Abstract. The early evolution of Ornithodira —the clade including pterosaurs and dinosaurs— is po... more Abstract. The early evolution of Ornithodira —the clade including pterosaurs and dinosaurs— is poorly known. Until a decade ago, the basal radiation of Dinosauromorpha, the clade including dinosaurs and birds, was poorly understood because of the poor fossil record restricted to specimens known from the Ladinian Chañares Formation in Argentina. Over the last years the discovery of several non-dinosaurian dinosauromorphs dramatically expanded this record and also demonstrated that this group —previously restricted to de Middle Triassic— survived at least well into the Norian. Although Norian non-dinosaurian dinosauromorphs have been reported from several regions around the world, the only known Norian non-dinosauriform dinosauromorphs —Dromomeron romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs and Dromomeron gregorii Nesbitt, Irmis, Parker, Smith, Turner, and Rowe— come from North America. We report here the first record from the Southern Hemisphere of a non-dinosauriform dinosauromorph, Dromomeron gigas sp. nov., from the Norian Quebrada del Barro Formation, northwestern Argentina. A phylogenetic analysis recovers Dromomeron gigas nested into the monophyletic group Lagerpetidae, and as the sister taxon to Dromomeron romeri. The inclusion of D. gigas within Lagerpetidae suggests that body size increased in this lineage over time, as was previously demonstrated for Dinosauriformes as a whole, and that lagerpetids reached a larger size than previously thought. Finally, the new finding provides new information on the basal radiation of Dinosauromorpha constituting the first record of a Norian association of dinosaurs with non-dinosauriform dinosauromorphs outside North America.
Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and... more Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations ...
Dinosaurs are unique among terrestrial tetrapods in their body sizes, which range from less than ... more Dinosaurs are unique among terrestrial tetrapods in their body sizes, which range from less than 3 gm in hummingbirds to 70,000 kg or more in sauropods. Studies of the microstructure of bone tissue have indicated that large dinosaurs, once believed to be slow growing, attained maturity at rates comparable to or greater than those of large mammals. A number of structural criteria in bone tissue have been used to assess differences in rates of osteogenesis in extinct taxa, including counts of lines of arrested growth and the density of vascular canals. Here, we examine the density of the cytoplasmic surface of bone-producing cells, a feature which may set an upper limit to the rate of osteogenesis. Osteocyte lacunae and canaliculi, the cavities in bone containing osteocytes and their extensions, were measured in thin-sections of primary (woven and parallel fibered) bone in a diversity of tetrapods. The results indicate that bone cell surfaces are more densely organized in the Saurisch...
ABSTRACT A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is des... more ABSTRACT A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described from the McCoy Brook Formation (Lower Jurassic: Hettangian) of Nova Scotia, Canada. It is most closely related to C. mcgilli Wu, 1994 from the Dark Red Beds of the Lower Lufeng Formation (Lower Jurassic) of Yunnan, China, and differs from the latter mainly in the absence of the hourglass-shaped constriction of the parietals, the shape of the suborbital fenestra, and in features of the marginal dentition. The new taxon is of ...
ABSTRACT The proterochampsids are a group of crocodile-like archosauri-forms usually considered a... more ABSTRACT The proterochampsids are a group of crocodile-like archosauri-forms usually considered as one of the potential successive sister taxa of the crown group Archosauria (e.g., Sereno and Ar-cucci, 1990; Sereno, 1991; Dilkes and Sues, 2009; Ezcurra et al., 2010). The clade is currently endemic to the late Middle and early Late Triassic of South America. The best-known proterochampsids come from the Chã nares and Ischigualasto formations, which belong to the Ischigualasto-Villa Uní on Basin (Fig. 1), which formed during the breakup of Gondwana (Uliana and Biddle, 1988; Ramos and Kay, 1991). The tetrapod assemblage from the Chã nares Formation (Anisian–early Car-nian; Desojo et al., 2011) has been usually interpreted to differ from that of the Ischigualasto Formation (late Carnian–earliest Norian; Martínez et al., 2011), and this traditional view also applies for proterochampsids. In the Chã nares Formation, the proterochampsid genera Chanaresuchus, Gualosuchus, and Tropidosuchus (Romer, 1971, 1972; Arcucci, 1990) are docu-mented, whereas in the Ischigualasto Formation only the genus Proterochampsa has been reported (Reig, 1959). The latter highlighted a complete replacement at a generic level among these proterochampsid assemblages in southwestern Pangaea. However, an almost complete proterochampsid skeleton from the Late Triassic Ischigualasto Formation is given a preliminarily description here and assigned to the traditionally Middle Triassic genus Chanaresuchus (cf. Sill et al., 1994). Accordingly, the new proterochampsid record reported here changes the pattern of macroevolutionary history of the group during the Middle to Late Triassic in southwestern Pangaea.
A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described fr... more A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described from the McCoy Brook Formation (Lower Jurassic: Hettangian) of Nova Scotia, Canada. It is most closely related to C. mcgilli Wu, 1994 from the Dark Red Beds of the Lower Lufeng Formation (Lower Jurassic) of Yunnan, China, and differs from the latter mainly in the absence of the hourglass-shaped constriction of the parietals, the shape of the suborbital fenestra, and in features of the marginal dentition. The new taxon is of ...
Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs bef... more Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago
Abstract. The early evolution of Ornithodira —the clade including pterosaurs and dinosaurs— is po... more Abstract. The early evolution of Ornithodira —the clade including pterosaurs and dinosaurs— is poorly known. Until a decade ago, the basal radiation of Dinosauromorpha, the clade including dinosaurs and birds, was poorly understood because of the poor fossil record restricted to specimens known from the Ladinian Chañares Formation in Argentina. Over the last years the discovery of several non-dinosaurian dinosauromorphs dramatically expanded this record and also demonstrated that this group —previously restricted to de Middle Triassic— survived at least well into the Norian. Although Norian non-dinosaurian dinosauromorphs have been reported from several regions around the world, the only known Norian non-dinosauriform dinosauromorphs —Dromomeron romeri Irmis, Nesbitt, Padian, Smith, Turner, Woody, and Downs and Dromomeron gregorii Nesbitt, Irmis, Parker, Smith, Turner, and Rowe— come from North America. We report here the first record from the Southern Hemisphere of a non-dinosauriform dinosauromorph, Dromomeron gigas sp. nov., from the Norian Quebrada del Barro Formation, northwestern Argentina. A phylogenetic analysis recovers Dromomeron gigas nested into the monophyletic group Lagerpetidae, and as the sister taxon to Dromomeron romeri. The inclusion of D. gigas within Lagerpetidae suggests that body size increased in this lineage over time, as was previously demonstrated for Dinosauriformes as a whole, and that lagerpetids reached a larger size than previously thought. Finally, the new finding provides new information on the basal radiation of Dinosauromorpha constituting the first record of a Norian association of dinosaurs with non-dinosauriform dinosauromorphs outside North America.
Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and... more Dinosaurs dominated the terrestrial ecosystems for more than 140 Myr during the Mesozoic era, and among them were sauropodomorphs, the largest land animals recorded in the history of life. Early sauropodomorphs were small bipeds, and it was long believed that acquisition of giant body size in this clade (over 10 tonnes) occurred during the Jurassic and was linked to numerous skeletal modifications present in Eusauropoda. Although the origin of gigantism in sauropodomorphs was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change. Here, we describe a new sauropodomorph from the Late Triassic of Argentina nested within a clade of other non-eusauropods from southwest Pangaea. Members of this clade attained large body size while maintaining a plesiomorphic cyclical growth pattern, displaying many features of the body plan of basal sauropodomorphs and lacking most anatomical traits previously regarded as adaptations ...
Dinosaurs are unique among terrestrial tetrapods in their body sizes, which range from less than ... more Dinosaurs are unique among terrestrial tetrapods in their body sizes, which range from less than 3 gm in hummingbirds to 70,000 kg or more in sauropods. Studies of the microstructure of bone tissue have indicated that large dinosaurs, once believed to be slow growing, attained maturity at rates comparable to or greater than those of large mammals. A number of structural criteria in bone tissue have been used to assess differences in rates of osteogenesis in extinct taxa, including counts of lines of arrested growth and the density of vascular canals. Here, we examine the density of the cytoplasmic surface of bone-producing cells, a feature which may set an upper limit to the rate of osteogenesis. Osteocyte lacunae and canaliculi, the cavities in bone containing osteocytes and their extensions, were measured in thin-sections of primary (woven and parallel fibered) bone in a diversity of tetrapods. The results indicate that bone cell surfaces are more densely organized in the Saurisch...
ABSTRACT A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is des... more ABSTRACT A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described from the McCoy Brook Formation (Lower Jurassic: Hettangian) of Nova Scotia, Canada. It is most closely related to C. mcgilli Wu, 1994 from the Dark Red Beds of the Lower Lufeng Formation (Lower Jurassic) of Yunnan, China, and differs from the latter mainly in the absence of the hourglass-shaped constriction of the parietals, the shape of the suborbital fenestra, and in features of the marginal dentition. The new taxon is of ...
ABSTRACT The proterochampsids are a group of crocodile-like archosauri-forms usually considered a... more ABSTRACT The proterochampsids are a group of crocodile-like archosauri-forms usually considered as one of the potential successive sister taxa of the crown group Archosauria (e.g., Sereno and Ar-cucci, 1990; Sereno, 1991; Dilkes and Sues, 2009; Ezcurra et al., 2010). The clade is currently endemic to the late Middle and early Late Triassic of South America. The best-known proterochampsids come from the Chã nares and Ischigualasto formations, which belong to the Ischigualasto-Villa Uní on Basin (Fig. 1), which formed during the breakup of Gondwana (Uliana and Biddle, 1988; Ramos and Kay, 1991). The tetrapod assemblage from the Chã nares Formation (Anisian–early Car-nian; Desojo et al., 2011) has been usually interpreted to differ from that of the Ischigualasto Formation (late Carnian–earliest Norian; Martínez et al., 2011), and this traditional view also applies for proterochampsids. In the Chã nares Formation, the proterochampsid genera Chanaresuchus, Gualosuchus, and Tropidosuchus (Romer, 1971, 1972; Arcucci, 1990) are docu-mented, whereas in the Ischigualasto Formation only the genus Proterochampsa has been reported (Reig, 1959). The latter highlighted a complete replacement at a generic level among these proterochampsid assemblages in southwestern Pangaea. However, an almost complete proterochampsid skeleton from the Late Triassic Ischigualasto Formation is given a preliminarily description here and assigned to the traditionally Middle Triassic genus Chanaresuchus (cf. Sill et al., 1994). Accordingly, the new proterochampsid record reported here changes the pattern of macroevolutionary history of the group during the Middle to Late Triassic in southwestern Pangaea.
A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described fr... more A new sphenodontian lepidosaur referable to Clevosaurus Swinton, 1939, C. bairdi, is described from the McCoy Brook Formation (Lower Jurassic: Hettangian) of Nova Scotia, Canada. It is most closely related to C. mcgilli Wu, 1994 from the Dark Red Beds of the Lower Lufeng Formation (Lower Jurassic) of Yunnan, China, and differs from the latter mainly in the absence of the hourglass-shaped constriction of the parietals, the shape of the suborbital fenestra, and in features of the marginal dentition. The new taxon is of ...
Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs bef... more Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago
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