New Mexico Geological Society Annual Spring Meeting — Abstracts

A diverse vertebrate-fossil assemblage of late Campanian age was collected from the Mesa Portales area southwest of Cuba, in Sandoval County, New Mexico. The fossils come from shoreline sandstones close to the contact of the Pictured Cliffs Sandstone and Fruitland Formation, and, depending on how that contact is defined, could be assigned to either lithostratigraphic unit. The vertebrate fossils are clearly an allochtonous, hydraulically concentrated assemblage, primarily of selachian teeth. The fossils were collected by surface collecting and screenwashing sediment from a site less than 150 yards square and sifting through roughly 180 pounds of ant hill armor from a mound of Western Harvester Ant, Pogonomymex occidentalis (Cressen, 1865), within that area. The predominantly selachian fauna includes Squalicorax kaupi, S. pristodontis, Meristodonoides nov. sp., Ischyrhiza mira, Onchosaurus sp., Protolamna borodini, Rhinobatis incertus, Squatina sp., and Scapanorhynchus sp., as well as other lamnids, ptychotrygonids, orectolobids and other taxa that have yet to be described, which include Pseudohypolophus sp., Cretolamna sp., Parasquatina sp., and Cantioscyllium sp. In addition to these taxa, species of orectolobids and ptychotrygonids are present, but have yet to be researched. In addition to selachian teeth, 158 selachian dermal denticles of various types were also recovered. Scapanorhynchids are by far the most numerous taxon found at the site. Teleosts include Enchodus petrosus and Protosphyraena sp. Non-piscine fossils include inoceramid bivalves, plesiosaurs, ?mosasaurs, ?crocodilians, turtles and a single mammal tooth. This is the youngest extensive selachian-dominated site in the New Mexico Upper Cretaceous.

Although much has yet to be done concerning all of the taxa from this site, numbers of various species have been recorded. It must be noted that given the taphonomy of the site, the majority of fossils have been abraded and broken. Counts of numbers of teeth of some of the taxa present are:

Scapanorhynchus sp. 6099 (This number could change as further study might divide this genus into two species)

Ischirhyza mira 294 (oral teeth, 36 rostral spines)

Meristodonoides nov. sp. 78

Onchosaurus sp. 28

Protolamna borodini 31

Rhynobatus incertus. 3

Squalicorax kaupi 33

Squalicorax pristodontus. 8

Squatina sp. 88

Encodespetrosus 201

Protosphyraena sp. 72

There are two species of sharks in the genus Squalicorax found at the Mesa Portales site. It was previously thought that there was a linear development of squalicorid species from S. falcatus to S. kaupi, ending with S. pristidontus. However, both Cappetta (2006) and Jambura et al. (2023) noted that the latter two species had been found in the same deposits. The Mesa Portales site confirms these observations.Squalicorids were large sharks, possibly reaching a total length of 15 meters. Their serrated teeth were efficient in tearing the flesh of their prey. The feeding strategies of this genus are debated, with some claiming that they scavenged on already dead animals such as dinosaurs that had died and floated out to sea, as well as marine reptiles that had perished in the open ocean. Others maintain that they were active hunters, going after other kinds of sharks, as well as larger teleosts and mosasaurids. A third view is that they relied on both strategies,

The most common selachian at the Mesa Portales site is Scapanorhynchus sp. This taxon is problematic as to its identification as a species, because the vast majority of the teeth are broken. However, a very diagnostic morphology of scapanorhynchids is the lingual face of the main cusp having pronounced striations. This morphology was used to identify even small segments of these teeth. But, upon further investigation of the more complete teeth, it was discovered that the accessory cusps were also lingually striated, a feature found on only S. minimus, a species found exclusively in Europe. Some of these teeth also had short labial striations. Teeth of Cretodus and some species of Carcharias also have short labial striations, but never in conjunction with the lingual striations that are found on all of the cusps of the teeth found at Mesa Portales. Further investigation is needed to determine this scapanorhynchid species.

Protolamna borodini is a shark that is not often found in New Mexico. This genus is seen as a basal form of the order Lamniformes. It is recognized by teeth with a heavily striated labial face, with both the main cusp and the accessory cusps displaying these striations. The teeth are small, reaching 3-4 mm in height. This genus arose in the Early Cretaceous (Valanginian) and disappeared towards the end of the Late Cretaceous (Maastrichtian).

References:

1. Cappetta, H. 2006. Elasmobranchii post-Triadici (index generum et specierum). Pp. 1–472 in W. Riegraf (ed.) Fossilium Catalogus I: Animalia 14. Backhuys, Leiden.Cresson, E. T. 1865.Catalogue of Hymenoptera in the collection of the Entomological Society of Philadelphia, from Colorado Territory.Proceedingsof the Entomological Society of Philadelphia, 4: 426-488.Jambura, Patrick L., Sergey V. Solonin, Samuel LA Cooper, Eduard V. Mychko, Maxim S. Arkhangelsky, Julia Türtscher, Manuel Amadori, Sebastian Stumpf, Alexey V. Vodorezov, and Jürgen Kriwet. 2023. Fossil marine vertebrates (Chondrichthyes, Actinopterygii, Reptilia) from the Upper Cretaceous of Akkermanovka (Orenburg Oblast, Southern Urals, Russia). Cretaceous Research 155, 105779.