Palaeontologists make key contributions to our understanding of evolution on the large scale, macroevolution. These themes address fundamental questions of interest to many: why do groups come and go through time, and is evolution progressive? New tools allow us to explore changes in diversity (taxon richness), relative abundance (ecological significance), and disparity (morphospace occupation; range of adaptation). Palaeobiologists have focussed on mass extinctions, but clade origins are more important for an understanding of evolution: they are creative, and they shape the biosphere. Our special contribution is to combine cladistic phylogenetic information with classic stratigraphic and statistical approaches to drill deep into the major events of the past.
Current projects include these:
- Amniote origins and climate change. Amniotes (= reptiles + birds + mammals) originated some 315 million years ago, in the Late Carboniferous, at the time of the great coal forests. Dominant land vertebrates were amphibians of all sizes that lived in and around the water. The first reptiles laid eggs with shells and so broke the link with permanent water, and could live in new environments. Our new work has revealed footprints of amniotes older than the oldest skeletal fossils, and importantly that the switch-over from amphibian-dominated to reptile-dominated faunas happened at the time of the rather sudden decline of the great coal forests, 305 million years ago.
- The Permo-Triassic bottleneck. During the Permo-Triassic mass extinction, somewhere between 80 and 96% of species disappeared, meaning that only 4-20% of species survived. The severity of the
crisis was as great for life on land as in the sea, as we have found in detailed studies of terrestrial tetrapods. In a new study of dicynodonts, dominant herbivores before and after the event, Marcello Ruta and Mike Benton find that, while Triassic diversity recovered more or less to pre-extinction levels, disparity did not. The post-extinction dicynodonts radiated from the two or
three lineages that survived the crisis, and their range of morphology and adaptation was remarkably less than for the Late Permian forms. This is the first numerical demonstration of a macroevolutionary bottleneck effect.
- Adaptation, competition, and chance in the origin of dinosaurs. We explored the rates of evolution and patterns of morphospace occupation during the first 50 million years of dinosaurian evolution. The two disputed models for dinosaurian origins are the 'competitive' and 'opportunistic' models, and the latter is supported by the fossil record. There were two extinction events, one at the Carnian-Norian boundary, some 225 Mya, when dominant herbivores (rhynchosaurs, dicynodonts, chiniquodontids) died out, and the second at the Triassic-Jurassic boundary, when dominant carnivores (crurotarsans) died out. In the interval between these two events, the Norian and Rhaetian stages of the Late Triassic, dinosaurs lagged behind crurotarsans in evolutionary rate and in morphospace occupation. Not much changed after the disappearance of crurotarsans. In this case, the disparity of Dinosauria apparently increased more slowly than their diversity during the first 50 Myr of their evolution.
- Numerical study of the evolution of dinosaurs. We produced the first supertree of Dinosauria in 2002 and a second in 2008. This provided the basis for an investigation of patterns of dinosaurian evolution. By considering rates of evolution and statistically significant diversification shifts, we found that dinosaurs did most of their fast evolving early, in the first third of their history. They continued to radiate steadily through the Mid and Late Jurassic and Cretaceous, but they apparently did not profit from the Cretaceous Terrestrial Revolution, the time when angiosperms, insects, and smaller vertebrates radiated explosively. Further, dinosaurs seemed to continue at steady diversity levels to their end at the end-Cretaceous mass extinction, neither declining, nor expanding in diversity.
- Evolution of Mesozoic marine reptiles. Five major clades of amniotes dominated Mesozoic seas: the Ichthyopterygia and Sauropterygia, which originated in the Early Triassic, and radiated sporadically throughout the Mesozoic, as well as marine crocodiles (especially Jurassic thalattosaurs), marine turtles, and marine lizards (mosasaurs). New studies allow us to explore the waxing and waning of these groups, importantly exploring changes in diversity and disparity, evolutionary rates, and responses to extinction crises. New work shows that ichthyosaurs were hit hard by the end-Triassic mass extinction and their morphospace occupation did not recover in the Early Jurassic, fllowing this bottleneck.
- Evolution of pterosaurs. The Mesozoic flying reptiles were important components of terrestrial ecosystems through the Late Triassic to Late Cretaceous. Early pterosaurs, commonly called 'rhamphorhynchoids', include a variety of Late Triassic to Early Cretaceous forms, but the derived pterodactyloids of the Late Jurassic and Cretaceous showed most range in morphology and body size. A new study shows that, although diversity shows remarkable vicissitudes, doubtless relfecting a very patchy fossil record, disparity was highest in the Early Cretaceous, a time of remarkable experimentation within the group.
- The island rule and dwarfing. It is often said that animal body size changes according to geographic area. On islands, it seems that many large animals become smaller, and small animals may beciome larger. Classic examples are dwarf elephants and dwarf deer found on Mediterranean islands in the Pleistocene and Holocene. New work has extended the concept to dinosaurs, and especially the Late Cretaceous dinosaurs from Romania.
- Dinosaur names, error, and biodiversity. What if more than 50% of named species are false? The first dinosaur genus, Megalosaurus was named in 1824, and the rate of naming of new species has risen from one or two a year at that time, to about 20 per year now. This means that a new dinosaur is named every two weeks.
Are all these new species valid, or are palaeontologists getting carried away by the intense public fascination in dinosaurs? Getting the species list right is essential not only for palaeontologists, but also for evolutionary biologists. If you want to investigate origins, diversifications, and extinctions, you must have an accurate list of taxa. The same is true for studies of modern biodiversity and for setting conservation priorities. New studies on the fossil
record of dinosaurs shed some light, and give some hope that taxonomists are doing their job
better now than 100 years ago.