Bruce A. Schumacher

FOSSIL MARINE VERTEBRATES FROM THE LOWERMOST GREENHORN LIMESTONE (UPPER CRETACEOUS: MIDDLE CENOMANIAN) IN SOUTHEASTERN COLORADO

Abstract Fossiliferous deposits of the Greenhorn Limestone (Upper Cretaceous) are found on the Comanche National Grassland in southeastern Colorado. The lowest portion of the Greenhorn Limestone, the base of the Lincoln Limestone Member, contains calcarenite beds rich in disarticulated remains of marine vertebrates. We examined the composition of the vertebrate paleofauna from one particular location (Tobe locality) through surface collecting and acid solution of rocks. The paleofauna is taxonomically diverse, consisting of 22 chondrichthyans, at least 15 osteichthyan fishes, and six aquatic reptiles. Molluscan taxa indicate that the basal Lincoln Limestone in southeastern Colorado was deposited sometime between middle Middle Cenomanian (ca. 95 Ma) and late Middle Cenomanian (ca. 94.7 Ma), and the composition of the vertebrate fauna (primarily chondrichthyan taxa) agrees with this interpretation. The Tobe locality was situated far from the shorelines of the Western Interior Seaway, with relatively high-energy waves impinging upon the seafloor, concentrating biogenic remains. Fossil remains within the calcarenite appear to have undergone minor time-averaging, based upon erosional rounding and breakage present in numerous specimens. Nevertheless, the deposit contains one of the best representative Middle Cenomanian vertebrate communities so far described from the Western Interior Seaway of North America. Considering that the paleofauna consists of taxa diverse in size, morphology, and inferred ecology, the trophic structure of the paleocommunity was probably complex.

A ‘Woollgari-Zone Mosasaur’ (Squamata; Mosasauridae) from the Carlile Shale (Lower Middle Turonian) of Central Kansas and the Stratigraphic Overlap of Early Mosasaurs and Pliosaurid Plesiosaurs

Anew specimen of russellosaurine mosasaur (FHSM VP-17564) from the middle Fairport Chalk Member (Collignoniceras woollgari zone, early Middle Turonian), Carlile Shale of central Kansas is among the earliest records of a derived mosasaur known from the North American Western Interior Seaway. Unequivocally the oldest recorded occurrence of a russellosaurine mosasaur from Kansas, and one of only four Kansas occurrences consisting of more than a single element, the specimen includes seven successive proximal caudal vertebrae bearing articulating hemal facets. No demonstrable characters ally the specimen to a more refined taxonomic subgroup, although the relative dimensions of the vertebral centra are significantly longer than equivalent caudal series in Platecarpus and Tylosaurus. This suggests that the tails of early russellosaurines were composed of a lesser number of longer caudal vertebrae than their successors, with less hydrodynamic adaptation from the terrestrial condition. Size of the vertebrae in relation to Platecarpus and Tylosaurus indicates a relatively large animal roughly 3.2 m in total body length, although this may be an over-estimate given the difference in vertebral dimensions as compared to more derived russellosaurines of the Niobrara Formation. The middle of the Fairport Chalk interval continues to produce a large number of vertebrate specimens despite the rarity of exposures. A pliosaurid plesiosaur skull (FHSM VP-321) is known from similar geographic and stratigraphic position as FHSM VP-17564, and the record of pliosaurs (FHSM VP-17469) continues into younger Blue Hill Shale (Prionocyclus hyatti zone, middle Middle Turonian) demonstrating with certainty that pliosaurs and early mosasaurs inhabited the same stretch of seaway for roughly one million years.

On the skull of a pliosaur (Plesiosauria; Pliosauridae) from the Upper Cretaceous (Early Turonian) of the North American Western Interior

Abstract A partial pliosaur skull reveals anatomical features poorly known for Plesiosauria. The specimen lacks recorded contextual information, however preservational character and nannofossil analysis (earliest Turonian) suggests origination from the upper Greenhorn Limestone. The vomers bifurcate near their posterior termination, and are overlapped by long, slender extensions of the anterior pterygoids. The latter character has only recently been noted and may be variously developed in many if not all plesiosaurs. Large foramina commonly referred to as the ‘internal nares’ are formed at the juncture of the palatines, vomers, and maxillae. Configuration of the palatal elements differs in several ways from the only well established pliosaurid taxon of the North American Cretaceous, Brachauchenius. Particular differences include a more visibly elongate and lanceolate shape of the vomers, palatines which do not project as far anteriorly (similarly for the ‘internal nares’), and a squared posterior termination of the palatines. A relatively elongate muzzle and homodont dentition differ from the shorter muzzles and caniniform teeth of Kronosaurus and Plesiopleurodon. The specimen is also not referable to the poorly founded taxon Polyptychodon. With no clearly similar material for reference, the specimen may expand the definition of Brachauchenius as currently known or could represent an undescribed taxon. Although perhaps not of taxonomic significance, the specimen is impressively large with an estimated total skull length of 1.75 m, the largest known for a North American plesiosaur.

The earliest record of the Late Cretaceous plethodid fish, Thryptodus (Teleostei: Tselfatiiformes), from central Kansas

Abstract We note the occurrence of the Late Cretaceous teleostean genus Thryptodus from the Lincoln Limestone Member (upper Middle Cenomanian) of the Greenhorn Limestone in Russell County, Kansas. The specimen, here referred to Thryptodus cf. T. zitteli Loomis, is represented by fused premaxillae. The specimen is noteworthy because it marks the oldest record of the genus.

A New Late Cretaceous Marine Vertebrate Assemblage from the Lincoln Limestone Member of the Greenhorn Limestone in Southeastern Colorado

The Lincoln Limestone Member of the Greenhorn Limestone is a mid-Cenomanian (94.7–95 Ma) deposit of the Late Cretaceous Western Interior Seaway in North America. Fossiliferous rock samples from the Lincoln Limestone were collected at a new locality (‘Table Mesa locality’) in southeastern Colorado. Acid treatment of the rock samples yielded at least 29 vertebrate taxa, including 13 chondiichthyan and 14 osteichthyan fishes as well as two reptilian (squamate) taxa. The taxonomic composition of the fauna broadly resembles previously described mid-Cenomanian localities in North America that further demonstrates the high taxonomic homogeneity of vertebrates in the Western Interior Seaway. Although the occurence of a terrestrial lizaid is noteworthy, proportions of common taxa at the Table Mesa locality are particularity similar to another Lincoln Limestone locality situated about 100 km to the west where remains of bony fishes also dominate.

Highly specialized suspension-feeding bony fish Rhinconichthys (Actinopterygii: Pachycormiformes) from the mid-Cretaceous of the United States, England, and Japan

We re-define the Cretaceous bony fish genus Rhinconichthys by re-describing the type species, R. taylori, and defining two new species; R. purgatorensis sp. nov. from the lowermost Carlile Shale (middle Turonian), southeastern Colorado, United States, and R. uyenoi sp. nov. from the Mikasa Formation (Cenomanian), Middle Yezo Group, Hokkaido, Japan. Rhinconichthys purgatoirensis sp. nov. is designated on a newly discovered specimen consisting of a nearly complete skull with pectoral elements. Only known previously by two Cenomanian age specimens from England and Japan, the North American specimen significantly extends the geographic and stratigraphic range of Rhinconichthys. The skull of Rhinconichthys is elongate, including an expansive gill basket, and estimated maximum body length ranges between 2.0 and 2.7 m. Rhinconichthys was likely an obligate suspension-feeder due to its derived cranial morphology, characterized by a remarkably large and elongate hyomandibula. The hyomandibula mechanically acts as a lever to thrust the jaw articulation and hyoid arch both ventrally and anterolaterally during protraction, thus creating a massive buccal space to maximize filtering of planktonic prey items. Cladistic analysis supports a monophyly of suspension-feeding pachycormids including Rhinconichthys, but further resolution within this clade will require more information through additional fossil specimens.

Bombers and Bivalves: First North American occurrence of the rudist Durania sp. (Bivalvia: Radiolitidae) in the Upper Cretaceous (Cenomanian) Greenhorn Limestone of southeastern Colorado

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member, Greenhorn Limestone (Upper Cenomanian) are attributable to Durania cf. D. cornupastoris. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Cretaceous Western Interior and the only Cenomanian record of rudist Durania in North America. Discovered in 2011, the specimen was unearthed by aerial bombing at a training facility utilized during World War II. The appearance of rudist bivalves at mid-latitudes coincident with marked change in marine sediments likely represents the onset of mid-Cretaceous global warming.

Of Bombers and Bivalves: North American Cenomanian Occurrence of the Rudist Durania sp. (Bivalvia: Radiolitidae) in the Upper Cretaceous Greenhorn Limestone of Southeastern Colorado

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member, Greenhorn Limestone (Upper Cenomanian) are attributable to Durania cf. D. cornupastoris. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Cretaceous Western Interior and the only Cenomanian record of rudist Durania in North America. Discovered in 2011, the specimen was unearthed by historic aerial bombing during World War II. The appearance of rudist bivalves at mid-latitudes coincident with marked change in marine sediments likely represents the onset of mid-Cretaceous global warming.