Introduction
The lower Cambrian faunas of western Mongolia display a very high level of diversity and disparity (Korobov, Reference Korobov1989; Esakova and Zhegallo, Reference Esakova and Zhegallo1996; Zhuravlev and Naimark, Reference Zhuravlev and Naimark2005). This pattern is related to the highly complicated tectonic history of the Central Asian Orogenic Belt, which now includes Mongolia, Kazakhstan, the Altay-Sayan Foldbelt, and Transbaikalia (Fig. 1.1). Two principal models for the development of this mosaic region have been developed. Both models attribute the tectonic development of Mongolia during the Neoproterozoic–early Paleozoic to the accretion and collision of several terranes that were previously recognized as tectonic zones or provinces within the same marine basin (Amantov, Reference Amantov and Marinov1963; Marinov, Reference Marinov1970; Blagonravov and Zaytsev, Reference Blagonravov and Zaytsev1972; Marinov et al., Reference Marinov, Zonenshayn and Blagonravov1973). The names of these zones are still in use, but their affinity to the same marine basin is rejected.
Figure 1. Location and geological setting of the Seer Ridge section. (1) Tectonic map of western Mongolia, showing key Cambrian sections (modified after Badarch et al., Reference Badarch, Cunningham and Windley2002); dashed-line rectangular box indicates location shown in (2). (2) Locality map. (3) Outcrops of sandy siltstone overlying archaeocyathan reefs in the upper Burgasutay Formation. (4) Stratigraphic column and fossil horizons of the Seer Ridge section and a correlation of lower Cambrian sections in western Mongolia (modified after Korobov, Reference Korobov1980, Reference Korobov1989).
The first hypothesis suggests that Mongolia spanned a number of different volcanic arcs (Lake, Khan-Khukhiy, Ider and Dzhida zones) and cratonic terranes (former Zavkhan and Khuvsgul zones), which originated as Precambrian crustal fragments, rifted from North China, drifted across the Paleo-Asian Ocean, and finally collided with and accreted to Siberia (Zonenshain et al., Reference Zonenshain, Kuz'min and Kononov1985; Mossakovsky et al., Reference Mossakovsky, Ruzhentsev, Samygin and Kheraskova1993; Zhou et al., Reference Zhou, Wilde, Zhao and Han2018). Alternatively, another hypothesis attributes the formation of the entire Central Asian Orogenic Belt to the growth of giant subduction-accretion complexes along a single migrating magmatic arc, which was formed during rifting of the combined Baltica-Siberia craton (Sengör et al., Reference Sengör, Natal'in and Burtman1993). However, the latter hypothesis is neither supported by the distribution pattern of facies nor by faunal affinities, emphasizing a close similarity between faunas of different ages in South China and Siberia. Terreneuvian small shelly fauna in South China and Mongolia shared much in common (Yang et al., Reference Yang, Sreiner and Keupp2015). By contrast, later archaeocyaths and trilobites from Mongolia Cambrian Series 2 strata shared identical species with the Altay-Sayan Foldbelt, Transbaikalia, and the Siberian Platform (Korobov, Reference Korobov1989; Debrenne et al., Reference Debrenne, Maidanskaya and Zhuravlev1999). The suggestion that some Mongolian terranes originally belonged to South China and/or Tarim is further supported by a variety of tectonostratigraphic and paleomagnetic data (Levashova et al., Reference Levashova, Meert, Gibsher, Grice and Bazhenov2011) and by detrital and xenocrystic age spectra (Rojas-Agramonte et al., Reference Rojas-Agramonte, Kröner, Demoux, Xia, Wang, Donskaya, Liu and Sun2011). Although the paleotectonic situation is more complicated, some models suggest a long lasting and continuous sequence of events leading to the accretion and amalgamation of numerous volcanic arcs, backarc/forearc basins and associated subduction complexes, as well as crustal and other terranes for the formation of Mongolia (e.g., Badarch et al., Reference Badarch, Cunningham and Windley2002; Kheraskova et al., Reference Kheraskova, Didenko, Bush and Volozh2003; Janoušek et al., Reference Janoušek, Jiang, Buriánek, Schulmann and Hanžl2018).
An underestimation of faunal data has played a huge part in weakening tectonic models, and therefore suggestions made on the places of origin and migration routes of terranes. Thus, tectonic models need better paleontological underpinning. The Neoproterozoic–lower Cambrian successions and faunas of the Zavkhan terrane are relatively well studied (e.g., Voronin et al., Reference Voronin, Voronova, Girgor'eva, Drozdova and Zhegallo1982; Wood et al., Reference Wood, Zhuravlev and Chimed Tseren1993; Brasier et al., Reference Brasier, Shields, Kuleshov and Zhegallo1996; Smith et al., Reference Smith, Macdonald, Petach, Bold and Schrag2016; Yang et al., Reference Yang, Steiner, Schifauer, Selly, Wu, Zhang and Liu2020; Steiner et al., Reference Steiner, Yang, Hohl, Li and Donoghue2021; Topper et al., Reference Topper, Betts, Dorjnamjaa, Li, Li, Altanshagai, Enkhbaatar and Skovsted2022). In contrast, our knowledge of facies and fossils from other former tectonic zones is still almost in its infancy. A few archaeocyaths and small shelly fossils from the Lake, Khan-Khukhiy, and Khuvsgul regions have been described (Vologdin, Reference Vologdin1940; Zhuravleva, Reference Zhuravleva and Zhuravleva1972; Voronin, Reference Voronin1988; Esakova and Zhegallo, Reference Esakova and Zhegallo1996; Demidenko et al., Reference Demidenko, Zhegallo, Parkhaev and Shuvalova2003; Malakhovskaya, Reference Malakhovskaya2014), and some trilobite faunas were described from the Lake, Ider, and Khuvsgul regions (Dumicz et al., Reference Dumicz, Tomczykowa and Wójcik1970; Blagonravov et al., Reference Blagonravov, Zaytsev, Korobov and Pokrovskaya1971; Korobov, Reference Korobov1980, Reference Korobov1989; Korovnikov and Lazarev, Reference Korovnikov and Lazarev2021). The youngest of these assemblages is represented almost exclusively by paradoxidid trilobites of the Khovd Aimak (region), the Mongolian Altay and assigned to the former middle Cambrian Amgan Stage of Siberia (Dumicz et al., Reference Dumicz, Tomczykowa and Wójcik1970). They occur in a succession of mostly siliciclastic (sandstone and conglomerate) and volcanic (dolerite and doleritic tuff) strata and are restricted to siliceous shales. Unfortunately, the assemblages appear to belong to a separate block that is devoid of any older or younger fossils. At present, strata bearing similar trilobites in Siberia belong either to the uppermost Cambrian Stage 4, Series 2, or to the lower Wuliuan Stage, Miaolingian Series (Geyer, Reference Geyer2019). In addition, an undescribed oryctocephalid assemblage from the top of the Udzhigin-Gol Formation on the Khuvsgul microcontinent (Korobov, Reference Korobov1980) may correspond to the uppermost part of the lower Cambrian. It is possible that an extreme rarity of trilobites younger than the early Cambrian Stage 4 in Mongolian terranes was related to intense volcanic activity (Fig. 1.4).
Here we report a new uppermost Cambrian Series 2 trilobite assemblage from Seer Ridge (northwestern Mongolia) that occurs in a continuous fossiliferous Cambrian Series 2 succession of the Lake Zone and is the youngest rich Cambrian trilobite assemblage in Mongolia. The assemblage includes 13 genera of nine families. Additionally, this assemblage provides new data on the paleobiogeographic affinities of Cambrian faunas of Mongolia.
Geological setting
The section, known as the Seer Southern Reef, lies on the northeast slope of the Seer Ridge, the northern shore of Khar-Us Lake in the Great Lake Depression, western Mongolia (Drozdova, Reference Drozdova1980; Fig. 1.2). The basal lower Cambrian volcanic Tsol-Ula Formation and the lower, siliciclastic-carbonate subformation of the Burgasutay Formation are not present here. The section exposes the upper Burgasutay Formation, which only consists of five principal members delimited by a fault from below (Fig. 1.3, 1.4).
The lower member (~50 m thick) is composed of dark green doleritic porphyry. It is succeeded by a conformable contact with a thick (~500–600 m) second member, above which is a unit composed of alternating green doleritic porphyry, tuffs, and fine- to medium-grained tuffstone and gray siliceous siltstone. The third member (15–20 m thick) overlies the volcanic layers with an erosive contact and is represented by dark-gray lenticular calcareous mudstone, sandy calcareous mudstone, and wackestone with thin tuffaceous interbeds.
The fourth member (~200 m thick) lies conformably on red and brown fine- to medium-grained sandstone and siltstone. It is entirely represented by white archaeocyathan-calcimicrobial reefal limestone. Many of archaeocyaths occur in life position and are covered by abundant marine synsedimentary fibrous cement, which displays a microwaved texture (up to 0.5 m thick) and rare calcimicrobial crusts to form an in-situ framework (mostly cementstone).
The fifth, upper, member (>500 m thick) lies atop the reef with a slightly erosional concordant contact. This member consists of a frequent rhythmic alternation of blue calcareous mudstone, gray gravelstone with quartz gravel, carbonate intraclasts, and calcareous cement, green to blueish-black shale, fine- to coarse-grained sandstone, and black doleritic tuffstone. Each of these layers is 0.05–0.2 m thick. Sandstone and siltstone interlayers bear abundant trilobite cranidia, pygidia, and even intact carapaces restricted to the bedding surfaces. The trilobite fossils are almost exclusively oriented convex side up. These skeletal remains stand out from the host rock as bright yellow and orange colors and most are concentrated in the lower 10 m of the member, which consists of intraclast-bearing sandy siltstone and black or green shale intercalated with thin limestone layers (Fig. 1.3, 1.4).
Age and paleobiogeographic affinities
Previously discovered trilobite faunas of the Lake Zone were assigned to earlier middle Atdabanian–early Toyonian or Cambrian Stage 3–lower Stage 4 intervals (Korobov, Reference Korobov1980, Reference Korobov1989). The two youngest, lower Toyonian, trilobite assemblages or faunal beds were described by Korobov (Reference Korobov1980, Reference Korobov1989), namely, the Kooteniella ventricosa–Chilometopus–Solontzella bed in the upper Ak-Bashi Formation of the nearby Ak-Bashi Island, and the Laminurus planus–Kootenina and Edelsteinaspis–Kooteniella ventricosa beds in the middle Burgasutay Formation of Seer Ridge. These faunal beds yielded trilobites of the genera Alokistocare, Chilometopus, Chondragraulos, Edelsteinaspis, Erkelina, Kootenia, Kootenina (=Olenoides), Kooteniella, Laminurus, Pegetides, Piriforma (=Dinesus), and Solontzella. An approximately coeval trilobite assemblage from the Khuvsgul terrane (Udzhigin-Gol Formation) also was described (Korobov, Reference Korobov1989; Korovnikov and Lazarev, Reference Korovnikov and Lazarev2021). In this area, trilobites are mostly represented by the genera Redlichia, Inouyina, Edelsteinaspis, Dinesus, Kootenia, Lermontoviella, Parapoulsenia, Chondragraulos, and Onchocephalina. This level is roughly correlated with the middle Cambrian Stage 4.
The new trilobite assemblage from Seer Ridge is found in the uppermost Burgasutay Formation and includes the genera Amecephalus, Catinouyia, Chondragraulos, Dinesus, Eoptychoparia, Kootenia, Ogygopsis, Olenoides, Pagetides, and Proerbia, as well as uncertain antagmid, dorypygid, and weymouthiid genera. Because the index middle–late Toyonian trilobite genus Edelsteinaspis occurs in the Edelsteinaspis–Kooteniella ventricosa bed of the middle Burgasutay Formation (Korobov, Reference Korobov1989), the new assemblage in the upper part of this formation is thought to be younger than the Toyonian and rather represents the lower Amgan. Among fossils only assigned to genus level (Dinesus, Eoptychoparia, Kootenia, Ogygopsis, Olenoides, Pagetides, Proerbia), only Proerbia has a range within Stage 4, with the other genera abundant in both Wuliuan and the Stage 4. Among fossils assigned to species level, Chondragraulos minussensis Lermontova, Reference Lermontova and Vologdin1940, has a range entirely restricted to the upper Stage 4, and Amecephalus laticaudum (Resser, Reference Resser1939a) can be extended to the Wuliuan (Chernysheva Reference Chernysheva1961; Egorova and Savitsky, Reference Egorova and Savitsky1969; Egorova et al., Reference Egorova, Shabanov, Rozanov, Savitsky, Chernysheva and Shishkin1976; Korovnikov and Shabanov, Reference Korovnikov and Shabanov2016; Pegel et al., Reference Pegel, Egorova, Shabanov, Korovnikov and Luchinina2016, and other references in systematic paleontology). Therefore, all these taxa are known to range into the upper Stage 4 (Lermontovia grandis Zone to Ovatoryctocara-Schistocephalus [=Enixus] Zone), and it would seem more reasonable on the balance of evidence to suggest a Stage 4 age assignment. However, given that most of the specimens are only preliminarily identified, we cannot rule out that this assemblage may have a younger age (Wuliuan) than we suspected now.
With a high diversity of dorypygids and the presence of Amecephalus, Chondragraulos, Dinesus, and Proerbia, the early Amgan fauna of the Lake Zone shows a close similarity to the Agata Horizon of the Altay-Sayan Foldbelt (e.g., Pokrovskaya, Reference Pokrovskaya1959; Repina et al., Reference Repina, Khomentovsky, Zhuravleva and Rozanov1964; Repina and Romanenko, Reference Repina and Romanenko1978; Repina, Reference Repina and Zhuravleva1980; Astashkin et al., Reference Astashkin, Belyaeva, Esakova, Osadchaya, Pakhomov, Pegel’, Repina, Rozanov and Zhuravlev1995), as well as to the Ovatoryctocara-Enixus Zone of the open marine (eastern) basin on the Siberian Platform (e.g., Chernysheva, Reference Chernysheva1961; Khomentovsky and Repina, Reference Khomentovsky and Repina1965; Egorova et al., Reference Egorova, Shabanov, Rozanov, Savitsky, Chernysheva and Shishkin1976; Pegel, Reference Pegel2000). Although the majority of genera and species occur in a Siberian provenance (Siberian Platform and Altay-Sayan Foldbelt), the Lake Zone assemblage has its own biofacies identity. While some lower Amgan index trilobites such as Chondranomocare, Pseudanomocarina, and Schistocephalus (=Enixus) are fairly common in Siberia, none of them appears in the Lake Zone. Instead, the local assemblage includes inouyiids, which represent a group previously found only in East Gondwana (North and South China, Indian Himalaya) (Peng et al., Reference Peng, Hughes, Heim, Sell, Zhu, Myrow and Parcha2009; Yuan et al., Reference Yuan, Li, Mu, Lin and Zhu2012, Reference Yuan, Zhu and Zhang2016). This means that the Lake Zone volcanic arc was still under influence of faunal migrations from East Gondwana, even at the end of the early Cambrian. Extreme rarity of trilobites younger than the middle Cambrian Stage 4 (Wuliuan and upper stages) in Mongolian terranes possibly was related to intense volcanic activity (Fig. 1.4). Trilobite records of the early Wuliuan Stage are limited to an Eccaparadoxides assemblage from the western accretionary wedge of the Lake Zone (Dumicz et al., Reference Dumicz, Tomczykowa and Wójcik1970) and an undescribed oryctocephalid assemblage from the top of the Udzhigin-Gol Formation in Khuvsgul microcontinent (Korobov, Reference Korobov1980).
Paleoecology
The Seer Sothern Reef locality is an interesting example of a large archaeocyathan reef lacking typical kalyptrate structures (Rowland and Shapiro, Reference Rowland, Shapiro, Kiessling, Flügel and Golonka2002). No distinct smaller single or stacked mound-like buildups that are similar to those in Siberia, Australia, or Laurentia (James and Kobluk, Reference James and Kobluk1978; James and Gravestock, Reference James and Gravestock1990; Kruse et al., Reference Kruse, Zhuravlev and James1995) are recognized in the entire mound. Another specific feature of the Seer Reef is its principal composition of solitary archaeocyaths and synsedimentory fibrous calcite cement, enveloping their cups, with subdued calcimicrobal patches. Mud fillings are restricted to the reef top and some areas of lava breccia development. Such composition indicates higher-energy conditions (James and Gravestock, Reference James and Gravestock1990; Kruse et al., Reference Kruse, Zhuravlev and James1995).
Another important factor that shaped the sedimentological and ecological composition of the reefs could have been a tectonically active regime, which led to a relatively rapid sinking of the entire island area with reefs. The presence of contemporary lava breccias bearing reefal fragments and covered with younger reefal strata shows that the reef formed on an active island arc. Under such conditions, the reefal community survived by rapid growth in order to escape submergence to the depths. The reef probably was initiated during a time when volcanic activity ceased, which allowed localized cementation and stabilization of seafloor sediments favoring archaeocyathan larval settlement.
Muds are restricted mostly to the top of the buildup, which indicates that reef archaeocyath growth rates had slowed late in the reef development. Presumably, the island arc drowned and finally became a site of massive volcanic-siliciclastic accumulation, while the reefal communities vanished. A new muddy environment, probably enriched by particulate organic matter due to nutrient-rich volcanic dust fertilization, became a site for the proliferation of an abundant and diverse trilobite fauna.
Materials and methods
All specimens described in this paper were collected from the lower Cambrian Burgasutay Formation along the Seer Ridge section, Khovd region, western Mongolia (Fig. 1.2), and are housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, China (NIGP 200755–200780). Light photographs were taken using a Nikon D810 camera fitted with a Nikon AF-S Nikkor 105 mm lens. Images were processed using Adobe Photoshop to adjust tone, contrast, and brightness. The morphological terminology employed here follows that of Whittington et al. (Reference Whittington, Chatterton, Speyer, Fortey, Owens and Kaesler1997), and the systematic framework is based on Jell and Adrain (Reference Jell and Adrain2002). Measurements were made parallel to and normal to the sagittal line, the directions of which are referred to as sagittal (sag.)/exsagittal (exs.) and transverse (tr.), respectively. The abbreviations for lateral glabellar lobes from posterior to anterior are LO–L4, and lateral glabellar furrows are SO–S4.
Repositories and institutional abbreviations
The following abbreviations of repositories are used below: Central Scientific Research Geological Exploration Museum (Chernyshev Museum), St. Petersburg, Russia (CNIGR); Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China (NIGP); Borissiak Palaeontological Institute, Russian Academy of Sciences, Moscow, Russia (PIN); United States National Museum, Washington, D.C., USA (USNM).
Systematic paleontology
Class Trilobita Walch, Reference Walch1771
Order Eodiscida Kobayashi, Reference Kobayashi1939
Superfamily Eodiscoidea Raymond, Reference Raymond1913
Family Eodiscidae Raymond, Reference Raymond1913
Genus Pagetides Rasetti, Reference Rasetti1945
Type species
Pagetides elegans Rasetti, Reference Rasetti1945, Cambrian, Series 2, Ville Guay Conglomerate, Québec, Canada.
Pagetides cf. P. conicus Korobov, Reference Korobov1989
Figure 2.1–2.10
- cf. Reference Korobov1989
Pagetides conicus Korobov, p. 57–59, pl. I, figs. 10–12.
Figure 2. Eodiscinid trilobites from the uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia. (1–10) Pagetides cf. P. conicus Korobov, Reference Korobov1989. (1–4, 9) Cranidia, (1) NIGP 200755a, (2) 200756a, (3) 200756b, (4) 200777, (9) 200774a; (5–8, 10) pygidia, (5) NIGP 200759a, (6) 200755b, (7) 200756c, (8) 200774b, (10) 200774c. (11, 12) Weymouthiid gen. and sp. indet. pygidia, NIGP 200758 and 200774d, respectively. Scale bars = 1 mm.
Holotype
Cranidium PIN 4726/10 (Korobov, Reference Korobov1989, pl. XIII, fig. 10), Cambrian Series 2, lower Toyonian, Laminurus planus-Kootenina bed, Burgasutay Formation, the Seer Ridge, western Mongolia.
Occurrence
Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia.
Material
Five cranidia and five pygidia (NIGP 200755a, 200755b, 200756a–c, 200759a, 200774a–c, 200777) from the Seer Ridge, western Mongolia.
Remarks
The Mongolian specimens have a well-developed occipital furrow but no basal glabellar and pygidium axial spines, hence closely resemble Pagetides conicus Korobov, Reference Korobov1989, from the same site. The narrower pygidial axis was probably preserved as an internal mold, which distinguishes the new specimen from typical specimens. Moreover, some Mongolian pygidia show more obvious pleural furrows, suggesting that expression of the pygidial pleural furrows varies from individual to individual within the species, as previously recognized in other eodiscids (Blaker and Peel, Reference Blaker and Peel1997; Geyer and Peel, Reference Geyer and Peel2011). Because of the different state of preservation between compacted and noncompacted specimens, it is appropriate to assign these specimens to Pagetides conicus. Some Macannaia (Jell, Reference Jell1975) from Siberia, such as M. sibiricus (Lazarenko, Reference Lazarenko1959) and M. spinosus (Lazarenko, Reference Lazarenko1959), also resemble Mongolian specimens in having an occipital furrow and short occipital spines. However, the pygidium axis of Macannaia has a teardrop-shaped back and hangs over the marginal border of the pygidium (Lazarenko, Reference Lazarenko1959; Rasetti, Reference Rasetti1966; Jell, Reference Jell1975).
Family Weymouthiidae Kobayashi, Reference Kobayashi1943
Weymouthiid gen. and sp. indet.
Figure 2.11, 2.12
Material
Two pygidia (NIGP 200758, 200774d) from the Seer Ridge, western Mongolia.
Remarks
The Mongolian specimens are similar to Cobboldites Kobayashi, Reference Kobayashi1943 (possibly, including Litometopus Rasetti, Reference Rasetti1966) in having effaced axial rings and pleural furrows, and a deep border furrow. However, the Mongolian pygidia have a conical axis, which rather is a feature of Runcinodiscus Rushton in Bassett et al., Reference Bassett, Owens and Rushton1976, and Morocconus (Geyer, Reference Geyer1988). Notably, the Mongolian specimens also show a spine at the pygidial terminus. Therefore, these specimens may represent a new genus if a better material becomes available.
?Order Corynexochida Kobayashi, Reference Kobayashi1935
Family Dorypygidae Kobayashi, Reference Kobayashi1935
Genus Kootenia Walcott, Reference Walcott1889
Type species
Bathyuriscus (Kootenia) dawsoni Walcott, Reference Walcott1889, Cambrian, Miaolingian, Wuliuan, Stephen Formation, British Columbia, Canada.
Kootenia spp.
Figure 3
Description
Subrectangular glabella moderately convex, slightly expanding forward and well curved anteriorly; lateral glabellar furrows deep, reaching the anterior border furrow; occipital furrow well defined, other glabellar furrows only faint indications. Anterior border narrow (sag.) and convex, anterior area reduced to narrow depression between border and eye ridge. Palpebral area of the fixigena moderate broad, ~50% of the cranidial width across midlength. Palpebral lobes gently arcuate, defined by shallow furrow, centered at about glabellar midlength, length ~20% of the glabellar (sag.). Eye ridge short and faint. Posterolateral projections short (exs.) and narrow (tr.), subtriangular in outline. Pygidium semicircular except for marginal spines, length 5.6–25 mm (sag., n = 6), ~53–63% of the width (tr.). Convex axis moderately tapering, extending to posterior margin, with four rings and a terminal piece; first three inter-ring furrows clearly defined, and the terminal one shallow and poorly defined. Pleural fields with four pairs of pleural furrows and three weak interpleural furrows; furrows of small specimens nearly uniform in width and depth, shallowing posteriorly in large specimens. Wide border with shallow furrow and five pairs of short to moderately long marginal spines; slender spines nearly uniform in length and subequally spaced; the tip of third spine does not extend beyond the smooth terminus of the pygidium. Surface smooth and without ornament.
Figure 3. Kootenia spp. from the uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia. (1, 2) Cranidia NIGP 200762 and 200759b, respectively; (3–9) pygidia, (3) NIGP 200778 showing post-depositional deformation with inferred strain ellipse, (4) 200764, (5) 200765a, (6) 200756d, (7) 200774e, (8) 200757a, (9) 200763. All scale bars = 5 mm.
Material
Two cranidia and seven pygidia (NIGP 200756d, 200757a, 200759b, 200762, 200763, 200764, 200765a, 200774e, and 200778) from the Seer Ridge, western Mongolia.
Remarks
The key characteristics of Kootenia from the Seer Ridge are: (1) pygidial axis displaying four rings and terminus; and (2) five pairs of pygidial spines, short to moderately long, the tip of third spine does not extend beyond the smooth terminus of the pygidium. Korobov (Reference Korobov1989) described K. hirsuta Suvorova, Reference Suvorova1964, K. rotundata Rasetti, Reference Rasetti1948, K. tersa Ergaliev in Ergaliev and Pokrovskaya, Reference Ergaliev and Pokrovskaya1977, and a new species, K. lata Korobov, Reference Korobov1989, from the Seer Ridge. However, most of these species were identified by cranidia, and only one pygidium possessing four pairs of spines was assigned to K. hirsuta. Therefore, the new specimen does not show similarity with known species of Kootenia from the same site. More than 30 species of Kootenia from the lower–middle Cambrian boundary interval have been described in Siberia and adjacent areas (see list in Yuan et al., Reference Yuan, Zhu and Zhang2016), of which six species—K. abacanica (Poletaeva, Reference Poletaeva, Usov and Vasil'ev1936), K. siberica Lermontova, Reference Lermontova and Vologdin1940, K. florens Suvorova, Reference Suvorova1964, K. rasilis Suvorova, Reference Suvorova1964, K. mirabile Ergaliev in Ergaliev and Pokrovskaya, Reference Ergaliev and Pokrovskaya1977, and K. tersa Ergaliev in Ergaliev and Pokrovskaya, Reference Ergaliev and Pokrovskaya1977—have five pairs of pygidial spines. Kootenia siberica and K. florens are similar to Mongolian Kootenia in the length of pygidial spines, but these specimens differ from Mongolian ones in their longer occipital spine and tuberculate cranidia. Given that >100 species of Kootenia have been named (Sundberg, Reference Sundberg1994; Yuan et al., Reference Yuan, Zhu and Zhang2016), this group needs a proper revision, and Kootenia species from the Seer Ridge are left in open nomenclature.
Ogygopsis Walcott, Reference Walcott1889
Type species
Ogygia klotzi Rominger, Reference Rominger1887, Cambrian, Miaolingian, Wuliuan, Stephen Formation, British Columbia, Canada.
Ogygopsis cf. O. virgata (E. Romanenko in Romanenko and Romanenko, Reference Romanenko and Romanenko1962)
Figure 4.5, 4.6
- cf. Reference Romanenko and Romanenko1962
Kootenia virgata E. Romanenko in Romanenko and Romanenko, p. 19, pl. 1, figs. 9–11.
- cf. Reference Blaker and Peel1997
Ogygopsis virgata; Blaker and Peel, p. 80, figs. 46–48 (see for synonymy).
- cf. Reference Geyer and Peel2011
Ogygopsis virgata; Geyer and Peel, p. 477, fig. 9C–H.
Figure 4. Dorypygid trilobites from the uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia. (1) Cranidium of Olenoides sp. indet, NIGP 200760. (2, 3) Cranidia of dorypygid gen. and sp. indet., NIGP 200765b and 200766, respectively. (4, 7, 8) Ogygopsis sp. indet. (4) nearly complete exoskeleton NIGP 200767; (7, 8) pygidia NIGP 200769 and 200771, respectively. (5, 6) Pygidia of Ogygopsis cf. O. virgata (E. Romanenko in Romanenko and Romanenko, Reference Romanenko and Romanenko1962), NIGP 200768 and 200770, respectively. Scale bars = 5 mm in (1–3, 5–8); 10 mm in (4).
Holotype
Originally assigned holotype specimen has not been traced.
Material
Two nearly complete pygidia (NIGP 200768, 200770) from the Seer Ridge, western Mongolia.
Remarks
The Mongolian pygidia show six pairs of wide pleural furrows, six axial rings, and two pairs of marginal spines, similar to O. virgata (E. Romanenko in Romanenko and Romanenko, Reference Romanenko and Romanenko1962) by Blaker and Peel (Reference Blaker and Peel1997), and distinct from other species of this genus. The other specimen with similar furrows and axial rings has anterior marginal spines poorly preserved, but its undulated posterior margin is reminiscent of O. virgata. The Mongolian material appears to lack the distinct interpleural furrows characteristic of the species, therefore we only make a tentative comparison and do not make a definitive assignment.
Ogygopsis sp. indet.
Figure 4.4, 4.7, 4.8
Material
Two incomplete pygidia and a nearly complete exoskeleton (NIGP 200767, 200769, 200771) from the Seer Ridge, western Mongolia.
Remarks
These specimens have smooth margins without spines and six pairs of pleural furrows, which is reminiscent of Ogygopsis batis (Walcott, Reference Walcott1916). However, except for the former two traits, another important character, the numbers of axial rings, cannot be confirmed, and we suggest open nonculture for the Mongolia specimens.
Olenoides Meek, Reference Meek1877
Type species
Paradoxides? nevadensis Meek, Reference Meek1877, Cambrian, Miaolingian, Drumian, Wheeler Formation, Utah, USA.
Olenoides sp. indet.
Figure 4.1
Material
A cranidium (NIGP 200760) from the Seer Ridge, western Mongolia.
Remarks
The single Mongolian dorypygid cranidium is characterized by a glabella with well-expressed lateral glabellar furrows. These features align the cranidium with Olenoides. However, because >80 species of Olenoides are identified primarily based on pygidial features (Yuan et al., Reference Yuan, Zhao, Li and Huang2002), we prefer to place the new specimen under a more open nonculture, and we tentatively assign it as Olenoides sp. indet.
Dorypygid gen. and sp. indet.
Figure 4.2, 4.3
Material
Two cranidia (NIGP 200765b, 200766) from Seer Ridge, western Mongolia.
Remarks
The smooth and broadly cylindrical glabella and extremely narrow preglabellar field suggest these specimens should be assigned to the Dorypygidae rather than another group. The broad swollen glabella is reminiscent of Kooteniella Lermontova, Reference Lermontova and Vologdin1940, but this genus has an almost spherical glabella with the highest point and widest measure. The lack of a pygidium also makes specific comparison difficult, thus the taxonomic placement is left open.
Family Dinesidae Lermontova, Reference Lermontova and Vologdin1940
Proerbia Lermontova, Reference Lermontova and Vologdin1940
Type species
Proerbia prisca Lermontova, Reference Lermontova and Vologdin1940, Cambrian Series 2, Stage 4, Kutorgina Formation, Siberian Platform, Russia.
Proerbia sp.
Figure 5.1, 5.2
Material
Two incomplete cranidia (NIGP 200761, 200772b) from Seer Ridge, western Mongolia.
Figure 5. Cranidia of Dinesid and ‘Pychopariid’ trilobites from the uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia. (1, 2) Proerbia sp., NIGP 200761 and 200772b, respectively. (3) Dinesus sp. indet., NIGP 200776. (4–7) Catinouyia heyunensis n. sp. (4) Counterpart of paratype NIGP 200779; (5) paratype NIGP 200772a; (6) holotype NIGP 200773; (7) counterpart of paratype NIGP 200755d. (8, 9) Amecephalus laticaudum (Resser, Reference Resser1939a), NIGP 200762b and 200762c, respectively. Scale bars = 2 mm in (1–3, 8, 9); 5 mm in (4–7).
Remarks
Cylindrical glabella with four well-expressed lateral furrows and wide preglabelar area with three swellings allow identification of this material as Proerbia. The Mongolian specimens differ from other Proerbia species in having a convex palpebral area obviously wider than the glabella, effaced eye ridges, and upturned palpebral lobes. This Mongolian Proerbia sp. is somewhat similar to P. angarensis Dalmatov in Yazmir et al., Reference Yazmir, Dalmatov and Yazmir1975, from the Ogne Formation (Maolingian, Wuliuan) of western Transbaikalia, which differs in having flat palpebral areas and a long occipital spine. The wide and convex palpebral areas suggests that Proerbia sp. possibly represents a new species. However, it would be premature to establish a new species until better specimens are found.
Dinesus Etheridge, Reference Etheridge1896
Type species
Dinesus ida Etheridge, Reference Etheridge1896, Cambrian, Miaolingian, Knowsley East Shale, Victoria, Australia.
Dinesus sp. indet.
Figure 5.3
Material
Single cranidium (NIGP 200776) from the Seer Ridge, western Mongolia.
Remarks
Axial furrows branching forward adjacent to the anterior part of the glabella, narrow or almost vanishing preglabellar area, effaced eye ridge, isolated triangular lobes adjacent to the anterior end of the glabella, and the short palpebral lobe situated anterior to the mid-length of the cranidium support the placement of this single incomplete cranidium in Dinesus. However, due to the distortion of preglabellar area and the damage of palpebral lobe, it is difficult to assign this specimen to any species.
Order Unknown
Family Inouyiidae Zhang, Reference Zhang1963
Remarks
The faint or effaced border and furrow, wide preglabellar area with a periclinal swelling in front of the glabella, and posterior border and border furrow bending backward opposite to the posterior end of palpebral lobe (Zhang, Reference Zhang1963; Yuan et al., Reference Yuan, Zhu and Zhang2016) are diagnostic of the Inouyiidae Zhang, Reference Zhang1963. The Bolaspididae Howell in Harrington et al., Reference Harrington, Henningsmoen, Howell, Jaanusson, Lochman-Balk and Moore1959, from the Miaolingian of Laurentia are also characterized by a swelling on the preglabellar field, but the inouyiids have a pair of wide shallow oblique furrows in the preglabellar field starting from the anterior lateral corner of the glabella. Currently, 13 genera are assigned to the Inouyiidae (Jell and Adrain, Reference Jell and Adrain2002; Yuan et al., Reference Yuan, Li, Mu, Lin and Zhu2012, Reference Yuan, Zhu and Zhang2016), all of which are restricted to Cambrian strata of East Gondwana.
Catinouyia Zhang and Yuan, Reference Zhang and Yuan1981
Type species
Catinouyia typica Zhang and Yuan, Reference Zhang and Yuan1981, Cambrian, Miaolingian, Wuliuan, Kochaspis-Ruichengella (=Asteromajia hsuchuangensis) Zone, Hulusitai Formation, North China.
Remarks
Catinouyia is considered to be a junior synonym of Inouyia Walcott, Reference Walcott1911, by Peng (Reference Peng2021) because the principal difference between these two genera is solely the anterior margin. However, the anterior margin has always been regarded as an important character to distinguish genera within Inouyiidae (Yuan et al., Reference Yuan, Li, Mu, Lin and Zhu2012, Reference Yuan, Zhu and Zhang2016), so we consider that the genus name Catinouyia is still valid. Except for the type species, two inouyiid species, Catinouyia jiawangensis Qiu et al., Reference Qiu, Lu, Zhu, Bi and Lin1983, and C. dasonglinensis Yuan and Gao in Yuan et al., Reference Yuan, Zhu and Zhang2016, with convex and narrow anterior border, also have been assigned to Catinouyia.
Catinouyia heyunensis Sun, Yang, and Zhao, new species
Figure 5.4–5.7
Holotype
Cranidium (NIGP 200773) from uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia.
Paratypes
Three cranidia (NIGP 200755d, 200772a, 200779) from the type locality and stratum.
Diagnosis
An inouyiid trilobite with rectangle cranidium, broad (tr.) frontal area, convex and narrow anterior border, straight border furrow, broad (tr.) palpebral area and straight eye ridges.
Description
Cranidium obviously broad (tr.), moderately convex, rectangular in outline; length (tr.) 11.1–12.3 mm, ~60–65% of the width at the palpebral lobes. Glabella moderately convex, truncated conical, occupying ~60% of total cranidial length and ~27% of total cranidial width; three pairs of lateral glabellar furrows (SO–S2) present, moderately deep, adjacent slightly to the axial furrow, progressively less well expressed from SO to S2; SO (occipital furrow) consisting of a pair of well-developed lateral sections and a median section slightly bending forward medially; S1 long, shallow and wide, directed backward from axial furrows and curved; S2 short, very shallow or indistinct; occipital ring convex, narrowing distally, with a short (sag.) occipital spine posteromedially; axial furrow moderately deep; preglabellar furrow deep, wide, and straight, separated from the axial furrow by eye ridge. Preglabellar area long, wide, convex, nearly rectangular in outline, ~6 times as wide as anterior border, with faint caeca; two wide shallow furrows extending from anterior corner of glabella, separating a low subrounded swelling from preocular field; straight anterior border furrow well defined, narrow, and deep; anterior border narrow and gently convex. Palpebral area of the fixigena fairly broad, transversely ~70% the cranidial width across S1; strongly convex, highest near palpebral lobe, slightly sloping down towards axial furrow. Palpebral lobes crescent shaped, upturned, clearly convex; anterior end located about opposite S2, posterior tips located about opposite midpoint of L1. Eye ridges elevated, well developed, straight, moderately posteriorly directed from glabella, nearly horizontal from anterior corner of glabella. Posterior border furrow wide and deep, widening outward, posterior border narrow and convex; posterior border and border furrow bending backward opposite to posterior end of palpebral lobe. Anterior branch of facial suture convergent from palpebral lobe; posterior branch short, extending outwards and backwards.
Etymology
From the Chinese pinyin ‘Heyun’, the ancient Chinese word for the Lake Zone.
Remarks
Catinouyia heyunensis n. sp. is distinguished from C. typica Zhang and Yuan, Reference Zhang and Yuan1981, and C. jiawangensis Qiu et al., Reference Qiu, Lu, Zhu, Bi and Lin1983, by a wider preglabellar field, narrower anterior border, and longer (tr.) eye ridges. Catinouyia dasonglinensis Yuan and Gao in Yuan et al., Reference Yuan, Zhu and Zhang2016, differs from Catinouyia heyunensis n. sp. in having a wider glabella with less-forward taper, narrower palpebral areas, and eye ridge directed slightly more posterolaterally.
The paleogeographic distribution of inouyiids has been confined to North China, the Yangtze Platform, and Indian Himalaya (Peng et al., Reference Peng, Hughes, Heim, Sell, Zhu, Myrow and Parcha2009; Yuan et al., Reference Yuan, Li, Mu, Lin and Zhu2012, Reference Yuan, Zhu and Zhang2016), therefore, the discovery of inouyiids in Mongolia reveals a wider distribution of this family.
Family Alokistocaridae Resser, Reference Resser1939
Amecephalus Walcott, Reference Walcott1924
Type species
Ptychoparia piochensis Walcott, Reference Walcott1886, Cambrian, Miaolingian, Wuliuan, Chisholm Formation, Nevada, USA.
Amecephalus laticaudum (Resser, Reference Resser1939)
Figures 5.8, 5.9, 6.1
- Reference Resser1939a
Alokistocare laticaudum Resser, p. 17, pl. 4, figs. 15–19.
- part Reference Resser1939b
Alokistocare euchare Resser, p. 51, pl. 2, figs. 11, 12.
- part Reference Resser1939b
Poulsenia granosa Resser, p. 59, pl. 13, figs. 20, 21.
- Reference Lazarenko and Shvedov1962
Alokistocare faceta Lazarenko, p. 66, pl. VIII, figs. 12, 13.
- Reference Fritz1968
Amecephalus laticaudum; Fritz, p. 227, pl. 40, figs. 17–23.
- Reference Egorova and Savitsky1969
Alokistocare faceta; Egorova and Savitsky, p. 239, pl. 43, fig. 5.
- Reference Egorova and Savitsky1969
Alokistocare laticaudum; Egorova and Savitsky, p. 241, pl. 43, fig. 4.
- Reference Repina, Lazarenko, Meshkova, Korshunov, Nikiforov and Aksarina1974
Alokistocare faceta; Repina et al., p. 175, pl. L, figs. 3, 4.
- Reference Egorova, Shabanov, Rozanov, Savitsky, Chernysheva and Shishkin1976
Alokistocare laticaudum; Egorova et al., p. 128, pl. 11, fig. 21; pl. 16, figs. 1–3; pl. 18, figs. 10, 11; pl. 22, fig. 2.
- Reference Sundberg2005
Amecephalus laticaudum; Sundberg, Reference Sundberg2005, fig. 6.6, 6.12.
- Reference Foster2011
Amecephalus laticaudum; Foster, fig. 7.4–7.7.
- Reference Pegel, Egorova, Shabanov, Korovnikov and Luchinina2016
Alokistocare laticaudum; Pegel et al., p. 126, pl. 27, figs. 1, 1a, 2.
Figure 6. Cranidia of ‘Pychopariid’ trilobites from the uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia. (1) Amecephalus laticaudum (Resser, Reference Resser1939a), NIGP 200774g; (2) Chondragraulos (Chondragraulos) minussensis Lermontova, Reference Lermontova and Vologdin1940, NIGP 200775; (3) antagmid gen. and sp. indet. NIGP 200757b; (4–7), Eoptychoparia sp. indet., NIGP 200780, 200774f, 200755c, and 200757c, respectively. Scale bars = 2 mm.
Holotype
Cranidium USNM 96517 (Resser, Reference Resser1939a, pl. 4, fig. 18), Cambrian, Miaolingian, Wuliuan, Spence Shale, locality 55e, Wasatch Mountains, Utah, USA.
Occurrence
Albertella to Glossopleura zones (lower Wuliuan), Pioche and Spence shales, Chisholm Formation, Great Basin, USA; Lermontovia grandis to Ovatoryctocara-Enixus zones (upper Cambrian Stage 4), Sekten, Elanka, Udachny, and Morgunovo formations, Siberian Platform, Russia; uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia.
Material
Three cranidia (NIGP 200762b, 200762c, 200774g) from the Seer Ridge, western Mongolia.
Remarks
The cranidia from the Lake Zone are similar in size and overall morphology to those of Amecephalus laticaudum (Resser, Reference Resser1939a) from the Siberian Platform (Egorova and Savitsky, Reference Egorova and Savitsky1969; Repina et al., Reference Repina, Lazarenko, Meshkova, Korshunov, Nikiforov and Aksarina1974; Egorova et al., Reference Egorova, Shabanov, Rozanov, Savitsky, Chernysheva and Shishkin1976; Pegel et al., Reference Pegel, Egorova, Shabanov, Korovnikov and Luchinina2016). The principal difference between the Lake Zone and Siberian Platform specimens is that the Siberian Platform specimens have a more well-defined anterior border. However, this difference is the result of preservation because the Siberian cranidia are preserved primarily in carbonate lithologies while the Mongolian cranidia are flattened in siliciclastics. Siberian representatives of A. laticaudum have a poorly defined anterior border furrow, concave anterior border, and swollen frontal area that encroaches onto the anterior border furrow, which is similar to specimens of Amecephalus from Laurentia (Resser, Reference Resser1939b; Fritz, Reference Fritz1968; Sundberg, Reference Sundberg1999, Reference Sundberg2005, Reference Sundberg2020).
Family Utiidae Kobayashi, Reference Kobayashi1935
Chondragraulos Lermontova, Reference Lermontova and Vologdin1940
Subgenus Chondragraulos Lermontova, Reference Lermontova and Vologdin1940
Type species
Chondragraulos minussensis Lermontova, Reference Lermontova and Vologdin1940, Cambrian Series 2, Stage 4, Kutorgina Formation, Siberian Platform, Russia.
Chondragraulos (Chondragraulos) minussensis Lermontova, Reference Lermontova and Vologdin1940
Figure 6.2
- Reference Lermontova and Vologdin1940
Chondragraulos minussensis Lermontova, p. 143, pl. XLIV, figs. 10, 10a.
- Reference Egorova, Shabanov, Rozanov, Savitsky, Chernysheva and Shishkin1976
Chondragraulos minussensis; Egorova et al., p. 100, pl. XII, fig. 21 (see for synonymy).
- Reference Pegel, Egorova, Shabanov, Korovnikov and Luchinina2016
Chondragraulos (Chondragraulos) minussensis; Pegel et al., p. 111, pl. 23, figs. 7–10 (see for synonymy).
Lectotype
Cranidium CNIGR 9182 (Lermontova, Reference Lermontova and Vologdin1940, pl. XLIV, fig. 10), Cambrian Series 2, Stage 4, “Potekhino limestone,” Kuznetsky Alatau, Altay-Sayan Foldbelt, Russia.
Occurrence
Lermontovia grandis to Ovatoryctocara-Enixus zones (upper Cambrian Stage 4), Amga, Erkeket, Elanka, Kharatas, Kutorgina, Nouyo, Sekten, and Shumnoy formations, Siberian Platform, Russia; Kooteniella-Edelsteinaspis Zone to Mundybash Horizon (upper Cambrian Stage 4), Barangol and Karabulun formations, Altay-Sayan Foldbelt, Russia; uppermost Cambrian Stage 4, Yanguda Formation, Transbaikalia, Russia; uppermost Cambrian Stage 4, Burgasutay Formation, Lake Zone, Seer Ridge, western Mongolia.
Material
Single cranidium (NIGP 200775) from Seer Ridge, western Mongolia.
Remarks
Chondragraulos (Chondragraulos) minussensis Lermontova, Reference Lermontova and Vologdin1940, is common in upper Cambrian Stage 4 strata of the Siberian Platform, Altay-Sayan Foldbelt and Transbaikalia. Another common species of this genus, C. (C.) granulatus Chernysheva, Reference Chernysheva1961, differs from the Mongolian species by a wide, less convex cranidium, distinct lateral glabellar furrows, and a sharper tapering of the glabella towards the preglabellar furrow.
Family Antagmidae Hupé, Reference Hupé1953
Antagmid gen. and sp. indet.
Figure 6.3.
Material
Single cranidium (NIGP 200757b) from the Seer Ridge, western Mongolia.
Remarks
The convex anterior border and deep, well-defined, continuous border furrow of this cranidium are typical features of antagmids. Unlike antagmids previously reported from Siberia such as Onchocephalina Repina, Reference Repina and Khalfin1960, the Mongolian specimen has a wider preglabellar area, but the broken palpebral area of the specimen prevents a precise taxonomic determination.
Family incertae sedis
Eoptychoparia Rasetti, Reference Rasetti1955
Type species
Eoptychoparia normalis Rasetti, Reference Rasetti1955, boulders with Cambrian fossils within the Lévis Formation, near Lévis, Quebec, Canada.
Eoptychoparia sp. indet.
Figure 6.4–6.7
Material
Four cranidia (NIGP200755c, 200757c, 200774f, 200780) from the Seer Ridge, western Mongolia.
Remarks
The absence of a distinct median swelling at the anterior border and a well-enough expressed plectral swelling in front of the glabella in the Mongolian specimens suggests their assignment to Eoptychoparia Rasetti, Reference Rasetti1955, rather than to Onchocephalus Resser, Reference Resser1937. Of several Eoptychoparia species reported from Siberia and adjacent areas (Geyer and Peel, Reference Geyer and Peel2011), Mongolian specimens are most similar to E. manifesta Lazarenko, Reference Lazarenko and Shvedov1962, in having a narrower glabella with only three lateral glabellar furrows. However, the distinctly upturned and brim-like anterior border commonly shown in E. manifesta cannot be observed in the Mongolia specimens due to their poor preservation. Thus, the Mongolian specimens are not assigned to any definite species.
Acknowledgments
This research was supported by the National Key Research and Development Program of China (2022YFF0800100), the Research Funds for the Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, the Fundamental Research Funds for the Central Universities (No.0206-14380137), and the National Natural Science Foundation of China (41921002, 41872011, 42072006). We are grateful for help during fieldwork and logistics by Anaad Chimidtseren in Mongolia. We thank J-L. Yuan, J. Gao, and anonymous reviewers for constructive comments and suggestions.
Declaration of competing interests
The authors declare none.
