Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic an... more Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic and phylogenetic purposes under the assumption that differences in the number of teeth are largely explained by interspecific variation. Although phylogeny is almost certainly one of the greatest factors influencing tooth count, the relative role of intraspecific variation is difficult, and often impossible, to test in the fossil record given the sample sizes available to palaeontologists and, as such, is best investigated using extant models. Intraspecific variation (largely manifested as size-related or ontogenetic variation) in tooth counts has been examined in extant squamates (lizards and snakes) but is poorly understood in archosaurs (crocodylians and dinosaurs). Here, we document tooth count variation in two species of extant crocodylians (Alligator mississippiensis and Crocodylus porosus) as well as a large varanid lizard (Varanus komodoensis). We test the hypothesis that variation...
The holotype of Saurornitholestes robustus (SMP VP-1955) from the upper Kirtland Formation (De-na... more The holotype of Saurornitholestes robustus (SMP VP-1955) from the upper Kirtland Formation (De-na-zin Member), originally identified as a dromaeosaurid, is here re-identified as an indeterminate troodontid theropod. The frontal has no diagnostic dromaeosaurid characters, but is shown to have several features unique to troodontids among deinonychosaurs, including the shallow lateral wall defining the fossae for the olfactory system, the exclusion of the supratemporal fossa from the dorsal surface of the frontal, and a raised orbital rim. The size and morphology of SMP VP-1955 is also consistent with better-known troodontid material from the late Campanian of Alberta, and as preserved, the Kirtland specimen is indistinguishable from comparable Alberta material. The revised identification of SMP VP-1955 provides the first non-dental skeletal record of a troodontid from the Kirtland Formation, and a rare record of this clade from the Kirtlandian land-vertebrate age.
Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known la... more Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known largely from isolated teeth, which have typically been referred to taxa based on more complete material from earlier Campanian strata. An almost complete maxilla with well-preserved dentition and an associated dentary from the Hell Creek Formation of Montana are used to establish a new dromaeosaurid taxon in the latest Maastrichtian, immediately prior to the end-Cretaceous extinction event. Acheroraptor temertyorum gen. et sp. nov. is differentiated from other dromaeosaurids on the basis of a hypertrophied postantral wall that projects posteriorly into the antorbital fenestra, a maxillary fenestra positioned low in the antorbital fossa and directly posterior to the promaxillary fenestra, and distinctive dentition with marked apicobasal ridges. The new material allows a dromaeosaurid from the Maastrichtian of North America to be placed within a phylogenetic framework for the first time. Phylogenetic analysis suggests Acheroraptor is a velociraptorine that is more closely related to Asian dromaeosaurids, including Tsaagan and Velociraptor, than it is to Dromaeosaurus, Saurornitholestes, or any other taxon from North America. As part of the Lancian Tyrannosaurus–Triceratops fauna, A. temertyorum is the latest occurring dromaeosaurid. Its relationships and occurrence suggest a complex historical biogeographic scenario that involved multiple, bi-directional faunal interchanges between Asia and North America during the Late Cretaceous.
Taphonomic biases dictate how organisms are represented in the fossil record, but their effect on... more Taphonomic biases dictate how organisms are represented in the fossil record, but their effect on studies of vertebrate diversity dynamics is poorly studied. In contrast to the high diversity and abundance of small-bodied animals in extant ecosystems, small-bodied dinosaurs are less common than their large-bodied counterparts, but it is unclear whether this reflects unique properties of dinosaurian ecosystems or is due to taphonomic biases.
Isolated small theropod teeth are abundant in vertebrate microfossil assemblages, and are frequen... more Isolated small theropod teeth are abundant in vertebrate microfossil assemblages, and are frequently used in studies of species diversity in ancient ecosystems. However, determining the taxonomic affinities of these teeth is problematic due to an absence of associated diagnostic skeletal material. Species such as Dromaeosaurus albertensis, Richardoestesia gilmorei, and Saurornitholestes langstoni are known from skeletal remains that have been recovered exclusively from the Dinosaur Park Formation (Campanian). It is therefore likely that teeth from different formations widely disparate in age or geographic position are not referable to these species. Tooth taxa without any associated skeletal material, such as Paronychodon lacustris and Richardoestesia isosceles, have also been identified from multiple localities of disparate ages throughout the Late Cretaceous. To address this problem, a dataset of measurements of 1183 small theropod teeth (the most specimen-rich theropod tooth dataset ever constructed) from North America ranging in age from Santonian through Maastrichtian were analyzed using multivariate statistical methods: canonical variate analysis, pairwise discriminant function analysis, and multivariate analysis of variance. The results indicate that teeth referred to the same taxon from different formations are often quantitatively distinct. In contrast, isolated teeth found in time equivalent formations are not quantitatively distinguishable from each other. These results support the hypothesis that small theropod taxa, like other dinosaurs in the Late Cretaceous, tend to be exclusive to discrete host formations. The methods outlined have great potential for future studies of isolated teeth worldwide, and may be the most useful non-destructive technique known of extracting the most data possible from isolated and fragmentary specimens. The ability to accurately assess species diversity and turnover through time based on isolated teeth will help illuminate patterns of evolution and extinction in these groups and potentially others in greater detail than has previously been thought possible without more complete skeletal material.
Morphology and Evolution of Turtles (Brinkman, Holroyd, and Gardner, eds.), May 2012
A new species of Neurankylus (N. lithographicus sp. nov.) is described on the basis of skull and ... more A new species of Neurankylus (N. lithographicus sp. nov.) is described on the basis of skull and shell material from the Santonian-aged Milk River Formation, Alberta, Canada. The genus Neurankylus is also rediagnosed on the basis of the Milk River material and new material pertaining to the type species, N. eximius. N. eximius has previously been considered to represent a long-lived and cosmopolitan taxon. New material, though, provides insight into the range of morphological variation present in these species and indicates that many specimens referred to Neurankylus eximius may be distinct species. The species recognized in this paper have a much more restricted geographical and temporal range than has been suggested previously. Phylogenetic analysis of all known baenid taxa, including all described species of Neurankylus and several basal paracryptodiran taxa of uncertain affinities, was undertaken and sug-
Documenting variation in theropod dinosaurs is usually hindered by the lack of a large sample siz... more Documenting variation in theropod dinosaurs is usually hindered by the lack of a large sample size and specimens representing several ontogenetic stages. Here, variation within 140 disassociated and seven in situ tyrannosaur teeth from the Upper Cretaceous (lower Maastrichtian) monodominant Albertosaurus sarcophagus (Theropoda: Tyrannosauridae) bonebed is documented. This sample represents the largest data set of teeth from one population of A. sarcophagus containing both adult and juvenile specimens. Tooth variation was assessed using multivariate analyses (principal component, discriminant, and canonical variate analyses). Heterodonty in the teeth of A. sarcophagus contributes to the large amount of variation in the data set. Premaxillary teeth are significantly different from maxillary and dentary teeth, but there is no quantifiable difference between a priori identified maxillary and dentary teeth. Juvenile and adult teeth of A. sarcophagus show apparent quantitative differences that are size dependent on closer investigation, suggesting a cautious approach when interpreting multivariate analyses to identify novel tooth morphologies. Multivariate analyses on teeth of A. sarcophagus and published tooth data from other North American tyrannosaurid species reveals species-level clusters with little separation. The degree of separation among tooth clusters may reveal a phylogenetic signal in tyrannosaurid teeth.
Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic an... more Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic and phylogenetic purposes under the assumption that differences in the number of teeth are largely explained by interspecific variation. Although phylogeny is almost certainly one of the greatest factors influencing tooth count, the relative role of intraspecific variation is difficult, and often impossible, to test in the fossil record given the sample sizes available to palaeontologists and, as such, is best investigated using extant models. Intraspecific variation (largely manifested as size-related or ontogenetic variation) in tooth counts has been examined in extant squamates (lizards and snakes) but is poorly understood in archosaurs (crocodylians and dinosaurs). Here, we document tooth count variation in two species of extant crocodylians (Alligator mississippiensis and Crocodylus porosus) as well as a large varanid lizard (Varanus komodoensis). We test the hypothesis that variation...
The holotype of Saurornitholestes robustus (SMP VP-1955) from the upper Kirtland Formation (De-na... more The holotype of Saurornitholestes robustus (SMP VP-1955) from the upper Kirtland Formation (De-na-zin Member), originally identified as a dromaeosaurid, is here re-identified as an indeterminate troodontid theropod. The frontal has no diagnostic dromaeosaurid characters, but is shown to have several features unique to troodontids among deinonychosaurs, including the shallow lateral wall defining the fossae for the olfactory system, the exclusion of the supratemporal fossa from the dorsal surface of the frontal, and a raised orbital rim. The size and morphology of SMP VP-1955 is also consistent with better-known troodontid material from the late Campanian of Alberta, and as preserved, the Kirtland specimen is indistinguishable from comparable Alberta material. The revised identification of SMP VP-1955 provides the first non-dental skeletal record of a troodontid from the Kirtland Formation, and a rare record of this clade from the Kirtlandian land-vertebrate age.
Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known la... more Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known largely from isolated teeth, which have typically been referred to taxa based on more complete material from earlier Campanian strata. An almost complete maxilla with well-preserved dentition and an associated dentary from the Hell Creek Formation of Montana are used to establish a new dromaeosaurid taxon in the latest Maastrichtian, immediately prior to the end-Cretaceous extinction event. Acheroraptor temertyorum gen. et sp. nov. is differentiated from other dromaeosaurids on the basis of a hypertrophied postantral wall that projects posteriorly into the antorbital fenestra, a maxillary fenestra positioned low in the antorbital fossa and directly posterior to the promaxillary fenestra, and distinctive dentition with marked apicobasal ridges. The new material allows a dromaeosaurid from the Maastrichtian of North America to be placed within a phylogenetic framework for the first time. Phylogenetic analysis suggests Acheroraptor is a velociraptorine that is more closely related to Asian dromaeosaurids, including Tsaagan and Velociraptor, than it is to Dromaeosaurus, Saurornitholestes, or any other taxon from North America. As part of the Lancian Tyrannosaurus–Triceratops fauna, A. temertyorum is the latest occurring dromaeosaurid. Its relationships and occurrence suggest a complex historical biogeographic scenario that involved multiple, bi-directional faunal interchanges between Asia and North America during the Late Cretaceous.
Taphonomic biases dictate how organisms are represented in the fossil record, but their effect on... more Taphonomic biases dictate how organisms are represented in the fossil record, but their effect on studies of vertebrate diversity dynamics is poorly studied. In contrast to the high diversity and abundance of small-bodied animals in extant ecosystems, small-bodied dinosaurs are less common than their large-bodied counterparts, but it is unclear whether this reflects unique properties of dinosaurian ecosystems or is due to taphonomic biases.
Isolated small theropod teeth are abundant in vertebrate microfossil assemblages, and are frequen... more Isolated small theropod teeth are abundant in vertebrate microfossil assemblages, and are frequently used in studies of species diversity in ancient ecosystems. However, determining the taxonomic affinities of these teeth is problematic due to an absence of associated diagnostic skeletal material. Species such as Dromaeosaurus albertensis, Richardoestesia gilmorei, and Saurornitholestes langstoni are known from skeletal remains that have been recovered exclusively from the Dinosaur Park Formation (Campanian). It is therefore likely that teeth from different formations widely disparate in age or geographic position are not referable to these species. Tooth taxa without any associated skeletal material, such as Paronychodon lacustris and Richardoestesia isosceles, have also been identified from multiple localities of disparate ages throughout the Late Cretaceous. To address this problem, a dataset of measurements of 1183 small theropod teeth (the most specimen-rich theropod tooth dataset ever constructed) from North America ranging in age from Santonian through Maastrichtian were analyzed using multivariate statistical methods: canonical variate analysis, pairwise discriminant function analysis, and multivariate analysis of variance. The results indicate that teeth referred to the same taxon from different formations are often quantitatively distinct. In contrast, isolated teeth found in time equivalent formations are not quantitatively distinguishable from each other. These results support the hypothesis that small theropod taxa, like other dinosaurs in the Late Cretaceous, tend to be exclusive to discrete host formations. The methods outlined have great potential for future studies of isolated teeth worldwide, and may be the most useful non-destructive technique known of extracting the most data possible from isolated and fragmentary specimens. The ability to accurately assess species diversity and turnover through time based on isolated teeth will help illuminate patterns of evolution and extinction in these groups and potentially others in greater detail than has previously been thought possible without more complete skeletal material.
Morphology and Evolution of Turtles (Brinkman, Holroyd, and Gardner, eds.), May 2012
A new species of Neurankylus (N. lithographicus sp. nov.) is described on the basis of skull and ... more A new species of Neurankylus (N. lithographicus sp. nov.) is described on the basis of skull and shell material from the Santonian-aged Milk River Formation, Alberta, Canada. The genus Neurankylus is also rediagnosed on the basis of the Milk River material and new material pertaining to the type species, N. eximius. N. eximius has previously been considered to represent a long-lived and cosmopolitan taxon. New material, though, provides insight into the range of morphological variation present in these species and indicates that many specimens referred to Neurankylus eximius may be distinct species. The species recognized in this paper have a much more restricted geographical and temporal range than has been suggested previously. Phylogenetic analysis of all known baenid taxa, including all described species of Neurankylus and several basal paracryptodiran taxa of uncertain affinities, was undertaken and sug-
Documenting variation in theropod dinosaurs is usually hindered by the lack of a large sample siz... more Documenting variation in theropod dinosaurs is usually hindered by the lack of a large sample size and specimens representing several ontogenetic stages. Here, variation within 140 disassociated and seven in situ tyrannosaur teeth from the Upper Cretaceous (lower Maastrichtian) monodominant Albertosaurus sarcophagus (Theropoda: Tyrannosauridae) bonebed is documented. This sample represents the largest data set of teeth from one population of A. sarcophagus containing both adult and juvenile specimens. Tooth variation was assessed using multivariate analyses (principal component, discriminant, and canonical variate analyses). Heterodonty in the teeth of A. sarcophagus contributes to the large amount of variation in the data set. Premaxillary teeth are significantly different from maxillary and dentary teeth, but there is no quantifiable difference between a priori identified maxillary and dentary teeth. Juvenile and adult teeth of A. sarcophagus show apparent quantitative differences that are size dependent on closer investigation, suggesting a cautious approach when interpreting multivariate analyses to identify novel tooth morphologies. Multivariate analyses on teeth of A. sarcophagus and published tooth data from other North American tyrannosaurid species reveals species-level clusters with little separation. The degree of separation among tooth clusters may reveal a phylogenetic signal in tyrannosaurid teeth.
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