The Post-Extinction Anji Biota (Zhejiang, China) and a Wider Hirnantian Sponge Mega-Community
1 Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; acutipuerilis@yahoo.co.uk;
2 Department of Natural Sciences, Amgueddfa Cymru-National Museum Wales, Cathays Park, Cardiff CF10 3LP, UK;lucy@asoldasthehills.org;
3 CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Nanjing 210008, China; ydzhang@nigpas.ac.cn;
4 School of Geosciences and Info-Physics, Central-South University, Changsha 410083, China; wwhever@126.com.
The recently-discovered Anji Biota of Zhejiang (Botting et al., 2017a, b) preserves an unprecedented deep-water Hirnantian fossil assemblage, preserved mainly through pyritisation. The black mudstone interval hosting the fossils is around 10 m thick in the Anji area, and represents deepening of the basin during post-glacial sea-level rise. Graptolites constrain the age to the middle or later part of the Metabolograptus persculptus Biozone, the last graptolite biozone of the Ordovician. The community was entirely dominated by a hyper-diverse assemblage of sponges, representing both early-branching and modern groups, and preserved with detailed skeletal remains and soft tissues (Fig. 1A). Aside from abundant graptolites, associated fossils are limited to occasional orthocone nautiloids, and rare examples of other exceptionally preserved taxa such as eurypterids and other arthropods.
The initial description of the fauna was limited to a few sites in one region, although a similar but less diverse assemblage has been described from 100 km away in Anhui Province (Li et al., 2015). Recent work has increased both the local diversity of sites in the Anji area and the lateral extent of the sponge community across South China. In the Anji Biota itself, additional collections have increased the total diversity of sponges to an estimate of significantly over 100 species, with many new taxa expected from further collecting. One new site shows a depth sequence from the shallow-water Aegiromenella Fauna into the sponge community, with an intermediate assemblage including problematic echinoderms. The absence of brachiopods, trilobites and other shelly taxa from the offshore community appears to be a real pattern rather than a taphonomic artefact.
Figure 1 Hirnantian(M.persculptus Biozone)sponges from South China.A.Mass assemblage of an undescribed reticulosan from the Anji Biota, Zhejiang Province (see Botting et al., 2017a); B, C. Undescribed sponges from a new locality in Jiangsu Province. Scale bars=5 mm.
Additional work elsewhere in South China has revealed similar diverse, abundant sponge faunas from the M.persculptus Biozone in several provinces and a municipality:Jiangsu (Fig. 1B, C), Hunan, Hubei and Chongqing. A site with very early Silurian sponges is also known from Sichuan Province, but has not yet been investigated. The preservation of these faunas differs considerably between sites, although pyritisation is a consistent component.All the faunas are preserved in M. persculptus Biozone mudstones, some weathered (as in Anhui; Li et al., 2015) and some not. In all new sites, the fossiliferous interval is shorter than that in Anji, where the entire succession is greatly expanded, and thick sandstone deposits represent the Hirnantian lowstand. In the thinner sequences, the lowstand is represented mainly by condensed cherty mudstone deposits. This difference relates to the presence of an eroding landmass to the south, and a major open shelf area to the north, with preservation of the sponges related to an interval of rapid sediment input during transgression.
The implications of the Anji Biota and its equivalents remain unclear in global terms, but the extent of the fauna across 2000 km of South China suggests that this sponge mega-community was a widespread and perhaps global (at least at low latitudes) phenomenon. There are a few reports of M. persculptus Biozone sponge spicules from other areas, including Thailand (Agematsu et al., 2006), Austria (Štorch & Schönlaub, 2012), and Nevada, USA (Melchin et al., 2013). As sponge spicules are stated to be common at some of these sites, it is possible that further collecting would yield articulated material. We encourage investigations of Ordovician-Silurian boundary sections globally in order to assess whether the offshore sponge proliferation seen in South China is a global pattern, and if so, what its implications might be for post-extinction recovery.
References
Agematsu, S., Sashida, K., Salyapongse, S., Sardsud, A. 2006. Ordovician—Silurian boundary graptolites of the Satun area, southern peninsular Thailand. Paleontological Research, 10:207—214.
Botting, J. P., Muir, L. A., Zhang, Y., Ma, X., Ma, J., Wang, L., Zhang, J., Song, Y., Fang, X. 2017a. Flourishing sponge-based ecosystems after the end-Ordovician mass extinction. Current Biology, 27: 556—562.
Botting, J. P., Zhang, Y., Muir, L. A. 2017b. Discovery of missing link between demosponges and hexactinellids confirms palaeontological model of sponge evolution. Scientific Reports, 7(1): 5286.
Li, L. X., Feng, H. Z., Janussen, D., Reitner, J. 2015. Unusual Deep Water sponge assemblage in South China — witness of the end-Ordovician mass extinction. Scientific Reports, 5:16060.doi:10.1038/srep16060.
Melchin, M. J., Mitchell, C. E., Holmden, C., Štorch, P. 2013. Environmental changes in the Late Ordovician-Early Silurian: review and new insights from black shales and nitrogen isotopes. GSA Bulletin, 125: 1635-1670.
Storch, P., Schönlaub, H. P. 2012. Ordovician-Silurian boundary graptolites of the southern Alps, Austria. Bulletin of Geosciences, 87(4): 755-766.