Animal phyla – 500 million years ago and today

By the end of the Cambrian, every major animal phylum known today was firmly established. Primitive members of animal phyla found in the Burgess Shale include the following:

Annelida: These elongate, many-segmented animals are represented today by earthworms and leeches. The annelid body is covered by a thin flexible cuticle that is not moulted after the adult stage is reached.

Arthropoda: Today, arthropods are the most diverse of all animal groups. Characterized by a segmented body, rigid exoskeleton, and jointed limbs, this group is represented today by insects, spiders, centipedes, and crustaceans. Arthropods grow by shedding their exoskeleton (a process called moulting), which can harden or even mineralize in some cases (such as in crabs).

Brachiopoda: Brachiopods are bottom-dwelling marine suspension-feeding animals enclosed in a two-part shell. Most forms attach to a surface (either the sea floor or on other organisms) via a flexible cylindrical organ called a pedicle. Although some species still survive, the phylum was hit hard by the Permian mass extinction about 250 million years ago.

Chordata: Chordates are a group of animals united by the possession of a notochord. Members of this phylum include mammals, fish, and reptiles with their defining backbone and spinal chord.

Ctenophora: Ctenophores are radially organized animals with a simple body plan superficially resembling that of a jellyfish. Living representatives are termed "comb jellies" because they have 8 comb-like rows of cilia (small elongated extensions of cells which can reach up to 2 mm) to propel them through the water.

Echinodermata: The echinoderms form a distinctive group of mostly benthic animals characterized by a mineralized skeleton. Almost all adult echinoderms exhibit fivefold symmetry. Modern groups include sea stars, sea urchins, and sea lilies (crinoids).

Mollusca: A large group of animals, today characterized by a cavity-forming mantle. Modern molluscs include snails, squids, and clams.

Onychophora (Lobopoda): Worm-like animals with unspecialized pairs of non-jointed limbs. The modern onychophorans (velvet worms) are all terrestrial.

Porifera: The sponges are among the most primitive animals; their simple body is not organized into true tissues. Sponges are mostly bottom-dwelling suspension feeders, and many forms possess a supporting mesh-work of fine needle-like spicules composed of various minerals.

For more information on these animal phyla and their Burgess Shale representatives click here.

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