Modern birds are typified by the presence of feathers, complex evolutionary innovations that were... more Modern birds are typified by the presence of feathers, complex evolutionary innovations that were already widespread in the group of theropod dinosaurs (Maniraptoriformes) that include crown Aves. Squamous or scaly reptilian-like skin is, however, considered the plesiomorphic condition for theropods and dinosaurs more broadly. Here, we review the morphology and distribution of non-feathered integumentary structures in non-avialan theropods, covering squamous skin and naked skin as well as dermal ossifications. The integumentary record of non-averostran theropods is limited to tracks, which ubiquitously show a covering of tiny reticulate scales on the plantar surface of the pes. This is consistent also with younger averostran body fossils, which confirm an arthral arrangement of the digital pads. Among averostrans, squamous skin is confirmed in Ceratosauria (Carnotaurus), Allosauroidea (Allosaurus, Concavenator, Lourinhanosaurus), Compsognathidae (Juravenator), and Tyrannosauroidea (Santanaraptor, Albertosaurus, Daspletosaurus, Gorgosaurus, Tarbosaurus, Tyrannosaurus), whereas dermal ossifications consisting of sagittate and mosaic osteoderms are restricted to Ceratosaurus. Naked, non-scale bearing skin is found in the contentious tetanuran Sciurumimus, ornithomimosaurians (Ornithomimus) and possibly tyrannosauroids (Santanaraptor), and also on the patagia of scansoriopterygids (Ambopteryx, Yi). Scales are surprisingly conservative among non-avialan
Abstract Although upper Campanian dinosaur assemblages are well-known from Alberta, Montana, sout... more Abstract Although upper Campanian dinosaur assemblages are well-known from Alberta, Montana, southern Utah, and New Mexico, specimens from Wyoming and central and eastern Utah are very rare. This area constitutes a biogeographic break between northern and southern biogeographic provinces, so any specimens from this region are critical to understanding the origin, evolution, and limits of upper Campanian biogeographic zones on the west margin of the Western Interior Seaway. We report the discovery of a theropod dinosaur partial hindlimb from the Book Cliffs area northeast of Green River, Utah. The specimen was recovered from the Palisade coal zone in the Neslen Formation (Mesaverde Group), which is dated to the mid-Campanian based on ammonite biostratigraphy and radioisotopic age constraints. The specimen, comprising a partial fibula, the distal half of metatarsal II, a complete metatarsal IV, and a partial metatarsal V, can be assigned to Tyrannosauridae based on a number of synapomorphies, including a bipartite iliofibularis tubercle on the fibula and a teardrop shaped articular surface for metatarsal III on the medial surface of the distal portion of metatarsal IV. This is the first unambiguous tyrannosaurid dinosaur reported from the Mesaverde Group and represents an important biogeographic record situated between northern and southern upper Campanian vertebrate assemblages. Specifically, we identify morphological evidence on the pes that separates northern (Montana and Alberta) and southern (southern Utah and New Mexico) tyrannosaurid dinosaurs, and suggests that the Book Cliffs specimen belongs to the northern group. This implies that either the biogeographic boundary between the northern and southern Campanian assemblages lies somewhere between central and southern Utah or that the Book Cliffs taxon represents a northern emigrant in the southern assemblage.
Fossil caddisfly (Insecta: Trichoptera) larval cases are preserved in some Eocene Green River For... more Fossil caddisfly (Insecta: Trichoptera) larval cases are preserved in some Eocene Green River Formation tufa, and well organized arrays of these calcified larval cases have been misidentified as “Oocardium tufa”. The arrays of caddisfly larval cases occur as superimposed layers of vertically oriented cases. The cases are parallel to each other, closely packed, equal in size, and are oriented perpendicular to the bedding planes of the tufa. In thin section, the cases are composed of detrital carbonate particles, ooids and quartz grains arranged in a single layer along the walls of the cases. The size and arrangement of the particles in the walls of the fossil caddisfly larval cases compares favorably with modern caddisfly larval cases. Correct identification of the origin of this unique tufa fabric enables us to better understand the important role of metazoans (Trichopteran larvae) in some Eocene Lake Gosiute paleoenvironments. Caddisfly larvae played direct and indirect roles in the deposition and erosion of tufa in Lake Gosiute during the deposition of the Laney Member of the Green River Formation.
Medusaceratops lokii Ryan, Russell, and Hartman, 2010 is an enigmatic taxon of ceratopsid represe... more Medusaceratops lokii Ryan, Russell, and Hartman, 2010 is an enigmatic taxon of ceratopsid represented by partial parietals from the Mansfield bonebed in the Campanian Judith River Formation, Montana. Originally, all ceratopsid material collected from this bonebed was referred to the centrosaurine ceratopsid Albertaceratops, but subsequently two parietals were designated the types of the chasmosaurine, M. lokii, in part, because they were interpreted to have three epiparietals bilaterally. Here we describe new material from the bonebed that allows a systematic revision of the taxon. A revised reconstruction of the frill, informed by newly discovered parietals, reveals that M. lokii had a broad midline ramus and at least five epiparietals (ep) around the margin of the frill, both traits that are characteristic of Centrosaurinae. From medial to lateral, the epiparietal ornamentation consists of a small, variably procurving epiparietal (ep 1), an anterolaterally curving pachyostotic hoo...
This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of... more This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-si...
Allosaurus is one of the best known theropod dinosaurs from the Jurassic and a crucial taxon in p... more Allosaurus is one of the best known theropod dinosaurs from the Jurassic and a crucial taxon in phylogenetic analyses. On the basis of an in-depth, firsthand study of the bulk of Allosaurus specimens housed in North American institutions, we describe here a new theropod dinosaur from the Upper Jurassic Morrison Formation of Western North America, Allosaurus jimmadseni sp. nov., based upon a remarkably complete articulated skeleton and skull and a second specimen with an articulated skull and associated skeleton. The present study also assigns several other specimens to this new species, Allosaurus jimmadseni, which is characterized by a number of autapomorphies present on the dermal skull roof and additional characters present in the postcrania. In particular, whereas the ventral margin of the jugal of Allosaurus fragilis has pronounced sigmoidal convexity, the ventral margin is virtually straight in Allosaurus jimmadseni. The paired nasals of Allosaurus jimmadseni possess bilateral...
Establishing the origin and function of unusual traits in fossil taxa provides a crucial tool in ... more Establishing the origin and function of unusual traits in fossil taxa provides a crucial tool in understanding macroevolutionary patterns over long periods of time. Ceratopsian dinosaurs are known for their exaggerated and often elaborate horns and frills, which vary considerably between species. Many explanations have been proposed for the origin and evolution of these 'ornamental' traits, from predator defence to socio-sexual dominance signalling and, more recently, species recognition. A key prediction of the species recognition hypothesis is that two or more species possessing divergent ornamental traits should have been at least partially sympatric. For the first time to our knowledge, we test this hypothesis in ceratopsians by conducting a comparison of the morphological characters of 46 species. A total of 350 ceratopsian cladistic characters were categorized as either 'internal', 'display' (i.e. ornamental) or 'non display'. Patterns of divers...
... LITERATURE CITED. Brochu, CA 2003. Osteology of Tyrannosaurus rex: insights from a nearly com... more ... LITERATURE CITED. Brochu, CA 2003. Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. ... CrossRef. Cifelli, RL, RL Nydam, JG Eaton, JD Gardner, and JI Kirkland. 1999. ...
The Upper Cretaceous (middle-late Campanian) Wahweap Formation of southern Utah contains the olde... more The Upper Cretaceous (middle-late Campanian) Wahweap Formation of southern Utah contains the oldest diagnostic evidence of ceratopsids (to date, all centrosaurines) in North America, with a number of specimens recovered from throughout a unit that spans between 81 and 77 Ma. Only a single specimen has been formally named, Diabloceratops eatoni, from the lower middle member of the formation. Machairoceratops cronusi gen. et sp. nov., a new centrosaurine ceratopsid from the upper member of the Wahweap Formation, is here described based on cranial material representing a single individual recovered from a calcareous mudstone. The specimen consists of two curved and elongate orbital horncores, a left jugal, a nearly complete, slightly deformed braincase, the left squamosal, and a mostly complete parietal ornamented by posteriorly projected, anterodorsally curved, elongate spikes on either side of a midline embayment. The fan-shaped, stepped-squamosal is diagnostic of Centrosaurinae, how...
ABSTRACT—The theropod genus and species Ornithomimus velox was erected based on a partial hind li... more ABSTRACT—The theropod genus and species Ornithomimus velox was erected based on a partial hind limb recovered from the late Maastrichtian Denver Formation. A partial manus, which likely belonged to the same individual, was also recovered and described in the same paper. Ornithomimus edmontonicus is the only other valid species currently recognized in the genus. The validity of Ornithomimus velox has been questioned due to its fragmentary nature and because the diagnostic features identified when the species was erected are now considered characteristic for the family. The pes and manus of Ornithomimus velox were never fully prepared. In the original description, reconstructed metatarsals were figured, but manual phalanges, although preserved, were not described. Here we describe and reevaluate Ornithomimus velox based upon a new preparation of the specimen. Metacarpal proportions are diagnostic for the genus, with metacarpal I longer than metacarpal II, which in turn is longer than metacarpal III. The pes and manus of Ornithomimus velox are smaller than in the type specimen of Ornithomimus edmontonicus. Ornithomimus velox can be distinguished from Ornithomimus edmontonicus based on the robusticity of the pes. Material previously referred to Ornithomimus velox from the Kaiparowits Formation of Utah is older and morphologically distinct and does not represent the same species. The morphological disparity and temporal separation observed in specimens referred to Ornithomimus edmontonicus suggest that it may represent a species complex. The redescription and diagnosis of Ornithomimus velox provides a new framework to investigate ornithomimid systematics.
A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal fri... more A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal frill shape among ceratopsid dinosaurs early in their evolutionary radiation. The new taxon is described based on two apomorphic squamosals collected from approximately time equivalent (approximately 77 million years old) sections of the upper Judith River Formation, Montana, and the lower Dinosaur Park Formation of Dinosaur Provincial Park, Alberta. It is referred to Chasmosaurinae based on the inferred elongate morphology. The typical chasmosaurine squamosal forms an obtuse triangle in dorsal view that tapers towards the posterolateral corner of the frill. In the dorsal view of the new taxon, the lateral margin of the squamosal is hatchet-shaped with the posterior portion modified into a constricted narrow bar that would have supported the lateral margin of a robust parietal. The new taxon represents the oldest chasmosaurine from Canada, and the first pre-Maastrichtian ceratopsid to have ...
Modern birds are typified by the presence of feathers, complex evolutionary innovations that were... more Modern birds are typified by the presence of feathers, complex evolutionary innovations that were already widespread in the group of theropod dinosaurs (Maniraptoriformes) that include crown Aves. Squamous or scaly reptilian-like skin is, however, considered the plesiomorphic condition for theropods and dinosaurs more broadly. Here, we review the morphology and distribution of non-feathered integumentary structures in non-avialan theropods, covering squamous skin and naked skin as well as dermal ossifications. The integumentary record of non-averostran theropods is limited to tracks, which ubiquitously show a covering of tiny reticulate scales on the plantar surface of the pes. This is consistent also with younger averostran body fossils, which confirm an arthral arrangement of the digital pads. Among averostrans, squamous skin is confirmed in Ceratosauria (Carnotaurus), Allosauroidea (Allosaurus, Concavenator, Lourinhanosaurus), Compsognathidae (Juravenator), and Tyrannosauroidea (Santanaraptor, Albertosaurus, Daspletosaurus, Gorgosaurus, Tarbosaurus, Tyrannosaurus), whereas dermal ossifications consisting of sagittate and mosaic osteoderms are restricted to Ceratosaurus. Naked, non-scale bearing skin is found in the contentious tetanuran Sciurumimus, ornithomimosaurians (Ornithomimus) and possibly tyrannosauroids (Santanaraptor), and also on the patagia of scansoriopterygids (Ambopteryx, Yi). Scales are surprisingly conservative among non-avialan
Abstract Although upper Campanian dinosaur assemblages are well-known from Alberta, Montana, sout... more Abstract Although upper Campanian dinosaur assemblages are well-known from Alberta, Montana, southern Utah, and New Mexico, specimens from Wyoming and central and eastern Utah are very rare. This area constitutes a biogeographic break between northern and southern biogeographic provinces, so any specimens from this region are critical to understanding the origin, evolution, and limits of upper Campanian biogeographic zones on the west margin of the Western Interior Seaway. We report the discovery of a theropod dinosaur partial hindlimb from the Book Cliffs area northeast of Green River, Utah. The specimen was recovered from the Palisade coal zone in the Neslen Formation (Mesaverde Group), which is dated to the mid-Campanian based on ammonite biostratigraphy and radioisotopic age constraints. The specimen, comprising a partial fibula, the distal half of metatarsal II, a complete metatarsal IV, and a partial metatarsal V, can be assigned to Tyrannosauridae based on a number of synapomorphies, including a bipartite iliofibularis tubercle on the fibula and a teardrop shaped articular surface for metatarsal III on the medial surface of the distal portion of metatarsal IV. This is the first unambiguous tyrannosaurid dinosaur reported from the Mesaverde Group and represents an important biogeographic record situated between northern and southern upper Campanian vertebrate assemblages. Specifically, we identify morphological evidence on the pes that separates northern (Montana and Alberta) and southern (southern Utah and New Mexico) tyrannosaurid dinosaurs, and suggests that the Book Cliffs specimen belongs to the northern group. This implies that either the biogeographic boundary between the northern and southern Campanian assemblages lies somewhere between central and southern Utah or that the Book Cliffs taxon represents a northern emigrant in the southern assemblage.
Fossil caddisfly (Insecta: Trichoptera) larval cases are preserved in some Eocene Green River For... more Fossil caddisfly (Insecta: Trichoptera) larval cases are preserved in some Eocene Green River Formation tufa, and well organized arrays of these calcified larval cases have been misidentified as “Oocardium tufa”. The arrays of caddisfly larval cases occur as superimposed layers of vertically oriented cases. The cases are parallel to each other, closely packed, equal in size, and are oriented perpendicular to the bedding planes of the tufa. In thin section, the cases are composed of detrital carbonate particles, ooids and quartz grains arranged in a single layer along the walls of the cases. The size and arrangement of the particles in the walls of the fossil caddisfly larval cases compares favorably with modern caddisfly larval cases. Correct identification of the origin of this unique tufa fabric enables us to better understand the important role of metazoans (Trichopteran larvae) in some Eocene Lake Gosiute paleoenvironments. Caddisfly larvae played direct and indirect roles in the deposition and erosion of tufa in Lake Gosiute during the deposition of the Laney Member of the Green River Formation.
Medusaceratops lokii Ryan, Russell, and Hartman, 2010 is an enigmatic taxon of ceratopsid represe... more Medusaceratops lokii Ryan, Russell, and Hartman, 2010 is an enigmatic taxon of ceratopsid represented by partial parietals from the Mansfield bonebed in the Campanian Judith River Formation, Montana. Originally, all ceratopsid material collected from this bonebed was referred to the centrosaurine ceratopsid Albertaceratops, but subsequently two parietals were designated the types of the chasmosaurine, M. lokii, in part, because they were interpreted to have three epiparietals bilaterally. Here we describe new material from the bonebed that allows a systematic revision of the taxon. A revised reconstruction of the frill, informed by newly discovered parietals, reveals that M. lokii had a broad midline ramus and at least five epiparietals (ep) around the margin of the frill, both traits that are characteristic of Centrosaurinae. From medial to lateral, the epiparietal ornamentation consists of a small, variably procurving epiparietal (ep 1), an anterolaterally curving pachyostotic hoo...
This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of... more This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-si...
Allosaurus is one of the best known theropod dinosaurs from the Jurassic and a crucial taxon in p... more Allosaurus is one of the best known theropod dinosaurs from the Jurassic and a crucial taxon in phylogenetic analyses. On the basis of an in-depth, firsthand study of the bulk of Allosaurus specimens housed in North American institutions, we describe here a new theropod dinosaur from the Upper Jurassic Morrison Formation of Western North America, Allosaurus jimmadseni sp. nov., based upon a remarkably complete articulated skeleton and skull and a second specimen with an articulated skull and associated skeleton. The present study also assigns several other specimens to this new species, Allosaurus jimmadseni, which is characterized by a number of autapomorphies present on the dermal skull roof and additional characters present in the postcrania. In particular, whereas the ventral margin of the jugal of Allosaurus fragilis has pronounced sigmoidal convexity, the ventral margin is virtually straight in Allosaurus jimmadseni. The paired nasals of Allosaurus jimmadseni possess bilateral...
Establishing the origin and function of unusual traits in fossil taxa provides a crucial tool in ... more Establishing the origin and function of unusual traits in fossil taxa provides a crucial tool in understanding macroevolutionary patterns over long periods of time. Ceratopsian dinosaurs are known for their exaggerated and often elaborate horns and frills, which vary considerably between species. Many explanations have been proposed for the origin and evolution of these 'ornamental' traits, from predator defence to socio-sexual dominance signalling and, more recently, species recognition. A key prediction of the species recognition hypothesis is that two or more species possessing divergent ornamental traits should have been at least partially sympatric. For the first time to our knowledge, we test this hypothesis in ceratopsians by conducting a comparison of the morphological characters of 46 species. A total of 350 ceratopsian cladistic characters were categorized as either 'internal', 'display' (i.e. ornamental) or 'non display'. Patterns of divers...
... LITERATURE CITED. Brochu, CA 2003. Osteology of Tyrannosaurus rex: insights from a nearly com... more ... LITERATURE CITED. Brochu, CA 2003. Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. ... CrossRef. Cifelli, RL, RL Nydam, JG Eaton, JD Gardner, and JI Kirkland. 1999. ...
The Upper Cretaceous (middle-late Campanian) Wahweap Formation of southern Utah contains the olde... more The Upper Cretaceous (middle-late Campanian) Wahweap Formation of southern Utah contains the oldest diagnostic evidence of ceratopsids (to date, all centrosaurines) in North America, with a number of specimens recovered from throughout a unit that spans between 81 and 77 Ma. Only a single specimen has been formally named, Diabloceratops eatoni, from the lower middle member of the formation. Machairoceratops cronusi gen. et sp. nov., a new centrosaurine ceratopsid from the upper member of the Wahweap Formation, is here described based on cranial material representing a single individual recovered from a calcareous mudstone. The specimen consists of two curved and elongate orbital horncores, a left jugal, a nearly complete, slightly deformed braincase, the left squamosal, and a mostly complete parietal ornamented by posteriorly projected, anterodorsally curved, elongate spikes on either side of a midline embayment. The fan-shaped, stepped-squamosal is diagnostic of Centrosaurinae, how...
ABSTRACT—The theropod genus and species Ornithomimus velox was erected based on a partial hind li... more ABSTRACT—The theropod genus and species Ornithomimus velox was erected based on a partial hind limb recovered from the late Maastrichtian Denver Formation. A partial manus, which likely belonged to the same individual, was also recovered and described in the same paper. Ornithomimus edmontonicus is the only other valid species currently recognized in the genus. The validity of Ornithomimus velox has been questioned due to its fragmentary nature and because the diagnostic features identified when the species was erected are now considered characteristic for the family. The pes and manus of Ornithomimus velox were never fully prepared. In the original description, reconstructed metatarsals were figured, but manual phalanges, although preserved, were not described. Here we describe and reevaluate Ornithomimus velox based upon a new preparation of the specimen. Metacarpal proportions are diagnostic for the genus, with metacarpal I longer than metacarpal II, which in turn is longer than metacarpal III. The pes and manus of Ornithomimus velox are smaller than in the type specimen of Ornithomimus edmontonicus. Ornithomimus velox can be distinguished from Ornithomimus edmontonicus based on the robusticity of the pes. Material previously referred to Ornithomimus velox from the Kaiparowits Formation of Utah is older and morphologically distinct and does not represent the same species. The morphological disparity and temporal separation observed in specimens referred to Ornithomimus edmontonicus suggest that it may represent a species complex. The redescription and diagnosis of Ornithomimus velox provides a new framework to investigate ornithomimid systematics.
A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal fri... more A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal frill shape among ceratopsid dinosaurs early in their evolutionary radiation. The new taxon is described based on two apomorphic squamosals collected from approximately time equivalent (approximately 77 million years old) sections of the upper Judith River Formation, Montana, and the lower Dinosaur Park Formation of Dinosaur Provincial Park, Alberta. It is referred to Chasmosaurinae based on the inferred elongate morphology. The typical chasmosaurine squamosal forms an obtuse triangle in dorsal view that tapers towards the posterolateral corner of the frill. In the dorsal view of the new taxon, the lateral margin of the squamosal is hatchet-shaped with the posterior portion modified into a constricted narrow bar that would have supported the lateral margin of a robust parietal. The new taxon represents the oldest chasmosaurine from Canada, and the first pre-Maastrichtian ceratopsid to have ...
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Papers by Mark Loewen