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Small Shelly Fauna


Abstract

Mineralised skeletons of uncertain affinity, which appear just before the beginning of the Cambrian, are collectively referred to as the ‘small shelly fauna’ (or ‘fossils’). Whereas many of these fossils are dissociated sclerites that give few clues as to the nature of the animals that bore them, in a few cases sclerites have been found in life associations or preserved with soft-bodied remains which provide indications of a body plan, even if the animal cannot confidently be assigned to any living phylum.

Keywords: small shelly fossils, small shelly fauna, Vendian, Cambrian, Tommotian

Introduction

Stimulated in part by an international endeavour to define the Precambrian-Cambrian boundary for the global geological time scale, investigations across the world, including Siberia, China, Europe, and Australia, have yielded an unexpected range of mineralised microfossils. Dubbed the ‘small shelly fauna’ (SSFs) in a 1975 review paper by Matthews & Missarzhevsky, they appear just before the beginning of the Cambrian, increasing in numbers and diversity towards the Tommotian. Most are either tiny shells or else the disarticulated components, called sclerites, of an overall protective armour composed of many such parts, called a scleritome (Bengtson et al. 1990; Runnegar 1992, p. 66).

The most common skeletal materials are calcium carbonate (aragonite or calcite) and varieties of calcium phosphate. Many of the latter may originally have been carbonates, phosphatised during preservation.

The oldest of these to occur abundantly are Cloudina and some allied genera: small, conical fossils made of calcium carbonate, first appearing in the Vendian. While it is not known what kind of organism produced Cloudina, and many other SSFs are equally problematic, some of the Cambrian representatives have been found in life associations or preserved with soft-bodied remains, and thus tied back to a firm systematic placement. One such is Microdictyon, which is now known to be an onychophoran (see more below).

Some other examples (Dinomischus, paleoscolicids, halkieriids, etc.) are less well-known, but at least provide indications of a body plan, even if the animal cannot confidently be assigned to any living phylum (After Valentine 1995, pp. 90-91.)

 
 

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  • Life on a Young Planet - Andrew Knoll

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[Purella Zone? – mentioned in Brasier et al.]

Chronology

First Appearance

Among the oldest records of Cloudina is the occurence in the Stirling Quartzite of California (Langille 1974) and persisting into the Cambrian. Anabarites and Cambrotubulus are other Vendian SSF taxa, known from Siberia and Mongolia.

Abundant calcified fossils occur in the terminal Proterozoic Nama Group, Namibia.
http://eaps.mit.edu/sedlab/projects/fossils.html - page removed; content (right) retrieved from Google cache Tomographic Fossil Reconstruction The color images are three-dimensional reconstructions of the earliest-known calcareous metazoans from the late Precambrian. The fossils were discovered by our group during systematic stratigraphic studies of facies distributions in the Nama Group. The grey-scale images contain examples from same group of fossils, as photographed in cross-section, as they are found in the rock. It is guessed that these organisms had soft calcareous encasings because some appear to have been deformed before final lithification (see arrows). It is hoped that further study of these organisms, undertaken in part by making three-dimensional reconstructions, will uncover most of the primary morphologies. Ultimately our goal is to establish the affinity of these metazoans to contemporary Ediacaran faunas and later Cambrian shelly faunas (if there is any).
The Nama SSFs "are constrained to occur stratigraphically below the first occurrence of Phycodes pedum, overlap in range with known Ediacaran fossil impressions, and extend between ash beds dated at 549 and 543 Ma. At least six distinct morphotypes have been identified to date, suggesting that the proportional representation of calcareous taxa in the terminal Proterozoic fauna was not markedly different from that observed in younger periods. New taxa include branching forms up to 1 cm in diameter that display hexagonal symmetry. This symmetry recalls the tri-radiate geometry of several prominent terminal Proterozoic and basal Cambrian taxa, including Tribrachidium and Anabarites. Other forms include open cups possibly representative of simple asconoid (vase-shaped) sponges; simple tubes that are functionally (if not necessarily phylogenetically) comparable to sabellid worm tubes, possibly indicating the presence of anterior tentacles or lophophore-like appendages; and more complex multichambered forms. In terms of abundance, the simple cups and tubes are most numerous, followed by the complex branching and multichambered forms, and then the previously described Cloudina. All calcified fossils in the Nama appear to represent benthic organisms that lived in close association with thrombolitic and stromatolitic reefs and biostromes, suggesting a strong substrate preference. Generally, they are rare or absent in other facies. Although the forms with hexagonal symmetry may be related to Anabarites, most Nama calcareous fossils dissappear near the PC-C boundary and do not appear to be closely related to Cambrian small shelly fossils. For the most part, they are as enigmatic as the canonical Ediacaran biota" (Grotzinger et al. 1998, Abstract).

Last Appearance

It is misleading to think of the SSFs as suffering extinction, or making a last appearance in any conventional sense, because they are not a real taxonomic group. Some, as we have seen, are onychophorans, others probably molluscs, and still others certainly neither. The small shelly fauna is defined by, as much as anything, its unfamiliarity. Thus, as the component organisms evolved to become more modern, so they become more familiar, and the SSF concept loses its cohesion. It is rare to encounter any mention of ‘SSFs’ in sediments younger than Atdabanian age, when the ‘familiar’ Paleozoic faunas - trilobites and so on - make their appearance.

Some Representative Taxa

Cloudina

Cloudina Germs 1972 and the allied genera comprising the family Cloudinidae Hahn & Pflug 1985; small, conical fossils made of calcium carbonate. Cloudinids are the oldest shelly animal fossils to appear abundantly in the fossil record, first appearing in the Stirling Quartzite of California (Langille 1974) and persisting into the Cambrian.

It is not known what kind of organism produced Cloudina.

Halkieria and Wiwaxia

 

Microdictyon

[Text box for Microdictyon – show the disarticulated sclerites as well as the complete organism]

[Was this in the Hou & Ramskold Nature article which ‘uprighted’ Hallucigenia?]

Evolutionary Significance

Paraphrase: "Small shelly fossils near the base of the Cambrian mark the transition to a skeletonized fauna and the metazoan-dominated Phanerozoic fossil record. The recovery of articulated specimens composed of multiple sclerites discussed above, such as Wiwaxia, Halkieria and Microdictyon, suggests that much of the remaining "small shelly fauna" represent elements similarly employed as ectodermal armor in bilaterian Metazoa that have yet to be recovered in an articulated form. In addition, recent finds of cap-shaped shells composed of fused spicules of the early Cambrian age (Bengtson 1992) support an argument of fusion of individual skeletal elements to form sclerites, plates, or shells (Haas 1981)."
[MaiLinEvo, CamExp - Quote some of the above + "Thus, the engrailed data reported here, in combination with previous scenarios for the formation of ectodermal armor and recent fossil discoveries, suggest a singular evolution of invertabrate skeletons near the base of the Cambrian, followed by subsequent loss in several lophotrochozoan and ecdysozoan lineages." Implication for CamExp continues: "Such an interpretation, if substantiated, would have important implications for the Cambrian radiation, as it would constrain readily fossilizable exoskeletons to a single lineage, leading to a monophyletic clade of bilaterians. This would lead to a closer association of the bilaterian diversification with the base of the Cambrian."]

Quote: "Onychophorans are thought to be the sister taxon of arthropods and are segmented. However, onychophorans lack engrailed expression in their dermis. Instead, expression is observed in the posterior half of the developing limb and in a segmental pattern in the lateral mesoderm. The limb staining suggests shared ancestry of the onychophoran and arthropod limbs. However, given the close relationship of Arthropoda and Onychophora, and their segmented body plans, the lack of segmental ectodermal expression in Onychophora suggests that the ancestral role of engrailed was not segmentation; this absence may be a consequence of evolutionary loss of skeletons. Onychophoran dermis lacks a chitinous cuticle; thus Onychophora lack an exoskeleton. Furthermore, Cambrian fossils thought to be stem group onychophorans, such as Microdictyon, Hallucinogenia, and Xenusion, bear skeletal elements above the limb on each segment. Therefore, the absence of engrailed transcription in the ectoderm of modern Onychophora could well be a consequence of evolutionary loss of exoskeletal elements coincident with the loss of a component of engrailed expression associated with skeletal boundaries. In addition, the Cambrian Palaeoscolex, an ecdysozoan (Conway Morris 1997), bears more extensive ectodermal sclerites than moderm pripulids, and Wiwaxia and Halkieria (Conway Morris & Peel 1995) bear more armor than many moden lophotrochozoa. These observations are consistent with the loss of skeletal elements in a number of lineages since the early Cambrian Period."

(All from Jacobs et al. 2000, p. 345)

References

Bengtson, S. 1992: The Cap-Shaped Cambrian Fossil Maikhanella and the Relationship Between Coeloscleritophorans and Molluscs. Lethaia 25: 401-420.

Conway Morris, S. 1997: The Cuticular Structure of the 495-Myr-Old Type Species of the Fossil Worm, Palaeoscolex, P. piscatorum (?Priapulida). Zoological Journal of the Linnaean Society 119: 69-82.

Conway Morris, S.; Peel, J.S. 1995: Articulated Halkieriids from the Lower Cambrian of North Greenland and Their Role in Early Protostome Evolution. Philosophical Transactions of the Royal Society of London, Ser. B Biol. Sci. 347: 305-358.

Grotzinger, John P.; Watters, W.; Knoll, Andrew H.; Smith, O. 1998: Diverse Calcareous Fossils from the Ediacaran-Age Nama Group, Namibia. Abstracts with Programs, Geological Society of America, 20 (7): A-147.

Haas, W. 1981: Evolution of Calcareous Hardparts in Primitive Molluscs. Malacologia, 21: 403-418.

Jacobs, David K.; Wray, Charles G.; Wedeen, Cathy J.; Kostriken, Richard; DeSalle, Rob; Staton, Joseph L.; Gates, Ruth D.; Lindberg, David R. 2000: Molluscan engrailed Expression, Serial Organization, and Shell Evolution. Evolution and Development, 2:6, pp. 340-347.

Matthews, S.C.; Missarzhevsky, V.V. 1975: Small shelly fossils of late Precambrian and early Cambrian age: a review of recent work. Journal of the Geological Society 131: 289-304.

Runnegar, Bruce 1992: Chapter 3 – Evolution of the Earliest Animals.  In, Schopf, J. William (ed) Major Events in the History of Life.  Jones and Bartlett.

Valentine, James W. 1995: Late Precambrian bilaterians: Grades and clades. In Fitch, Walter M.; Ayala, Francisco J. 1995: Tempo and mode in evolution: Genetics and paleontology 50 years after Simpson. National Academy of Sciences, pp. 87-107.


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