GSM Technical talk (in collaboration with the Geology Department, UM and Institute Geology Malaysia): “The Ordovician Radiation in Reef Ecosystems” - Qi-jian LI (Nanjing Institute of Geology and Palaeontology)

Event Code : GSMTT 07-18-2017 Venue Name : Geology Department, University of Malaya Host Name : Coordinator Name : Date From : 2017-07-18

Dr. Qi-Jian Li is a geologist and biologist. His main interest focuses on the origin of metazoan 3D reefs in the Early Ordovician. He has just started his new project with Dr. Masatoshi Sone (Geology, UM) on Malaysian Ordovician sedimentology. This project is funded by the Chinese Academy.

The Ordovician Period (438–443 million years ago) witnessed the most important diversification event of marine life in Earth history — the so-called Great Ordovician Biodiversification Event (GOBE). The event had a significant influence in level-bottom communities as well as reefs. It was thought that a unique transition from microbial to metazoan-dominated reefs took place in the Middle Ordovician. However, although Early Ordovician reefs were still mainly microbial-dominated, sessile metazoans already began to occupy shallow reef communities. This is documented by lithistid sponge-Calathium reefs of the Upper Hunghuayuan Formation (early Floian) from 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 were secondary reef builders. Morphological, constructional and functional analyses provide evidence that Calathium was a hypercalcified sponge 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.

We also discovered the oldest labechiid stromatoporoid (Cystostroma) in one of the lithistid sponge-Calathium reefs at Zhangzhai, South China. Cystostroma occurs as dome-shaped coenosteum with undulating thin laminae. The stromatoporoids and cryptic sponges attached on the walls of Calathium as encrusters and cryptobionts. Cystostroma might have originated in intraskeletal crypts and later expanded their habitats to open surfaces. Beyond the oldest Cystostroma, keratose sponges, Pulchrilamina and bryozoans also benefited from the microhabitats provided by Calathium. This finding supports that increasing topographic complexity contributed by novel metazoan builders promoted the diversification of reefs and ultimately of the whole benthic fauna. It also implies that hypercalcified sponges may have played a more important role in Early Ordovician reefs than usually assumed.

Late Ordovician reefs from southeast China offer another piece of evidence for tracing the trajectory of reef development after the main pulse of the radiation. In the nearshore areas, coral-stromatoporoid reefs are exposed in the Xiazhen Formation (late Katian) 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 dominate the first unit, whereas tabulate corals 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 terrigenous 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. We conclude that the waxing and waning of the reefs in South China was strongly controlled by this regional tectonic event. In the offshore areas, we found the oldest sphinctozoan-coral-microbial reef from the Upper Sanqushan Formation (late Katian) of southeast China, which is also the first report of Ordovician sphinctozoans from South China. Paleogeographically, the studied site belongs to the north margin of Zhe-Gan platform, which developed in a deep-subtidal setting. This is not the preferred habitat of most metazoan reef-builders. The sponges occur in a >120m thick reef that is mainly constructed by calcimicrobes with a low abundance of in situ metazoans, predominantly sphinctozoan sponges and rugose corals. In contrast to the shallower coral-stromatoporoid reefs from the nearshore areas, this sphinctozoan-bearing reef possessed a low diversity of metazoans, indicating that the first reef-building sphinctozoans might have had a larger fundamental niche but were competitively inferior to many other colonial organisms during the Late Ordovician. The detrital input derived from the weathering of volcanic rocks might have been a crucial factor to boost the formation of thick and abundant microbialites on the Zhe-Gan platform during late Katian. By using sampling-standardized occurrence data from the Paleobiology Database, the results support that the Ordovician Radiation might have been an extension of Cambrian diversification dynamics. The Cambrian diversification was halted by a prolonged greenhouse episode. And high temperatures might have played a critical role in inhibiting metazoan reefs, creating an ecological bottleneck during the middle and late Cambrian.