Sachitida

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

(Image from Dzik, 1993)

[edit] Introduction

Halkieria evangelista is one of a number of the strange armoured "coat of mail" creatures that inhabited the early and middle Cambrian oceans. These creatures were protected by a prickly armour of calcareous scales. Usually these scales are found disarticulated, and they constitute much of the so-called "small shelly fauna" that predominates in the earliest Cambrian. Only in rare instances, such as the preservation resulting from a lagerstätte, do we get a glimpse of the entire animal.

As can be seen from the above illustration, Halkieria evangelista was a very bizarre creature indeed; almost counter-intuitive in appearance. Assuming the reconstruction (Bengston and Conway Morris [1984] based it on Wiwaxia) is accurate, the animal has a flattened body with a soft, rubbery underside, like a slug, which it used to crawl across the substrate.

The scales or sclerites which made up the dorsal surface were hollow and had a stem which was inserted into the back making a flexible chain-mail armor with which to defend against predators. The sclerites were arranged in three broad zones with a different shaped scale in each zone. The sides of the animal were covered with knife-shaped scales, the edges of the belly had sickle-shaped scales. But the most unusual thing about this animal were the two limpet-shaped shells attached to the dorsal surface, at either end of the animal. It is hard to see what purpose these relatively tiny shells might have served; perhaps they were vestigial structures.

[edit] Interpretations and Phylogentic relationships

Jerzy Dzik (1993) considers the Sachitida were primitive mollusks. He points out the similarity between the shell of the helcionellid Bemulla and that of Halkieria (although Glenn Morton suggests instead that the posterior shell is nearly identical to the shells of the earliest Cambrian brachiopods, like Obolella (Moore, Lalicker and Fisher, 1952 p. 224) ). Dzik also tentatively suggests a homology between Halkieria's two shells and the mollusk conch and operculum.

Jan Bergström (1989) considers that coat of mail animals like Halkieria and Wiwaxia represent a group of late surviving, perhaps pseudosegmented, "procoelomates", the ancestors of all higher (coelomate body plan) animals. These creatures developed sclerites as part of the general process of early Cambrian acquisition of fossiliferous hard parts by many phyla.

Conway Morris (1998, fig. above from therein) postulates the halkieriids as an intermediate common ancestor between annelids and brachiopods. As evidence for this, he points out that the edges of brachiopod shells have chitinous bristles, called setae, which extend away from the shell. The microscopic structure of the setae is identical to that of the chaetae of polychaete worms. Both of these could have evolved from the scales of halkieriids. In some of the brachiopods the setae are segmented as is the case with halkieriid scales.

The brachiopod development might have proceeded as follows. When attacked, Halkieria curled up between the two shells for protection. It does seem to me that these shells - at least from the illustration - are too small to have accommodated the animal in times of danger. Eventually the animal remained between the two shells and took up a sessile life. In support of this hypothesis, he observes that the primitive living brachiopod Neocrania early in life crawls across the ocean bottom, and as it matures, it folds itself in two and secretes a shell, like other brachiopods.

In contrast, the annelids opted for mobility. They lost their shells altogether, if indeed they ever had any. The Middle Cambrian Wiwaxia was a descendent of the halkieriids that developed very long spines instead of shells. From Wiwaxia it is only a relatively small step to spiny but more conventional worms like Canadia spinosa. Dzik however retains Wiwaxia among the halkieriids and the latter among the mollusks, or proto-mollusks.

More evidence for molluscan relationships is from a recent paper (Sceltema & Ivanov, 2002 - abstract reproduced below). Ironically, some 13 years earlier, Bergström had made a very similar observation, and suggested that rather than halkieriids having a mollusk connection, aplacophorans may be late surviving procoelomates!

It is probably meaningless to try to pigeonhole the halkieriids and other early protostomes. As with land plants like Ibyka and Hyenia, which contain characteristics of several distinct plant divisions, the halkieriids and their kin represent a common ancestral type, in which the distinction between mollusk, annelid, and brachiopod had not yet emerged. To an early Cambrian observer, there would be no difficulty in classifying them; they would all fit into a neat and well-defined monophyletic clade. It is only from out perspective of 500 million years of rigidly phyla-specific evolution that early coat of mail creatures seem strange.

[edit] Information

Linnaean ranking: Order

Author: He, 1980 in Yin et al. 1980

Stratigraphic range: Early and Middle Cambrian

Distribution: Cosmopolitan

Lifestyle: Vagile benthos

[edit] References

Bengston, S., & Conway Morris, S. 1984. A comparative study of Lower Cambrian Halkieria and Middle Cambrian Wiwaxia. Lethaia 17: 307-329.

Bergström, J. 1989. Metazoan evolution around the Precambrian-Cambrian transition. In The early evolution of Metazoa and the significance of problematic taxa (A. M. Simonetta & S. Conway Morris, eds.) Cambridge University Press.

Conway Morris, S. 1998. The Crucible of Creation. Oxford.

Dzik, J. 1993. Early Metazoan Evolution and the Meaning of its Fossil Record. In Evolutionary Biology vol. 27 (Max K. Hecht et al., eds.). Plenum Press, New York.

Scheltema, A. H., & D. L. Ivanov. 2002. An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils. Invertebrate Biology 121(1):1-10.

Abstract. A tiny neomenioid postlarva (Neomeniomorpha, or Solenogastres) collected from the water column 3 to 6 m above the east Pacific seamount Fieberling Guyot has 6 iterated, transverse groups of spicules and 7 regions devoid of spicules between the transverse groups and the anterior- and posteriormost spicules. Three pairs of ventral, longitudinal zones with columns of single spicules, each pair with its own distinctive spicule morphology, lack transverse iteration. The 7 regions bare of spicules are compared to shell fields in developing polyplacophorans, and spicule arrangement is compared to sclerite arrangement on the Cambrian fossils Wiwaxia corrugata and Halkieria evangelista and to the spines and shell plates of the Silurian Acaenoplax hayae. The term iteration is used to denote processes that result in both metameric segments and repeated ectodermal skeletal structures. Iterative morphogenesis was probably present in bilateral animals before the Cambrian. Comparisons of iterated ectodermal skeletal structures among fossil and extant forms are suggested to indicate evolutionary relationship.

[edit] Credits

MAK020510