Some annelids have small [chitinous] jaws and teeth which also may be preserved as fossils. These dental structures are called scolecodonts and are microfossils.
[Phylum] Arthropoda
The arthropods are one of the more advanced groups of invertebrates, and they are known from the [Cambrian] to the Recent ([Pl. 34]). Modern representatives of this group include the crabs, shrimp, crayfish, insects, and spiders. Arthropods vary greatly in size and shape and are among the most abundant of all animals. They have become successfully adapted to a wide [variety] of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a [chitinous] [exoskeleton] which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.
Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein.
Subphylum Trilobitomorpha
The members of this subphylum are extinct arthropods which were most abundant during early [Paleozoic] time.
CLASS TRILOBITA.—
The trilobites are a group of exclusively marine arthropods which derive their name from the typical three-lobed appearance of their bodies ([fig. 21]a). The [trilobite] body is divided into a central or axial [lobe] and two [lateral] lobes. The body of the animal was encased in a [chitinous] [exoskeleton]. The top part of this exterior covering, the [carapace], is very thick, and it is this part of the trilobite that is usually preserved.
Plate 34
[FOSSIL] ARTHROPODS
