A HISTORY OF LIFE ON EARTH 97 Figure 5 7 Members of the Ediacaran fauna A Mawsonites spriggi may be a cnidarian relative of sea anemones although this is not certain B The actual relationship of the wormlike Dickinsonia costata to later animals is unknown A O The Natural History Museum London B O Ken Lucas Visuals Unlimited Paleozoic Life The Cambrian Explosion The Paleozoic era began with the Cambrian period starting about 542 Mya For the first 10 Myr or so animal diversity was low then during a period of just 10 to 25 million years almost all the modern phyla and classes of skeletonized marine animals as well as many extinct groups appeared in thefossil record Tlus interval marks the first appearance of brachiopods Figure 5 8A trilobites Figure 5 8B and other classes of arthropods various classes of molluscs echinoderms and many others The remarkably well preserved Cambrian fauna of the Burgess Shale of British Columbia Figure 5 9 and the chapter opening photograph includes animals rather different from any that succeeded them The Cambrian diversification included the earliest jawless agnathan vertebrates the recently discovered early Cambrian Haikouichthys had eyes gill pouches notochord segmented musculature and other features resembling those of larval lampreys Figure 5 10A Shu et al 2003 and the late Cambrian conodonts had teeth made of cellular bone Figure 5 10B Most of the fundamentally different body plans often called Bauplane German for construction plans or blueprints known in animals apparently evolved during the Cambrian perhaps the most dramatic adaptive radiation in the history of life The reasons for this diversification called the Cambrian explosion have been the subject of vigorous debate Erwin 1991 Lipps and Signor 1992 How and why did so many great changes evolve in such a short time By applying a molecular clock to the DNA sequence divergence among the living animal phyla Gregory Wray and colleagues 1996 showed that the phyla actually originated long before their first appearances in the fossil record perhaps 1000 Mya Before the explosion however most animals either lacked skeletons or were so small that their Precambrian remains have not been found The Cambrian explosion thus consists of rapid diversificationwithin clades that had evolved much Figure 5 8 Two animal groups brachiopod Mucrospirifer p phylum e A 98 CHAPTER 5 A Artist s reconstrucof two of the peculiar of the Cambrian Burgess Shale A Opabitiia probably an arthropod 8 W i u xperhaps ia related to annelid worms Characteristics confirmed in multiple fossils can be illustrated in a single drawing which therefore often can convey structures more clearly than a photograph of a single fossil specimen For that reason taxa are often illustrated by drawings rather than by photographs in this chapter Drawings by Marianne Collins from Gould 1989 Figure 5 9 earlier diversification that included the evolution of shells and skeletons A combination of genetic and ecological causes may account for this diversification Knoll and Carroll 1999 Knoll 2003 Regulatory genes that govern the differentiation of body parts such as the Hox genes see Chapter 20 may have undergone major evolutionary changes at this time Moreover the extinction of the archaic Ediacaran animals which may have been brought about by a decrease in oxygen levels released the survivors from competition allowing them to diversify just as mammals did after the demise of dinosaurs see Chapter 7 Molecular phylogenetic studies show that animals are most closely related to the unicellular choanoflagellates Sponges phylum Porifera which have inany choanoflagellate like cells are the sister group of the other animals known collectively as Metazoa Figure 5 11 The radially symmetrical Cnidaria jellyfishes corals and the Ctenophora comb jellies are basal branches relative to the Bilateria bilaterally symmetrical animals with a head often equipped with mouth appendages sensory organs and a brain The Bilateria include three major branches the deuterostomes in which the blastopore formed during gastrulation becomes the anus and two groups of protostomes in which the blastopore becomes the mouth The largest deuterostome phyla are the Echinodermata starfishes and relatives and the Chordata including the vertebrates tunicates and amphioxus Protostomes form two major clades the Ecdysozoa arthropods nematodes and some smaller phyla and the Lophotrochozoa molluscs annelid worms brachiopods and a variety of other groups The end of the Cambrian 500 Mya was marked by mass extinction The trilobites of which there had been more than 90 Cambrian families were greatly reduced and several classes of echinoderms became extinct As Stephen Jay Gould 1989 emphasized if the early vertebrates had also succumbed we would not be here today The same may be said about every point in subsequent time had our ancestral lineage been among the enormous number of lineages that became extinct humans would not have evolved and perhaps no other form of life like us would have either Figure 5 10 Early vertebrates of the Cambrian A Photograph and drawing of one of the earliest known vertebrates Haikouichthys of the early Cambrian The drawing calls attention to features interpreted as eye notochord vertebral elements dorsal fin esophagus gill pouches ventral fin and anus indicating a postanal tail region characteristic of vertebrates B Bony toothlike structures of Cambrian conodonts Conodonts were slender finless chordates that are thought to be related to agnathans jawless vertebrates such as lampreys A photo courtesy of D G Shu after Shu et al 2003 B courtesy of James Davison M D A HISTORY OF LIFE ON EARTH Embryonic Vertebrata vertebrates becomes the anus Cephalochordata amphioxus Urochordata tunicates Hemichordata acorn worms Echinodermata starfishes sea urchins etc 3 cell layers bilateral I re 1 7 Bryozoa moss animals lv etry locomotion Figure 5 11 A recent estimate of relationships among animal phyla based on the sequence of genes encoding ribosomal RNA Some relationships among phyla are uncertain and are shown as unresolved polytomies i e multiple branches from a single stem as opposed to a totally dichotomous branching pattern seen here in the deuterostome lineage After Adoutte et al 2000 Brachiopoda brachiopods Platyhelminthes flatworms Pogonophora tubeworms Rotifera rotifers Annelida segmented worms J 2 cell layers radial symmetry becomes the mouth Nematoda nematode worms Tardigrada