Early Evolution of Nautiloid Faunas Through the Cambrian-Ordovician Transition: A General Overview
1 CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Nanjing 210008, China;
2 University of Chinese Academy of Sciences, Beijing 100049, China.
The early evolution and diversity history of nautiloids in the Late Cambrian has been poorly known and rarely addressed. Smith & Caron (2010) reported the possibly oldest cephalopod Nectocaris pteryx Conway Morris from the Burgess Shale of Canada and considered it to be the stem-group of Cephalopoda, but the phylogenetic role of the species has been disputed (Kröger et al., 2011). However, it is clear that the undisputed oldest nautiloid fauna in the world occurs in the lower part of the Fengshan Formation (late Jiangshanian, Furongian, Cambrian), Shandong Province, North China (Walcott, 1905). The fauna includes the representative genus Plectronoceras Ulrich and Foerste, which was assigned to the Family Plectronoceratidae in the Order Plectronocerida (Fig. 1;Chen et al.,1979a).The species of Plectronoceras are typified by a small and cyrtoconic external shell with simple septa and a ventral siphuncle, short and straight septal necks, and expanded segments (Chen et al., 1979a; Chen & Qi, 1982). This occurrence is followed by the first explosive diversification of nautiloids in the middle part of the Fengshan Formation, a horizon corresponding roughly to the uppermost Jiangshanian to unnamed Stage 10 of the Cambrian. The diverse nautiloid faunas include more than 30 genera belonging to 10 families and 4 orders, as recorded in China, Kazakhstan, Siberia, and the U.S.A. (Table 1).
In the latest Furongian (Cambrian), the nautiloids rapidly reached their first diversity peak after their origination at a slightly older age. The Order Plectronocerida, represented by 3 families, the Plectronoceratidae, Balkoceratidae and Protactinoceratidae, have been regarded as the earliest nautiloids (Mutvei et al., 2007). In contrast, the Order Ellesmerocerida is the largest group of Cambrian nautiloids, containing 5 families: Ellesmeroceratidae, Acaroceratidae, Huaiheceratidae, Protocycloceratidae and Xiashanoceratidae. Furthermore, Yanhecerida and Endocerida are also distinctive components of late Cambrian nautiloid faunas (Chen & Teichert, 1983a; Li, 1984). The remarkable diversification of nautiloids in the Furongian indicates that they were probably among the dominant planktonic and nektonic organisms in the late Cambrian oceans, in association with the agnostid trilobites and conodonts.However,in the latest Cambrian Mictosaukia Biozone,the‘Late Trempealeauan Eclipse' occurred (Stage 10, Furongian; Chen & Teichert, 1983a), a possible extinction event characterized by the abrupt and cryptic disappearance of nearly all the existing nautiloid faunas.Only a few forms,including Ectenolites and Clarkoceras(Ellesmeroceratidae)and some elements of Protocycloceratidae and Endocerida survived into the Early Ordovician.
Figure 1 Range chart of early nautiloid faunas through the Cambrian and Ordovician transition and the diversity of some other major marine organisms during the onset of the GOBE (Great Ordovician Biodiversification Event). Data sources: acritarchs, Servais et al., 2016; graptolites, Zhang et al., 2010;trilobites(China),Zhou&Zhen,2008.Fossil illustrations:1.Nectocaris pteryx Conway Morris (Smith&Caron,2010;British Columbia,Canada);2.Plectronoceras cf.cambria(Walcott)(Chen et al.,1979a;Shandong,China);3.Protactinoceras magnitubulum Chen and Qi(Chen et al.,1979a;Shandong, China); 4. Xiaoshanoceras jini Chen and Teichert (Chen &Teichert, 1983b; Zhejiang, China);5.Huaiheceras hanjiaense Zou and Chen(Chen et al.,1979b;Anhui,China);6.Ectenolites petilus Chen and Qi(Chen&Qi,1982;Anhui,China);7.Acaroceras endogastrum Chen et al.(Chen et al.,1979b;Anhui,China);8.Yanheceras endogastrum Chen(Chen et al.,1979a;Guizhou,China);9.Zhuibianoceras zhejiangense Li(Li,1984;Zhejiang,China).
The palaeoecology of Cambrian nautiloids is poorly known, largely due to poor specimen preservation and the remarkably intermittent occurrences within their fairly long stratigraphic ranges. A recent study of the internal microstructure of plectronocerids and ellermerocerids showed that they had developed only a low capacity for jet-power swimming, which suggested a probable lifestyle of crawling or moving slowly on the sea-floor, and possibly also slow vertical migration (Mutvei et al., 2007).
Table 1 Global distribution of Cambrian nautiloids. Data sources: Flower, 1964; Yochelson et al., 1973;Chen & Qi, 1982; Peng & Chen, 1983; Chen & Teichert, 1983a, 1983b; Li, 1984.
Tremadocian (Early Ordovician) nautiloid faunas were dominated by the few representatives of Ellesmerocerida and Endocerida, descended from their Furongian ancestors. However, the earliest representatives of Tarphycerida, Orthocerida, and Actinocerida, although low-diversity, originated during the latest Tremadocian and Floian (Flower, 1976). The early Middle Ordovician was the time of greatest morphologic differentiation in the history of nautiloid evolution. Species of the five orders that occurred in the Early Ordovician, Ellesmerocerida, Endocerida, Tarphycerida, Orthocerida and Oncocerida, persisted into the early Middle Ordovician. Of these taxa, the families Endoceratidae, Baltoceratidae, Protocycloceratidae, and Orthoceratidae diversified significantly and dominated at this time. Two new orders, Discosorida and Actinocerida, first appeared in this period, adding to the increasingly abundant nautiloid faunas.
As the dominant predator in late Cambrian and Ordovician oceans, nautiloids may have played a critical role in the oceanic foodweb of the Cambrian-Ordovician transition and in the onset of the GOBE. After their origination in the early Furongian, nautiloids diversified rapidly in the late Furongian and reached two diversity peaks in a stepwise pattern. It is noteworthy that the rapid diversification of nautiloids in the late Cambrian coincides with the origination and radiation of several other groups during the middle to late Cambrian (e.g. graptolites and radiolarians, Fig. 1; Zhang et al., 2010; Servais et al., 2016). Furthermore, in the latest Cambrian (unnamed Stage 10), most of the previously existing nautiloids (more than 30 genera) disappeared suddenly, with only 2 surviving into the earliest Ordovician. This suggests that there may have been a nautiloid extinction event at the Cambrian-Ordovician transition, as proposed by Chen & Teichert (1983a). Coincidentally, there was also a mass extinction of trilobites in the latest Cambrian, as evidenced by their lowest diversity (Fig. 1; Zhou & Zhen, 2008), exactly corresponding to the ‘Late Trempealeauan Eclipse'. In the late Early to Middle Ordovician, nautiloids diversified significantly and eventually reached their diversity peaks (Kröger & Zhang, 2009), at almost the same pace as other groups of organisms (e.g. acritarchs; Servais et al., 2010), suggesting close coevolution between these marine organisms.
Acknowledgements Financial support from the Chinese Academy of Sciences (Nos. XDPB05, XDB10010100), the National Natural Science Foundation of China (Nos. 41290260, 41521061) and the Ministry of Science and Technology of China (No. 2013FY111000) is acknowledged. This is a contribution to IGCP Project 653.
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