- Email: [email protected]
Evolution of shelly fossils and the end of the late precambrian
Precambrian Research, 29 (1985) 45--52 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
45
EVOLUTION OF SHELLY FOSSILS AND THE END OF THE LATE PRECAMBRIAN
JIANG ZHIWEN Institu te of Geologic Sciences of Yunnan Province (People's Republic of China)
(Received May 16, 1984 ; revision accepted August 2, 1984)
ABSTRACT
Jiang, Z., 1985. Evolution of shelly fossils and the end of the late Precambrian. Precambrian Res., 29: 45--52. The emergence of metazoan mollusca is a turning point in the history of the development of life. The evolution of shelly organisms, which took place close to the late Precambrian--Cambrian boundary laid the foundations of biologic features that have persisted ever since. This evolution was a break-through in terms of the co-ordinated interaction of environmental and biotal factors. In this paper a "multi-source" theory is presented for the origin of the earliest Cambrian shelly biota. It is suggested that the initial shelly biota underwent three stages of development. The global distribution of the biota is discussed and three different biogeographical provinces are delineated. The data conform with available palaeomagnetic data. The upper limit of the late Precambrian should be defined on the basis of evolutionary changes in the shelly biota. As currently defined the Precambrian--Cambrian boundary is diachronous on a global scale.
INTRODUCTION In existing g e o c h r o n o l o g i c a l timescales the stages b e f o r e the C a m b r i a n are p o o r l y defined. This is m a i n l y because o f a p o o r u n d e r s t a n d i n g o f the early stages o f m e t a z o a n e v o l u t i o n . The e m e r g e n c e o f m e t a z o a n s (as r e p r e s e n t e d by the Ediacaran fauna) m a r k s the beginning o f a very specific k i n d o f life f o r m with organs a n d sexual d i f f e r e n t i a t i o n . These early m e t a z o a n s are c o m m o n l y preserved in clastic s e d i m e n t s at the t o p o f marginal geosynclinal f o r m a t i o n s . Such f o r m s are k n o w n f r o m Australia, S o u t h Africa, N o r t h America, the E u r o p e a n part o f the U.S.S.R., w e s t e r n E u r o p e and elsewhere. Similar fossils have been r e p o r t e d f r o m the u p p e r Sinian in the m i d d l e - u p p e r parts o f the D e n g y i F o r m a t i o n o f the Yangzi P l a t f o r m in China. The e m e r g e n c e o f the E d i a c a r a n f a u n a saw the realization o f specializ a t i o n o f cell f u n c t i o n s . Such organisms were especially capable o f a d a p t i o n to c h a n g i n g e n v i r o n m e n t a l c o n d i t i o n s , leading to the e v o l u t i o n of still higher life f o r m s .
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
46 EMERGENCE OFTHESHELLYFAUNA The emergence of a shelly fauna marks an event of profound significance in the evolution of life on Earth. The change was no more than a conversion of biological type and yet it led to a structural improvement that proved to be a basic element in all succeeding animals. With the exception of the arthropoda virtually all invertebrates had primitive representatives in these early biotas. The evolution of shells took place about 600 Ma ago. At that time the level of free oxygen in the atmosphere was probably about 10% of present values (Cloud, 1976) and was sufficient to support respiratory circulation of living things in an aqueous medium. With further perfection of internal organs and the circulating fluid, the necessity of direct contact between the organism and the oxygen source was obviated. The outer surfaces were gradually reinforced as an adaptation to the ever-changing environment, and as a means of protection from predators. At the beginning of the Cambrian it is likely that sea-water was rich in phosphorus, calcium, manganese and silicon and t h a t the elements P, F, Ca, I, Si, B, Br etc. were relatively abundant, providing the materials necessary for the production of early shells. I have made analyses of small shelly fossils from various places and stratigraphic levels. Invariably the shells are composed of calcium phosphate. The P2Os c o n t e n t of such shells generally exceeds 2070 which is much greater than the c o n t e n t of the surrounding rocks (Jiang Zhiwen, 1984). In the course of biological mineralization which led to the formation of calcium phosphate shells, the question as to whether the biota could secrete alkaline phosphatase is of crucial importance (Tasch, 1973; Chave and Erhen, 1979). Such high molecule proteinase played the role of a catalyst in an alkaline environment, causing the Keratic-like conchiolin, initially secreted by the biota, to be linked with inorganic phosphorus, and then, with the help of biochemical activities, solid calcium phosphate shells were formed. It was easier for this evolution to take place in the mollusca because they had a paralaea mantle with specific shell-making functions. Therefore hyolitha, gastropoda, monoplancophorans, rostraconcha etc., all of which belong to the mollusca, occupy a predominant position among the shelly biota of the earliest Cambrian. Once shell-making evolution had occurred, the process was very vigorous and rapid, so much so, that it has led to a sort of illusion. It appears to have taken place rapidly at the beginning of the Cambrian so that an abundance of shelly biota was brought into being. It has been described as an " a b r u p t " process, but this is, at least in part, merely an artifact of our ignorance and the limits imposed by the available research tools. The Ediacara fauna represents a type of marine animal which came into being during a period of large scale transgression following widespread glaciation during the Varangan ice age. The records of fossil metazoa in the late Precambrian are relatively sparse. Interruption of the development of life is, however,
47 impossible. This is borne out by the presence of abundant trace fossils. It is likely that a transitional biota existed between the Ediacaran and the Meishucunian biotas. This is a research topic that deserves more work. The earlier history of the earliest Cambrian shelly biota that existed before the preservation of body fossils of arthropoda can be roughly divided into three stages represented by the three assemblages of the Meishucunian; the lowest is the Anabarites-Circotheca-Protohertzina, the middle the Paragloborilus-Siphogonuchites and the upper, Sinosachites-Eonovitatus. These represent, respectively, the early, middle and terminal growth stages of the Meishucunian shelly biota. The lower biota has a m o n o t o n o u s composition, is small in body, few in number and primitive in structure, all showing evolutionary conservatism. The middle assemblage is characterised by large quantities and diverse biota with larger forms and higher differentiation of shell structure. Most of the described varieties belong to this biota. Most of the upper assemblage show the sign of decadence and a few were distinctively declined. At a slightly higher level, trilobites and archaeoostracoda, primitive arthropoda, make their appearance heralding the arrival in the biological world of a completely new stage. PROVINCIALISM OF THE SMALL SHELLY BIOTAS Fossified biota had more differentiated organs than mollusca, were multifunctional organisms and had multi-use hard parts, thus enhancing their adaptibility to changing environments. Once established in their habitat they could radiate rapidly to greater and greater areas. Their distribution included not only broad platformal areas such as the Yangzi and Tarim Platforms, the Siberian and East European Platforms of the U.S.S.R. etc., but also geosynclinal environments such as East Altai of Mongolia, the Adelaide geosyncline of Australia etc. The provincial distribution of Meishucunian biota (Fig. 1) lends itself to a threefold subdivision (Jiang Zhiwen, 1984). (1) Meishucunian small shelly faunal province as represented by the Meischucun section in China. The three developmental stages of the earliest Cambrian shelly biota are very distinct from each other. This province falls into two main regions: (a) the western Asia--Pacific region, beginning at the Flinders Ranges in Australia, passing north of Vietnam to the western part of China's Upper Yangzi Platform, east Yunnan, to the eastern part of the Yangzi Gorges in Hubei (south of Shangxi), then curving westward across Chinglin Mt. to the mid-west part of the Tarim Platform, extending along the Tienshan Mts. to end in the Karatau region in the middle Asian part of the U.S.S.R. The belt-like extension intersects the present day equator at about 45 ° ; and {b) West-European and North American region, including the southern end of Sweden, northern Germany, western France, northern Spain, Northern Ireland, Newfoundland, the Mackenzie Mountains of Canada and northwestern Mexico.
48 | Y •
:,,
'
//
": q
/
~
).~.
-"
r.
x\
\,j"
- -
] I . . . . .
2
/
" . -
I '
,
,
i '
Fig. 1. Diagram showing global distribution of the earliest shelly metazoans. 1. Meischucurt type Shelly biota distribution region. 2. Altal-Sayan type Archaeocyathus-Alga distribution region. 3. Aldan type shelly-Archaeocyathus distribution region. A.E., Ancient equator (conjectural). (2) Aldan-type mixed small shelly fauna- archaeocyathid province, represented by some sections on the middle reaches of the Aldan River. Included in this province are the Anabar Massif of the Northwest Siberian Platform, the long narrow strip covering China's northeastern Yunnan province, northwestern Guichow, northern Sichuan and the Adelaide Geosyncline in Southern Australia. At these places the shelly biota is mixed, with primitive single, and rare double-walled axchaeocyathus. The division of the shelly biota into zones is not distinct. The lower assemblage of the first Meishucunian biota is c o m m o n l y lost and the middle and upper assemblages may occur together. (3) Altai-Sayan-type mixed archaeocyathus-algae biological province. In the vicinity of the Altai-Sayan Mts., the Khiya River of Kuzhetsk-Altai, and in the Eastern Sayan Mts., there axe Lower Cambrian outcrops containing the most ancient archaeocyathus fossil zone (Nachorocyathus marinskii), 93--225 m beneath the oldest trilobites. Zhuravleva et al. (1979) classified this as Atdabanian. It is possible that a contrast can be made with the upper Meishucunian. These three biological provinces have a well-defined global distribution. In the eastern hemisphere they are distributed in three belts: the southern, middle and northern belts. The middle belt is the Altai-Sayan-type archaeocyathus-algae province, located approximately at the palaeoequator. The southern belt is the Meishucunian-type small shelly fossil province, accompanied by phosphatic sediments and located in middle or high pa-
49 laeolatitudes. The northern belt and the east Yunnan--Mongolia belt is a mixed region of small shelly fossils and archaeocyathus, located close to the palaeotropics. The palaeogeographical framework based on biological distribution conforms to available palaeomagnetic data (Jiang Zhiwen, 1984). The distribution in the western hemisphere is rather sporadic. The western Europe--North American belt and the California--Mexico belt are similar to the southern belt of the eastern hemisphere. The Moroccan region may have been close to the palaeoequator (it includes archaeocyathus). "MULTI-SOURCE" THEORY ON THE ORIGIN OF SMALL SHELLY FOSSILS At the beginning of the Cambrian there had emerged an a b u n d a n t multip h y l u m shelly biota which was able to develop rapidly then spread over the whole world. Local genera and species made up a rather large proportion of this biota. The individual biological provinces and their subregions, even individual sections, all have unique local forms in addition to the main assemblage. This is one of the major distinctions between the initial small shelly fossils and the succeeding biota. The difference was chiefly related to the presence of physical barriers perhaps related to plate tectonic movements. When conditions were essentially ripe in all aspects for shelly evolution there were different types of biological sources. Where there were essentially identical internal causes and slightly different external causes, evolution took place in different ways. Living things were apt to originate evolve and develop initially in the most congenial environments. In earliest Cambrian times it is likely that there were different biological sources. This view is here called a "multi-source" theory. The eastern Yunnan region was the source of small shelly fossils. The Aldan River area in Siberia in the U.S.S.R. was the source of the archaeocyathids. THE PROBLEM OF THE PRECAMBRIAN--CAMBRIAN BOUNDARY
The discovery of the Ediacara and Meishucunian faunas has helped to bring into focus the obscure evolutionary genealogy of the late Precambrian and contributes to the solution of problems of late Precambrian chronology and the question of the Precambrian--Cambrian boundary. What age is the end of the late Precambrian? Where should the Precambrian--Cambrian boundary be located? These are two aspects of a longstanding geological problem. The crux of the matter lies in the choice of principles to be observed and methods to be adopted. There are several points of view which might be summarized as follows: {1) the late Precambrian is the youngest chronological unit of the Cryptobiotic which ought to terminate with the initial growth of macroscopic life forms with differentiated organs; (2) acceptance of the idea that this period is the youngest chronological
~ngyin~ :iania n | Yuhucun
Meishucunian Format ion
B a i y a n - [iaowai. Z h o n g ~hae :oushan yicun
Stage
Dahai
: h i u n g - Stag~ o~ "nussu o C
C h i u n g c h u s su Formation Badao- Tuanwan shan
Form.
8 5'
Mem.
3 Assen ~lag¢ =r
5' O
ubisse~
CO
blage
o
m
Baltic
"~ [Rowno ~"
Group
Lontova
~7
Glebovo
i~orizon horizon
o
I-,.
o~
horizon O"
o
o~
m N
=
ommot ian
c~-I~.cl- l-~i~.
Stage
N
• O" O
~D 0 ~.
o Q ooct-
e-
~ o
5"
~,.
~. 0
o o
I
~ o
o
m
O'q I ~
I~1
0
m c~
I~"
I
0
~" 0
i.~.
O'q 0
o
IJo
i.j.
I
0 0 0 o
O
I'~"
0
~
Q
~
Backbone Ranges Format ion
Anahareo Formation
~.~
~.0(~ 0 ~
m
0g
51 unit of the C r ypt obi ot i c while considering that its termination is the beginning o f the Phanerobiotic forms; (3) this period is the youngest chronological unit of the Proterozoic. Since its termination marks the beginning of the Palaeozoic it ought to be placed at the b o t t o m of the Cambrian; (4) the end of this period marks the termination of early biota in Earth history and the alternation of later biota, particularly the alternation of s p o r o p h y t e s and g a m e t o p h y t e s in the botanical world; (5) the terminal p o i n t of this period should be fixed at the place where i m p o r t a n t events in Earth history t o o k place. All of these points of view have c o n t r i b u t e d to discussions of the position o f the top of the late Precambrian. It must be acknowledged, however, that man y o f the changes that occur in the inorganic world are not of global significance. By contrast the organic world shows evidence of gradual irreversible sequential evolutionary changes, the record of which has provided the major means of subdivision of the Phanerozoic record. These biological subdivisions and correlations have been enhanced and quantified in recent years by the use o f geochronology and palaeomagnetic techniques. The upper and lower boundaries of a chronostratigraphic b o u n d a r y should, where possible, c o n f o r m with i m p o r t a n t events in the evolution of organisms. The " f o s s i f i c a t i o n " event is among the most i m p o r t a n t in the history of biological evolution. The upper bounda r y of the late Precambrian should be placed where there is clear evidence of biological evolution, i.e., at the base of the stratum containing the oldest shelly assemblages. This view has already been p r o p o u n d e d by bounda r y workers from various countries. The problems of the Precambnan--Cambrian b o u n d a r y are, however, far from being settled. One very i m p o r t a n t consideration is the time at which the shelly fossils emerged. L o o k e d at on a large scale, this event should have been a ppr oxi m a t e l y globally synchronous but when looked at on a local scale it seems more reasonable that ossification was initiated at different times in di f f er e nt evolutionary centres. Each form had its unique laws o f origin, d e v e l o p m e n t and evolution. In addition, there is the problem of time involved in diffusion out from places of origin. For each individual region a suitable biota is chosen as the standard, the same biota might not be suitable for a n o t h e r area. Even forms already selected as type genera and species should n o t be regarded as absolute standards for their study I. Circotheca-Anabarites-Protohertzina Assemblage, II Paragloborilus-Siphogonuchites Assemblage, III Sinosachites-Eonovitatus Assemblages. 1. Anabarites primitivus Subassemblage, 2. Circotheca longiconica Subassemblage, 3. Barbitositheca ansatus Subassemblage, 4. Anabarites trisulcatus Subassemblage, 5. Siphogonuchites triangulatus Subassemblage, 6. Paragloborilus subglobosus Subassemblage, 7. Eonovitatus longevanginatus Subassemblage, 8. Sinosachites flabireformis Subassemblage. A. Aldanocyathus sunnaginicus-Tiksitheca licis Zone, B. Lapworthella tortuosa Zone, C. Lapworthella bella Zone, D. Dokidocyathus lenaicus-Majatheco tumefecta Zone.
52 n e e d s t o be e x t e n d e d b o t h in s p a c e a n d t i m e . T h e use o f d i f f e r e n t research t e c h n i q u e s a n d d i f f e r e n t degrees o f scientific k n o w l e d g e m i g h t also result in d i f f e r e n t p o i n t s o f view. In m y o p i n i o n t h e b o u n d a r y s h o u l d be r e g a r d e d f o r t h e p r e s e n t as a s o m e w h a t w a v y line ( T a b l e I). A f t e r m u c h careful research we m a y select a s t r a t o t y p e b o u n d a r y w h i c h will t h e n r e p r e s e n t t h e c u r r e n t s t a t e o f k n o w l e d g e . We should, h o w e v e r , retain a s u f f i c i e n t l y flexible a t t i t u d e t h a t we are p r e p a r e d t o revise such b o u n d a r i e s in a m a n n e r c o n s i s t e n t w i t h d e v e l o p i n g t e c h n i q u e s a n d t h e e v e r - e x p a n d i n g field o f human knowledge.
REFERENCES
Chave, K. and Erben, H.K., 1979. Biomineralization. In: R.W. Fairbridge and D. Jublonsk (Editors), The Encyclopedia of Paleontology. Dowden, Hutchinson and Ross, Inc. Cloud, D.E., Jr., 1976. Beginnings of biospheric evolution and their biogeochemical consequence.Paleobiology, 2: 351--387. Jiang Zhiwen, 1984. Bioaction in the earliest Cambrian phosphorite deposit. Professional papers of fifth international field workshop and seminar on phosphorite (in Chinese). Jiang Zhiwen, 1984. Global distribution of the earliest shelly metazoans. Geol. Mag., 121. Jiang Zhiwen, 1984. Evolution of Early Shelly Metazoans and Basic Characteristics of Meishucun Fauna. In: Professional Papers of Stratigraphy and Palaeontology, No. 13, Geol. Publ. Hou., Peking, China (in Chinese). Tasch, P., 1973. Paleobiology of the Invertebrates, Date Retrieval from the Fossil Record. Wiley and Sons, Inc. Zhuravleva, I.T. (Editor), 1979. Biostratigrafiya i paleontologiya nizhnego Kembriya (Biostratigraphy and palaeontology of the Lower Cambrian) (in Russian). Trudy Institute of Geology and Geophysics, SO AN SSSR, Issue 406.
45
EVOLUTION OF SHELLY FOSSILS AND THE END OF THE LATE PRECAMBRIAN
JIANG ZHIWEN Institu te of Geologic Sciences of Yunnan Province (People's Republic of China)
(Received May 16, 1984 ; revision accepted August 2, 1984)
ABSTRACT
Jiang, Z., 1985. Evolution of shelly fossils and the end of the late Precambrian. Precambrian Res., 29: 45--52. The emergence of metazoan mollusca is a turning point in the history of the development of life. The evolution of shelly organisms, which took place close to the late Precambrian--Cambrian boundary laid the foundations of biologic features that have persisted ever since. This evolution was a break-through in terms of the co-ordinated interaction of environmental and biotal factors. In this paper a "multi-source" theory is presented for the origin of the earliest Cambrian shelly biota. It is suggested that the initial shelly biota underwent three stages of development. The global distribution of the biota is discussed and three different biogeographical provinces are delineated. The data conform with available palaeomagnetic data. The upper limit of the late Precambrian should be defined on the basis of evolutionary changes in the shelly biota. As currently defined the Precambrian--Cambrian boundary is diachronous on a global scale.
INTRODUCTION In existing g e o c h r o n o l o g i c a l timescales the stages b e f o r e the C a m b r i a n are p o o r l y defined. This is m a i n l y because o f a p o o r u n d e r s t a n d i n g o f the early stages o f m e t a z o a n e v o l u t i o n . The e m e r g e n c e o f m e t a z o a n s (as r e p r e s e n t e d by the Ediacaran fauna) m a r k s the beginning o f a very specific k i n d o f life f o r m with organs a n d sexual d i f f e r e n t i a t i o n . These early m e t a z o a n s are c o m m o n l y preserved in clastic s e d i m e n t s at the t o p o f marginal geosynclinal f o r m a t i o n s . Such f o r m s are k n o w n f r o m Australia, S o u t h Africa, N o r t h America, the E u r o p e a n part o f the U.S.S.R., w e s t e r n E u r o p e and elsewhere. Similar fossils have been r e p o r t e d f r o m the u p p e r Sinian in the m i d d l e - u p p e r parts o f the D e n g y i F o r m a t i o n o f the Yangzi P l a t f o r m in China. The e m e r g e n c e o f the E d i a c a r a n f a u n a saw the realization o f specializ a t i o n o f cell f u n c t i o n s . Such organisms were especially capable o f a d a p t i o n to c h a n g i n g e n v i r o n m e n t a l c o n d i t i o n s , leading to the e v o l u t i o n of still higher life f o r m s .
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
46 EMERGENCE OFTHESHELLYFAUNA The emergence of a shelly fauna marks an event of profound significance in the evolution of life on Earth. The change was no more than a conversion of biological type and yet it led to a structural improvement that proved to be a basic element in all succeeding animals. With the exception of the arthropoda virtually all invertebrates had primitive representatives in these early biotas. The evolution of shells took place about 600 Ma ago. At that time the level of free oxygen in the atmosphere was probably about 10% of present values (Cloud, 1976) and was sufficient to support respiratory circulation of living things in an aqueous medium. With further perfection of internal organs and the circulating fluid, the necessity of direct contact between the organism and the oxygen source was obviated. The outer surfaces were gradually reinforced as an adaptation to the ever-changing environment, and as a means of protection from predators. At the beginning of the Cambrian it is likely that sea-water was rich in phosphorus, calcium, manganese and silicon and t h a t the elements P, F, Ca, I, Si, B, Br etc. were relatively abundant, providing the materials necessary for the production of early shells. I have made analyses of small shelly fossils from various places and stratigraphic levels. Invariably the shells are composed of calcium phosphate. The P2Os c o n t e n t of such shells generally exceeds 2070 which is much greater than the c o n t e n t of the surrounding rocks (Jiang Zhiwen, 1984). In the course of biological mineralization which led to the formation of calcium phosphate shells, the question as to whether the biota could secrete alkaline phosphatase is of crucial importance (Tasch, 1973; Chave and Erhen, 1979). Such high molecule proteinase played the role of a catalyst in an alkaline environment, causing the Keratic-like conchiolin, initially secreted by the biota, to be linked with inorganic phosphorus, and then, with the help of biochemical activities, solid calcium phosphate shells were formed. It was easier for this evolution to take place in the mollusca because they had a paralaea mantle with specific shell-making functions. Therefore hyolitha, gastropoda, monoplancophorans, rostraconcha etc., all of which belong to the mollusca, occupy a predominant position among the shelly biota of the earliest Cambrian. Once shell-making evolution had occurred, the process was very vigorous and rapid, so much so, that it has led to a sort of illusion. It appears to have taken place rapidly at the beginning of the Cambrian so that an abundance of shelly biota was brought into being. It has been described as an " a b r u p t " process, but this is, at least in part, merely an artifact of our ignorance and the limits imposed by the available research tools. The Ediacara fauna represents a type of marine animal which came into being during a period of large scale transgression following widespread glaciation during the Varangan ice age. The records of fossil metazoa in the late Precambrian are relatively sparse. Interruption of the development of life is, however,
47 impossible. This is borne out by the presence of abundant trace fossils. It is likely that a transitional biota existed between the Ediacaran and the Meishucunian biotas. This is a research topic that deserves more work. The earlier history of the earliest Cambrian shelly biota that existed before the preservation of body fossils of arthropoda can be roughly divided into three stages represented by the three assemblages of the Meishucunian; the lowest is the Anabarites-Circotheca-Protohertzina, the middle the Paragloborilus-Siphogonuchites and the upper, Sinosachites-Eonovitatus. These represent, respectively, the early, middle and terminal growth stages of the Meishucunian shelly biota. The lower biota has a m o n o t o n o u s composition, is small in body, few in number and primitive in structure, all showing evolutionary conservatism. The middle assemblage is characterised by large quantities and diverse biota with larger forms and higher differentiation of shell structure. Most of the described varieties belong to this biota. Most of the upper assemblage show the sign of decadence and a few were distinctively declined. At a slightly higher level, trilobites and archaeoostracoda, primitive arthropoda, make their appearance heralding the arrival in the biological world of a completely new stage. PROVINCIALISM OF THE SMALL SHELLY BIOTAS Fossified biota had more differentiated organs than mollusca, were multifunctional organisms and had multi-use hard parts, thus enhancing their adaptibility to changing environments. Once established in their habitat they could radiate rapidly to greater and greater areas. Their distribution included not only broad platformal areas such as the Yangzi and Tarim Platforms, the Siberian and East European Platforms of the U.S.S.R. etc., but also geosynclinal environments such as East Altai of Mongolia, the Adelaide geosyncline of Australia etc. The provincial distribution of Meishucunian biota (Fig. 1) lends itself to a threefold subdivision (Jiang Zhiwen, 1984). (1) Meishucunian small shelly faunal province as represented by the Meischucun section in China. The three developmental stages of the earliest Cambrian shelly biota are very distinct from each other. This province falls into two main regions: (a) the western Asia--Pacific region, beginning at the Flinders Ranges in Australia, passing north of Vietnam to the western part of China's Upper Yangzi Platform, east Yunnan, to the eastern part of the Yangzi Gorges in Hubei (south of Shangxi), then curving westward across Chinglin Mt. to the mid-west part of the Tarim Platform, extending along the Tienshan Mts. to end in the Karatau region in the middle Asian part of the U.S.S.R. The belt-like extension intersects the present day equator at about 45 ° ; and {b) West-European and North American region, including the southern end of Sweden, northern Germany, western France, northern Spain, Northern Ireland, Newfoundland, the Mackenzie Mountains of Canada and northwestern Mexico.
48 | Y •
:,,
'
//
": q
/
~
).~.
-"
r.
x\
\,j"
- -
] I . . . . .
2
/
" . -
I '
,
,
i '
Fig. 1. Diagram showing global distribution of the earliest shelly metazoans. 1. Meischucurt type Shelly biota distribution region. 2. Altal-Sayan type Archaeocyathus-Alga distribution region. 3. Aldan type shelly-Archaeocyathus distribution region. A.E., Ancient equator (conjectural). (2) Aldan-type mixed small shelly fauna- archaeocyathid province, represented by some sections on the middle reaches of the Aldan River. Included in this province are the Anabar Massif of the Northwest Siberian Platform, the long narrow strip covering China's northeastern Yunnan province, northwestern Guichow, northern Sichuan and the Adelaide Geosyncline in Southern Australia. At these places the shelly biota is mixed, with primitive single, and rare double-walled axchaeocyathus. The division of the shelly biota into zones is not distinct. The lower assemblage of the first Meishucunian biota is c o m m o n l y lost and the middle and upper assemblages may occur together. (3) Altai-Sayan-type mixed archaeocyathus-algae biological province. In the vicinity of the Altai-Sayan Mts., the Khiya River of Kuzhetsk-Altai, and in the Eastern Sayan Mts., there axe Lower Cambrian outcrops containing the most ancient archaeocyathus fossil zone (Nachorocyathus marinskii), 93--225 m beneath the oldest trilobites. Zhuravleva et al. (1979) classified this as Atdabanian. It is possible that a contrast can be made with the upper Meishucunian. These three biological provinces have a well-defined global distribution. In the eastern hemisphere they are distributed in three belts: the southern, middle and northern belts. The middle belt is the Altai-Sayan-type archaeocyathus-algae province, located approximately at the palaeoequator. The southern belt is the Meishucunian-type small shelly fossil province, accompanied by phosphatic sediments and located in middle or high pa-
49 laeolatitudes. The northern belt and the east Yunnan--Mongolia belt is a mixed region of small shelly fossils and archaeocyathus, located close to the palaeotropics. The palaeogeographical framework based on biological distribution conforms to available palaeomagnetic data (Jiang Zhiwen, 1984). The distribution in the western hemisphere is rather sporadic. The western Europe--North American belt and the California--Mexico belt are similar to the southern belt of the eastern hemisphere. The Moroccan region may have been close to the palaeoequator (it includes archaeocyathus). "MULTI-SOURCE" THEORY ON THE ORIGIN OF SMALL SHELLY FOSSILS At the beginning of the Cambrian there had emerged an a b u n d a n t multip h y l u m shelly biota which was able to develop rapidly then spread over the whole world. Local genera and species made up a rather large proportion of this biota. The individual biological provinces and their subregions, even individual sections, all have unique local forms in addition to the main assemblage. This is one of the major distinctions between the initial small shelly fossils and the succeeding biota. The difference was chiefly related to the presence of physical barriers perhaps related to plate tectonic movements. When conditions were essentially ripe in all aspects for shelly evolution there were different types of biological sources. Where there were essentially identical internal causes and slightly different external causes, evolution took place in different ways. Living things were apt to originate evolve and develop initially in the most congenial environments. In earliest Cambrian times it is likely that there were different biological sources. This view is here called a "multi-source" theory. The eastern Yunnan region was the source of small shelly fossils. The Aldan River area in Siberia in the U.S.S.R. was the source of the archaeocyathids. THE PROBLEM OF THE PRECAMBRIAN--CAMBRIAN BOUNDARY
The discovery of the Ediacara and Meishucunian faunas has helped to bring into focus the obscure evolutionary genealogy of the late Precambrian and contributes to the solution of problems of late Precambrian chronology and the question of the Precambrian--Cambrian boundary. What age is the end of the late Precambrian? Where should the Precambrian--Cambrian boundary be located? These are two aspects of a longstanding geological problem. The crux of the matter lies in the choice of principles to be observed and methods to be adopted. There are several points of view which might be summarized as follows: {1) the late Precambrian is the youngest chronological unit of the Cryptobiotic which ought to terminate with the initial growth of macroscopic life forms with differentiated organs; (2) acceptance of the idea that this period is the youngest chronological
~ngyin~ :iania n | Yuhucun
Meishucunian Format ion
B a i y a n - [iaowai. Z h o n g ~hae :oushan yicun
Stage
Dahai
: h i u n g - Stag~ o~ "nussu o C
C h i u n g c h u s su Formation Badao- Tuanwan shan
Form.
8 5'
Mem.
3 Assen ~lag¢ =r
5' O
ubisse~
CO
blage
o
m
Baltic
"~ [Rowno ~"
Group
Lontova
~7
Glebovo
i~orizon horizon
o
I-,.
o~
horizon O"
o
o~
m N
=
ommot ian
c~-I~.cl- l-~i~.
Stage
N
• O" O
~D 0 ~.
o Q ooct-
e-
~ o
5"
~,.
~. 0
o o
I
~ o
o
m
O'q I ~
I~1
0
m c~
I~"
I
0
~" 0
i.~.
O'q 0
o
IJo
i.j.
I
0 0 0 o
O
I'~"
0
~
Q
~
Backbone Ranges Format ion
Anahareo Formation
~.~
~.0(~ 0 ~
m
0g
51 unit of the C r ypt obi ot i c while considering that its termination is the beginning o f the Phanerobiotic forms; (3) this period is the youngest chronological unit of the Proterozoic. Since its termination marks the beginning of the Palaeozoic it ought to be placed at the b o t t o m of the Cambrian; (4) the end of this period marks the termination of early biota in Earth history and the alternation of later biota, particularly the alternation of s p o r o p h y t e s and g a m e t o p h y t e s in the botanical world; (5) the terminal p o i n t of this period should be fixed at the place where i m p o r t a n t events in Earth history t o o k place. All of these points of view have c o n t r i b u t e d to discussions of the position o f the top of the late Precambrian. It must be acknowledged, however, that man y o f the changes that occur in the inorganic world are not of global significance. By contrast the organic world shows evidence of gradual irreversible sequential evolutionary changes, the record of which has provided the major means of subdivision of the Phanerozoic record. These biological subdivisions and correlations have been enhanced and quantified in recent years by the use o f geochronology and palaeomagnetic techniques. The upper and lower boundaries of a chronostratigraphic b o u n d a r y should, where possible, c o n f o r m with i m p o r t a n t events in the evolution of organisms. The " f o s s i f i c a t i o n " event is among the most i m p o r t a n t in the history of biological evolution. The upper bounda r y of the late Precambrian should be placed where there is clear evidence of biological evolution, i.e., at the base of the stratum containing the oldest shelly assemblages. This view has already been p r o p o u n d e d by bounda r y workers from various countries. The problems of the Precambnan--Cambrian b o u n d a r y are, however, far from being settled. One very i m p o r t a n t consideration is the time at which the shelly fossils emerged. L o o k e d at on a large scale, this event should have been a ppr oxi m a t e l y globally synchronous but when looked at on a local scale it seems more reasonable that ossification was initiated at different times in di f f er e nt evolutionary centres. Each form had its unique laws o f origin, d e v e l o p m e n t and evolution. In addition, there is the problem of time involved in diffusion out from places of origin. For each individual region a suitable biota is chosen as the standard, the same biota might not be suitable for a n o t h e r area. Even forms already selected as type genera and species should n o t be regarded as absolute standards for their study I. Circotheca-Anabarites-Protohertzina Assemblage, II Paragloborilus-Siphogonuchites Assemblage, III Sinosachites-Eonovitatus Assemblages. 1. Anabarites primitivus Subassemblage, 2. Circotheca longiconica Subassemblage, 3. Barbitositheca ansatus Subassemblage, 4. Anabarites trisulcatus Subassemblage, 5. Siphogonuchites triangulatus Subassemblage, 6. Paragloborilus subglobosus Subassemblage, 7. Eonovitatus longevanginatus Subassemblage, 8. Sinosachites flabireformis Subassemblage. A. Aldanocyathus sunnaginicus-Tiksitheca licis Zone, B. Lapworthella tortuosa Zone, C. Lapworthella bella Zone, D. Dokidocyathus lenaicus-Majatheco tumefecta Zone.
52 n e e d s t o be e x t e n d e d b o t h in s p a c e a n d t i m e . T h e use o f d i f f e r e n t research t e c h n i q u e s a n d d i f f e r e n t degrees o f scientific k n o w l e d g e m i g h t also result in d i f f e r e n t p o i n t s o f view. In m y o p i n i o n t h e b o u n d a r y s h o u l d be r e g a r d e d f o r t h e p r e s e n t as a s o m e w h a t w a v y line ( T a b l e I). A f t e r m u c h careful research we m a y select a s t r a t o t y p e b o u n d a r y w h i c h will t h e n r e p r e s e n t t h e c u r r e n t s t a t e o f k n o w l e d g e . We should, h o w e v e r , retain a s u f f i c i e n t l y flexible a t t i t u d e t h a t we are p r e p a r e d t o revise such b o u n d a r i e s in a m a n n e r c o n s i s t e n t w i t h d e v e l o p i n g t e c h n i q u e s a n d t h e e v e r - e x p a n d i n g field o f human knowledge.
REFERENCES
Chave, K. and Erben, H.K., 1979. Biomineralization. In: R.W. Fairbridge and D. Jublonsk (Editors), The Encyclopedia of Paleontology. Dowden, Hutchinson and Ross, Inc. Cloud, D.E., Jr., 1976. Beginnings of biospheric evolution and their biogeochemical consequence.Paleobiology, 2: 351--387. Jiang Zhiwen, 1984. Bioaction in the earliest Cambrian phosphorite deposit. Professional papers of fifth international field workshop and seminar on phosphorite (in Chinese). Jiang Zhiwen, 1984. Global distribution of the earliest shelly metazoans. Geol. Mag., 121. Jiang Zhiwen, 1984. Evolution of Early Shelly Metazoans and Basic Characteristics of Meishucun Fauna. In: Professional Papers of Stratigraphy and Palaeontology, No. 13, Geol. Publ. Hou., Peking, China (in Chinese). Tasch, P., 1973. Paleobiology of the Invertebrates, Date Retrieval from the Fossil Record. Wiley and Sons, Inc. Zhuravleva, I.T. (Editor), 1979. Biostratigrafiya i paleontologiya nizhnego Kembriya (Biostratigraphy and palaeontology of the Lower Cambrian) (in Russian). Trudy Institute of Geology and Geophysics, SO AN SSSR, Issue 406.