BIOGEOGRAPHY OF THE MESOZOIC ERA
fte Triassic period marks the beginning of the Mesozoic Era, and began about 252 MYA. During the Triassic, Pangaea persisted, surrounded by the world-wide ocean Panthalassa and partially bisected by the Tethys Sea (Fig. 7.7). fte climate was mostly hot and dry, although there was more moisture near the poles, and periods of increased rainfall near the equator. ftere was little or no glaciation during the Triassic. Plant life during the period was dominated by conifers and other seed plants; the seed fern Glossopteris (Fig. 2.10) was abundant in the southern hemisphere. fte large mass extinction at the end of the Permian opened many new ecological niches that led to adaptive radiations. In the oceans, more modern corals appeared, and ammonites remained abundant, descended from ancestors that survived the mass extinction. Reptiles diversified in the oceans and on land, ultimately evolving into the diverse forms that are the reason the Mesozoic is often referred to as the “age of reptiles.” Plesiosaurs and ichthyosaurs became abundant and diverse in the marine world.
Figure 7.7: Pangaea and oceans about 249 million years ago at the Permian- Triassic boundary. (Wikipedia ‘Paleo-Tethys Ocean;’ Attribution: http://www-sst. unil.ch/research/plate_tecto/alp_tet.htm)
In the terrestrial environment, early semiaquatic archosaurs would diversify from their humble beginnings to eventually give rise to such dominant groups as dinosaurs and the first flying vertebrates, the pterosaurs. Archosaurs became dominant in the Triassic; their ability to eliminate nitrogenous wastes in the form of uric acid, which contains little water, was advantageous in the arid Triassic climate. fteropod dinosaurs appeared in the Triassic; this group would eventually give rise to the birds. Cynodonts, a group of therapsids that first appeared in the Permian, would increase in importance in the Triassic and eventually lead to the evolution of mammals.
fte Jurassic Period began about 201 MYA; its beginning is marked by the Triassic-Jurassic mass extinction. During this event, over half of all species went extinct, including all archosaurs other than dinosaurs and crocodylomorphs. Many amphibian species went extinct as well. ftis extinction event opened many ecological niches and made way for the diversification of the dinosaurs, which would become dominant in the Jurassic. During this period, the climate changed significantly; this was caused to a great extent by the splitting of Pangaea into two supercontinents, the northern Laurasia and southern Gondwana. ftis created more coastline relative to continental interior, leading to a general shift from arid to wetter conditions. As was the case in the Triassic, the Jurassic climate was generally warm, with little glaciation.
In the oceans, the dominant vertebrates were fish and marine reptiles such as ichthyosaurs and plesiosaurs. Many planktonic groups appeared and diversified. High levels of precipitated calcium carbonate produced large carbonate hardgrounds on the sea floor. ftis led to the increased importance of encrusting and boring communities that caused bioerosion of these hardgrounds. Rudists, a group of bivalve molluscs, appeared in the Jurassic and would become major components of reef communities. On land, many groups of ferns were present. Conifers and cycads were also common, as well as ginkgos. fte latter group is represented today by one relict species, Ginkgo biloba, a species that is found in the wild in China, but grown as an ornamental throughout the world. fte flowering plants, or angiosperms, appeared in the late Jurassic, but would not achieve dominance until the Cretaceous. Reptiles, and dinosaurs in particular, were the dominant vertebrates (Fig. 7.8). Sauropods, huge plant-eating dinosaurs such as Brachiosaurus and Brontosaurus, roamed the fern thickets and forests, and were preyed on by predatory theropods such as Torvosaurus and Allosaurus. Within the theropod clade, the first birds appeared in the late Jurassic. fte first mammals also appeared, primarily in the form of nocturnal insectivores.
Figure 7.8: Various dinosaurs roamed forests of similarly large conifers during the Jurassic period. (Wikipedia ‘Jurassic;’ Attribution: Gerhard Boeggemann)
fte Cretaceous Period began about 145 MYA and spanned 79 million years, making it the longest period in the Phanerozoic Eon. ftis period would see the continued separation of the landmasses, with Laurasia and Gondwana breaking up into the recognizable present day continents, although in different positions than they are today. With the exception of a cooling trend that began in the late Jurassic and extended into the early Cretaceous, the Cretaceous Period was relatively warm. ftis warming trend is thought to have been due primarily to the greenhouse effect of carbon dioxide produced by volcanic activity.
In the oceans, modern sharks and ray-finned fishes appeared. Marine reptiles such as ichthyosaurs and plesiosaurs (Fig. 7.9) remained dominant through most of the Cretaceous, but near the end of the period ichthyosaurs would decline, to be replaced by the mososaurs. Rudist bivalves were abundant, as were straight-shelled cephalopods in the genus Baculites. Sea stars and other echinoderms flourished, and diatoms diversified as well. On land, some groups of conifers and ferns continued to thrive as well. Angiosperms continued to spread and diversify, becoming the dominant group of plants by the end of the period. ftis coincided with the continued diversification of insects; coevolutionary relationships between angiosperms and pollinating beetles, flies, butterflies and moths, and especially bees have provided important selective pressures driving diversification. Among the vertebrates, mammals continued to radiate. But reptiles, in particular the dinosaurs, continued to remain dominant. ftese included such well-known dinosaurs as Tyrannosaurus rex, Triceratops, and Velociraptor of Jurassic Park movie fame. fte pterosaurs declined in the latter part of the period; this was once thought to be due to competition with birds, but patterns of bird diversification do not appear to correspond to pterosaur decline.
Figure 7.9: Kronosaurus queenslandicus preying on another plesiosaur,
Woolungasaurus. (Wikipedia ‘Cretaceous;’ Attribution: Dmitry Bogdanov)
fte Cretaceous was brought to a close by the Cretaceous-Paleogene mass extinction. While other factors may have played a role, the widely accepted cause of this mass extinction is an asteroid impact on the coast of Yucatán, Mexico. ftis hypothesis was first proposed by Nobel Prize-winning physicist Luis Alvarez (1911–1988) and his research team, which discovered extremely high concentrations of the element iridium (an element found in high concentrations in many asteroids) in Cretaceous-Paleogene boundary sedimentary layers worldwide. Since then, their hypothesis has been supported by the presence of tiny droplets of crystalized molten rock at the boundary layer, and, finally, the discovery of a very large crater on the Yucatán Peninsula.
Such an impact would have created a cloud of dust and aerosols in the air that would have dramatically decreased the amount of sunlight reaching the Earth. fte disturbance would have taken years to dissipate. Extinction of photosynthetic plants and phytoplankton was accompanied by disappearance of the herbivores dependent on them, and consequently their predators. Most famously, the non-avian dinosaurs went extinct, although some paleontologists believe that some survived into the Paleocene Epoch. ftis would be an example of a “dead clade walking;” a taxon that survives a mass extinction and persists for a few million years, but never recovers and eventually goes extinct. However, the evidence for Paleocene dinosaurs is scant, and most paleontologists reject this argument. In the marine environment, ca. 75% of species went extinct; among land animals, only insects and other invertebrates, and small vertebrates, survived. Detritivores (animals that feed on non-living organic material, or detritus), and animals that depended on detritivores as food, fared relatively well compared to herbivores and their predators. ftis makes sense, for most of the available food would have been in the form of dead, rather than living, plants and animals. Recovery of biodiversity from this event was slow, but eventually the profusion of available ecological niches led to the large evolutionary radiations of the Paleogene Period.