Kenshu Shimada

Dentition of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli from the Niobrara Chalk of Kansas

ABSTRACT Some specimens of the Late Cretaceous shark, Cretoxyrhina mantelli (Agassiz), from the Niobrara Chalk in Kansas have been found with partial or nearly complete sets of teeth. Analysis of these dentitions offers a better understanding of the sharks systematic position and tooth function. Cretoxyrhina mantelli has a lamnoid dentition with closely spaced tooth rows without any overlap, and with a tendency toward monognathic heterodonty. The best specimen shows four symphysial, two anterior, three intermediate, five lateral, and at least seven posterior teeth in the upper dental series, and one? symphysial, three anterior, five lateral, and at least ten posterior teeth in the lower. Some variation in individual tooth morphology is noted. Dental features support current taxonomic assignments of C. mantelli at ordinal, familial, and generic levels. Cretoxyrhina mantelli probably stabbed and anchored its food with large anterior teeth, and cut food primarily with its more distally located teeth.

Skeletal anatomy of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli from the Niobrara Chalk in Kansas

ABSTRACT Some specimens of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli (Agassiz), from the Niobrara Chalk in Kansas are represented by partial skeletons. The vertebral and neurocranial structures of C. mantelli suggest its placement in Lamniformes. The pectoral fins of C. mantelli may be plesodic, possibly indicating that the taxon is an “advanced” lamnoid. The total vertebral count of C. mantelli, approximately 230, is a unique number among lamniforms, supporting the view based on tooth-based taxonomy that this taxon is a distinct species. The total length of large C. mantelli is estimated to be 5 m. Very large C. mantelli possibly attained a total length of about 6 m or slightly more. Cretoxyrhina mantelli possessed a conical head with a blunt snout and large eyes. The body form of C. mantelli could have resembled that of extant Carcharodon carcharias.

Ontogenetic parameters and life history strategies of the late Cretaceous lamniform shark, Cretoxyrhina mantelli, based on vertebral growth increments

Abstract Cretoxyrhina mantelli is a Late Cretaceous lamniform shark that possibly resembled the modern white shark (Carcharodon carcharias) in size, body form, and feeding strategies. Here, a previously identified set of growth increment (= periodic marker) bands in a vertebra from an individual of Cretoxyrhina mantelli (5 m in total length [TL]) is used as a model to quantitatively infer the ontogenetic development and life history strategies of the fossil shark. The results indicate that its length at birth was large, and that the early part of its life history was marked by a comparably rapid growth, which could have helped the neonatal C. mantelli to efficiently exploit prey items while deterring potential predators. The von Bertalanffy growth function (VBGF) experimentally fitted to the data gives the following estimated growth parameters: the length at birth (L0) of 1.28 m TL, the asymptotic length (L∞) of 6.91 m TL, and the rate constant with units of reciprocal time (k) of 0.073 yr−1. The large maximum length indicates that individuals may have weighed 3,400 kg. The VBGF parameters also indicate that the longevity of C. mantelli was possibly up to about 38 years. This study shows that many aspects of ontogeny and life history of C. mantelli are similar to those of the modern white shark, and that a single shark fossil may offer a wealth of biological information.

A partial skeleton of the Late Cretaceous lamniform shark, Archaeolamna kopingensis, from the Pierre Shale of western Kansas, U.S.A.

ABSTRACT All previous records of the lamniform shark, Archaeolamna kopingensis, are based on isolated teeth. Here we describe a partial skeleton from the Sharon Springs Formation of the Pierre Shale Group of western Kansas, U.S.A. The specimen includes portions of the upper and lower jaws with articulated teeth. The dentition consists of two files of upper and lower anterior teeth that, together with a single file of intrabullar intermediate teeth, are housed in a dental bulla, as well as multiple files of lateral teeth, along with at least two files of lower symphysial teeth and a single file of upper symphysial teeth. The intrabullar intermediate tooth is slightly shorter than the other anterior teeth and has a median cusp with distinctive distal curvature. The dental sequence of A. kopingensis is unique among both extinct and extant lamniforms. Associated with the jaws are fragments of the neurocranium and multiple vertebral centra. A sagittal section through a centrum shows that this shark deposited 18 annual marker bands after its birth and adult size was attained by the 10th band. The robust but penetrating tooth morphology and large jaw circumference suggest that A. kopingensis likely fed upon large prey items.

A remarkable case of a shark-bitten elasmosaurid plesiosaur

KENSHU SHIMADA,*12 TAKANOBU TSUIHIJI,3 TAMAKI SATO,4 and YOSHIKAZU HASEGAWA5; Environmental Science Program and Department of Biological Sciences, DePaul University, 2325 North Clifton Avenue, Chicago, Illinois 60614, U.S.A., kshimada@depaul.edu; 2Sternberg Museum of Natural History, Fort Hays State University, 3000 Sternberg Drive, Hays, Kansas 67601, U.S.A.; 3National Museum of Nature and Science, 3-23-1 Hyakuhin-cho, Shinjuku-ku, Tokyo 169-0073, Japan, taka@kahaku.go.jp; 4Department of Astronomy and Earth Sciences, Tokyo Gakugei University, 4-1-1 Nukui-Kita-Machi, Koganei City, Tokyo 184-8501, Japan, tsato@u-gakugei.ac.jp; 5Gunma Museum of Natural History, 1674-1 Kamikuroiwa, Tomimoka, Gunma 370-2345, Japan, hasegawa@gmnh.pref.gunma.jp

Partial skull of Late Cretaceous durophagous shark, Ptychodus occidentalis (Elasmobranchii: Ptychodontidae), from Nebraska, U.S.A.

ABSTRACT Ptychodus (Elasmobranchii: Ptychodontidae) is an enigmatic durophagous shark known from Cretaceous marine deposits nearly worldwide based primarily on isolated teeth. Here, we describe a specimen of P. occidentalis Leidy from the Greenhorn Limestone (middle Cenomanian—early Turonian) in Nebraska, U.S.A. The specimen does not offer any new taxonomic information concerning the unresolved ordinal placement of Ptychodus. However, it is significant because it preserves a partial skull including teeth, incomplete jaws, and neurocranial fragments as well as placoid scales and vertebrae, all of which provide a wealth of new anatomical information. The specimen contains at least 267 teeth and shows that the two dental plates are anteroposteriorly elongate. The partial paired Meckels cartilages show a long, fused jaw symphysis and elongate jaw rami. The distribution of presumed neurocranial fragments and placoid scales with respect to the position of the dental plates suggests that the shark had a broad head with a narrow, subterminal mouth. The morphology of placoid scales suggests that the shark was a rather sluggish swimmer and had a stout, streamlined body similar to that in the extant orectolobiform sharks (e.g., Ginglymostoma). Based on the jaw length, the individual of P. occidentalis was approximately 2 m in total length (TL), while some of the largest Ptychodus known from younger Cretaceous horizons in the North American Western Interior likely attained at least 7 m TL. Ptychodus occidentalis is inferred to be an opportunistic generalist (rather than a hard prey specialist) that fed on a variety of readily available prey using inertial suction feeding.

A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States

ABSTRACT n Eorhincodon casei from Russia and Megachasma comanchensis from the United States are two Cretaceous taxa initially described as putative planktivorous elasmobranchs, but the type specimens of these two taxa were subsequently reinterpreted to represent taphonomically abraded teeth of an odontaspidid, Johnlongia Siverson (Lamniformes: Odontaspididae). Here, we redescribe the type materials of ‘E. casei’ and ‘M. comanchensis’ and describe additional specimens of these species from other Late Cretaceous localities in Russia and the United States. These specimens demonstrate that (1) the two fossil taxa are valid species; (2) they warrant the establishment of a new genus of presumed planktivorous sharks, Pseudomegachasma, gen. nov., to accommodate the two species; and (3) the new genus is sister to Johnlongia and together constitute a new subfamily Johnlonginae, subfam. nov., tentatively placed in the family Odontaspididae sensu stricto. This taxonomic placement indicates that the putative planktivorous clade was derived from a presumed piscivorous form (Johnlongia), with an implication that Pseudomegachasma, gen. nov., evolved a plankton-eating habit independent of the four known planktivorous elasmobranch clades (Rhincodontidae, Megachasmidae, Cetorhinidae, and Mobulidae). It also indicates that planktivorous diets evolved independently at least three times in the order Lamniformes (i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and more significantly, Pseudomegachasma, gen. nov., would represent the oldest known plankton-feeding elasmobranch in the fossil record. The present fossil record suggests that Pseudomegachasma, gen. nov., evolved in a relatively shallow-water environment in Russia in the early Cenomanian or earlier and subsequently migrated to the North American Western Interior Seaway by the mid-Cenomanian.

A New Gigantic Pycnodont Fish from the Juana Lopez Member of the Upper Cretaceous Mancos Shale of New Mexico, U.S.A.

KENSHU SHIMADA,*12 THOMAS E. WILLIAMSON,3 and PAUL L. SEALEY3; Environmental Science Program and Department of Biological Sciences, DePaul University, 2325 North Clifton Avenue, Chicago, Illinois 60614, U.S.A., kshimada@depaul.edu; 2Sternberg Museum of Natural History, Fort Hays State University, 3000 Sternberg Drive, Hays, Kansas 67601, U.S.A.; 3New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104, U.S.A., thomas.williamson@state.nm.us, Ammonoidea@comcast.net

A new elusive otodontid shark (Lamniformes: Otodontidae) from the lower Miocene, and comments on the taxonomy of otodontid genera, including the ‘megatoothed’ clade

Abstract We describe a new large otodontid lamniform shark, Megalolamna paradoxodon gen. nov. et sp. nov., chronostratigraphically restricted to the early Miocene (Aquitanian–Burdigalian). This new species is based on isolated teeth found from five globally distributed localities: the Jewett Sand in southern California, USA; the Pungo River Formation of North Carolina, USA; the Chilcatay Formation of Peru; the Oi Formation in Mie Prefecture, Japan; and the O’oshimojo Formation in Nagano Prefecture, Japan. Extrapolations based on available published data on modern macrophagous lamniforms suggest that the largest specimen of M. paradoxodon gen. nov. et sp. nov. possibly came from an individual that measured at least 3.7 m in total length. All specimens came from deposits in the mid-latitudinal zones representing shallow-water, shelf-type, coastal environments. Its dentition likely exhibited monognathic heterodonty suited for capturing and cutting relatively large prey (e.g. medium-sized fishes). We recommend the genus Otodus to include sharks of the ‘megatoothed’ (e.g. megalodon) lineage in order to avoid Otodus paraphyly. We also propose the following phylogenetic hypothesis: [Kenolamnau2009+u2009[Cretalamnau2009+u2009[Megalolamna + Otodus]]]. ZooBank LSID for the genus Megalolamna is: urn: lsid:zoobank.org:act:B4791DEF-4D96-4FEB-9B7B-0EF816B96079 ZooBank LSID for the species Megalolamna paradoxodon is: urn:lsid:zoobank.org:act:7D3D7442-53C6-43A2-9E8D-6339729565B6

A New Fossil Megamouth Shark (Lamniformes, Megachasmidae) from the Oligocene—Miocene of the Western United States

ABSTRACT n The extant megamouth shark, Megachasma pelagios (Lamniformes: Megachasmidae), is a large filter-feeding fish. We here describe a new species of Megachasma, M. applegatei, sp. nov., a putative sister species of the extant M. pelagios, based on isolated teeth from late Oligocene—early Miocene (late Chattian—Aquitanian) marine deposits in California and Oregon, U.S.A. Although showing a megachasmid tooth design, teeth of M. applegatei, sp. nov., exhibit a wide morphological range and are reminiscent to those of odontaspidid sharks with strong heterodonty. Megachasma applegatei, sp. nov., could have commonly measured approximately 6 m in total length and likely had a wide range of diet, possibly including small fishes and planktonic invertebrates. The fossil record indicates that either M. applegatei, sp. nov., was broadly adapted to a wide bathymetric tolerance or was a nektopelagic feeder over both deep and shallow water habitats.