Ecdysozoa arthropods

Key points

• Arthropods - such as lobsters, spiders, beetles and trilobites - have legs, a segmented body plan with jointed appendages and the ability to molt.

• The first major arthropod faunas of the Early Cambrian appear bizarre by modern standards but probably were no more morphologically different to each other than are living faunas.

• A number of arthropod-like animals in the Ediacara biota suggest an ancient origin for the phylum.

• Trilobites appeared in the Early Cambrian and during the Paleozoic evolved advanced visual systems and enrolment structures while pursuing a variety of benthic and pelagic life styles.

• The largest arthropods were the chelicerates and included the giant eurypterids that patrolled marine marginal environments during the Silurian and Devonian.

• Myriapods represent the earliest terrestrial body fossils in the Mid Ordovician, but trackways indicate euthycarcinoids (i.e. stem-group mandibulates) moved onto land even earlier, in the Late Cambrian.

• Insects first appeared during the Early Devonian and diversified rapidly; there are probably 10 million species of living insects.

• Insects had probably already evolved flight before the Mid Carboniferous, when giant dragonflies patrolled the forests.

• The crustaceans include many familiar groups such as crabs, lobsters and shrimps, together with the barnacles and ostracodes.

• Much of our knowledge of the early history of the phylum has come from exceptionally preserved fossils from the Cambrian Burgess Shale, Chengjiang and Sirius Passet faunas.

Whence we see spiders, flies, or ants entombed and preserved forever in amber, a more than royal tomb.

Francis Bacon, English philosopher (1561-1626)

Trilobitomorph: trilobite

Trilobitomorph: trilobite

Crustacean: ostracode

Crustacean: ostracode

Chelicerate: scorpion

Chelicerate: scorpion

Crustacean: decapod

Crustacean: decapod

Crustacean: barnacle

Insect: palaeopteran

Crustacean: decapod

Crustacean: decapod

Crustacean: barnacle

Insect: palaeopteran

Figure 14.1 Some of the main arthropod groups: a variety of forms based on a simple body plan of a tough exoskeleton and jointed limbs.

Insect: coleopteran

Insect: coleopteran

Figure 14.1 Some of the main arthropod groups: a variety of forms based on a simple body plan of a tough exoskeleton and jointed limbs.

ARTHROPODS: INTRODUCTION

Arthropods are a very common and spectacularly diverse group of legged invertebrates accounting for about three-quarters of all species living on the planet today, largely because of the phenomenal abundance of the insects. The basic body plan - conspicuously segmented, with jointed appendages adapted for feeding, locomotion and respiration -together with a tough exoskeleton, first appeared during the Early Cambrian and has since been exploited by a huge variety of living and fossil arthropods that pursue many lifestyles. All members of this phylum have both segmented bodies and appendages (Fig. 14.1); moreover the animal is differentiated into a head, thorax and abdomen, with often the head and thorax fused to form the cephalothorax. The possession of mandibles, or hard mouthparts, equipped many arthropods with the ability to process a wide variety of foods.

The arthropod exoskeleton is constructed mainly from the organic substance chitin. This is often hardened or sclerotized by calcium carbonate or calcium phosphate, so the potential for preservation is excellent across the group. The exoskeleton acts as a base for the attachment of locomotory muscles, permitting rapid movement, and is not usually mineralized. Although many arthropods undergo metamorphosis, virtually all the main groups grow by molting or ecdysis; first the endoskeleton is dissolved and second the old exoskeleton is detached along sutures while the new exoskeleton is generated. Exuviae, or cast-off coverings, are all that remain of the previous skeleton or cuticle of the animal: one arthropod can thus produce many potential skeletal fossils in its lifetime.

During a geological history of at least 540 million years, the five subphyla of arthropods (Box 14.1) have adapted to life in marine, freshwater and terrestrial environments. For a long time their closest living relatives were thought to be the segmented annelid worms, but new studies show that the closest sister group of arthropods is a clade of unsegmented worms that includes the priapulids and the nematodes or round worms. Their segmentation may thus either have arisen independently to that of the annelids, or may have been inherited from a very deep ancestor to both groups.

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