Classifying animals

A family tree for organisms (implied in most systems of classification) provides a series of events that must have happened in the past in a known order. Classification of the metazoa is therefore a key element in deducing information about their origin and early history. For example, it is now known that animals with a gut evolved relatively late in the history of metazoans. This information makes the discovery of possible fecal pellets produced from a gut-bearing animal in rocks up to 1.9 billion years old an extremely important one, though, as usual, it is the subject of intense dispute.

Recently, large strides have been made in producing a generally agreed classification for all animals. This points to various early events in the evolution of multicellular animals and the order in which they occurred (Fig. 15.5). Dates derived from molecular clocks can be added to suggest the timing of the important events implied by it. The main problem with this classification is the place in it filled by Ediacaran organisms. Whilst their location within such a classification is unclear, the geological evidence they offer about the origin of animals remains difficult to interpret.

Sponges

Bilateria

Mollusks Arthropods

Echinoids Hemichordates Chordates

Jellyfish, corals Annelid worms

Mollusks Arthropods

Echinoids Hemichordates Chordates

940 Ma

1100 Ma

1100 Ma

700 Ma

600-1000 Ma

670-1200 Ma

Single common ancestor for all metazoans

Fig. 15.5 A recent classification of multicellular animals. This classification divides animals into two broad groups - those with bilateral symmetry and those with radial symmetry. There is a large genetic gap between the two groups. The position of Ediacaran organisms is uncertain. Dates added to the diagram represent molecular estimates of time since divergence. Note the wide variation in these estimates.

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