Evolution__3rd_Edition

536 PART 5 / Macroevolution
Protostoma
Bilateria
Deuterostoma
Arthropoda
Nematoda
Priapula
Mollusca
Annelida
Platyhelminthes
Brachiopoda
Chordata
Echinodermata
Cnidaria
Porifera
Figure 18.5
The fossil record of most major animal groups begins in the
Cambrian. The horizontal lines to the right of each taxon show
the times when the group is represented in the fossil record for
the Cambrian and late Precambrian. The thick, continuous
lines show the “crown group” fossils a fossils descended from
the same common ancestor as modern members of the taxon.
The broken lines show the “stem group” fossils a fossils that
are members of the taxon but that branched off before the last
common ancestor of the modern members of the taxon. Notice
that the first fossils of all taxa except the Cnidaria and Porifera
(and Nematoda) date in or near the early Cambrian. The
Late Mid Early
Cambrian
Orders
Classes
ancestor at about 1,200 million years ago. The common ancestor of all animals would
then have lived earlier still. (The Bilateria include all the animal groups in Figure 18.5
except sponges and Cnidaria.)
How can we reconcile the fossil and molecular dates? Either (or both) could be
wrong in some way. However, many biologists suspect that they are both correct. The
molecular evidence tells us the date of the common ancestor, whereas the fossil evidence
tells us when each animal group arose in its modern form. There could have been
a period before the fossils were deposited when ancestors of each group existed but
?
? (Biomarkers)
490 500 510 543 Diverse Ediacarans
610 (Myr)
Burgess Chengjiang
Sirius Passet
Nama
CaCO 3
Late
Neoprotozoic
Doushantuo
PO 4
80
60
40
20
Number of classes and orders
Oldest radially
symmetric
impressions
histograms show the same event in terms of the numbers
of orders and classes (the numbers are on the y-axis).
Again, note the rapid increase in the early Cambrian.
The phylogenetic relations of the taxa are shown to the
left. Dates are along the bottom, along with times of major
fossil deposits. Doushantuo PO 4 and Nama CaCO 3 refer
to sites where the fossils are phosphates and calcium
carbonates. The animal groups shown here are the main
groups for which a reasonably good fossil record exists.
Other animal groups also exist but are smaller, or have
less certain fossil records. Modified, with permission of the
publishers, from Knoll & Carroll (1999).
..