Early Life IV: Life in the Seas

When most of the Cambrian creatures died out, they were replaced by an enormous variety of new creatures which thrived in the warm, shallow seas of the Ordovician and Silurian Periods. Some of those creatures, like starfish, sea lilies, and corals are still around today. The first coral reefs appeared 450 million years ago during the Ordovician Period.(1)


1. Artistic impression of ancient coral reef

During the Ordovician and Silurian Periods predators became experts at chasing and catching their prey. One group of predators were the nautiloids. They had excellent eyesight and long, grasping tentacles for catching food. Nautiloids were fast swimmers and moved around by shooting water in and out of their shells.(1)


2. Artistic impression of a nautiloid

The most ferocious predators were the eurypterids, or sea scorpions. The largest of these was the giant Pterygotus, which grew up to two meters (six foot six inches) long. It had sharp pincers in front of its mouth and used its tail as a paddle for moving quickly through the water.(1) Recent evidence, however, has scientist questioning if the sea scorpions really were such ferocious predators. (3)


3. Artistic impression of the Silurian sea scorpion Acutiramus cummingsi

At the beginning of the Ordovician Period, 510 million years ago, the first fish appeared. They were the first creatures with a backbone to support their bodies; the first vertebrates. These first fish had no jaws for opening and closing their mouth. They lived at the bottom of the sea, where they could suck up small particles of food from the seabed.(1)


4. Artistic impression of various Acanthodians

The first fish with jaws appeared during the Silurian Period. They are known as acanthodians. Because of their jaws, they could use their mouths for grasping and biting, so they could eat a much greater variety of food. Many of them became hunters.(1)


5. Artistic impression of various Acanthodians

A group of fish called placoderms, or ‘plated skins’, were particularly fierce hunters. Some were gigantic and had powerful jaws lined with sharp, jagged plates of bone.(1)


6. Artistic impression of Dunkleosteus terreli, one of the largest placoderms that have ever live.

After the Ordovician and Silurian came the Devonian. This period was the age of fish. (2) It was during this period that sharks first appeared in the oceans of the world. (1)


7. Early shark

Most fish that are alive today have bony skeletons. Almost all of these bony fish belong to a group known as ‘ray fins’. They have delicate, fan-shaped fins supported by fine, bony rods (or rays). A few bony fish belong to a group known as ‘fleshy fins’. Their thick fins are mainly bone and muscle, with a fringe of fine rays around the edges. It was from these fishes that the first land-living vertebrates evolved.(1)



8. Fossil of the fleshy-finned fish Eusthenopteron foordi


References:

1. The Usborne Internet-Linked Encyclopedia of World History, p. 28-31, 2002
2. Ackroyd, Peter, The Beginning, p. 28-43, 2003, Dorling Kinderley, London
3. Choi, Charles Q., Ancient 8-Foot Sea Scorpions Probably Were Pussycats, 30 December 2010, Last accessed: 6 March 2011, http://www.livescience.com/10353-ancient-8-foot-sea-scorpions-pussycats.html (With thanks to my anonymous commenter for keeping me up to date!)

Picture credits:

All pictures from Wikipedia Commons