Palæos:		Procoelomates
INVERTEBRATES		Procoelomates

Page Back	Unit Home	Animal Kingdom	Metazoa	Mikko's Phylogeny Home
Page Next	Page Up	Invertebrates	Kingdoms Index	Vertebrates Index

The "Procoelomates"

Among the many remarkable animals of the early Cambrian are those characterized by a protective covering of small hollow scales or sclerites. In most cases of course no trace of the animal is found, and the disarticulated sclerites are hard to associate with a particular organism. These tiny cap-shaped or scalelike skeletal elements make up the so called "small shelly fossils" that are so predominant in assemblages of Tommotian, Atdabanian and Botomian age (the so-called "Tommotian biota").

Although the creatures that bore these scales remained a mystery for some time (and in many cases still do!), it is now known that many of these tiny shelly/skeletal elements belonged to small creeping slug-like animals. They formed a sort of interlocking "coat of mail", like medieval chain mail, that protected the animal against predators.

These animals cannot be easily classified in any modern phylum, but instead form a patchwork or mosaic of characteristics of several phyla, especially the molluscs, annelids, and brachiopods, regarding which they appear to be a sort of common ancestor. Cladists would divide these creatures according to their modern relations (which may be a very difficult task, because often very little is known of them. In view of their close resemblance to each other however, another approach is to group them together in a single paraphyletic phylum, the "Procoelomata" of Bergström or the "Promollusca" of Kinman. Ironically, Bergström's term is probably too broad, since it would seem that these coat-of-mail animals are ancestral to the Lophotrochozoa (the brachiopod- annelid- mollusc clade) only, rather than coelomate animals in general; whilst Kinman's term is certainly too narrow, since they were not solely mollusc cousins. However, I have decided to retain "Procoelomata" as this is the oldest term used to designate these creatures as a group.

Although most Procoelomata died out with the early Cambrian Botomian mass-extinction (one of the largest but least known extinction events in Earth's history), a few continued to thrive right up until the Devonian, and perhaps even the Carboniferous period

Representative Orders

classification mostly according to Valid PMPD Orders

Class: Machaeridia 
	Order: Hercollepadida 
	Order: Turrilepadida 

Class: Coeloscleritophora Bengtson & Missarzhevsky, 1981 
	Order: Chancelloriida Walcott, 1920
	Order: Sachitida 
	Order: Halkieriida (syn with Sachitida?)
	Order: Siphogonuchitida

Class: Cribricyathea 
	Order: Conoidocyathida 
	Order: Cribricyathida 
	Order: Vologdinophyllida 

Class: Machaeridia 
	Order: Hercollepadida 
	Order: Turrilepadida (= Lepidocoleomorpha) 

Class: Thylacocephala 
	Order: Concavicarida 
	Order: Conchyliocarida 

Class: Tommotiida Missarzhevsky, 1970 
	Order: Mitrosagophora Bengtson, 1977 
	Order: Coleolida 
	Order: Hyolithelminthes 
	Order: Paiutiida 
	Order: Sabelliditida 
	Order: Volborthellida 

Class and Order indet. 
	Family Anabaritidae Missarzhevsky, 1974

References

Jan Bergström, 1989 "Metazoan evolution around the Precambrian-Cambrian transition", in The early evolution of Metazoa and the significance of problematic taxa, ed. by Alberto M. Simonetta and Simon Conway Morris, Cambridge University Press

Jan Bergström, 1989, The Origin of Animal Phyla and the New Phylum Procoelomata, Lethaia 22: 259-69.

Page Back

Page Next

images not loading? | error messages? | broken links? | suggestions? | criticism?
contact webmaster

page uploaded 10 May 2002
checked ATW050808
page by M. Alan Kazlev
bars and buttons from Jelane's families of graphics

Unless otherwise noted,
the material on this page is licensed under a
Creative Commons License.