2.1 Introduction

Approximately three-fourths of all known animals are arthropods-they constitute the largest phylum in the animal kingdom (estimated at well over one million extant species). Forms familiar to us axe the insects, spiders, crabs and trilobites. Many of the first complex fossilized animals were arthropods and, if their rapid adaptations to changing environments are any indication, arthropods will probably be the last living animals on Earth. They have exploited every known environment; arthropods are found at all depths in marine, brackish, and fresh waters. They are common in hot springs, saline lakes, ephemeral ponds, and underground rivers. On dry land they are found in deserts, forests, grasslands and tundra. Many of the species are adapted to parasitism. Indeed, of all animals with the possible exception of humans, the arthropods have made the greatest impact on the Earth's biosphere.

Arthropods have segmented bodies with two or more distinct regions termed tagma. The body is covered by a tough, sometimes flexible, chitinous exoskeleton that serves for both protection and the attachment of muscles and other soft tissues. This covering is a simple device that may explain in large part the startling success of the group. It can be mineralized selectively to enhance its protective function and yet maintain sufficient articulating ability. It is the basis for a variety of arthropod innovations, from claws and antennae to wings. In terrestrial arthropods, it serves as an excellent barrier to dehydration.

The external nature of the exoskeleton does have one limitation-it does not grow continuously with the internal soft parts of the animal. To increase in size, many arthropods must periodically shed the exoskeleton and secrete a larger one, a process known as molting or ecdysis. The discarded molts, or exuviae are often preserved as fossils.

Each tagma of an arthropod, including those fused together, typically bears a pair of jointed appendages. In the head region, one or two pairs are modified into long sensory structures called antennae or antennules. Tooth-like jaw appendages termed mandibles, are also usually present. Maxillae are limbs in the head region that are modified to pass food to the mouth. Most of the other appendages function as walking or swimming limbs; in aquatic forms they may have attached gills. In one group (the chelicerates), the first pair of appendages have pincer-like arrangements at their tips termed chelicerae, and there are no antennae.

Arthropod classification and phylogeny have always been controversial. It is obvious, for many reasons that we cannot go into here, that the arthropods are all descended from polychaete (Phylum Annelida; segmented, chitinous marine worms) ancestors. The controversy centers around whether all the arthropod groups share a single annelid ancestor, or if there have been several instances of annelid worms giving rise to an "arthropod" group (most likeley by the development of tagma from the segmented condition). For years, conventional wisdom has held that the Arhtropoda constitute a polyphyletic group, and each major group of arthropods therefore represents a separate phylum. More recent phylogenetic work however, both molecular and morphological (based primarily on Cambrian and other fossil taxa), seems to support arthropod monophyly. In this lab we will therefore list Arthropoda as a superphylum, and the individual groups as separate phyla. The phyla are Trilobitomorpha, Crustacea, Chelicerata and Uniramia. There are also a number of minor and obviously related phyla that we will not be able to cover. Incidentally, monophyly of the Arthropoda makes the evolution of the arthropods the single largest adaptive radiation in the history of eukaryotic life.

12.2 Phylum Trilobita

Trilobites are exclusively Paleozoic fossils found throughout the world. Though among the eaxliest of the fossilized invertebrates, trilobites are in many ways the most advanced of the already complex arthropods. Because of their extreme diversity of form, and their abundance in certain rocks, trilobites are always popular fossils for study.

The term "trilobite" refers to the division of the anterior tagma into three longitudinal parts: an axial lobe in the center, and two flanking pleural lobes. Trilobites are also divided transversely into three tagms: the head (cephalon), a middle section called the thorax, and a "tail", properly termed the pygidium. Like most arthropods, these parts are covered with a hard exoskeleton, which in trilobites is made of CaCO₃, chitin, and sometimes CaPO₄.

The cephalon of trilobites bears a variety of structures important for both the classification of the phylum and for the interpretation of their life modes. The axial portion of the cepahlon is inflated to varying degrees and termed the glabella. Most of the phylum have two eyes on either side of the glabella, each of which usually contains several lenses. All trilobites, except the agnostids, have facial sutures on the cephalon that mark where the exoskeleton would split during ecdysis. These sutures are faint lines that follow three patterns; protoparian and hypoparian sutures are confined to the cephalic margin. Proparian sutures are mostly dorsal but do not meet the posterior dorsal margin of the cephalon. Opisthoparian sutures are predominantly dorsal and do meet the posterior dorsal cephalic margin. The portions of the cephalon on both sides of the glabella and inside the boundaries of the proparian and opisthoparian facial sutures are called fixed cheeks. The two dorsal cephalic parts outside the sutures are termed free cheeks. The posterior lateral corners of the cephalon sometimes bear a set of long extensions called genal spines. The hypostome is a plate found on the underside of the cephalon in front of the mouth.

The trilobite thorax consists of at least two (usually many more) unfused, articulated segments. Those portions that are part of the pleural lobes are called pleura (singular pleuron). Pleural spines are pointed lateral extensions of the pleura.

The pygidium of trilobites is made of anywhere from one to thirty fused segments. It may also bear spines along the margin.

Trilobite appendages have only been found rarely-their exoskeleton was not mineralized like the rest of the body. The limbs of some trilobites were biramous, meaning that there were two appendages (exopodite and endopodite) extending from the basal unit (protopodite). Very often one of the extensions is a gill. A few trilobites have been found with attached antennae.

Trilobite eyes are especially interesting because they are the most ancient fossilized visual systems (though, contrary to popular claims by some paleontologists, they most likely do not represent the oldest visual systems, nor do they tell us much about the evolution of animal sight). Great variation in morphology, structure and size of trilobite eyes suggests differential use of vision among different types. The eyes are usually crescentic in shape, but can be globose, conical, stalked or fused anteriorly into a single band. The composition of the eye lenses account for their unusual preservation. Each lens is composed of a single calcite crystal oriented with its principal optic axis (the c axis) normal to the visual surface. Such precise crystal orientation is functionally desirable because it eliminates polarized rays and does not produce a double image. There are two basic types of trilobite eyes: holochroal and schizochroal. Holochroal eyes are characterized by close packing of biconvex lenses beneath a single cornea. These lenses are generally hexagonal in outline and range in number from one to more than 15,000! Schizochroal eyes on the other hand are an aggregated type of eye. These eyes are made up of a few to more than 700 relatively large, thick lenses, each covered by a separate cornea. Each lens is positioned in a cylindrical mounting and is separated from its neighbours by cuticular-type material.

12.2.1 Classification

Many of the major groups of trilobites appeared rather suddenly in the early Cambrian with derived characters and non-obvious interrelationships. A previous scheme of classification used the type of facial sutures as a primary taxonomic character, but subsequent work showed that these features were often very derived. We will use the system adopted in the Treatise of Invertebrate Paleontology;

• cephalic axial characteristics (number of segments and structure of pleural lobes), facial suture patterns, and 3) pygidial type. We are still far from a "natural classification".

Some of the most exceptional specimens of Cambrian trilobites come from the Burgess Shale in British Columbia, Canada. It is from these Konservat-Lagerstatten forms that we have gotten much of our understanding of trilobite soft anatomy. This aids in functional morphological analysis, as well as systematic study of these extinct critters. The Burgess Shale itself is interpreted as being deposited, in part, as a slumped block of fine-grained material. The slumping is an important aspect of the preservation, for it placed the entire block into anaerobic water, not only killing the organisms, but keeping them from serious decay and postmortem scavenging. Rapid burial of the block entombed them and the fine-grained dark grey-black fossil-rich rocks are the result.

12.2.2 Taxonomy

Order Agnostida

• The agnostids are the smallest of the trilobites. An individual has only two or three thoracic segments, and its pygidium is approximately equal to its cephalon in size and shape. Agnostids were probably planktonic.

• These were the first trilobites, appealing in the earliest Cambrian. They are large and spinose, with crescentic eyes and a relatively wide cephalon. Redlichiids also have a very small to rudimentary pygidium.

• This large order of trilobites is virtually impossible to characterize-it may in fact be an entirely artificial and polyphyletic group. The most we can say is that many have a forward-tapering glabella, more than three thoracic segments, and a relatively small pygidium.

• This group is again difficult to define. Most phacopids have a medium-sized to large pygidium, but even this is a generalization.