major animal groups appear for the first time in the fossil
record some 545 million years ago in a relatively short period
of time known as the Cambrian explosion. Of great worry
to Darwin, the explanation of this sudden, apparent explosion
persists today as a shadowy mystery. While some scientists believe
there was indeed an explosion of diversity (the so-called punctuated
equilibrium theory elaborated by the late Stephen J Gould -
Models In Paleobiology, 1972), others believe that such
rapid acceleration of evolution is not possible; they posit
that there was an extended period of evolutionary progression
of all the animal groups, the evidence for which is lost in
scanty fossil record.
theory of the Cambrian Explosion holds that, beginning some
545 million years ago, an explosion of diversity led to the
appearance over a relatively short period of 5 million to 10
million years of a huge number of complex, multi-celled organisms.
Moreover, this burst of animal forms led to most of the major
animal groups we know today, every extant Phylum. It is also
postulated that many forms appeared that would rightfully deserve
the rank of Phylum, both appeared in the Cambrian, and subsequently,
rapidly disappeared. Natural selection in many cases favored
larger size, for example, hard skeletons to provide structural
support - hence, the Cambrian gave rise to the first shelly
animals and animals with exoskeletons (e.g., the trilobites),
and the size of many animals "exploded".
in fact, the Cambrian Explosion is true, environmental factors
probably were prominent in defining the selective pressures
leading to diversification on a vast
scale. By the start of the Cambrian, a large supercontinent
comprising all land on Earth was breaking up into smaller land
masses. This increased the area of continental shelf and produced
shallow seas, and a diversity of environments in which animals
debate about whether the evolutionary "explosion"
of the Cambrian was as sudden and spontaneous as it appears
persists today. The discovery of new pre-Cambrian and Cambrian
fossils help, as these transitional forms support the hypothesis
that diversification was well underway before the Cambrian began.
More recently, the sequencing of the genomes of thousands of
life forms is revealing just how many genes and the proteins
they encode have been conserved from the Precambrian. The explosion
of external form in the fossil record is what we see, but more
gradual adaptation was taking place at the molecular level.
the famous Lagerstatte of Cambrian time, the Burgess Shale of
Canada and Chengjiang, in Yunnan Province, China are the best
known, having a great diversity of benthic or burrowing creatures.
Less well known is our own state of Utah where similar Cambrian
forms are found. If fact, a larger number of species to be found
in the Wheeler and Marjum Formations of Utah than the Burgess
Shale, though the fossils are far less abundant. The link below
allows viewing of some of these intriguing Cambrian fossils:
Animals of Utah