Page 270 - Introduction to Paleobiology and The Fossil Record
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ORIGIN OF THE METAZOANS 257
The ribbon worms, or nemertines, are char- 2 Interpretations of Ediacaran biotas are as
acterized by a long anterior sensory probos- far from a consensus as ever. Why are the
cis. The majority are marine, although some Ediacara organisms so diffi cult to classify
inhabit soil and freshwater. Although the and understand?
bizarre Amiskwia from the Middle Cambrian 3 The identification of embryos and trace
Burgess Shale was assigned to this group, fossils are both important evidence of
recent opinion suggests it is merely conver- animal life. How can both be used to indi-
gent on the nemertine body shape. Some of cate the presence of metazoan life?
the Tommotian animals may also be nemer- 4 Was the Cambrian explosion one of
tine worms. The nematodes or roundworms animals or fossils? How large was the role
are generally smooth and sac-like. of taphonomy in the manifestation of the
The priapulid worms are exclusively Cambrian explosion?
marine, short and broad with probosces 5 Within an interval of 100 million years the
(“noses”; singular, proboscis) covered in planet’s seafloors were changed for ever.
spines and warts. The Middle Cambrian Briefly compare and contrast the changing
Burgess Shale contains seven genera assigned seascapes through the Ediacaran, Cam-
to at least five families. The Burgess forms are brian and Ordovician periods.
all characterized by priapulid probosces, and
most have little in common with modern
forms. Nevertheless the most abundant taxon, Further reading
Ottoia, is very similar to the living genus Hal- Briggs, D.E.G. & Fortey, R.A. 2005. Wonderful strife:
icryptus. Elsewhere in the fossil record the systematics, stem groups, and the phylogenetic signal
Upper Carboniferous Mazon Creek fauna has of the Cambrian radiation. Paleobiology 31 (Suppl.),
yielded Priapulites, which has a distinctly 94–112.
modern aspect. Brusca, R.C. & Brusca, G.J. 2002. Invertebrates, 2nd
The annelid worms, such as the common edn. Sinauer Associates, Sunderland, MA.
earthworm and lugworm, have ring-like exter- Conway Morris, S. 2006. Darwin’s dilemma: the reali-
nal segments that coincide with internal parti- ties of the Cambrian explosion. Philosophical Trans-
tions housing pairs of digestive and reproductive actions of the Royal Society B 361, 1069–83.
organs; the nervous system is well developed Gould, S.J. 1989. Wonderful Life. The Burgess Shale
and the head has distinctive eyes. The annelid and the Nature of History. W.W. Norton & Co.,
New York.
body is ornamented by bristles that aid loco- Nielsen, C. 2003. Animal Evolution. Interrelationships
motion and provide stability. Most are preda- of the Living Phyla, 2nd edn. Oxford University
tors or scavengers living in burrows. The Press, Oxford, UK.
polychaetes or paddle worms have the most Valentine, J.W. 2004. On the Origin of Phyla. Univer-
complete fossil record; the record is enhanced sity of Chicago Press, Chicago.
by the relatively common preservation of ele-
ments of the phosphatic jaw apparatus known
as scolecodonts (see p. 359). Although some References
Ediacaran animals, such as Spriggina, have Aguinaldo, A.M.A. & Lake, J.A. 1998. Evolution of
been associated with the polychaetes, the fi rst multicellular animals. American Zoologist 38,
undoubted paddle worms are not known until 878–87.
the Cambrian. A diverse polychaete fauna has Bengtson, S. 2005. Mineralized skeletons and early
been described from the Burgess Shale; it even animal evolution. In Briggs, D.E.G. (ed.) Evolving
contains Canada spinosa, similar to some Form and Function. New Haven Peabody Museum of
living polychaetes. Natural History, Yale University, New Haven, CT, pp.
101–17.
Bottjer, D.J., Droser, M.L., Sheehan, P.M. & McGhee,
Review questions G.R. 2001. The ecological architecture of major
events in the Phanerozoic history of marine life. In
1 Traditional methods of reconstructing the Allmon, W.D. & Bottjer, D.J. (eds) Evolutionary
phylogeny of the early metazoans based Paleoecology. Columbia University Press, New York,
on morphology have encountered prob- pp. 35–61.
lems. Is the concept of body plans still Brasier, M.D. & McIlroy, D. 1998. Neonereites unise-
useful and if so, for what? rialis from c. 600 Ma year old rocks in western
