分子時計
Trilobites in the Ordovician were very different from their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive eurypterids and nautiloids while other trilobites such as "Aeglina prisca" evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the trinucleids developed a broad pitted margin around their head shields. Some trilobites such as "Asaphus kowalewski" evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely. Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.