Calcimicrobe-archaeocyath reefs from the Tianheban Formation (Stage 4, Cambrian) at the Xiachazhuang section of Yichang, Hubei and their palaeoecological significance
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Abstract:
Archaeocyaths are the first metazoans to play a reef-building role on Earth. They first appeared in the early Fortunian on the Siberian Platform and survived until their extinction during the Cambrian Age 4. The calcimicrobe-archaeocyath reefs from the Tianheban Formation (Stage 4) in Yichang, Hubei Province, are the youngest known calcimicrobe-archaeocyath reefs in South China. However, there are rare reports of Age 4 calcimicrobe-archaeocyath reefs worldwide. In this paper, we report calcimicrobe-archaeocyath reefs from the Tianheban Formation at the Xiachazhuang section, Yichang. This discovery provides new materials for the study of palaeoecological characteristics of calcimicrobe-archaeocyath communities of the Cambrian Stage 4. The reefs include Archaeocyathusyichangensis and various calcimicrobes, including Girvanella, Kordephyton, Botomaella, thick tubular cyanobacteria, Epiphyton, and Renalcis. Other associated organisms include trilobites, brachiopods, chancelloriids, and echinoderms. The reef-building organisms show the characteristics of a single archaeocyath species and high diversity of calcimicrobes. The relationship between calcified microproblematica Epiphyton and Renalcis, and Archaeocyathus may be similar to that between modern marine sponges and their associated microorganisms. Archaeocyathus and calcimicrobes are the main components of the reefs from the Tianheban Formation. Analysis of the reef-building modes shows that Archaeocyathus grow in situ and form the first-order framework, acting as bafflers. Then Girvanella cover the surfaces of Archaeocyathus to stabilize the underlying sediments. The smaller Epiphyton and Renalcis form small rigid frameworks. They can also co-grow with sheet or cluster Girvanella, and in the space among Archaeocyathus to form a secondary framework to strengthen the reefs. After forming a staggered reticular framework, the interstitial space is filled by smaller Renalcis and Epiphyton. Lastly, the cavities re filled with fine cements. The correlation of calcimicrobe-archaeocyath reefs of the Tianheban Formation from South China suggests that the community appearances of calcimicrobe-archaeocyath are slightly different, which may be related to the differences in hydrodynamic background. In addition, comparison of calcimicrobe-archaeocyath evolution from the early Cambrian shows that the diversity of archaeocyaths decreased significantly in the calcimicrobe-archaeocyath reef communities from Age 4, and the diversity of calcareous organisms increased compared with that from Age 3. These evolutionary trends are significant in revealing the decline of calcimicrobe-archaeocyath reefs.
