Introduction
The Mexican state of Sonora exposes a thick Paleozoic sedimentary basin, mostly composed of carbonate platform deposits that have been studied previously (e.g., Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952; Stewart et al., Reference Stewart, McMenamin and Morales-Ramirez1984, Reference Stewart, Amaya-Martínez, Palmer and Barth2002; Cuen-Romero et al., Reference Cuen-Romero, Valdez, Buitrón, Monreal, Sundberg, Montijo-González and Minjarez-Sosa2016, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022; Noriega-Ruiz et al., Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020). This study is based on the early–middle Cambrian Chihuarruita Hill section (San José de Gracia, east of Hermosillo area) in the central part of Sonora State (Figs. 1, 2). Nardin et al. (Reference Nardin, Almazán-Vásquez and Buitrón-Sánchez2009, and references therein) studied the fauna and stratigraphy of the Chihuarruita Hill section, which later also was studied by Cuen-Romero et al. (Reference Cuen-Romero, Valdez, Buitrón, Monreal, Sundberg, Montijo-González and Minjarez-Sosa2016, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020). The main objective of this paper is to provide the first comprehensive description of the poorly known brachiopod fauna recovered from lower–middle Cambrian limestone beds from Sonora State.
Figure 1. Geographic map of the studied area, El Chihuarruita Hill, San José de Gracia, Sonora, Mexico (modified after Cuen-Romero et al., Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022, fig.1).
Figure 2. Stratigraphic framework and section from the El Chihuarruita Hill locality, with sampled levels.
Geological setting and stratigraphy
The Mexican state of Sonora is bordered by the Gulf of California to the west, the state of Chihuahua to the east, and the south border of the United States of America (Arizona and New Mexico) to the north. The studied Chihuarruita Hill outcrop (29°17′2.10″N, 110°35′4.89″W), is located near the town of San José de Gracia, 40 km northeast of Hermosillo, the capital of Sonora (Fig. 1). Cuen-Romero et al. (Reference Cuen-Romero, Valdez, Buitrón, Monreal, Sundberg, Montijo-González and Minjarez-Sosa2016, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020) provided more detailed comprehensive reports on the geological setting and stratigraphy in the general area.
The stratigraphy described in this report is based on fieldwork on the lower–middle Cambrian, Chihuarruita Hill section in the San José de Gracia area, led by Sébastien Clausen and Juan José Palafox Reyes, Jesus Porifirio Sosa Leon, and Blanca Buitrón-Sánchez. The studied section extends approximately from 29°17′02″N, 110°35′00″W to 29°17′00″N, 110°34′5″W. The same area was studied previously by Nardin et al. (Reference Nardin, Almazán-Vásquez and Buitrón-Sánchez2009), and more recently by Cuen-Romero et al. (Reference Cuen-Romero, Valdez, Buitrón, Monreal, Sundberg, Montijo-González and Minjarez-Sosa2016, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020). Brachiopods were recovered from the Buelna and El Gavilán formations in the Chihuarruita Hill section (Figs. 1, 2; Table 1).
Table 1. Distribution of brachiopods in the sampled levels (see Fig. 2) from the Buelna and El Gavilán formations at the Chihuarruita Hill outcrop, northwest Sonora State, Mexico.
Table 2. Main dimensions and ratios of ventral and dorsal valves of Batenevotreta? mexicana n. sp. from the El Gavilán Formation (sample SJG 2/9). Measurements (in μm) have been made as follows: V = ventral valve, D = dorsal valve, L = sagittal length of shell, W = maximum width of shell, La = length of ventral apical process; Lc, Wc = length and width of cardinal muscle scars; Lb = length of dorsal median buttress; Ls = length of dorsal median septum.
The Buelna Formation contains various limestones, dolomites, siliciclastic limestone, siliciclastic dolomite, and a few siltstones and sandstones mostly found in its basal part. The few productive samples (samples SJG2/2 and 2/3; Fig. 2, Table 1) come from limestone beds that are overlaid by alternating sandstones and shales towards the top of the formation. The whole Buelna Formation is ~60 m thick. The overlying Cerro Prieto Formation consists of 22.5 m of oolithic beds, sometimes interrupted by centimetric layers of micritic mudstones, but did not yield any brachiopods.
The richly fossiliferous El Gavilán Formation (samples SJG 2/6, 2/7, 2/9, 2/12, 2/13, 3/7, 3/8, 3/9; Fig. 2; Table 1) contains various lithologies, and consists of thinly bedded limestones, siliciclastic limestones, limy siltstone, and siltstone. Its thickness in Chihuarruita Hill section is ~200 m, and the formation is unconformably overlain by Tertiary volcanic deposits.
Materials and methods
Limestone samples (average weight of 1.2 kg) were collected directly from the lower–middle Cambrian limestone beds from the Chihuarruita Hill section (Figs. 1, 2; Table 1). They were broken into fragments and dissolved, either with ~10% acetic acid when dealing with limestone or with ~8% formic acid for the slightly dolomitic limestone. The acid-resistant residues were sifted (>50 μm), dried, and the microfossils were picked from the residues under a stereomicroscope. The brachiopods were coated with palladium and observed and imaged with a Scanning Electron Microscope at the University of Lille, France, and at the Evolutionary Biology Center, University of Uppsala, Sweden.
Repositories and institutional abbreviations
The described and figured new material is housed in the collections of University of Lille (USTL = Université des Sciences et Technologies de Lille) following the recommendation of the International Commission on Zoological Nomenclature. Other type material referred to is housed in the Smithsonian National Museum of Natural History, Washington DC (USNM).
Systematic paleontology
Subphylum Linguliformea Williams et al., Reference Williams, Carlson, Brunton, Holmer and Popov1996
Class Lingulata Gorjansky and Popov, Reference Gorjansky and Popov1985
Superfamily Linguloidea Menke, Reference Menke1828
Family Eoobolidae Holmer, Popov, and Wrona, Reference Holmer, Popov and Wrona1996
Genus Eoobolus Matthew, Reference Matthew1902
Type species
Obolus triparilis Matthew, Reference Matthew1902 (subsequent designation by Rowell, Reference Rowell and Moore1965, p. H263); Drumian (Eccaparadoxides eteminicus Zone), Cape Breton Island, Canada.
Eoobolus sp.
Figure 3
Occurrence in Chihuarruita Hill outcrop
Wuliuan El Gavilán Formation (samples SJG 2/7, 3/7, 3/8).
Figure 3. Eoobolus sp. from the El Gavilán Formation (sample SJG 3/7), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1) (USTL4031-1) Dorsal valve exterior; (2, 3, 7) (USTL4031-2) (2) ventral valve interior, (3) detail of ventral pseudointerarea, (7) oblique lateral view; (4, 6) (USTL4031-3) (4) detail of dorsal pseudointerarea, (6) dorsal valve interior; (5, 8–11) (USTL4031-4) (5) ventral valve exterior, (8) detail of ornamentation, (9) detail of weak pustulose ornamentation on juvenile part, (10) detail of umbo, (11) detail of adult pustulose ornamentation.
Description
Shell equibiconvex elongate, subtriangular. Ventral valve acuminate with triangular pseudointerarea bisected by narrow and deep pedicle groove. Propareas elevated above the valve floor, bisected by flexure lines running close to the smooth outer margins of proparea. Dorsal valve elongate, suboval. Dorsal interarea mainly occupied by shallow concave median grove. External surface of post-metamorphic shell finely and densely pustulose. Ventral interior with weakly impressed visceral area. Posterolateral muscle scars well defined, emerging from under raised propareas. Ventral vascula lateralia almost straight, narrowly divergent. Dorsal interior with long median ridge. Posterolateral muscle scars well defined, emerging from under raised propareas.
Figured material
USTL4031-1–4.
Remarks
The scant available material from El Gavilán Formation (Wuliuan) is fragmentary, making detailed description and taxonomic discrimination difficult. The species exhibits characteristic features of Eoobolus, including an elevated ventral pseudointerarea with both a deep pedicle groove and well-developed flexure lines (Fig. 3.2, 3.3, 3.7), as well as a divided and raised dorsal pseudointerarea (Fig. 3.4, 3.6), and, most importantly, a finely pustulose postmetamorphic shell (Fig. 3.1, 3.5, 3.8–3.11). The poorly known Lingulella proveedorensis Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, and Lingulella sp., reported by McMenamin (Reference McMenamin1984) from the somewhat older Puerto Blanco Formation (Cambrian Age 3 to 4), appear to be similar in ornamentation and outline and probably represent Eoobolus, but cannot be compared in detail based on existing information. Devaere et al. (Reference Devaere, Clausen, Sosa-Leon, Palafox-Reyes, Buitrón-Sánchez and Vachard2019, figs. 13–16) more recently described material of Eoobolus sp. from the Puerto Blanco Formation; the ventral valve and ornamentation is closely similar to the younger Eoobolus sp. and may be conspecific.
Specimens referred to Lingulella proveedorensis Cooper and Lingulella sp. (probably also representing species of Eoobolus) have been briefly listed and illustrated by Cuen-Romero et al. (Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020), Ramírez-Valenzuela (Reference Ramírez-Valenzuela2019), and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020), but remain poorly understood.
Superfamily Acrotheloidea Walcott and Schuchert in Walcott, Reference Walcott1908
Family Acrothelidae Walcott and Schuchert in Walcott, Reference Walcott1908
Subfamily Acrothelinae Walcott and Schuchert in Walcott, Reference Walcott1908
Genus Acrothele Linnarsson, Reference Linnarsson1876
Type species
Acrothele coriacea Linnarsson, Reference Linnarsson1876 (subsequent designation by Oehlert, Reference Oehlert and Fischer1887, p. 1279); Cambrian (Miaolingian), Sweden.
Acrothele concava Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Figure 4
- Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Acrothele concava Cooper in Cooper et al., p. 44, pl. 12A, figs. 1–6.
Holotype
USNM116035d, a dorsal valve (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, pl. 12A, fig. 6) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.
Diagnosis
See Cooper in Cooper et al. (Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, p. 44).
Occurrence in Chihuarruita Hill outcrop
Wuliuan El Gavilán Formation (samples SJG 2/9, 2/13, 3/8).
Description
Shell ventribiconvex, transversely oval.
Ventral valve convex; subconical in lateral view with rounded margin, and with umbo at maximum height; well-defined sulcus on adult shell (Fig. 4.5). Ornamentation of fine concentric fila and knob-shaped pustules (Fig. 4.1–4.9). Ventral metamorphic shell (400–535 μm wide), oval with pitted ornamentation, and two low lobes, close to the center and extending to margin (Fig. 4.1–4.4). Oval pedicle foramen (120–170 μm long) not enclosed within metamorphic shell (Fig. 4.1–4.3). Median tubercle in front of pedicle foramen very poorly developed to absent (Fig. 4.1–4.3). Pseudointerarea procline and subtriangular, mostly defined by the absence of pustule-like ornamentation, and lacking well-defined intertrough.
Figure 4. Acrothele concava Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, from the El Gavilán Formation (sample SJG 2/9), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1–4) (USTL 4032-1) (1) Oblique posterior view of ventral valve exterior, (2) detailed view, (3) ventral metamorphic shell, (4) detail of edge of ventral metamorphic shell; (5–9) (USTL4032-2) (5) oblique lateral view of dorsal valve exterior, (6) detail of dorsal metamorphic shell, (7) detail of dorsal metamorphic shell, (8) pitted dorsal metamorphic shell, (9) pustulose adult ornamentation; (10) (USTL4032-3) dorsal valve interior; (11, 12) (USTL4032-4) (11) oblique lateral view of dorsal valve interior, (12) detail of umbo.
Ventral interior poorly preserved, with somewhat thickened area around foramen, and poorly defined vascula lateralia diverging anteriorly.
Dorsal valve flattened to slightly convex in lateral view, with subcircular margin and very shallow sulcus; fine concentric fila and pustule-like ornamentation (Fig. 4.5). Dorsal metamorphic shell marginal, oval (625–350 μm wide), with pitted ornamentation; single pair of spines at posterior margin of metamorphic shell (Fig. 4.5–4.7, 4.12). Pair of divergent, anteriorly inflated lobes terminated close to anterior margin of metamorphic shell (Fig. 4.5–4.8). Dorsal pseudointerarea poorly developed, orthocline to apsacline; median sulcus sometimes develops at edge of metamorphic shell (Fig. 4.10–4.12).
Dorsal interior poorly preserved, but with low median ridge (Fig. 4.10–4.12).
Figured material
USTL4032-1–4.
Remarks
The most distinguishing character of A. concava from the El Gavilán Formation (Wuliuan) is the unusual morphology of the metamorphic shell morphology (Fig. 4.1–4.8). Most well-known species of Acrothele have a distinctive low median tubercle in front of the pedicle foramen (e.g., Acrothele vertex Reed, Reference Reed1910; Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015, fig. 13C, D), but here it is very poorly developed to absent (Fig. 4.1–4.3); the ventral metamorphic shell of the Mexican species lacks the characteristic Acrothele spines, and only has two low raised lobes, close to the center and extending to margin (Fig. 4.1–4.3). Moreover, the dorsal metamorphic shell of A. concava only has a single pair of low spines in front of the posterior margin, and anterior to the spines, a pair of divergent raised and inflated lobes are present (Fig. 4.7, 4.12). This type of dorsal metamorphic shell is similar to that described from an unnamed problematic species of Acrothele from the Guzhangian of Novaya Zemlya (Holmer et al., Reference Holmer, Popov, Ghobadi Pour, Klishevich, Liang and Zhang2020, fig. 6E–H); in both these species, the dorsal lobes are considered to represent impressions from a single pair of metamorphic setal sacks (see also Zhang et al., Reference Zhang, Popov, Holmer and Zhang2018, fig. 8). In the type species, A. coriacea, the dorsal metamorphic shell has two pairs of well-developed spines (e.g., Rowell, Reference Rowell1980, pl. 2:6; Holmer and Popov, Reference Holmer, Popov, Williams, Brunton and Carlson2000, fig, 46.1). The species from Novaya Zemlya also lacks the paired ventral metamorphic spines, and has two inflated lobes. Most well-known species of Acrothele have a well-developed internal pedicle tube (e.g., Acrothele vertex Reed; Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015, fig. 13G, K), but this is absent in A. concava, where there is only a slightly thickened interior area around the foramen; a very similar morphology also is found in Acrothele? sp. from Novaya Zemlya (Holmer et al., Reference Holmer, Popov, Ghobadi Pour, Klishevich, Liang and Zhang2020, fig. 5A, C). However, the pustulose adult ornamentation and pitted ornamentation on the metamorphic shell of A. concava is unlike the species from Novaya Zemlya, and closely similar to that of other Acrothele species (Fig. 4.8, 4.9).
Cooper in Cooper et al. (1952, p. 45) provided a short comparative discussion of his species with A. colleni Walcott, Reference Walcott1912, from the middle Cambrian of Montana. Specimens possibly referable to Acrothele concava and Acrothele sp. also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020).
Genus Eothele Rowell, Reference Rowell1980
Type species
Acrothele spurri Walcott, Reference Walcott1908 (original designation by Rowell, Reference Rowell1980, p. 17); Cambrian (Series 2, Bonnia-Olenellus Zone), Nevada.
Eothele sp.
Figure 5
Occurrence in Chihuarruita Hill outcrop
Cambrian Stage 4 Buelna Formation (sample SJG 2/2), and Wuliuan El Gavilán Formation (samples 2/13, 3/7).
Figure 5. Eothele sp. from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1, 4, 5) (USTL4033-1) (1) Oblique posterior view of ventral valve exterior, (4) oblique lateral view of ventral apex, (5) detail of ventral metamorphic shell; (2) (USTL4033-2) oblique posterior view of ventral valve exterior; (3) (USTL4034-1) oblique posterior view of ventral valve exterior; (6) (USTL4034-2) ventral valve interior with pedicle tube; (7–9) (USTL4034-3) (7) ventral pedicle foramen, (8) oblique lateral view, (9) detail of metamorphic shell.
Description
All specimens represent incomplete ventral valves, showing only a part of the shell around the metamorphic shell and the foramen opening.
Ventral valve strongly conical. Pedicle opening (330–400 μm long, 160–240 μm wide) not enclosed within metamorphic shell; well-developed media tubercle bounding anterior margin of oval foramen opening; pitted metamorphic shell with two symmetrical tubercles anterior to apex (Fig. 5). Pedicle foramen opening strongly elongate oval, almost two times longer than wide, and continuing internally as a collar-like tube; pseudointerarea procline, poorly defined, lacking intertrough (Fig. 5). Pustulose adult ornamentation. Ventral interior with short pedicle tube (Fig. 5.8).
Figured material
USTL4033-1–3.
Remarks
Although the material from the Buelna Formation (Stage 4) is fragmented, the large elongate pedicle foramen (Fig. 5.1–5.5, 5.7, 5.8), and the ventral metamorphic shell with the strong median tubercle (Fig. 5.9) and pedicle tube (Fig. 5.6) show some similarities with the type species E. spurri (Walcott, Reference Walcott1908) from Nevada. However, due to the lack of a definite dorsal valve and the poor preservation of the ventral, it is kept under open nomenclature.
Order Acrotretida Kuhn, Reference Kuhn1949
Superfamily Acrotretoidea Schuchert, Reference Schuchert1893
Family Acrotretidae Schuchert, Reference Schuchert1893
Genus Linnarssonia Walcott, Reference Walcott1885
Type species
Obolella transversa Hartt in Dawson, Reference Dawson1868 (original designation by Walcott, Reference Walcott1885, p. 115); middle Cambrian of New Brunswick, Canada.
Linnarssonia arellanoi? (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952)
Figure 6
- ?Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Pegmatreta arellanoi Cooper in Cooper et al., p. 43, pl. 13C, figs. 7–12.
Figure 6. Linnarssonia arellanoi? (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952) from the El Gavilán Formation: (1) sample SJG 2/9; (2–4), sample SJG 2/13; (5, 6), sample SJG 3/8; (7–9), sample SJG 2/7), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1) (USTL4035-1) Dorsal valve exterior; (2, 4) (USTL4036-1) (2) dorsal valve interior, (4) detail of pseudointerarea and cardinal muscle scars; (3) (USTL4036-2) dorsal valve interior; (5, 6) (USTL4036-3) (5) dorsal view of complete articulated shell, (6) detail of umbo; (7–9) (USTL4037-1) (7) oblique lateral view of ventral valve interior, (8) detail of posterior interior and pedicle foramen, (9) detail of apical process, cardinal muscle scar.
Holotype
USNM116057d, a ventral valve (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, pl. 13C, fig. 7) from the El Tren Formation (Wuliuan), Arrojos Hills, Sonora.
Diagnosis
See Cooper in Cooper et al. (Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, p. 44).
Occurrence in Chihuarruita Hill outcrop
Cambrian Stage 4 Buelna Formation (sample SJG 2/2) and Wuliuan El Gavilán Formation (samples SJG 2/7, 2/9, 2/13, 3/7, 3/8, 3/9).
Description
Shell ventribiconvex and elongate oval in adults, ~80–112% as long as wide (Fig. 6.5).
Ventral valve (length 1.2–2.3 mm, width 1.2–2.3 mm) evenly convex in lateral view. Pseudointerarea apsacline, poorly defined. Intertrough short and well defined (Fig. 6.5–6.8). Pedicle opening oval and not enclosed within metamorphic shell.
Ventral interior with prominent boss-like apical process, extending anteriorly for ~36–40% of the total length of valve; process laterally bounded by apical pits and a pair of diverging vascula lateralia. The apical process is triangular in anterior view. Prominent oval cardinal muscle scars placed laterally, located close to the internal foramen (Fig. 6.7–6.9).
Dorsal valve (length 1.7–2.3 mm, width 1.8–2.5 mm) convex. Dorsal metamorphic shell located near posterior edge of shell (Fig. 6.6).
Dorsal valve with short median groove and orthocline propareas. Median groove almost four times wider than long. Well-developed median ridge extending anteriorly for ~64–83% of total valve length; well-developed median buttress fused with median ridge. Prominent cardinal and anterocentral muscle scars (Fig. 6.2–5.4).
Figured material
USTL4035-1, 4036-1–3, 4037-1.
Remarks
Linnarssonia and similar taxa (e.g., Pegmatreta and Hadrotreta) are known to be taxonomically complex genera because of the great morphological variation and general lack of detailed studies (see Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001); recent discussions on this and related problematic genera can be found in Peel et al. (Reference Peel, Streng, Geyer, Kouchinsky and Skovsted2016) and Ushatinskaya and Korovnikov (Reference Ushatinskaya and Korovnikov2019). Holmer and Popov (Reference Holmer, Popov, Williams, Brunton and Carlson2000) and Ushatinskaya and Korovnikov (Reference Ushatinskaya and Korovnikov2019) included Pegmatreta within Linnarssonia, and this is followed here. Linnarssonia arellanoi (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952) is a particularly poorly known species that has only ever been recorded from the type horizon in the slightly younger El Tren Formation. It has never been re-studied, but the illustrations and description by Cooper in Cooper et al. (1952, pl. 13C, figs. 7–12) are very similar in all morphological details—including the elongate oval shape of the shell, prominent boss-like apical process, and long dorsal median septum—to the slightly older material described here (Fig. 6). Pending re-study of the type material, they are questionably referred to the same species.
Linnarssonia arellanoi is similar in most characters to the somewhat younger L. ophirensis (Walcott, Reference Walcott1902) from the Drumian of Utah (Rowell, Reference Rowell1966), and pending further studies it is possible that they are conspecific.
Genus Hadrotreta Rowell, Reference Rowell1966
Type species
Acrotreta primaea Walcott, Reference Walcott1902 (original designation by Rowell, Reference Rowell1966, p. 12); lower to middle Cambrian (Stage 4 to Wuliuan) Pioche Shale, Nevada, USA.
Hadrotreta rara? (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952)
Figure 7
- ?Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Hadrotreta rara Cooper in Cooper et al., p. 42, pl. 12C, figs. 10–19.
Figure 7. Hadrotreta rara? (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952) from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1, 10) (USTL4038-1) (1) Dorsal valve exterior, (10) oblique posterior view of dorsal metamorphic shell; (2, 4) (USTL4038-2) (2) dorsal valve interior, (4) oblique lateral view; (3) (USTL4038-3) ventral valve exterior; (5) (USTL4038-4) oblique lateral view of dorsal valve interior; (6–9) (USTL4038-5) (6) oblique lateral view of ventral valve exterior, (7) detail of umbo, (8) detail of ventral metamorphic shell, (9) oblique lateral view of umbo; (11) (USTL4038-6) oblique lateral view of ventral valve interior.
Holotype
USNM116057d, a dorsal valve (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, pl. 12C, fig. 18) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.
Diagnosis
See Cooper in Cooper et al. (Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, p. 42).
Occurrence in Chihuarruita Hill outcrop
Cambrian Stage 4 Buelna Formation (samples SJG 2/2) and Wuliuan El Gavilán Formation (samples SJG 2/6, 2/9, 2/13, 3/7, 3/8, 3/9.).
Description
Shell ventribiconvex, subcircular in outline.
Ventral valve cone-shaped with rounded to sub-rectangular outline (length 863 μm–1.8 mm, width 858 μm–1.9 mm), ~71–102% as long as wide; maximum width about mid-valve (Fig. 7.3, 7.6). Ventral pseudointerarea poorly defined, subtriangular, and procline to apsacline, but with well-developed narrow intertrough that runs from the margin to the pedicle opening. Ventral metamorphic shell convex and circular. Oval pedicle foramen situated outside the metamorphic shell (Fig. 7.7–7.9). Ventral interior poorly preserved in most specimens, with well-defined pedicle tube, a pair of cardinal muscle scars, and well-developed apical process anterior to the foramen. Apical pits placed directly lateral to pedicle foramen (Fig. 7.11).
Dorsal valve convex, subcircular in outline (length 1.1–1.8 mm, width 1.2–2.1 mm), ~83–88% as long as wide, width with flat posterior part. Dorsal metamorphic shell with shallow median sulcus extending from metamorphic shell to anterior margin (Fig. 7.1, 7.10).
Dorsal pseudointerarea anacline, median groove occupying total width of posterior margin, but shortly developed along posterior-anterior axis. Median groove, wide, triangular, supported by median buttress, which is fused with a median septum. Triangular median septum extending for ~65–75% of the total valve length. Dorsal cardinal muscle scars bean-shaped, located directly anterior to propareas (Fig. 7.2, 7.4, 7.5).
Figured material
USTL4038-1–6.
Remarks
As discussed above, Hadrotreta can be difficult to distinguish from Linnarssonia (=Pegmatreta Bell, Reference Bell1941; Holmer and Popov, Reference Holmer, Popov, Williams, Brunton and Carlson2000). Pegmatreta rara Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, has never been re-studied, but the illustrations and description (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, pl. 12C, figs. 10–19) appear to be very similar in all morphological details—including the shape of the shell, dorsal pseudointerarea, and dorsal median septum (Fig. 7.1–7.6). The new Sonoran material is tentatively considered to be conspecific, pending restudy, and it is placed within Hadrotreta based on the presence of an oval pedicle foramen situated outside the metamorphic shell (Fig. 7.9), widely spaced dorsal cardinal scars (Fig. 7.2, 7.4, 7.5), and deep apical pits directly lateral to the internal pedicle tube (Fig. 7.11), as well as having a low dorsal median ridge (Fig. 7.4, 7.5). However, the ventral interior is comparatively poorly preserved in the available specimens and the characteristic boss-like apical process is not well seen (Fig. 7.11). Hadrotreta rara? appears to be most similar to the type species (see Liang et al., Reference Liang, Holmer, Duan, Xiaolin and Zhang2022), but has a longer dorsal median ridge, although most dorsal valves are fragmented anteriorly, and moreover, the “forked” septum with the dorsal anterior lateral scars (Fig. 7.2, 7.4, 7.5) are not as well developed as in other species of the genus.
Poorly known specimens referred to Pegmatreta rara also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020).
Genus Prototreta Bell, Reference Bell1938
Type species
Prototreta trapeza Bell, Reference Bell1938 (original designation by Bell, Reference Bell1938, p. 405); middle Cambrian (Bathyuriscus Zone), Montana, USA.
Prototreta sp.
Figure 8
Occurrence in Chihuarruita Hill outcrop
Wuliuan El Gavilán Formation (samples SJG 2/9, 2/13, 3/7, 3/8, 3/9).
Figure 8. Prototreta sp. from the El Gavilán Formation (sample SJG 2/13), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1–3) (USTL4039-1) (1) ventral valve exterior, (2) oblique posterolateral view, (3) detail of umbo, (4–6) (USTL4039-2) (4) oblique lateral view dorsal valve exterior, (5) oblique lateral view of umbo, (6) oblique lateral view of dorsal metamorphic shell; (7) (USTL4039-3) dorsal valve exterior; (8–10) (USTL4039-4) (8) dorsal valve exterior, (9) oblique lateral view, (10) oblique lateral view of dorsal pseudointerarea, (11, 12) (USTL4039-5) (11) oblique anterolateral view of ventral valve interior, (12) oblique anterolateral view of apical process and muscle scars.
Description
Shell ventribiconvex, with a plano-convex dorsal valve and a highly conical ventral valve, ~25–33% as high as wide. Ornamentation with fine rugae.
Ventral valve conical (length 1.2–2.8 mm, width 1.3–3.2 mm), ~87–97% as long as wide. Pedicle foramen not enclosed within metamorphic shell. Pseudointerarea strongly procline, with well-defined intertrough (Fig. 8.1–8.3).
Ventral interior with apical process forming ridge along posterior wall of shell. Pedicle tube penetrating apical process, which is often broken in the available specimens. Pair of apical pits located postero-laterally to internal pedicle opening. Parallel ventral vascula lateralia extending from apical pits, continuing alongside apical process, before diverging anteriorly. Well-developed oval cardinal muscle scars located postero-laterally to pedicle opening (Fig. 8.11, 8.12).
Dorsal valve circular in outline and almost planar in lateral view (length 1.7–2.9 mm, width 1.6–3.4 mm), ~83–104% as long as wide. Ornamentation of fine concentric growth lines. Metamorphic shell circular, with two parallel small ridges (Fig. 8.4–8.6).
Dorsal pseudointerarea well developed, long, and wide, occupying ~18–20% and 83–100% of total length and width of valve, respectively; anacline to orthocline, with triangular wide dorsal median groove and small propareas. Dorsal cardinal muscle scars located just anterior to propareas and median groove. Moderately high median ridge, extending for ~56–60% of the length of valve; triangular in lateral view with a blade-like structure on the posterior part of the crest (Fig. 8.7–8.10).
Figured material
USTL4039-1–5.
Remarks
Brock and Percival (Reference Brock and Percival2006) recently discussed and reviewed Prototreta, which is clearly in need of revision. Pending such revision, it is difficult to make a detailed comparison with other species of the genus, therefore the material is kept under open nomenclature. The dorsal median septum of the Sonoran Prototreta sp. is not very high and digitate, as compared with P. trapeza Bell, Reference Bell1938, and P. flabellum Bell, Reference Bell1941, but more similar to P. interrupta Bell, Reference Bell1941, and P. mimica Bell, Reference Bell1941, all from the Cambrian of Montana, in having a lower septum with an upper rod (Fig. 8.7–8.9).
Specimens referred to Prototreta sp. also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020).
?Family Scaphelasmatidae Rowell, Reference Rowell and Moore1965
?Genus Batenevotreta Ushatinskaya, Reference Ushatinskaya, Repina and Rozanov1992
Type species
Batenevotreta formosa Ushatinskaya, Reference Ushatinskaya, Repina and Rozanov1992 (original designation by Ushatinskaya, Reference Ushatinskaya, Repina and Rozanov1992, p. 87); middle Cambrian (Amgian), Altay, Russia.
Batenevotreta? mexicana new species
Figure 9
Holotype
USTL4040-6, a ventral valve (Figure 9.8, 9.11, 9.12) from Wuliuan El Gavilán Formation (sample SJG 2/9).
Figure 9. Batenevotreta? mexicana n. sp., from the El Gavilán Formation (sample SJG 2/9), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1, 4) (USTL4040-1) (1) Dorsal valve exterior, (4) oblique posterolateral view; (2, 5) (USTL4040-2) (2) dorsal valve interior; (3) (USTL4040-3) ventral valve exterior, (5), oblique anterolateral view; (6, 9, 10) (USTL4040-4) ventral valve interior, (9) oblique lateral view, (10) oblique posterior view; (7) (USTL4040-5) oblique lateral view of dorsal valve interior; (8, 11, 12) (USTL4040-6) holotype, oblique posterolateral view of ventral valve exterior, (11) oblique posterolateral view of ventral metamorphic shell, (12) detail of umbo.
Diagnosis
Shell transversely oval with strong, irregularly spaced rugae; ventral valve low conical; ventral pseudointerarea procline to catacline, poorly defined laterally, with shallow, poorly defined intertrough; foramen circular, not enclosed within metamorphic shell; apical process high, broad, boss-like, directly anterior to short pedicle tube; dorsal valve moderately convex; dorsal pseudointerarea poorly defined, very short, raised high above valve floor, with shallow very poorly defined median groove; dorsal interior with small, poorly developed median buttress and low median ridge; dorsal cardinal muscle fields large, transversely oval, thickened.
Occurrence in Chihuarruita Hill outcrop
Wuliuan El Gavilán Formation (sample SJG 2/9).
Description
Shell unequally biconvex, rounded, transversely oval, with strong, irregularly spaced rugae (Fig. 9.1, 9.3, 9.4).
Ventral valve low conical (length 1.0–1.2 mm, width 1.3–1.6 mm), ~76–92% as long as wide (Fig. 9.8–9.10). Maximum width in middle of shell length. Ventral pseudointerarea poorly defined laterally, procline to catacline; intertrough, shallow, poorly defined (Fig. 9.8). Ventral metamorphic shell well defined (~150–170 μm wide); pitted ornamentation poorly preserved (Fig. 9.11, 9.12). Foramen circular (~60 μm wide and long), not entirely enclosed within metamorphic shell (Fig. 9.11). Ventral interior with short pedicle tube, directly posterior to well-developed, high, boss-like apical process, extending for around one-third of the total valve length (Fig. 9.6, 9.10). Ventral cardinal muscle scars thickened and raised, short and wide extending for ~20% and 78% of total valve length and width, respectively (Fig. 9.6, 9.10). Apical pits small, located near posterior margin. Ventral vascula lateralia starting lateral to apical pits and diverging widely up to the mid-length of the valve (Fig. 9.6, 9.10).
Dorsal valve moderately convex (length 770 μm–1.66 mm, width 911 μm–1.62 mm), ~82–110% as long as wide (Fig. 9.4). Dorsal pseudointerarea very poorly defined, very short, raised high above valve floor, with shallow, very poorly defined median groove (Fig. 9.2, 9.5, 9.7).
Dorsal interior with small, poorly developed median buttress and low median ridge; dorsal cardinal muscle fields large, transversely oval, thickened, extending for ~25% and 90% of total valve length and width, respectively (Fig. 9.2, 9.5, 9.7). Dorsal median ridge low and long, extending for ~86% of total valve length. Dorsal anterolateral muscle scars located directly lateral to the median ridge (Fig. 9.2, 9.5, 9.7).
Etymology
From the occurrence in Mexico.
Figured material
USTL4040-1–6, from sample SJG 2/9.
Remarks
Batenevotreta is a rare and early member of the Scaphelasmatidae, which was originally described from the middle Cambrian of Altay, Russia, by Ushatinskaya (Reference Ushatinskaya, Repina and Rozanov1992), and later recorded from the late Cambrian of Kazakhstan by Koneva and Ushatinskaya (Reference Koneva and Ushatinskaya2010). Unfortunately, the type species, B. formosa Ushatinskaya, Reference Ushatinskaya, Repina and Rozanov1992, is not well known (Holmer and Popov, Reference Holmer, Popov, Williams, Brunton and Carlson2000). Percival and Kruse (Reference Percival and Kruse2014) most recently illustrated a questionable record from the mid-Cambrian of central Australia (southern Georgina Basin). In the species from Kazakhstan (Koneva and Ushatinskaya, Reference Koneva and Ushatinskaya2010), B. variabilis Koneva and Ushatinskaya, Reference Koneva and Ushatinskaya2010, and B. ivshini Koneva and Ushatinskaya, Reference Koneva and Ushatinskaya2010, the adults have a very large pedicle foramen, which is oval to slit-like, as in other members of the Scaphelasmatidae, and the metamorphic shells have a characteristic pitting with two distinctive sizes of pits. In contrast, the pedicle foramen of B.? mexicana n. sp. is circular and comparatively small, and moreover, the pitting of the metamorphic shells is not well preserved (Fig. 9.11, 9.12). The new Sonoran material is most similar in most other characters to the species from Kazakhstan and Russia, but assignation to the genus is kept questionable due to the differences in pedicle foramen and lack of information on metamorphic pitting.
Class Paterinata Williams et al., Reference Williams, Carlson, Brunton, Holmer and Popov1996
Order Paterinida Rowell, Reference Rowell and Moore1965
Superfamily Paterinoidea Schuchert, Reference Schuchert1893
Family Paterinidae Schuchert, Reference Schuchert1893
Remarks
The studied material includes very fragmentary specimens of Dictyonina sp., Micromitra sp., and Paterina sp. that are too poorly preserved to allow closer taxonomic discrimination (Fig. 10.10–10.12). A poorly preserved ventral valve, lacking preserved ornamentation, may also represent an unidentified Paterinidae (Fig. 10.13–10.15). Sonoran specimens of Micromitra sp. and Paterina sp. were described by Cooper in Cooper et al. (Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, p. 38–39, pl. 11B, figs. 7, 8, pl. 13A, figs. 1–3), as well as listed and illustrated by Cuen-Romero et al. (Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020).
Figure 10. Dictyonina minutipuncta (1–9) Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, from the Cambrian (Wuliuan) El Gavilán Formation: (1–6) sample SJG 2/9, (7–9) sample SJG 2/7); Paterina sp. (10–12) and an unidentified Paterinidae (13–15) from the Cambrian Stage 4 Buelna Formation (sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1–6) (USTL4041-1) (1) Ventral valve exterior, (2) oblique posterolateral view, (3) oblique posterolateral view of umbo, (4) detail of umbo, (5) detail of ventral metamorphic shell, (6) detail of adult ornamentation; (7–9) (USTL4042-1) oblique lateral view of ventral valve exterior, (8) detail of umbo, (9) detail of adult ornamentation; (10–12) (USTL4043-1) (10) oblique lateral view of ventral valve exterior, (11) detail of adult ornamentation, (12) detail of adult ornamentation; (13–15) (USTL4043-2) (13) ventral valve exterior, (14) oblique posterolateral view, (15) detail of umbo.
Genus Dictyonina Cooper, Reference Cooper1942
Type species
Trematis pannulus White, Reference White1874 (original designation by Cooper, Reference Cooper1942, p. 228); Cambrian (Stage 4–Wuliuan) Pioche Shale, Nevada, USA.
Dictyonina minutipuncta Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Figure 10.1–10.9
- Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952
Dictyonina minutipuncta Cooper in Cooper et al., p. 40, pl. 11A, figs. 1–6.
Holotype
USNM116054a, a ventral valve (Cooper in Cooper et al., Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, pl. 11A, fig. 5, 6) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.
Diagnosis
See Cooper in Cooper et al. (Reference Cooper, Arellano, Johnson, Okulitch, Stoyanow and Lochman1952, p. 40).
Occurrence in Chihuarruita Hill outcrop
Wuliuan El Gavilán Formation (samples SJG 2/7, 2/9, 2/13).
Description
Ventral valve strongly convex with prominent beak and short homeodeltidium (Fig. 10.1–10.9). Ornamentation of radiating rows of subhexagonal pits, increasing in size distally (Fig. 10.1–10.9). Metamorphic shell bi-lobate, with fine pustulose ornamentation (Fig. 10.5). Ventral pseudointerarea well developed, lacking pitted ornamentation.
Interior ventral valve poorly preserved.
Figured material
USTL4041-1, 4042-1, 4043-1, 2.
Remarks
The pitted ornamentation (Fig. 10.1–10.9) and shape of the ventral valve are closely similar to the material described by Cooper in Cooper et al. (1952, pl. 11A, fig. 5, 6) from the Arrojos Formation, and they are considered to be conspecific. The Sonoran species is also similar to Dictyonina pannula (White, Reference White1874) (see Liang et al., Reference Liang, Holmer, Duan, Xiaolin and Zhang2022). Specimens also have been listed and illustrated by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (Reference Noriega-Ruiz, Cuen-Romero, Enríquez-Ocaña, Sundberg, Monreal, Varela-Romero, Palafox-Reyes and Grijalva-Noriega2020).
?Subphylum Rhynchonelliformea Williams et al., Reference Williams, Carlson, Brunton, Holmer and Popov1996
?Indeterminate rhynchonelliform
Figure 11
Remarks
The studied material also includes possible rhynchonelliform brachiopods represented by internal molds that are too poorly preserved for closer taxonomic discrimination (Fig. 11).
Figure 11. ?Indeterminate rhynchonelliform, from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1–5) (USTL4043-3) (1) Internal mold of ventral valve, (2) detail of umbo, (3) detail of umbo, (4) oblique lateral view of umbo, (5) posterior view of umbo; (6, 7, 10, 11) (USTL4043-4) (6) internal mold of ventral valve, (7) detail of umbo, (10) oblique posterolateral view, (11), oblique posterior view; (8, 9) (USTL4043-5) (8) detail of umbo of internal mold of ventral valve, (9) posterior view.
Biogeographical affinities
During the Cambrian, the Sonora Region formed a part of the Caborca Terrain, which was an integral part of the southern margin of Laurentia, facing the South American sector of Gondwana (Torsvik and Cocks, Reference Torsvik and Cocks2017; Fig. 12.3).
Figure 12. Results of the pair-group cluster analysis for presence-absence data (Raup-Crick similarity) for the Cambrian linguliform genera (Cambrian Stages 4 and Wuliuan) from 26 localities (for details see Appendix; see also Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015). (1) Cambrian Stage 4 and Eothele Fauna; (2) Cambrian Wuliuan. (3) Paleogeographical reconstruction for Cambrian Stage 4 showing geographical distribution of the localities used in cluster analysis. Relative position of major early Paleozoic continents (Laurentia, Baltica, Gondwana, and Siberia) mainly after Cocks and Torsvik (Reference Cocks and Torsvik2002), with significant emendations for Australasian peri-Gondwana.
Two successive linguliform brachiopod assemblages have been recognized in the Sonoran deposits in the time interval between Cambrian Series 4 and the newly recognized Wuliuan Stage. The lowermost assemblage, which includes Dictyonina sp., Paterina sp., Eothele sp., Hadrotreta rara?, Linnarssonia arellanoi?, comes from the upper part of the Buelna Formation (Fig. 2; Table 1). Trilobite-based correlation (e.g., Cuen-Romero et al., Reference Cuen-Romero, Valdez, Buitrón, Monreal, Sundberg, Montijo-González and Minjarez-Sosa2016, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018, Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez- Ocaña, Hinojosa, Ochoa-Granillo, Grijalva-Noriega and Palafox Reyes2019, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2020, Reference Cuen-Romero, Reyes-Montoya and Noriega-Ruiz2022, and references herein) supports assignment of the unit to Cambrian Series 4. The younger El Gavilán Formation contains a more diverse linguliform brachiopod assemblage, including Acrothele concava, Batenevotreta? mexicana n. sp., Dictyonina minutipuncta, Eothele sp., Eoobolus sp., Hadrotreta rara?, Linnarssonia arellanoi?, Micromitra sp., Paterina sp., and Prototreta sp. (Fig. 2; Table 1). The El Gavilán Formation also contains a diverse trilobite fauna that suggests a Delamaran age (in terms of the Laurentian regional stratigraphical scheme). Although, the index fossil for the base of the global Wuliuan Stage and Miaolingian Series–Oryctocephlus indicus–is absent, Cuen-Romero et al. (Reference Cuen-Romero, Valdez-Holguín, Buitrón-Sánchez, Monreal, Enríquez-Ocaña, Aguirre-Hinojosa, Ochoa-Granillo and Palafox-Reyes2018) suggested that it should be provisionally placed in the lower part of the El Gavilán Formation. The Wuliuan or younger age of the linguliform brachiopod assemblage recovered the El Gavilán Formation is also supported by the occurrence of Acrothele. In the Cambrian succession of Himalayas, Acrothele makes it first appearance in the Kaotaia prachina Zone, which is definitely above the estimated position of the Wuliuan Stage base (Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015). The co-occurrence of Acrothele and Eothele also can be taken as indication of the Wuliuan age of the fauna.
To investigate biogeographical relations of the newly recovered Cambrian linguliform brachiopod faunas from the Caborca Terrain, a cluster analysis (Raup-Crick similarity) was performed using the computer program PAST (version 3.06; Hammer et al., Reference Hammer, Harper and Ryan2001; Fig. 12.1, 12.2). The data set (Appendix) used in analysis is modified from Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), with addition of the newly recovered faunas from Sonora and new data for the Wuliuan faunas from Siberia reported by Ushatinskaya and Korovnikov (Reference Ushatinskaya and Korovnikov2019). In spite of restrictions caused by the low generic richness of the faunas, as well as the heterogenous character of available data (already discussed by Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), it was possible to recover general patterns of biogeographical differentiation and biotic changes caused by increased migration and major biodiversification of linguliform faunas close to the base of the Wuliuan Stage (Fig. 12).
The first analysis is based on a data matrix including generic composition of 19 individual faunas (A1–A11 and B1–B8) from Cambrian Stage 4 (Fig. 12.1). The core of the analyzed matrix is derived from faunal lists, including 31 total genera, as presented by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), with addition of the faunal assemblage from the Caborca Terrane (Sonora, Buelna Formation; Appendix). The resulting cladogram mainly preserves general patterns shown by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015; Fig. 12.1); however, there are also some important differences, which require consideration.
Excluding the highly endemic fauna of Anti-Atlas, Morocco (B5; located on the margin of the North African sector of Gondwana), two major clusters (Cluster A and B) can be recognized (Fig. 12.1). Cluster A includes two second-order subclusters. The first cluster is formed by the Siberian faunas (B2 and A6) plus the faunal assemblage of the peri-Gondwanan Karatau-Naryn microplate (A2). The second cluster includes most, but not all, of the faunas grouped in previous analyses by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015) within the Schizopholis-Botsfordia Fauna, which is confined to tropical Gondwana, plus a single Laurentian faunal assemblage from Greenland (Fauna A4; Fig. 12.1). The latter abnormality probably was caused by the fact that Laurentian faunas from the lower to middle part of Cambrian Stage 4 are poorly known and strongly underrepresented, both in terms of geographical distribution and generic richness.
Cluster B includes faunal assemblages of the so-called Eothele Fauna (Fig. 12.1; Popov et al., Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015) as a separate Subcluster B2. Cluster B is composed of the individual faunas from the Australasian sector of Gondwana and Laurentian faunas. The latter form a distinct third-order subcluster, which also includes the faunal assemblage from Sonora (B8), clearly showing the Laurentian biogeographical signature. Subcluster B1 includes two individual faunas (A3 and A10; Fig. 12.1) from the Australian Sector of Gondwana, which in the previous analysis by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015) was included within the Schizopholis-Botsfordia Fauna. It is possible that these Australian faunas are among the precursors of the Eothele Fauna. The new biogeographic analysis confirms the earlier suggestion by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015) that the Eothele Fauna evolved late in Cambrian Stage 4 due to increased faunal migration within the southern tropical latitudes directed from Australasian Gondwana to Laurentia. Remarkably, the contemporaneous faunas of Siberia (A6), which formed an isolated continent mainly in the northern tropical latitudes, and North African sector of Gondwana (Fig. 12.1; B1, located in temperate southern latitudes) retained their individuality. Yet both contain Eothele, which was a short-lived taxon that proliferated close to the end of Cambrian Stage 4.
The second cluster analysis, which only comprises the Wuliuan Stage (Fig. 12.2), includes 41 genera representing 16 individual faunas mainly based on data published by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), with addition of new data on the linguliform faunas from Sonora and Siberia (Appendix). The results of the analysis also mainly preserved the general pattern that was found in the analysis published earlier by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015); however, there are notable differences addressed below.
As in the earlier biogeographical analysis by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), two major clusters (A and B) are recognized (Fig. 12.2). Cluster A includes diverse and highly endemic faunas of Kazakh terranes (C1 and C2), which are characterized by diverse micromorphic brachiopod taxa, predominantly acrotretides. However, the Baltic fauna (C8; Fig. 12.2) surprisingly appears within this cluster—a possible explanation could be that, unlike the Kazakh faunas, which represent genuine faunal associations, the Baltic fauna is based on a composite list, while characters of individual Baltoscandian faunal associations and their biofacies differentiation remain inadequately known. Thus, the observed grouping reflects heterogeneity of the original data set used in the analysis. Nevertheless, this clustering also suggests a relative isolation of linguliform brachiopod faunal assemblages that inhabited the Kazakh terranes and Baltic continent in relation to other contemporaneous faunas.
Other Wuliuan faunas form Cluster B, which is subdivided into two second-order subclusters (Fig. 12.2; B1 and B2). The Wuliuan linguliform faunal assemblage from Sonora, Caborca Terrain (C16) appears within the large third-order Subcluster B1a, together with numerous faunas from tropical Gondwana and associated volcanic arcs and microcontinents; this subcluster also includes the Siberian fauna (C10; Fig. 12.2). The faunal assemblage from Sonora shows closest similarity to the fauna of the Alai peri-Gondwana terrane.
The only other Wuliuan fauna from Laurentia, from the Pioche Shale of Utah (C5), occurs within the third-order Subcluster B1b of the peri-Iapetus faunas together with faunal assemblages Novaya Zemlya (C4; considered as part of Baltica continent) and New Brunswick (Avalonia), which was attached to the North African sector of Gondwana through the Cambrian (Cocks and Torsvik, Reference Cocks and Torsvik2002; Fig. 12.3). A cluster of the Wuliuan peri-Iapetus faunas also was recognized earlier by Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015), but that also included the Wuliuan fauna of Spain (C7), which was attached to the North African sector of Gondwana. In the new analysis, the latter fauna appears at the base of second-order Subcluster B (Fig. 12.3).
The biogeographical affinities of Laurentian faunas of Wuliuan age remain poorly resolved because of our insufficient knowledge. Further progress in biogeographical studies of the Cambrian faunas is not possible without proper documentation of individual faunas from Laurentia and Siberia, two major Cambrian continents that were distinctly isolated from the Gondwana supercontinent. A better understanding of biofacies differentiation of the Cambrian linguliform brachiopod faunas also should be taken in consideration in future analyses of their biogeographical affinities.
Acknowledgments
LD benefited from funding from the Alexander von Humboldt Foundation and SC from a grant from Spanish Ministry of Economics, Finance and Competitiveness CGL2017-87631-P. The study was supported by projects UNAM-DGAPA-PAPIIT No. IN105012, CONACyT No. 165826, and ECOS-France-Mexico No. M13U01, and by the National Natural Science Foundation of China (NSFC 41720104002, 41890844, 41425008, and 41621003), by Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB26000000), by Program of Introducing Talents of Discipline to Universities (the 111 Project D17013). The research of LEH was supported by the Swedish Research Council (VR Project no. 2018-03390) and by a Zhongjian Yang Scholarship from the Department of Geology, Northwest University, Xi'an. Many thanks to the editor S. Zamora, as well as the reviewers M. Mergl and R. Freeman for valuable comments that greatly improved the manuscript.
Appendix: Faunal lists for cluster analysis
Faunal lists for cluster analysis (Raup–Crick similarity) using the computer program PAST (version 3.06; Hammer et al., Reference Hammer, Harper and Ryan2001). The data set used in analysis is modified from Popov et al. (Reference Popov, Holmer, Hughes, Ghobadi Pour and Myrow2015) with addition of the newly recovered faunas from Sonora (Caborca: B8 and C15) and new data for the Wuliuan faunas from Siberia presented by Ushatinskaya and Korovnikov (Reference Ushatinskaya and Korovnikov2019).
Cambrian Stage 4, ‘Botsfordia-Schizopholis faunas’.—
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A1.—Gondwana; Antarctica, King George Island (South Shetland Islands), glacial erratics in the Early Miocene Cape Melville Formation (Holmer et al., Reference Holmer, Popov and Wrona1996); Eoobolus, Schizopholis, Vandalotreta.
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A2.—Karatau-Naryn microplate; Malyi Karatau Range, Redlichia chinensis - Kootenia gimmelfarbi Biozone (Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001); Botsfordia, Lingulellotreta, Linnarssonia, Palaeoobolus.
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A3.—Australasian segment of Gondwana; Northern Territories, composite list (Wiso, Georgina and Daly Basin) (Kruse, Reference Kruse1990, Reference Kruse1991, Reference Kruse1998): Westonia, Schizopholis, Kyrshabaktella, Vandalotreta, Micromitra?.
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A4.—Laurentia, North-East Greenland, Bastion and Ella Island formations (Skovsted and Holmer, Reference Skovsted and Holmer2005); Botsfordia, Eoobolus, Micromitra, Vandalotreta.
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A5.—South China, Shaanxi Province, Guojiaba and Xiannüdong formations (Li and Holmer, Reference Li and Holmer2004); Eohadrotreta, Eoobolus, Kyrshabaktella, Palaeobolus, Lingulellotreta.
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A6.—Siberia, Toyon Regional Stage (Pelman, Reference Pelman1977; Ushatinskaya and Malakhovskaya, Reference Ushatinskaya and Malakhovskaya2001); Botsfordia, Eoobolus, Kyrshabaktella, Linnarssonia, Paterina.
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A7.—Australasian segment of Gondwana; South Australia, Parara and Ramsey limestone formations (Ushatinskaya and Holmer, Reference Ushatinskaya, Holmer, Alexander, Jago, Rozanov and Zhuravlev2001); Eoobolus, Kyrshabaktella, Schizopholis, Vandalotreta.
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A8.—Sub-Himalaya; Botsfordia, Eoobolus, Neobolus, Schizopholis, Wynnia.
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A9.—Spity, Tethyan Himalaya; Parahio Formation, Haydenaspis parvarya Level; Aksarinaia, Eohadrotreta, Eoobolus, Paterina, Schizopholis.
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A10.—Australasian segment of Gondwana, central Australia, southern Georgina Basin, Thorntonia Limestone (Percival and Kruse, Reference Percival and Kruse2014), Aksarinaia, Dictyonina, Hadrotreta, Kostjubella, Kyrshabaktella, Micromitra, Schizopholis, Vandalotreta, Westonia, Wynnia?.
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A11.—South China, Yunnan Province, Wulongqing Formation, Guanshan Fauna (Zhao et al., Reference Zhao, Zhu, Babcock and Peng2011): Acanthotretella, Diandongia, Eoobolus, Lingulellotreta, Palaeobolus.
Transitional ‘Eothele faunas’.—
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B1.—Laurentia, Canada, Mackenzie Mountains (Voronova et al., Reference Voronova, Drozdova, Esakova, Zhegallo, Zhuravlev, Rozanov, Sayutina and Ushatinskaya1987) Lars, could you please supply list of the brachiopod genera for the “Toyonian” and if it is possible for basal Middle Cambrian as well. Dyeran Mackenzie Mountain (Bonnia-Olenellus zone) Laurentia (Voronova et al., Reference Voronova, Drozdova, Esakova, Zhegallo, Zhuravlev, Rozanov, Sayutina and Ushatinskaya1987); Palaeoschmidtites, Linnarssonia, Eothele, Micromitra, Paterina.
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B2.—Peri-Siberia, Altai-Sayany Region, ‘Toyon Stage’, composite list (Ushatinskaya and Malakhovskaya, Reference Ushatinskaya and Malakhovskaya2001; Botsfordia, Chakassilingula, Eothele, Kyrshabaktella, Linnarssonia, Oepikites.
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B3.—Australasian segment of Gondwana; western New South Wales, Coonigan Formation (Roberts and Jell, Reference Roberts and Jell1990), Eothele, Hadrotreta, Kleithriatreta, Dictyonina, Micromitra, Eoobolus (=Palaeoshcmidtites), Oepikites(?) (=Lingulella), Westonia.
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B4.—Australasian segment of Gondwana; north-western New South Wales, Wydjah Formation (Pimpira Member) (Brock and Percival, Reference Brock and Percival2006); Dictyonina, Eothele, Eoobolus, Micromitra, Prototreta.
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B5.—North African Sector of Gondwana; Morocco, composite list (Mergl, Reference Mergl1988; Streng, Reference Streng1999; Alvaro et al., Reference Álvaro, Ezzouhairi, Ayad, Charif, Popov and Ribeiro2008), Acanthotreta, Almohadella, Botsfordia, Eothele, Monophtalma, Vandalotreta.
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B6.—Laurentia, late Dyeran – early Delamaran, Nevada, Pioche Shale (Rowell, Reference Rowell1980), and Harkless Formation (Skovsted and Holmer, Reference Skovsted and Holmer2006); Dictyonina, Eothele, Hadrotreta.
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B7.—Laurentia, Nevada, Harkless Formation (Skovsted and Holmer, Reference Skovsted and Holmer2006); Eothele, Kyrshabaktella, Hadrotreta.
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B8.—Caborca Terrane, Sonora, Cerro Prieto Formation; Dictyonina, Eothele, Hadrotreta, Linnarssonia, Paterina.
Cambrian Stage 5, Acrothele Fauna
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C1.—Karatau-Naryn microplate; Malyi Karatau Range, Pernopsis? ultimus and Ptychagnostus intermedius biozones (Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001); Aksarinaia, Akmolina?, Canalilatus, Kotylotreta, Kyrshabaktella, Linnarssonia, Prototreta, Schizopholis, Stilpnotreta.
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C2.—North Tien-Shan microplate, Kargajly Formation (Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001); Anabolotreta?, Canalilathus, Conthylotreta, Kleithriatreta, Kotylotreta, Kyrshabaktella, Neotreta, Paterina.
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C3.—Chingiz-Tarbagatai island arcs; Tarbagatai Range, ‘Amga’ Stage (Popov et al., Reference Popov, Holmer and Gorjansky1996), Acrothele, Kleithriatreta, Kostjubella, Prototreta.
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C4.—Novaya Zemlya; Southern Island, Astafiev Formation (Popov, Reference Popov and Bondarev1985); Acrothele, Acrothyra, Hadrotreta.
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C5.—Laurentia; Utah, Pioche Shale (Rowell, Reference Rowell1980); Acrothele, Acrothyra, Aphelotreta, Dictyonina, Micromitra.
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C6.—Avalonia; New Brunswick (Walcott, Reference Walcott1912); Acrothele, Acrothyra, Botsfordia, Eoobolus, Palaeoobolus.
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C7.—Western Mediterranean peri-Gondwana; Spain, Láncara Formation (Wotte and Mergl, Reference Wotte and Mergl2007), Muerero Formation (Liñán and Mergl, Reference Liñán and Mergl1982); Acrothele, Dictyonina, Genetreta, Iberotreta, Luhotreta, Micromitra.
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C8.—Baltica, Sweden, Forsemölla Limestone Bed and erratic boulders (Streng, Reference Streng1999; Streng et al., Reference Streng, Holmer, Popov and Budd2007), and Sablinka Formation (Gertovo Member) (Khazanovich et al., Reference Khazanovitch, Popov and Melnikova1984); Acrothele, Canalilatus, Eoobolus, Kotylotreta, Linnarssonia, Monophtalma, Obolus, Oepikites, Vandalotreta.
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C9.—Alai terrane; Kyrgzstan, Alai Range, Pseudoanomocarina Beds (Aksarina, Reference Aksarina, Repina, Yaskovich, Aksarina, Petrunina, Ponilenko, Rubanov, Bolgova, Golikov, Hajrullina and Posochova1975); Acrothele, Dictyonina, Hadrotreta, Kleithriatreta, Linnarssonia, Micromitra, Paterina.
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C10.—Siberia, Amga Regional Stage, composite list (Pelman, Reference Pelman1977; Pelman and Pereladov, Reference Pelman and Pereladov1986; Korovnikov, Reference Korovnikov1998); Acrothele, Botsfordia, Eoobolus, Eothele, Erbotreta, Kostjubella Kyrshabaktella, Linnarssonia, Paterina, Prototreta.
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C11.—Spity, Tethyan Himalaya; Parahio Formation, Cambrian Stage 5; Acrothele, Amictocracens, Aphelotreta, Eohadrotreta, Hadrotreta, Linnarssonia, Oepikites, Paterina, Prototreta.
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C12.—Central Bohemia, Czech Republic, Jince Formation (Mergl and Šlehoferová, Reference Mergl and Šlehoferová1990; Mergl and Kordule, Reference Mergl and Kordule2008); Acrothele, Almohadella, Botsfordia, Hadrotreta, Lindinella, Luhotreta, Treptotreta?, Vandalotreta, Westonia.
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C13.—West Antarctica, Shackleton and Argentina ranges, Unnamed Cambrian Stage 5 (Popov and Solovyev, Reference Popov and Solovyev1981); Acrothele, Linnarssonia, Notiobolus.
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C14.—Australasian segment of Gondwana, central Australia, southern Georgina Basin, Arthur Creek Formation, lower part (Percival and Kruse, Reference Percival and Kruse2014), Acrothele (=Orbithele), Amictocracens, Anabolotreta, Chakassilingula, Kyrshabaktella, Linnarssonia, Micromitra, Picnotreta, Treptotreta, Stilpnotreta.
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C15.—South China Guizhou Province, Kaili Formation, Unnamed Cambrian Stage 5 (Zhao et al., Reference Zhao, Zhu, Babcock and Peng2011), Acrothele, Dictyonina?, Linnarssonia?, Palaeobolus (=Lingulepis, may also belong to Notiobolus) Paterina? (=Micromitra). Generic assignation of so-called ‘Lingulella’ and ‘Paterina’ cannot be proved from provided descriptions and illustrations.
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C16.—Caborca Terrane, Sonora, Arrojos Formation; Acrothele, Batenevotreta?, Dictyonina, Eothele, Eoobolus, Hadrotreta, Linnarssonia, Micromitra, Paterina, Prototreta.
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