Coeloscleritophora

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

A life-reconstruction of the armoured slug-like benthic animal Halkieria evangelista (Coeloscleritophora,Sachitida,Halkieriidae), Atdabanian of North-East Laurentia. Artwork © Stanton F. Fink

[edit] Introduction

This Cambrian group includes the Sachitida. They are characterized by having a large number of small sclerites. In fact, most members of the group are a large number of small sclerites, the actual body form being unknown. Where known, the body plan includes a prominent internal cavity and restricted basal foramen -- something like a balloon with scales.

The scales of coeloscleritophorans are distinctive in having a thin (<1µ) organic layer over the entire scale and an internal layer of aragonite fibers oriented along the long axis of the sclerite. Most, like Siphoguchites in the image (scale bar = 100µ) also have (1) bundles of external aragonite fibers inclined toward the tip which look like microscales, and (2) a rim around the base which lacks these aragonite bundles. See Porter (2005).

Recent specimens from Chenjiang suggest that the sclerites were embedded in a flexible, continuous layer of organic cuticle and stuck out "like cactus spines." In this model, both scales and cuticle were formed by an epithelial layer of living cells. Bengston & Hou (2001).

[edit] References

Bengston, S. 2004. Early skeletal fossils. In Neoproterozoic-Cambrian Biological Revolutions (J. H. Lipps & B. M. Waggoner, eds.). Pal. Soc. Papers 10: 67-77.

Bengtson, S., & Hou, X. 2001. The integument of Cambrian chancelloriids. Acta Palaeontologica Polonica 46 (1): 1–22.

[edit] Credits

ATW051231