Ward's World Activity Guides

During the Jurassic and Cretaceous, the marine ecosystem appears to have become increasingly "high energy," possibly owing to an increased nutrient runoff fostered by angiosperm plant weathering of soils. The planktonic ecosystem experi- enced dramatic evolution, and diatoms and coccoliths became exceptionally common and formed large amounts of sediment. Radiolarians flourished in offshore settings and planktonic foraminifera appeared for the first time in the Cretaceous. This rise of primary and secondary producers was matched by unprecedented evolution of the predatory consumer guilds. A second and perhaps more intense phase of predator-prey interaction termed the "Mesozoic marine revolution" yielded highly complex predator-prey interactions involving a host of new predators. Teleost fishes, some of them huge, shared the key advance of a swim bladder for buoyancy regulation and increased maneuverability. A host of more efficient preda- tory sharks appeared, and these were matched by an unprec- edented number of large predatory marine reptiles. Ichthyo- saurs, plesiosaurs, and sea-going crocodilians flourished in the middle-to-late Mesozoic Era, and mosasaurs (giant swimming relatives of the monitor lizards) diversified in the Cretaceous. With the exception of sharks and bony fishes, most of the large predators died out near or at the end of the Cretaceous. The niches left vacant from these large reptiles were later partially filled by predaceous whales beginning in the Eocene. However, the slightly later occurring baleen whales, the most gigantic animals on Earth, feed directly on plankton, which they sieve from the water. Terrestrial ecosystem The terrestrial ecosystem evolved late relative to the marine ecosystem. Spore-bearing land plants such as mosses and li- chens may have invaded rocky coastlines as far back as the Pro- terozoic; however, vascular land plants—tiny psilophytes—did not appear until the Late Ordovician or Silurian periods about 440 million years ago. These plants possessed the key adap- tive breakthrough of tracheids to allow conduction of fluids up the stem, but they lacked both roots and leaves. The first land invertebrate animals, probably relatives of the millipedes, ap- peared at about the same time, as evidenced by trace fossils in paleosols (ancient soil deposits) from the Ordovician of Penn- sylvania. Many new plant groups, including lycopods, sphenop- sids ("horsetails"), ferns, and the ancestors of gymnosperms, evolved during a very critical time in land plant evolution in the Devonian Period (about 416–359 million years ago). The old- est known fossil forests, supporting trees up to 15 m (50 ft) in height, are known from stumps preserved in place in paleosols in eastern New York. Late in the Devonian (approximately 365 million years ago), one group of trees also evolved the first seeds, a reproductive breakthrough that permitted invasion of drier, upland areas. Associated with the radiating land plants in the Devonian were some of the first spiders and insects. Also, at about this time, the first semiterrestrial vertebrates, the early tetrapods, ancestors of modern amphibians, evolved apparently from lobe-finned fishes or crossopterygians. These animals pos- sessed several key adaptive breakthroughs, including fully developed lungs and tetrapod limbs with digits, effective for walking on land. During the succeeding Carboniferous Period, coal swamps flourished in coastal wetlands fringing the rising Appalachian Mountains (in the eastern United States) and the correspond- ing Hercynian Mountains (in ancestral Europe). Coals, typically overlying heavily leached soils or underclays, were formed largely of primitive plants such as lycopods and seed-ferns. Casts of stumps of these plants at Joggins, Nova Scotia, have yielded skeletons of some of the oldest animals assignable to the reptiles. These animals possessed another key evolutionary breakthrough, amniotic eggs, with protective membranes and a shell to prevent desiccation of embryos; this feature allowed reptiles to become the first fully terrestrial animals. Drying of coal swamps during the following Permian Period led to a shift toward drier, upland vegetation, including conifers. Also, new groups of reptiles appeared, such as the mammal-like reptiles, a precursor of which included the pelycosaurs. Some, such as Dimetrodon, possessed specialized vertebral "sails," which pos- sibly aided in thermal regulation. The so-called mammal-like reptiles or therapsids also flourished in the Late Paleozoic. Both groups underwent extinction at the end of the Paleozoic Era. Also during the Carboniferous Period, primitive insects that were flightless evolved both wings and wing-folding ability, enabling a more fully aerial mode of life. During the Late Triassic Period, about 220 million years ago, the dinosaurs appeared. These animals dominated Mesozoic terrestrial faunas, but they coexisted with small, furry, and warm-blooded mammals, which also evolved in the Late Triassic from therapsid ancestors. Notably, flight evolved twice among the archosaurs. First, the pterosaurs (batlike reptiles that are a sister group to the dinosaurs) appeared in the Triassic; by the Cretaceous, these creatures evolved into the largest flying animals ever, with wingspans exceeding 15 m (50 ft). Paleontology (continued) + ward ' s science

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