extinct carpoids are an informal grouping of Paleozoic fossils that
closely resemble echinoderm, but lack radial symmetry. However,
their classification remains controversial and they have been variously
postulated to have been stem groups of other groups such as the
basal deuterostomes or the craniates, tunicates, acraniates and
chordates. Carpoids are known from the Middle Cambrian to Early
from Utah’s Marjum Formation, this fine carpoid fossil
dates to the Middle Cambrian when these animals first appeared.
speak to strange asymmetric body plan of these enigmatic creatures.
speculation about the problemmatic carpoids: Evolutionary
biologists have been interested in external asymmetry as a
of genetic quality. Research shows that animals choose their
partly on the basis of symmetry. Animals increase their genetic
contribution to the next generation when mates of higher genetic
quality are chosen. More symmetry implies better genes and begets
better prospects to pass on their genes, and so on.
paleontologist Richard Jefferies of the Natural History Museum
in London has posited that a progenitor of
all vertebrates suffered an evolutionary accident perhaps more
than 600 million years ago, and that vertebrate evolution ever
has been a struggle to regain symmetry that has been only partially
successful. The asymmetry of the internal organs within a symmetrical
exterior of vertebrates is a retained manifestation of this accident
that even 600 million years of selection has not eliminated.
in part, bases his theory on studies of carpoids. These small
creatures had spiny tails, bulbous heads and were covered with
spines. Having a body supported by calcitic plates, they resemble
such as starfish and their relatives. Jefferies presents a compelling
argument that the common ancestor of echinoderms and vertebrates
was a carpoid, which is supported by the many shared features of
echinoderms and vertebrate embryonic development.
are distinct from all other animals, extant and extinct, due
to their complete asymmetry, both internal and external.
body plan accident he hypothsizes would require it took place
in a common ancestor of echinoderms and vertebrates.
Carpoid's descendents retained the lop-sided external baud plan,
but the echinoderms first, and the vertebrates later, were re-shaped
through selection into
with adhering externally at least to
symmetry, while internal organs generally do not.
intriguing theory – we’ll need to
wait and see if now dirt cheap whole-genome sequencing-based modern
phylogenetics of extant echinoderms can shed light on the shadowy
period in the Precambrian when
Also see: Carpoid Evolution
R. P. S., 1986 The ancestry of the vertebrates(British Museum
of Natural History), London
LD, Degnan BM. Hemichordates and deuterostome evolution: robust
molecular phylogenetic support
for a hemichordate +
clade. Evol Dev. 1999 Nov-Dec;1(3):166-71.