(Stylophorans) are enigmatic fossils often found in Cambrian strata,
and prevalent in the fossil record from about 500 to 300 million
years ago. Debate has persisted for decades as to the group’s
evolutionary origins and placement. The three competing hypotheses
of Stylophora origins have been:
They are very primitive echinoderms with a mobile stalk or single
arm filled with muscle;
Stylophorans are highly derived echinoderms related to crinoids
possessing an ambulacrum with tube-feet and an oral tegmen with
They are neither primitive nor advanced echinoderms, but were
primitive chordates that
retained a calcite exoskeleton from an older common ancestor of
echinoderms and chordates, with the stalk
containing muscle, notochord and brain.
a recent publication in Nature (Nature 438, 351-354; 2005),
and Smith report evidence that support the first hypothesis and
reject the last two, based on study of a ceratocystid stylophoran
from the Middle Cambrian of Morocco. The weight of evidence now
holds that the enigmatic carpoids are, indeed, primitive echinoderms
and a stem group from the crinoids.
vali Cambrian Carpoid from Utah for another theory that
carpoids are a common ancestor between echinoderms and vertebrates.