OTHER LIFE IN THE JURASSIC

Besides dinosaurs, what other land animals were present during the Jurassic period?

Although the dinosaurs were the dominant animals, there were other animals that existed during the Jurassic period. Because of the relative stability of the climate and the lush vegetation, many land animals diversified and increased in numbers. But not all creatures survived through the Jurassic period; many became extinct, probably because competition for food increased. Below are short descriptions of the types of animals that existed at the time:

Amphibians

Frogs and salamanders: First modern frogs and salamanders appeared.

Reptiles

Turtles: First modern turtles appeared in the Early Jurassic period; they were able to retract their heads into their upper shell.

Lizards: First true lizards appeared in the Middle Jurassic period.

Crocodylians: First true crocodylians appear, and were small, three-foot- (one- meter-) long reptiles that walked on all fours; they had longer hind legs, indicating that their ancestors were bipedal.

Therapsids: Few families of therapsids, or mammal-like reptiles, lived into the Middle Jurassic period; the anomodonts and therocephalians no longer existed, but the cynodonts did survive into the Middle Jurassic.

Mammals

Small mammals: Small mammals became more profuse and diverse during the Jurassic period; they were still very small animals, about the size of a mouse or rat, with the largest the size of a cat; they were mostly nocturnal.

Triconodonts: Late Triassic to Late Cretaceous mammals; one of the oldest fossil mammals; three cusps of teeth in a straight row give them their name.

Haramyoids: Late Triassic to Middle Jurassic mammals; one of the oldest fossil mammals; their teeth had many cusps in at least two parallel rows.

Symmetrodonts: Late Jurassic to Early Cretaceous mammals; they had upper and lower cheek teeth with many cusps in a triangular pattern.

Docodonts: Middle to Late Jurassic period mammals; they had elaborate cheek teeth, with most of the cusps in a T-shape.

A reef shark swims near the Bahamas. As far back as the Jurassic period, sharks swam in Earth's oceans and were some of the deadlist predators of the deep (iStock).

Multituberculates: The multituberculates were the largest group of mammals in the Mesozoic, first appearing in the Late Jurassic period.

What were the major marine animals living during the Jurassic?

There were many marine animals that thrived during the Jurassic period, ranging from small seabed dwellers to large swimming predators. Most of these animals were similar to those found in the Triassic period, although many had diversified and increased in number during the Jurassic period.

Modern shark families developed at this time; bony fishes with symmetrical tails, the teleosts, diversified (they account for the great majority of modern fishes over 20,000 species). The first oysters evolved; modern squids and cuttlefishes appeared; and squid-like belemnites diversified. The ammonoids almost disappeared during the late Triassic extinctions; one family survived (out of eight) and quickly diversified during the Jurassic period.

The ichthyosaurs, such as the Ichthyosaurus and Stenopterygius, flourished in the Jurassic period oceans. Plesiosaurs were also abundant, such as the Muraenosaurus, a Late Jurassic period reptile with a 36-foot (11-meter) neck (it included 40 vertebrae); and the Liopleurodon, a short-necked pliosaur with a huge, elongated head was 39 feet (12 meters) long, with a 10-foot (3-meter) head.

What was so special about the evolution of the ichthyosaurs?

Ichthyosaurs were streamlined, dolphin-shaped reptiles, but their backgrounds differed from many reptiles living around them: These creatures ancestors went back to the sea after living on the land. Some scientists believe that ichthyosaurs were the first major group of reptiles to return to the sea. At first, they no doubt stayed close to the shoreline, just as seals and walruses do today. But after millions of years, the creatures went into the oceans, spread, and eventually became totally fish-shaped.

The oldest ichthyosaur fossils, and the most primitive so far, are 240-million- year-old fossils found in Japan. The primitive ichthyosaur, measuring about 9 feet (2.7 meters) long, probably lived its entire life in the water. Its shape was not yet like a dolphin; and its pelvis bone was still attached to the vertebrae, similar to those of land animals and dissimilar to the later ichthyosaurs. The primitive ichthyosaurs also had fins that were similar to the limbs of land reptiles (with splayed fingers). In other words, this primitive fossil shows the first step the ichthyosaurs took from the land to the oceans.

There was one other strange characteristic of the ichthyosaurs: For unknown reasons (as evident in the fossil record), about 135 million years ago, the animals began to fade away, becoming totally extinct between 90 and 100 million years ago. This was much earlier than the demise of the dinosaurs about 65 million years ago.

 
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