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the broadly simultaneous evolution of carbonate, phosphatic, siliceous, and agglutinated
skeletons in marine animals and protists. It has also been proposed that Cambrian rocks
document an explosion of fossils, made apparent by robust skeletons, rather than a true
diversification of animals in the oceans. Again, the growing record of latest Proterozoic
fossils and trace fossils discourages this idea. More likely, as Bengtson (1994) has
argued, the Cambrian diversification of mineralized skeletons was part and parcel of a
broader Cambrian radiation of animal diversity, with increases in predation pressure
favoring the evolution of protective armor.
Exceptionally well preserved Cambrian fossils such as those of the Burgess (Briggs et
al. 1994) and Chengjiang (Chen and Zhou 1997) biotas show that, then as now, most
animals did not precipitate mineralized skeletons; only about 15% of Burgess species
formed skeletons that would be preserved in conventional fossil assemblages (Bengtson
and Conway Morris 1992). Nonetheless, many clades did evolve mineralized skeletons
(Figure 5), and these gave rise to the predominant fossil record of the past 500 million
years. Skeletal diversity increased tremendously in the sponges, even though this clade
must have differentiated from other animals before Ediacaran fossils first entered the
record. The Cambrian diversity of siliceous spicules matches or exceeds anything known
from subsequent periods (Bengtson et al. 1994; Dong and Knoll 1996), and the carbonate-
precipitating archaeocyathids produced the first massively mineralized skeletons known
from the geological record. Indeed, by virtue of their skeletal evolution, archaeocyathids
qualify as the first metazoan reef formers, although many Cambrian build-ups remained
predominantly microbial (Riding and Zhuravlev 1998; Rowland and Shapiro 2002).
Anabaritids, small calcitic tubes with three-fold symmetry found in basal Cambrian
rocks, may have been cnidarian, but for the most part, Cambrian rocks contain only
Figure 5
. Summary of early skeleton evolution in metazoans, showing the diversity, skeletal
mineralogy, and stratigraphic distribution of Cambro-Ordovician animals. Skeleton data modified from
Bengtson and Conway Morris (1992); family diversity from Sepkoski (1982).
