Kenshu Shimada | DePaul University (original) (raw)

Papers by Kenshu Shimada

Research paper thumbnail of Supplementary material for 'Cautionary tales on the use of proxies to estimate body size and form of extinct animals'

Ecology and Evolution, 2024

Research paper thumbnail of Cautionary tales on the use of proxies to estimate body size and form of extinct animals

Ecology and Evolution, 2024

Body size is of fundamental importance to our understanding of extinct organisms. Physiology, eco... more 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 community composition and faunal turnover in past ecosystems and broad macroevolutionary trends. Many extinct animals are known only from incomplete remains, necessitating the use of anatomical proxies to reconstruct body size and form. Numerous limitations affecting the appropriateness of these proxies are often overlooked, leading to controversy and downstream inaccuracies in studies for which reconstructions represent key input data. In this perspective, we discuss four prominent case studies (Dunkleosteus, Helicoprion, Megalodon and Perucetus) in which proxy taxa have been used to estimate body size and shape from fragmentary remains. We synthesise the results of these and other studies to discuss nuances affecting the validity of taxon selection when reconstructing extinct organisms, as well as mitigation measures that can ensure the selection of the most appropriate proxy. We argue that these precautionary measures are necessary to maximise the robustness of reconstructions in extinct taxa for better evolutionary and ecological
inferences.

Research paper thumbnail of Figure 3 In The Extinct River Shark Glyphis Pagoda From The Miocene Of Myanmar And A Review Of The Fossil Record Of The Genus Glyphis (Carcharhiniformes: Carcharhinidae)

FIGURE 3. Dentition of extant Glyphis (A) compared with selected teeth of G. pagoda from the Mioc... more FIGURE 3. Dentition of extant Glyphis (A) compared with selected teeth of G. pagoda from the Miocene of Myanmar (B, C), and three separate morphological ranges in the dentition of Glyphis that alluded the total intraindividual variation of G. pagoda (1‒3). A, upper and lower dental series of extant G. glyphis in labial view (jaw symphysis to the left; after Compagno 1984, p. 509). B, isolated upper teeth of G. pagoda artificially aligned with size adjustment to show approximate correspondence with upper teeth of extant G. glyphis (top half of Fig. 3A): from left to right, NMMP-KU-IR 1522(2) (Fig. 2E), NMMP-KU-IR 1496 (Fig. 2C; image reversed), NMMP-KU-IR 1530(8) (Fig. 2M; image reversed), NMMP-KU-IR 1530(7) (Fig. 2K; image reversed), NMMP-KU-IR 1530(10) (Fig. 2O), NMMP-KU-IR 1530(11) (Fig. 2N). C, isolated lower teeth of G. pagoda artificially aligned with size adjustment to show approximate correspondence with lower teeth of extant G. glyphis (bottom half of Fig. 3A): from left to ...

Research paper thumbnail of Figure 1 In The Extinct River Shark Glyphis Pagoda From The Miocene Of Myanmar And A Review Of The Fossil Record Of The Genus Glyphis (Carcharhiniformes: Carcharhinidae)

FIGURE 1. Maps showing locations where Miocene shark fossils (Fig. 2) were collected. A, Myanmar ... more FIGURE 1. Maps showing locations where Miocene shark fossils (Fig. 2) were collected. A, Myanmar showing four major geological regions divided by dashed lines (1, Sino-Burman Ranges; 2, Central Tertiary Basin; 3, Indo-Burman Ranges; 4, Arakan Coastal Plain) and general area (Magway area; star) where three Miocene fossil localities in this present study are located; B, close-up view of Magway area (star in Fig. 1A) showing three Miocene fossil localities, TeB-PG1 locality (Obogon Formation of Pegu Group) and YS2 and OND localities (Irrawaddy sediments).

Research paper thumbnail of The Late Cretaceous Lamniform Shark, Serratolamna Serrata (Agassiz), from the Mooreville Chalk of Alabama

Serratolamna serrata (Agassiz) is a Late Cretaceous lamniform shark typically found in Maastricht... more Serratolamna serrata (Agassiz) is a Late Cretaceous lamniform shark typically found in Maastrichtian marine deposits. Here we describe seven isolated teeth of S. serrata from the Lower Campanian portion of the Mooreville Chalk in Alabama, and this report constitutes the first documentation of the species from this stratigraphic unit. The described teeth are significant because they represent the oldest (ca. 80 Ma) record of S. serrata in the world, and possibly suggest a North American origin for the taxon. Serratolamna serrata may have evolved from another lamniform, Cretalamna appendiculata (Agassiz).

Research paper thumbnail of The Extinction of Iconic Megatoothed Shark Otodus megalodon : Preliminary Evidence from ‘Clumped’ Isotope Thermometry

Research paper thumbnail of The relationship between the tooth size and total body length in the common thresher shark, Alopias vulpinus (Lamniformes: Alopiidae)

The relationship between the height of tooth crown (CH) and total body length (TL) in the modern ... more The relationship between the height of tooth crown (CH) and total body length (TL) in the modern common thresher shark, Alopias vulpinus (Bonnaterre) (Lamniformes : Alopiidae), is examined using regression analyses. The results suggest : 1) that an increase in the CH of each tooth through replacement is proportional to the increase in the TL, 2) that the CH can be used to predict the TL, and 3) that distally located teeth develop faster through replacement compared to mesially located teeth. The regression equations presented here can be used to estimate the TL of fossil Alopias .

Research paper thumbnail of Dentition of the modern basking shark, Cetorhinus maximus(Lamniformes: Cetorhinidae), and its paleontological and evolutionary implications

Research paper thumbnail of Associated skeletal and dental remains of a fossil odontaspidid shark (Elasmobranchii: Lamniformes)from the Middle Eocene Lillebælt Clay Formation in Denmark

Skeletal and dental remains of an odontaspidid shark from the Eocene Lillebælt Clay found within ... more Skeletal and dental remains of an odontaspidid shark from the Eocene Lillebælt Clay found within a volume of approximately 1.5 × 1.5 m × 2 m. They are all but one tooth interpreted to come from a single individual shark because of identical morphological characteristics and preservation among the vertebrae and teeth found within the small area at a single stratigraphic horizon. The fossil assemblage is now housed in the Natural History Museum of Denmark under the catalogue number DK541 after it was declared ‘Danekræ’ (Christensen & Hald 1991) in 2008. Associated remains of sharks are rare in the fossil record due to the cartilaginous nature of their skeleton. The fossil assemblage that largely constitutes a single shark individual is thus significant. The aim of this paper is to describe the fossil shark and to discuss its palaeobiological significance along with another shark that co-occurred as an isolated tooth. The anatomical terminology used here largely follows Ridewood (1921)...

Research paper thumbnail of Identity of a small symmetrical teeth of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli, from western Kansas, U.S.A

Research paper thumbnail of The relationship between the tooth size and total body length in the shortfin mako, Isurus oxyrinchus(Lamniformes: Laminidae)

Research paper thumbnail of A new pycnodont specimen (Actinopterygii: Pycnodontiformes) from the Upper Cretaceous of Big Bend National Park, Texas, USA, confirming the bony fish genus Macropycnodon as a junior synonym of Acrotemnus

Pycnodonts are an extinct group of bony fishes with crushing dentitions suited to feed on shelled... more 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 vomerine and both prearticular tooth plates was determined to taxonomically belong to the genus Macropycnodon, or possibly conspecific with M. streckeri (Hibbard) that is previously known only from a vomerine tooth plate. The complete set of dentition allowed us to support a previous study suggesting that the genus Macropycnodon is a junior synonym of Acrotemnus Agassiz, where the genus is now known from at least four species confined to the Turonian: 1) A. faba (UK); 2) A. streckeri (USA); 3) A. megafrendodon (USA); and 4) an unnamed species of Acrotemnus (Nigeria). The new specimen from Texas is here conservatively referred to as Acrotemn...

Research paper thumbnail of Types of tooth sets in the fossil record of sharks, and comments on reconstructing dentitions of extinct sharks

Reconstructing shark dentitions, using one or more sets of fossil teeth, has become a recent tren... more Reconstructing shark dentitions, using one or more sets of fossil teeth, has become a recent trend in shark paleontology. If a tooth set is defined as more than one tooth belonging to a single taxon, three major preservation types of tooth sets are recognized in the fossil record of sharks : isolated, associated, and semiassociated tooth sets. An isolated tooth set consists of isolated teeth and may represent more than one individual shark. An associated tooth set is a tooth set referable to one individual shark, and it can be either an articulated tooth set or disarticulated tooth set. A semiassociated tooth set consists of one or more disarticulated tooth sets and/or an isolated tooth set, and it includes two subtypes, feed-shed tooth set and coprolitic tooth set. A feed-shed tooth set consists of an assemblage of fallen teeth formed presumably during feeding, and a coprolitic tooth set represents teeth enclosed in a coprolitic matter. Complex taphonomic processes, including both ...

Research paper thumbnail of Body forms in sharks (Chondrichthyes: Elasmobranchii) and their functional, ecological, and evolutionary implications

Research paper thumbnail of The fossil record of extant elasmobranchs

Journal of Fish Biology

Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various ... more Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various anthropogenic pressures. The abundant fossil record of fossil taxa has allowed to trace the evolutionary history of modern elasmobranchs to at least 250 million years ago; however, exactly how far back the fossil record of living taxa goes has never been collectively surveyed. Here, we assess the representation and extent of the fossil record of elasmobranchs living in our oceans today by collecting their oldest records and quantifying First Appearance Dates (FADs) at different taxonomic levels (i.e., orders, families, genera and species), ecological traits (e.g., body size, habitat and feeding mechanism) and extinction risk (i.e., threatened, not threatened and data deficient). Our results confirm the robust representation of higher taxonomic ranks, with all orders, most families and over half of the extant genera having a fossil record. Further, they reveal that 10% of the current global species diversity is represented in the geological past. Sharks are better represented and extend deeper in time than rays and skates. While the fossil record of extant genera (e.g., the six gill sharks, Hexanchus) goes as far back as ~190 Ma, the fossil record of extant species (e.g., the sand shark, Carcharias taurus Rafinesque 1810) extends ~66 Ma. Although no significant differences were found in the extent of the fossil record between ecological traits, we found that species currently threatened have a significantly older fossil record than not threatened species. Our study demonstrates that the fossil record of extant elasmobranchs extends deep into the geologic time, especially in the case of threatened sharks. As such, the elasmobranch geological history has great potential to advance our understanding of how species currently facing extinction have responded to different stressors in the past, thereby providing a deep-time perspective to conservation. This article is protected by copyright. All rights reserved.

Research paper thumbnail of Body, jaw, and dentition lengths of macrophagous lamniform sharks, and body size evolution in Lamniformes with special reference to ‘off-the-scale’ gigantism of the megatooth shark, Otodus megalodon

Historical Biology

ABSTRACT Extinct lamniform sharks (Elasmobranchii: Lamniformes) are well represented in the late ... more ABSTRACT 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 would allow estimations of body, jaw, and dentition lengths of extinct macrophagous lamniforms from their teeth. These quantitative functions enable us to examine the body size distribution of all known macrophagous lamniform genera over geologic time. Our study reveals that small body size is plesiomorphic for Lamniformes. There are four genera that included at least one member that reached >6 m during both the Mesozoic and Cenozoic, most of which are endothermic. The largest form of the genus Otodus, O. megalodon (‘megatooth shark’) that reached at least 14 m, is truly an outlier considering that all other known macrophagous lamniforms have a general size limit of 7 m. Endothermy has previously been proposed to be the evolutionary driver for gigantism in Lamniformes. However, we contend that ovoviviparous reproduction involving intrauterine cannibalism, a possible synapomorphy of Lamniformes, to be another plausible driver for the evolution of endothermy achieved by certain lamniform taxa.

Research paper thumbnail of Integument-based inferences on the swimming ability and prey hunting strategy of the bigeye thresher shark, Alopias superciliosus (Lamniformes: Alopiidae)

Zoomorphology

Scanning Electron Microscope. Without her amazing patience, I would not have had the beautiful SE... more Scanning Electron Microscope. Without her amazing patience, I would not have had the beautiful SEM images displayed throughout this thesis. Another special thanks to the Northwestern Mouse Histology and Phenotyping Lab, especially G. E. Lord, who provided me with shark histology slides, something well out of the realm of mouse samples. Again, thanks to my committee members Drs. W. E. Aguirre and T. C. Sparkes as well as my thesis advisor, Dr. K. Shimada. Another special thanks to my graduate cohort and Shedd Aquarium coworkers, whose close kinship continued to push me to complete the project and always helped me pursue my goals and aspirations. They all provided me with advice from their fields of expertise and supported me through the entirety of this project.

Research paper thumbnail of A new large Late Cretaceous lamniform shark from North America, with comments on the taxonomy, paleoecology, and evolution of the genusCretodus

Journal of Vertebrate Paleontology

We describe a partial skeleton of the Late Cretaceous shark, Cretodus, collected from the Blue Hi... more 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 vertebral morphology affirms its placement into Lamniformes. With a strong tendency towards monognathic heterodonty, the dental morphology indicates that the specimen belongs to a new species, C. houghtonorum, sp. nov., increasing the total known species of Cretodus to five. The five species can be divided into three distinct groups: the longiplicatus/semiplicatus-grade, gigantea/houghtonorumgrade, and crassidens-grade. Cretodus, that successively evolved by broadening the tooth crown. The individual of C. houghtonorum, sp. nov., is estimated to be about 515 cm in total length (TL). Our vertebra-based growth analysis suggests that the shark was about 118 cm TL at birth and that the species had an estimated maximum growth length of 684 cm TL. The large size at birth indicates that the intrauterine cannibalism behavior of embryos seen in extant lamniforms had already evolved by the Late Cretaceous. Where C. houghtonorum, sp. nov., preferred nearshore environments, the specimen cooccurred with isolated teeth of Squalicorax and fragments of two dorsal fin spines of a hybodont shark, circumstantially indicating that the individual of Cretodus fed on the much smaller hybodont and was scavenged by Squalicorax.

Research paper thumbnail of The Late Cretaceous Chimaeroid Fish,Ischyodus bifurcatusCase (Chondrichthyes: Holocephali), from California, USA, and Its Paleobiogeographical Significance

Paleontological Research

Abstract. A nearly complete right mandibular tooth plate of Ischyodus bifurcatus Case (Holocephal... more Abstract. 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 reported from marine rocks deposited in epicontinental seas and continental shelf paleoenvironments of temperate latitudes in the Northern Hemisphere. Previous records of the species consist of specimens from Santonian to Maastrichtian strata of the US (Delaware, New Jersey, North Carolina, Georgia, Alabama, Arkansas, Montana, and Wyoming), Sweden, and European Russia. The tooth plate described herein is the first verifiable record of I. bifurcatus from California, and, more significantly, represents the only known definite Mesozoic record of Ischyodus from the entire North Pacific region.

Research paper thumbnail of The size of the megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), revisited

Historical Biology

Otodus megalodon (Lamniformes: Otodontidae) is a gigantic late Neogene shark that lived nearly wo... more 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 length (TL). In this study, I reexamined the ontogenetic development of teeth and the quantitative relationships between TL and the crown height of anterior teeth in the extant white shark, Carcharodon carcharias, with an ultimate goal to clarify the current state of the scientific understanding of the body size of O. megalodon. My study suggests that the use of upper anterior teeth is much preferred over lower anterior teeth when estimating TL. This study combined with previous studies suggests that the presently verifiable maximum TL estimates of O. megalodon (i.e. scientifically justifiable account based on museum specimens) are 14.2-15.3 m TL, where individuals of O. megalodon exceeding 15 m TL must have been exceptionally rare. Nevertheless, O. megalodon can still be regarded as one of the largest carnivores, if not superpredators, that have ever lived on Earth.

Research paper thumbnail of Supplementary material for 'Cautionary tales on the use of proxies to estimate body size and form of extinct animals'

Ecology and Evolution, 2024

Research paper thumbnail of Cautionary tales on the use of proxies to estimate body size and form of extinct animals

Ecology and Evolution, 2024

Body size is of fundamental importance to our understanding of extinct organisms. Physiology, eco... more 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 community composition and faunal turnover in past ecosystems and broad macroevolutionary trends. Many extinct animals are known only from incomplete remains, necessitating the use of anatomical proxies to reconstruct body size and form. Numerous limitations affecting the appropriateness of these proxies are often overlooked, leading to controversy and downstream inaccuracies in studies for which reconstructions represent key input data. In this perspective, we discuss four prominent case studies (Dunkleosteus, Helicoprion, Megalodon and Perucetus) in which proxy taxa have been used to estimate body size and shape from fragmentary remains. We synthesise the results of these and other studies to discuss nuances affecting the validity of taxon selection when reconstructing extinct organisms, as well as mitigation measures that can ensure the selection of the most appropriate proxy. We argue that these precautionary measures are necessary to maximise the robustness of reconstructions in extinct taxa for better evolutionary and ecological
inferences.

Research paper thumbnail of Figure 3 In The Extinct River Shark Glyphis Pagoda From The Miocene Of Myanmar And A Review Of The Fossil Record Of The Genus Glyphis (Carcharhiniformes: Carcharhinidae)

FIGURE 3. Dentition of extant Glyphis (A) compared with selected teeth of G. pagoda from the Mioc... more FIGURE 3. Dentition of extant Glyphis (A) compared with selected teeth of G. pagoda from the Miocene of Myanmar (B, C), and three separate morphological ranges in the dentition of Glyphis that alluded the total intraindividual variation of G. pagoda (1‒3). A, upper and lower dental series of extant G. glyphis in labial view (jaw symphysis to the left; after Compagno 1984, p. 509). B, isolated upper teeth of G. pagoda artificially aligned with size adjustment to show approximate correspondence with upper teeth of extant G. glyphis (top half of Fig. 3A): from left to right, NMMP-KU-IR 1522(2) (Fig. 2E), NMMP-KU-IR 1496 (Fig. 2C; image reversed), NMMP-KU-IR 1530(8) (Fig. 2M; image reversed), NMMP-KU-IR 1530(7) (Fig. 2K; image reversed), NMMP-KU-IR 1530(10) (Fig. 2O), NMMP-KU-IR 1530(11) (Fig. 2N). C, isolated lower teeth of G. pagoda artificially aligned with size adjustment to show approximate correspondence with lower teeth of extant G. glyphis (bottom half of Fig. 3A): from left to ...

Research paper thumbnail of Figure 1 In The Extinct River Shark Glyphis Pagoda From The Miocene Of Myanmar And A Review Of The Fossil Record Of The Genus Glyphis (Carcharhiniformes: Carcharhinidae)

FIGURE 1. Maps showing locations where Miocene shark fossils (Fig. 2) were collected. A, Myanmar ... more FIGURE 1. Maps showing locations where Miocene shark fossils (Fig. 2) were collected. A, Myanmar showing four major geological regions divided by dashed lines (1, Sino-Burman Ranges; 2, Central Tertiary Basin; 3, Indo-Burman Ranges; 4, Arakan Coastal Plain) and general area (Magway area; star) where three Miocene fossil localities in this present study are located; B, close-up view of Magway area (star in Fig. 1A) showing three Miocene fossil localities, TeB-PG1 locality (Obogon Formation of Pegu Group) and YS2 and OND localities (Irrawaddy sediments).

Research paper thumbnail of The Late Cretaceous Lamniform Shark, Serratolamna Serrata (Agassiz), from the Mooreville Chalk of Alabama

Serratolamna serrata (Agassiz) is a Late Cretaceous lamniform shark typically found in Maastricht... more Serratolamna serrata (Agassiz) is a Late Cretaceous lamniform shark typically found in Maastrichtian marine deposits. Here we describe seven isolated teeth of S. serrata from the Lower Campanian portion of the Mooreville Chalk in Alabama, and this report constitutes the first documentation of the species from this stratigraphic unit. The described teeth are significant because they represent the oldest (ca. 80 Ma) record of S. serrata in the world, and possibly suggest a North American origin for the taxon. Serratolamna serrata may have evolved from another lamniform, Cretalamna appendiculata (Agassiz).

Research paper thumbnail of The Extinction of Iconic Megatoothed Shark Otodus megalodon : Preliminary Evidence from ‘Clumped’ Isotope Thermometry

Research paper thumbnail of The relationship between the tooth size and total body length in the common thresher shark, Alopias vulpinus (Lamniformes: Alopiidae)

The relationship between the height of tooth crown (CH) and total body length (TL) in the modern ... more The relationship between the height of tooth crown (CH) and total body length (TL) in the modern common thresher shark, Alopias vulpinus (Bonnaterre) (Lamniformes : Alopiidae), is examined using regression analyses. The results suggest : 1) that an increase in the CH of each tooth through replacement is proportional to the increase in the TL, 2) that the CH can be used to predict the TL, and 3) that distally located teeth develop faster through replacement compared to mesially located teeth. The regression equations presented here can be used to estimate the TL of fossil Alopias .

Research paper thumbnail of Dentition of the modern basking shark, Cetorhinus maximus(Lamniformes: Cetorhinidae), and its paleontological and evolutionary implications

Research paper thumbnail of Associated skeletal and dental remains of a fossil odontaspidid shark (Elasmobranchii: Lamniformes)from the Middle Eocene Lillebælt Clay Formation in Denmark

Skeletal and dental remains of an odontaspidid shark from the Eocene Lillebælt Clay found within ... more Skeletal and dental remains of an odontaspidid shark from the Eocene Lillebælt Clay found within a volume of approximately 1.5 × 1.5 m × 2 m. They are all but one tooth interpreted to come from a single individual shark because of identical morphological characteristics and preservation among the vertebrae and teeth found within the small area at a single stratigraphic horizon. The fossil assemblage is now housed in the Natural History Museum of Denmark under the catalogue number DK541 after it was declared ‘Danekræ’ (Christensen & Hald 1991) in 2008. Associated remains of sharks are rare in the fossil record due to the cartilaginous nature of their skeleton. The fossil assemblage that largely constitutes a single shark individual is thus significant. The aim of this paper is to describe the fossil shark and to discuss its palaeobiological significance along with another shark that co-occurred as an isolated tooth. The anatomical terminology used here largely follows Ridewood (1921)...

Research paper thumbnail of Identity of a small symmetrical teeth of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli, from western Kansas, U.S.A

Research paper thumbnail of The relationship between the tooth size and total body length in the shortfin mako, Isurus oxyrinchus(Lamniformes: Laminidae)

Research paper thumbnail of A new pycnodont specimen (Actinopterygii: Pycnodontiformes) from the Upper Cretaceous of Big Bend National Park, Texas, USA, confirming the bony fish genus Macropycnodon as a junior synonym of Acrotemnus

Pycnodonts are an extinct group of bony fishes with crushing dentitions suited to feed on shelled... more 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 vomerine and both prearticular tooth plates was determined to taxonomically belong to the genus Macropycnodon, or possibly conspecific with M. streckeri (Hibbard) that is previously known only from a vomerine tooth plate. The complete set of dentition allowed us to support a previous study suggesting that the genus Macropycnodon is a junior synonym of Acrotemnus Agassiz, where the genus is now known from at least four species confined to the Turonian: 1) A. faba (UK); 2) A. streckeri (USA); 3) A. megafrendodon (USA); and 4) an unnamed species of Acrotemnus (Nigeria). The new specimen from Texas is here conservatively referred to as Acrotemn...

Research paper thumbnail of Types of tooth sets in the fossil record of sharks, and comments on reconstructing dentitions of extinct sharks

Reconstructing shark dentitions, using one or more sets of fossil teeth, has become a recent tren... more Reconstructing shark dentitions, using one or more sets of fossil teeth, has become a recent trend in shark paleontology. If a tooth set is defined as more than one tooth belonging to a single taxon, three major preservation types of tooth sets are recognized in the fossil record of sharks : isolated, associated, and semiassociated tooth sets. An isolated tooth set consists of isolated teeth and may represent more than one individual shark. An associated tooth set is a tooth set referable to one individual shark, and it can be either an articulated tooth set or disarticulated tooth set. A semiassociated tooth set consists of one or more disarticulated tooth sets and/or an isolated tooth set, and it includes two subtypes, feed-shed tooth set and coprolitic tooth set. A feed-shed tooth set consists of an assemblage of fallen teeth formed presumably during feeding, and a coprolitic tooth set represents teeth enclosed in a coprolitic matter. Complex taphonomic processes, including both ...

Research paper thumbnail of Body forms in sharks (Chondrichthyes: Elasmobranchii) and their functional, ecological, and evolutionary implications

Research paper thumbnail of The fossil record of extant elasmobranchs

Journal of Fish Biology

Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various ... more Sharks and their relatives (Elasmobranchii) are highly threatened with extinction due to various anthropogenic pressures. The abundant fossil record of fossil taxa has allowed to trace the evolutionary history of modern elasmobranchs to at least 250 million years ago; however, exactly how far back the fossil record of living taxa goes has never been collectively surveyed. Here, we assess the representation and extent of the fossil record of elasmobranchs living in our oceans today by collecting their oldest records and quantifying First Appearance Dates (FADs) at different taxonomic levels (i.e., orders, families, genera and species), ecological traits (e.g., body size, habitat and feeding mechanism) and extinction risk (i.e., threatened, not threatened and data deficient). Our results confirm the robust representation of higher taxonomic ranks, with all orders, most families and over half of the extant genera having a fossil record. Further, they reveal that 10% of the current global species diversity is represented in the geological past. Sharks are better represented and extend deeper in time than rays and skates. While the fossil record of extant genera (e.g., the six gill sharks, Hexanchus) goes as far back as ~190 Ma, the fossil record of extant species (e.g., the sand shark, Carcharias taurus Rafinesque 1810) extends ~66 Ma. Although no significant differences were found in the extent of the fossil record between ecological traits, we found that species currently threatened have a significantly older fossil record than not threatened species. Our study demonstrates that the fossil record of extant elasmobranchs extends deep into the geologic time, especially in the case of threatened sharks. As such, the elasmobranch geological history has great potential to advance our understanding of how species currently facing extinction have responded to different stressors in the past, thereby providing a deep-time perspective to conservation. This article is protected by copyright. All rights reserved.

Research paper thumbnail of Body, jaw, and dentition lengths of macrophagous lamniform sharks, and body size evolution in Lamniformes with special reference to ‘off-the-scale’ gigantism of the megatooth shark, Otodus megalodon

Historical Biology

ABSTRACT Extinct lamniform sharks (Elasmobranchii: Lamniformes) are well represented in the late ... more ABSTRACT 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 would allow estimations of body, jaw, and dentition lengths of extinct macrophagous lamniforms from their teeth. These quantitative functions enable us to examine the body size distribution of all known macrophagous lamniform genera over geologic time. Our study reveals that small body size is plesiomorphic for Lamniformes. There are four genera that included at least one member that reached >6 m during both the Mesozoic and Cenozoic, most of which are endothermic. The largest form of the genus Otodus, O. megalodon (‘megatooth shark’) that reached at least 14 m, is truly an outlier considering that all other known macrophagous lamniforms have a general size limit of 7 m. Endothermy has previously been proposed to be the evolutionary driver for gigantism in Lamniformes. However, we contend that ovoviviparous reproduction involving intrauterine cannibalism, a possible synapomorphy of Lamniformes, to be another plausible driver for the evolution of endothermy achieved by certain lamniform taxa.

Research paper thumbnail of Integument-based inferences on the swimming ability and prey hunting strategy of the bigeye thresher shark, Alopias superciliosus (Lamniformes: Alopiidae)

Zoomorphology

Scanning Electron Microscope. Without her amazing patience, I would not have had the beautiful SE... more Scanning Electron Microscope. Without her amazing patience, I would not have had the beautiful SEM images displayed throughout this thesis. Another special thanks to the Northwestern Mouse Histology and Phenotyping Lab, especially G. E. Lord, who provided me with shark histology slides, something well out of the realm of mouse samples. Again, thanks to my committee members Drs. W. E. Aguirre and T. C. Sparkes as well as my thesis advisor, Dr. K. Shimada. Another special thanks to my graduate cohort and Shedd Aquarium coworkers, whose close kinship continued to push me to complete the project and always helped me pursue my goals and aspirations. They all provided me with advice from their fields of expertise and supported me through the entirety of this project.

Research paper thumbnail of A new large Late Cretaceous lamniform shark from North America, with comments on the taxonomy, paleoecology, and evolution of the genusCretodus

Journal of Vertebrate Paleontology

We describe a partial skeleton of the Late Cretaceous shark, Cretodus, collected from the Blue Hi... more 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 vertebral morphology affirms its placement into Lamniformes. With a strong tendency towards monognathic heterodonty, the dental morphology indicates that the specimen belongs to a new species, C. houghtonorum, sp. nov., increasing the total known species of Cretodus to five. The five species can be divided into three distinct groups: the longiplicatus/semiplicatus-grade, gigantea/houghtonorumgrade, and crassidens-grade. Cretodus, that successively evolved by broadening the tooth crown. The individual of C. houghtonorum, sp. nov., is estimated to be about 515 cm in total length (TL). Our vertebra-based growth analysis suggests that the shark was about 118 cm TL at birth and that the species had an estimated maximum growth length of 684 cm TL. The large size at birth indicates that the intrauterine cannibalism behavior of embryos seen in extant lamniforms had already evolved by the Late Cretaceous. Where C. houghtonorum, sp. nov., preferred nearshore environments, the specimen cooccurred with isolated teeth of Squalicorax and fragments of two dorsal fin spines of a hybodont shark, circumstantially indicating that the individual of Cretodus fed on the much smaller hybodont and was scavenged by Squalicorax.

Research paper thumbnail of The Late Cretaceous Chimaeroid Fish,Ischyodus bifurcatusCase (Chondrichthyes: Holocephali), from California, USA, and Its Paleobiogeographical Significance

Paleontological Research

Abstract. A nearly complete right mandibular tooth plate of Ischyodus bifurcatus Case (Holocephal... more Abstract. 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 reported from marine rocks deposited in epicontinental seas and continental shelf paleoenvironments of temperate latitudes in the Northern Hemisphere. Previous records of the species consist of specimens from Santonian to Maastrichtian strata of the US (Delaware, New Jersey, North Carolina, Georgia, Alabama, Arkansas, Montana, and Wyoming), Sweden, and European Russia. The tooth plate described herein is the first verifiable record of I. bifurcatus from California, and, more significantly, represents the only known definite Mesozoic record of Ischyodus from the entire North Pacific region.

Research paper thumbnail of The size of the megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), revisited

Historical Biology

Otodus megalodon (Lamniformes: Otodontidae) is a gigantic late Neogene shark that lived nearly wo... more 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 length (TL). In this study, I reexamined the ontogenetic development of teeth and the quantitative relationships between TL and the crown height of anterior teeth in the extant white shark, Carcharodon carcharias, with an ultimate goal to clarify the current state of the scientific understanding of the body size of O. megalodon. My study suggests that the use of upper anterior teeth is much preferred over lower anterior teeth when estimating TL. This study combined with previous studies suggests that the presently verifiable maximum TL estimates of O. megalodon (i.e. scientifically justifiable account based on museum specimens) are 14.2-15.3 m TL, where individuals of O. megalodon exceeding 15 m TL must have been exceptionally rare. Nevertheless, O. megalodon can still be regarded as one of the largest carnivores, if not superpredators, that have ever lived on Earth.