Procoelomata

From Palaeos.org

Protostomia
`--o Lophotrochozoa [or Spiralia]
   `--+--Bryozoa [Ectoprocta]
      |--Platyzoa
      `--o Trochozoa 
         |==Coeloscleritophora† ("Procoelomata†")
         |--Nemertea [Nemertinea]
         |--o Eutrochozoa
         |  |?-Tullimonsterida †
         |  |--Mollusca
         |  |--Hyolitha †
         |  |--Sipuncula 
         |  `--Annelida
         `--o Brachiozoa
            |--Phoronida 
            `--Brachiopoda

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.

It is not certain that all these groups are related; it has been suggested for example that the Machaeridia may be closer to the Echinodermata.

• Class: Coeloscleritophora Bengtson & Missarzhevsky, 1981
  • Order: Chancelloriida Walcott, 1920
  • Order: Sachitida (incl. Halkieriida)
  • 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.

Credits

Palaeos com MAK020510; Palaeos org MAK061115