178 Evolution? The Fossils Say YES!
Thus we have seen that the “Cambrian explosion” is a myth. It is better described as the
Cambrian slow fuse. It takes from 600 to 520 million years ago before the typical Cambrian
fauna of large shelly organisms (especially trilobites) finally develops. Eighty million years is
not explosive by any stretch of the imagination! Not only is the explosion a slow fuse, but it
follows a series of logical stages from simple and small to larger and complex and mineral-
ized. First, of course, we have microfossils of cyanobacteria and other eukaryotes going back
to as far as 3.5 billion years ago and spanning the entire fossil record since that ancient time.
Then, about 600 million years ago, we get the first good evidence of multicellular animals,
the Ediacara fauna. They are larger and multicellular but did not have hard shells. The earli-
est stages of the Cambrian, the Nemakit-Daldynian and Tommotian stages, are dominated
not by the little shellies, which were just beginning to develop small mineralized skeletons.
FIGURE 7.4. A detailed examination of the stratigraphic record of fossils through the late Precambrian and
Cambrian shows that life did not “explode” in the Cambrian, but appeared in a number of steps spanning
almost 100 million years. The large soft-bodied Ediacaran fossils (fig. 7.2) appeared first at 600 million years
ago, in the late Precambrian (Vendian). Toward the end of their reign, we see the first tiny shelly fossils,
including the simple conical Cloudina and Sinotubulites. The Nemakit-Daldynian and Tommotian stages
are dominated by the “little shellies” (fig. 7.3) plus the earliest lamp-shells, or brachiopods, and the conical
spongelike archaeocyathans, and many burrows showing that wormlike animals without hard skeletons were
also common. Finally, in the third stage of the Cambrian (Atdabanian, around 520 million years ago), we see
the radiation of trilobites and a huge diversification in total number of genera (histograms on the right side
of the diagram). Thus the Cambrian explosion took over 80 million years to develop and was no “sudden”
event, even by geological standards. (Modified from Dott and Prothero 2010: fig. 9.14, and from Kirschvink
et al. 1997: fig. 1. Copyright © 1997 American Association for the Advancement of Science. Reprinted with
permission.)
Late490500510520530540550560Zone IIIZone IIZone IMegafossil zones (Ediacaran fauna)Cambrian
“explosion”570580590600MiddleGloudinaEdiacaran faunaSinotubulitesAnabarites
Arthropod tracesArchaeocyathansBrachiopodsDeep vertical burrowsTrilobitesTommotianAtdabanianBotomianNemakit-
DaldynianCambrianVendianEarly