Michael J. Everhart

100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas.

From Big Fish to Big Whales Whales are the largest animals today, and many feed on the abundant plankton, particularly diatoms, in the oceans. Whales arose and diversified in the Cenozoic, about 30 to 40 million years ago (see the Perspective by Cavin). Marx and Uhen (p. 993) show that their diversity parallels the diversity of diatoms and changes in ocean temperature. Whether there were large predators of plankton before whales has been enigmatic, because the fossil record during the Mesozoic (245 to 65 million years ago) is sparse. Friedman et al. (p. 990) now show that a group of large fish filled this role for nearly 100 million years in the Mesozoic. Although not as large as whales, these globally distributed fish were still several meters long. Their extinction at the Cretaceous-Paleogene boundary 65.5 million years ago may have cleared the seas for the evolution of whales. The extinction of widespread large plankton-eating fish led to the emergence of whales in the Cenozoic. Large-bodied suspension feeders (planktivores), which include the most massive animals to have ever lived, are conspicuously absent from Mesozoic marine environments. The only clear representatives of this trophic guild in the Mesozoic have been an enigmatic and apparently short-lived Jurassic group of extinct pachycormid fishes. Here, we report several new examples of these giant bony fishes from Asia, Europe, and North America. These fossils provide the first detailed anatomical information on this poorly understood clade and extend its range from the lower Middle Jurassic to the end of the Cretaceous, showing that this group persisted for more than 100 million years. Modern large-bodied, planktivorous vertebrates diversified after the extinction of pachycormids at the Cretaceous-Paleogene boundary, which is consistent with an opportunistic refilling of vacated ecospace.

An Elasmosaur with Stomach Contents and Gastroliths from the Pierre Shale (Late Cretaceous) of Kansas

Abstract A nearly complete skeleton of an elasmosaurid plesiosaur (NJSM 15435) from the Sharon Springs Member (Middle Campanian) of the Pierre Shale, Logan County, Kansas, is associated intimately with fragmentary fish remains and numerous gastroliths. The fish bones and gastroliths were located just behind the pectoral girdle in the abdominal region. Identifiable prey includes Enchodus and other small clupeomorph fishes. An isolated tooth of the anacoracid shark Squalicorax cf. S. pristodontus also was recovered in this area. Ninety-five gastroliths (6.8 kg) were present, with the largest stone measuring 15.1 × 8.5 × 5.7 cm (5.0 × 3.3 × 2.2 in.) and weighing 1.06 kg (2.3 lb.). Many of the gastroliths are composed of pink or gray Sioux Quartzite, which suggests that the source of these stones was about 600 km (475 mi) to the northeast of where the elasmosaur remains were discovered. The association of fragmentary fish remains and gastroliths within the abdomen of NJSM 15435 supports the contention that the stones aided in the breakdown of food in plesiosaurs.

Revisions to the Biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas

Abstract The Smoky Hill Chalk Member of the Niobrara Chalk was deposited in the Western Interior Sea during a span of about five million years extending from the upper Coniacian through the lower Campanian. Coincidentally, this period also encompasses much of the early evolution and radiation of the family Mosasauridae. Thousands of mosasaur specimens have been collected from the Smoky Hill Chalk Member of western Kansas since the first Yale College Scientific Expedition in 1870. Early workers viewed the entire Smoky Hill Chalk Member as the “Pteranodon beds” and were not able to provide accurate stratigraphic information. Even though mosasaurs are represented by numerous complete and well preserved specimens, the ranges of the various species could not be defined further without better stratigraphic data. In 1898, S. W. Williston first described the general distribution of mosasaurs within the Rudistes and Hesperornis beds of the Smoky Hill Chalk Member. Seventy years later, D. A. Russell grouped mosasaur species into upper and lower zones in the chalk and in 1990, J. D. Stewart incorporated D. E. Hattins stratigraphic data into his biostratigraphic zones and refined the occurrences of mosasaur species to a higher degree than had been done previously. B. A. Schumacher in 1993, and M. A. Sheldon in 1996, reviewed existing collections and provided further definition to mosasaur biostratigraphy. New specimens of Tylosaurus proriger, and Clidastes liodontus with accurate stratigraphic information reported by M. J. Everhart and co-workers in 1997 further expanded and refined the temporal distribution of these species within the Smoky Hill Chalk Member.

Gastroliths Associated with Plesiosaur Remains in the Sharon Springs Member of the Pierre Shale (Late Cretaceous), Western Kansas

Field work conducted in 1991 and 1998 recovered 47 gastroliths in association with the incomplete and disarticulated remains of a large plesiosaur (KUVP 129744). The specimen was discovered in the upper Sharon Springs Member of the Pierre Shale (Late Cretaceous), Logan County, Kansas. The gastroliths are unusually large in size when compared to those documented from other plesiosaur remains, and larger than those associated with the giant sauropod, Seismosaurus. The shapes of the gastroliths are consistent with similar sized stones that occur in river gravel. This specimen provides new data in regard to the range of sizes and the occurrence of gastroliths in these extinct marine reptiles.

Revised Vertebral Count in the “Longest-Necked Vertebrate” Elasmosaurus platyurus Cope 1868, and Clarification of the Cervical-Dorsal Transition in Plesiosauria

Elasmosaurid plesiosaurians are renowned for their immensely long necks, and indeed, possessed the highest number of cervical vertebrae for any known vertebrate. Historically, the largest count has been attributed to the iconic Elasmosaurus platyurus from the Late Cretaceous of Kansas, but estimates for the total neck series in this taxon have varied between published reports. Accurately determining the number of vertebral centra vis-à-vis the maximum length of the neck in plesiosaurians has significant implications for phylogenetic character designations, as well as the inconsistent terminology applied to some osteological structures. With these issues in mind, we reassessed the holotype of E. platyurus as a model for standardizing the debated cervical-dorsal transition in plesiosaurians, and during this procedure, documented a “lost” cervical centrum. Our revision also advocates retention of the term “pectorals” to describe the usually three or more distinctive vertebrae close to the cranial margin of the forelimb girdle that bear a functional rib facet transected by the neurocentral suture, and thus conjointly formed by both the parapophysis on the centrum body and diapophysis from the neural arch (irrespective of rib length). This morphology is unambiguously distinguishable from standard cervicals, in which the functional rib facet is borne exclusively on the centrum, and dorsals in which the rib articulation is situated above the neurocentral suture and functionally borne only by the transverse process of the neural arch. Given these easily distinguishable definitions, the maximum number of neck vertebrae preserved in E. platyurus is 72; this is only three vertebrae shorter than the recently described Albertonectes, which together with E. platyurus constitute the “longest necked” animals ever to have lived.

A new nodosaur specimen (Dinosauria: Nodosauridae) from the Smoky Hill Chalk (Upper Cretaceous) of western Kansas

Abstract The right ulna and radius of a small nodosaur were recovered from the Smoky Hill Chalk Member (Upper Santonian) of the Niobrara Formation in October, 2000. Based on the similarity of the specimen in comparison with the holotype of Niobrarasaurus coleii, and the relatively small size of the remains, the bones are considered to be those of a juvenile N. coleii. The presence of two parallel scratch marks on the distal shaft of the radius, and the partially digested appearance of the proximal and distal ends of both bones, suggest that the lower limb had been detached from the carcass as the result of scavenging, most likely by the large lamniform shark, Cretoxyrhina mantelli. Although the remains of terrestrial vertebrates are not unknown from sediments deposited in the Late Cretaceous Western Interior Sea, additional discoveries are infrequent and valuable sources of information regarding the terrestrial fauna of the time.

A bitten skull of Tylosaurus kansasensis (Squamata: Mosasauridae) and a review of mosasaur-on-mosasaur pathology in the fossil record

Abstract Aside from large sharks such as Cretoxyrhina mantelli, there are no other marine species known from the Late Cretaceous which could challenge the role of mosasaurs as apex predators. From the evidence of the injuries observed on mosasaur remains, however, it is likely that they occasionally fought with or were attacked by other mosasaurs. Sometimes the injuries incurred in this combat were non-fatal and show evidence of healing prior to the death of the mosasaur, while in other instances, the injuries were apparently fatal. The well-preserved skull and lower jaws of the holotype specimen of Tylosaurus kansasensis (FHSM VP-2295) in the collection of the Stemberg Museum of Natural History exhibit readily visible punctures and gouges on the frontal, left prefrontal, right dentary and right articular that are attributable to the bite of a larger mosasaur. These deep and unhealed bite marks on the skull, as well as a possible broken neck, suggest that this individual died from injuries received when its skull was bitten and possibly crushed by another mosasaur.

Ptychodus mammillaris (Elasmobranchii) and Enchodus cf. E. shumardi (Teleostei) from the Fort Hays Limestone Member of the Niobrara Chalk (Upper Cretaceous) in Ellis County, Kansas

Abstract Vertebrate fossils are rare components of the Fort Hays Limestone Member (uppermost Turonian - Lower Coniacian) of the Upper Cretaceous Niobrara Chalk. Herein, two fish taxa are described that are new to the unit: Ptychodus mammillaris Agassiz and Enchodus cf. E. shumardi Leidy. These additions increase the total number of documented vertebrate species to 11 for the Fort Hays Limestone and provide additional insights into the paleoecology (e.g. community structure) of the Western Interior Seaway during the deposition of the Fort Hays Limestone.

New Data on Cranial Measurements and Body Length of the Mosasaur, Tylosaurus nepaeolicus (Squamata; Mosasauridae), from the Niobrara Formation of Western Kansas

Abstract Tylosaurus nepaeolicus (Cope 1874) is one of the least well known of the five species of mosasaurs that are recognized from the lower Smoky Hill Chalk Member (upper Coniacian) of the Niobrara Formation in western Kansas. In describing the type material, Cope stated that this species was one-third or less the size of T. proriger (Cope 1869), a species that appeared during the Santonian and is well represented in the middle and upper chalk. Additional data provided by Russell from a review of specimens in the American Museum of Natural History and the Yale Peabody Museum showed that most T. nepaeolicus material is somewhat larger than the type specimen, but is significantly smaller than adult T. proriger specimens. Measurements of two additional T. nepaeolicus skulls, an articulated series of Tylosaurus sp. vertebrae and other material in the Sternberg Museum of Natural History suggest that this species approached 8–9 m in size by the end of the Coniacian and was approximately the same size as a large T. proriger reported from the lower Santonian. Although time and other morphological features separate T. nepaeolicus from T. proriger, the two species are closer in size than previously reported.

Occurrence of Ptychodus mammillaris (Elasmobranchii) in the Fairport Chalk Member of the Carlile Shale (Upper Cretaceous) of Ellis County, Kansas

Abstract Although more vertebrate species have been reported from the Fairport Chalk Member (Middle Turonian) than from the overlying Blue Hill Shale and Codell Sandstone members (upper Middle Turonian) of the Carlile Shale in Kansas, the number is relatively small compared to that from the deeper water fauna of the Smoky Hill Chalk Member (Upper Coniacian to Lower Campanian) of the Niobrara Chalk. Ptychodontids are poorly represented in the Fairport Chalk, both in number of specimens and species. Here we report the first occurrence of Ptychodus mammillaris from a single tooth collected in the Fairport Chalk of Ellis County. This specimen adds new data concerning the faunal diversity of the Western Interior Sea during the Middle Turonian.