The Rough Guide to Evolution (Rough Guide SciencePhenomena) by Unknown

Author:Unknown
Language: eng
Format: mobi
Published: 2011-08-18T19:30:00+00:00

Limnoscelis, a five-foot-long reptile-like amphibian. During the Permian period, terrestrial tetrapods evolved into the first large herbivores and carnivores.

Life in the Mesozoic

The end-Permian mass extinction, sometimes labelled the Great Dying, eliminated as much as 96 percent of marine species and three quarters of terrestrial organisms. Trilobites, eurypterids and graptolites all disappeared. Among fish, just a few families pulled through into the first period of the Mesozoic, the Triassic (251–199Ma). Trematosaurids became the only amphibians ever to take to the sea. Some reptiles, the sauropterygians, also adjusted to aquatic environments, growing flippers on specially adapted shoulders; the most primitive sauropterygians, pachypleurosaurs, resembled elongated semi-aquatic lizards. Another early group, the nothosaurs, lived like seals, feeding in water and basking on beaches. Some aquatic reptiles, such as the placodonts, grew armour plating, while others, the thalattosaurs, developed sleek elongated bodies up to four metres long. Towards the end of the Triassic, the first plesiosaurs appeared: long-necked, carnivorous, fully aquatic reptiles that were to flourish later in the Mesozoic. Another star of the Mesozoic oceans, the ichthyosaurs (large dolphin-like reptiles), took to the sea early in the Triassic and then diversified. The largest of these, Shonisaurus sikanniensis, was equivalent in size to a sperm whale.

In what has been dubbed the Triassic takeover, various archosaurs steadily took over from synapsids as the dominant large land animals: these include the rauisuchians (large predators with a distinctive erect stance), the crocodile-like phytosaurs, the plant-eating armoured aetosaurs and the carnivorous poposaurs. Rhynchosaurs, stocky beak-snouted herbivorous sauropsids, also flourished in Triassic terrestrial environments. In the late Triassic, the first turtles appeared and one group of archosaurs, the ornithodira, split into two lineages whose descendents would characterize the rest of the Mesozoic: the pterosaurs and the dinosaurs. Eoraptor, a small two-legged carnivore discovered in the early 1990s is probably the best-known candidate for the first dinosaur. Also, in the Late Triassic, a new offshoot of the cynodont lineage appeared, which gave rise to the mammals (although what counts as a mammal and what counts as a “mammaliform” remains a matter of semantics). Morganucodon watsoni (named after Glamorgan in Wales), is the best example of an abundant, early proto-mammal from the Late Triassic.

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