LAB 6

Chapter 8: Superphylum Arthropoda

8.1 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 are 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.

Figure 8.1 Some of the main arthropod groups: a variety of forms based on a simple body plan of a tough exoskeleton and jointed limbs (from Benton, 1997).

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 likely by the development of tagma from the segmented condition). For years, conventional wisdom has held that the Arthropoda 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.

8.2 Phylum Trilobita

Figure 8.2 Flexicalymene meeki, Waynesville Formation, Upper Ordovician (from Levi-Setti, 1993)

Trilobites are exclusively Paleozoic fossils found throughout the world. Though among the earliest 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 tagma: 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 members 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.

Figure 8.3 Triobite eyes. (A) Holochroal eye composed of closely packed lenses (Levi-Setti, 1993) (B) Schizochoroal eye with lenses separated by solid cuticle. (from Prothero, 1998).

8.2.1 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".

It has gradually become evident through ontogenetic studies that phylogenetic classification of the trilobites must be based on many characters and great care is necessary in their selection. Part of the phylum has been analyzed cladistically and it will probably be one of the first invertebrate phyla to be completely analyzed cladistically (along with the brachiopods and perhaps echinoderms). This is because the arthropods have more discrete morphological characters in general compared to such groups as the molluscs.

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.

8.2.2 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.

Order Redlichiida

• 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.

Order Ptychopariida

• 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.

Order Phacopida

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

8.2.3 Terminology
cephalon doublure schizochroal thorax pygidium gill branch exuviae holaspis	glabella suture fringe axial furrow biramous enrollment protaspis	free cheeks holochroal genal spine pleura walking leg ecdysis meraspis

8.3 Phylum Crustacea

With the possible exception of the trilobites, the crustaceans are the most common arthropods in the fossil record. They are almost entirely aquatic, being found in fresh, brackish and marine waters (terrestrial isopods and semi-terrestrial crabs are the only exceptions). Crustaceans have two pairs of antennae, and one pair of antennules. All appendages, except the antennules, are biramous, or secondarily uniramous. Three classes are important as fossils. 12.3.1 Classification

Class Ostracoda

• These unusual crustaceans have a bivalved shell that completely encases the body. They are usually small, about 0.5 to 4.0 mm long, but some species in the past grew to over 20 mm in length. Ostracodes can be found in practically any aqueous environment, from muddy puddles after a rainstorm to the deep sea. They have a variety of shell forms, and are common fossils in Ordovician through Quaternary sediments. These characteristics make them quite useful in biostratigraphy. They are also used frequently in paleoenvironmental reconstructions.

Figure 8.4 Top: generalized ostracode with left valve removed to show appendages. Bottom Left: exterior view of right valve of Microcytherura (Pleistocene x 70). Bottom Right: exterior view of left valve of Orionina (Pliocene, x 58) (from Boardman et al 1987).

Class Cirripedia

• These are the barnacles, which are sedentary, attached, shelled animals, rather different from other crustaceans. They may attach to almost any firm foothold in the shallow sea, from rocks and shells to boat hulls, wharves and whales. Their own shell consists of a series of overlapping plates, the top two or more of which are movable. To feed, the animal opens the top of its shell and extends its legs, which then kick to direct food-bearing currents to the mouth. The disarticulated plates of the shell are commonly found in nearshore sediments; scars are sometimes observed on mollusc shells where barnacles were attached. Most barnacles are only a few millimeters across, but a few grow much wider and are up to 12 cm long!

Figure 8.5 Sessile (acorn) barnacles, Semibalanus balanoides (from Brusca, 1990).

Class Malacostraca

• This group includes the shrimp, lobsters, crabs, and a host of minor forms. They bear appendages on all body segments, the first pair of which (behind the antennae) are usually modified into claws (chelipeds or chelae). Although the Malacostracans are diverse, almost all have 20 segments, with six fused to form the head, eight the thorax, and six the abdomen. The subclass is almost entirely aquatic, living in fresh and marine waters.

Figure 8.6 The diversity of malacostracan crustaceans is illustrated by these representative taxa: (A) The phyllocarid Ceratiocaris (Silurian); (B) The syncarid Palaeocaris (Carboniferous); (C) The eocarid Tealliocaris (Carboniferous); (D) The hoplocarid Acanthosquilla (Recent); (E) The decapod Eryma (Jurassic) (Clarkson, 1993; from Prothero, 1990).

8.4 Phylum Chelicerata

The chelicerates (sometimes called the Cheliceriformes) are one of the most successful animal phyla, including such familiar animals as spiders, scorpions, ticks and mites. They are also one of the oldest, being well established in the Cambrian. They are distinguished generally by having the first two tagma fused, and in general eight limbs. The first one or two pairs of limbs are often modified for feeding.

8.4.1 Classification

Class Merostomata

The typical merostome body is divided into two parts; a cephalothorax (or prosoma), and an abdominal tagma, the opisthosoma. They have anterior claws (chelicerae), and a spike like extension at the posterior end (telson). The opisthosomal appendages are biramous, with one branch serving as a gill. Merostomes are divided into two subclasses:

• Xiphosura: this group is characterized by a prominent telson and a relatively wide, subrounded prosoma. It includes the horseshoe crab and its fossil ancestors (the limulids), along with a primitive trilobite-like order known as aglaspids. Xiphosurans are now entirely marine, but some of the Paleozoic forms lived in freshwater.

Figure 8.7 Morphology of the recent horse shoe crab Limulus; Left: dorsal side , Right: ventral side (from Boardman et al, 1987).

• Eurypterida: this extinct group includes the largest known arthropods ever; some eurypterids ("sea scorpions") were over three meters in length! They have an elongate body with a spatula- shaped or spike-like telson, some form of chelicerae, and usually a pair or more of oar-like appendages. Eurypterids are among the most-studied of fossil arthropods because they are usually preserved with impressions of their important appendages (unlike most trilobites). This group was predominanlty brackish and freshwater. While their range is Ordovician to Permian, eurypterids are most common in Silurian and Devonian deposits.

Figure 8.8 Morphology of Eurypterus from the Silurian; Left: dorsal side; Right: ventral side (from Boardman et al, 1987).

Class Arachnida

Arachnids include the scorpions, spiders, ticks, mites and other related forms. The typical arachnid body is divided into a cephalothorax and an abdomen (except in ticks and mites, where they are fused together). The cephalothorax bears six pairs of uniramous appendages (only one extension from the basal protopodite), the first two of which are modified for feeding, and the last four for locomotion. Like insects, all arachnids are air-breathing. The earliest known arachnid is a well preserved scorpion from the Silurian of Sweden. The scorpions were at this time an exclusively freshwater group!

8.5 Phylum Uniramia

The Uniramia, so named for their exclusively uniramous appendages, include the millipedes, centipedes and insects. This phylum represents the single most successful evolutionary event in the history of multicellular life.

8.5.1 Classification

Class Myriapoda

• Subclass Diplopoda: These worm-like arthropods are often lumped with the Chilopoda (see below) into the Myriapoda (="10,000 legs"). Diplopods are the common millipedes found under rocks and logs in fairly moist climates. They are rather cylindrical and bear two pairs of legs on each segment. Diplopods were apparently originally a freshwater group, which may explain their common occurrence in the fossil record. They were one of the first animals to venture onto dry land, perhaps sometime in the Silurian.

Figure 8.9 A millipede from East Africa (from Brusca, 1990).

• Subclass Chilopoda: Chilopods are the centipedes, which differ from the diplopods (millipedes) mainly in having only one pair of appendages on each segment. The first pair of appendages behind the head is claw-like and functions as a set of poisoned pincers-larger centipedes can inflict very painful bites, and have been known to cause human fatalities on occassion. The chilopods appear to always have been terrestrial. They are most commonly found as fossils in Tertiary amber deposits.

Figure 8.10 A California centipede (from Brusca, 1990).

Class Insecta

The insects are by far the most diverse and common arthropods today. The typical insect body is divided into three distinct tagma: the head (composed of six segments), thorax (three segments), and the abdomen (eleven segments, usually with only ten apparent). The head bears a pair compound eyes, antennae, mandibles, and maxillae. Each of the three segments of the thorax have uniramous (unbranched) legs and, in most insects, the posterior two thoracic segments bear one or two pairs of wings.

Insects appear to have developed from an arthropod group that was already breathing air. They are mostly terrestrial-the few aquatic insects are usually able to live in the water by trapping air bubbles against their respiratory openings (spiracles). Terrestrial insects are one of only two groups of animals that are truly adapted to terrestrial life, in that they manage water to an extent that allows them to explore the terrestrial habitat fully (the other group is the Amniota, which includes Mammalia and Reptilia).

Wingless insects are known from rocks as old as Devonian, and the winged forms appear first in the Pennsylvanian. Most Paleozoic insects are known from swamp deposits, and most Mesozoic and Cenozoic fossils from air-fall tuffs and amber (although there are several significant Lagerstaten).

Figure 8.11 Diversification of the insects (from Prothero, 1990).

8.6 Terminology
segmentation abdomen opisthosoma cuticle mandibles biramous	metamerism cephalothorax carapace chitin gnathobases uniramous	thorax prosoma exoskeleton trachaea chelicerae

8.7 Questions

Phylum Trilobita

1. Look at the numerous specimens. Note that some of these specimens seem to have been molts. How can you tell? Note also that the specimens vary considerably in size. What accounts for this variation? Sketch an individual and label the following: glabella, cephalon, thoracic segment, pygidium.

2. Flexicalyrnene is one of the earliest genera to have the capability to enroll. Some specimens show this nicely. What advantage can you think of which might have led to selection for enrollment? Phacops is another trilobite from the Devonian which enrolled.

3. These samples show how densely fossils can accumulate in sediment. Does this necessarily reflect the real density of the living population. Why or why not? (Think of energy regimes, and also note the disarticulation and fragmentation of these fossils).

Other Arthropods

4. Barnacles are among the most abundant modern crustacean groups. They also have a geologic history (Ord.), but they are rarely seen in rocks older than the Tertiary. Look at these fossil specimens. What physiological and environmental factors may control their preservation?

5. Examine the examples of a modern horseshoe crab and look at the fossil representatives. How have they changed in appearance?

6. Look at the examples of fossil insects on display. Note how diversified their morphologies are. Give a scenario for how these insects were preserved.

Comparative anatomy of arthropods

You will be given handouts that contain schematic drawings of representatives of major groups of arthropods: trilobites (a trilobite), insects (cockroach), crustaceans (crayfish), chelicerates (a scorpion and a eurypterid).

Arthropod anatomy can be said to be variations on the themes of segmentation and specialization; some segments are repeated while others are fused; some limbs remain generalized while others perform specialized functions.

1. Label the following morphological features on the schematic drawings (some features may not be present in all groups):

Cephalon

Thorax

Eyes

Antennae

Walking limbs

Feeding limbs

Swimmerets

Wings

2. For each group illustrated, is it:

1. marine or terrestrial? What’s it the evidence?
2. If marine, benthic or pelagic? What is the evidence?
3. If terrestrial, walking or flying? What is the evidence?

3. Arthropods are often said to be the most “successful” group of animals. But that, of course, depends on one’s criterion of “evolutionary success”. Briefly discuss two criteria that could be used to measure evolutionary success. How do arthropods measure up by each criterion?