sea floor. In trilobites, especially from the Cambrian,long genal spines were developed. These supported the weight of the whole exoskeleton,the thorax and pygidium being carried higher than the rest of the body prevented themtouching the sea floor and kept total bodily contact to a minimum. This higher carriage ofthe thorax and the pygidium is found primarily in the primitive trilobites only.Trilobites' jointed legs identify them as members of the phylum: Arthropoda, oneof the most successful of all animal groups. They were also among the first animals topossess the segmented outer skeletons. The combination of these physical features gavetrilobites distinct advantages in mobility, feeding, and self-protection. For any species oftrilobite its survival depended on obtaining a sufficient amount of food, on protecting itselffrom predators and other dangers in its environment, and successively completing moltingcycles and reaching maturity to enable it to produce young. In most instances, both sexes of trilobites occur separately. Males commonly passsperm to females in sealed packets called spermatophores. The males lay these packets onthe ground, and the females later pick them up, or the male deposits them into the female'sgenital opening. The fertilized eggs hatch after days, weeks, months, and even years. Thelarvae have little or no physical resemblance to adults. Larvae may also inhabit differentenvironments and eat different foods than their parents. The life spans of trilobites rangefrom a few weeks to several decades. The trilobite population sharply declined ca. 510 Maand finally became extinct ca. 250 Ma. The reason for their extinction is not clear,although it would seem that their decline may be related to the first arrival of sharks andother primitive fish in the Silurian and Devonian periods which may have found them agood source of food. The Trilobites fought back for a time, growing spines during theDevonian period but thes...
