Using creation science to demonstrate evolution 2: morphological continuity within Dinosauria

Creationist literature claims that sufficient gaps in morphological continuity exist to classify dinosaurs into several distinct baramins (‘created kinds’). Here, I apply the baraminological method called taxon correlation to test for morphological continuity within and between dinosaurian taxa. The results show enough morphological continuity within Dinosauria to consider most dinosaurs genetically related, even by this creationist standard. A continuous morphological spectrum unites the basal members of Saurischia, Theropoda, Sauropodomorpha, Ornithischia, Thyreophora, Marginocephalia, and Ornithopoda with Nodosauridae and Pachycephalosauria and with the basal ornithodirans Silesaurus and Marasuchus. Morphological gaps in the known fossil record separate only seven groups from the rest of Dinosauria. Those groups are Therizinosauroidea + Oviraptorosauria + Paraves, Tazoudasaurus + Eusauropoda, Ankylosauridae, Stegosauria, Neoceratopsia, basal Hadrosauriformes and Hadrosauridae. Each of these seven groups exhibits within‐group morphological continuity, indicating common descent for all the group’s members, even according to this creationist standard.     

Biology of the sauropod dinosaurs: the evolution of gigantism

The herbivorous sauropod dinosaurs of the Jurassic and Cretaceous periods were the largest terrestrial animals ever, surpassing the largest herbivorous mammals by an order of magnitude in body mass. Several evolutionary lineages among Sauropoda produced giants with body masses in excess of 50 metric tonnes by conservative estimates. With body mass increase driven by the selective advantages of large body size, animal lineages will increase in body size until they reach the limit determined by the interplay of bauplan, biology, and resource availability. There is no evidence, however, that resource availability and global physicochemical parameters were different enough in the Mesozoic to have led to sauropod gigantism. We review the biology of sauropod dinosaurs in detail and posit that sauropod gigantism was made possible by a specific combination of plesiomorphic characters (phylogenetic heritage) and evolutionary innovations at different levels which triggered a remarkable evolutionary cascade. Of these key innovations, the most important probably was the very long neck, the most conspicuous feature of the sauropod bauplan. Compared to other herbivores, the long neck allowed more efficient food uptake than in other large herbivores by covering a much larger feeding envelope and making food accessible that was out of the reach of other herbivores. Sauropods thus must have been able to take up more energy from their environment than other herbivores. The long neck, in turn, could only evolve because of the small head and the extensive pneumatization of the sauropod axial skeleton, lightening the neck. The small head was possible because food was ingested without mastication. Both mastication and a gastric mill would have limited food uptake rate. Scaling relationships between gastrointestinal tract size and basal metabolic rate (BMR) suggest that sauropods compensated for the lack of particle reduction with long retention times, even at high uptake rates. The extensive pneumatization of the axial skeleton resulted from the evolution of an avian‐style respiratory system, presumably at the base of Saurischia. An avian‐style respiratory system would also have lowered the cost of breathing, reduced specific gravity, and may have been important in removing excess body heat. Another crucial innovation inherited from basal dinosaurs was a high BMR. This is required for fueling the high growth rate necessary for a multi‐tonne animal to survive to reproductive maturity. The retention of the plesiomorphic oviparous mode of reproduction appears to have been critical as well, allowing much faster population recovery than in megaherbivore mammals. Sauropods produced numerous but small offspring each season while land mammals show a negative correlation of reproductive output to body size. This permitted lower population densities in sauropods than in megaherbivore mammals but larger individuals. Our work on sauropod dinosaurs thus informs us about evolutionary limits to body size in other groups of herbivorous terrestrial tetrapods. Ectothermic reptiles are strongly limited by their low BMR, remaining small. Mammals are limited by their extensive mastication and their vivipary, while ornithsichian dinosaurs were only limited by their extensive mastication, having greater average body sizes than mammals.     

Sauropod diversity in the Late Cretaceous of southwestern Europe: The lessons of odontology

Recent discoveries in southern France and northern Spain suggest that the morphology of titanosaurian teeth shows much greater variations that previously thought. It is suggested that the different morphotypes are informative at specific or generic level and that titanosaurian genera may indeed be recognized by their isolated teeth. It is also confirmed that juvenile titanosaurian teeth have a rather uniform, cylindrical morphology. Four different morphotypes are described for the Ibero-Armorican Island in the Late Cretaceous.     

Growth curve of Psittacosaurus mongoliensis Osborn (Geratopsia: Psittacosauridae) inferred from long bone histology

The skeleton undergoes substantial histological modification during ontogeny in association with longitudinal growth, shape changes, reproductive activity, and fatigue repair. This variation can hinder attempts to reconstruct life history attributes for individuals, particularly when only fossil materials are availble for study. Histological examinations of multiple elements throughout development provide a means to control for such variability and facilitate accurate life history assessments. In the present study, the microstructure of various major long bones of the ceratopsian Psittacosaurus monogoliensis Osborn were examined from a growth series spanning juvenile through adult developmental stages. The first reconstruction of a growth curve (mass vs. age) for a dinosaur was made for this taxon using a new method called Developmental Mass Extrapolation. The results suggest P. mongoliensis : (I) had an S-shaped growth curve characteristics of most extant vertebrates, and (2) had maximal growth rates that exceeded extant reptiles and marsupials, but were slower than most avian and eutherian taxa.     

Increases in sampling support the southern Gondwanan hypothesis for the origin of dinosaurs

Dinosaurs were ubiquitous in terrestrial ecosystems through most of the Mesozoic and are still diversely represented in the modern fauna in the form of birds. Recent efforts to better understand the origins of the group have resulted in the discovery of many new species of early dinosaur and their closest relatives (dinosauromorphs). In addition, recent re‐examinations of early dinosaur phylogeny have highlighted uncertainties regarding the interrelationships of the main dinosaur lineages (Sauropodomorpha, Theropoda and Ornithischia), and questioned the traditional hypothesis that the group originated in South Gondwana and gradually dispersed over Pangaea. Here, we use an historical approach to examine the impact of new fossil discoveries and changing phylogenetic hypotheses on biogeographical scenarios for dinosaur origins over 20 years of research time, and analyse the results in the light of different fossil record sampling regimes. Our results consistently optimize South Gondwana as the ancestral area for Dinosauria, as well as for more inclusive clades including Dinosauromorpha, and show that this hypothesis is robust to increased taxonomic and geographic sampling and divergent phylogenetic results. Our results do not find any support for the recently proposed Laurasian origin of dinosaurs and suggest that a southern Gondwanan origin is by far the most plausible given our current knowledge of the diversity of early dinosaurs and non‐dinosaurian dinosauromorphs.     

On the origin of high growth rates in archosaurs and their ancient relatives: Complementary histological studies on Triassic archosauriforms and the problem of a “phylogenetic signal” in bone histology

Three possible hypotheses could explain the polarity of the histological features of basal archosauriform and archosauromorph reptiles: either, the fibrolamellar complex is basal; or, the lamellar-zonal complex is basal or finally, the condition varied, and each complex evolved more than once in these early groups. The answer to this question would have broad implications for our understanding of the physiological, ecological, and behavioral features of the first archosaurs. To this end, we sampled the bone histology of various archosauriforms and basal archosaurs from the Triassic and Lower Jurassic: erythrosuchids, proterochampsids, euparkeriids, and basal ornithischian dinosaurs, including forms close to the origin of archosaurs but poorly assessed phylogenetically. The new data suggest that the possibility of reaching and maintaining very high growth rates through ontogeny could have been a basal characteristic of archosauriforms. This was partly retained (at least during early ontogeny) in most lineages of Triassic pseudosuchians, which nevertheless generally relied on lower growth rates to reach large body sizes. This trend to slower growth seems to have been further emphasized among Crocodylomorpha, which may thus have secondarily reverted toward more generalized reptilian growth strategies. Accordingly, their “typical ectothermic reptilian condition” may be a derived condition within archosauriforms, homoplastic to the generalized physiological condition of basal amniotes. On the other hand, ornithosuchians apparently retained and even enhanced the high growth rates of many basal archosauriforms during most of their ontogenetic trajectories. The Triassic may have been a time of “experimentation” in growth strategies for several archosauriform lineages, only one of which (ornithodirans) eventually stayed with the higher investment strategy successfully.     

The first 50Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity

The evolutionary radiation of dinosaurs in the Late Triassic and Early Jurassic was a pivotal event in the Earth's history but is poorly understood, as previous studies have focused on vague driving mechanisms and have not untangled different macroevolutionary components (origination, diversity, abundance and disparity). We calculate the morphological disparity (morphospace occupation) of dinosaurs throughout the Late Triassic and Early Jurassic and present new measures of taxonomic diversity. Crurotarsan archosaurs, the primary dinosaur 'competitors', were significantly more disparate than dinosaurs throughout the Triassic, but underwent a devastating extinction at the Triassic-Jurassic boundary. However, dinosaur disparity showed only a slight non-significant increase after this event, arguing against the hypothesis of ecological release-driven morphospace expansion in the Early Jurassic. Instead, the main jump in dinosaur disparity occurred between the Carnian and Norian stages of the Triassic. Conversely, dinosaur diversity shows a steady increase over this time, and measures of diversification and faunal abundance indicate that the Early Jurassic was a key episode in dinosaur evolution. Thus, different aspects of the dinosaur radiation (diversity, disparity and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50Myr of dinosaur evolution is more complex than often considered.