Mesozoic Era Paleobiology

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Mesozoic Era (245 to 65 mya)

One of the most striking events in the Mesozoic Era was the rise to dominance of dinosaurs in terrestrial ecosystems. The Mesozoic lasted from 245 to 65 million years ago, and is divided into three periods. The Mesozoic, which derives its name from the Greek with a rough meaning of middle animals, began after the Permian extinction and ended with the Cretaceous extinction. It comprises the Triassic, Jurassic and Cretaceous Periods. The Mesozoic is most famed for the Dinosaurs, and popular lexicon considers it the Age of the Dinosaurs (or Reptiles). The dinosaurs together with reptiles of all sizes ranging from the gigantic to the merely intimidating by human size standards, dominated the terrestrial environments. The flowering plants, or angiosperms, appeared in the Mesozoic. Importantly, by the end of the Mesozoic, the body plans and corresponding genetic developmental blueprints that define modern life forms were in place, although mammalian avian forms were still quite primitive.

The fossil record supports a theory of plate tectonics now widely accepted. At the start of the Mesozoic, nearly all of Earth's land is believed to have been united in a single supercontinent called Pangaea. Over the Mesozoic Era, Pangaea split into a northernmost continent Laurasia and a southernmost continent Gondwana. Laurasia then divided into North America and Eurasia, while Gondwana eventually separated into the four modern-day continents of South America, Africa, Australia and Antarctica. The Triassic’s climate was probably hot and humid, and thus particularly suitable for reptiles.

For 140 million years prior to the Cenozoic Era, dinosaurs held dominion over the land, as their stage was prepared by the great extinction at the end of the Permian Period. The cause and timing of dinosaur ascent have been broadly debated, but evidence is accumulating that a bolide strike rendered the earth only survivable by the hardiest of terrestrial animals, the dinosaurs.

The Permian-Triassic (P/T) Extinction Event marked the end of the Permian Period of the Paleozoic Era, and the start of the Triassic Period of the Mesozoic Era. The P/T extinction decimated the brachiopods, corals, echinoderms, mollusks, and other invertebrates. The last surviving trilobite Order, the Proetids, also did not survive. The P/T event set the stage for adaptative radiation in both land and marine environments. Corals belonging to hexacorallia appeared.

Among the enchinodermata, the inadunate crinoids, which had barely survived the end-Permian extinction with one family, finally disappeared. While crinoids were the most abundant group of echinoderms from the early Ordovician to the late Paleozoic, they nearly went extinct during the Permian-Triassic extinction. All the post-Paleozoic crinoids, namely the Articulata, are presumed to be a monophyletic clade that originated from the inadunate Order Cladida. However there seems to be a great gap between the morphologies of articulates and Paleozoic crinoids. Thus, a single genus of crinoid is known from the early Triassic, and is ancestral to all "deep water" extant articulate crinoids.

Other invertebrates, notably the bivalves, ammonoids and brachiopods recovered to dominate the marine environment, and the squid-like Belemites appeared and became abundant. New groups of echinoderms appeared as well. Marine reptiles were highly diverse, including the Sauropterygia, nothosaurs, pachypleurosaurs, placodonts, and the first plesiosaurs. The ichthyosaurs appeared in the early Triassic, and radiated into huge, marine-dominating species. Seed plants dominated the land, especially conifers to the north and the Glossopteris, or seed ferns, to the south. The first flowering plants (the Angiosperms) probably evolved during the Triassic

The Archosauromorph reptiles, and most notably the archosaurs, diapsid reptiles that first appeared during the late Permian, inexorably displaced the synapsids that had dominated the land during the Permian. The plant-eating dinosaurs that appeared during the mid-to-late Triassic were the first ornithischians, or bird-hipped dinosaurs, and were not larger than turkeys. The early dinosaurs got bigger, faster and more ferocious, and preyed on and eventually eliminated many of the larger reptiles so that only the smallest survived. The first flying vertebrates, the pterosaurs appeared in the Triassic. The most primitive mammals probably appeared in the late Triassic, descending from a mammal-like suborder of theraspid reptiles known as cynodonts.

The Triassic period closed with an extinction event that particularly affected marine life, including decimation of marine reptiles, except the surviving ichthyosaurs and plesiosaurs. A quarter of invertebrate families met extinction, as well as the conodonts. Actually, many extinction events punctuated the Triassic, which are believed to have provided additional selective pressures fostering the dinosaur radiation into emptied niches. Triassic land was predominately the supercontinent Pangea (meaning all the land) located near the equator. Sediments from the Triassic in the deeper ocean have been mainly subsumed by subduction of oceanic plates, leaving but a sparse marine fossil record. Dinosaurs would thus go on to be increasingly diverse and dominating in the subsequent Jurassic and Cretaceous Periods. The stereospondyl labyrithodont amphibians, a group that had been successful through the Triassic, suddenly and dramatically disappeared. Among other tetrapods, several reptilian orders also became extinct, including the protosaurs, nothosaurs, and placodonts.

Jurassic Period (208 to 146 mya) - dinosaurs rule the land (Jurassic Fossils)

While the dinosaurs appeared in the Triassic, it was during the Jurassic that they prodigiously radiated and ascended to be the rulers of the land. Dinosaurs are a clade of reptiles defined by somewhat ambiguous criteria. Compared with other reptiles, the dinosaur hind limbs are beneath the body. The pelvis extends vertically so that the hip socket vertically carries the load, rather than on lateral loading other reptiles. In recent years dinosaurs have been viewed as transitional between ordinary reptiles (especially crocodiles) and the birds. The fossil record supports the appearance of the large theropod dinosaurs within 10,000 years after the Triassic-Jurassic boundary.

The immense plant-eating dinosaurs (the sauropods) were ubiquitous and were the prey of the large theropods, including Ceratosaurs, Megalosaurs, and Allosaurs. Among plantae, Gymnosperms (especially conifers, Bennettitales and cycads) and ferns are common providing abundant food for the sauropods. Birds evolved during the late Jurassic. The pterosaurs, the flying reptiles, were common in the Jurassic. Fish and reptiles dominated marine environs. The ichthyosaurs, plesiosaurs, and marine crocodiles flourished, as did bivalves, belemnites, brachiopods, echinoids, starfish, sponges and ammonites among the invertebrates. As a general rule, the mammals remained diminutive and backstage during the Jurassic.

Cretaceous Period (146 to 65 mya) - (Cretaceous Fossils)

During the Cretaceous, the rays, modern sharks and teleosts, or the ray-finned fish became widespread and diverse. The marine reptiles persisted, including the ichthyosaurs in the in the Lower and Middle of the Cretaceous, the plesiosaurs throughout the Cretaceous, and the mosasaurs that dominated the Upper Cretaceous. Baculites, a straight-shelled ammonite, flourished in the seas. The Cretaceous also saw the first radiation of marine diatoms in the oceans.

The archosaurian reptiles, particularly the dinosaurs, continue to dominate the land. Climate changes due to the breakup of Pangaea allowed flowers and grasses to appear for the first time. The most well known dinosaurs, Tyrannosaurus rex, Triceratops, Velociraptor and Spinosaurus all lived in the Cretaceous. Pterosaurs remain common until the Upper Cretaceous when competition occurs from evolving birds. Mammals persist in their backstage existence among life on land. Insects became even more diverse as the first ants, termites and butterflies appeared, along with aphids, grasshoppers, and gall wasps. Another important Hymenopteran insect, the eusocial bee appeared, which was integral to and symbiotic with the appearance of flowering plants.

The Cretaceous ended at the so-called KT boundary, or the Cretaceous-Tertiary (K-T or KT) extinction event, that occurred some 65.5 million years ago. While the duration of this extinction remains unknown, half of all life’s genera disappeared; most famous was the extinction of the non-avian dinosaurs. Though many theories exist for the cause, the most widely-accepted is an impact on the Earth of an immense body from space.

Cenzoic Era Paleobiology Continued