Jaws vs jawless: battle for the seas

Science - Pete Wilton | 07 Jul 11. 400 million years ago jawless vertebrates filled the oceans but today they are limited to only a few species: boneless, parasitic creatures such as lampreys and hagfishes. So what happened to this 'lost tribe' of ancient mariners? Were they the victims of environmental change or thrust aside by jawed newcomers? And how did fish evolve jaws anyway? Research reported in this week's Nature , by a team including Oxford University scientists, investigates the rise of jawed fish and gives us an ideal jumping off point to imagine what a dive through the ancient oceans would be like. Voyage to the Silurian - 'A scuba diver transplanted to the Silurian would find the kinds of vertebrates swimming around in those ancient seas alien and unfamiliar,' Matt Friedman of Oxford University's Department of Earth Sciences, co-author tells me. 'At this time, jaw-bearing creatures with backbones (our closest relatives) were something of an oddity, with most vertebrate diversity tied up in various groups of bizarre armoured fishes without jaws.' These jawless fishes were the ' ostracoderms ' ('bony skinned ones'), so called because they were covered in a protective armour made out of plates of bone. Typically between 15 and 60cm long, they had gills and balancing organs, and are thought to have sucked food into their mouths using a muscular pharynx. They shared the balmy Silurian and Devonian seas with some familiar neighbours, such as sea lilies, snails, and corals, but also more bizarre animals; trilobites, giant predatory sea scorpions (at up to two metres across enough to give any time-travelling diver a fright) and the shelled relatives of squid and octopus - of which the chambered nautilus is the only surviving example.