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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
www.nature.com/scientificreports
Unusual Deep Water sponge
assemblage in South China—
Witness of the end-Ordovician
mass extinction
Lixia Li1, Hongzhen Feng2, Dorte Janussen3 & Joachim Reitner4
There are few sponges known from the end-Ordovician to early-Silurian strata all over the world,
and no records of sponge fossils have been found yet in China during this interval. Here we report
a unique sponge assemblage spanning the interval of the end-Ordovician mass extinction from the
Kaochiapien Formation (Upper Ordovician-Lower Silurian) in South China. This assemblage contains
a variety of well-preserved siliceous sponges, including both Burgess Shale-type and modern type
taxa. It is clear that this assemblage developed in deep water, low energy ecosystem with less
competitors and more vacant niches. Its explosion may be related to the euxinic and anoxic condition
as well as the noticeable transgression during the end-Ordovician mass extinction. The excellent
preservation of this assemblage is probably due to the rapid burial by mud turbidites. This unusual
sponge assemblage provides a link between the Burgess Shale-type deep water sponges and the
modern forms. It gives an excellent insight into the deep sea palaeoecology and the macroevolution
of Phanerozoic sponges, and opens a new window to investigate the marine ecosystem before and
after the end-Ordovician mass extinction. It also oers potential to search for exceptional fossil biota
across the Ordovician-Silurian boundary interval in China.
e end-Ordovician mass extinction is the earliest and the second largest among the Big-ve in
Phanerozoic1. It occurred near the end of the Ordovician period (between 445.6 Ma and 443.7 Ma), and
had a devastating inuence on the marine ecosystem. It is estimated that about 85% of marine species
became extinct during this crisis2. Aer nearly 40 years investigation in a global scale, there is a con-
sensus that the end-Ordovician mass extinction was stepwise and episodic, and two phases have been
recognized, with about one million years in between. Such biotic event was thought to be coincident
with the Late Ordovician glaciation that took place mainly in Gondwana causing dramatic changes of
global sea level and climate together with an oceanic turnover2–5. Many fossils have been found from the
rocks chronostratigraphically corresponding to the interval from the rst episode of the end-Ordovician
mass extinction to the latest Ordovician, such as brachiopods, trilobites, corals, graptolites and so
on6–8. Sponges are rare in this interval, and only lithistids and stromatoporoids have been sporadically
documented9–11. Although many new discoveries show that the non-lithistid spicular sponges (espe-
cially the Burgess Shale-type faunas) are known from a broad range of times, and not restricted to
the Cambrian, there are still large gaps in the fossil records of non-lithistid spicular sponges during
the Late Ordovician12–14. So far, no sponges have been reported from the Late Ordovician in China.
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology,
School of Earth Sciences and Engineering, Nanjing
Department of Geobiology, Centre for Geosciences, Faculty of Geosciences and Geography, Georg-
R
A
P
OPEN
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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
Fortunately, a new sponge assemblage bearing diverse and abundant well-preserved siliceous sponges
was discovered from the Kaochiapien Formation (Upper Ordovician-Lower Silurian) of South China. It
evolved slightly earlier than the second episode of the end-Ordovician mass extinction, ourished rap-
idly during the latest Hirnantian, and disappeared in South China at the beginning of the Silurian. is
assemblage remarkably enhances our understanding of the marine ecosystem shortly before and aer
the end-Ordovician mass extinction event. With a brief discussion of the key taxa we also evaluated the
relationship between our sponge assemblage and their modern counterpart in light of the morphological
analysis, and explored the environmental background and taphonomic model of the sponge assemblage.
Results
e sponge fossils have been collected in Kaochiapien Formation at Beigong of Jingxian County, south-
ern Anhui Province, east China (Fig.1). It is a continuous and fossiliferous Ordovician-Silurian bound-
ary section with well-exposed outcrop. is locality is interpreted palaeogeographically to be part of the
Lower Yangtze Platform, adjacent to the Xiang-Wan Basin5, characterized by graptolitic shale facies both
near the top of Ordovician and the base of the Silurian. Abundant and diversied fossils were found at
this section, including the well-preserved sponges and other groups such as graptolites, brachiopods,
trilobites, cephalopods, echinoderms, ostracods, bivalves, and some indeterminate fossils. e sponge
assemblage occurs mainly in the basal Kaochiapien Formation, which is characterized by black sili-
ceous mudstone. e rocks show bluish-grey or greyish-white in color aer weathering. e Kaochiapien
Formation conformably overlies the Wufeng Formation that contains black siliceous and calcareous
mudstone, yielding abundant shelly fossils such as brachiopods and trilobites.
According to the graptolites from the Wufeng and Kaochiapien formations, three graptolite biozones
could be recognized in ascending order: Metabolograptus extraordinarius Biozone, Metabolograptus per-
sculptus Biozone, and Akidograptus ascensus Biozone. e index fossils M. persculptus and A. ascensus are
not found here, but other important species such as Avitograptus avtitus, Normalograptus laciniosus, N.
normalis, Neodiplograptus shanchongensis, and Ne. modestus conrm the presence of the M. persculptus
Biozone and A. ascensus Biozone at Beigong section.
e sponges reported here belong to the M. persculptus Biozone and the lower part of the A. ascen-
sus Biozone, indicating that they are of latest Ordovician (late Hirnantian) to earliest Silurian (early
Rhuddanian) age, just extending across the Ordovician-Silurian boundary. But those shelly fossils
(brachiopods and trilobites) are conned to the upper M. extraordinarius Biozone, suggesting a latest
Ordovician (late Hirnantian) age (Fig.2).
Environmental interpretation. e presence of graptolites, brachiopods, trilobites, bivalves and echi-
noderms from the O-S transitional rocks suggests that the Beigong fauna is a typical Ordovician marine
fauna. e core taxa of the Hirnantia fauna, Aegiromena ultima, Paromalomena polonica, and Fardenia
sp., occur in our collection with low diversity and small sizes (between 5–10 mm), indicating an oshore,
deep-water living environment with very poor nutrition supply6,15,16. e Hirnantia-Dalmanitina fauna
at Beigong could be dierentiated into three parts: the lower part is from the black calcareous mudstone
(SF8-SF9), and characterized by rich Dalmanitina as well as one brachiopod (Oxoplecia? incasfauta Rong),
representing a shallow water environment (about 20–30 m). ere are no complete sponges found in this
part, and only a few spicules are scattered in the rocks. e middle part is also from the black calcareous
Figure 1. Locality map of Ordovician-Silurian boundary succession at Beigong, Anhui Province, South
China. (a) Location of Anhui within China. (b) Location of studied section within Anhui indicated by a red
star. Image created using CorelDRAW X5.
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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
mudstone (SF10), but abundant in brachiopods together with a few trilobites. Brachiopods are dominated
by Aegiromena ultima, typical of the Paromalomena-Aegiromena community with lower diversity and
small shell sizes indicating a relatively deeper environment (about 60–150 m)17. No sponges were found
in this part. e upper part is from the calcareous-siliceous mudstone (SF11-SF12), and is characterized
by large trilobites (mainly Platycoryphe). Dalmanitina is also found in this part, but much less than in the
lower part. e appearance of large trilobites shows that the oxygen level on the sea oor becomes higher
during this interval18. Sponges are very rare in this part, with only a few scattered spicules in the rocks.
Above these shelly beds, only sponges and graptolites were found, indicating a large scale transgression
occurred during the interval when sponges ourished.
Overall, the Ordovician-Silurian transitional sequence at Beigong is not only characterized by
shallow-water platform deposits (before the second pulse of the mass extinction) but also showing a
hint of deep-water facies (aer the second pulse of the mass extinction), which might be consistent
with the unique palaeogeographic position of the section, stretching across the marginal Lower Yangtze
Platform to the Xiang-Wan Basin. It is why the fossil assemblage at Beigong shows transitional anity
between platform and basin. It might be reasonable to infer that, during the latest Ordovician and the
earliest Silurian, Beigong developed an open marine ecosystem on the platform margin, with relatively
deep-water, low-energy, and oxygen-decient conditions.
Sponges. Diversied and well-preserved sponges occur in the black siliceous mudstone of the basal
Kaochiapien Formation at Beigong. However, the spicules are preserved in late diagenetic iron hydrox-
ides and end-product of pyrite oxidation. erefore the spicules have lost some of their original form
and structure. Pyrite precipitation as a product of microbial sulfate reduction in former spicule-moulds
is characteristic of anaerobic sedimentary conditions. e sponges at Beigong systematically include two
classes: Demospongiae (about 43% of the total) and Hexactinellida (about 57%). At least 15 types of
articulated hexactinellids have been found, dominated by a few species in one family, with a more diverse
assemblage of rare elements. e most abundant group is the vasiform or spheroidal hexactinellids, with
thin body wall composed of regularly arranged stauractines (rare pentactines) parallel to the principal
axis of the sponge body, usually forming quadrules in six orders, which is identied here as a new taxon
of protospongids (Fig.3a). e second group of hexactinellids is related to rossellimorph hexactinellids
and is classied also as a new taxon, showing spherical growth form. Skeletal net consists of dierent
Figure 2. Outline of biostratigraphy (graptolite biozones) and lithostratigraphy of the Beigong section;
the black arrows indicate horizons yielding sponge fossils. Image drawn by Lixia Li.
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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
Figure 3. Representative sponges from the Beigong section. (a) SF14-26, Protospongiid showing typical
budding behavior known from lyssakine Hexactinellida; (b–d) SF14-1, b, Hexasterophorid rossellimorph
Hexactinellida with articulated stauracts and hexacts as well as characteristic pentactine dermal layer; (c)
magnied view of (b) (black arrow), showing the characteristic pentactine dermal layer; (d) an isolate
pentact spicule; (e,f), SF13-131, (e), oracospongia sp.; f is close up of e, showing inated stauracts and
swollen hexacts in varying sizes; (g,i), SF13-124, (g), an isolate tetractinal spicule (triaene); (i), Cylindrical
Tetractinellid demosponge preserved as oval compression with two layers of the body; (h,j), SF14-35, (h)
an isolate monaxonal spicule (style); (j) Axinellid-type demosponge with plumose arrangement consisted of
short styles; (k) SF14-8, Halichondrid-type demosponge with extremely long styles as Dragmaxia-type; (l)
Autochthonous spiculite in the black shale (sp-spicules; oh-oxidation halo). (m) Autochthonous spiculite
preserved in goethite aer pyrite, the black arrow indicates spicule of the complete sponge. Scale bars:
(a,b,e,i–k) 5 mm; (c), (g,h) 1 mm; d, f: 500 μ m; (l,m) 200 μ m. Photographs: Lixia Li (a–k), Joachim Reitner
(l,m).
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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
types of hexactins, mainly stauracts paratangentalia in quadruled arrangement. Classical hexactins are
also present. e outer margin is reinforced by pentactines with their four longer paratangential rays
along the periphery, and the shorter one pointing toward to the center of the sponge body (Fig.3b–d).
Another group of hexactinellids with globular form is assigned to oracospongia Mehl, 199619. Its spic-
ulation consists mainly of irregularly oriented inated stauractines and hexactines. e upper part of
sponge body is composed of delicate spicules, while the basal part is reinforced by stout spicules, with a
few delicate ones scattered sporadically (Fig.3e,f).
Demosponges are also crucial component of this assemblage, with about 19 morphotypes having been
found. According to spicule geometries and skeletal structures, two subclasses, Tetractinomorpha and
Ceractinomorpha, have been recognized. e most distinctive group of Tetractinomorpha (Spirophorida)
is identied as a new taxon closely related with modern tetillids, showing cylindrical form with walls of
anatriaenes, triaenes, and long styles arranged in plumose pattern. Choanosomal skeleton is composed of
randomly arranged anatriaenes and triaenes. Most of the individuals are preserved as oval compressions
(Fig. 3g,i). A group of axinellid-type demosponges with hemispherical shape is composed of plumose
arranged short styles, which radiate from the center of the sponge body, with all the pointed end of
spicules pointing outward (Fig.3h,j). Bundles of extremely long styles occur too, which are 80–120 μ m in
diameter, more or less grouped in fascicles. It was identied here as halichondrid-type Ceractinomorpha
(Fig.3k).
Although most of the specimens are articulated, a few dispersed siliceous spicules are also present,
including monacts, diacts, stauracts, pentacts and hexacts (Fig.3l,m). Only a small proportion of spec-
imens are shown here. Detailed taxonomic description and phylogenetic interpretation are in progress
and not the topic of the presented paper.
Palaeoecological implication. e observed sponge assemblage is dominated by lyssakine “so”
hexactinellids (ca. 60%). Four major key taxonomic groups and related species were observed. Most of
the specimens are articulated and show their pristine spicular inventory and anatomy. erefore it was
possible to determine the taxa in detail. e observed assemblage is a mixture of “old” Cambrian taxa
(protospongids, thoracospongids) and modern-type rossellimorph hexactinellids. All of these hexactinel-
lids are adapted to so bottoms and known normally from low-energy deeper water settings. Rossellid
hexactinellids are today common in deep water conditions oen in cold polar environments20,21.
Also the spiculated demosponge assemblage exhibits a very modern character22. Surprisingly spi-
rophorid demosponge taxa closely related to tetillids dominate the demosponge assemblage. It is the
rst occurrence of this demosponge taxon in the fossil record. Sponges with axinellid and halichondrid
spicule arrangements are already known from the lower Cambrian (e.g. Chengjiang Biota)23. However,
the forms discussed here are dierent and show morphological coincidences with modern type hali-
chondrid/axinellid demosponges. Also these sponges are adapted to so bottoms and low-energy envi-
ronments. e entire sponge assemblage exhibits many coincidences with modern deep sea sponge
communities and gives an excellent insight into the Late Ordovician deep sea palaeoecology.
Discussion
As a group of sessile lter-feeding metazoans, sponges require relatively strict physical and chemi-
cal environment for their habitats. e mass disappearance of sponges during the rst episode of the
end-Ordovician mass extinction was thought to be related to the harsh climate and active upwelling with
toxic components like enrichment of H2S2,9. Till early Silurian, sponges gradually recovered from the cri-
sis in both richness and diversity9. Due to the sparse fossil record, it seems that there is an “evolutionary
gap” of sponges from the rst episode of the mass extinction to the beginning of Silurian. Fortunately,
the sponges at Beigong are chronostratigraphically in this “gap”. ey rst appeared at the beginning of
the M. persculptus Biozone, subsequent to the rst extinction episode, then evolved quickly and our-
ished during and aer the second pulse of the mass extinction. At the beginning of Silurian, the sponges
disappeared rapidly. It should be noted that the sponges studied in this paper are only found at Beigong,
southern Anhui, and no similar materials have been reported in any other places in South China and
elsewhere in the world.
Why did sponges settle and ourish at Beigong during the end-Ordovician mass extinction? As men-
tioned above, Beigong had an environmental background of deeper water and low energy aer the sec-
ond pulse of the mass extinction, which was favorable to the siliceous sponges24. Besides, the diversity
of marine ecosystem and the community types during the “survival-recovery interval” (i.e. the time
between two pulses of the mass extinction) decreased sharply with much fewer predators and large
numbers of nearly vacant niches25. In the Beigong area, only a few brachiopods and trilobites survived
the rst pulse of the mass extinction, leaving a unique opportunity for sponges to evolve and ourish
during and aer the second pulse of the mass extinction. Furthermore, a noticeable transgression is
recognized to occur aer the second pulse26 when sponges were ourishing in the Beigong area. So, it
might be reasonable to propose that the explosion of sponges at Beigong should be related to this trans-
gressive event. More recently, in light of analysis of geochemical redox indicators (such as iron specia-
tion, molybdenum concentrations, pyrite framboid size distribution and sulfur isotopes), Hammarlund
et al.27 suggested that the end-Ordovician mass extinction is linked to widespread euxinia together with
near-global anoxia. In their model, during transgression in the late Hirnantian, the anoxic and weakly
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SCIENTIFIC RepoRts | 5:16060 | DOI: 10.1038/srep16060
sulphidic water reached the continental shelves, aecting shallow benthos and causing extinction in
shallower habitats28. Interestingly, this model is tested by the evolution of fauna and sedimentology from
the Beigong section. Aer the second episode, the sea level rise triggered the migration of the deep
anoxic water to the platform. is probably made the deep water sponge communities move to the rel-
atively shallow water in order to “escape” from the new anoxic conditions (Fig.4). It is suggested that
the emergence of sponges in the marginal platform may be related to the expansion of the euxinic and
anoxic condition. With transgression going on, sponge fauna disappeared, which may be triggered by the
widespread anoxic and suldic water column conditions during the early Silurian time27,29.
e preservation of nearly complete sponge specimens is unusual and related to two taphonomic
processes, i.e. low to zero oxygen in the deep water column and sediment in association with occasional
ne-grained mud turbidites (described in detail by Einsele & Kelts30) which cover the sponges rapidly
(Fig.4). Mud turbidites exhibit sometimes little silt fraction at the base and thin laminated mud/clay-rich
above. ese types of thin (1–2 cm) turbidites are observed in close relationship with the sponge-layers.
is taphonomic model is also recorded the Cambrian Lagerstätte Chengjiang Biota, which yields high
diversity and well-preserved so-bodied fossils. As noted by Zhao et al.31, the exceptional preserved
fossils of Chengjiang Biota were smothered by storm-generated mud clouds and buried rapidly with
low bioturbation and low hydrodynamic disturbance. It shows couplets of thin layers in their mudstone
samples, which were supposed to be subjected to dierent taphonomic processes, looking similar to our
thin layer mud turbidite sediment. is kind of taphonomic model prevents the pre- or syn-burial decay
and promotes the complete preservation both in Chengjiang and Beigong.
Methods
All gured specimens are deposited in Paleontology and Stratigraphy Laboratory, Department of Earth
Science, Nanjing University, and indicated by their accession numbers. Locality details for all the fos-
sils are saved in Nanjing University, and can be provided by the authors upon request. Photographs
were made with SONY A350 digital reex camera, and Zeiss SteREO Discovery V8 stereomicroscope
(transmitted- and reected light) linked to an AxioCam MRc 5-megapixel camera. Figures2 and 4 were
conducted by using Drawing tablet (WACOM, Intuos, PTK-840). Digital photographs were processed in
Adobe Photoshop CS3 and composite images were stitched together using CorelDraw X5.
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Acknowledgements
We thank Prof. Renbin Zhan for constructive comments on dras and stimulating discussion. is work
was nancially supported by the National Natural Science Foundation of China (NSFC) (Nos. 41372017,
41221001, 41290260), and the State Key Laboratory of Palaeobiology and Stratigraphy (LPS) (No. 143103).
is paper is also a contribution to the IGCP 591 Project “e Early to Middle Paleozoic Revolution”.
Author Contributions
L.X.L. and H.Z.F. conducted the eldwork; L.X.L., D.J. and J.R. contributed to the interpretation of the
fossils. L.X.L. wrote the paper with input from J.R.
Additional Information
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Li, L. et al. Unusual Deep Water sponge assemblage in South China—Witness
of the end-Ordovician mass extinction. Sci. Rep. 5, 16060; doi: 10.1038/srep16060 (2015).
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