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SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

September 2012 Vol.55 No.9: 1427–1444 doi: 10.1007/s11430-012-4458-4

Evolution and paleogeography of Eospirifer (Spiriferida, Brachiopoda) in Late Ordovician and Silurian ZHAN RenBin1*, JIN JiSuo2, LIANG Yan1 & MENG LingKai1 1

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; 2 Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada Received November 14, 2011; accepted February 20, 2012

Eospirifer, the oldest known genus of the spiriferide group (Brachiopoda), first appeared on the Zhe-Gan Platform of South China paleoplate during late Katian (Late Ordovician) represented by E. praecursor. It survived the end-Ordovician mass extinction, and reappeared on the Upper Yangtze Platform of South China paleoplate by the end of Ordovician near the upper part of the Normalograptus persculptus Biozone. Starting from the beginning of Silurian, Eospirifer experienced some morphological innovations and expanded its geographical distribution substantially. It reached its species diversity acme and the widest geographic distribution in Wenlock (middle Silurian), with the diversity hotspots in Laurentia, Avalonia, and Baltica. Various shell size frequency curves of E. praecursor under different paleogeographic settings suggest that this pioneer species of Eospirifer, with several macroevolutionary novelties, adopted a range of life strategies to adapt to the changing environments during early spiriferide evolution. There are also some morphological macroevolutionary trends during the evolutionary history of Eospirifer from Late Ordovician to the end of Silurian, such as the ever enlarging shell sizes and the width/length ratios from late Katian to Wenlock, but decreasing apparently of both parameters from Wenlock to Pridoli. Eospirifer, macroevolution, Ordovician-Silurian, species diversity, paleogeographic distribution Citation:

Zhan R B, Jin J S, Liang Y, et al. Evolution and paleogeography of Eospirifer (Spiriferida, Brachiopoda) in Late Ordovician and Silurian. Sci China Earth Sci, 2012, 55: 427–1444, doi: 10.1007/s11430-012-4458-4

Brachiopods are among the most important constituents of the Paleozoic Evolutionary Fauna for most of the Paleozoic Era [1, 2], and the spiriferides constitute one of the major orders of brachiopods that thrived in the benthic fauna from the Silurian to Permian periods [3, 4]. The eospiriferines are the oldest known group of the order Spiriferida, and their early evolution holds the key to the subsequent radiation of the spiriferides during the Silurian and Devonian periods. The focus of this study is to investigate the macroevolution of Eospirifer in Late Orodovician and Silurian, the oldest known genus of the order, with emphasis on its early evolu-

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2012

tion, radiation, and paleogeographic dispersal, and to explore its implication for the brachiopod macroevolution in general.

1 Type species and the earliest known species of Eospirifer Eospirifer was named by Schuchert [5] based on Spirifer radiatus Sowerby [6] from the Wenlock Limestone of Dudley, England. The internal structure of this type species remained little known until St. Joseph [7] described in detail the interiors of Eospirifer radiatus, particularly its crural structure and spiralia. Rong and Zhan [8] further investiearth.scichina.com

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gated the type species by serially sectioning well-preserved shells of E. radiatus from Gotland and reconstructed its cardinalia, crura, and spiralia (Figure 1(d), (e)), as part of their study of a series of Late Ordovician and Silurian eospiriferine taxa worldwide. These authors also proposed the first model for the early evolution of the Eospiriferinae. In current definition, Eospirifer radiatus (Sowerby [6]) is characterized by a medium to large, transversely extended shell with fine radial ornamentation (capillae) and a well-delimited dorsal fold and ventral sulcus. The internal features include a primitively striated cardinal process, and a pair of variously developed jugal processes that do not unite to form a jugum. The spiralia are pointed to the cardinal extremities as typical of all spiriferides (Figure 1(d), (e)). The earliest known species of Eospirifer, E. praecursor, came from the upper Changwu Formation (late Katian, Late Ordovician) at Pengli of Hejiashan, Jiangshan County, western Zhejiang Province, East China [3] (Figure 2). It was first discovered in the greenish yellow mudstone and all specimens are preserved as external and internal moulds. Thus its spiralia were unknown when the species was initially described and established. Unlike the Silurian Eospirifer, E. praecursor has a minute shell that is not trans-

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versely extended like a typical spiriferide in Devonian or younger, with a narrow interarea, a nearly flat-bottomed, shallow ventral sulcus and nearly flat-topped, strongly developed dorsal fold [4]. One year after its initial discovery in 1991, thousands of well-preserved conjoined valves (loose specimens) of Eospirifer praecursor were found by the senior author from the Xiazhen Formation (corresponding rocks of the Changwu Formation [9]) at Zhuzhai of Qunli, Yushan County, northeastern Jiangxi Province, about 50 km southwest of the type locality of E. praecursor (Figure 2). Serial sections of more than 20 individuals revealed the presence of spiriferide-type spiralia and hence validated the occurrence of Eospirifer in Late Ordovician [8, 10] (Figure 1(a)–(c)). Compared with the type species Eospirifer radiatus, E. praecursor has many primitive characters, such as its minute size, slightly elongate outline, weak and much finer costellae, lack of a comb-like, striated cardinal process, and a short spiralium with up to four whorls [8]. Despite these differences, we still treat the species E. praecursor as a primitive representative of Eospirifer mainly because its general spiriferide morphology, such as straight hingeline, ventral sulcus and dorsal fold, ornament of fine costellae

Figure 1 Reconstruction of internal structures of two Eospirifer species. (a)－(c) Ventral, lateral and dorsal views of the spiralia of Eospirifer praecursor Rong, Zhan et Han [3] from the upper Changwu Formation (late Katian), Jiangshan County, western Zhejiang Province, South China paleoplate; (d), (e) dorsal and lateral views of the spiralia of Eospirifer radiatus [6] from the Much Wenlock Limestone (late Wenlock), Dudley, West Midlands, England [8].

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Figure 2 (a) Location map of the earliest known Eospirifer in the border region of Zhejiang and Jiangxi provinces, East China (“JCY area”). Solid triangles show the localities of Eospirifer praecursor, and solid rectangles indicate the type locality and flourishing site of Eospirifer praecursor. (b) Reconstruced paleogeogaphical cross section from Zhuzhai (Yushan, Jiangxi Province) to Fengzu (Jiangshan, Zhejiang Province) with the type locality and flourishing site of E. praecursor indicated.

(capillae), and postero-laterally pointing spiralia with a pair of weak jugal processes, all of which are identical to Eospirifer rather than to any other genera in the spiriferide group. The lack of comb-like, striated cardinal process of E. praecursor is probably owing to its small shell size compared with the type species E. radiatus. Measurements are made for thousands of conjoined valves from the Zhe-Gan Platform and hundreds of external and internal moulds from the Zhexi Slope. The sizefrequency curves of populations from the platform and the slope are different substantially, with the platform population (YZ3-2) being strongly right-skewed, whereas the slope population (FD76) strongly left-skewed (Figure 3). The lack of a symetrical normal (bell-curve) distribution of shell sizes in two population types suggests that neither was an

incumbent population that had occupied the habitats for a prolonged geological interval. These were most likely opportunistic populations that invaded the very shallow and deep water environments when environmental conditions are favorable (Figure 3). The platform population was dominated by immature individuals developed not long after the pulse of larval settlement. The slope population, in contrast, was dominated by gerontic shells, with an impoverished input of new settlements (Figure 3(a)). Another contributing factor to the generally smaller shell size of E. praecursor on the Zhe-Gan Platform may have been the high environmental stress associated with the settlements of Eospirifer in the boundary zone between intertidal and subtidal zones, as indicated by the abundant desiccation polygons and microbial mats [11].

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Figure 3 Size-frequency of Eospirifer praecursor at two different collections in the JCY area. Shell size refers particularly to the shell width of each individual. (a) Collection YZ3-2 from the lower Xiazhen Formation at Zhuzhai, Yushan County, northeastern Jiangxi Province; (b) Collection FD76 from the top Changwu Formation at Pengli, Jiangshan County, western Zhejiang Province.

2 Temporal and spatial occurrences of Eospirifer Eospirifer has already been reported from more than 25 paleoplates or terranes in the Late Ordovician and Silurian rocks, notably Laurentia, Avalonia, Baltica, Siberia and adjacent terranes, Kazakhstanian terranes, South China, North China, Australia, northern Gondwana and periGondwana microplates. Some of those documented species of Eospirifer are actually something else rather than Eospirifer itself (see refs. [3, 8, 12] for examples). In order to summarize the temporal and spatial occurrences of Eospirifer around the world, we also check as many of those related references as possible and make our own decision on counting or excluding those reported Eospirifer species. Among those verified occurrences, some blocks have a particularly rich Eospirifer fossil record, such as Laurentia, South China, and Avalonia. It is now generally accepted that the main morphological characters of Eospirifer include the following: biconvex shell with prominent ventral sulcus and dorsal fold, hingeline straight, ornament of fine radial costellae (capillae), dental plates well-developed, cardinal process lacking or primitive comb-like, crural plates absent in some early species, and spiralia with a pair of variously developed jugal processes. 2.1

Late Katian (Late Ordovician)

The first appearance of Eospirifer praecursor in South China is marked by two puzzles: first, it appeared as a full-fledged spiriferide, without any obvious ancestral forms to show the transition from an atrypide- or athyridide-type spiralia to the spiriferide type; second, it dispersed almost

instantaneously to occupy a wide range of substrate settings, from deep-water, fine-grained siliciclastic substrate (BA 4; represented by the Changwu Formation), through mid-shelf carbonate (and siliciclastics, represented by the Sanjushan Formation), to very shallow water, intertidal argillaceous carbonate bottom conditions (BA1-2, represented by the Xiazhen Formation) (Figure 2). Paleogeographically, the Xiazhen Formation was formed on the Zhe-Gan Platform and the Changwu Formation on the Zhexi Slope during late Katian in the border region of Zhejiang and Jiangxi provinces [13]. The Xiazhen Formation consists of peritidal carbonates, with intervals of pervasive desiccation polygons in its lower part at Zhuzhai section (Figure 2), indicating at least periodic intertidal conditions; the formation also contains some siliciclastic interbeds. The Changwu Formation comprises solely siliciclastic deposits (such as mudstone, siltstone and thin beds of sandstone) except at only a few localities on the upper Zhexi Slope (e.g., Tanshi of Jiangshan) where some nodular limetone or argillaceous limestone interbeds occur in the upper part of the Changwu Formation (Figure 2). Eospirifer praecursor forms a nearly monospecific community on the intertidal, mud-cracked surfaces in the lower Xiazhen Formation at Zhuzhai of Yushan [11]. At all the sections of the Xiazhen Formation, both moulds and shells are present: moulds in the mudstone beds and well-preserved shells in the argillaceous limestone and calcareous mudstone beds (Figure 4). The plot of shell frequency revealed some interesting features of population dynamics: (1) Predominance of immature shells, as indicated by the strongly right-skewed (or positively skewed) curve (Figure 3(a)). (2) High-abundance and low diversity association. At Zhuzhai section, about 7500 loose specimens were collected

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Figure 4 Section of the Xiazhen Formation at Zhuzhai, Yushan County, northeastern Jiangxi Province, showing the preservation of conjoined shells or moulds of Eospirifer praecursor at different horizons with changing lithofacies. a, mudstone; b, calcareous mudstone; c, biotic limestone; d, argillaceous limestone. The number 1 to 5 under the “Benthic Assemblages” refers to the relative water depth of each assemblage from near shore shallow water (BA 1) to off shore deeper water (BA 5). The size of the arrows indicates the relative abundance of Eospirifer praecursor at each horizon.

in 1 m2 area of the argillaceous limestone beds with well-developed desiccation polygons. The occurrence of other species here is negligible in terms of the number of specimens. The shell assemblage at Zhuzhai section occurs in the lower part of the section, and is probably the oldest sample among the collections of E. praecursor in the upper Katian of South China. In contrast, samples of E. praecursor from the deepwater Changwu Formation show an opposite pattern of population dynamics (Figure 3(b)). The size frequency curve is very strongly left-skewed (or negatively skewed), indicating the predominance gerontic shells in the population. All individuals are preserved as external and internal moulds, and scattered in the siliciclastic rocks. Although the overall abundance is much lower than that at Zhuzhai, the


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brachiopod species diversity is apparently higher than that of the platform settings with several genera preserved together with Eospirifer praecursor, such as Wangyuella, Epitomyonia, Kassinella, and Foliomena. So far, only a few specimens of E. praecursor have been found in the widespread mudmound facies of the Sanjushan Formation. It remains unclear whether this is due to collection bias because it is usually difficult to collect brachiopods from the massive, cliff-forming carbonate facies. Despite its oldest known age and the greatest abundance in the peritidal Xiazhen Formation, the large populations of Eospirifer praecursor probably do not represent the center of origin for Eospirifer (or the Order Spiriferida). This interpretation is based on the following fossil data. (1) The strongly left-skewed shell size frequency suggests opportunistic species populations that dispersed periodically to the high-stressed peritidal environment from the yet unknown stable habitat of E. praecursor. The intertidal zone is characterized by severe environmental stressors such as fluctuating water level with daily subaerial exposure, unstable salinity, and shifting substrate sediments (especially during storms). The brachiopod larvae probably settled here periodically when the conditions are tolerable, but most obviously did not reach adult stage before the population was killed by local environmental disturbances. It is unlikely, therefore, that a species could originate under such erratic ecological conditions, where the continuous existence of an array of populations of a species could not be maintained. (2) The deep-water populations in the Changwu Formation, characterized by left-skewed shell size frequency, imply a preferential accumulation of adult to gerontic shells, without a constant supply of larvae and successive settlement. In a sustainable species population, such continuous input of larvae and growing shells would produce an approximately normally distributed size frequency curve (i.e., a symmetrical bell curve). Thus, the deep-water population dynamic feature indicates that a limited number of larvae were successfully recruited, either by local production or dispersal from other places. The limiting factors in a deep-water setting usually involve low levels of oxygen and food supply, and a lack of current for larval dispersal. Therefore, the deep-water populations of E. praecursor, in a different manner, also represent periodic invasions of this opportunistic species. Because of the relatively stable physical conditions, most shells were able to reach the adult stage. Obviously, this is not a setting where continuous populations could have been maintained for the origin of a new species. (3) Temporally, Eospirifer praecursor disappeared from all known localities of South China from the upper Paraorthograptus pacificus Biozone, the Normalograptus extraordinarius-N. ojsuensis Biozone, and the lower Normalograptus persculptus Biozone [14, 15]. After a hiatus of

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about 2.3 million years (Fan Junxuan, 2011, personal communication), Eospirifer sp. reappeared in the upper Kuanyinchiao Formation (middle Undulograptus persculptus Biozone, late Hirnantian) at Honghuayuan, Tongzi County, northern Guizhou Province, Southwest China. This hints a possibility that an Eospirifer source area existed outside the South China paleoplate, such as an island fauna, which is regarded as the most likely cradles of new species [16]. If such a source fauna existed, it would have been relatively close to South China paleoplate, as indicated by the frequent temporal and spatial recurrences of Eospirifer’s populations in Late Ordovician rocks of this paleogeographic region, but not in other regions. And the dispersal of Eospirifer from that "cradle" might be related to the environmental warming. Middle Ashgill, i.e., late Katian, is the time when global warming occurred [17], and when the third (also the last) diversity acme of the great Ordovician biodiversification event (GOBE) took place in many regions of the world, including South China paleoplate. The second dispersal of Eospirifer from that source fauna happened immediately after the extinction of the widespread Hirnantia Fauna also because of the global warming which caused the icecap melt down and a major sea level rise. This time, Eospirifer settled and became widespread on the Yangtze Platform of South China paleoplate and also expanded rapidly to many regions around the world in Early Silurian. The large collections of E. praecursor from the bedding planes show a wide range of morphological variations, such as the shell outline (e.g., length/width ratio), convexity, the development of fold and sulcus, the size of the ventral interarea, as well as aspects of internal structures [8]. Despite the variations, several features of this pioneer species of Eospirifer suggest that it was still an ecological generalist, in contrast to most of the later forms specialised to living in soft muddy substrate. For example, the shell of E. praecursor is very small, not transversely extended, and the delthyrium is entirely open without deltidal plates. This implies that it probably had a large and strong pedicle and was able to attach to a variety of substrate from hard ground, firm ground, to soft mud ranging from peritidal to deeper water environments. Its relatively small shell, and perhaps a correspondingly small body mass, is characteristic of an r-strategist (survival through rapid reproduction rate) and an ecological opportunist that invades into new habitats whenever the conditions are suitable and reaches maturity quickly. Eospirifer praecursor seems to have possessed the generalist and opportunistic ecological traits and life strategies, as indicated by 1) their occurrences in a wide range of substrates from the Zhe-Gan Platform to the upper Zhexi Slope of South China paleoplate [11]; 2) the abnormal distribution patterns (either strongly left- or right-skewed) of shell size frequency curve (Figure 3); and 3) high-frequency and low diversity benthic shelly assemblages, especially in peritidal settings, as discussed above.


2.2

Hirnantian

Available fossil data indicate that Eospirifer is absent from South China paleoplate or elsewhere for much of the Hirnantian, from the upper Paraorthograptus pacificus Biozone, the Normalograptus extraordinarius-N. ojsuensis Biozone, and the lower Normalograptus persculptus Biozone [14, 15]. At Honghuayuan of Tongzi County, northern Guizhou Province, Southwest China, some specimens of Eospirifer sp. [15] are present in the yellowish brown mudstone of the upper Kuanyinchiao Formation (chronostratigraphically corresponding to the upper middle part of the Undulograptus persculptus Biozone, late Hirnantian). All specimens are preserved as external and internal moulds, but they are similar to the late Katian E. praecursor of eastern South China in shell size, outline, interarea, fold and sulcus, and fine radial costellae (capillae). Paleoenvironmental barriers inhibited Eospirifer dispersal during the Ordovician-Silurian transition. The absence of Eospirifer from the early and middle Hirnantian and its reappearance in the late Hirnantian suggest that the earliest known representative of the spiriferides preferred a warm water environment. This interpretation is in agreement with several lines of paleoecological evidence. (1) During late Katian, Eospirifer successively established across the entire range of continental shelf environments, from intertidal zone (BA1) to outer shelf (BA4) water depths and from siliciclastic to carbonate substrates in the border region of Zhejiang and Jiangxi provinces (South China paleoplate). Thus, its disappearance during the earlymiddle Hirnantian is unlikely attributable to sea-level drop. (2) The major phase of oceanic cooling occurred during the early and middle Hirnantian. Climate warming caused the decay of the Gondwana icecap, and renewed sea level rise started in late Hirnantian, as has been demonstrated by stable isotope excursions [18–20]. (3) During Hirnantian, limestone deposits with abundant and diverse shelly benthos (the cosmopolitan Hirnantia Fauna) were widespread on the Yangtze Platform and adjacent areas of South China paleoplate, although interbeds of organic-rich shale were also ubiquitous. This implies that the Hirnantian depositional environments were characterized by prolonged episodes of well-oxygenated water mass. This is in sharp contrast to the succession of the late Katian Wufeng black shale that accumulated across much of the Yangtze Platform during a protracted period of oceanic anoxia. Therefore, the lack of Eospirifer during the earlymiddle Hirnantian could not have been the result of anoxic events. These lines of arguments suggest that the cooled oceanic water mass during the Hirnantian glaciation was the major barrier hindering the dispersal of Eospirifer from its refugium or evolutionary cradle, which was close to the South China paleoplate and remains to be discovered.

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Rhuddanian

Eospirifer went through a local to regional paleogeographic dispersal during this time interval. The oldest known Silurian representative of Eospirifer occurs in Bed D (lower Rhuddanian) of the Daurich Mountains of Tajikistan, which is below the Virgiana barrandei-bearing Bed F of the same area [21]. In South China, there are several sections and localities with occurrence of Eospirifer although only two species have been recognized [3, 22]. Besides, the other two paleoplates with Eospirifer, Australia and Kazakhstanian terranes, were both not far away from South China during the Early Silurian (Figure 5), indicating that the macroevolution of Eospirifer was still in its early stage. (1) Kazakhstanian terranes Eospirifer cinghzicus Borisiak [24]; lower Alpeis Formation (latest Rhuddanian); Akdombak Mountain, Chingiz Range, Kazakhstan (lectotype selected in ref. [25]). Eospirifer cf. radiatus Menakova [21]; Bed D (lower Rhuddanian, below Virgiana horizon in Bed F); Daurich Mountain, Zeravshan-Gissar Range, Tajikistan. (2) South China Eospirifer sinensis Rong, Xu et Yang [22]; base Xiang-

Figure 5


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shuyuan Formation (late Rhuddanian-early Aeronian); Leijiatun, Shiqian County, northeastern Guizhou, SW China. Eospirifer sp.; Rong et al. [3]; Wulipo Formation; Wulipo, Yanjiazhai, Meitan, northeastern Guizhou, SW China. (3) Australia Eospirifer tasmaniensis Sheehan and Baarli [26]; Arndell Sandstone (lower Rhuddanian, acuminatus Biozone [27]); Range Road Section, Tasmania, Australia. Eospirifer sp.; Sheehan and Baarli [26]; Arndell Sandstone (lower Rhuddanian, acuminatus Biozone [27]); Westfield Quarry, Tasmania, Australia.

The limited dispersal of Eospirifer between tectonic plates and terranes during Rhuddanian was probably the result of abrupt climate warming and widespread oceanic anoxia. Thick sequences of organic-rich black shale of Rhuddanian age are known to be predominant in South China, Australia, Avalonia, and northern Laurentia. High sea level and reduced presence of oceanic islands as stepping stones, as well as anoxic and perhaps sulphidic-euxinic oceanic watermass, would have been an effective ecological barrier against the successful long-distance dispersal of Eospirifer. A similar paleobiogeographical pattern has been ob-

Paleogeographical distribution of Eospirifer in six time intervals from Late Ordovician to the latest Silurian. Base maps are based on ref. [23].

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served for other Rhuddanian brachiopods and shelly benthos [28–30]. Early Rhuddanian brachiopods, in particular, were mostly Late Ordovician holdover taxa within each paleogeographical regions and comprised a mixture of endemic, opportunistic, and generalist forms [28, 31]. The Rhuddanian brachiopod fauna of South China, for example, has very few genera in common with those of Avalonia, Baltica, Siberia, and Laurentia, especially the taxa that newly originated after the Ordovician mass extinction. Virgiana, Stricklandia, Zygospiraella are among the small number of the earliest Silurian genera that dispersed across several tectonic plates before the end of Rhuddanian, with each taxon appearing in some or all of these regions: Laurentia, Avalonia, Baltica, Siberia, Kazakhstanian terranes [32–34]. This was referred to as the “intercontinental migration of the Virgiana Fauna”, the first major pulse of brachiopod dispersal during the Early Silurian [35]. In contrast, Eospirifer appears to have been confined to the "eastern" part of Gondwana and peri-Gondwana realm during this interval. 2.4

Aeronian

The species diversity of Eospirifer increased notably during this interval, with a corresponding expansion of its paleogeographic distribution. Its presence persisted in South China and Kazakhstanian terranes, but also expanded to Tuva (peri-Siberia), North China, and western North Greenland (marginal Laurentia). Both species diversity and general abundance of Eospirifer remained centred in South China, characterized by a broader distribution on the Upper Yangtze Platform, spanning an area of several thousand square kilometers, and a very high species diversity (from two species in the former interval to seven species of this interval). Six out of the seven species are new in the southwestern part of South China paleoplate. By mid-Aeronian, Eospirifer became relatively common and diverse also in peri-Siberia (Altai and Tuva) and Kazakhstanian terranes. In Laurentia and some other regions, however, the occurrence of Eospirifer was mostly sporadic during this interval. It is eqaully notable that it disappearred from Australia starting from this interval. The geographic expansion and cosmopolitanism of Eospirifer during the Aeronian was part of a widespread intercontinental dispersal of benthic shelly faunas. In terms of brachiopods, for example, the well-known Pentamerus and Stricklandia communities achieved a cosmopolitan distribution by mid-Aeronian [36, 37]. Eospirifer, pentamerides (e.g., Pentamerus, Stricklandia, Kulumbella), Eocoelia, and many other brachiopods with Aeronian characteristics show a similar pattern of westward migration during this time interval, which has been referred to as the “second pulse” of Early Silurian faunal dispersal [35]. It is not well understood what environmental or paleobiological factors triggered the accelerated faunal dispersal. Stable isotope data


suggest that the Aeronian was punctuated by a pulse of oceanic cooling [38], which may have resulted in improved oceanic circulation and consequently intercontinental faunal exchanges. (1) Laurentia Eospirifer quinqueplicatus Poulsen [39]; Cape Schuchert Formation; St. George Fiord, western North Greenland. (2) Kazakhstanian terranes Eospirifer fusus Borisiak [24]; Pentamerus Beds; central Kazakhstan. Eospirifer kassini Borisiak [24]; upper Alpeis Formation (Aeronian) [25]. (3) Siberia and adjacent terranes (Salair, Altai, Tuva) Eospirifer tuvaensis Chernyshev [40]; Kyzylchirinskie Beds; Kyzyl-Chiraa, Tuva [41]. Eospirifer tuvaensis altaicus Kulkov in Kulkov and Severgina [41]; Chinetkinsky Formation (Aeronian); Central Altai [41]. Eospirifer cf. radiatus Kulkov et al. [42]; Kyzylchirinskie Beds; Tuva, Kazakhstan. Eosirifer dedjanovi Kulkov in Kulkov and Severgina [41]; Chinetkinsky Formation (Aeronian); Gorny Altai [41]. (4) South China Eospirifer minutus Rong and Yang [43]; middle Xiangshuyuan Formation (early Aeronian); Yingwuxi, Sinan County, northeastern Guizhou, SW China. Eospirifer? plicatus Xian and Jiang [44]; middle lower Xiangshuyuan Formation (early Aeronian); Leijiatun, Shiqian County, northeastern Guizhou, SW China. Eospirifer sinanensis Jiang in Xian and Jiang [44]; upper lower Xiangshuyuan Formation (early Aeronian); Yingwuxi, Sinan County, northeastern Guizhou, SW China. Eospirifer sinensis Rong, Xu and Yang [22]; base Xiangshuyuan Formation (late Rhuddanian-early Aeronian); Leijiatun, Shiqian County, northeastern Guizhou, SW China. Eospirifer transversalis Rong and Yang [45]; middle Xiangshuyuan Formation (early Aeronian); Donghuaxi, Sinan County, northeastern Guizhou, SW China. Eospirifer sp.; Rong and Yang [45]; middle Xiangshuyuan Formation (early Aeronian); Leijiatun, Shiqian, northeastern Guizhou, SW China. Eospirifer sp.; Zeng et al. [46]; Zhangwan Formation (Aeronian); Shiyanhe, Xichuan, Henan, Central China. (5) North China Eospirifer sinensis dasifiliformis Fu [47]; lower Zhaohuajing Formation; Zhaohuajing, Tongxin County, Ningxia, North China. (6) Gondwana and peri-Gondwana Eospirifer radiatus; Gigout [48]; Morocco, northern Africa.

2.5

Telychian

During the Telychian, Eospirifer was represented by 12 species from seven tectonic plates and terranes, amongst which only one species is a holdover from the previous interval. For this interval, although only three species are recognized in Laurentia, Eospirifer became well established along the eastern seaboard of that supercontinent, leaving a rich fossil record in North Greenland, Anticosti Island, New York, Pennsylvania, Maryland, Indiana, Kentucky, etc. The

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most widely distributed species is Eospirifer radiatus, the type species, which was reported from many localities along the east coast of North America. Besides, E. radiatus of Telychian age has also been reported from Avalonia, Baltica, and peri-Gondwana terranes. In other regions, Eospirifer was present in Avalonia (New Brunswick and New World Island of Newfoundland), Baltica (Gotland, Estonia, and Podolia), Kazakhstanian terranes (Tien Shan), and South China. Compared with the early stages, the diversity and paleogeographic extent of Eospirifer in South China went through a major decline during the Telychian. This was most likely related to the tectonic uplift of the South China paleoplate and thus the loss of favourable shallow marine habitats on much of the Yangtze Platform [13, 49–51]. One of the striking changes in the paleogeographical pattern of Eospirifer is the shift of its diversity and abundance hotspot from South China to Laurentia by early Telychian time. On Anticosti Island, for example, Eospirifer occurs as shell beds (mainly preserved in situ) in numerous localities from the upper Jupiter to Chicotte formations [52]. The worldwide paleogeographical coverage of Eospirifer during the Telychian, however, is similar to that of the Aeronian. On a broader scale, the Telychian brachiopod faunas can also be regarded as a continuation of the Aeronian ones in many regions. The characteristic Early Silurian pentameride faunas, for example, are similarly represented by the Pentamerus-Pentameroides transition, and the Stricklandia-Costistricklandia transition in Laurentia, Avalonia, Baltica, and peri-Siberia. This interval is also marked by the diversification and wide dispersal of atrypoids (e.g., Gotatrypa, Septatrypa, Lissatrypa, and their related genera). The late forms of the well-known Eocoelia lineage are also reported widely in Laurentia, Avalonia, Baltica, and periSiberia. In the large paleocontinents such as Laurentia, Eospirifer appears to occur primarily in continental-margin shelves and platforms, and has rarely been found in epicontinental inland seas. During the Early Silurian, the eastern and northwestern margins of North America today were located in middle to relatively high tropical latitudes, and the inland basins (e.g., the Hudson Bay and Williston basins) were close to the paleoequator. The distribution pattern, therefore, suggests that the genus preferred continental shelf settings in mid-tropical latitudes. The somewhat “over-heated” inland basins along the paleoequator were not favourable for the life strategies of Eospirifer. (1) Laurentia Eospirifer consobrinus Poulsen [53]; Offley Island Formation; north coast of Offley Island, Cape Godfred Hansen, northern Greenland. Eospirifer radiatus; Gillette [54]; Williamson Shale; New York, USA; Lesley [55]; Clinton Shale; Pennsylvania, USA; Tillman [56]; Osgood Formation (Telychian); Indiana, USA; Prouty and Swartz [57]; Rose Hill Formation (upper Telychian; Maryland,


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USA (see Helfrich [58]); Foerste [59]; West Union Formation (= Bisher Member, Telychian); Kentucky, USA; Twenhofel [60]; upper Jupiter Formation (Cybele-Pavillon members) and Chicotte Formation; Anticosti Island, Canada. Eospirifer cf. radiatus; Boucot and Thompson [61]; Clough Formation; westcentral New Hampshire, USA. (2) Avalonia Eospirifer radiatus; Shrock and Twenhofel [62]; Pike Arm Formation (mainly of Telychian age), northern Newfoundland, Canada. Eospirifer sp.; Boucot et al. [63]; Long Reach Formation; south New Brunswick, Canada. (3) Baltica Eospirifer marklini (de Verneuil [64]); Bassett and Cocks [65]; Lower Visby Beds; Gotland, Sweden. Eospirifer profusus Rubel [66]; Adavere Stage (Telychian); Estonia. Eospirifer radiatus; Nikiforova [67]; Kitaigorod Formation; Podolia. (4) Kazakhstanian terrranes Eospirifer cinghizicus; E. silirscum; E. schidertensis; Wang [68]; Mishigou Formation and its equivalents (Telychian); northern Tien Shan. (5) South China Eospirifer songkanensis Wu; Rong and Yang [43]; upper Shiniulan Formation and Leijiatun Formation; northeastern Guizhou, SW China. Eospirifer subradiatus Wang [69]; from the fine grained yellowish green sandstone near the first fault, Xiushan Formation (Telychian); Changning, Sichuan, SW China. (6) Gondwana and peri-Gondwana Eospirifer radiatus; Gigout [48]; Morocco, northern Africa. Eospirifer cf. radiatus; Telychian age; Iberia, northwestern Spain [70].

2.6

Sheinwoodian

Eospirifer attained its macroevolutionary climax during this interval when it had a much higher level of worldwide species diversity and a slightly wider paleogeographical distribution (compared with that of the earlier interval). It is now known from Laurentia, Baltica, Avalonia, Kazakhstanian terranes, South China, Gondwana, and peri-Gondwana microplates. Among the 21 species recognized in this interval, only three are holdovers from the previous interval, indicating a high origination rate (Table 1). The diveristy hotspot of Eospirifer during this interval remained in Laurentia, but centred primarily in the eastern part of North America. The genus was still fairly common and diverse in South China, and also underwent a moderate radiation in Avalonia and Baltica manifested by the comparatively high species diversity, but its occurrence in the Tarim block was sporadic, like its presence in North China during the Aeronian. In the Gondwana and peri-Gondwana realm, Eospirifer became most common in the Bohemia microplate, such as those reported from the Motol Formation of the Prague Basin [71]. The success of Eospirifer during Sheinwoodian coincided with an episode of global cooling now recognized for

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Table 1 Species diversity, palaeogeographic distribution, and other statistics of Eospirifer species at each interval during its macroevolutionary history from Late Ordovician to the end of Siluriana) Chronostratigraphy

Silurian

Ordvician

Pridoli Ludfordian Ludlow Gorstian Homerian Wenlock Sheinwoodian Telychian Llandovery Aeronian Rhuddanian Hirnantian Upper Katian

Number of species 5 13 12 21 21 12 16 6 1 1

Number of sustained species 4 10 4 13 3 1 1 0 0 0

Number of paleoplates or terranes 4 7 9 9 8 7 7 4 1 1

Rate of origination Rate of extinction (%) (%) 20  23.1 69.2 66.7 16.7 38.1 81 85.7 38.1 91.7 75 93.8 93.8 100 83.3 100 100 100 100

a) The “sustained species” means the species extended from the former interval. The originating species means the species newly occurred in the current interval, and the extinct species refers to the species of this interval that does not extend upward to its younger interval.

this time interval, especially for the early Sheinwoodian (for a recent summary see ref. [72]). This may have been the result of a paleoecological expansion of Eospirifer into coolwater depositional settings. This interpretation seems to agree with the continued success of Eospirifer in Gondwana and peri-Gondwana regions during the Wenlock in general. (1) Laurentia Eospirifer (Acutilineolus) acutolineatus Amsden [73]; Fitzhugh Member, Clarita Formation; Arkansas, USA. Eospirifer (Acutilineolus) inferatus Amsden [74]; Marble City Member, Quarry Mountain Formation; eastern Oklahoma, USA. Eospirifer acutolineatus acutolineatus Amsden [73]; St. Clair Formation; Batesville District, Arkansas, USA. Eospirifer pentagonus Amsden [73]; Fitzhugh Member, Clarita Formation; Oklahoma, USA. Eospirifer radiatus Gillette [54]; Rochester Shale, Herkimer Sandstone; New York, USA; Lesley [55]; Clinton Shale; Pennsylvania, USA; Nettleroth [75]; Louisville Limestone; Indiana, USA; Prouty and Swartz [57]; Rochester Formation; Maryland, USA; Hall and Clarke [76]; Racine Dolomite; Wisconsin, USA; Foerste [77]; Massie Clay; Ohio, USA; Bolton [78]; Rochester Formation, and Ancaster chert of Goat Island Member of the Lockport Formation; southwestern Ontario, Canada. Eospirifer sp.; Amsden [79]; Clarita Member of the Chimneyhill Formation; Oklahoma, USA. Eospirifer sp.; Amsden [79]; St. Clair Formation; Arkansas, USA. Eospirifer sp.; Bolton [78]; De Cew Formation; southwestern Ontario, Canada. Eospirifer sp.; Sheehan [80]; Solis Limestone; Quadalupe section, Chihuahua, Mexico. Eospirifer sp.; Barnes et al. [81]; Starcke Limestone (Wenlock age according to Boucot in Barnes et al. [81]); Llano uplift, central Texas, USA. (2) Avalonia Eospirifer radiatus; Twenhofel [82]; Aroostook Limestone; Aroostook County, northern Maine, USA; Beecher and Dodge [83]; Ames Knob Formation (426 Ma, Sheinwoodian-Homerian boundary interval; see ref. [84]); coastal Maine, USA. Eospirifer cf. radiatus; Twenhofel [82]; Ashland Limestone (= Aroostook Limestone); Aroostook County, northern Maine, USA. Eospirifer stonehousensis McLearn [85]; Maehl [86]; French River Formation (lower Wenlock); Nova Scotia, Canada.

Eospirifer sp.; Boucot et al. [63]; Long Beach Formation; coastal New Brunswick, Canada. (3) Baltica Eospirifer radiatus; Hede [87]; Hogklint Bed, Slite Group, Halla ls., Mulde Marl; Gotland, Sweden. Eospirifer radiatus globosus (Salter [88]); Bassett and Cocks [65]; Slite Beds; Gotland, Sweden. Eospirifer togatus Barrande [89]; Vascautanu [90]; Schistes Marneaux a Strophomenides; Podolia. (4) Kazakhstanian terranes Eospirifer sp.; Boucot [12]; locality 14 of Arpishmebulag Series; Tien-Shan. Eospirifer radiatus; Leleshus [91]. lower Wenlock; southern Tien-Shan. (5) Junggar (part of Kazakhstanian terranes) Eospirifer radiatus; Rong et al. [50]; Sharbur Formation; Xinjiang, NW China. (6) South China Eospirifer dilectus Rong and Yang [43]; lower Xiushan Formation; northeastern Guizhou, SW China. Eospirifer radiatus; Zeng [92]; Shamao Group; western Hubei, South China; Rong and Chen [93]; Shamao Formation; southern Hubei, central China. Eospirifer subradiatus (Wang); Zeng [92]; Shamao Group; Xianfeng County, central Hubei, central China. Eospirifer xianfengensis Zeng [92]; Shamao Group; Datianba, Xianfeng County, central Hubei, central China. (7) Gondwana and peri-Gondwana Eospirifer pollens (Barrande [89]); Havlíček [71]; upper Motol Formation; area between Hills Kolo and Branžovy, Kozolupy, Bohemia, Czech Republic. Eospirifer praesecans Havlíček [94]; Havlíček [71]; Motol Formation; area between Lužce and Lodĕnice, Bohemia, Czech Republic. Eospirifer radiatus; Havlíček [71]; Tuffaceous limestone, Motol Formation; Tetín and “V Kozle” near Beroun, Bohemia, Czech Republic; Gigout [48]; Morocco, northern Africa.

2.7 Homerian Eospirifer had essentially the same paleogeographic pattern and species diversity during Homerian (late Wenlock) as in Sheinwoodian. The continued success of Eospirifer was reflected also by the large number of species that extended

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their range from the Sheinwoodian interval (13 out of 21), and by its widest paleogeographic distribution in nine tectonic plates or terranes. Its centre of diversity and abundance remained in Laurentia, where 13 species were reported from many localities, mostly in eastern North America, that is, more than half of the species of this interval are from Laurentia paleoplate. Besides, Eospirifer was also common in Baltica, Avalonia, South China, and periGondwana microplates (e.g., Bohemia) in this interval. Its distribution in South China became slightly smaller than that of the former interval confined to the northeastern Upper Yangtze Platform, such as Hubei Province [92]. (1) Laurentia Eospirifer acuolineatus acutolineatus Amsden [73]; Fitzhugh Member, Clarita Formation; Oklahoma, USA Eospirifer acutolineatus pentagonus Amsden [73]; St. Clair Limestone Formation; Batesville District, Arkansas, USA. Eospirifer (Acutilineolus) inferatus Amsden [73]; Marble City Member, Quarry Mountain Formation; eastern Oklahoma, USA. Eospirifer radiatus Gillette [54]; Rochester Shale; New York, USA; Lesley [55]; Clinton Shale; Pennsylvania, USA; Nettleroth [75]; Louisville Limestone; Indiana, USA; Prouty and Swartz [57]; Rochester Formation; Maryland, USA; Foerste [77]; Massie Clay; Ohio, USA; Hall and Clarke [76]; Racine Dolomite; Wisconsin, USA; Amsden [74]; Marble City Member, Quarry Mountain Formation; eastern Oklahoma, USA (Amsden identified his specimens as this species with a question mark); Bolton [78]; Rochester Formation, and Ancaster chert of Goat Island Member of the Lockport Formation; southwestern Ontario, Canada. Eospirifer cf. radiatus; Twenhofel [82]; Ashland Limestone; Aroostook County, northern Maine, USA. Eospirifer cf. eudora Northrop [95]; Gascons Formation (middle Wenlock-middle Ludlow); Gaspé, Canada. Eospirifer sp.; Amsden [79]; Clarita Member of the Chimneyhill Formation; Oklahoma, USA. Eospirifer sp.; Amsden [79]; St. Clair Formation; Arkansas, USA. Eospirifer sp.; Bolton [78]; De Cew Formation; southwestern Ontario, Canada. Eospirifer sp.; Sheehan [80]; Solis Limestone; Quadalupe section, Chihuahua, Mexico. Eospirifer sp.; Boucot [12]; late Wenlock to Ludlow [96, 97]; vicinity of Ciudad Victoria, State of Tamaulipas, Mexico. Eospirifer sp.; Lenz [98]; Road River Formation; Canadian Cordillera, Canada. (2) Avalonia Eospirifer globosus; Much Wenlock Limestone (Homerian); Dudley, West Midland [99]. Eospirifer radiatus; Much Wenlock Limestone (Homerian); Dudley, West Midland [99]; Twenhofel [82]; Aroostook Limestone; Aroostook County, northern Maine, USA; Beecher and Dodge [83]; Ames Knob Formation (426 Ma, SheinwoodianHomerian boundary interval [84]); coastal Maine, USA. (3) Baltica Eospirifer globosus; Hede [100]; Lower Visby Marl; Gotland, Sweden. Eospirifer radiatus; Shergold and Bassett [101]; Wenlock Limestone; Wenlock Edge, Shropshire, England; Hede [87]; Hogklint Limestone, Slite Group; Halla ls., Mulde Marl; Gotland,


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Sweden. Eospirifer radiatus globosus (Salter [88]); Bassett and Cocks [65]; Slite Beds, Wenlock Limestone; Dudley, Gotland, Sweden. Eospirifer togatus Barrande [89]; Vascautanu [90]; Schistes Marneaux a Strophomenides; Podolia. (4) Siberia and adjacent terranes (Salair, Altai, Tuva) Eospirifer radiatus; Kulkov [102]; Chagyrsky beds, upper Wenlock; Altai. Eospirifer parvus Kulkov [102]; Chagyrsky beds (?), upper Wenlock; Altai. Eospirifer decorus Kulkov [102]; Chagyrsky beds (?), upper Wenlock; Altai. (5) Kazakhstan and adjacent terranes Eospirifer sp.; Boucot [12]; locality 14 of Arpishmebulag Series; Tien-Shan. (6) Junggar (part of Kazakhstan terranes) Eospirifer radiatus; Rong et al. [50]; Sharbur Formation; Xinjiang, NW China. (7) South China Eospirifer radiatus; Rong and Chen [93]; Shamao Formation; southern Hubei, central China. Eospirifer xianfengensis Zeng [92]; Shamao Group; Datianba, Xianfeng County, central Hubei, central China. (8) Gondwana and peri-Gondwana Eospirifer devonicans Havlíček [94]; Kopanina Beds (according to the author, it is of Ludlow age); Czech Republic. Eospirifer pollens (Barrande [89]); Havlíček [71]; upper Motol Formation; area between Hills Kolo and Branžovy, Kozolupy, Bohemia, Czech Republic. Eospirifer praesecans Havlíček [94]; Havlíček [71]; Motol Formation; area between Lužce and Lodĕnice, Bohemia, Czech Republic. Eospirifer radiatus; Havlíček [71]; tuffaceous limestone, Motol Formation; Tetín and “V Kozle” near Beroun, Bohemia, Czech Republic; Gigout [48]; Morocco, northern Africa.

2.8

Gorstian

By early Ludlow, Eospirifer experienced a sharp drop in species diversity and abundance, although its distribution remained global and fairly wide, covering nine tectonic plates or regions. Among 12 species recognized for this interval, only four are holdovers from the previous interval, indicating a comparatively high origination rate (Table 1), contrasting against the overall drop of species diversity. The decrease in species diversity and in the number of occurrences was most pronounced in Laurentia (from 13 species in Homerian to three in Gorstian), with only a couple of species recorded along the eastern margin of the paleocontinent, such as in Tennessee and the Gaspé Peninsula. Besides, Eospirier sp. from the Ciudad Victoria region can only be counted as a dubious occurrence in Laurentia because the associated fauna was only distantly related to the North American fauna [96, 97]. The appearances of Eospirifer in Australia, Sibumasu, and Baltica can be categorized as sporadic during Gorstian, and it disappeared from the Kazakhstanian terranes in this interval. In contrast, Eospirifer persisted successfully in the peri-Gondwana terranes, continuing its diversity level and paleogeographic pattern as

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for the Wenlock time. (1) Laurentia Eospirifer foggi Foerste [77]; Lobelville Formation (Brownsport Group, Ludlow); Tennessee, USA. Eospirifer cf. eudora Northrop [95]; Gascons Formation (middle Wenlock to middle Ludlow [103]); Gaspé, Canada. Eospirifer sp.; Boucot [12]; late Wenlock to Ludlow [96, 97]; vicinity of Ciudad Victoria, State of Tamaulipas, Mexico. (2) Avalonia Eospirifer radiatus; Boucot [12]; Watkins [104]; Cocks [99]; upper Llandovery to lower Ludlow; many places in UK; Shergold and Bassett [101]; Lower and basal Middle Elton Beds; Wenlock Edge, Shropshire, England. (3) Baltica Eospirifer radiatus; Bassett and Cocks [65]; Upper Visby Beds to Klinteberg Beds; Gotland, Sweden. Eospirifer cf. radiatus; Khodalevitch [105]; striatus Beds; Urals, Russia. (4) South China Eospirifer radiatus; Rong and Chen [93]; Shamao Formation; southern Hubei, central China. Eospirifer tingi Grabau [106]; Tsin [107]; Gaozhaitian Formation; Wudang, Guiyang, Guizhou Province, southwest China. Eospirifer uniplicatus Tsin [107]; Gaozhaitian Formation; Wudang, Guiyang, Guizhou Province, southwest China. (5) Sibumasu Eospirifer cf. radiatus; Reed [108]; Namhsim ss.; Myanmar (Burma). (6) Australia Eospirifer eastoni Gill [109]; Garratt [110]; Dargile Formation; Melbourne Trough, Victoria, Australia. (7) Gondwana and peri-Gondwana Eospirfer contortus Havlíček [94]; Ludlow; Czech Republic. Eospirifer devonicans Havlíček [94]; Kopanina Beds (according to the author, it is of Ludlow age); Barrande area, Bohemia, Czech Republic. Eospirifer radiatus; Gigout [48]; Morocco, northern Africa. Eospirifer cf. tenuis (Barrande); Havlíček [94]; Kopanina Beds (according to the author, it is of Ludlow age); Barrande area, Bohemia, Czech Republic.

2.9

Ludfordian

The species diversity of this interval is similar to that of the Gorstian, but its paleogeographical distribution became slightly narrower, known from seven paleoplates or terranes, including Laurentia, Avalonia, Baltica, South China, Australia, Gondwana and peri-Gondwana terranes. Besides, it was reported from the Fukuji Formation (Ludfordian) of central Japan for the first time [111]. There are two Eospirifer faunal characteristics for this interval: firstly, 10 out of 13 speices recognized for Ludfordian are holdovers from the previous interval, indicating a very low origination rate during this interval (Table 1); secondly, Eospirifer became extinct in Sibumasu during the Ludfordian. (1) Laurentia Eospirifer foggi

Foerste

[77];

Lobelville

Formation


(Brownsport Group, Ludlow); Tennessee, USA. Eospirifer sp.; Boucot [12]; late Wenlock to Ludlow [96, 97]; vicinity of Ciudad Victoria, State of Tamaulipas, Mexico. (2) Avalonia Eospirifer plicatellus var. interlineatus Lindström; Walmsley [112]; Lower Llangibby Beds; Usk inlier, Monmouthshire, England. Eospirifer radiatus (Sowerby [6]); Cocks [99]; upper Bringewood Beds; many places in UK. (3) Baltica Eospirifer cf. radiatus; Khodalevitch [105]; striatus Beds; Urals, Russia. (4) South China (+ Japan) Eospirifer radiatus; Rong and Chen [93]; Shamao Formation; southern Hubei, central China. Eospirifer tingi Grabau [106]; Tsin [107]; Gaozhaitian Formation; Wudang, Guiyang, Guizhou Province, southwest China; Grabau [113]; Miaokao Formation; Qujing, Yunnan Province, southwest China. Eospirifer uniplicatus Tsin [107]; Gaozhaitian Formation; Wudang, Guiyang, Guizhou Province, southwest China. Eospirifer variplicatus Ohno [111]; Fukuji Formation (the author thought the age was Early Devonian); Hida Massif, central Japan. (5) Australia Eospirifer eastoni Gill [109]; Garratt [110]; Dargile Formation; Melbourne Trough, Victoria, Australia. (6) Gondwana and peri-Gondwana Eospirfer contortus Havlíček [94]; Ludlow; Czech Republic. Eospirifer devonicans Havlíček [94]; Kopanina Beds (according to the author, it is of Ludlow age); Barrande area, Bohemia, Czech Republic. Eospirifer radiatus; Gigout [48]; Morocco, northern Africa. Eospirifer cf. tenuis (Barrande); Havlíček [94]; Kopanina Beds (according to the author, it is of Ludlow age); Barrande area, Bohemia, Czech Republic.

2.10

Pridoli

Eospirifer experienced another major decline in both species diversity and paleogeographical distribution during the last stage of the Silurian, with only sporadic occurrences now known from South China, Australia, and Kazakhstan (Balkhash). Among the small number of taxa, four out of five species are holdovers from the previous interval, indicating a very low origination rate. It disappeared from Laurentia where it flourished through much of the Silurian, particularly during the Wenlock. Eospirifer was similarly absent in Avalonia and Baltica. Its reported occurrences are putative and questionable in Gondwana and peri-Gondwana. The drop in Eospirifer species diversity and geographical distribution was caused, at least partly, by a global regression and much reduced areas of epeiric seas during the Pridoli [114]. It is notable that the paleogeographical pattern of Eospirifer during the Pridoli mirrors that of the Rhuddanian, with its occurrences beginning, and ending, in nearly the same places, i.e., Australia, South China, and Kazakhstanian terranes.

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(1) Kazakhstanian terranes Eospirifer cf. radiatus; Olenicheva [115], mid Tokrausky horizon (Pridoli); Balkhash. (2) South China Eospirifer tingi Grabau [106]; Grabau [113]; Miaokao Formation; Qujing, Yunnan Province, southwest China. Eospirifer uniplicatus Tsin [107]; Gaozhaitian Formation; Wudang, Guiyang, Guizhou Province, southwest China. (3) Australia Eospirifer eastoni Gill [109]; Garratt [110]; Dargile Formation; Melbourne Trough, Victoria, Australia. (4) Gondwana and periGondwana Eospirifer radiatus; Gigout [48]; Morocco, northern Africa.

3 Discussion: Macroevolution of Eospirifer during Late Ordovician and Silurian Based on the data we have, Eospirifer first appeared in South China during the last climax of the GOBE, although its immediate ancestor and the centre of origin remain to be investigated. In external morphology, the generally planoconvex or ventri-biconvex shells, without laterally extended, spiriferide-type wings, show a certain degree of resemblance to some Late Ordovician atrypides [116, 117], although none of the early atrypides have the fine capillae of Eospirifer. In terms of the spiralia, both Copper [116] and Popov et al. [117] described early atrypides with medially directed spiralia with only a few whorls. Moreover, the Late Ordovician athyridides Kellerella and Nikolaespira have been shown by Popov et al. [117] to have laterally directed spiralia tilting towards the cardinal extremities. The wide range of variations in internal structures suggests that the orientation of the spiralia in Late Ordovician atrypides and athyridides was highly unstable. It is thus possible that the postero-laterally directed spiriferide spiralia could have evolved from either an atrypide or an athyridide, with some adjustment and stabilization of spiral orientation. In particular, it would have required only a very minor adjustment for the spiralia to change from the Kellerella- or Nikolaespiratype to Eospirifer-type. The small-shelled, pioneer species of Eospirifer appeared suddenly in South China and successfully colonized a wide range of substrate conditions, from lower intertidal, carbonate mudflat (lower BA1 or upper BA2) to deeper water, siliciclastic mud bottom (BA4 or slightly deeper together with the typical Foliomena fauna [3, 118]) during late Katian [8, 11, 119]. From its early evolution, Eospirifer preferred muddy substrate but warm-water environments. This is reflected in its disappearance from the lower and middle Hirnantian and recurrence in the upper Hirnantian of South China paleoplate. Although its refugium remains to be found, Eospirifer survived the Hirnantian mass extinction pulses, and rapidly expanded and radiated during the Silurian [31, 50]. Starting from the lower Rhuddanian, all representatives


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of Eospirifer are substantially different from its Late Ordovician counterparts by having a much bigger and wider shell, which is generally larger than 10 mm with length/width ratio usually less than 1.0 (Figure 6). During the Aeronian, Eospirifer went through further geographical dispersal, spreading to North China, peri-Siberia terranes (Tuva, Altai), and Laurentia (Greenland), in addition to its Rhuddanian occurrences, and its species diversity nearly tripled compared with that of the previous interval. The centre of species diveristy and abundance, however, remained in South China. By Telychian time, there appeared a major change in Eospirifer paleobiogeography, with its species diversity hotspot shifted from South China to Laurentia, where it flourished in relatively deep-water, muddy-bottom, carbonate depositional environments in the continental shelf and pericratonic epeiric seas of eastern North America. The westward dispersal of the genus is also reflected in its occurrences in Avalonia (northern Newfoundland and New Brunswick, maritime Canada), and Baltica (Podolia and Gotland). This paleobiogeographical pattern persisted through the Wenlock, with the species diversity and abundance hotspot centered in Laurentia, although Eospirifer became increasingly common and diverse in Baltica and

Figure 6 Morphological changes of Eospirifer in shell sizes (width) (a) and width/length ratios (b) from the Late Ordovician to the latest Silurian. Data from the species of each time interval, and mean value is introduced.

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Avalonia. Wenlock is the time when Eospirifer reached its highest species diversity and the widest paleogeographic distribution (Figure 7). Paleogeographically, the radiation hotspot of Eospirifer was in Laurentia where Eospirifer flourished during the Sheinwoodian and Homerian, represented by numerous species and very abundant individuals. Avalonia and Baltica also experienced moderate levels of Eospirifer radiation (e.g., refs. [65, 82, 99]). Comparatively, South China and Kazakhstanian terranes, where early evolution of Eospirifer took place, were poorly represented by Eospirifer during its macroevolutionary acme. From Ludlow to Pridoli, the species diversity descreased gradually and steadily worldwide, manifested by a generally decreasing specific origination rate and an increasing extinction rate (Figure 8), while its paleogeographic distribution became increasingly limited and sporadic (Figure 7). From the Late Ordovician to the terminal Silurian, Eospirifer experienced three dramatic changes in species diversity and paleogeographic distribution accordingly: one across the Hirnantian/Rhuddanian boundary and the other two at the beginning and the end of Wenlock respectively. The rapid increase in species diversity and expansion of paleogeographic distribution in the Early Silurian were accompanied by a high origination rate and low extinction rate.


But throughout the Silurian, the species diversity and paleogeographic distribution do not seem to have a direct relationship to its specific origination and extinction rates. Some of the modifications appear to have been the result of allopatric speciation in different tectonic plates, or in different regions of a single plate. (1) Increase in shell size from the Late Ordovician to the Wenlock. All Late Ordovician representatives are generally smaller than 5 mm, whereas most of the Silurian species before Ludlow are larger than 10 mm, and some even close to 30 mm in width. (2) Increase in the strength of capillae. The Ordovician species have extremely fine capillae (about 16－18 lines per mm), which make the shell appear smooth to the naked eye (Figure 1). The Silurian species have increasingly coarser capillae over a much larger shell (about 4－5 lines per mm), which make them easily seen. (3) Increase in the number of whorls of the spiralia. In the Late Ordovician species, the number of whorls usually does not exceed four, most commonly two or three in average-sized shells. By the earliest Silurian (Rhuddanian– Aeronian), the number increased drastically to 10 or more due to major increases in shell size and especially shell width (e.g., the type species Eospirifer radiatus, see ref. [8]).

Figure 7 Changes in species diversity, paleogeographical distribution and the number of sustained species of each interval from its previous interval of Eospirifer from the Late Ordovician to the latest Silurian, showing a nearly symmetrical pattern of the rise and fall in both macroevolutionary proxies during the Silurian.

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Figure 8 Changes in origination rates and extinction rates of Eospirifer during the Late Ordovician and Silurian, showing the fluctuation of both rates, and the generally decreasing origination and increasing extinction rates.

(4) Reduction in the angle between the jugal processes. It is about 116° in E. praecursor (late Katian), about 70° in E. sinensis (Rhuddanian), about 42° in E. cf. radiatus (Telychian), and about 30° in E. radiatus (Wenlock) [8]. In addition to the species diversification within Eospirifer, the genus is interpreted to have been the ancestral stock for many other eopiriferine genera during the Silurian and Early-Middle Devonian, such as the Early Silurian Striispirifer, Janius, Nikiforovaena, Cyrtia and Yingwuspirifer [3, 8]. Thus, it can be said that the minute and simple shell of Eospirifer praecursor contained a wide range of genetic variability and evolutionary potential to give rise to a vastly successful group of brachiopods (Order Spiriferida) from the Silurian to the Triassic.

4 Conclusions The oldest pioneer species of spiriferides, Eospirifer praecursor, demonstrated great adaptive potential during its earliest evolution and successfully invaded a wide range of habitats during the Late Ordovician, from shallow (BA1–2) to relatively deep (BA4) water environments, from silici-

clastic to carbonate muddy substrates, and from the Zhe-Gan Platform to the upper Zhexi Slope. During the Early Silurian, the Eospirifer diversity hotspot shifted from South China (Rhuddanian–Aeronian) to North America (Telychian-Homerian), with its macroevolutionary climax reached during the Wenlock, followed by a gradual decline from Ludlow to the early Middle Devonian. During the Silurian, the species diversity and geographical extent of Eospirifer show a similar pattern of nearly symmetrical rise and fall, with a plateaued peak in Wenlock time (Figure 7). The rapid increase in species diversity in Llandovery was consistent with the high origination rates and low extinction rates (Figure 8), but the decrease in species diversity and the shrinking paleogeographical distribution were accompanied by the largely decreasing origination rates and increasing extinction rates in the Late Silurian. Up to now, Eospirifer has been documented from almost all major continents (paleoplates or terranes) from the Late Ordovician to the Middle Devonian, except for Antarctica and South America. In North America (Laurentia), Eospirifer had the highest species diversity, the widest paleogeographical distribution, the longest stratigraphical range, and the most abundant occurrences and individuals.

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Prof. Rong Jiayu’s continued encouragement and guidance in the study of eospiriferine brachiopods are greatly appreciated. Two anonymous reviewers and Dr. Miao Desui carefully read the early version of our manuscript and gave us very good suggestions that have improved the manuscript. This study was supported by Chinese Academy of Sciences (Grant No. KZCX2-YW-Q05-01), National Natural Science Foundation of China (Grant No. 40825006), the CAS/SAFEA International Partnership Program for Creative Research Teams, and the State Key Laboratory of Paleobiology and Stratigraphy. This paper is also a contribution to IGCP 591 (The Early to Middle Paleozoic Revolution).


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