Page 248 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 235

ORIGINS AND CLASSIFICATION The fi rst metazoans: when and what?

When did the first complex animals, the meta- Life on our planet has been evolving for nearly
zoans, appear on Earth and what did they 4 billion years. Molecular data suggest meta-
look like? How could complex, multicelled zoans have probably been around for at least
animals evolve from the undifferentiated 600 myr (Fig. 10.1), during which time,
single-celled organisms of most of the Pre- according to some biologists, as many as 35
cambrian? Why did they take almost 4 separate phyla have evolved. Five lines of evi-

billion years to appear? These questions have dence have figured prominently in the search
puzzled scientists, including Charles Darwin, for the earliest metazoans: body fossils, trace
for over two centuries. In the last few decades fossils, fossil embryos, the molecular clock
a range of multidisciplinary techniques, from and biomarkers.
molecular biology to X-ray tomography, has Much controversy still surrounds the timing
helped generate new testable hypotheses of their origin. Was there a long cryptic inter-
regarding the origins of our early ancestors. val of metazoan evolution prior to the Edia-
Apart from the fossil evidence of metazoan caran – a time when we do not fi nd fossils
body and trace fossils, the investigation of preserved, either because the animals lacked
minute fossil embryos, carefully calibrated preservable bodies, or they were small, or
molecular clocks and more recently biomark- perhaps a combination of both? Or, as the
ers have placed the investigation of Precam- recalibrated molecular clocks suggest, can
brian life at the top of many scientifi c animal origins be tracked back only to the
agendas. Ediacaran, when there was also a sudden rise

C Crinozoa
535 Deuterostomia Eleutherozoa
200 metazoan orders Hemichordata
metazoan classes
C Pteriomorpha
604 molecular clock estimates C Nuculoida
160 C molecular clock calibration 579 Vetigastropoda
feeding larva 545 C
non-feeding larva 549 493 Spiralia Sorbeochoncha
Number of classes/orders 80 604 561 546 494 Ecdysozoa Spionidea + Serpulidea
Oman biomarker record
Phyllodicida
120
Hoplonemertea
Heteronemertea
Arthropoda
634
“Porifera” Cnidaria Calcispongia
Anthozoa
543 Priapulida
664
40 Hydrozoa

N-D T AB/T M Late Early M Late Demospongia
Cryogenian Ediacaran
Early
Cambrian Ordovician
700 635 600 575 555 542 513 501 488 472 461 443 Ma

Figure 10.1 Time scale and tempo of early animal evolution: the key metazoan groups are shown with
the putative age of their last common ancestor, together with an estimate of the respective numbers of
classes and orders indicated against a stratigraphy indicating key biological and chemical events. N–D,
Nemakit-Daldynian; T, Tommotian; A, Atdabanian; B/T, Botomian. (Courtesy of Kevin Peterson.)