Qi-jian (启剑) Li (李)

This study provides an overview and discussion of controls on the distribution of organic reefs during the Early Ordovican Period, in the Yangtze Platform, a region of epicontinental sedimentary rocks in South China. The Yangtze Platform was located in low latitudes during the Early Ordovician and recorded rich and diverse reefs through that time. During the late Tremadocian Epoch, dolomitic and stratiform stromatolites were common in supratidal to intertidal zones of the western Yangtze Platform, while columnar stromatolites formed in deeper waters of the eastern Yangtze Platform. Skeletal-dominated reefs occurred in upper subtidal settings of the central Yangtze Platform. A transition from microbial-dominated to metazoan-dominated reefs with shallowing-upward cycles was evident, indicating that the composition of the main reef-builders was driven mainly by water depth. Increasing metazoan competition during the Great Ordovician Biodiversification Event reduced the abundance of mic...

As an obconical macrofossil with porous double-wall, Calathium was commonly present in reefs of Early to early Middle Ordovician age. The Calathium-bearing reefs thrived globally during the Early Ordovician, but this ecosystem collapsed in Middle Ordovician. A rare case of Calathium-microbial reefs was found from the middle part of the Yijianfang Formation (Darriwilian, late Middle Ordovician) of the Bachu area, located in the northwestern margin of the Tarim Basin, northwestern China. Surrounded by bioclastic grainstones, patch reefs here are well-developed and vary in size, with 1–4 m in thickness and 2–18 m in diameter. Three facies types are distinguished within the reef limestones: (1) Calathium-microbial framestone, (2) echinoderm-Calathium bafflestone, and (3) bryozoan-microbial bindstone. As a dominant type, the Calathium-microbial framestone shows a three-dimensional skeletal framework that is mainly constructed by Calathium and stabilized by microbialites. Although most specimens are toppled, Calathium displays well-developed lateral outgrowths, which connected individuals of the same species. Morphological characters of Calathium in thin sections show that calathids are hypercalcified sponges rather than receptaculitid algae. Unlike the Early Ordovician lithistid sponge-Calathium reefs, the Tarim reefs studied herein contain very few lithistid sponges. Instead, bryozoans are fairly common and act as the most important non-microbial encrusters, attaching to the walls of Calathium. Noticeably, the co-occurrence of pelmatozoans and Calathium is similar to those reported from Late Ordovician calathids-echinoderm reef communities from Tennessee. The presence of Calathium-microbial reefs in Tarim indicates that locally calathids were major reef builders, at least before the latest Middle Ordovician.

Although sphinctozoans, multi-chambered
hypercalcified sponges, can be traced from the Cambrian,
their reef-building capacity in the Early Paleozoic
appears limited. The oldest sphinctozoan-coral-microbial
reef is documented here, from the Upper Sanqushan Formation
(Late Katian) of southeast China. This is also the
first report of Ordovician sphinctozoans from South China.
The sponges occur in a >120-m-thick reef that is mainly
constructed by calcimicrobes (Kordephyton, Renalcis, and
Epiphyton) with a low abundance of in situ metazoans, predominantly
sphinctozoan sponges (Corymbospongia) and
rugose corals (mostly Palaeophyllum and Streptelasma).
Tetradium is the only tabulate coral preserved in growth
position. Crinoids and brachiopods are common reef dwellers.
Dasycladacean algae are rare and probably transported
and stromatactis is abundant. Bio- and litho-facies in this
area as well as the characteristics of the microbialite suggest
that the reef developed in a deeper subtidal setting that
was unfavorable for most metazoan reef builders. In contrast
to the high-energy stromatolite-sphinctozoan reefs
from the Late Silurian, this case represents a low-energy community, indicating that the first reef-building sphinctozoans
might have originated in a relatively deep-water
environment on seamounts of a back-arc basin during the
Late Ordovician.

Lithistid sponge-Calathium-microbial reefs were widespread on the Yangtze Platform during the Early Ordovician and are well studied. However, the biological affinity and reef-building role of Calathium in these communities has remained unclear up to now. We document lithistid sponge-Calathium reefs from the Upper Hunghuayuan Formation (early Floian) at Huanghuachang in Hubei, South China. These reefs have a three-dimensional skeletal framework that is mostly produced by Calathium and lithistid sponges. Calathium had a critical role in reef construction, as demonstrated by well-developed lateral outgrowths, which connected individuals of the same species and with lithistid sponges. Bryozoans, stromatoporoids and microbial components are secondary reef builders. Morphological, constructional and functional analyses provide evidence that Calathium was a sponge-grade metazoan rather than a receptaculitid alga as previously thought. At the dawn of the Ordovician Radiation, these small-scaled patch reefs thus represent the initial rebound of metazoan-dominated reefs after the late-Early Cambrian archaeocyath reef crisis. Gradual global cooling through the Early Ordovician may have been a key driver for the recovery metazoan reefs.

Several Late Ordovician (late Katian) reef complexes are known from the border area of Jiangxi and Zhejiang provinces in southeast China. We studied two coral–stromatoporoid reefs exposed in the Xiazhen Formation at Zhuzhai (Yushan, Jiangxi). The reefs have a combined thickness of 7.4 m and are metazoan-dominated with most reef-builders in growth position. Stromatoporoids and tabulate corals constitute the framework of the reefs. Stromatoporoids (mostly Clathrodictyon) dominate the first unit and show a vertical increase in proportion and dominance from the middle part to the top of the unit, whereas tabulate corals (dominated by Catenipora and Agetolites) are the main reef-builders in the second unit where stromatoporoids are rare. We attribute this change to a greater tolerance of tabulate corals to turbidity, allowing them to thrive in the muddy facies of the upper unit. This facies change is probably related to the increasing terrestrial input from the northwestward expansion of the Cathaysian Land during the late Katian. The Cathaysian orogeny also led to a short-term exposure of the sea floor in the study area, which terminated the reef growth.