The Cambrian radiation of shelly fossils

TREE vol. 7, no. 3, March

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21, 13-55 3 Lloyd, D.G. and Bawa. KS. (1984) Evol. Biol. 17, 255-338 4 Ross, M. ( 19901 Trends Ecol. Evol. 5, 43-47 5 Bawa, K.S. and Webb, C.J. (1983) Evolution 37. 1272-l 282 6 Charnov, E.L. ( 1988) Theor. Popul. Biol. 34, 38-46 7 Kakehashi, M. and Yasushi, H. (19871 Plant Species Biof. 2, I-13 8 Charlesworth, D. and Charlesworth, B.

1. Theor. Biol. 118, 321-325 I4 Lloyd, D.G. II9871 Funct Ecol. I, 83-89 I5 Lloyd, D.G. II9871 in The Evolution of Sex: An Examination of Current Ideas (Levin, B.R. and Michod, R.E., eds), pp. 233-237, Sinauer Associates I6 Charlesworth, D. and Morgan, M.T. (1991) Philos. Trans. R. Sot. London Ser. B 332, 91-102 I7 Goldman, D.A. and Willson, M.F. (1986) Bot. Rev. 52, 157-194 41, 591-598 I8 McKone, M. (1987) Evolution I9 Harder, L.D. and Thomson, I.D. (1989) Am. Nat. 133. 323-344 20 Burd, M. and Allen, T.F.H. (19881

9 Charnov, E.L. (1979) Proc. Nat/ Acad. Sci.

Evolurion 42, 403-407 21 Schoen, D.G. and Stewart, S.C. (1986) Evolution 40. I 109-l I 20

USA 76, 2480-2484

22 Thomson, I.D. and Thomson, B.A. ( 1989)

IO Charnov. E.L. II9821 The Theory of Sex Allocation, Princeton University Press II Lloyd, D.G. (1984) in Perspectives on Plant Population Ecology IDirzo, R. and Sarukhan, I., eds), pp. 277-300, Sinauer Associates I2 Charlesworth. D. and Charlesworth, B. I I9871 Evolution 4 I ) 948-968 I3 Charnov, E.L. and Bull, 1.1.( I9861

Evolution 43, 657-661 23 Young, H.I. and Stanton, M.L. (1990)

( 1981I Biol. 1. Linn. Sot. 15, 53-73

Ecology 7 I I 536-547 24 Denslow, I.S. (I9871 Can. 1. Bat. 65, 1229-1235 25 Howe, H.F. and Swallwood, 1. (1982) Annu. Rev. Ecol. Syst. 13, 201-228 26 Schoen, D.I. ( 1982) Oecologia 53, 255-257

The CambrianRadiationof Shelly Fossils A.Y. Rozanov One of tCre mast significant events in tCle molluscs, brachiopods history of tGteorganic world was tCle acquigroups have now been sition by animals of tCle ability to build u sheleton. TGris is of special interest Gecause Stratigraphical problems the overwhelming majority of linown maior groups (phyla) ucquired that ability during a very short period (five to seven million years), early in the Cambrian. Recent fossil finds, especially in northern Asia, are adding much detail to our knowledge of this period. Though a very rapid appearance of the skeletal fauna has been suspected for a long time, a real notion of the variety of the first skeletal fossils was not revealed until the 1960~‘,~. Following the early publications and the introduction of the idea of ‘the Tommotian fauna’ (the fauna of the Tommotian stage, about 570 million years ago), there was a burst of investigations in many countries on ancient skeletal fossils. Hundreds of papers and a number of monograph+I4 on archaeocyaths, A.Y. Rozanov is at the Palaeontological Institute of the USSRAcademy of Sciences, Profsoyuznaya ul. 123, I 17868Moscow 832 I, Russia.

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There are differences of opinion concerning ‘the beginning of the Cambrian’, for two reasons. The first is a difference in the choice of the level to be taken for the beginning of the Cambrian; the second is differing notions of correlation of Precambrian and Cambrian boundary transitional deposits (and events) in various regions. In the first case, I mean the beginning of the Tommotian time; there are suggestions of drawing the Cambrian boundary one stage lower (along the bottom of the Nemakit-Daldyn or Rovno stages of the Vendian), but because of changes in ‘trace fossils’, not in skeletal fauna15. In the second case, the contradictions are in correlations of the Precambrian and Cambrian boundary sediments of the East-European and Siberian Platforms. Some scientists16J7 assume that the Lontova horizon (regional stage) of the East-European Platform is older

27 Bell, C. ( 19851 Proc. R. Sot. London

1992

Ser.

B 244, 223-265 28 Geber, M.A. and Charnov, E.L. (1986) I. Theor. Biol. 118, 33-43 29 Charnov, E.L. 119861Heredity 56, 119-121

30 Morgan, M.T. and Barrett, SCH. II9891 I. Evol. Biol. 2, 183-203 31 Ritland, C. and Ritland, K. ( 19891 Am. /. Bot. 76, I73 l-l 739 32 Cruden, R.W. and Lyon, D.L. (1985) Oecologia 66, 299-306 33 Cumaraswamy, A. and Bawa, K.S. ( 19891 P/ant Syst. Evol. 168, 59-69 34 Lovett Doust, I. and Cavers, P.B. ( 1982) Can. 1. Bat. 60, 2530-2534 35 Spira, P.T. 119801 Am. /. Bat. 67, 278-284 36 Cruden. R.W. ( 1977) Evolution 3 I, 32-46 37 Preston, R.E. (19861 Am. /. Bot. 73, 1732-l 737 38 Vasek, F.C. and Weng, V. II9881 Syst. Bot. 13, 336-350 39 Wyatt, R. ( 1984) Syst. Bot. 9, 432-440 40 Queller, DC. (19841 Evolution 38, 1148-1151 41 Willson. M.F. and Ruppel, K.P. (19841 Can. 1. Bot. 62, 799-805 42 Lemen, C. (19801 Oecologia 45, 156-159 43 Charnov, E.L. II9871 Evol. Ecol. I, 30-36

than the Tommotian stage, but this Even if it has not been confirmed. proves to be so, it is not of much importance for estimation of the scale and rate of the Cambrian skeletal fauna radiation. A very few solitary skeletal forms are present on the Lontova horizon, and their diversity is not great. Though the Cambrian radiation was very rapid, the first symptoms of the ability of organisms to build a skeleton appeared much earlier (Fig. I). Recently, there have been some problematic finds from the Siberian Ripheants (about 1600-800 million years ago), but the first animals with a true skeleton appear in the upper parts of the Precambrian. They cannot all be assigned with certainty to any of the known groups. They include: the so-called sabelliditids, found on the East-European Platform and in Siberia; Redkinia, known so far only on the East-European Platform; and, perhaps, Cloudina and similar organisms from South Africa and China. The stratigraphic position of the latter organisms is not yet clear within the frame of time scales used in the East-European Platform, and it is doubtful whether a number of finds from North America and other regions should be assigned to Cloudinaig. @ 1992.

Elsewr

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Publishers

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7REE vol. 7, no. 3, March

7992 Phosphatic (2)

ln

Carbonate (3)

Atdabanian

Tommotian lemakit-Daldin ROwlO Kotlin

Redklno

Vllchany

Fig.

I. The appearance in time of shelly fossils and mineral types of skeleton

Early Cambrian fossils At the very base of the Cambrian, the occurrence of sponges, archaeocyaths, radiocyathids, differentgroups of molluscs (Fig. 2) and brachiopods has been established with certainty. There is also a large number of problematic organisms conventionally named small shelly fossils (SSF). In general, SSFs may be divided into three formal groups”,20. The first of these comprises tubular fossils which are usually compared with polychaetes, pogonophorans and, more seldom, with protists (foraminifers), coelenmol1uscs2.3.5,6.9.l I ,I 3.20 terates and They are anabaritids, hyolithel: minths (Fig. 31, sabelliditids, mongolotubulids, platysolenitids and many others. Standing somewhat apart are the hyoliths, compared most often with molluscs2,“, but also considered an independent phylum2’. Conodontomorph forms mainly have a tubular shape, too (Fig. 4); the presence of specific microstructure in them is the reason for their comparison with conodonts. The conodontomorphs might, however, be assigned to the second group, which is represented by forms differing in shape, but all with an internal cavity (Fig. 5). Some of these represent separate sclerites of individual organisms. The sclerites covered the surface of animals that are very difficult to reconstruct. This is shown

especially vividly by the latest finds, in Greenland, of complete specimens22. At first, finds of these morphologically diverse forms produced an impression of great variety of SSFs in the early Cambrian. However, discovery of scleritomes (conglomerations of separate sclerites) showed that our notion of the diversity should be considerably reduced, since the body of an animal might have been covered by sclerites of different morphology. The third group includes the rest of the problematic fossils with various morphologies. They might be both independent skeletons and parts of skeletons of organisms as yet unknown. There are two main lines of interpretation of these fossils: worms (sensu Iato) or molluscs. The complexity of interpretation of these forms is illustrated by Sunnaginia (Fig. 6a) and Tumulduria23 (Fig. 6~). At the beginning of the Atdabanian, somewhat later than these three groups, the first trilobites and other arthropods appear, and at the end of the Atdabanian we find the first real echinoderms24. Amongst these there is another conventional group, represented by conical forms with various types of inner cavity and interpreted as polychaetes, coelenterates or independent phyla. They are volborthellids, Agmata, Hydroconozoa and others. There is also a purely

conventional group of plates and button-shaped fossil forms such as Hadimopanelia (Fig. 71, and others. Thus, in a comparatively short period (five to seven million years), practically all known types of invertebrate acquired the ability to

Fig. 2. (al Spicule (sponge), x 5, Botomian stage; Ibl Girphanovella, x I, Botomian stage. (c-f) Molluscs: (cl Khairkhania, X 29,Tommotian stage; fdl Latouchella, X 13, Tommotian stage; le) Watsonella, X IO, Atdabanian stage; IfI Postacantella, x 23, Atdabanian stage.

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Fig. 3. la) Coleoh, X 18, Tommotian (b) Anabarites, X 17, Tommotian stage.

stage;

build skeletons. The first skeletal organisms were mostly very small, not usually exceeding a few millimetres in length. In the majority of groups, the first appearance of skeletal forms is represented by several distinct genera and species; such is the state of matters with achaeocyaths, molluscs and others. This diversity suggests that these phyla may have had a long Precambrian history before skeletonization developed. A similar picture is observed, in the early Atdabanian time, in different arthropod groups3,25. An exception is the inarticulate brachiopods, represented by only one species early in the Cambrian14. Skeletal chemistry

X 18, Tommotian Fig. 4. la) Protohertzina, Ib) Fomitchella, x 40, Tommotian stage.

stage;

Fig. 5. (al Chance//aria, X 34, Tommotian Ibl Tommotia x 21, Tommotian stage.

stage;

Fig. 6. (a)

Sunnaginia, X X 28, Tommotian stage.

24, Tommotian stage; (b)

The sequence of emergence of skeletal formations differing in their chemical composition (Fig. I) is very interesting. In the middle Vendian (Redkino time), skeletal formations consisting of organic chemicals were found. At the end of the Vendian (early Nemakit-Daldyn time, Fig. I), fossils with phosphate ffrancolite Ca,,(P,C)O,F,) and (Anabarites) probably carbonate skeletons2” have been found. Practically on the boundary with the Cambrian, numerous groups with carbonate (CaCO,) and silicate (SiO,) skeletons appeared. Strange as it may seem, true agglutinated shells are known only from the Atdabanian. To some extent, such a sequence is consistent with historic ideas of biomineralization and with the data on the presence of an organic matrix necessary for building an overwhelming majority of mineral skeletons27. However, the reliability of these notions of chemical composition of ancient skeletal organism shells is not very

Mobergella,

x 16, Tommotian stage; (cl Tumulduria,

1992

great; skeleton composition has not been investigated in many ancient fossils, and a number of inferences were made from indirect features, mostly on the basis of reaction to acids by shells during their release from the rocks. Biogeographyand geologicalchange Judging by the available data, the beginning of the ‘Cambrian radiation’ of skeletal organisms is probably connected with suitable environmental conditions in particular geographic regions28. These were mainly epicontinental shallow-water seas with warm carbonate sedimentation, located within one huge continent of Palaeopangea28 (or Rodinia as named by the McMenamins29). This is evidenced by characteristic lithological peculiarities and specific textural features of the rocks. Conjectured water temperatures for the basins at that time correspond to those of the tropics today28. The main centres of appearance and diversification of the ancient skeletal faunas correspond to parts of today’s Asia (Siberia, Mongolia, China, Kazakhstan, etc.). There is a narrow zone on the Siberian Platform, only several scores of kilometres in width but over a thousand kilometres long, which we suspect to be the centre of origin of a number of groups, in particular archaeocyaths’2,28. It is situated between two large different basins: a salt-bearing (evaporite) one and an open-sea, epicontinental one. In this transitional zone of extreme shallow-water conditions the first bioherms (reefs) in the history of our planet were built by animals with skeletons: archaeocyaths. The centre of origin of a small number of groups, such as platysolenitids, volborthellids and lukatiellas having an agglutinated or silicate skeleton, may have been basins with terrigenous sedimentation. These basins might be supposed to be located in an extratropical zone (boreal or notalianl. Of great interest is the relationship between the scleritization process of the organic world and the background of global abiotic changes20,2a30. These changes include: a change at the end of the

TRE~vol.

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Fig. 7. Hadimopanella, stage.

x

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315, Atdabanian

Vendian or early Cambrian from dolomite sedimentation to limestone (that is, a substitution of Ca for CaMg carbonates); the appearance of enriched, primarily red-coloured limestones in considerable amounts; and large-scale salt accumulation. One of the most interesting peculiar global features is a phosphorite accumulation, whose maximum in time and space is closely connected with a wide development of SSF with phosphatic skeletons3L,32. References

I Rozanov, A.Y. and Missarzhevsky, V.V. ( 1966) Biostratigraphy and Fauna of Lower Cambrian Horizons (Transactions of the Geological Institute, Vol. 1481, Nauka 2 Rozanov, A.Y. et a/. (1969) Tommotian Stage and the Cambrian Lower Boundary Problem (Transactions of the Geological

CmXino.: “....-.“.!,”

Institute, Vol. 2061, Nauka 3 Sokolov, B.S. and Zhuravleva, I.T., eds ( 19831 Lower Cambrian Stage Subdivision of Siberia, Atlas of Fossils (Transactions of the Institute of Geology and Geophysics, Vol. 5581, Nauka 4 Luo Huilin et a/. (1984) Sinian-Cambrian Boundary Stratotype Sect/on at Meishucun, /inning, Yunnan, China, People’s Publishing House, Yunnan, China 5 Valkov, A.K. ( 1987) Biostratigraphy of the Lower Cambrian of the Eastern Part of the Siberian Platform ( Yudoma-Olenek region), Nauka 6 Yu Wen (1987) in Stratigraphy and Palaeontology of System/c Boundaries in China, Precambrian-Cambrian Boundary, pp. 19-344, Nanjing University Press [Chinese] 7 Geyer. G. (1986) Lethaia 67. 55-l I8 8 Kerber, M. (1988) Palaeontogr. Abt. A 202, 127-203 9 Qian Yi and Bengtson, S. (1989) Foss. Strata 24, I-156 IO Hamdi, B. (1989) Geol. Surv. /ran, Rep. 59, l-53 I I Missarzhevsky, V.V. ( 1989) Oldest Skeletal Fossils and Stratigraphy of Precambrian and Cambrian Boundary Beds (Transactions of the Geological Institute, Vol. 443), Nauka I2 Debrenne. F., Rozanov, A.Y. and Zhuravlev, A.Y. ( 19901 Regular Archaeocyaths (Cahiers de Pa/aeon tologie), Editions du CNRS, Paris I3 Bengtson, S., Conway Morris, S., Cooper, B.J., Ielf, P.A. and Runnegar, B.N. ( 1990) Early Cambrian Fossils from South Australia, Association of Australasian Palaeontologists. Brisbane I4 Pelman, Y.L. (1977) Early and Middle Cambrian Inarticulate Brachiopods from the Siberian Platform (Transactions of the Institute of Geology and Geophysics, Vol. 3161, Nauka I5 Narbonne, G., Myrow, P., Landing, E. and

*...~....I_#X” .. ......I.I....,......^....iL...“.....“. .I...Il....”I....._I.........

Anderson, M. (1987) Can. 1. Earth Sci. 24, 1277-1293 I6 Khomentovsky, V.V. f 1976) The Vendian (Transactions of the Institute of Geology and Geophysics. Vol. 243), Nauka 17 Moczydlowska, M. and Vidal, C. ( 1988) Geology 16, 166-168 I8 Sayutina. T.A. and Vilmova, Y.S. ( 1990) Paleonto/. 1. 3, 100-104 19 Grant, S.W.F. (I9901 Am. /. Sci. 290A, 26 l-294 20 Hoffman, A. and Netecki, M., eds ( 1986) Problematic Fossil Taxa, Clarendon Press 21 Sysoev. V.A. ( 1984) Palaeontol. Mag. 3, 3-14 22 Conway Morris, S. and Peel, J.S. (1990) Nature 345, 802-805 23 Bengtson, S. et a/. (1987) Lethaia 20, 361-370 24 Rozhnov, S., Fedorov, A. and Sayutina, T Palaeontol. Mag. (in press) 25 Rozanov, A.Y. and Sokolov. B.S., eds ( 1984) Lower Cambrian Stage Subdivision, Stratigraphy, Nauka 26 Brasier, M.D. ( 1990) Ceol. Sot. Spec. Pub/. 52, 289-303 27 Lowenstam, H.A. and Weiner, S. ( 1989) On Biomineralization, Oxford University Press 28 Rozanov, A.Y. ( 1986) What Happened 600 Million Years Ago, Nauka 29 McMenamin, M. and McMenamin, D. ( 1990) The Emergence of Animals: The Cambrian Breakthrough, Columbia University Press 30 Cowie, I. and Brasier, M.D., eds ( 19891 The Precambrian-Cambrian Boundary (Oxford Monographs on Geology and Geophysics No. /2). Clarendon Press 31 Rozanov. A.Y. and Zhegallo, E.A. (19891 Lithol. Miner. Resour. 3, 67-82 32 Cook, P. and Shergold, I., eds ( 19861 Phosphate Deposits of the World. Vol. I: Proterozoic and Cambrian Phosphor&, Cambridge University Press

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