Procedia Environmental Sciences 10 ( 2011 ) 1540 – 1545

doi: 10.1016/j.proenv.2011.09.245

Available online at www.sciencedirect.com

Conference Title

Initial Study of Biofacies Characteristics of Sinoceras Formation in Wansheng District of Chongqing

Guobin Li1 , Chensi2, Dewan Li2, and Xieneng Li2 1State Land and Resource of Wansheng, Chongqing 400800

2Key Laboratory of Geology for Mineral Resources & Environment, Chongqing Administration of Land, Resources and Housing, Chongqing 400042, China

E-mail: GUOTEACH@tom.com

Abstract

This paper conducts further study in sedimentary environment of Ordovician and living environment of Sinoceras in Wansheng Stone Forest through the analysis of biofacies characteristic of Sinoceras of this district. Combined with the study of mollusk growth pattern, this paper initially studies the reason why Sinoceras has large individual scale.

Keywords: Sinoceras; Ordovician; sedimentary facies; living environment

1. Introduction

In South China, Ordovician [1] is widely exposed with complete sequence and rich fossils. In the stratum of this region, a large number of Sinocerases are found. Paleontologic fossils found in Wansheng Stone Forest in Chongqing are mainly Chinese Sinoceras, which is also called “Orthoceras”. Walking in the Forest, people can find lots of paleontologic fossils, in the shape of shrimp and with the color of auratus and isabelline, in the stones---“Orthoceras”. They are petrified and evolved from the marine organism 500 million years ago. According to the researches, most of the fossils in the world are grey or with no color. This kind of colorful and well-preserved fossil is rare. As “Chinese Sinoceras” organisms have short living geological time, its fossil can be used to identify stratum sedimentation age. They are regarded by geologist as “standard fossil”. The Orthoceras in the Stone Forest, rich in number, has large individual scale and clear outlined giant cephalopod fossil. So, they are rare and delicate treasure.

This paper, addressing the Sinoceras characteristic in Wansheng Stone Forest, initially studies the reason why this kind of giant cephalopod has large individual scale. Combined with the biofacies of Sinoceras, it analyzes its living environment and relation of its large body..

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2. Formation of Sinoceras

Sinoceras is a marine soft-bodied mollusk without backbones (cephalopoda Nautilus), which is a kind of marine carnivorous swimming animal lived 450 million years ago (early paleontology Ordovician). It has calcareous shell (like rocket). In the front of the shell is soft flesh (head). In the head, there are tentacles distributed in circles (like present octopus or cuttlefish). The functions of tentacles are like feet, capturing food, crawling and swimming (so the paleontologists call it cephalopod). While swimming, the conehead is toward front, and the head can spray water. The reactive force of water spraying can make it move rapidly. After death, the flesh part was rot partially, and the calcareous shell was preserved in the ocean, becoming the present Sinoceras fossil.

Sinocerases are distributed in Central and Southern China. Its main characteristic are: medium-sized shell, large tapered and amplification degree of 1.6-1.9; the diameter of shell has abrupt increase to the oral area; the height of gas cell is 1/2 of neck; septum inferior fovea depth can reach half of the gas cell; surface of shell is fine and even waving streaks; siphuncle is thin, near to center of shell; short septum neck, less than 1/3 of gas cell height (figure 1)

Fig.1 fossil specimen of Sinoceras in Wansheng District

3. Living Environment Analysis

Biofacies refers to the combination of bioecology and sedimentary characteristic that can reflect specific palaeoenvironment. The researches on biofacies and its partition not only can improve stratum partition and comparison precision, reconstruct palaeoenvironment and restore basin paleogeography characteristic, but also has significance on researches of continental biota, paleontology geographic characteristic and sea level changes[2-4].

Researches related to cephalopod of Ordovician are rare. Systematically collect and analyze cephalopod researches of Ordovician in dozens of provinces of South China on the basis of studying cephalopoda ecologic characteristic in part of South China. And then, analyze in depth the cephalopod ecologic characteristic of Ordovician in South China and its relation with sedimentary facies, and after that, conduct studies on biofacies of cephalopoda of Ordovician in this region and its partition [5].

According to the biofacies characteristic of Sinoceras and combined with lots of materials, the diversity of marine organisms of Ordovician and its food chain (web) features are analyzed.

Ecologic Diversity of Marine Organisms of Ordovician. There were many types of marine organisms in Ordovician with various shapes, taking on a colorful and complex scene. In the ecologic space utilization aspect, marine organisms in Cambrian period were mainly sessile organisms on the bottom of ocean, epifauna or plankton. They have already owned a certain ecologic diversity, but most of

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diversity did not continue or evolve. To the end of Cambrian period, only a few epifauna and plankton (such as trilobita) survived. In Ordovician, marine biologic types have expanded on the basis of that in Cambrian period. In mesa environment, there were depth delve (bivalve), superficial delve (polychetes, part of brachiopoda and trilobita), low sessile organism (brachiopoda, bivalve, etc), high sessile organism (petiolate echinodermata, polyzoan, coral, dendroidea), lying habitat (hyolithes), vagile organism (trilobite, bivalve, gastropods and Ostracoda) and so on, forming a ecologic layering from the bottom of ocean to the top of it. Besides, in shelf water, oceanic water and slope environment, there were swimming cephalopoda and swimming (or floating) graptolthines, acritarchs, chitinozoa and radiolarian, etc. Therefore, from bottom of ocean to the surface water in Ordovician, from shore land to deepwater slope area, ecospheres were all occupied effectively, and every ecosphere had various organisms of different categories [6-7].

In organisms of different ecologic types in Ordovician, sessile organism held the superior position, such as the brachiopod, echinodermata, bryozoans, coral and sponge (bottleneck stone) and so on which were multiplied greatly. Most of them followed this ecologic method. They moved and got food in different levels of the ocean. Meanwhile, endobiont started to grow. Some (such as lamellibranch) could burrow and drill to more than 12cm depth to the bottom of ocean (Luquan Qiaojia Fm of Yunnan). In Ordovician, swimming and floating organisms began to rise (large-sized swimming organism Anomalocaris in Cambrian period was extinct at the end of Cambrian period), such as large-scaled swimming organism—cephalopoda and floating graptolithina. They were experiencing rapid evolvement, taking important role in oceanic ecologic area of different depth. Besides, conodont animals have developed in large size in Ordovician. Though the ecologic method of it is not sure, its shape pattern has taken on distinct diversity.

Food Chain of Marine Organisms of Ordovician. As regard of food intake, the most distinct characteristic of marine organisms of Ordovician was the huge emergence of various filter feeders. Their food were suspended objects or small-sized floating organisms, for example, part of the lamellibranch, rostroconchs and phosph brachiopoda werer filtering at the superficial level of the ocean; most lamellibranch, monoplacophora and gastropods were filtering food at the bottom of ocean; calcareous shell brachiopod and so on were filtering food at the upper bottom of ocean with ciliary rings; coral and bryozoans were taking food at different level of the ocean with tentacles; sea lily and cystoids could take food at the top level of the bottom of ocean with the tentacles because of their long stalks; graptolite were filtering food in the open and far oceanic water with ciliary tentacles. All of these formed the food-taking ecologic partition in different level of bottom of ocean [8-10]. The rapid growth of large number of filter organisms and food-taking ecologic partition expansion are the main characteristic of organism great radiation of Ordovician.

As for feeding habit, on the basis of taking micro plant as food in Cambrian period, some predators existed in Ordovician, such as cephalopodain in far oceanic water and open water, Ostracoda, agnatha and starfish swimming on the bottom, etc. Part of them could directly used all kinds of trapping devices to get food [11]. Besides, plant-eating organisms were on increase distinctly. Except for Monoplacophora and Ostracoda appeared in Cambrian period, urchin, gastropods (partially), Ostracoda and arthropod in Class Malacostraca (such as walnut shrimp) and others have been added in Ordovician. Meanwhile, the organisms fed in deposit mud at the bottom of ocean continued to multiply, such as trilobite, part of gastropods and Ostracoda, etc. In a word, the appearance of many predators at the bottom of ocean and in far oceanic water and increase of phytophagous animal groups made the food structure of marine organisms in Ordovician more complete.

In the ocean of Ordovician, it was possible that a relative stable and complex food web or food chain was formed. However, at present, the evidence for this is insufficient with weak and a few researches. Right now, there are two research methods: first is getting evidence from the paleoecology, such as fossil

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evidence in preying, parasitism, symbiotic relationship and other aspects. According to the function analysis on graptolite theca collar [12], as well as researches on coprolite with graptolite broken branches in stratum of Silurian period [13], the experts assumed that graptolite might be the food of many other fossil classes. Most possible predators included cephalopoda, arthropod and so on. The graptolite broken specimens found in Zhejiang China and Sweden in recent years [14] have provided new evidence for the symbiotic relationship of graptolite and other organisms. In the food habit of trilobite, preying was an original method, mainly limited in types of the early cambrian period. The types in Ordovician mainly included particle-eating and filtering, etc [15]. Therefore, even though the trilobite in Ordovician could move freely at the bottom of the ocean, they might not the predators, but the prey one. Many brachiopod of Ordovician were sedentary to the bottom, living by filtering. They might be the prey ones either. The brachiopod fossil in the late Ordovician stratum of America preserved their scar after the prey by cephalopoda [16]. The brachiopod group of late Ordovician in Western Zhejiang and Gandong even preserved the real example of ceratocricoid parasitizing on shells of brachiopod [17]. Besides, the evidence for preying of brachiopod by cephalopoda could also be seen in the USA stratigraphic record of Carboniferous [18]. The predator and prey relation between them might be established in the early Ordovician. In Sweden, brachiopod in Ordovician also had scar evidence after being preyed, but the exact predator cannot be determined [19].

Fish (Agnatha) had few records in Ordovician, only be seeing in a few spots in Hubei. Besides, the specimens are broken, normally a few scales or bones, etc. Their feeding habit is not clear yet, but possible mud or filtering. The conodont animal classification position was a mystery for a long time before. But with the discovery of specimens with mollusk in Scotland and South Africa in recent years, it can be determined that conodont animal is a chordate between Agnatha and Gnathostomata. It is a predator [20]. But its food relation with other organisms is not clear yet. The stratum of Ordovician has few directly evidence for relation between predator and prey, but there are many evidences for symbiotic relationship between organisms, such as the bryozoans, Pedunculata echinodermata, brachiopod and other symbiotic relationship preserved in the sponge fossil of Ordovician in Argentina [12]. The above evidences show that in the marine organisms of Ordovician, the filtering organisms took the lead, and the symbiotic relationship between organisms was frequent. So it should be a relatively harmonious scene at that time.

Another way to study food chain of Ordovician is the correlation of group evolvement of different organisms. The existing data shows that different organisms of Ordovician had disparities in the starting time of radiation with diversified evolvement tendency and track. Through comparison analysis of these data, it could be assumed indirectly that there were symbiotic relationships among organisms. If there was antibiotic relationship between two organisms, the “loss for one, and gain for another” was shown in the diversity curve; if they had reciprocal relation, “simultaneous loss and gain” was shown; if they had only commensalism relation or no relation, there was no correlation in the diversity curve. However, from the current situation, the researches in this aspect are insufficient.

According to the diversity of marine organisms of Ordovician and it food chain (web) characteristic, this paper believes Sinoceras was on top of food chain (web). Meanwhile, the diversity of marine organisms and its large quantity have provided sufficient food for Sinoceras, offering a good condition for its growth.

4. Growth Characteristic of Sinoceras.

According to the Sinoceras characteristic in Wansheng district and combined with the above shell membrane characteristic of mollusk, this paper makes assumptions respectively on psychological structure and growth model of Sinoceras.

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Characteristic of Sinoceras Sinoceras had hard shell with straight or spirally coiled, and the surface of its shell had wave-like cross striation. Inner the shell, there were lots of cross vein. The length of shell could reach more than 2 meters. Most of them were between several decades centimeters to 1 meters. When dissecting longitudinally, it could be seen that it was pointing to the slender pyramidal siphuncle in the beak; and in the transverse section center, the round siphuncle could be found. Compared with the diameter of shell, siphuncle was nearer to the center, and some to the margin. Normally, the shell was medium-sized with the shape of long tapered. Its amplification degree was 1.6-1.9; the diameter of shell had abrupt increase to the oral area; the height of gas cell is 1/2 of neck; septum inferior fovea depth could reach half of the gas cell; surface of shell was fine and even waving streaks; siphuncle was thin, near to center of shell; short septum neck, less than 1/3 of gas cell height.

Growth Characteristic Analysis of Sinoceras Mollusk was animal with trilaminar germs, symmetrical sides and deuterocoel. Shell membrane of mollusk was formed by the calcium carbonate secreted by epitheliums. The structure of shell had three layers. The outer layer was horny layer, which was thin, transparent and glossing and made of conchiolin. Normally, it would not be eroded by acid-base, so it could protect the shell. The middle layer was lamella, also called prismatic layer pearl shell, taking most of the shell, and it was made of calcite. The inner layer was bottom shell, also called pearl layer. It was glossing and made of leaf-like aragonite. The outer and middle layer were formed and secreted by mantle margin. It could grow in line with the animal, but the thickness would not change; inner layer was secreted by the whole mantle, and its thickness would increase with the growth of individual.

According to the researches on mollusk growth model and combined with the characteristic of Sinoceras fossil, it could be judged initially that its growth model met that of mollusk. With the growth of Sinoceras, it would continuously make calcareous secretion, forming the membrane in the middle and outer layer. Meanwhile, in the further growth process, the secretion would be discharged to its head along with the existing membrane. Part of the secretion would remain on the original membrane, thickening its outer and middle layer membrane. After that, gradual calcification gave birth to hard shell. In the process of outer membrane formation, secretion would gradually be hardened along with the shell around the shell. In the hardening process, the loop transverse striation would be formed. Therefore, the preserved fossil specimens of Sinoceras had taken on the structural characteristic of its transverse striation.

Due to the growth characteristic of Sinoceras, strong shell protection and on the top of food chain (web), its growth process had no threat from others and it could grow to the predator with large body.

5. Conclusions

Through the analysis of fossil specimens of Sinoceras in Wansheng Stone Forest and combined with the biofacies characteristic of Sinoceras, after reading lots of materials, it can be initially decided that it was in the top of the food chain (web) in oceanic environment of Ordovician. It had no threat from other organisms, so the condition for its large body was available. Meanwhile, the ocean harbored various organisms in Ordovician, providing sufficient food for its growth. This also became a condition for its large body

Combined with the researches on mollusk growth characteristic and model, it could be determined that the growth model of Sinoceras could ensure its continuous growth on the condition of sufficient food.

Therefore, from the above analysis, it can be initially judged that the prerequisites of large body Sinoceras can make it grow into a big one.

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