Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 239

Bilaterian triplobastic, but protostome and deuterostome lineages seem to first
appear around 570 Mya ( Erwin, 1999 ). According to Peterson et al. (2007) , the last
common ancestor (LCA) of protostomes and deuterostomes evolved 580 Mya.
These Ediacaran forms, known as the “vendobiont” phylum, have no confirmed
relationship with the Cambrian forms. Together with radiates and bilaterians, they
form their own Pre-Cambrian group before gradually becoming extinct at the outset
of the Cambrian period. According to Peterson et al.:
Ediacaran “survivors” in the Cambrian can be counted on the fingers of one hand.
Peterson et al. (2003)
If this is true, the group may be considered a failed experiment in multicellular
life, although there is evidence that a small number of the Ediacaran forms survived
into the Cambrian. The extinction of the Ediacaran biota coincided with the onset of
the Cambrian fauna about 542 Mya, which marks the beginning of the Phaneorzoic
eon. Among the suggested causes for the demise of the Ediacaran fauna are the
evolution of the more successful Cambrian forms, which preyed on immobile soft-
bodied Ediacaran species ( Bengtson, 2002 ), as well as a global short-term anoxia,
and widespread methane release ( Seilacher, 1984 ).
Cambrian Explosion
For five-sixths of its existence, life on Earth evolved slowly, mainly in the form of
unicellular organisms and in accordance with the Darwinian principle of gradual-
ism. Then abruptly, about 543 Mya, a new, eumetazoan fauna emerged in a still little
understood way.
Metazoan life began with the fossil record of the Cambrian 1 period, dated at 542-
485 Mya. Almost all the extant and several extinct metazoan phyla and body plans
emerged in geologically rapid succession (phylum Bryozoa was the last to evolve
by the end of the Cambrian about 490 Mya) within the evolutionarily short span of
about 10 million years from about 530-520 Mya (i.e., within less than 2% of the time
from the base of the Cambrian to the present) ( Conway Morris, 2000; Roth et al.,
2007; Valentine et al., 1999 ).
Little high-level morphological innovation occurred during the subsequent 500 mil-
lion years in that much of animal disparity, as measured by the Linnean taxonomic
ranking, was achieved early in the radiation.
Erwin et al. (2011)
In 1948, Preston Cloud (1912-1991) concluded that the discontinuation of the
Cambrian fauna from the Pre-Cambrian was not an illusion that derived from the
1 Cambrian derives from Cambria, the Latinized form of the Cymry, the Celtic name of the Welsh people.
The term Cambrian period was introduced in 1836 by paleontologist Adam Sedgwick (1785-1873), one
of Darwin's mentors, who studied the fossil fauna of the period in rocks from Wales.