the southern US and previously undescribed specimens from the Flor de Alba Limestone Member of the Pozas Formation in Puerto Rico. Identified rudists from the GCP comprise the Monopleuridae Munier-Chalmas, 1873, including Gyropleura Douvillé, 1887, as well as Radiolitidae d’Orbigny, 1847, including Biradiolites cardenasensis Böse, 1906, Durania Douvillé, 1908, Durania maxima (Logan, 1898), Radiolites Lamarck, 1801, Guanacastea jamaicensis (Trechmann, 1924), Radiolites acutocostata (Adkins, 1930), and Sauvagesia Choffat, 1886. Integrating rudist occurrences within well-established GCP biostratigraphy allows for extension of upper ranges of D. maxima and R. acutocostata into the late Campanian, and lower ranges of B. cardenasensis and G. jamaicensis into the early Campanian. Identified rudists from Puerto Rico comprise the Hippuritidae Gray, 1848, and include Barrettia monilifera Woodward, 1862, which supports the age of the Flor de Alba Limestone Member of the Pozas Formation as middle Campanian. Combined taxonomic, biostratigraphic, and paleobiogeographic analyses indicate there is no rudist fauna endemic to the GCP, and the region marks the northeastern range of the Caribbean genera Biradiolites, Durania, Guanacastea, Gyropleura, Radiolites, and Sauvagesia during the Campanian and Maastrichtian. The new occurrences help inform future updates of Late Cretaceous sea surface current reconstructions for the Caribbean and Western Interior Seaway, USA.  

Over 200 Upper Cretaceous rudists from the GCP were available for study–146 specimens from the Alabama Museum of Natural History Paleontology Collections (ALMNH:Paleo:), University of Alabama, Tuscaloosa, Alabama, and 63 specimens from the Mississippi Museum of Natural Science (MMNS), Mississippi Department of Wildlife, Fisheries, and Parks, Jackson, Mississippi. Specimens were housed in Alabama and Mississippi, yet they were collected from a broad geographic area across the east and west provinces of the GCP, including Alabama, Mississippi, Arkansas, and Texas (see Supplemental Data 1 for list of specimen identification numbers; information on precise collecting localities are available on request from respective museums). Formations of the Selma Group, which cover the East GCP, yielded the majority of rudist specimens at 200, including 151 specimens from the Mooreville, 17 from the Demopolis, four from the Ripley, 27 from the Prairie Bluff, and one from the Owl Creek (Table 1, left column). Formations of the Taylor Group, which span the West GCP and is equivalent in age to the lower part of the Selma Group, yielded nine specimens, including four from the Ozan, four from the Annona, and one from the Marlbrook (Table 1, center column).

Many of the GCP specimens were donated by avocational collectors or not found exactly in situ, though 75% of specimens are well-resolved stratigraphically. Regarding ALMNH:Paleo: material, 64% of specimens were sourced from the Harrell Station Paleontological Site (HSPS). HSPS has a precise stratigraphic placement based on foraminifera, ostracodes, and nannoplankton biostratigraphy, as recently reviewed by Ehret and Harrell (2018). Located in central Alabama, HSPS possesses over 140 acres of exposed chalk gullies and yielded the most rudists of any single site at 94 specimens. Regarding MMNS material, if not from in situ, specimens were stratigraphically traceable to within a meter or so of source points. In addition, localities yielding specimens contributed by hobbyists were field checked.

Puerto Rico material consisted of 50 samples from the Flor de Alba Limestone Member of the Pozas Formation (Table 1). Specimens were collected from one locality near Flor de Alba Hacienda, which is southwest of Ciales, Puerto Rico. The rock exposure consisted of a limestone layer in a cave ceiling that was near a meter in thickness and followed along an adjacent valley. Flor de Alba figured specimens, reference fossil, and rock materials, and locality data are housed in the Indiana University Paleontology Collections (IUPC), Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana, USA. Specimens are assigned IUPC numbers 101110-101113 for figured specimens and 101114-101160 for reference specimens.

Specimens were photographed and described in terms of taxonomy and taphonomy. Each specimen was identified to the lowest taxonomic rank using presence of identifiable morphologic characters, including height and width of radial bands, presence or absence of a ligamental ridge, shape of shell costae, outer layer shell thickness, and shell geometry. When possible, specimens were measured to determine the posterior-anterior diameter, dorsal-ventral length, costae width, and taxa-specific traits such as beading and rays in Barrettia specimens or radial band width in Radiolitidae genera (Supplemental Data 2). The preservation type of each specimen was documented as original shell material, permineralized, or preserved as a mold/cast. The relative completeness of each shell was assessed qualitatively and shell sections that were not present were also noted. The degree of shell breakage was assessed qualitatively and noted in terms such as the most ventral section of the right valve. If epibionts were preserved with the shell, the number, shell location, and size were described. If a specimen possessed more than one preserved rudist then the total number was counted, and the shell orientation was documented. Finally, the determined taxonomic designations were placed within the lithostratigraphic and biostratigraphic framework provided in the literature of the GCP and Caribbean (Figure 2). GCP specimens were not cut or sectioned because high-quality rudist specimens from that region are rare. Furthermore, these specimens are primarily found as weathered lower valves that are not encased in matrix, and the myophores, upper valves, and teeth are usually not preserved. Thus, they do not require transverse sections to identify the shell shape, whereas many rudist specimens from the Caribbean or Central America are encased in dense limestone matrix and need to be cut to view shell morphology. Also, given that most GCP specimens are missing upper valves, teeth, and myophores, there is far less usefulness in cutting transverse sections and permanently damaging the specimens.

Repositories and institutional abbreviations.—Figured and reference specimens examined in this study are deposited in the following institutions: Alabama Museum of Natural History (ALMNH:Paleo:), University of Alabama, Tuscaloosa, Alabama, USA; Mississippi Museum of Natural Science (MMNS), Mississippi Department of Wildlife, Fisheries, and Parks, Jackson, Mississippi, USA; and Indiana University Paleontology Collections (IUPC), Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana, USA.

Text file and CSV file.