Types

Holotype: complete specimen facing the ventral valve (CORD-PZ 33014-2). Paratypes: ventral valve (CORD-PZ 33001-4) and dorsal valve (CORD-PZ 33015-3), plus tens of more or less complete shells and larger fragments (CORD-PZ 33001-1–6, 33001-9–14, 33002-11, 33004-6, 33014-1, 330014-2, 33015-1–8, and 33015-11–14).

Diagnosis

Lingulella? small-sized, moderately thick-walled, broadly ovate in outline and dorsibiconvex with scattered internal davisate pits; dorsal valve with fine and long median ridge; ventral pseudointerarea low, with distinct flexure lines; pedicle groove narrowly triangular; ventral central scars large, slightly oblique; exterior with fine concentric growth lamellae and radial ribbon-like striation.

Description

Shell dorsibiconvex, inequivalved, subtriangular in outline, with moderately thick shell wall. Adult shells 4 to 5 mm long according to recorded fragments, but the only complete ventral valve, representing a juvenile specimen, is 2 mm long, 125% as long as wide. Larval shell not clearly differentiated from postlarval shell (Fig. 3.4).

Figure 3

Lingulella melonica Mergl and Herrera n. sp., from the Lampazar Formation, Angosto de Lampazar locality. All are SEM photographs. Bar scale length 100 μm, except for (16) (20 μm) and (18, 19) (10 μm). (1, 13) ventral valve exterior (1) and detail of postlarval shell surface (13), holotype, CORD-PZ 33014-2; (2, 5) ventral valve exterior (2) and oblique view (5), CORD-PZ 33001-1; (3) dorsal valve exterior, CORD-PZ 33001-2; (4) larval shell of small ventral valve, CORD-PZ 33015-7; (6) incomplete dorsal valve exterior, CORD-PZ 33001-3; (7) ventral valve exterior in anterior view, CORD-PZ 33015-14; (8, 15) ventral valve interior (8) and detail of pseudointerarea (15), paratype, CORD-PZ 33001-4; (9) incomplete ventral valve interior, CORD-PZ 33001-5; (10, 16) dorsal valve oblique view (10) and detail of microornament (16), CORD-PZ 33015-2; (11, 12) incomplete ventral valve interior (12) and internal oblique view (11), CORD-PZ 33001-6; (14) dorsal valve interior in oblique view, paratype, CORD-PZ 33015-3; (17, 18, 19) ventral valve interior in oblique view (17), anterior part of median tongue of visceral area showing epithelial cell moulds (18), and detail of davisate pits between pedicle nerve imprints (19), CORD-PZ 33015-13.

Dorsal valve with rounded posterior margin and subcircular anterior margin, evenly convex in transverse and axial profiles. Maximum width at shell midlength. Dorsal pseudointerarea short, deeply U-shaped, with broadly triangular median groove. Anterior edge of pseudointerarea weakly elevated above valve floor and shortly excavated (Fig. 3.17). Dorsal valve interior with fine and long median ridge, disappearing anterior to shell midlength (Fig. 3.14). Central and anterior muscle scars small and poorly impressed posterior to shell midlength.

Ventral valve slightly acuminate, with apical angle ca. 110°, transverse profile rather convex apically, becoming depressed anteriorly; axial profile weakly and unevenly convex. Pedicle groove shallow, narrowly triangular, two to three times longer than wide, with anterior edge slightly elevated above level of visceral area. Propareas weakly raised above valve floor having inner propareas wider than outer propareas. Anterior edges of propareas subtending 90° angle, with edges of each inner proparea weakly excavated anteriorly (Fig. 3.8, 3.9, 3.12, 3.15). Slightly curved growth lines distinct on surfaces of both propareas. Pedicle groove surface smooth axially although with rough flanks. Visceral area weakly defined, with anterior border limited by large, short, broad and slightly oblique central muscle scars. Narrow and undivided muscle scars flanking posterolateral borders of visceral area. Vascula lateralia broad, gently diverging anteriorly. Imprints of pedicle nerve thin, impressed as weakly diverging groove at posterior half of ventral visceral area. Surface of visceral area shows weakly imprinted epithelian cell molds with flat surface, mostly hexagonal outline and ca. 20 μm diameter. Two low, mound-like clusters of small imprints, axially to central muscle imprints, are two scars of the gastroparietal band. Shell interior shows numerous, irregularly distributed subcircular deep davisate pits ca. 20–50 μm in diameter. Pits penetrate through several laminae of the shell (Fig. 3.17, 3.18, 3.19).

Shell exterior almost smooth, with five to ten unevenly spaced, weak concentric lamellae, and distinctive radial ribbon-like microornament (Fig 3.13, 3.16). This consists of alternating flattened strips and shallow grooves 7 to 10 μm wide.

Etymology

After the shell outline, suggesting the shape of watermelon seeds.

Remarks

The general morphology of this species is the same as in many early Paleozoic, minute Lingulella-like species; there are tens of species that need revision since Walcott’s (Reference Walcott1912) monograph. These species differ from L. davisi (M‘Coy, Reference M’Coy1851) as revised by Sutton et al. (Reference Sutton, Bassett and Cherns2000a), such that their attribution to Lingulella Salter is not definitive, and their generic attribution should be regarded as questionable. A differently pitted external micro-ornament in obolids of similar size and shape has been considered valid for the erection of Wahwahlingula Popov, Holmer and Miller, Reference Popov, Holmer and Miller2002. However, this new species is comparatively smooth.

Lingulella? melonica n. sp. is similar to L. similis Walcott, Reference Walcott1898 from the late Cambrian of South Dakota (Robson and Pratt, Reference Robson and Pratt2007) in terms of the outline and general shape of the ventral interarea. The new species has more clearly defined flexure lines, lacks a peripheral limbus apparent in L. similis, and has a fine dorsal median septum, which is not present in L. similis. Unfortunately, the microornament of L. similis is not clear.

Another similar species is L.? incurvata Popov, Holmer and Miller, Reference Popov, Holmer and Miller2002 from the lower Tremadocian of Utah (Popov et al., Reference Popov, Holmer and Miller2002). Both species have strongly elongated inequivalved shells, an elongate, deeply concave ventral pseudointerarea with a narrow and shallow pedicle groove and a fine long dorsal median ridge. The main difference between these species is a broader shell outline and a less acuminate ventral valve in the new species. This difference is somewhat problematical because Popov et al. (Reference Popov, Holmer and Miller2002) suggested that specimens of L.? incurvata belong to juveniles. However, these shells show broader outline and correspond to the shell size of L.? incurvata. The ventral pseudointerarea of L.? melonica is comparatively shorter than that in L.? incurvata. There are no data about the postlarval shell microornament of L.? incurvata besides the statement that the post-larval shell is smooth (Popov et al., Reference Popov, Holmer and Miller2002). We agree with Popov et al. (Reference Popov, Holmer and Miller2002) that L.? incurvata, Oepikites? elongatus Popov and Khazanovich, 1989 in Popov et al., Reference Popov, Khazanovitch, Borovko, Sergeeva and Sobolevskaya1989, and likely L. similis may represent a new genus, to which L.? melonica n. sp. could also be assigned, but until the microornament of all those taxa is taken into account, its erection is premature.

Genus Libecoviella Mergl, Reference Mergl1997

Remarks

In the original diagnosis of the genus (Mergl, Reference Mergl1997, Reference Mergl2001) no information about microornamentation was provided. The new species shows a microornament consisting of rheomorphic folds and scattered globose deep pits. This microornament confirms the attribution of Libecoviella to Obolinae King, Reference King1846 (see Cusack et al., Reference Cusack, Williams and Buckman1999).

Libecoviella lenticula Mergl and Herrera, new species

Figure 4.1–4.19

Types

Holotype: complete specimen facing the ventral valve with partly preserved original shell (CORD-PZ 33019). Paratypes: complete specimen facing the ventral valve with partly preserved original shell and showing transverse profile (CORD-PZ 33018), and incomplete dorsal valve (CORD-PZ 33013-3).

Diagnosis

Medium-sized, thin-walled, equally and gently biconvex Libecoviella with small, broadly triangular and anteriorly excavated ventral pseudointerarea; narrow, deep and short pedicle groove; almost perfect divaricate ornament on late post-larval shell surface, developed from subconcentric growth fila of early post-larval shell; muscle scars weakly impressed and situated posteriorly to shell mid-length; sparse and prominent coarse pitting over the whole internal surface of both valves.

Occurrence

Furongian, Lampazar Formation 15 m below the lowest quartzitic beds of the overlying Cardonal Formation, Angosto de Lampazar, Salta Province (24º 34’ 00’’ S; 65º 45’ 40’’ W).

Description

Shell equally biconvex, rectimarginate, evenly thin-walled (50 μm thick at shell mid-length), up to 12 mm long. Dorsal valve elongately oval, with broadly rounded posterior margin, evenly curved sides and semicircular anterior margin. Transverse and axial valve profiles weakly convex, with gently but distinctly flattened axial sector (Fig. 4.9). Dorsal pseudointerarea short, gently elevated above valve floor, ~50% as long as valve width. Shallow median groove large, with broadly crescentic outline and undercut anterior edge. Propareas orthocline, short, poorly raised above valve floor, having free undercut anterior edge near upon shell margin (Fig. 4.10). Broadly rhomboidal visceral area poorly defined by shell thickening. Pair of narrow undivided muscle scars placed in posterolateral part of visceral area near shell margin (Fig. 5). Central muscle scars medium-sized, elongately oval, placed posteriorly to valve midlength. Anterior projection short, with pair of anterolateral muscle scars near shell center (Fig 4.12). Weak median ridge developed between central muscle scars (Fig 4.12). Vascular system not impressed apart of short proximal part of vascula lateralia.

Ventral valve elongately oval, 117%–120% as long as wide, evenly and moderately convex in axial profile, with maximum depth at shell midlength, acute posterior margin with apical angle approximately 100°, almost straight posterolateral margins, evenly curved sides and semicircular anterior margin (Fig. 4.7). Transverse shell profile subcarinate near beak, forward turning into low evenly convex profile. Ventral pseudointerarea almost orthocline, developed as broadly triangular short platform, halved by short and deep pedicle groove, slightly expanding anteriorly, with smooth floor not elevated above shell floor (Fig. 4.7, 4.11). Inner propareas broad, gently sloping axially, with gently curved deeply excavated anterior edges, and subtending approximately 150°. Flexure lines poorly defined. Outer propareas narrow, orthocline. Growth lines distinct on inner propareas surface, with coarser lines distinct also on sloping sides of pedicle groove. Outer propareas bare. Visceral field broadly rhomboidal, 30% as long as valve. Central muscle scars are oblique, weakly impressed, with anterior limits subtending almost 80°. Vascular system not preserved.

Ornamentation of early post-larval shell consisting of regularly arranged concentric raised growth fila. These fila become less regular forwards and laterally, achieving more and more divaricate zigzag terrace line pattern. Divaricate terrace lines regularly cover entire shell surface in large shells but axial shell sector of very large shells may lack this divaricate ornament. Slopes of terrace lines facing towards shell margins always steep, often with overhanging edge, while posterior slopes of terrace lines much weaker and broader. Rugellae-like growth concentric fila at apical region of shell less than 0.1 mm apart. Distances of terrace lines range from 0.1 to 0.15 mm. Low, short and regular concentric growth lamellae developed anteriorly on large shells.

Microornament consisting of rheomorphic folds, especially distinct at slopes and bottoms of terrace line interspaces, as well as of pitting, made up of circular, semiglobose deep pits, scattered on an otherwise almost smooth shell surface (Fig 4.18, 4.19). Pit size ranges from 2 to 3 μm, with maximum diameter of 5 μm. Size of pits on slopes of interspaces usually bigger than on facing upper surface of terrace lines. Groups of a few pits may form more or less discrete radial rows (Fig 4.17).

Inner shell surface bearing numerous, irregularly scattered, up to 70 μm large, deep subcircular davisate pits (Fig. 4.10, 4.15). Pits anteriorly arranged into concentric rows, which follow the course of growth lamellae.

Etymology

After the lenticular transverse profile of the shell.

Remarks

The irregular zigzag ornament of terrace lines derived from the concentric raised lines is a comparatively uncommon feature in Obolidae. However, as pointed out by Savazzi (Reference Savazzi1986), the divaricate zigzag ornament represents a functional analogy. Terrace lines are present in diverse and taxonomically probably unrelated obolids, being often derived from a fine rugellate ornament as in Rebrovia Popov and Khazanovitch, 1989 and Westonisca Havlíček, Reference Havlíček1982, or from a spacing of triangular pustulose ornament as in Kacakiella Mergl, Reference Mergl2001 and Spinilingula Cooper, Reference Cooper1956. The divaricate ornament of the new species is similar to that of Westonia Walcott, Reference Walcott1901 and Libecoviella Mergl, Reference Mergl1997 and also to taxonomically unrelated and stratigraphically much younger Glyptoglosella Cooper, Reference Cooper1956. Westonia differs from the new species by an anteriorly more expanded pedicle groove and much finer and more regular zigzag lines. This unique zigzag ornament is known in the majority of species referred to Westonia (Walcott, Reference Walcott1912; Ulrich and Cooper, Reference Ulrich and Cooper1938; Gorjansky, Reference Gorjansky1969; Legrand, Reference Legrand1969; Mergl and Šlehoferová, Reference Mergl and Šlehoferová1990; Popov and Gorjansky, Reference Popov and Gorjansky1994; Mergl, Reference Mergl2002) but it is often restricted to the posterior, posterolateral or lateral parts of the shells (Holmer and Popov, Reference Holmer and Popov2000). The new species has the zigzag ornament less regular, tracing the course of growth lines. The divaricate lines of the new species are developed on almost the entire shell surface except on the early postlarval shell and the anteromedian sector of the largest valves.

Libecoviella accommodates two species known from the sandstones of assumed Tremadocian age in Bohemia, Czech Republic. The type species L. arachne (Barrande, Reference Barrande1879) differs from the new species by a subtriangular shell outline and a distinct internal striation of the shell interior. L. ovata (Havlíček, Reference Havlíček1982) differs by a development of oblique terrace lines devoid of zigzag pattern in the posterolateral parts of the shell and by a weakly trilobate anterior shell margin.

The pitted microornament observed on the post-larval shell surface of the new species is, as far as known, unique among the obolids. This pitting had not been reported in any species referred to Libecoviella or Westonia. The pitted surface is known in zhanatellids, but also in Hyperobolus Havlíček, Reference Havlíček1982, which was originally referred to the Obolinae King, 1828 (Popov and Holmer, Reference Popov and Holmer1994). Hyperobolus, together with other morphologically related genera (Paldiskia Gorjansky, Reference Gorjansky1969, Rosobolus Havlíček, Reference Havlíček1982, Thysanotos Mickwitz, Reference Mickwitz1896) was referred to the Zhanatellidae due to the pitted microornament of the postlarval shell (Holmer and Popov, Reference Holmer and Popov2000). However, the taxonomic relevance of pitted microornament should not be overemphasized. The variability in the density and shape of pits indicates that the superficial pitting is a common feature in more groups of linguloids and simply reflects preservational potential of vesicles (Williams, Reference Williams2003). The use of only this feature as a criterion for the separation of the Zhanatellidae from other similar linguloid families is, according the present authors, a weak argument. Therefore, the new species is referred to the Obolinae and not to the Zhanatellidae, despite the pitted microornament.

Family Elkaniidae Walcott and Schuchert, 1908

Genus Saltaia Mergl and Herrera, new genus

Type species

Saltaia lampazarensis Mergl and Herrera, n. gen. n. sp., by monotypy.

Diagnosis

Thick-walled and moderate biconvex shell, weakly raised visceral areas, small central and anterolateral muscle scars in the dorsal valve interior, concentric macroornament crossed by uneven radial plications, and microornament of rhombic regularly arranged pits.

Etymology

After the Salta Province, northwestern Argentina.

Remarks

The new genus is most similar to Broeggeria Walcott, Reference Walcott1902 but differs from that by a thick shell, external radial plications, and a rhombic microornament. Others Furongian to Tremadocian elkaniids differ by distinctly elevated visceral fields and prominent dorsal valve convexity as Elkania Ford, Reference Ford1886 and Lamanskya Moberg and Segerberg, Reference Moberg and Segerberg1906 and by a finely rugellate ornament as Elkanisca Havlíček, Reference Havlíček1982 although these genera have similar microornament with rhombic pits. The late Cambrian Pseudodicellomus Bell in Bell and Ellinwood, Reference Bell and Ellinwood1962 differs by the microornament of circular pits and by the broadly triangular pedicle groove. However, attribution of Pseudodicellomus to Elkaniidae is assumed problematic (Holmer and Popov, Reference Holmer and Popov2000). The presence of rhomboidal pits justifies the attribution of Saltaia to the elkaniids, although the new genus is by its general morphology similar to some late Cambrian and Early Ordovician obolids and zhanatellids.

Saltaia lampazarensis Mergl and Herrera, new species

Figure 6.1–6.14

Types

Holotype: ventral valve with partly preserved original shell (CORD-PZ 33028). Paratype, dorsal valve with partly preserved original shell (CORD-PZ 33026).

Diagnosis

Shell moderately biconvex, slightly inequivalved, subcircular; ventral pseudointerarea orthocline, short, with propareas separated by distinct flexure lines; narrow and deep pedicle groove; dorsal pseudointerarea forming short crescentic strip along posterior shell margin; visceral areas weakly elevated, not extending anterior to midlength; dorsal vascula media thin, long; macroornament of concentric fila with evenly sized wavy radial plications; postlarval shell with finely pitted microornament of regularly spaced rhombic concave pits.

Occurrence

Furongian, Lampazar Formation, 15 m below the lowest quartzitic beds of the overlying Cardonal Formation; Angosto de Lampazar, Salta Province, Argentina (24º 34’ 00’’ S – 65º 45’ 40’’ W).

Description

Shell dorsibiconvex, rectimarginate, thick-walled (200–300 μm thick in shell midlength), 9 mm long in largest specimen.

Dorsal valve subcircular, with broadly rounded posterior margin, evenly rounded sides and less rounded anterior margin. Transverse profile gently convex. Axial profile moderately convex. Dorsal pseudointerarea short and low, forming broad arcuate strip along posterior margin (Fig. 6.4). Visceral area poorly defined. Central muscle scars small, elongately oval, placed relatively close to each another at about shell midlength, posteriorly extended into subparallel grooves delimitating margins of visceral area (Fig. 6.9). Oval anterolateral muscle scars half-sized, located close and anteromedianly to central scars (Fig. 6.9). Thin, rapidly diverging proximal trunks of vascula media extend from anterior lobes of anterolateral muscle scars (Fig. 6.9). Proximal part of vascula lateralia broadly divergent, branched into thinner short perpendicular secondary canals.

Ventral valve subcircular, as long as wide, with gently pointed apex at posterior margin, evenly curved sides and less curved anterior margin. Maximum width and height at shell midlength. Apical angle ca. 110°. Valve moderately convex in lateral profile, with same convexity in transverse profile at shell midlength. Posterior part of valve clearly arched along shell axis, flanked by flattened sides. Almost orthocline ventral pseudointerarea developed as broadly triangular short platform, halved by short, deep and narrowly triangular pedicle groove. Propareas weakly divided by weak flexure lines (Fig. 6.1, 6.2). Inner and outer propareas equally broad. Fine growth lines covering the surface of propareas and flanks of pedicle groove. Visceral area broadly rhomboidal, ~30% as long as valve (Fig. 7). Central muscle scars short and wide, slightly oblique, well-impressed, located in the posterior third of the valve (Fig. 6.3, 6.6). Vascula lateralia distinct, diverging at about 80° of each other, with proximal trunks subparalell with shell sides. Distal part of each trunk turns anteromedianly at about shell midlength (Fig. 6.11). Thin secondary branches present at both sides of vascula lateralia are poorly preserved.

Ornamentation of mature shell of weak growth fila (ca. 12 per 1 mm) and widely spaced growth lamellae. Radial ornament of irregular plications of uneven size separated by slightly broader interspaces (Fig. 6.5, 6.12). Postlarval shell covered by shallow evenly and uniformly concave pits having rhombic or lenticular outlines. Pits width ranges from 5 to 20 μm, but majority of pits ca. 10 wide. Length:width of pits varies from 0.3 to 0.7. Pits arrangement remarkably regular (Fig. 6.13), generally forming oblique lines, which results of regular secretion of uniformly-sized lenticular vesicles. However, this regular pattern is disrupted when groups of larger or smaller pits are present (Fig. 6.14). This irregularity is quite common along the course of radial plications while smoother shell surface shows higher level of regularity in pits size and arrangement.

Etymology

After the type area of occurrence.

Material

Three ventral valves and three dorsal valves (CORD-PZ 33008, 33011, 33012, 33026–33028) and several small fragments (CORD-PZ 33013-4, and 33014-3–33014-6).

Order Siphonotretida Kuhn, Reference Kuhn1949

Superfamily Siphonotretoidea Kutorga, Reference Kutorga1848

Family Siphonotretidae Kutorga, Reference Kutorga1848

Genus Schizambon Walcott, Reference Walcott1884

Schizambon cardonalis Mergl and Herrera, new species

Figure 8.1–8.23

Types

Holotype: ventral valve internal mould CORD-PZ 33007). Paratype, dorsal valve CORD-PZ 33005-1).

Diagnosis

Schizambon small subcircular, with steeply apsacline ventral pseudointerarea, distinct, narrowly triangular raised fold in ventral valve, narrowly triangular pedicle track and prominent concentric fila with superimposed concentric rows of small prostrate hollow spines.

Occurrence

Furongian, Lampazar Formation, 15 m below the lowest quartzitic beds of the overlying Cardonal Formation; Angosto de Lampazar, Salta Province, Argentina (24º 34' 00" S - 65º 45' 40" W).

Description

Shell small, thin to moderate thick walled, up to 4 mm wide. Maximum width at shell midlength. Dorsal valve subcircular to broadly elliptical, with less curved posterior margin, which occupies ~50% of shell width. Valve weakly convex, with maximum height in posterior one-fifth, having weakly flattened median sector and depressed posterolateral extremities (Fig. 8.13). Apex placed at steeply sloping posterior shell surface. Dorsal pseudointerarea narrow and short, almost orthocline platform covered by few coarse growth lines (Fig. 8.14, 8.18). Median groove very short, resting almost vertically and having acute edge at contact with propareas (Fig. 8.18). Posterior valve floor very steeply sloping. Undivided muscle scars (=outside lateral, transmedian, and middle lateral muscles) small, placed posterolaterally. Central muscle scars oval, placed posterolaterally to shell midlength. Short and weak median ridge present in visceral area. Anterolateral muscle scars located in anterior third of valve, placed close to each other. Vascula lateralia weakly defined, extending from anterior of undivided posterior muscle scars. No clearly defined peripheral brim or papillate strip along inner shell margin.

Ventral valve elongately elliptical, 105%–110% as long as wide, with subangular posterior margin, less curved lateral margins, and subcircular anterior margin. Valve gently convex in transverse and axial profiles having maximum depth at posterior third. Shell convexity decreases forwards from shell midlength. Apex pointed, raised above posterior valve margin. Apical chamber deeply excavated. Ventral pseudointerarea low and catacline in juvenile shells, becoming steeply apsacline in large shells. Pseudointerarea occupies ca. 55%–60% of shell width, with posterior axial part having posteriorly slightly arched surface and lower, flat and narrowly triangular propareas (Fig. 8.13). Coarse growth lines covering surface of pseudointerarea. External pedicle opening subcircular, up to 0.2 mm in diameter in large individuals. Pedicle opening situated at anterior end of narrowly triangular pedicle track extending from apex (Fig. 8.7, 8.16). Gently elevated median fold occupying ~25% of shell width developed anterior to pedicle opening (Fig. 8.1, 8.4). This fold less apparent anteriorly in large shells. Ventral valve interior with large internal pedicle foramen, 0.2 mm wide in large shells, encircled by swollen collar (Fig. 8.19). Two narrowly triangular raised lobes form internal face of pedicle track. Visceral area weakly defined. Central muscle scars large, oblique, weakly impressed, placed posterior to shell midlength (Fig. 8.3). Pair of minute scars placed anteromedian to central scars. Proximal trunks of broad vascula lateralia subparallel with shell margin, converging towards each other from shell midlength (Fig. 8.3).

Ornamentation of early postlarval shell consists of regular concentric growth fila, moderately raised and arranged in regular intervals, numbering ca. 25 fila per 1 mm. Concentric ornament becomes less distinct anteriorly, and is outbalanced by coarser radially arranged fila (Fig. 8.5, 8.10). Growth lamellae weak, densely crowded anteriorly being rare in posterior half of shell. Prostrate hollow spines strictly uniformly sized, having external diameter ca. 50 μm, inclined at a 10°–30° angle toward the shell surface (Fig. 8.22). Preserved maximal length of spines 0.4 mm. Spines arranged in discrete concentric rows, with individual spines 0.10 to 0.15 apart. In general, spines aligned in concentric rows show radial arrangement over the shell surface, with spines on posterolateral shell surface directed posterolaterally to posteriorly (Fig. 8.10, 8.16).

Etymology

After densely spinose surface similar to cactus locally named cardón.

Material

Four ventral valves (CORD-PZ 33006, 33007, 33009, 33010) and tens of shell fragments (CORD-PZ 33002-1, 33002-3 to 33002-10, 33004-1, 33004-2, 33004-7, 33005-1 to 33005-3, 33005-6, and 33005-8).

Remarks

Schizambon Walcott, Reference Walcott1884 is represented by ~20 species, some of them rather poorly preserved, from the upper Cambrian to the Lower Ordovician. These species are known from many paleocontinents (Walcott, Reference Walcott1884; Ulrich and Cooper, Reference Ulrich and Cooper1938; Popov and Holmer, Reference Popov and Holmer1994; Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001, Reference Holmer, Popov, Streng and Miller2005; Mergl, Reference Mergl2002; Popov et al., Reference Popov, Holmer and Miller2002; González-Gómez, Reference González-Gómez2005), but most occurrences are from ancient tropic and temperate climatic belts. Many descriptions are not sufficient for critical comparison lacking the important information about the spinose ornament. There is often noted concentric, radial, or both ornaments devoid of hollow spines, but this feature is likely only preservational, at least in some species. Secondary abrasional or taphonomic lacks of spines accommodating setae is probable feature in other siphonotretid genera from the Ordovician to the latest Lower Devonian (Mergl, 2000, Reference Mergl2002; Popov et al., Reference Popov, Bassett, Holmer and Ghobadi Pour2009; Mergl and Vodrážková, Reference Mergl and Vodrážková2012). Many of our shell fragments show striking and nice radial or concentric ornaments with only locally preserved groups of spines or spine bases, indicating abrasion and/or exfoliation of the primary shell layer with spines prior the fossilization or during the extraction of shells from rocks.

The new species is similar to Schizambon convexus Gonzalez-Gómez, 2005 from the Val d’Homs Formation (upper Cambrian) of southern France (Gonzalez-Gómez, 2005). This species has similar outline and concentric ornament, but differs from the new species in the absence of spines, shorter pedicle track, and more prominent radial ornament. Another closely related species is S. borealis Ulrich and Cooper, Reference Ulrich and Cooper1938, which also has a prominent triangular elevated fold on the ventral valve but differs from the new species by a more transverse outline, the absence of distinct concentric rows of spines, a larger pedicle foramen and a smaller size being only 3 mm wide (Ulrich and Cooper, Reference Ulrich and Cooper1938). Schizambon delicatula Ulrich and Cooper, Reference Ulrich and Cooper1938, has finer ornamentation. Schizambon australis Ulrich and Cooper, Reference Ulrich and Cooper1938 from northwestern Argentina has much prominent radial ornament and its ventral valve lacks a distinct triangular fold. Other species listed by Ulrich and Cooper (Reference Ulrich and Cooper1938) are less similar to the new species and are not commented herein. Schizambon reticulatus MacKinnon, 1976, differs by the presence of a reticulate ornament and the absence of the median fold (Shergold et al., Reference Shergold, Cooper, MacKinnon and Yochelson1976). Two Lower Ordovician species from Estonia (Gorjansky, Reference Gorjansky1969) differ by having a larger shell size, larger pedicle foramina and by lack of ventral triangular folds. Schizambon typicalis Walcott, Reference Walcott1884 (Utah, Barn Canyon Member, Cordylodus lindstromi Zone to lower part of C. angulatus Zone; Popov et al., Reference Popov, Holmer and Miller2002) differs from the new species by having a more prominent radial ornament and the absence of the ventral triangular fold. Schizambon obtusus Holmer, Popov, Streng, and Miller, Reference Holmer, Popov, Streng and Miller2005 (Utah, Barn Canyon Member, upper half of C. angulatus Zone; Holmer et al., Reference Holmer, Popov, Streng and Miller2005) differs from the new species by a transverse outline and a shorter pedicle track. In addition, this species has exterior covered by nodes and pustules, and by solid blunt “spines” on intersection of concentric lamellae and radial parvicostellae. Holmer et al. (Reference Holmer, Popov, Streng and Miller2005) suggested that neither S. obtusus nor S. typicalis have the true hollow siphonotretoid spines. Unlike these species, the new species has well-developed hollow spines (Fig. 8.22).

Order Acrotretida Kuhn, Reference Kuhn1949

Superfamily Acrotretoidea Schuchert, Reference Schuchert1893

Family Acrotretidae Schuchert, Reference Schuchert1893

Genus Eurytreta Rowell, Reference Rowell1966

Eurytreta harringtoni Mergl and Herrera, new species

Figure 9.1–9.20

Types

Holotype: ventral valve (CORD-PZ 33003-1). Paratype, dorsal valve (CORD-PZ 33003-7).

Diagnosis

Eurytreta with subplanar, moderately sulcate dorsal valve, high conical ventral valve, weakly apsacline ventral pseudointerarea, distinct peripheral brim in dorsal valve and broad and moderately high dorsal median ridge.

Occurrence

Furongian, Lampazar Formation, 15 m below the lowest quartzitic beds of the overlying Cardonal Formation; Angosto de Lampazar, Salta Province, Argentina (24º 34’ 00’’ S – 65º 45’ 40’’ W).

Description

Shell up to 3 mm wide, with broadly and weakly unisulcate commissure, fairly variable in convexity and outline, and ~80% as wide as long. Dorsal valve broadly oval, with regularly rounded posterolateral margins, gently rounded posterior margin, and less rounded anterior margin although always curved; subplanar to slightly concave in anterior view. Maximum shell width at or immediately anterior to shell midlength. Valve thickened posteriorly, with thin shell around anterior and lateral peripheries in large individuals. Broad sulcus already apparent in early mature shell in large specimens (Fig. 9.11). Larval shell 150 μm wide, weakly convex with large subcentral mound, and two smaller and lower anterolateral mounds weakly defined (Fig. 9.9, 9.19). Boundary of larval shell defined with prominent step-like growth line. Surface of larval shell bears fine hemisphaerical pits that become progressively smaller toward its periphery (Fig. 9.19, 9.20). Dorsal interarea large, 40%–50% as wide as valve and ~15% long as valve. Propareas anacline, narrowly triangular, broadly divergent, with posterolaterally sloping surfaces (Fig. 9.18). Median groove widely triangular to broadly rhomboidal in large specimens, anteriorly excavated, with edges diverging at 110°–130° (Fig. 9.12). Median septum extends up to 75% of valve length, being highest from shell midlength to its anterior end. Septum stout posteriorly becoming much thinner forward extending from wide low median buttress (Fig. 9.16, 9.17). Marginal brim prominent, flat, distinctly developed, ~20% as long as valve, bordered by steep slope from visceral area (Fig. 9.17). Visceral area well defined, 60% as long as valve. Cardinal scars large, obliquely oval, placed relatively close one to another, with highly elevated surface separated from median ridge by deep pits (Fig. 9.16, 9.17). Small and circular anterocentral scars well defined being located posterior to shell midlength. Vascula lateralia thin, broadly diverging between cardinal and anterocentral scars, branched into several canals from both sides of main trunk. Vascula media are thin canals paralleling median septum, with two distinct lateral canals (Fig. 9.17).

Ventral valve highly conical, with highest point at apex, evenly rounded margins, straight to weakly convex lateral slopes and gently to moderately convex anterior slope. Valve thickened apically with thinner peripheral parts. Apical angle of shell is 60°–70° in anterior view. Ventral pseudointerarea gently apsacline, slightly curved in lateral view, narrowly triangular, occupying ~30% of shell width. Moderately widening interridge with flat surface accentuated by coarse growth lines, extending dorsally into shortly arcuate tongue (Fig. 9.3). Ventral larval shell small, with weakly defined margins. Its surface bears same type of pitting as dorsal larval shell. Pedicle opening rounded and placed within larval shell (Fig. 9.15). Ventral valve interior with low triangular apical process. Cardinal muscle scars located on posterolateral sector of internal valve slope, with weakly convex surface (Fig. 9.5). Vascula media prominent, consisting of broad, straight and deeply impressed and gently arcuate undivided canals, diverging anteriorly ~45° and extending beyond shell midlength (Fig. 9.5).

Ornamentation consisting of fine distant growth lines interrupted in irregular intervals by fine growth lamellae.

Etymology

After H. J. Harrington for his studies of the Paleozoic fauna in the area.

Material

Hundreds of valves, mostly heavily fragmented (CORD-PZ 33002-2, 33003-9, 33004-3–5, 33004-8, 33005-4, 33005-7, 33014-7, and 33015-9).

Remarks

The morphology of the shell is consistent with the redefinition of Eurytreta Rowell, Reference Rowell1966 by Streng et al. (Reference Streng, Mellbin, Landing and Keppie2011): “narrow ventral pseudointerarea, low dorsal median ridge, dorsal cardinal muscle scars placed close one to another, rather weak external ornament, and hemisphaerical pits on the larval shell.” Many species of this genus have been described from the upper Cambrian to the Middle Ordovician, mainly from sites of low–latitude paleocontinents. These species from Laurentia (Rowell, Reference Rowell1966; Krause and Rowell, Reference Krause and Rowell1975; Holmer et al., Reference Holmer, Popov, Streng and Miller2005), Baltica (Biernat, Reference Biernat1973; Popov and Holmer, Reference Popov and Holmer1994), Kazakhstan (Popov and Holmer, Reference Popov and Holmer1994; Holmer et al., Reference Holmer, Popov, Koneva and Bassett2001), and periphery of Gondwana (Owens et al., Reference Owens, Fortey, Cope, Rushton and Bassett1982; Sutton et al., Reference Sutton, Bassett and Cherns2000b) are discussed by Streng et al. (Reference Streng, Mellbin, Landing and Keppie2011). Eurytreta harringtoni n. sp. is distinct from the majority of the species referred to this genus by the highly conical shell, weakly apsacline ventral pseudointerarea, and the distinct peripheral brim in the dorsal valve interior. The most similar species is Eurytreta chabakovi (Lermontova, 1933) from the Lower Ordovician Akbulaksai and Alimbet formations of the South Urals (Popov and Holmer, Reference Popov and Holmer1994). This species shares with E. harringtoni a sulcate dorsal valve and a highly conical apsacline ventral valve. Illustrated specimens (Popov and Holmer, Reference Popov and Holmer1994, fig. 83) of E. chabakovi differ by a less sulcate dorsal valve, a lower dorsal median ridge, a weaker dorsal internal brim, and more transverse outline of the dorsal valve. The ventral valve of E. chabakovi has only weakly impressed vascular system. The topotypic specimens of Eurytreta curvata Walcott, Reference Walcott1902 figured by Popov and Holmer (Reference Popov and Holmer1994) differ from the new species by a lower ventral valve having a posteriorly curved apex and a deeper, and more transverse dorsal valve lacking a broad sulcus. Eurytreta sabrinae (Callaway, Reference Callaway1877) was revised by Sutton et al. (Reference Sutton, Bassett and Cherns2000b), who noted subevenly convex, sometimes weakly sulcate dorsal valve, unlike the new species, which shows a deep and broad dorsal sulcus. The specimens of E. cf. sabrinae from Mexico (Streng et al., Reference Streng, Mellbin, Landing and Keppie2011) differs from the new species in having more closely spaced dorsal cardinal scars and a thinner median septum. Eurytreta cf. fillmorensis Holmer, Popov, Streng, and Miller, Reference Holmer, Popov, Streng and Miller2005 can be distinguished from the new species by a more apsacline ventral pseudointerarea, having the interridge with variably deflected growth lamellae: dorsally, ventrally, or not deflected at all (Streng et al., Reference Streng, Mellbin, Landing and Keppie2011). These growth lamellae are always dorsally deflected in the new species paralleling the short dorsal tongue-like extension of the interridge at the posterior shell margin. Another difference of E. harringtoni is the absence of flattening of the ventral larval shell surface anterior to the short pedicle tube. The dorsal valve of E. cf. fillmorensis differs by the higher and thinner median septum and the posteriorly more protruding dorsal beak.

Figure 4

Libecoviella lenticula Mergl and Herrera n. sp., from the Lampazar Formation, Angosto de Lampazar locality. Specimens coated with ammonium chloride, except (10, 11) and (13–19), which are SEM photograps. Bar scale length 1 mm, except for (13–17) (100 μm) and (18, 19) (10 μm). (1) large ventral valve, CORD-PZ 33017; (2) almost complete dorsal valve, CORD-PZ 33016-4; (3) incomplete dorsal valve, CORD-PZ 33016-1; (4) fragment of ventral valve, CORD-PZ 33016-2; (5, 9) ventral valve of complete shell (5) and its transverse profile (9), paratype, CORD-PZ 33018; (6) ventral valve of complete shell, holotype, CORD-PZ 33019; (7) ventral valve of complete shell, CORD-PZ 33020; (8) incomplete ventral valve, CORD-PZ 33016-3; (10) dorsal valve interior, paratype, CORD-PZ 33013-3; (11) ventral valve interior, CORD-PZ 33013-2; (12) dorsal valve internal mould, CORD-PZ 33021; (13) fragment of shell showing terrace lines, CORD-PZ 33001-7; (14, 16–19) fragment of shell showing terrace lines (14) and details of rheomorphic and pitted microornament (16–19), CORD-PZ 33005-5; (15) dorsal valve interior showing large internal pits, CORD-PZ 33013-1.

Figure 5

Dorsal and ventral valve interior reconstructions of Libecoviella lenticula Mergl and Herrera n. sp., from the Lampazar Formation.

Figure 6

Saltaia lampazarensis Mergl and Herrera, n. sp., from the Lampazar Formation, Angosto de Lampazar locality; specimens coated with ammonium chloride except (13) and (14), which are SEM photographs. Bar scale length 1 mm, except (13) (50 μm) and (14) (10 μm). (1) fragment of ventral valve interior, CORD-PZ 33022; (2) fragment of ventral valve showing lateral extremity of proparea, CORD-PZ 33023; (3) exfoliated incomplete ventral valve, CORD-PZ 33024; (4) fragment of dorsal valve showing median groove, CORD-PZ 33025; (5, 12) dorsal valve exterior (5) and detail of radial plicate ornament (12), paratype, CORD-PZ 33026; (6, 7) ventral valve exterior with partly exfoliated shell (6) and oblique view (7), CORD-PZ 33027; (8) slightly exfoliated ventral valve, CORD-PZ 33008; (9) dorsal valve, partly exfoliated, CORD-PZ 33012; (10) dorsal valve interior, CORD-PZ PZ 33011; (11) deeply exfoliated ventral valve, holotype, CORD-PZ 33028; (13) fragment of shell showing microornament, CORD-PZ 33013-4. (14) fragment of shell showing microornament, CORD-PZ 33014-6.

Figure 7

Dorsal and ventral valve interior reconstructions of Saltaia lampazarensis Mergl and Herrera n. sp., from the Lampazar Formation.

Figure 8

Schizambon cardonalis Mergl and Herrera n. sp., from the Lampazar Formation, Angosto de Lampazar locality. Specimens (1–4) coated with ammonium chloride, remaining are SEM photographs. Bar scale length 500 μm, except (1–4) (1 mm), (16, 20–22) (100 μm), and (23) (50 μm). (1) internal mold of small ventral valve, CORD-PZ 33010; (2) internal mold of ventral valve, CORD-PZ 33009; (3) internal mold of ventral valve showing vascula lateralia, holotype, CORD-PZ 33007; (4) ventral valve interior, CORD-PZ 33006; (5, 9, 21, 22) dorsal valve exterior (5) and oblique view (9), details of larval shell (21) and of shell ornamentation (22), paratype, CORD-PZ 33005-1; (6, 13) dorsal valve exterior (6) and oblique view (13), CORD-PZ 33002-1; (7, 11) ventral valve exterior (7) and oblique view (11), CORD-PZ 33002-8; (8, 12, 16) ventral valve exterior (8), anterolateral view (12), and detail of pedicle track (16), 33005-2; (10, 20) dorsal valve exterior (10) and detail of larval shell (20), CORD-PZ 33005-3; (14, 18) incomplete dorsal valve interior (14) and oblique view (18), CORD-PZ 33002-3; (15) incomplete ventral valve interior, CORD-PZ 33004-2; (17) ventral valve interior, CORD-PZ 33004-1; (19) incomplete ventral valve interior in oblique view, CORD-PZ 33002-4; (23) detail of hollow spine, CORD-PZ 33002-10.

Figure 9

Eurytreta harringtoni Mergl and Herrera n. sp., from the Lampazar Formation, Angosto de Lampazar locality. All are SEM photographs. Bar scale length 500 μm, except (15) (50 μm), (19) (25 μm), and (20) (10 μm). (1, 6, 10, 15) Partially broken ventral valve in ventral (1), lateral (6), and posterior views (10), and detail of larval shell (15), CORD-PZ 33004-3; (2, 4) ventral valve exterior (4) and posterolateral view (2), holotype, CORD-PZ 33003-1; (3) incomplete ventral valve showing pseudointerarea, CORD-PZ 33002-2; (5) ventral valve interior, CORD-PZ 33003-5; (7) incomplete ventral valve showing pseudointerarea, CORD-PZ 33003-4; (8, 11) dorsal valve exterior (8) and anterolateral view, CORD-PZ 33005-4; (9, 14, 19, 20) dorsal valve exterior (9), oblique view (14), larval shell (19), and detail of pitting on the larval shell (20), CORD-PZ 33004-4; (12) incomplete dorsal valve interior, CORD-PZ 33003-6; (13) incomplete dorsal valve exterior, CORD-PZ 33004-5; (16–18) oblique (16), dorsal valve interior (17), and anterior (18) views, paratype, CORD-PZ 33003-7.