Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in th... more Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in the fossil record, yet information on growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we describe the partial skull of a juvenile chasmosaurine, attributed to Arrhinoceratops brachyops, within the context of more mature specimens of this species, to better understand the ontogenetic transformations therein. We show that as A. brachyops matured, the postorbital horncores became longer and shifted from a posterior to an anterior inclination, the deltashaped frill epiossifications became lower and fused to the underlying frill, and the face became more elongate. In these respects, A. brachyops closely resembled Triceratops, suggesting that these ontogenetic changes may have been common to all long-horned chasmosaurines. However, an event-paired cladistic analysis of Chasmosaurinae using a standardized matrix of 24 developmental characters reveals that the relative timing of ontogenetic events in Arrhinoceratops was more like that of Chasmosaurus, particularly in the relatively late reduction in scalloping around the frill margins. Thus, the ontogenetic similarities between Arrhinoceratops and Triceratops appear to be plesiomorphic, partly related to the retention of the elongate postorbital horncores, which are primitive for Ceratopsidae. This study elucidates the otherwise contentious evolutionary relationships of Arrhinoceratops, and highlights the importance of ontogenetic data for resolving phylogenies when morphological data from adults alone are inadequate.
The early collecting history of dinosaurs and other fossil vertebrates in Western Canada during t... more The early collecting history of dinosaurs and other fossil vertebrates in Western Canada during the 1870s and 1880s is poorly documented. Initial finds were made by the British North American Boundary Commission and the Geological Survey of Canada in modern Saskatchewan and Alberta but, beyond a few well-publicized examples, little is known about precisely what was found and where. Much of the collected material is now housed in the collections of the Canadian Museum of Nature in Gatineau, Quebec, and a recent survey of these historic finds allows for the first comprehensive narrative regarding their identity and procurement. The collection is heavily biased towards vertebral centra and phalanges, reflective of both taphonomic and collecting biases. Given current understanding of Upper Cretaceous assemblages of North America, ornithomimids and small theropods are overrepresented, whereas ceratopsids and ankylosaurs are underrepresented. Fossils from the Belly River Group are best re...
Despite strong evidence for sexual selection in various display traits and other exaggerated stru... more Despite strong evidence for sexual selection in various display traits and other exaggerated structures in large extinct reptiles, such as dinosaurs, detecting sexual dimorphism in them remains difficult. Their relatively small sample sizes, long growth periods, and difficulties distinguishing the sexes of fossil specimens mean that there are little compelling data on dimorphism in these animals. The extant gharial (Gavialis gangeticus) is a large and endangered crocodylian that is sexually dimorphic in size, but males also possesses a sexually selected structure, the ghara, which has an osteological correlate in the presence of a fossa associated with the nares. This makes the species a unique model for potentially assessing dimorphism in fossil lineages, such as dinosaurs and pterosaurs, because it is a large, slow-growing, egg-laying archosaur. Here we assess the dimorphism of G. gangeticus across 106 specimens and show that the presence of a narial fossa diagnoses adult male gha...
Ontogeny—the growth and development of an organism—is among the more poorly understood aspects of... more Ontogeny—the growth and development of an organism—is among the more poorly understood aspects of the life history of mosasaurs, largely owing to a dearth of fossil material from young individuals. We describe the partial and complete skulls of two subadult individuals of the mosasaurid Tylosaurus proriger from the upper Smoky Hills Chalk Member of the Niobrara Formation in Kansas. We include the more complete of the two specimens in an allometric analysis to better understand proportional changes of the skull through growth. Although our small sample size produces several instances of ‘soft isometry’, we recover the length of the edentulous rostrum as significantly negatively allometric, and quadrate height as significantly positively allometric. In light of our findings, we go on to consider the question of whether T. kansasensis represents an immature ontogimorph of T. nepaeolicus, but find no convincing evidence that this is the case.
Choristoderes are extinct neodiapsid reptiles that are well known for their unusual cranial anato... more Choristoderes are extinct neodiapsid reptiles that are well known for their unusual cranial anatomy, possessing an elongated snout and expanded temporal arches. Although choristodere skulls are well described externally, their internal anatomy remains unknown. An internal description was needed to shed light on peculiarities of the choristodere skull, such as paired gaps on the ventral surface of the skull that may pertain to the fenestra ovalis, and a putative neomorphic ossification in the lateral wall of the braincase. Our goals were: (i) to describe the cranial elements of Champsosaurus lindoei in three dimensions; (ii) to describe paired gaps on the ventral surface of the skull to determine if these are indeed the fenestrae ovales; (iii) to illustrate the morphology of the putative neomorphic bone; and (iv) to consider the possible developmental and functional origins of the neomorph. We examined the cranial anatomy of the choristodere Champsosaurus lindoei (CMN 8920) using high-resolution micro-computed tomography scanning. We found that the paired gaps on the ventral surface of the skull do pertain to the fenestrae ovales, an unusual arrangement that may be convergent with some plesiosaurs, some aistopods, and some urodeles. The implications of this morphology in Champsosaurus are unknown and will be the subject of future work. We found that the neomorphic bone is a distinct ossification, but is not part of the wall of the brain cavity or the auditory capsule. Variation in the developmental pathways of cranial bones in living amniotes was surveyed to determine how the neomorphic bone may have developed. We found that the chondrocranium and splanchnocranium show little to no variation across amniotes, and the neomorphic bone is therefore most likely to have developed from the dermatocranium; however, the stapes is a pre-existing cranial element that is undescribed in choristoderes and may be homologous with the neomorphic bone. If the neomorphic bone is not homologous with the stapes, the neomorph likely developed from the dermatocranium through incomplete fusion of ossification centres from a pre-existing bone, most likely the parietal. Based on the apparent morphology of the neomorph in Coeruleodraco, the neomorph was probably too small to play a significant structural role in the skull of early choristoderes and it may have arisen through non-adaptive means. In neochoristoderes, such as Champsosaurus, the neomorph was likely recruited to support the expanded temporal arches.
Modern megaherbivore community richness is limited by bottom-up controls, such as resource limita... more Modern megaherbivore community richness is limited by bottom-up controls, such as resource limitation and resultant dietary competition. However, the extent to which these same controls impacted the richness of fossil megaherbivore communities is poorly understood. The present study investigates the matter with reference to the megaherbivorous dinosaur assemblage from the middle to upper Campanian Dinosaur Park Formation of Alberta, Canada. Using a meta-analysis of 21 ecomorphological variables measured across 14 genera, contemporaneous taxa are demonstrably well-separated in ecomorphospace at the family/subfamily level. Moreover, this pattern is persistent through the approximately 1.5 Myr timespan of the formation, despite continual species turnover, indicative of underlying structural principles imposed by long-term ecological competition. After considering the implications of ecomorphology for megaherbivorous dinosaur diet, it is concluded that competition structured comparable ...
Several published censuses have noted the presence of two tyrannosaurids, Daspletosaurus sp. and ... more Several published censuses have noted the presence of two tyrannosaurids, Daspletosaurus sp. and Albertosaurus sarcophagus, within the Upper Cretaceous Horseshoe Canyon Formation of Alberta. Although A. sarcophagus is known from more than a dozen major discoveries in these strata, Daspletosaurus sp. is known from just a single problematic skeleton (lacking most of the skull) of a young individual. Here we describe and figure this skeleton, and marshal a variety of osteohistologic, morphometric, and phylogenetic methods to accurately determine its taxonomic status. Although none of these methods individually provides convincing evidence regarding the affinities of the specimen, together (and including other historical and biostratigraphic considerations) they strongly imply that the skeleton instead pertains to a young A. sarcophagus. In this way, we show that only a single species of tyrannosaurid is definitively present in the Horseshoe Canyon Formation, greatly simplifying interpretations of tyrannosaurid evolution and ecology in this setting. Anat Rec, 00:000-000, 2019.
Ankylosaurus magniventris is an iconic dinosaur species often depicted in popular media. It is kn... more Ankylosaurus magniventris is an iconic dinosaur species often depicted in popular media. It is known from relatively fragmentary remains compared with its earlier and smaller relatives such as Euoplocephalus and Anodontosaurus. Nevertheless, the known fossils of Ankylosaurus indicate that it had diverged significantly in cranial and postcranial anatomy compared with other Laramidian ankylosaurines. In particular, the dentition, narial region, tail club, and overall body size differ substantially from other Campanian–Maastrichtian ankylosaurines. We review the anatomy of this unusual ankylosaur using data from historic and newly identified material and discuss its palaeoecological implications.
We describe a new species of nanhsiungchelyid turtle, Basilemys morrinensis, based on a nearly co... more We describe a new species of nanhsiungchelyid turtle, Basilemys morrinensis, based on a nearly complete shell from the Horsethief Member (lower Maastrichtian) of the Horseshoe Canyon Formation of Alberta. The species is intermediate in age between the Campanian forms B. variolosa and B. gaffneyi and the upper Maastrichtian forms B. sinuosa and B. praeclara. It is also intermediate in its morphology, possessing a unique suite of both plesiomorphic (e.g., divided extragulars) and derived (e.g., square epiplastral beak, pygal wider than long) traits. It is further characterized by an autapomorphic square cervical scale. Phylogenetic analysis assuming parsimony recovers B. morrinensis in a polytomy with B. variolosa and B. gaffneyi, outside the clade formed by the upper Maastrichtian forms B. sinuosa and B. praeclara. The holotype of Basilemys morrinensis provides the first evidence that this genus reached large size (»1 m long) in the Horseshoe Canyon Formation and was not diminutive as previously thought based on less complete shell material. Although Basilemys is usually regarded as terrestrial in habit based on its skull and limb morphology, we note that the low profile of its shell is a derived feature usually indicative of an aquatic mode of life. This suggests that there is yet much to learn about the life habits of this interesting turtle.
Evidence for sexual dimorphism is extremely limited in the non-avian dinosaurs despite their high... more Evidence for sexual dimorphism is extremely limited in the non-avian dinosaurs despite their high diversity and disparity, and despite the fact that dimorphism is very common in vertebrate lineages of all kinds. Using body size data from both Alligator mississippiensis and Rhea americana, which phylogenetically bracket the dinosaurs, we demonstrate that even when there is strong dimorphism in a species, random sampling of populations of individuals characterized by sustained periods of growth (as in the alligator and most dinosaurs) can result in the loss of this signal. Dimorphism may be common in fossil taxa but very hard to detect without ontogenetic age control and large sample sizes, both of which are hampered by the limitations of the fossil record. Signal detection may be further hindered by Type III survivorship, whereby increased mortality among the young favours the likelihood that they will be sampled (unless predation or taphonomic bias acts against this). This hindrance is counteracted by taphonomic bias against small body size. These, and other considerations relating to behavior and ecology, provide powerful reasons to suggest that sexual dimorphism in dinosaurs may be very difficult to detect in almost all currently available samples. Similar issues are likely also applicable to many fossil reptiles, or animals more generally.
The demonstration of sexual dimorphism in the fossil record can provide vital information about t... more The demonstration of sexual dimorphism in the fossil record can provide vital information about the role that sexual selection has played in the evolution of life. However, statistically robust inferences of sexual dimorphism in fossil organisms are exceedingly difficult to establish, owing to issues of sample size, experimental control, and methodology. This is particularly so in the case of dinosaurs, for which sexual dimorphism has been posited in many species, yet quantifiable data are often lacking. This study presents the first statistical investigation of sexual dimorphism across Dinosauria. It revisits prior analyses that purport to find quantitative evidence for sexual dimorphism in nine dinosaur species. After the available morphological data were subjected to a suite of statistical tests (normality and unimodality tests and mixture modeling), no evidence for sexual dimorphism was found in any of the examined taxa, contrary to conventional wisdom. This is not to say that d...
The cranial anatomy of the flat-skulled hadrosaurine Edmontosaurus regalis (Ornithischia: Hadrosa... more The cranial anatomy of the flat-skulled hadrosaurine Edmontosaurus regalis (Ornithischia: Hadrosauridae) is extensively described here, based on the holotype and paratype collected from the middle part of the Horseshoe Canyon Formation in southern Alberta. Focus is given to previously undocumented features of ontogenetic and phylogenetic importance. This description facilitates overall osteological comparisons between E. regalis and other hadrosaurids (especially E. annectens), and revises the diagnosis of E. regalis, to which a new autapomorphy (the dorsal half of the jugal anterior process bearing a sharp posterolateral projection into the orbit) is added. We consider the recently named Ugrunaaluk kuukpikensis from the upper Campanian/lower Maastrichtian of Alaska a nomen dubium, and conservatively regard the Alaskan material as belonging to Edmontosaurus sp.. A phylogenetic analysis of Hadrosauroidea using maximum parsimony further corroborates the sistertaxon relationship between E. regalis and E. annectens. In the strict consensus tree, Hadrosaurus foulkii occurs firmly within the clade comprising all non-lambeosaurine hadrosaurids, supporting the taxonomic scheme that divides Hadrosauridae into Hadrosaurinae and Lambeosaurinae. Within Edmontosaurini, Kerberosaurus is posited as the sister taxon to the clade of Shantungosaurus + Edmontosaurus. The biogeographic reconstruction of Hadrosaurinae in light of the time-calibrated cladogram and probability calculation of ancestral areas for all internal nodes reveals a significantly high probability for the North American origin of the clade. However, the Laramidia-Appalachia dispersals around the Santonian-Campanian boundary, inferred from the biogeographic scenario for the North American origin of Hadrosaurinae, are in conflict with currently accepted paleogeographic models. By contrast, the Asian origin of Hadrosaurinae with its relatively low probability resulting from the biogeographic analysis is worth seriously considering, despite the lack of fossil material from the Santonian and lower Campanian of Asia. Extra fossil collecting in appropriate geographic locations and stratigraphic intervals of Asia and Europe will help to clarify the biogeographic dynamics of hadrosaurine dinosaurs in the near future.
A partial skull (CMN 8804) of a ceratopsid from the upper unit of the Campanian Oldman Formation ... more A partial skull (CMN 8804) of a ceratopsid from the upper unit of the Campanian Oldman Formation of Alberta is the first Canadian example of the newly established Nasutoceratopsini, a new subclade of Centrosaurinae defined as the stem-based clade of centrosaurine ceratopsids more closely related to Nasutoceratops titusi than to Centrosaurus apertus. The new clade is diagnosed, in part, by having a parietosquamosal frill lacking modified epimarginals; a small nasal horncore; large, rostrolaterally directed postorbital horncores; and a relatively short, deep face. Although the CMN 8804 taxon closely resembles Nasutoceratops, its phylogenetic position within Nasutoceratopsini is unresolved. The CMN 8804 taxon would have been contemporaneous with dinosaurs from the lower portion of the Dinosaur Park Formation 200 km to the northwest in Dinosaur Provincial Park. The presence of the CMN 8804 taxon in Alberta, and the approximately contemporaneous Nasutoceratops in Utah, indicates that the...
Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in th... more Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in the fossil record, yet information on growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we describe the partial skull of a juvenile chasmosaurine, attributed to Arrhinoceratops brachyops, within the context of more mature specimens of this species, to better understand the ontogenetic transformations therein. We show that as A. brachyops matured, the postorbital horncores became longer and shifted from a posterior to an anterior inclination, the deltashaped frill epiossifications became lower and fused to the underlying frill, and the face became more elongate. In these respects, A. brachyops closely resembled Triceratops, suggesting that these ontogenetic changes may have been common to all long-horned chasmosaurines. However, an event-paired cladistic analysis of Chasmosaurinae using a standardized matrix of 24 developmental characters reveals that the relative timing of ontogenetic events in Arrhinoceratops was more like that of Chasmosaurus, particularly in the relatively late reduction in scalloping around the frill margins. Thus, the ontogenetic similarities between Arrhinoceratops and Triceratops appear to be plesiomorphic, partly related to the retention of the elongate postorbital horncores, which are primitive for Ceratopsidae. This study elucidates the otherwise contentious evolutionary relationships of Arrhinoceratops, and highlights the importance of ontogenetic data for resolving phylogenies when morphological data from adults alone are inadequate.
The early collecting history of dinosaurs and other fossil vertebrates in Western Canada during t... more The early collecting history of dinosaurs and other fossil vertebrates in Western Canada during the 1870s and 1880s is poorly documented. Initial finds were made by the British North American Boundary Commission and the Geological Survey of Canada in modern Saskatchewan and Alberta but, beyond a few well-publicized examples, little is known about precisely what was found and where. Much of the collected material is now housed in the collections of the Canadian Museum of Nature in Gatineau, Quebec, and a recent survey of these historic finds allows for the first comprehensive narrative regarding their identity and procurement. The collection is heavily biased towards vertebral centra and phalanges, reflective of both taphonomic and collecting biases. Given current understanding of Upper Cretaceous assemblages of North America, ornithomimids and small theropods are overrepresented, whereas ceratopsids and ankylosaurs are underrepresented. Fossils from the Belly River Group are best re...
Despite strong evidence for sexual selection in various display traits and other exaggerated stru... more Despite strong evidence for sexual selection in various display traits and other exaggerated structures in large extinct reptiles, such as dinosaurs, detecting sexual dimorphism in them remains difficult. Their relatively small sample sizes, long growth periods, and difficulties distinguishing the sexes of fossil specimens mean that there are little compelling data on dimorphism in these animals. The extant gharial (Gavialis gangeticus) is a large and endangered crocodylian that is sexually dimorphic in size, but males also possesses a sexually selected structure, the ghara, which has an osteological correlate in the presence of a fossa associated with the nares. This makes the species a unique model for potentially assessing dimorphism in fossil lineages, such as dinosaurs and pterosaurs, because it is a large, slow-growing, egg-laying archosaur. Here we assess the dimorphism of G. gangeticus across 106 specimens and show that the presence of a narial fossa diagnoses adult male gha...
Ontogeny—the growth and development of an organism—is among the more poorly understood aspects of... more Ontogeny—the growth and development of an organism—is among the more poorly understood aspects of the life history of mosasaurs, largely owing to a dearth of fossil material from young individuals. We describe the partial and complete skulls of two subadult individuals of the mosasaurid Tylosaurus proriger from the upper Smoky Hills Chalk Member of the Niobrara Formation in Kansas. We include the more complete of the two specimens in an allometric analysis to better understand proportional changes of the skull through growth. Although our small sample size produces several instances of ‘soft isometry’, we recover the length of the edentulous rostrum as significantly negatively allometric, and quadrate height as significantly positively allometric. In light of our findings, we go on to consider the question of whether T. kansasensis represents an immature ontogimorph of T. nepaeolicus, but find no convincing evidence that this is the case.
Choristoderes are extinct neodiapsid reptiles that are well known for their unusual cranial anato... more Choristoderes are extinct neodiapsid reptiles that are well known for their unusual cranial anatomy, possessing an elongated snout and expanded temporal arches. Although choristodere skulls are well described externally, their internal anatomy remains unknown. An internal description was needed to shed light on peculiarities of the choristodere skull, such as paired gaps on the ventral surface of the skull that may pertain to the fenestra ovalis, and a putative neomorphic ossification in the lateral wall of the braincase. Our goals were: (i) to describe the cranial elements of Champsosaurus lindoei in three dimensions; (ii) to describe paired gaps on the ventral surface of the skull to determine if these are indeed the fenestrae ovales; (iii) to illustrate the morphology of the putative neomorphic bone; and (iv) to consider the possible developmental and functional origins of the neomorph. We examined the cranial anatomy of the choristodere Champsosaurus lindoei (CMN 8920) using high-resolution micro-computed tomography scanning. We found that the paired gaps on the ventral surface of the skull do pertain to the fenestrae ovales, an unusual arrangement that may be convergent with some plesiosaurs, some aistopods, and some urodeles. The implications of this morphology in Champsosaurus are unknown and will be the subject of future work. We found that the neomorphic bone is a distinct ossification, but is not part of the wall of the brain cavity or the auditory capsule. Variation in the developmental pathways of cranial bones in living amniotes was surveyed to determine how the neomorphic bone may have developed. We found that the chondrocranium and splanchnocranium show little to no variation across amniotes, and the neomorphic bone is therefore most likely to have developed from the dermatocranium; however, the stapes is a pre-existing cranial element that is undescribed in choristoderes and may be homologous with the neomorphic bone. If the neomorphic bone is not homologous with the stapes, the neomorph likely developed from the dermatocranium through incomplete fusion of ossification centres from a pre-existing bone, most likely the parietal. Based on the apparent morphology of the neomorph in Coeruleodraco, the neomorph was probably too small to play a significant structural role in the skull of early choristoderes and it may have arisen through non-adaptive means. In neochoristoderes, such as Champsosaurus, the neomorph was likely recruited to support the expanded temporal arches.
Modern megaherbivore community richness is limited by bottom-up controls, such as resource limita... more Modern megaherbivore community richness is limited by bottom-up controls, such as resource limitation and resultant dietary competition. However, the extent to which these same controls impacted the richness of fossil megaherbivore communities is poorly understood. The present study investigates the matter with reference to the megaherbivorous dinosaur assemblage from the middle to upper Campanian Dinosaur Park Formation of Alberta, Canada. Using a meta-analysis of 21 ecomorphological variables measured across 14 genera, contemporaneous taxa are demonstrably well-separated in ecomorphospace at the family/subfamily level. Moreover, this pattern is persistent through the approximately 1.5 Myr timespan of the formation, despite continual species turnover, indicative of underlying structural principles imposed by long-term ecological competition. After considering the implications of ecomorphology for megaherbivorous dinosaur diet, it is concluded that competition structured comparable ...
Several published censuses have noted the presence of two tyrannosaurids, Daspletosaurus sp. and ... more Several published censuses have noted the presence of two tyrannosaurids, Daspletosaurus sp. and Albertosaurus sarcophagus, within the Upper Cretaceous Horseshoe Canyon Formation of Alberta. Although A. sarcophagus is known from more than a dozen major discoveries in these strata, Daspletosaurus sp. is known from just a single problematic skeleton (lacking most of the skull) of a young individual. Here we describe and figure this skeleton, and marshal a variety of osteohistologic, morphometric, and phylogenetic methods to accurately determine its taxonomic status. Although none of these methods individually provides convincing evidence regarding the affinities of the specimen, together (and including other historical and biostratigraphic considerations) they strongly imply that the skeleton instead pertains to a young A. sarcophagus. In this way, we show that only a single species of tyrannosaurid is definitively present in the Horseshoe Canyon Formation, greatly simplifying interpretations of tyrannosaurid evolution and ecology in this setting. Anat Rec, 00:000-000, 2019.
Ankylosaurus magniventris is an iconic dinosaur species often depicted in popular media. It is kn... more Ankylosaurus magniventris is an iconic dinosaur species often depicted in popular media. It is known from relatively fragmentary remains compared with its earlier and smaller relatives such as Euoplocephalus and Anodontosaurus. Nevertheless, the known fossils of Ankylosaurus indicate that it had diverged significantly in cranial and postcranial anatomy compared with other Laramidian ankylosaurines. In particular, the dentition, narial region, tail club, and overall body size differ substantially from other Campanian–Maastrichtian ankylosaurines. We review the anatomy of this unusual ankylosaur using data from historic and newly identified material and discuss its palaeoecological implications.
We describe a new species of nanhsiungchelyid turtle, Basilemys morrinensis, based on a nearly co... more We describe a new species of nanhsiungchelyid turtle, Basilemys morrinensis, based on a nearly complete shell from the Horsethief Member (lower Maastrichtian) of the Horseshoe Canyon Formation of Alberta. The species is intermediate in age between the Campanian forms B. variolosa and B. gaffneyi and the upper Maastrichtian forms B. sinuosa and B. praeclara. It is also intermediate in its morphology, possessing a unique suite of both plesiomorphic (e.g., divided extragulars) and derived (e.g., square epiplastral beak, pygal wider than long) traits. It is further characterized by an autapomorphic square cervical scale. Phylogenetic analysis assuming parsimony recovers B. morrinensis in a polytomy with B. variolosa and B. gaffneyi, outside the clade formed by the upper Maastrichtian forms B. sinuosa and B. praeclara. The holotype of Basilemys morrinensis provides the first evidence that this genus reached large size (»1 m long) in the Horseshoe Canyon Formation and was not diminutive as previously thought based on less complete shell material. Although Basilemys is usually regarded as terrestrial in habit based on its skull and limb morphology, we note that the low profile of its shell is a derived feature usually indicative of an aquatic mode of life. This suggests that there is yet much to learn about the life habits of this interesting turtle.
Evidence for sexual dimorphism is extremely limited in the non-avian dinosaurs despite their high... more Evidence for sexual dimorphism is extremely limited in the non-avian dinosaurs despite their high diversity and disparity, and despite the fact that dimorphism is very common in vertebrate lineages of all kinds. Using body size data from both Alligator mississippiensis and Rhea americana, which phylogenetically bracket the dinosaurs, we demonstrate that even when there is strong dimorphism in a species, random sampling of populations of individuals characterized by sustained periods of growth (as in the alligator and most dinosaurs) can result in the loss of this signal. Dimorphism may be common in fossil taxa but very hard to detect without ontogenetic age control and large sample sizes, both of which are hampered by the limitations of the fossil record. Signal detection may be further hindered by Type III survivorship, whereby increased mortality among the young favours the likelihood that they will be sampled (unless predation or taphonomic bias acts against this). This hindrance is counteracted by taphonomic bias against small body size. These, and other considerations relating to behavior and ecology, provide powerful reasons to suggest that sexual dimorphism in dinosaurs may be very difficult to detect in almost all currently available samples. Similar issues are likely also applicable to many fossil reptiles, or animals more generally.
The demonstration of sexual dimorphism in the fossil record can provide vital information about t... more The demonstration of sexual dimorphism in the fossil record can provide vital information about the role that sexual selection has played in the evolution of life. However, statistically robust inferences of sexual dimorphism in fossil organisms are exceedingly difficult to establish, owing to issues of sample size, experimental control, and methodology. This is particularly so in the case of dinosaurs, for which sexual dimorphism has been posited in many species, yet quantifiable data are often lacking. This study presents the first statistical investigation of sexual dimorphism across Dinosauria. It revisits prior analyses that purport to find quantitative evidence for sexual dimorphism in nine dinosaur species. After the available morphological data were subjected to a suite of statistical tests (normality and unimodality tests and mixture modeling), no evidence for sexual dimorphism was found in any of the examined taxa, contrary to conventional wisdom. This is not to say that d...
The cranial anatomy of the flat-skulled hadrosaurine Edmontosaurus regalis (Ornithischia: Hadrosa... more The cranial anatomy of the flat-skulled hadrosaurine Edmontosaurus regalis (Ornithischia: Hadrosauridae) is extensively described here, based on the holotype and paratype collected from the middle part of the Horseshoe Canyon Formation in southern Alberta. Focus is given to previously undocumented features of ontogenetic and phylogenetic importance. This description facilitates overall osteological comparisons between E. regalis and other hadrosaurids (especially E. annectens), and revises the diagnosis of E. regalis, to which a new autapomorphy (the dorsal half of the jugal anterior process bearing a sharp posterolateral projection into the orbit) is added. We consider the recently named Ugrunaaluk kuukpikensis from the upper Campanian/lower Maastrichtian of Alaska a nomen dubium, and conservatively regard the Alaskan material as belonging to Edmontosaurus sp.. A phylogenetic analysis of Hadrosauroidea using maximum parsimony further corroborates the sistertaxon relationship between E. regalis and E. annectens. In the strict consensus tree, Hadrosaurus foulkii occurs firmly within the clade comprising all non-lambeosaurine hadrosaurids, supporting the taxonomic scheme that divides Hadrosauridae into Hadrosaurinae and Lambeosaurinae. Within Edmontosaurini, Kerberosaurus is posited as the sister taxon to the clade of Shantungosaurus + Edmontosaurus. The biogeographic reconstruction of Hadrosaurinae in light of the time-calibrated cladogram and probability calculation of ancestral areas for all internal nodes reveals a significantly high probability for the North American origin of the clade. However, the Laramidia-Appalachia dispersals around the Santonian-Campanian boundary, inferred from the biogeographic scenario for the North American origin of Hadrosaurinae, are in conflict with currently accepted paleogeographic models. By contrast, the Asian origin of Hadrosaurinae with its relatively low probability resulting from the biogeographic analysis is worth seriously considering, despite the lack of fossil material from the Santonian and lower Campanian of Asia. Extra fossil collecting in appropriate geographic locations and stratigraphic intervals of Asia and Europe will help to clarify the biogeographic dynamics of hadrosaurine dinosaurs in the near future.
A partial skull (CMN 8804) of a ceratopsid from the upper unit of the Campanian Oldman Formation ... more A partial skull (CMN 8804) of a ceratopsid from the upper unit of the Campanian Oldman Formation of Alberta is the first Canadian example of the newly established Nasutoceratopsini, a new subclade of Centrosaurinae defined as the stem-based clade of centrosaurine ceratopsids more closely related to Nasutoceratops titusi than to Centrosaurus apertus. The new clade is diagnosed, in part, by having a parietosquamosal frill lacking modified epimarginals; a small nasal horncore; large, rostrolaterally directed postorbital horncores; and a relatively short, deep face. Although the CMN 8804 taxon closely resembles Nasutoceratops, its phylogenetic position within Nasutoceratopsini is unresolved. The CMN 8804 taxon would have been contemporaneous with dinosaurs from the lower portion of the Dinosaur Park Formation 200 km to the northwest in Dinosaur Provincial Park. The presence of the CMN 8804 taxon in Alberta, and the approximately contemporaneous Nasutoceratops in Utah, indicates that the...
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Papers by Jordan Mallon