Page 250 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 237

suggesting that the embryos examined are sial. For example, the last common ancestor
those, at best, of stem-group metazoans of the bilaterians, the metazoan clade exclud-
(Hagadorn et al. 2006); they could equally ing the sponges and cnidarians, has been vari-
well be fungi or rangeomorphs (enigmatic ously placed at anywhere between 900 and
frond-like fossils). Nonetheless the Doushan- 570 Ma. Why is there such a spread of ages
tuo embryos, although unplaced taxonomi- in a seemingly exact science? The rates of
cally, provide our earliest body fossil evidence molecular evolution in various groups are
for probable metazoan life, albeit very basal, unfortunately not constant. The vertebrates
and a fascinating insight into embryologic appear to have reduced their rates of molecu-
processes in deep time (Donoghue 2007) lar change through time. So, using the slow
(Box 10.1). vertebrate rates of molecular evolution to
calibrate the date of origin of Bilateria gives
dates that are too ancient (900 Ma). On the
Molecular evidence other hand, using mean bilaterian rates of
molecular evolution gives a date (570 Ma)
Not only have the morphologies of organisms that is more in keeping with evidence from
evolved with time, but so too have their mol- the fossil record (e.g. Budd & Jensen 2000)
ecules. This forms the basis of the concept of and thus makes the Cambrian explosion much
the molecular clock (see p. 133). The molecu- more of an explosion of animals rather than
lar clock has opened up tremendous possibili- fossils (Peterson et al. 2004). Nevertheless the
ties to date, independently of direct fossil most recent molecular clock data (Peterson
evidence, the times of divergence of say the et al. 2008) suggest a major phase of meta-
mammals from the reptiles or the brachio- zoan radiation within the Ediacaran, prior to
pods from the mollusks. Nevertheless, that in the Cambrian. This radiation probably
attempts to date the divergences of the various set the agenda for metazoan macroevolution
groups of metazoans have proved controver- for the rest of geological time.

Box 10.1 Synchrotron-radiation X-ray tomographic microscopy

Fossil embryos from the Upper Neoproterozoic and Cambrian are providing some important clues
about the origin and early evolution of the metazoans. They are, however, tiny and notoriously hard
to study. Nevertheless Phil Donoghue and his colleagues (2006) are beginning to accumulate a large
amount of new information on the composition, structure and cell division within these minute
organisms together with their modes of preservation. Synchrotron-radiation X-ray tomographic
microscopy (SRXTM) has provided a whole new way of scanning embryos without actually destroy-
ing them (Fig. 10.3). The embryos, most of them 1 mm across or smaller, are held steady in a high-
energy beam of photons, and multiple “slices” are produced, spaced a few microns apart. Using
imaging software, these slices can be combined to create a detailed three-dimensional model of the
internal structure of the fossil. Embryos assigned to the bilaterian worm, Markuelia, together with
Pseudooides, variously show the process of cell cleavage and development of possible blastomeres,
clusters of cells produced by cell division after fertilization, rather than yolk pyramids, which are
more typical of the arthropods. This high-tech methodology has already demonstrated a real prospect
for identifying the animals themselves and charting their early stages of development, some 600 Ma.
It also can reject the claims that such fossils were the planula larvae of cnidarians, minute bilaterians
or the early stages of gastrulation (see p. 240) of hydrozoans or bilaterians. It has, however, been
recently suggested that many of these embryonic structures were created by bacteria (see p. 190).
But not all.
Read more about this topic at http://www.blackwellpublishing.com/paleobiology/.

Continued