Procoelomata

From Palaeos

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