Page 264 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 251

METAZOA
Diploblasts Triploblasts
Coelenterates Ecdysozoans Lophotrochozoans Deuterostomes

Vendobionts Clenophores Stem-group cniderians Anthozoans Other cnidarians Placozoarts Stem-group triploblasts Stem-group ocdysozoans Priapulida Nematodes Lobopodians/Anomalocaridids Arthropods Stem-group lophotrochozoens Platyhelminthes Moullusks Hallcerilds Brachiopods Phoronids Stem-group deuterostomes Hemichordates Echinoderms Cephalochordates Chordates

Fungi Sponges Annelids δ °C
Ma PDB
510
Wheeler
Formation
Burgess Shale
520
CAMBRIAN Kaili

530 Chengjiang
Sirius Passet
Abundant shelly ?
540 fossils
Shelly fossils
Cloudina ?
550 “Advanced”
Ediacara faunas
VENDIAN
560 Mistaken Point
faunas
First known fossil
?End of ice age
570 ?First fossil Key transitions
650 metazoans

Figure 10.14 Stratigraphic distribution of Late Precambrian and Early Paleozoic metazoan taxa, some
13
key morphological transitions and the carbon isotope record (δ C). PDB, Vienna Pee Dee beleminite,
the standard material for relative carbon isotope measurements. (Based on various sources.)
record, together with a revised molecular preserved in the fossil record (Lieberman
clock (see p. 133), have suggested an alterna- 2001).
tive hypothesis. The current Lower to Middle Much of our knowledge of the Cambrian
Cambrian fossil record displays the sequential explosion is derived from three spectacular,
and orderly appearance of successively more intensively-studied Lagerstätte assemblages:
complex metazoans (Budd 2003), albeit rather Burgess (Canada), Chengjiang (China) and
rapidly (Fig. 10.16), and the timing is closely Sirius Passet (Greenland). The diversities of
matched by revised molecular time scales the Cambrian “background” faunas are gen-
(see p. 235; Peterson et al. 2004). Neverthe- erally much lower and arguably contain less
less there is some suggestion from the biogeo- morphologically different organisms. Recon-

graphic patterns of trilobites that the diver- structions of these seafloors are possible (Fig.
gence of many metazoan lineages may have 10.17). But whereas the Cambrian explosion
already begun 30–70 myr earlier (Meert & provided higher taxa, in some diversity, the
Lieberman 2004) and speciation rates during Ordovician radiation generated the sheer
the explosion were not in fact so incredible biomass, biodiversity and biocomplexity that
compared with those of other diversifi cations would fill the world’s oceans.