To begin, the sequence of geological time intervals across the Precambrian
to Cambrian boundary are as follows: Vendian (~600 my ago to base of the
Cambrian), Early Cambrian (consisting of the Manykaian, Tommotian,
Atabanian, Botomian, and Toyonian in ascending order), Middle Cambrian, and
Upper Cambrian.
The Earliest body fossils of metazoans occur near the base of the Vendian.
Sponges are known from the early Vendian ~580 my ago. The embryos of
probable bilaterians have been found in rocks of slightly younger age ~570
my ago. The Ediacaran fauna begins about ~565 my ago and includes mollusks
(Kimberella), coelenterates, probable echinoderms, and a number of forms of
uncertain affinity. The first unquestioned burrows appear in the
Precambrian at about the same time as the body fossils and increase in
diversity and complexity over time. There have been several reports of
burrows, not widely accepted (due to uncertainties in dating or ambiguous
morphologies), that are claimed to be considerably older.
The Tommotian is the time of appearance of the diverse "small shelly"
fauna. These fossils include a wide variety of tiny cap-shaped, scale-like
and tubular mineralized fossils. Until relatively recently the biological
affinity of most were unknown. It is now known that many belonged to
mollusks, armoured slug-like animals with similarities to both mollusks and
annellid worms, and to armoured lobopods. The latter is one of the early
groups in the line toward arthropods. The earliest known fossil lobopod is
reported as basal Cambrian which is probably either the Tommotian or the
Manykaian.
The earliest accepted trilobites fossils appear in the Atdabanian of
Siberia. The pre-Atdabanian fossil faunas are in fact referrred to as the
pre-Trilobite faunas. The anomalocaridids appear in the Chengjiang faunas
of late Atdabanian age. The gilled lobopod fossil to which I referred was
from a formation of late Atdabanian to early Botomian age. The diverse
arthropods of the Burgess shale are much later in the middle Middle
Cambrian.
The proposed evolutionary sequence of lobopod - armoured lobopod - gilled
lobopod - anomalocaridid - "true" arthropod, is thus not inconsistent with
the known fossil record of early metazoans.
The only proposed Precambrian arthropod of which I am aware is from the
Ediacaran. It was based on simple impressions and what were interpretated
as parallel scratch marks from arthropod appendages. The scratch marks
have since been interpreted as radula scrape marks from the primitive
mollusk Kimberella. I have not seen any discussion of the proposed
arthropd fossil by other workers since it was proposed. As far as I am
aware no others have accepted that identification.
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www-personal.ksu.edu/~kbmill/