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The paper describes the brachiopod fauna recovered from the outer-ramp deposits developed …


Biology Articles » Paleobiology » Paleoecology » An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications » Systematic paleontology

Systematic paleontology
- An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications

Systematic paleontology (by J.L. Benedetto)

All figured and cited specimens are deposited in the Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina (prefix CEGH-UNC).

Order Orthida Schuchert and Cooper, 1932

Suborder Orthidina Schuchert and Cooper, 1932

Superfamily Orthoidea Woodward, 1852

Family Nanorthidae Havlícek, 1977

Genus Archaeorthis Schuchert and Cooper, 1931

Type species. Orthis electra Billings, 1865

Archaeorthis sp.
Figures 5.A-5.K

Material and occurrence. Three fragmentary ventral valves and two dorsal valves CEGH-UNC 21801-21805; two internal molds of dorsal valve, CEGH-UNC 21806 and 21832. Uppermost levels of the San Juan Formation, Quebrada Potrerillos.

Description. Medium size, largest specimen 11,6 mm wide, ventribiconvex, subelliptical shells (average length/width ratio 0,84). Cardinal extremities rounded, obtuse; maximum width at mid-length. Ventral valve moderately and evenly convex, maximum convexity in the posterior third. Ventral interarea planar, strongly apsacline (almost orthocline in one specimen), slightly higher than the dorsal interarea. Delthyrium open, large, unmodified. Dorsal valve gently convex with a shallow broad sulcus originating at umbo. Dorsal interarea narrow, planar, anacline. Ornament multicostellate with rounded primary costellae, finer second-order costellae originating at valve mid-length, and in some specimens third-order costellae developed incipiently at valve margin; number of costellae increases almost exclusively by intercalation; in mature specimens the count is of 6-7 costellae per 2 mm.
Ventral valve interior with strong teeth supported by very short dental plates extending anteriorly into medially convergent muscle-bounding ridges. Crural fossettes shallow, subcircular. Muscle field subrhomboidal buttressed anteriorly by a prominent, elongate oval premuscular callosity (the anterior extension of callosity and other proportions cannot be determined because of the incompleteness of available specimens).
Dorsal valve interior with subtriangular concave notothyrial platform strongly raised above the valve floor and continuous with a stout, rounded median dorsal ridge extending anteriorly to about one-half valve length. Cardinal process absent. Brachiophores thickened distally, widely divergent, bounding deep crenulated dental sockets excavated partially into the posterior wall of the valve; in a largest specimen they are floored by stout incipient fulcral plates (figure 5.F). Dorsal muscle field with subtriangular anterior adductor scars and smaller, elongate oval posterior scars; anterior pair of adductor scars penetrates as a wedge between the median dorsal ridge and the inner side of the posterior pair.
Remarks. The Potrerillos specimens are assigned to the genus Archaeorthis rather than to the closely related Nanorthis on the basis of the prominent premuscular thickening in the ventral valve. In addition, both teeth and brachiophores are strong, like in the former genus. A distinctive feature of our specimens is the morphology of the dorsal muscle field, with the posterior portion of the anterior pair located between the median ridge and the posterior adductor scars, like in the genera Nothorthis and Ranorthis . The only other species of Archaeorthis recorded from the San Juan Formation is A . sanroquensis Benedetto, which comes from the lowermost part of the unit, of latest Tremadoc-earliest Arenig age (Benedetto, 2001a). It differs from the Potrerillos sample in that number of costellae increases mainly by bifurcation, the less prominent dorsal median ridge, and the dorsal adductor scars configuration, which are arranged in front each to another so that the contact between them is almost perpendicular to the dorsal ridge instead of oblique. A. parviuscula Ulrich and Cooper 1938, from the Sarbach Formation of North America, resembles Archaeorthis sp. in the strong dorsal median ridge but differs in its smaller size, finer ornament and less developed premuscular callosity.

Family Orthidiellidae Ulrich and Cooper, 1936

Genus Orthidium Hall and Clarke, 1892

Type species. Orthis gemmicula Billings, 1862.

Orthidium guandacolensis sp. nov.
Figures 5.L-5.W

2001a. Orthidium sp. Benedetto, p. 6, pl. 1, figs. 22-31.

Holotype. A dorsal valve CEGH-UNC 21811.
Paratypes. Two conjoined specimens, three ventral valves, three dorsal valves, one external mold and one internal mold of ventral valve, and five internal molds of dorsal valves, CEGH-UNC 21812-21826, 21831.

Derivation of name. Refers to the Guandacol village.
Type locality and type stratum. San Juan Formation, uppermost levels exposed at Quebrada Potrerillos, Guandacol area.
Diagnosis. Small, up to 5,6 mm wide, strongly ventribiconvex Orthidium with proportionally high curved ventral interarea. Costellate radial ornament with 5-6 rounded costellae per mm; concentric growth lamellae irregularly distributed. Ventral interior with large massive teeth. Cardinal process large, plate-like, trilobed distally. Dorsal muscle field divided by a median ridge initially broad and rounded becoming higher and blade-like anteriorly.
Description. Shell minute, up to 5,6 mm wide, strongly ventribiconvex. Maximum width slightly beyond the hinge line; cardinal extremities almost orthogonal. Shell almost as long as wide, but in mature and ephebic specimens a ventral ?trail' is developed because of allometric growth, the valve becoming proportionally long and narrow (figures 5.Q, 5.R). Ventral valve strongly convex averaging 42% as deep as wide, non sulcate or with shallow, smoothly rounded sulcus on the anterior half. Ventral interarea proportionally high, curved, transversely striated, apsacline. Delthyrium broad, triangular. Dorsal valve gently convex, sulcate, with very low, anacline interarea. Ornament finely costellate, with 5-6 rounded costellae per mm and up to 50 costellae on the entire valve. Concentric lamellae irregularly distributed, impersistent, more numerous near the periphery of valve.
Ventral interior with large massive teeth supported by thick, receding dental plates almost completely masked by shell deposits filling the lateral apical cavities. Ventral muscle field confined to the deep delthyrial cavity, weakly elevated above the valve floor. Diductor scars very narrow, continued anteriorly by straight, slightly divergent vascula media; adductor field broad, triangular.
Dorsal interior with large subquadrangular, plate-like cardinal process directed posteriorly to occupy almost the entire delthyrial and notothyrial openings in conjoined valves; distally it is trilobate, with a small rounded median crest and smooth lateral lobes. Brachiophores thickened, fused with the sides of the cardinal process forming a massive arched structure. Dental sockets deep, hemispherical. Dorsal muscle field strongly impressed, rhomboidal, extending anteriorly for one-half to two-third valve length, divided longitudinally by a median ridge initially broad and rounded becoming higher and blade-like anteriorly; anterior adductor scars large, subtriangular, located on either side of the median ridge, bounded laterally by low ridges; posterior pair pear-shaped, about of equal size of slightly smaller than the anterior ones, separated from them by a low, oblique ridge.
Remarks. The Potrerillos specimens clearly differ from type species O. gemmiculum (Billings) in its less transverse and strongly ventribiconvex shell and its much shallower dorsal sulcus. Internally both species are similar, although in the North American species the cardinal process seems to be smaller and less compressed anteroposteriorly. Orthidium bellulum Ulrich and Cooper (1938) from the Upper Pogonip Group of Nevada differs in its more transverse outline and stronger growth lamellae. The internal features of the closely related O . Fimbriatum Cooper (1956), from the Table Head Group of Newfoundland, are unknown. O . guandacolensis sp. nov. has some features in common with O. Barnesi Ross (1970), from the Antelope Valley Limestone (Arenig), especially the comparatively narrow outline and the strongly ventribiconvex lateral profile. However, O. barnesi can be differentiated by its coarser ornament and the slightly auriculate cardinal extremities which coincide with the maximum shell width. Internally, the plate-like cardinal process is similar in both species, even though in the Precordilleran species it is subrectangular instead of triangular in posterior view. In view that other internal features were not adequately described and illustrated in the 1970 Ross' paper, a more detailed comparison with this species is impracticable.
Among the San Juan Formation species, O . Guandacolensis sp. nov. is very similar to the specimens referred to Orthidium sp., from the Monorthis cumillangoensis Zone (middle Arenig) at Cerro Cumillango (Benedetto, 2001a), in its external ornament, elongate shell outline, unusually high ventral interarea and strong teeth. The single fragmentary dorsal valve available from Cerro Cumillango is also somewhat similar. Thus it is probable that the Cumillango specimens are conspecific with O. guandacolensis . The late Tremadoc species O. prominens Benedetto, from the lower part of the San Juan Formation (Benedetto et al ., 2003) clearly differs in its smaller size (rarely exceeds 4 mm wide), lower ventral interarea and cardinal process with a highly developed median elevation. Perhaps the main feature distinguishing the two species is the morphology of the dorsal median ridge, which is uniformly narrow and high in O. prominens , whereas in O. guandacolensis it is initially broad and low becoming blade-like anteriorly. The early Llanvirn species O. geniculatum Herrera and Benedetto is readily distinguished by its more transverse outline, stronger and regularly-spaced concentric lamellae, geniculate shell, and slender and more divergent brachiophores.

Superfamily Plectorthoidea Schuchert and Le Vene, 1929

Family Platystrophiidae Schuchert and Le Vene, 1929

Genus Ffynnonia Neuman and Bates, 1978

Type species. Pleurorthis costatus Bates, 1968.

Ffynnonia sp.
Figure 5.X

Material and occurrence. One dorsal valve, CEGH-UNC 21810. Uppermost levels of the San Juan Formation, Quebrada Potrerillos.

Description. Shell transverse, 12,2 mm wide and 7,1 mm long. Maximum width slightly beyond the straight hinge line. Cardinal extremities rounded. Dorsal valve moderately convex, with well-defined median fold. Ornament coarsely costellate, with six primary costellae on flanks and four on the fold; ribs bifurcate near the mid-valve length and close to the anterior margin. Internal features unknown.
Remarks. Fynnonia is a highly distinctive taxon of the upper part of the San Juan Formation (late Arenig-earliest Llanvirn) in the central part of the basin (Benedetto and Herrera, 1986; Benedetto, 2001a). It has also been recorded in the Famatina basin, from the Suri Formation (early Arenig) (Benedetto, 2003a). Externally, it can be recognized by its transverse shell bearing a well-defined dorsal fold and facicostellate radial ornament. On the basis of these features the single specimen recovered from the Potrerillos area is tentatively assigned to Ffynnonia , the available material being too limited for specific identification.

Order Strophomenida Öpik, 1934

Suborder Strophomenidina Öpik, 1934

Superfamily Plectambonitoidea Jones, 1928

Family Taffiidae Schuchert and Cooper, 1931

Subfamily Taffiinae Schuchert and Cooper, 1931

Genus Taffia Butts, 1926

Type species. Taffia planoconvexa Butts, 1926.

Subgenus Taffia ( Chaloupskia ) Neuman in Neuman and Bruton, 1989

Type species. Chaloupskia scabrella Neuman in Neuman and Bruton, 1989.

Taffia ( Chaloupskia ) minima sp. nov.
Figures 6.A-6.J

Holotype. A silicified dorsal valve, CEGH-UNC 21774.
Paratypes. Two conjoined valves, seven ventral valves and two dorsal valves, CEGH-UNC 21775-21785. Three external molds of dorsal valve, CEGH-UNC 21800, 21808 and 21809, two internal molds of ventral valve, CECG-UNC 21786 and 21799, and two of dorsal valve, CEGH-UINC 21787-21788.

Derivation of name. Refers to small size of shells.
Type locality and type stratum. San Juan Formation, uppermost levels exposed at Quebrada Potrerillos, Guandacol area.

Material assigned. Two ventral valves from the lower member (parted limestoners) of the Gualcamayo Formation, Quebrada Gualcamayo, CEGH-UNC 21789-21790.

Diagnosis. Minute, ventribiconvex species of Taffia ( Chaloupskia ) with subequally multicostellate ornament formed by 26-35 narrow, subangular, mostly simple ribs. Pseudodeltidium confined to the posterior half of delthyrium. Comae absent. Ventral muscle field bounded anteriorly by a premuscular callosity. Notothyrial platform narrow merged anteriorly with a broad rounded median ridge. Simple, blade-like cardinal process always present.
Description. Shell minute, up to 9 mm wide (normally 5-6 mm wide), ventribiconvex; growth markedly allometric, so that juvenile specimens tend to be considerably more transverse than the largest ones (average length/width ratio in specimens up to 6 mm wide: 0,60). Hinge line long, straight, extending into small ears at cardinal extremities. Ventral valve moderately and evenly convex, slightly carinate; flanks nearly flat to slightly concave. At the front of largest specimens there is an angular, dorsally directed geniculation. Umbo minute, not raised beyond hinge line or barely protruding from them in larger specimens. Ventral interarea planar, large, apsacline. Delthyrium triangular, its posterior half closed by a gently arched pseudodeltidium. Dorsal valve gently convex, with well defined median sulcus narrow posteriorly but becoming broad and shallow anteriorly. Dorsal interarea low, planar, about 40 % as high as the ventral one, anacline to almost catacline. Chilidium not preserved in available material. Ornament multicostellate, with 26-35 narrow, subangular, mostly simple ribs (density 7-8 ribs per 2 mm), coarser and rounded anteriorly; a few costellae added on the anterior third by intercalation and bifurcation. Concentric ornament of fine, closely spaced growth lines, more marked on the anterior third of valve. Comae not developed.
Ventral interior with stout, bluntly triangular teeth supported by thick, strongly receding dental plates; inner face of teeth bearing shallow, elongate oval crural fossettes. Ventral muscle field subtriangular, confined to the delthyrial chamber, with arcuate anterior margin and bounded anteriorly by a subcircular thickening attaining one half of valve length. Vascula media narrow, straight, moderately divergent, originated at the anterolateral ends of muscle field.
Dorsal interior with robust, laterally compressed brachiophores diverging at about 90º, bounding deep, semiconical dental sockets. Notothyrial platform narrow, elevated, merged anteriorly with a broad rounded median ridge fading at valve midlength. Cardinal process simple, blade-like, slightly thickened anteriorly. Poorly impressed dorsal muscle field consisting of smaller elongate oval anterior pair of adductor scarsand larger subcircular posterior ones.
Remarks. In the revision by Ulrich and Cooper (1938), Taffia is characterized as a taffiid having planoconvex or concavoconvex profile, costellate ornament, well-developed chilidium and pseudodeltidium, and orthoid cardinalia lacking cardinal process. With respect to the latter feature, Neuman (1976) noted that in a few specimens of the type species T . planoconvexa Butts, from the Oddenville Formation (lower Arenig, Alabama) the cardinal process is variably developed, leading Cocks and Rong (1989, 2000) to introduce in the diagnosis of Taffia the statement "usually without simple cardinal process (but rarely present)". However, in our new species the cardinal process is a persistent character, like in those specimens attributed to Taffia anomala Benedetto and Herrera (1986), from the lower Llanvirn of the Precordillera, in which it varies from a low to thick ridge. Another feature that differentiates the Potrerillos specimens from the type material of Taffia is the biconvex lateral profile of shells, the dorsal valve being always gently convex. In both features our material approaches to the genus Chaloupskia erected by Neuman (in Neuman and Bruton, 1989) to include taffiids from the Arenig Lower Hovin Group (Hølonda, Norway) characterized by its planoconvex to vetribiconvex profile, persistent blade-like cardinal process and exteriors of largest specimens overgrown by comae, the latter being considered as the principal diagnostic feature. Although in the revised edition of the Treatise Cocks and Rong (2000) synonymized Chaloupskia with Taffia it seems adequate to retain it as a valid subgenus of Taffia to account for the above mentioned differences. Besides the type species Taffia ( Chaloupskia ) scabrella Neuman and Taffia sp. from the Summerford Group, Newfoundland (Neuman, 1976), the Precordilleran species Taffia anomala should also be included in this subgenus on the basis of its prominent comae and its well-developed cardinal process ( cf . Benedetto and Herrera, 1986, pl. 3, figs. 1, 2, 6). T . ( Chalaoupskia ) minima sp. nov. differs from T. ( C .) scabrella in its smaller size, equally costellate ornament and narrower dorsal ridge. Taking into account that in T. ( C .) scabrella comae appear in specimens longer than 8 mm, their absence in our material can be attributed to the small size of specimens. The Precordilleran species T . ( C .) anomala Benedetto and Herrera clearly differs from T . ( C .) minima in its larger size, narrower sharp-crested and less numerous costellae (numbering 18-24 on the entire surface) and the presence of comae on the periphery of both valves. In addition, the pseudodeltidium in T. ( C .) anomala covers almost entirely the delthyrium while in T. ( C .) minima it is restricted to the posterior half.

Subfamily Leptellinae Williams, 1965

Genus Leptella Hall and Clarke, 1892

Type species. Leptaena sordida Billings, 1862.

Subgenus Leptella ( Petroria ) Wilson, 1926

Type species. Petroria rugosa Wilson, 1926.

Diagnosis (Revised). Subgenus of Leptella with well developed comae on exteriors of both valves, especially on their anterior half ; external surface smooth or with fine, widely-spaced radial costellae.

Leptella ( Petroria ) ventrocristata sp. nov.
Figures 6.K-6.S

Holotype. A ventral valve CEGH-UNC 21798.
Paratypes. Two conjoined specimens, two ventral valves and one dorsal valve, CEGH-UNC 21791-21795; one external mold of ventral valve, CEGH-UNC 21833; one internal mold of dorsal valve and six internal molds of ventral valves, CEGH-UNC 21796, 21797, 21807, and 21827-21830.

Derivation of name. Refers to the presence of a ventral subperipheral rim.
Type locality and type stratum. San Juan Formation, uppermost levels exposed at Quebrada Potrerillos, Guandacol area.
Diagnosis. Small to medium-sized species of Leptella ( Petroria ) with prominent comae on the anterior half of both valves and fine, widely separated radial costellae. Ventral interior with stout, transversely elliptical, unsupported teeth. Interior of ventral valve with elevated, anteriorly bilobed subperipheral platform. Dorsal platform weakly elevated.
Description. Small to medium-sized, up to 13 mm wide (normally 8-10 mm wide), strongly concavoconvex, transversely semielliptical shells (average length/width ratio of ventral valve: 0,72). Cardinal extremities acute, mucronate in young specimens. Ventral valve strongly and evenly convex, maximum thickness at mid-length. Ventral interarea large, curved, transversely striated, orthocline. Delthyrium broad, triangular, almost entirely closed by a strongly arched pseudodeltidium enclosing apically a small rounded foramen. Dorsal valve strongly concave, deflected ventrally near the margin. Dorsal interarea planar, slightly narrower than ventral, transversely striated, hypercline. Notothyrium bounded laterally by thick chilidial plates and covered posteriorly by a small arched chilidium. Prominent shell overgrowths or ?comae' invariably present on the anterior half of both valves forming 3-7 roughly concentric rows. External surface free of comae bearing widely separated fine costellae visible on well-preserved external molds (figure 6 K).
Ventral interior with stout, transversely elliptical, unsupported teeth. Ventral muscle field short, confined to the deep delthyrial cavity, surrounded by thick secondary deposits; adductor and diductor muscle scars indistinguishable from each other. Elevated, anteriorly bilobed, slightly undercut subperipheral platform extending anteriorly for about 3/4 valve length, its periphery radially striated and interrupted by several gaps corresponding to the passage of vascular trunks. Vascular system deeply impressed, saccate, with straight, slightly diverging vascula media which bifurcate near the margin.
Dorsal interior with rod-like, thin brachiophores projecting ventrally. Robust, widely divergent socket ridges merged posteriorly with thickened chilidial plates. Dental sockets shallow, semiconical. Dorsal platform weakly elevated medially, bilobed, radially striated, divided longitudinally by a narrow ridge which is initially broad and rounded becoming thinner and higher anteriorly, not continued beyond the platform. Dorsal muscle field poorly impressed, semielliptical, slightly elevated above the valve floor.
Remarks. Cooper (1956) provided a detailed description and discussion of the type material of Petroria rugosa Wilson. He stated that Petroria is closely related to Leptella Hall and Clarke from which differs in few characters, mainly the presence of ?prominent wavy lamellae' (comae) on the external surface, the presence of a callosity anterior to the ventral muscle field, and the median septum extending anteriorly to the ?visceral disk' (platform). Although the latter feature has been considered diagnostic of the subgenus Leptella ( Petroria ) by Cocks and Rong (1989, 2000), Benedetto and Herrera (1993) noted that in specimens of Leptella ( Leptella ) variabilis Benedetto and Herrera the anterior prolongation of the dorsal septum is highly variable, ranging from absent to fully developed. Such a variation has also been observed in populations of Petroria ( Petroria ) rugosa acuta Benedetto and Herrera (1986, pl. 2, figs. 8 and 11). Moreover, in some well-established species of Leptella ( Leptella ) such as L. ( L .) musculosa Williams and Curry and L. ( L .) alata Benedetto and Herrera, the median septum is often well developed outside the platform. The premuscular thickening in the ventral valve also is a variable feature. It is well developed in the type species L. ( L .) sordida (Billings) as well as in the Precordilleran species L. ( L .) alata , L. ( L .) variabilis and L . ( L .) plana (Benedetto and Herrera, 1993) but it is absent in the Irish species L . ( L .) musculosa . On the other hand, a premuscular callus is present in Leptella ( Petroria ) rugosa elevata Benedetto and Herrera. The absence of dental plates was considered a distinguishing feature of Petroria by Williams and Curry (1985), but in both genera dental plates are usually very short and masked by secondary deposits. As Williams and Curry (1985) noted, Petroria and Leptella are very similar internally, but they can readily be distinguished externally as Petroria has prominent shell overgrowths or comae. The same criterion was used by the author to separate the two subgenera (Benedetto, 2001a, p. 15).
The new species L . ( P .) ventrocristata differs from the type species L. ( P .) rugosa Wilson in having a conspicuous platform in the ventral valve. The closest species is L . ( P .) rugosa acuta Benedetto and Herrera, from the top of the San Juan Formation at the Cerro Viejo area (latest Arenig/earliest Llanvirn), which shares the presence of a ventral platform, but usually it is weaker and confined to the sides of the valve. This is probably a synapomorphy that links phylogenetically the two species. In addition, L. ( P .) rugosa acuta differs from L . ( P .) ventrocristata sp. nov. in its larger shell size, more convex and less transverse ventral valve and strongly elevated dorsal platform. The internal features of P . austrina Ross are unknown in the type material from the Antelope Valley Limestone leading Ross (1972) to refer questionably this species to Petroria . The specimens assigned to L. ( P .) cf. austrina Ross by Ross and James (1987), from a Cow Head Group boulder (Newfoundland; lower Orthidiella Zone, upper Arenig), differ in their strongly concavo-convex shell, sulcate dorsal valve, less prominent comae, and shorter dorsal platform.

Subfamily Pelonomiinae Cocks and Rong, 1989

Genus Pelonomia Cooper, 1956

Type species. Orthis delicatula Billings, 1865.

Pelonomia ? sp.
Figures 6.Z-6.Z2

Material and occurrence. One external mold of dorsal valve and one internal mold of dorsal valve; one fragmentary internal mold of ventral valve, CEGH-UNC 21859-21861. Lower part of the Gualcamayo Formation, Quebrada Los Sapitos.

Description. Subrectangular shell, 9,5 mm wide and 6,1 mm long. Cardinal extremities right-angled, coinciding with maximum width. Ventral valve gently convex, at least in the posterior third (the only region preserved in the available specimen). Dorsal valve gently concave posteriorly becoming flat in the anterior half. Ornament subequally parvicostellate, with 6-7 narrow rounded costellae in 2 mm separated by wide flat interespaces bearing one or rarely two finer costellae. Ventral interarea planar, catacline, with delthyrium covered apically by an incipient pseudodeltidium; dorsal interarea catacline, approximately one third as high as ventral. Chilidium not preserved in the available specimen.
Ventral interior too poorly preserved for description of internal features. Dorsal interior with widely divergent, thin, blade-like socket ridges bounding narrow, slit-like dental sockets. Cardinal process very small located near the medial junction of socket ridges. A flat, slightly raised area is present in front of the cardinalia. Median ridge and platform absent. Dorsal muscle field not impressed.
Remarks. This small plectambonitoid is difficult to place sistematically because of the very limited available material. The absence of platform and ridges inside the dorsal valve, and the small cardinal process indicate closest affinities with the enigmatic Pelonomiinae, erected by Cocks and Rong (1989) to include the monotypic genus Pelonomia Cooper 1956. In size, outline, shell profile, ornament and cardinalia features, the Potrerillos specimens are comparable to P . delicatula (Billings), from the Cow Head Formation (Llanvirn). They differ from this species in lacking spine-like pseudopunctae along the dorsal internal sulcus, which is considered a diagnostic feature of this genus. In addition, our material lacks of dorsal sulcus and ventral fold, and for this reason the Potrerillos specimens are referred questionably to Pelonomia .

Order Billingsellida Schuchert, 1893

Suborder Clitambonittidina Öpik, 1934

Superfamily Polytoechioidea Öpik, 1934

Family Tritoechiidae Ulrich and Cooper, 1936

Genus Tritoechia Ulrich and Cooper, 1936

Subgenus Tritoechia ( Tritoechia ) Ulrich and Cooper, 1936

Type species. Deltatreta typica Schuchert and Cooper, 1932.

Tritoechia ( Tritoechia ) sp.
Figures 6.T-6.Y

Material and occurrence. One fragmentary conjoined specimen, two ventral valves and two dorsal valves, two internal molds of ventral valve and three internal molds of dorsal valve, CEGHUNC 21834-21844. Uppermost levels of the San Juan Formation, Quebrada Potrerillos.

Description. Shell up to 19 mm wide, semielliptical (average length/width ratio 0,61). Cardinal extremities slightly obtuse. Ventral valve subpyramidal, gentil convex in lateral view. Beak pointed, slightly curved. Ventral interarea planar, very high (averaging 45% as high as wide), radially striated, apsacline, forming with the commissural plane an angle of 45?-60?. Pseudodeltidium strongly arched, semiconical, bearing a circular apical foramen. Dorsal valve gently convex, non-sulcate or bearing a broad shallow sulcus on its anterior half. Dorsal interarea very low, planar, strongly anacline. Notothyrium entirely covered by an arched chilidium. Ornament subequally multicostellate, with 7-8 rounded costellae in 2 mm near the anterior margin. Interior of ventral valve with plate-like, semielliptical teeth supported by short receding dental plates subperpendicular to valve floor. Ventral muscle field subrectangular, with narrow elongate median adductor scars more elevated than the broader lateral diductor scars. A faint median ridge starts in front of the muscle field fading at about 3/4 valve length. Dorsal interior with triangular, posteriorly inclined, anteriorly free notothyrial platform bounded laterally by high chilidial plates. Cardinal process thick, posteriorly enlarged. Dental sockets slit-like, deep, open laterally, bounded by thick gently arched socket ridges running subparallel to the posterior margin. Dorsal median ridge very short, thick, merged with the front of notothyrial platform and confined to a depression below them. Dorsal muscle field poorly impressed, large, bisected by faint radial transmuscle septa.
Remarks. Although this form is relatively abundant in the etched residues, all available specimens are fragmentary. Their multicostellate ornament indicates that they are attributable to Tritoechia ( Tritoechia ) rather than to the recently erected subgenus Tritoechia ( Parvitritoechia ) Benedetto which possesses a distinctive markedly parvicostellate ornament. The Potrerillos species differs from T. ( T .) prima Benedetto (in Benedetto et al ., 2003), from the lower part of the San Juan Formation (upper Tremadoc), among other features, in its higher and more inclined interarea. The lower Arenig species T. ( T .) gigas Benedetto can be distinguished by its larger size, less inclined and shorter ventral interarea, and longer and strongly raised ventral muscle field. In size, ornament and interarea inclination our material approximates to T. ( T .) mollesensis Benedetto (2003a), from approximately coeval volcanosedimentary beds of Famatina Range (Molles Formation). Available material, however, is too incomplete for more accurate comparisons.


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