Trophic position of Otodus megalodon and great white sharks through time revealed by zinc isotopes

Diet is a crucial trait of an animal’s lifestyle and ecology. The trophic level of an organism indicates its functional position within an ecosystem and holds significance for its ecology and evolution. Here, we demonstrate the use of zinc isotopes (δ66Zn) to geochemically assess the trophic level in diverse extant and extinct sharks, including the Neogene megatooth shark (Otodus megalodon) and the great white shark (Carcharodon carcharias). We reveal that dietary δ66Zn signatures are preserved in fossil shark tooth enameloid over deep geologic time and are robust recorders of each species’ trophic level. We observe significant δ66Zn differences among the Otodus and Carcharodon populations implying dietary shifts throughout the Neogene in both genera. Notably, Early Pliocene sympatric C. carcharias and O. megalodon appear to have occupied a similar mean trophic level, a finding that may hold clues to the extinction of the gigantic Neogene megatooth shark.publishedVersio

Paleobiology of the Late Cretaceous Shark, Cretoxyrhina Mantelli (Lamniformes: Cretoxyrhinidae), from Kansas

Most extinct modern- level elasmobranchs, including most selachians from the Upper Cretaceous deposits of Kansas, are known only from isolated teeth. However, some specimens of the extinct shark, Cretoxyrhina mantelli (Lamniformes: Cretoxyrhinidae), represent reasonably complete individuals. Unlike studying isolated teeth, examination of associated skeletal and dental elements, combined with taphonomic and biostratigraphic data, provides robust information about the paleobiology of C. mantelli. The neurocranial structure, calcification pattern of centra, vertebral count, and dental formula indicate that C. mantelli is a valid taxon. The scale morphology suggests that C. mantelli was a fast swimming pelagic shark. The total body length of a large C. mantelli is estimated to be 4.5 to 5 m, but very large individuals probably reached about 5.5 m. Synthesis of all anatomical information suggests that the body form of C. mantelli resembled that of extant Carcharodon carcharias or Lamna nasus, whereas the feeding mechanics of Cretoxyrhina mantelli was probably similar to that of extant Isurus. Because some cretoxyrhina specimens provide evidence of ingestion of large active vertebrates (including teleosts, mosasaurs, and possibly plesiosaurs), C. mantelli most likely occupied the apex of the food chain; however, it was probably consumed frequently by anacoracids after death. C. mantelli is characterized herein as an offshore shark

Dentitions of lamniform sharks: Homology, phylogeny, and paleontology.

Dentitions of lamniform sharks: Homology, phylogeny, and paleontology

Marine vertebrates from the Hartland Shale (Upper Cretaceous: Upper Cenomanian) in southeastern Colorado, USA

The Hartland Shale Member of the Greenhorn Limestone was deposited in the middle of the Late Cretaceous Western Interior Seaway of North America. Rock samples rich in micro-vertebrate fossils were collected from the lower part of the Hartland Shale (ca. 94.6 Ma: early Late Cenomanian) in southeastern Colorado, USA. Through acid treatment of the rock samples, 25 marine vertebrate taxa are identified including chondrichthyans, osteichthyans, and a reptile. Chondrichthyans are represented by seven species: Ptychodus anonymus, Squalicorax curvatus, Carcharias saskatchewanensis, Archaeolamna kopingensis, Cretoxyrhina mantelli, Cretomanta canadensis, and Rhinobatos incertus. Osteichthyan fishes consist of 17 taxa: Micropycnodon kansasensis, cf. Palaeobalistum sp., Caturidae indet., Protosphyraena sp., Plethodidae indet., Elopopsis sp., Pachyrhizodus minimus, cf. Pachyrhizodus sp., Albulidae indet., Cimolichthys nepaholica, Enchodus cf. E. gladiolus, E. cf. E. shumardi, Apateodus sp., and four unidentified teleosts. The only reptilian recognized is the small aquatic lizard Coniasaurus crassidens (Dolichosauridae). The taxonomic composition of the Hartland Shale fauna is similar overall to the extensively sampled, underlying mid-Cenomanian Lincoln Limestone fauna in Colorado and Kansas, although the occurrence of Apateodus and Cimolichthys from the Hartland Shale is notable as they represent geologically the oldest record for the two genera. The vertebrates identified are mostly carnivores that include piscivorous and durophagous forms, providing new insights into the trophic structure of the palaeocommunity

Skeletal convergence in thunniform sharks, ichthyosaurs, whales, and tunas, and its possible ecological links through the marine ecosystem evolution.

Tunas, lamnid sharks, modern whales, and derived ichthyosaurs converged on the thunniform body plan, with a fusiform body, lunate caudal fin, compressed peduncle, and peduncle joint. This evolutionary convergence has been studied for a long time but little is known about whether all four clades share any skeletal characteristics. Comparisons of vertebral centrum dimensions along the body reveal that the four clades indeed share three skeletal characteristics (e.g., thick vertebral column for its length), while an additional feature is shared by cetaceans, lamnid sharks, and ichthyosaurs and two more by lamnid sharks and ichthyosaurs alone. These vertebral features are all related to the mechanics of thunniform swimming through contributions to posterior concentration of tail-stem oscillation, tail stem stabilization, peduncle joint flexibility, and caudal fin angle fixation. Quantitative identifications of these features in fossil vertebrates would allow an inference of whether they were a thunniform swimmer. Based on measurements in the literature, mosasaurs lacked these features and were probably not thunniform swimmers, whereas a Cretaceous lamniform shark had a mosaic of thunniform and non-thunniform features. The evolution of thunniform swimming appears to be linked with the evolution of prey types and, in part, niche availability through geologic time

The oldest fossil record of the megamouth shark from the late Eocene of Denmark and comments on the enigmatic megachasmid origin

The megamouth shark (Lamniformes: Megachasmidae) has sporadic occurrences both in the present-day oceans and in the fossil record. In this paper, we describe a new megachasmid, Megachasma alisonae sp. nov., on the basis of a morphologically distinct tooth collected from the Pyt Member of the late Eocene Søvind Marl Formation at Moesgård Strand in Denmark, that represents the geologically oldest known Megachasma. The tooth likely came from an individual that measured somewhere between 1.3 and 3.5 m long, and its morphology and chipped cusp tips suggest that it possibly fed on macro-zooplankton and small fishes that had hard skeletal components. Its occurrence in the mid-Priabonian Pyt Member at least suggests that the shark inhabited a relatively deep, open marine environment about 36 Ma ago. This Eocene specimen is significant because it illustrates the dental condition of early megachasmids, which is distinctively odontaspidid-like morphologically