The Triassic period was named from three distinct layers found throughout Germany and northwest Europe. In Late Permian the cynodonts (‘dog teeth’) appeared. Generally less than 60 cm (2 ft) long, these animals became more abundant in the Triassic. There is evidence on the snouts of these creatures that they had sensitive whiskers. Whiskers are specialised hairs and hair on other parts of the body would have provided insulation and temperature control. Towards the end of the Triassic the cynodonts became smaller and smaller and apparently more and more mammal-like, until right at the end of the Triassic the earliest true mammals appeared in the form of tiny 10 cm (4 in) long shrew-like creatures such as Megazostrodon (200 mya). But then, instead of radiating and dominating the world, they were supplanted by the dinosaurs which remained supreme for the next 160 million years.
Mesozoic forests were populated by tree ferns and seed plants that were up to 60 metres (200 ft) high. During the Jurassic, conifers became the dominant land plants and in the Early Cretaceous the first flowering plants or angiosperms (‘vessel seeds’) evolved to become the dominant land floras. Plants provided the food base for the Mesozoic land animals.
The major advance was the perfection of seeds. There are two phases in seed plant reproduction: fertilisation and dispersal. Conifers and other plants produce enormous numbers of pollen grains which are released to blow in the wind until by chance some of the grains reach the pollen receptor of a female plant of the same species. This is a very wasteful process and a major factor in the later evolution of plants was the manipulation of animals to do the job instead.
The Early Mesozoic pollinators were the insects. Strong scents and brightly coloured flowers to attract the insects perhaps evolved around the same time as the provision of rewards such as nectar. Then the alternative method evolved in which animals carry the seeds away and drop them in a suitable place for them to grow. Animals can feed on the pollen or nectar, browse on the plants or simply brush against them as they pass. Animals tend to eat more plant material than they can possibly return in services to the plant. Angiosperms therefore developed a variety of structures and chemicals to protect themselves, such as spines, stings, contact irritants and internal poisons.
Sauropterygians (‘lizard flippers’) first appeared at (or just before) the beginning of the Mesozoic and flourished until the end of the era. These aquatic reptiles had a shoulder modification that allowed powerful flipper strokes. Pachypleurosaurs (‘thick ribbed lizards’) were around 60 cm (2 ft) long and lived in near-shore environments. These were succeeded by the nothosaurs (‘false lizards’), which grew to be several metres long and moved into shallow waters. Placodonts (‘tablet teeth’) were very sturdy with a short neck and trunk compared to nothosaurs and some developed heavy armour: probably, like the living walrus, they swam to the shallow seafloor to dig up molluscs and crush them between their powerful teeth.
The plesiosaurs (‘ribbon teeth’, named after the long ribbon-like backbone found in some) appeared in the Early Jurassic and were much more fully aquatic than the nothosaurs and placodonts. Many plesiosaurs had long necks and small heads. An average adult was about three metres (10 ft) long, but there were giants: Elasmosaurus (‘thin plate lizard’) was 14 metres (46 ft) long. Alongside the long-necked plesiosaurs there was another type known as the pliosaurs (‘more lizards’), which were mainly larger and more compact animals; Kronosaurus (‘lizard of Kronos’) was 12.8 metres (42 ft) long.
Ichthyosaurs (‘fish lizards’) appeared in the Early Triassic, flourished during the Jurassic and became extinct during the Late Cretaceous. These were the supreme swimming reptiles. An early form, Mixosaurus (‘mixed lizard’) had a long tail with a low fin’. This suggests that unlike the plesiosaurs which apparently used their limbs as paddles for locomotion in the water, this animal swam with side-to-side movements of its tail like a fish. Some forms became enormous. In the Late Triassic, Shonisaurus, named after the Shoshone Mountains where the fossils were found, was 15 metres (50 ft) long.
The ancestors of the archosaurs (‘ruling lizards’), the group that includes crocodiles, pterosaurs (‘winged lizards’), dinosaurs and birds, appeared in the Permian. True archosaurs appeared in the Late Permian and Early Triassic: Erythrosuchus (‘red crocodile’) and Archosaurus were land animals; Proterosuchus (‘first crocodile’) and Rutiodon (‘wrinkled tooth) were aquatic carnivores closely resembling crocodiles.
The first true crocodiles appeared in the Late Triassic: Terrestrisuchus (‘land crocodile’) was a rather long-legged land predator; Protosuchus and Notochampsa were more conventionally crocodile-shaped and probably at least partly aquatic.
There was wide variety of land-based archosaurs in the Late Triassic: Saurosuchus (‘lizard crocodile’) was very mobile but totally quadrupedal; Ornithosuchus (‘bird crocodile’) had a long tail counterbalancing the front part of the body with the hind legs bearing the total weight of the body and as a result this animal became a swift bipedal runner. Once this trait had started, then towards the end of the Triassic there was a minor burst of types. These included smaller carnivores such as the agile Lagosuchus (‘rabbit crocodile’) and the possibly tree-dwelling insectivores which may have been close to the pterosaurs, the first genuine flying reptiles.
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