(Geologisch-Pal？ Mü nster Institute of Ontology, Germany, address: D-48 149.
The summary of the comprehensive paleoecology of Late Annian Qingyan in Guizhou Province is based on the continuous quantitative analysis of different fossil assemblages. In the late period of Animir, the area experienced the transition from deep water to shallow water and from deep water to shallow water. First of all, it has developed from a deeper water phase containing only a small amount of plankton to a shallow water phase with high heterogeneity characterized by a very rich benthic community. At first, several brachiopod-dominated communities with relatively low differentiation were established. After that, the high diversity community dominated by small bivalves and gastropods began to flourish. These communities may be closely related to the aquatic areas of macroalgae. After some turbulence and another replacement of these two community types, the facies has developed into a facies corresponding to deeper water conditions with only a small amount of plankton.
Comprehensive paleoecology; Qingyan; Anni period of middle Triassic in Guizhou
1 Introduction
Qingyan is a small town about 30km south of Guiyang (photo 1). The main research area marked on the map is located in the northeast of Qingyan Town, covering the belt-shaped area from the river to the foot of Lion Mountain. It is equivalent to the section passing through the upper part of the Anniian period and is called "Yuping Sub-Formation" in the regional stratigraphic table (table 1). The main research site is the "Bangtoupo" section with high fossil yield (also wrongly called "Radar Slope"), which belongs to the middle section of the radar slope section in stratigraphy. Here, there are few opportunities to study the details and high resolution of rich and highly differentiated fossil assemblages. The thickness of the upper part of Anniian in Qingyan area is 396 meters, and the Anniian sequence on Qingyan is deposited on the slope of the transition from carbonate platform to open basin.
In the lower part of the Leidapo profile, the upper part of the Annihilistic period begins with thin mudstone, which contains a small amount of deep-sea fossils (bivalves Posidonia, Daonella and Cephalopoda), and trace fossils are produced in a few horizons. The units deposited in shallow water, especially those well exposed on the slope of the club head, developed from this deep-water facies. These thin mudstones are more or less marl, mostly slightly silty, and the thin interbeds with more calcium (marl to marl) contain abundant, high-resolution and quantitative shallow sea fossils. They mainly represent epiphytic benthic and stable soft basement, and are rich in * * * combinations of most smaller groups. Finally, the transition from shallow water conditions to deep water facies is a thick sequence composed of thin mudstone, including the upper part of radar slope profile and the whole Qing Yu profile. Especially in Qing Yu section, these mudstones can be partially turned into marl. The sequence also contains a small number of deep-water fossils (bivalves Posidonia, Daonella, Cephalopoda), with only a few layers of trace fossils. Due to the poor outcrop conditions, it is impossible to make a detailed quantitative study of this transitional sequence.
Bangtoupo fossil assemblage
The brachiopod, bivalve and gastropod are the main assemblages in the shallow sea of Bangtoupo. The diversity of brachiopods is low, but some of their species can have rich individuals. On the other hand, bivalves and gastropods are often highly differentiated, but there is no dominant single species. The following will analyze these three categories respectively. Sponges, corals, annelids, reptiles, mosses, sea lilies, sea urchins, ammonites, nautilus and calcareous algae constitute a small part of this fossil assemblage in varying degrees, and they are treated as a unit for the sake of clarity. As mentioned above, micro-animals composed of foraminifera, ostracods, sea snakes and other taxa have not been studied in detail. Figure 3 shows the four units of each sample and * * * combination, that is, the relative abundance of brachiopods, bivalves, gastropods and "other groups" corrected by size, the relative abundance of brachiopods, bivalves and gastropods (not corrected by size), and the species diversity.
Fig. 2 Paleogeographic map of Miko Wu in south-central Guizhou (according to references [1 1] and [7]).
The first sample ({1}) still shows the obvious influence of the deeper water phase below. It represents a mixture of deep-water bivalves (Posidonia) and shallow benthic molecules. The second sample ({2}) already contains a clear benthic assemblage, but there are no dominant species, mainly composed of bivalves and brachiopods, with a small amount of gastropods.
At first, brachiopods were obviously the most important family, and several different combinations of brachiopods with low differentiation were developed. In these * * * combinations, brachiopods far exceed 50% of all combinations (Na = 61.8% ~ 77.5%). At first, the naked snail was obviously dominant ({3}), and then it transited to the * * generation combination ({5}, {6}) which was dominated by the naked snail with two teeth.
From the * * * combination of sample {7}, the * * * combination dominated by brachiopods began to decline. Brachiopods are still the most abundant species, but considering the relative abundance and species diversity, bivalves and gastropods have become more important. Just like the following combinations, no single species is dominant. In the * * * combination {8}, the relative abundance of bivalves and gastropods further increased, and now bivalves are the most abundant species, followed by brachiopods.
This development led to the emergence of a highly diverse community of small benthic invertebrates in the middle of Bangtoupo section. Each sample of these * * * combinations contains almost 120 species, and bivalves and gastropods are obviously dominant (the total number of na is slightly lower than 70%) ({9} to {12}). The two categories show roughly the same number of species. The diversity of gastropods with high scores is particularly obvious. The importance of brachiopods is greatly reduced. In these * * * combinations, no single species occupies a significant position. Although the individual components of samples {9} to {12} are slightly different, all their combinations should be regarded as this type. In these four * * * combinations, the frequency of Solenopora is higher, which deserves attention.
Figure 3
A—— the relative abundance of brachiopods, bivalves, gastropods and "other groups" after size correction; B—— relative abundance of brachiopod, bivalve and gastropod (without size correction); C refers to the species diversity of each sample/fossil combination.
Compared with the previous unit, Gastropoda lost a large part of its importance ({13}), followed by more * * * highly differentiated dominant species combinations ({14} to {16}). At first, bivalves were the most important class, and gastropods were also an important part of the * * * combination. In the * * * combination {14}, microalgae are extremely rich again. In the * * * combination {16}, brachiopods replaced bivalves as the most abundant phylum, but the species diversity was still high.
Except for minor changes, the general trend is to move towards a new stage of low diversity * * * biological combination dominated by brachiopods. In the * * * combination {17}, bivalves increased again, becoming the most abundant phylum, and microalgae were more common again. The * * * combination of sample {18} still showed a high degree of differentiation, but brachiopods were obviously dominant. The most abundant brachiopod is Asia. In the later * * * combination, brachiopods became more important, while bivalves, especially gastropods, became less important, just like the diversity of all species. A noteworthy phenomenon is that corals show great importance (about 7%) in the * * * combination {19}. Brachiopods in biota {20} and {2 1} are obviously dominant (na=76.7%, 84.3%), showing a low degree of diversity, while Koken is the dominant fossil species. In sample {2 1}, its relative abundance even reaches about 70% of the total * * * combination.
Towards the top of the slope section of the club head, the composition trend is opposite. Bivalves and gastropods become important again, with a high degree of species differentiation and no dominant species. Generally speaking, bivalves, gastropods and brachiopods are equally important. However, the number of species in these * * * combinations did not fully reach the degree of differentiation in samples {9} to {12}. In addition, compared with these samples, brachiopods are more important and bivalves are less important.
Although superficial observation shows some similarities, each different combination of * * * is dominated by different species, or at least determined by different species. This can be applied to different combinations of brachiopods, and also to bivalve-gastropod combinations with high heterogeneity in different parts of the profile.
5 Comprehensive paleoecological interpretation
The benthic assemblage of Bangtoupo is mainly composed of superbenthos. Shallow burrowing benthos and semi-benthos account for a small part. Few cephalopods swim. Most creatures are fixed. Most moving surface animals consist of gastropods. Considering the nutritional environment, suspended substance eaters are obviously the dominant category. In addition, there are herbivores (gastropods), scavengers, scavengers and omnivores, a few sediment eaters, and a few micro carnivores and dominant carnivores. This * * * biological combination is composed of organisms that need soft substrates and other organisms that need hard and stable substrates. Attached organisms can only use secondary hard matrix to attach to the shells or skeletal remains of other organisms.
The composition and burial state of the fossil assemblage with uneven height in Bangtoupo indicate a shallow sea environment with normal salinity and generally low energy, only occasionally affected by short-term high-energy events in the form of storm surges. All these conditions can be realized in the shallow sea habitat, which is protected at the upper part of the slope and separated from the carbonate platform, and is called a basin (it may be a terrace, but it is more likely to be a small basin in this position) (Figure 4). Creatures live in the transparent area above the wave base of storm surge and below the wave base of optimal climate. In the Middle Triassic, the climate here changed from subtropical arid climate to tropical arid climate [15]. The sea is warm. * * * The percentage of organisms that eat suspended solids in the biota is very high, indicating that the water is crystal clear, shallow, slightly turbulent and rich in oxygen and nutrients. The seabed is stable with fine-grained sediments. The deposition rate is generally low, assuming that the steady and slow deposition particle flow is the main one.
Fig. 4 Paleogeographic position of protected shallow sea habitat of Bangtoupo community on the upper slope (not to scale).
The high diversity of small invertebrates, especially bivalves and gastropods, represents the remains of communities living in large algae shoals (Figure 5). Algal beaches change the environment in different ways, providing highly differentiated and nutritious protective habitats for highly heterogeneous small invertebrates, which is equivalent to modern algae beaches [1]. In particular, the high diversity of small gastropods indicates the algae beach habitat [23], because a large number of gastropods need a large number of niches, and it is difficult for the exposed seabed to provide so many niches. Even if there are no preserved fresh algae, it can be concluded from the composition of fauna that the habitat of soft sediment community is characterized by macroalgae. This can be used for most small gastropods, and also for many different small animals that feed on suspended substances. These animals that feed on suspended matter use algae as a fixed substrate, such as bivalves with feet. In addition, the algae beach also provides excellent living conditions for many different organisms living directly on the seabed or in an endogenous benthic way. Organisms that need a stable or hard substrate use the stems of large algae, which are attached to shells and other bones on the seabed and shells of living invertebrates. The seabed may be covered by mosaic blocks with different properties. This mosaic has dense large plant vegetation in some areas and thin vegetation in some areas. Among the separated blocks of macroalgae and various small invertebrates, brachiopods, larger suspended bivalves, sea lilies and other animals prefer to live in areas with poor vegetation. On the algae beach, the species diversity is obviously higher than that in the area with poor vegetation.
Summary of comprehensive palaeoecological restoration of high heterogeneity algae beach community.
The relatively low degree of differentiation is dominated by brachiopods. On the one hand, it is dominated by animals that eat suspended substances in nutritional structure, on the other hand, it is likely to represent the remains of communities living on the seabed with poor vegetation (Figure 6). For those organisms that need a solid and stable substrate for fixation, suitable substrates are provided by bones on the seabed and shells of other invertebrates.
compare
J.Wendt and F. T. F ü rich [16] put forward the phase analysis of southern alps cassina Formation (from upper Latin stage to lower Carney stage). Considering their phase states, the high diversity of small invertebrates and the well-preserved fauna, Qingyan quiet period has some characteristics of cassina Formation. In the similarity described by cassina Formation, the closest three points exist in shallow sea basin facies with marl and clay deposits, and occasionally there is marl interlayer, in which the in-situ growth of soft basement-algae beach combination occurs. However, the composition of this * * * combination of Bangtoupo is not like the * * * combination of algae beach rich in gastropods in cassina Group [5]. More precisely, there are some similarities between Qingyan and cassina's reef assemblage, which is also rich in gastropods. However, the * * * biota in Bangtoupo can rule out the possibility of reef fauna remains, because there are no reefs and reefs in the late period of the whole Qingyan fauna, and some potential reef-building organisms such as corals, sponges or calcareous algae only constitute a small part of this * * biota. Although it is similar to the cassina Formation, the conditions in the late animal kingdom are obviously different from those in southern alps.
Fig. 6 Schematic diagram of comprehensive restoration of paleoecology of poorly differentiated benthic vegetation communities.
Summary of comprehensive restoration of palaeoecology
Fig. 7 is a general picture of the comprehensive palaeoecology of Qingyan at good night. In the early period of good night, the study area developed from deep-water facies containing a small amount of plankton to shallow water conditions of protected habitats on the uphill slope (Bangtoupo profile).
At first, several different brachiopod dominant communities with low differentiation were established on the seabed with poor vegetation. It is estimated that the shallow water body and the protection of shallow sea habitats have increased, and large algae have come here to breed. Gradually, the algae beach with increasing density may develop until the seabed becomes an uneven and patchy mosaic structure. Some areas have dense macroalgae vegetation, while others have sparse vegetation. Parallel to this development, small invertebrates with great differentiation, especially bivalves and gastropods, also began to flourish. For them, seaweed beaches provide ideal living conditions. In these biomes (in the middle of Bangtoupo section), the importance of brachiopods is greatly reduced. In a later period, another gradual transition led to the retreat to the absolutely dominant brachiopods and significantly reduced differentiated communities. This community once again lives on the seabed with poor vegetation, which may be due to the deepening of seawater (the upper part of the Bangtoupo section). In the future, another development of high diversity community may be partly related to the development of macroalgae vegetation.
Finally, the gradual increase of seawater depth ended the prosperity of shallow benthos. There was a transition from shallow water to deeper water, and there were few plankton in deeper water, which lasted until the middle and late period of the whole good night period. It was only at the beginning of the Latin period that the depth of the sea became smaller and the shallow sea fauna appeared again.
Fig. 7 General situation of comprehensive palaeoecology of Qingyan Goodnight Ninian period.
Thank you very much. Professor Chen Jinhua (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences) worked with me in the field and gave me all kinds of help during my stay in China. I am also very grateful to Dr. F.Strauch (Institute of Geology and Paleontology, University of Mü nster) for his support in Germany. I also thank Dr. M.Bertling for reviewing the manuscript for me. I would also like to thank the German People's Research Association for its financial support for this research project. This paper will be part of the doctoral thesis of Mü nster University.
(translated by Pan Yuntang, translated by Li Wenxiao)
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