Adaptive radiation in sauropod dinosaurs: bone histology indicates rapid evolution of giant body size through acceleration

The well-preserved histology of the geologically oldest sauropod dinosaur from the Late Triassic allows new insights into the timing and mechanism of the evolution of the gigantic body size of the sauropod dinosaurs. The oldest sauropods were already very large and show the same long-bone histology, laminar fibro-lamellar bone lacking growth marks, as the well-known Jurassic sauropods. This bone histology is unequivocal evidence for very fast growth. Our histologic study of growth series of the Norian Plateosaurus indicates that the sauropod sistergroup, the Late Triassic and early Jurassic Prosauropoda, reached a much more modest body size in a not much shorter ontogeny. Increase in growth rate compared to the ancestor (acceleration) is thus the underlying process in the phylogenetic size increase of sauropods. Compared to all other dinosaur lineages, sauropods were not only much larger but evolved very large body size much faster. The prerequisite for this increase in growth rate must have been a considerable increase in metabolic rate, and we speculate that a bird-like lung was important in this regard.     

The origin and early evolution of birds

Birds evolved from and are phylogenetically recognized as members of the theropod dinosaurs; their first known member is the Late Jurassic Archaeopteryx, now represented by seven skeletons and a feather, and their closest known non-avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved from the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree-climbing characters, and its wing planform and wing loading do not resemble those of gliders. The ancestors of birds were bipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. Apart from a perching foot and some skeletal fusions, a great many characters that are usually considered ‘avian’ (e.g. the furcula, the elongated forearm, the laterally flexing wrist and apparently feathers) evolved in non-avian theropods for reasons unrelated to birds or to flight. Soon after Archaeopteryx, avian features such as the pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws with a reversed, fully descended and opposable hallux, indicate improved flying ability and arboreal habits. In the further evolution of birds, characters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are more diverse and numerous than thought previously and the most diverse known group of Cretaceous birds, the Enantiornithes, was not even recognized until 1981. The vast majority of Mesozoic bird groups have no Tertiary records: Enantiornithes, Hesperornithiformes, Ichthyornithiformes and several other lineages disappeared by the end of the Cretaceous. By that time, a few Linnean ‘Orders’ of extant birds had appeared, but none of these taxa belongs to extant ‘families’, and it is not until the Paleocene or (in most cases) the Eocene that the majority of extant bird ‘Orders’ are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden ‘bottleneck’ in diversity that fostered the early Tertiary origination of living bird ‘Orders’.     

The ankylosaurid dinosaurs of the Upper Cretaceous Baruungoyot and Nemegt formations of Mongolia

The discovery of a new ankylosaurid skull with some unusual features from the Baruungoyot Formation of Mongolia prompted a systematic review of ankylosaurid specimens from the Baruungoyot and Nemegt formations. Dyoplosaurus giganteus was found to possess no diagnostic features and is regarded as a nomen dubium. The holotype of Tarchia kielanae (previously synonymized with Tarchia gigantea) has one autapomorphy, an accessory postorbital ossification with surrounding furrow, and Tar. kielanae is here considered a valid species, making the combination Tar. gigantea unnecessary. An accessory postorbital ossification is also found in the holotype of Minotaurasaurus ramachandrani, and this species is here considered a junior synonym of Tar. kielanae. The newly described skull from the Baruungoyot Formation forms the holotype of a new genus and species, Z araapelta nomadis gen. et sp. nov. , diagnosed by unusual bilayered ornamentation on the squamosal horn and extensive postocular ornamentation. Two distinct tail club handle morphotypes are present in the Nemegt Formation and probably represent two different species. However, it is impossible to assign either tail club morphotype to the single valid species from the formation, Saichania chulsanensis, because of a lack of overlapping material. A revised phylogenetic analysis including newly identified characters found Zaraapelta nomadis to be most closely related to Tar. kielanae. © 2014 The Linnean Society of London     

The first diplodocid from Asia and its implications for the evolutionary history of sauropod dinosaurs

Abstract: An isolated anterior caudal vertebra from the Qingshan (= Ch'ing shan) Formation (Early Cretaceous) of Shandong Province, China, is redescribed and shown to be an advanced diplodocid sauropod. This specimen possesses several derived character states that are typically observed in advanced diplodocoids or diplodocids, including the following: a mildly procoelous centrum; a deep pit‐like pneumatic fossa immediately below the caudal rib; wing‐ or fan‐shaped caudal ribs; and complex lamination of the neural spine. The neural spine is apomorphically short and the centrum is short relative to its height compared to those of other diplodocids, which, when coupled with the specimen’s unique geographical location and stratigraphical age, suggests that it probably represents a new taxon. This caudal vertebra provides the first convincing evidence that diplodocids were present in Asia, perhaps as a result of the dispersal of neosauropod lineages from Europe and/or North America during the Early Cretaceous. The discovery of a member of the Diplodocidae in the Early Cretaceous also indicates that this clade did not become extinct at the Jurassic/Cretaceous boundary as previously supposed.     

On an Ornithomimid dinosaur (Saurischia, Ornithomimosauria) from the Cenomanian of Fergana

A fragmentary coracoid of Ornithomimidae indet. from the Lower Cenomanian of the Abshir River (Kyrgyzstan) is described. This is the first record of this group in a Cenomanian locality of Fergana. The coracoid from the Abshir locality is similar in the lateral deflection of the glenoid to an endemic group of Asiatic Ornithomimidae that includes Anserimimus, Gallimimus, and taxa from the Cenomanian and Turonian of western Uzbekistan. It is most similar to the unnamed ornithomimid from the Cenomanian of Karakalpakia in having a distinct vertical crest that borders anteriorly a depression for the coracobrachialis brevis muscle. Dinosaurs recorded in the Cenomanian of Fergana are reviewed.     

Maximum Bite Force and Prey Size of Tyrannosaurus rex and Their Relationships to the Inference of Feeding Behavior

The feeding behavior of the theropod dinosaur Tyrannosaurus rex is investigated through analysis of two variables that are critical to successful predation, bite force and prey body mass, as they scale with the size of the predator. These size-related variables have important deterministic effects on the predator’s feeding strategy, through their effects on lethal capacity and choice of prey. Bite force data compiled for extant predators (crocodylians, carnivorans, chelonians and squamates) are used to establish a relationship between bite force and body mass among extant predators. These data are used to estimate the maximum potential bite force of T. rex, which is between about 183,000 and 235,000 N for a bilateral bite. The relationship between maximum prey body mass and predator body mass among the same living vertebrates is used to infer the likely maximum size of prey taken by T. rex in the Late Cretaceous. This makes it possible to arrive at a more rigorous assessment of the role of T. rex as an active predator and/or scavenger than has hitherto been possible. The results of this analysis show that adult Triceratops horridus fall well within the size range of potential prey that are predicted to be available to a solitary, predaceous T. rex. This analysis establishes boundary conditions for possible predator/prey relationships among other dinosaurs, as well as between these two taxa.     

Osteohistological insight into the early stages of growth in Mussaurus patagonicus (Dinosauria,Sauropodomorpha)

Here, we describe the bone histology of juvenile specimens of the basal sauropodomorph Mussaurus patagonicus and interpret its significance in terms of the early growth dynamics of this taxon. Thin sections from three juvenile specimens (femur length, 111–120 mm) of Mussaurus were analysed. The sampled bones consist of multiple postcranial elements collected from the Late Triassic Laguna Colorada Formation (El Tranquilo Group, Patagonia). The cortical bone is composed of fibrolamellar bone tissue. Vascularisation is commonly laminar or plexiform in the long bones. Growth marks are absent in all the examined samples. The ‘epiphyses’ of long bones are all formed by well-developed hypertrophied calcified cartilage. The predominance of woven-fibred bone matrix in cortical bones indicates a fast growth rate in the individuals examined. Moreover, given the existence of growth marks in adult specimens of Mussaurus, as in other sauropodomorphs, and assuming that the first lines of arrested growth was formed during the first year of life, the absence of growth marks in all the bones suggest that the specimens died before reaching their first year of life. Compared with the African taxon Massospondylus carinatus (another basal sauropodomorph for which the bone histology has been previously studied), it appears that Mussaurus had a higher early growth rate than Massospondylus.     

Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?

Abstract

Two recent studies have independently recovered Pisanosaurus mertii – long thought to represent the oldest known member of Ornithischia – within Silesauridae. These finds are expanded upon here, as are the implications of this hypothesis. Based upon these finds, it now appears that Ornithischia was absent in the Triassic Period entirely, which constitutes a major incongruence between the fossil record and current phylogenetic hypotheses, particularly the traditional model of dinosaur interrelationships in which Ornithischia and Saurischia are sister-taxa. It has been suggested previously that Ornithischia was simply a rare component of Late Triassic faunas, or that perhaps the clade’s ecology or geographic distribution were not conducive to producing a fossil record. Here I propose that phylogeny could hold the solution to this problem. I examine how an alternative position for Ornithischia – nested either within Theropoda or Sauropodomorpha – could be the reason behind their later appearance and relative rarity in the Early Jurassic. An Early Jurassic origin of Ornithischia would force us to consider that the anatomical similarities between ornithischians and Early Jurassic taxa might not be convergences, and to broaden the current datasets of early dinosaurs to test these ideas.