Paleontologia fossils英文电子书.pdf

BGS©NERC Click on the fossil names above or below to find out more Ammonite | Belemnite | Bivalve | Brachiopod | Coral Crinoid | Echinoid | Fish | Foraminifera | Ostracod | Plant | Trilobite BGS©NERC Trilobites rank among the most important of early animals. Our knowledge of them has been gained from the study of their fossils, usually impressions left of their shells after burial in sediment which subsequently hardened into rock. They appeared abruptly in the early part of the Cambrian Period, and came to dominate the Cambrian and early Ordovician seas. A prolonged decline then set in before they finally became extinct in the Permian Period, about 250 million years ago. Trilobites were arthropods—like crustaceans, spiders and insects. They are divided into three parts from head to tail (head, thorax and tail) and from side-to-side (the central axis and two lateral lobes). The word trilobite refers to the side-to-side partition, not the head, thorax Specimen of Calymene from the Silurian Period. Calymene was able to roll into a ball for protection. and tail. BGS©NERC Lifestyles of trilobites: the painting shows how trilobites from different periods lived in the sea. In shallow waters amongst the bivalves (1), crinoids (2), alg