Kenshu Shimada

• DePaul University

Otodus megalodon (Lamniformes: Otodontidae) is an iconic Neogene shark, but the lack of well-preserved skeletons has hampered our understanding of various aspects of its biology. Here, we reassess some of its biological properties using a new approach, based on known vertebral specimens of O. megalodon and 165 species of extinct and extant neoselac...

In the current genomic era, palaeontology remains crucial for understanding macroevolutionary patterns and dynamics. Despite a relatively rich fossil record, the early evolution of lamniform sharks, a group including iconic predators like the white shark and the extinct †Otodus megalodon, remains poorly understood due to the scarcity of complete fo...

The Neogene period (23–2.6 Ma) is characterized by the presence of a rich fossil record of bony and cartilaginous fishes worldwide, including localities in Japan. However, despite the rich and diverse fossil record of Neogene fishes in Japan, changes in diversity over time remain uninvestigated. This preliminary study aimed to analyze the diversifi...

Supplementary data used in this study, including a review of over 60 cases of body size estimation controversies in fossil animals.

Body size is of fundamental importance to our understanding of extinct organisms. Physiology, ecology and life history are all strongly influenced by body size and shape, which ultimately determine how a species interacts with its environment. Reconstruction of body size and form in extinct animals provides insight into the dynamics underlying comm...

The megatooth shark, †Otodus megalodon, which likely reached at least 15 m in total length, is an iconic extinct shark represented primarily by its gigantic teeth in the Neogene fossil record. As one of the largest marine carnivores to ever exist, under�standing the biology, evolution, and extinction of †O. megalodon is important because it had a s...

The evolution of the extinct megatooth shark, Otodus megalodon, and its close phylogenetic relatives remains enigmatic. A central question persists regarding the thermophysiological origins of these large predatory sharks through geologic time, including whether O. megalodon was ectothermic or endothermic (including regional endothermy), and whethe...

The late Neogene megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), is mostly known for its gigantic teeth and vertebrae. Re-examination of the rock matrix surrounding a previously described associated tooth set of O. megalodon from the upper Miocene of Japan resulted in the observation of numerous fragments of tessellated calcified cart...

The Niubu fossil locality in Chiayi County, southern Taiwan is best known for its rich early Pleistocene marine fossils that provide insights into the poorly understood past diversity in the area. The elasmobranch teeth at this locality have been collected for decades by the locals, but have not been formally described and have received little atte...

Jaw mechanics of lamniform sharks were examined three‐dimensionally to analyze the variability in jaw shape and the evolution of the jaw system based on the extant macrophagous species. Three‐dimensional lever analysis was applied to lamniform jaws to calculate bite force at each tooth relative to maximum input force from jaw adductor muscles for i...

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the 15N/14N ratio of enameloid-bound organic matter (δ15NEB) as a trophic level proxy by comparison to dentin collagen δ15N and apply this method to the fossil record to reconstruct the trophic level of the megatoot...

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...

The Smoky Hill Chalk Member of the Upper Cretaceous Niobrara Formation is a sedimentary deposit formed in the Western Interior Seaway of North America. Here, we examined a fossiliferous bonebed from an uncertain horizon within the Smoky Hill Chalk in Gove County, Kansas, U.S.A. Over 40,450 taxonomically identifiable specimens were collected, consis...

The Juana Lopez Member of the Carlile Shale is a calcarenite rock that became deposited within the Western Interior Seaway of North America during the Late Cretaceous approximately 90 million years ago (early late Turonian). In this study, rock samples collected from a Juana Lopez Member locality in southeastern Colorado, USA, were dissolved with a...

The late Neogene megatooth shark, Otodus megalodon, is known mainly from its gigantic teeth and possibly reached 18–20 m in total length (TL). We re-examine the previously proposed body size trends and nursery areas of O. megalodon by confining the previously used samples to upper anterior teeth offering more reliable TL estimates, and by taking pa...

The megatooth shark, Otodus megalodon, is an iconic Neogene lamniform shark known only from its teeth and vertebrae. Its thermophysiology is previously inferred to have been regionally endothermic, like the extant lamnids that are active predatory lamniforms. By considering the entire Lamnidae as the ecological and physiological analogue to O. mega...

Sibert and Rubin (Reports, 4 June 2021, p. 1105) claim to have identified a previously unidentified, major extinction event of open-ocean sharks in the early Miocene. We argue that their interpretations are based on an experimental design that does not account for a considerable rise in the sedimentation rate coinciding with the proposed event, nor...

Based on a nearly complete skeleton from the Arlington Member (mid-Cenomanian) of the Upper Cretaceous Woodbine Formation in Texas, USA, a new fossil fish, Bardackichthys carteri gen. et sp. nov. (Actinopterygii: Ichthyodectiformes), is described. Our phylogenetic analyses show the suborder Ichthyodectoidei (containing Amakusaichthys, Chirocentrite...

A lag deposit between the Tocito Sandstone and Mulatto Tongue of the Upper Cretaceous Mancos Shale in Sandoval County, New Mexico, USA, contains a fossil assemblage of late Turonian–early Coniacian chondrichthyans and osteichthyans. This assemblage consists primarily of isolated teeth that derive from at least 26 taxa including: Meristodonoides sp....

Pycnodonts are an extinct group of bony fishes with crushing dentitions suited to feed on shelled macroinvertebrates. In this paper, we described a partial skeleton of a large pycnodont collected from the middle Turonian portion of the Boquillas Formation within Big Bend National Park in Brewster County, Texas, USA. The specimen that includes the v...

The extinct megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), is known primarily from its gigantic teeth in the late Neogene marine fossil record. It is known to reach at least 14.1‒15.3 m in length, but its reproductive biology and ontogenetic growth pattern have remained largely in the realm of speculation. Here, we examined increment...

Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various anthropogenic pressures. The abundant fossil record of fossil taxa has allowed the tracing of the evolutionary history of modern elasmobranchs to at least 250 MYA; nonetheless, exactly how far back the fossil record of living taxa goes has never been co...

In this paper, a shark-bitten partial tibiotarsus of a hesperornithiform bird is described from the Upper Cretaceous of Grant County, South Dakota, U.S.A. Whether the bite marks represent a predatory attack or scavenging is uncertain, but they are attributed to an anacoracid shark, Squalicorax cf. S. falcatus. The specimen is significant because it...

Extinct lamniform sharks (Elasmobranchii: Lamniformes) are well represented in the late Mesozoic‒Cenozoic fossil record, yet their biology is poorly understood because they are mostly represented only by their teeth. Here, we present measurements taken from specimens of all 13 species of extant macrophagous lamniforms to generate functions that wou...

Alopias is a group of lamniform sharks characterized by a highly elongate caudal fin with three known extant species: Alopias pelagicus (pelagic thresher shark), Alopias superciliosus (bigeye thresher shark), and Alopias vulpinus (common thresher shark). Alopias pelagicus and A. vulpinus are considered fast swimmers and use their caudal fin to hunt...

Shark nurseries are essential habitats for shark survival. Notwithstanding the rich fossil record of the modern great white shark (Carcharodon carcharias, GWS), its use of nursery areas in the fossil record has never been assessed before. Here, we analysed the fossil record of the GWS from three South American Pliocene localities, assessed body siz...

Sharks are among the oldest vertebrate lineages in which their success has been attributed to their diversity in body shape and locomotor design. In this study, we investigated the diversity of body forms in extant sharks using landmark-based geometric morphometric analyses on nearly all the known (ca. 470) extant sharks. We ran three different ana...

SMU 76938 is a nearly complete skeleton of a fossil bony fish housed in Southern Methodist University in Dallas, Texas, USA. It was collected from the Tarrant Formation (middle Cenomanian) of the Upper Cretaceous Eagle Ford Group in Tarrant County, Texas, an area where it was near the East Texas Embayment of the Western Interior Seaway during the L...

We describe a partial skeleton of the Late Cretaceous shark, Cretodus, collected from the Blue Hill Shale (middle Turonian) in north-central Kansas, U.S.A. It consists of 134 disarticulated teeth, 61 vertebrae, 23 placoid scales, and fragments of calcified cartilage. The scale morphology suggests that Cretodus was a rather sluggish shark, and the v...

Lamniformes (Chondrichthyes: Elasmobranchii) is a group of sharks that consists of 15 extant species with a wide range of morphological diversity. The most rarely captured lamniform is Odontaspis noronhai, and many aspects of its biology remain unknown to date. In this study, the skeletal anatomy of a previously described specimen of O. noronhai wa...

Otodus megalodon (Lamniformes: Otodontidae) is a gigantic late Neogene shark that lived nearly worldwide in tropical-temperate regions. Its gigantic teeth have captivated imaginations of the scientific community and general public alike, where the most commonly cited maximum size range of O. megalodon in scientific literature is 18–20 m in total le...

Squaliformes (dogfish sharks) is a large elasmobranch order with six families (Centrophoridae, Dalatiidae, Etmopteridae, Oxynotidae, Somniosidae, and Squalidae) and over 100 species. A previous study suggested all squaliforms share one basic body plan unlike most other shark orders that have body shape diversity. Here, we used landmark-based geomet...

Micropycnodon kansasensis (Hibbard and Graffham 1941) is an extinct bony fish that lived in the Western Interior Seaway of North America during the Late Cretaceous. In this paper, we describe previously unreported anatomical features of the fossil fish based on a partial skeleton from the Smoky Hill Chalk Member of the Niobrara Chalk in Gove County...

Thryptodus zitteli Loomis (Actinopterygii: Tselfatiiformes) is an enigmatic Late Cretaceous bony fish with a massive blunt rostrum that lived in the Western Interior Seaway of North America. Here, a second species of Thryptodus, T. loomisi sp. nov., is described on the basis of two skulls and some isolated rostra from the Britton Formation (Cenoman...

Through elasmobranch (sharks and rays) evolutionary history, gigantism evolved multiple times in phylogenetically distant species, some of which are now extinct. Interestingly, the world's largest elasmobranchs display two specializations found never to overlap: filter feeding and mesothermy. The contrasting lifestyles of elasmobranch giants provid...

A nearly complete right mandibular tooth plate of Ischyodus bifurcatus Case (Holocephali: Chimaeroidei) is reported from the Point Loma Formation (upper Campanian) of the Upper Cretaceous Rosario Group in southern California, USA. The individual is estimated to have measured nearly 1 m in total body length. Remains of I. bifurcatus have been report...

Elasmobranchii is a clade of chondrichthyans (cartilaginous fishes) that comprises sharks, skates and rays represented today by approximately 1,200 species. Chondrichthyans have a long evolutionary history dating back to the Late Ordovician (ca. 450 million years ago [Mya]) based on isolated dermal denticles (Janvier 1996). Other remains such as ar...

Lamniformes is a small order of sharks consisting of only 15 extant species but a highly diverse group, including a wide interspecific variation range in their caudal fin shape. A previous study has suggested that caudal fins of lamniforms can be grouped into two types. Type 1 fins have a high aspect ratio and high heterocercal angles, characterize...

ABSTRACT.—Alopias (Alopiidae: thresher sharks) is a group of lamniform sharks with a highly elongate caudal fin, consisting of three extant species: A. pelagicus, A. superciliosus, and A. vulpinus. The relative brain size (‘encephalization’), relative development of five major brain areas (i.e., telencephalon, diencephalon, mesencephalon, cerebellu...

The Jetmore Chalk Member of the Greenhorn Limestone is a Late Cretaceous rock unit deposited under the Western Interior Seaway in North America. We here report an assemblage of vertebrate fossils collected through surface collecting and bulk sampling at a new Jetmore Chalk locality in Republic County, Kansas, U.S.A. Taxonomically, it consists of di...

We describe seven associated skeletal remains of Ischyrhiza mira, a Late Cretaceous sclerorhynchid sawfish, from the Campanian‒lower Maastrichtian of Tennessee and Alabama, U.S.A., to decipher its paleobiology. Ischyrhiza mira had about 16 or 17 functional spines and about the same number of replacement spines on each side of the rostrum in which t...

Ischyrhiza mira is a Late Cretaceous sclerorhynchid sawfish (Chondrichthyes: Elasmobranchii) that is primarily known by its rostral spines, but its paleobiology remains largely uncertain. Here, we describe seven associated skeletal remains of I. mira from the Campanian-lower Maastrichtian of Tennessee and Alabama, USA. Our study suggests that I. mi...

The Jetmore Chalk Member of the Greenhorn Limestone is a Late Cretaceous rock unit deposited under the Western Interior Seaway in North America. Recently, multiple vertebrate fossils were collected from the stratigraphic member through surface collecting at a new locality in Republic County, Kansas. Taxonomically, they consist of diverse carnivorou...

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 D...

The megamouth shark (Lamniformes, Megachasmidae) has enigmatic occurrences both in the present-day oceans and in the fossil record, where the geologically oldest record of megachasmids in literature is represented by the teeth of Megachasma applegatei from the late Oligocene (late Chattian; ca. 23 Ma) of the western USA. NHMUK PV P73711 is a previo...

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 Nor...

We redescribe an extinct river shark, Glyphis pagoda (Noetling), on the basis of 20 teeth newly collected from three dif-ferent Miocene localities in Myanmar. One locality is a nearshore marine deposit (Obogon Formation) whereas the other two localities represent terrestrial freshwater deposits (Irrawaddy sediments), suggesting that G. pagoda from...

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 Upper Cretaceous (Cenomanian) Mikasa Formation, Middle Yezo Group,...

Pentanogmius Taverne (Actinopterygii: Tselfatiiformes) is a Late Cretaceous bony fish. Here, the diagnosis for the genus is emended and a new species, P. fritschi sp. nov., described. The new species is based on a nearly complete skeleton from the Britton Formation (upper Cenomanian–lower Turonian) of the Eagle Ford Shale in Dallas County, Texas, U...

Reported here is the first collective description of a marine vertebrate assemblage from the Codell Sandstone Member (middle Turonian) of the Upper Cretaceous Carlile Shale in Jewell County, Kansas. The Codell Sandstone was deposited during a regression of the Western Interior Seaway, and the fossil locality is described as a relatively shallow, ne...

The Fairport Chalk (middle Turonian) of the Carlile Shale is an Upper Cretaceous rock unit deposited in the middle of the Western Interior Seaway during the early regressive phase of the Seaway following its peak transgressive phase. The Sternberg Museum of Natural History in Hays, Kansas, U.S.A., houses a collection of marine vertebrate fossils fr...

The Blue Hill Shale Member of the Carlile Shale is a Middle Turonian (ca. 90 Ma) nearshore deposit formed during the regressive phase of the Late Cretaceous Western Interior Seaway of North America. Over 900 identifiable specimens of fossil vertebrates were recovered from a locality in northeastern Nebraska that include at least 40 taxa, comprising...

The Pfeifer Shale Member of the Greenhorn Limestone is a Late Cretaceous (early Turonian) rock unit deposited in the Western Interior Seaway in North America. Approximately 50 isolated skeletal and dental elements of fossil fishes were recently recovered from the lower part of the Pfeifer Shale at a locality in south-central Republic County, Kansas...

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)...

Pachyrhizodus caninus Cope is an extinct bony fish found in various Upper Cretaceous marine deposits in North America, but very little has been known about its paleobiology. This paper describes a nearly complete skeleton of P. caninus from the Britton Formation (Cenomanian – lower Turonian) of the Eagle Ford Shale in Texas, USA. The fish specimen...

Eorhincodon casei from the lower Cenomanian of Belgorod Province in European Russia and Megachasma comanchensis from the mid-Cenomanian of Colorado in the United States are two Cretaceous nominal taxa initially described as possible planktivorous elasmobranchs. However, the type specimens of these two taxa were subsequently re-interpreted to repres...

The Jetmore Chalk Member of the Greenhorn Limestone is an early Turonian (Late Cretaceous) rock unit deposited under the Western Interior Seaway in North America in which its fossil vertebrate contents were poorly known previously. Recently, over 250 isolated teeth comprising a minimum of seven chondrichthyan taxa and nine osteichthyan taxa were re...

The Greenhorn Limestone is a rock formation composed primarily of chalky limestone and shale that was deposited in the Late Cretaceous Western Interior Seaway of North America. Many dental and skeletal remains of fossil fishes were collected from the basal Greenhorn Limestone (early Turonian; ca. 93 Ma) at a locality along the western border (Plymo...

ABSTRACT—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 Cal...

The Graneros Shale was deposited in the Late Cretaceous Western Interior Seaway of North America during the mid-Cenomanian (ca. 95 Ma). In this study, we report an assemblage of fossil vertebrates from the middle portion of the Graneros Shale in southeastern Nebraska. The fauna consists of at least 11 taxa, including eight chondnchthyan and one ost...

The Greenhorn Limestone was deposited in the middle of the Upper Cretaceous Western Interior Seaway of North America. Based on materials collected from southeastern Nebraska, we report an assemblage of fossil marine fishes from the upper half (Jetmore and Pfeifer members) of the formation that represents the peak transgressive phase of the seaway....

A set of associated vertebrae and teeth of a fossil shark was collected from the lower Lutetian (Middle Eocene) part of the Lillebælt Clay Formation in Denmark. Its vertebral morphology indicates that the individual belongs to an odontaspidid lamniform shark. Although it is here identified as Odontaspididae indet., its tooth morphology suggests tha...

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...

We describe a specimen of an Upper Cretaceous shark, Cardabiodon sp. (Lamniformes: Cardabiodontidae), from Kansas. This specimen, that consists of a set of teeth, pieces of calcified cartilage, and placoid scales, represents the first associated cardabiodontid material from North America. The combination of direct observation and radiographic exami...

The Graneros Shale was deposited in the Late Cretaceous Western Interior Seaway of North America during the mid-Cenomanian (ca. 95 Ma). Vertebrate fossils are known to occur in the rock formation, but there is no detailed report collectively documenting the composition of the vertebrate fauna. In this study, we collected many remains of fossil fish...

Lamniformes is a small shark group consisting of 15 extant species with remarkably diverse lifestyles and a wide range in heterocercal tail morphology. The caudal fin morphology must be related to their lifestyle because the tail is a main locomotive structure in sharks, but such relationships have remained largely uninvestigated. Here, the morphol...

The stratigraphic and paleogeographic distribution of the suspension-feeding pachycormiform fish Bonnerichthys is reviewed. Fossils attributable to this genus are known from the Western Interior Seaway (Niobrara Formation of Kansas, Sharon Springs Formation of Kansas, Nebraska, and North Dakota, DeGrey Formation of South Dakota, and possibly Mobrid...

Ptychodus (Elasmobranchii, Ptychodontidae) is an enigmatic durophagous shark that lived in Cretaceous seas. Based on multiple articulated tooth plates of Ptychodus mortoni from the Niobrara Chalk in Kansas, the dental pattern of P. mortoni and its paleobiological implications were examined. Each tooth plate consists of one medial tooth row and abou...

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 roc...

On 16 November 2006, a female juvenile megamouth shark, Megachasma pelagios, was caught off the coast of Mexico in the Pacific Ocean, near Sebastián Vizcaíno Bay. This specimen, that has informally been referred to as "Megamouth No. 38", measured 2265 mm in total length. It represents the third smallest female recorded for this taxon and the first...

On 16 November 2006, a female juvenile megamouth shark, Megachasma pelagios, was caught off the coast of Mexico in the Pacific Ocean, near Sebastián Vizcaíno Bay. This specimen, that has informally been referred to as "Megamouth No. 38", measured 2265 mm in total length. It represents the third smallest female recorded for this taxon and the first...

The Lincoln Limestone Member of the Greenhorn Formation is a mid-Cenomanian sedimentary rock unit deposited under the Late Cretaceous Western Interior Seaway (WIS) in North America. Fossiliferous rock samples from the basal Lincoln Limestone, that formed sometime between 95 and 94.7 Ma, were obtained from a locality (‘Table Mesa locality’) in south...

Lungfishes (dipnoans) are lobe-finned fishes that have lived from the Devonian to Recent. Here, we describe a previously unreported Mesozoic lungfish species assigned to the genus Ceratodus on the basis of an isolated upper tooth from the Western Interior of North America. The morphology of the tooth specimen clearly suggests that it represents an...

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...

The Western Interior Seaway (WIS) was an epicontinental sea that extended north–south through the middle of North America during the last half of the Cretaceous, linking the Tethys Sea to the south with the boreal paleo-Arctic Ocean to the north. Aptian–Maastrichtian sedimentary deposits from the seaway crop out in the Western Interior Basin of Can...

Ptychodus mortoni Mantell is a Late Cretaceous shark that possessed pavement-like tooth plates that were used to feed on hard-shelled macroinvertebrates (durophagy). Here, we describe a new specimen of P. mortoni from the Fort Hays Limestone Member of the Niobrara Chalk in Kansas, USA, that consists of associated teeth, placoid scales, and a portio...