Troodontidae is a clade of small, lightly built maniraptorans known from Cretaceous deposits of Asia and North America. These theropods have serrated teeth, raptorial hands, and an enlarged sickle-shaped claw on the foot. This chapter discusses the taxonomy and diagnostic features of troodontids. It also discusses phylogenetic hypotheses of troodontid relationships. Diagnostic remains of troodontids are largely restricted to Central Asia and China, and the origin and most of the evolutionary history of the clade was endemic to that continent. Only a single derived troodontid taxon, Troodon formosus, occurs in North America.
The phylogenetic position of the Indian gharial (Gavialis gangeticus) is disputed - morphological characters place Gavialis as the sister to all other extant crocodylians, whereas molecular and combined analyses find Gavialis and the false gharial (Tomistoma schlegelii) to be sister taxa. Geometric morphometric techniques have only begun to be applied to this issue, but most of these studies have focused on the exterior of the skull. The braincase has provided useful phylogenetic information for basal crurotarsans, but has not been explored for the crown group. The Eustachian system is thought to vary phylogenetically in Crocodylia, but has not been analytically tested. To determine if gross morphology of the crocodylian braincase proves informative to the relationships of Gavialis and Tomistoma, we used two- and three-dimensional geometric morphometric approaches. Internal braincase images were obtained using high-resolution computerized tomography scans. A principal components analysis identified that the first component axis was primarily associated with size and did not show groupings that divide the specimens by phylogenetic affinity. Sliding semi-landmarks and a relative warp analysis indicate that a unique Eustachian morphology separates Gavialis from other extant members of Crocodylia. Ontogenetic expansion of the braincase results in a more dorsoventrally elongate median Eustachian canal. Changes in the shape of the Eustachian system do provide phylogenetic distinctions between major crocodylian clades. Each morphometric dataset, consisting of continuous morphological characters, was added independently to a combined cladistic analysis of discrete morphological and molecular characters. The braincase data alone produced a clade that included crocodylids and Gavialis, whereas the Eustachian data resulted in Gavialis being considered a basally divergent lineage. When each morphometric dataset was used in a combined analysis with discrete morphological and molecular characters, it generated a tree that matched the topology of the molecular phylogeny of Crocodylia.
Abstract This revised edition of this book continues in the same vein as the first but encompasses recent spectacular discoveries that have continued to revolutionize this field. A thorough scientific view of current world research, the volume includes comprehensive coverage of dinosaur systematics, reproduction, and life history strategies, biogeography, taphonomy, paleoecology, thermoregulation, and extinction. It contains definitive descriptions and illustrations of these magnificent Mesozoic beasts. The first section of the book begins with the origin of the great clade of these fascinating reptiles, followed by separate coverage of each major dinosaur taxon, including the Mesozoic radiation of birds. The second part of the volume navigates through broad areas of interest. Here we find comprehensive documentation of dinosaur distribution through time and space, discussion of the interface between geology and biology, and the paleoecological inferences that can be made through this link.
The fossil record of tyrannosauroid theropods is marked by a substantial temporal and morphological gap between small-bodied, Barremian taxa, and extremely large-bodied taxa from the latest Cretaceous. Here we describe a new tyrannosauroid, Xiongguanlong baimoensis n. gen. et sp., from the Aptian–Albian Xinminpu Group of western China that represents a phylogenetic, morphological, and temporal link between these disjunct portions of tyrannosauroid evolutionary history. Xiongguanlong is recovered in our phylogenetic analysis as the sister taxon to Tyrannosauridae plus Appalachiosaurus , and marks the appearance of several tyrannosaurid hallmark features, including a sharp parietal sagittal crest, a boxy basicranium, a quadratojugal with a flaring dorsal process and a flexed caudal edge, premaxillary teeth bearing a median lingual ridge, and an expanded axial neural spine surmounted by distinct processes at its corners. Xiongguanlong is characterized by a narrow and elongate muzzle resembling that of Alioramus . The slender, unornamented nasals of Xiongguanlong are inconsistent with recent hypotheses of correlated progression in tyrannosauroid feeding mechanics, and suggest more complex patterns of character evolution in the integration of feeding adaptations in tyrannosaurids. Body mass estimates for the full-grown holotype specimen of Xiongguanlong fall between those of Late Cretaceous tyrannosaurids and Barremian tyrannosauroids, suggesting that the trend of increasing body size observed in North American Late Cretaceous Tyrannosauridae may extend through the Cretaceous history of Tyrannosauroidea though further phylogenetic work is required to corroborate this.
Journal Article Fossils, Phylogeny, and Taxonomic Rates of Evolution Get access Michael J. Novacek, Michael J. Novacek Search for other works by this author on: Oxford Academic PubMed Google Scholar Mark A. Norell Mark A. Norell Search for other works by this author on: Oxford Academic PubMed Google Scholar Systematic Biology, Volume 31, Issue 4, December 1982, Pages 366–375, https://doi.org/10.1093/sysbio/31.4.366 Published: 01 December 1982
Although Mesozoic fossils are quite common in the Gobi Desert of Central Asia, it is often difficult to correlate among different localities because of a dearth of rocks amenable to absolute dating. Specifically, correlating between the eastern Gobi Desert and more western localities has been challenging. Here we give a Santonian-Campanian age for the enigmatic Zos Canyon beds in the Nemegt basin. This is based on the occurrence of the primitive ornithopod dinosaur Haya griva at both eastern Gobi exposures of the Javkhlant Formation and the Zos Canyon locality.
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