Evolution, 4th Edition

442 CHAPTER 17

more than 700 Mya, when sponges and cnidarians formed distinct lineages (FIG-

U R E 17.8). Sequence data also show that divergence among many bilaterian lineages

occurred well before 541 Mya [29]. But none of these living phyla, except for sponges,

has been recorded in the fossil record before the start of the Cambrian period, 541 Mya.

The Cambrian period begins the Paleozoic era—with a bang. For the first 10 My

or so of the Cambrian period, starting about 541 Mya, animal diversity was low.

Then, during a period of about 20 My, almost all the modern phyla and classes of

skeletonized marine animals, as well as many extinct classes, appeared in the fos-

sil record. This interval marks the first appearance of brachiopods, trilobites and

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Demospongiae

Hemichordata

Echinodermata

Vertebrata

Nemertea

Brachiopoda

Mollusca

Annelida

Bryozoa

Priapulida

Onychophora

Arthropoda

Cnidaria

800 700

Proterozoic Phanerozoic

Cryogenian Ediacaran O

600

Time (Mya)

C

541 485

FIGURE 17.8 Phylogeny of some animal phyla and their occurrence in the fossil re-

cord. The thick portion of each branch represents time from the earliest known speci-

men to the present. The time estimates for the branch points when lineages diverged

from common ancestors are based on time-calibrated DNA sequence differences.

Note that no fossil specimens of any modern phylum have been recorded before the

Cambrian, although DNA sequences imply that some lineages diverged long before

that. (After [29].)

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