Carpoid Castericystis from Marjum Formation

The 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 Devonian.

Coming from Utah’s Marjum Formation, this fine carpoid fossil dates to the Middle Cambrian when these animals first appeared. The pictures speak to strange body plan of this enigmatic creature.

Interesting speculation about the enigmatic carpoids: Evolutionary biologists have been interested in external asymmetry as a measure of genetic quality. Research shows that animals choose their mates partly on the basis of symmetry. Animals increase their genetic contribution to the next generation when mates of higher genetic quality are chosen. More symmetrical implies better genes and begets better prospects to pass on their genes.

British 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 since 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 manifestation of this 600 million years of selection.

Jefferies, in part, bases his theory on studies of carpoids. These small strange creatures had spiny tails, bulbous heads and were covered with spines. Having a body supported calcitic plates, they resemble modern echinoderms 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.

Carpoids are distinct from all other animals, extant and extinct, because of their complete asymmetry, internally and externally. Jefferies has interprets the asymmetry as an evolutionary body plan accident in the common ancestor of echinoderms and vertebrates. Carpoids descendents retained the lop-sided plan, but the echinoderms, and later the vertebrates were re-shaped into animals with external symmetry, but retaining asymmetry of the internal organs.

An intriguing theory – we’ll need to wait and see if modern phylogenetics can shed light on the shadowy period in the Precambrian when asymmetry appeared.

Jefferies R. P. S., 1986 The ancestry of the vertebrates British Museum (Natural History), London
Bromham LD, Degnan BM. Hemichordates and deuterostome evolution: robust molecular phylogenetic support for a hemichordate + echinoderm clade. Evol Dev. 1999 Nov-Dec;1(3):166-71.