Jawed fish are collectively known as gnathostomes. During Silurian (named after the Silures, a Celtic tribe that once inhabited south Wales) the first true-jawed fish acanthodians (spiny sharks) appeared. They were rarely longer than about 15 cm (6 inches). In contrast, Pterygotus, one of the largest of the eurypterids, grew to over 2.3 metres (7.5 ft) long.
The earliest land plant fossils appear to show a strong resemblance to parts of modern liverworts. Early vascular land plants (named for their internal canals through which water and nutrients circulate) grew to only a few centimetres tall but with deeper root systems than earlier plants they were able to colonise more of Earth’s surface. These plants included Cooksonia (named after Isabel Cookson), which has a simple, branching form; and Steganotheca, a more bush-like plant.
Plant debris on land tends to be nutritious and to provide protection from solar radiation. It is therefore not surprising that the first land animals were small arthropods such as millipedes and insects that could eat the debris, followed by larger arthropods such as centipedes and spiders that ate them.
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