Late Maastrichtian-Danian Mytilids and Pinnids (Mollusca: Bivalvia) from Northern Patagonia, Argentina

Miguel Griffin^1,2, Ana Parras^1,2 and Silvio Casadío^1,2

¹Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina
²Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina. miguelgriffin@aol.com

Abstract. The Upper Cretaceous - lower Paleogene rocks from northern Patagonia yield a rich fauna of marine mollusks that may be useful in elucidating paleobiogeographic and paleoenvironmental changes taking place across the K/P boundary in the region. Most of the mollusk faunas remain poorly understood, as the preservation is generally deficient. This paper describes three new species of mytilids (Septifer mayai n. sp., Mytilaster ? edseli n. sp. and Gregariella amara n. sp.), and records four species of pinnids, one of which is new (Plesiopinna pampeana n. sp.). All the material comes from the Maastrichtian Jagüel Formation, and the Maastrichtian/Danian Roca Formation, exposed in the provinces of Río Negro, Mendoza and La Pampa (Argentina). A survey of the stratigraphic distribution of the material described shows that the only Maastrichtian species that survived into the Danian was Gregariella amara.

Resumen. Mytílidos y Pínnidos (Mollusca: Bivalvia) del Maastrichtiano Tardío - Daniano del Norte de Patagonia, Argentina . Las rocas del Cretácico Superior-Paleógeno inferior expuestas en el norte de Patagonia contienen una rica fauna de moluscos cuyo estudio es importante para comprender los cambios paleobiogeográficos y paleoambientales producidos en la región durante el límite K/P. Sin embargo, una porción considerable de esta fauna permanece poco estudiada, probablemente a causa de su pobre preservación. En este trabajo se describen tres especies nuevas de mytílidos (Septifer mayai n. sp., Mytilaster ? edseli n. sp. y Gregariella amara n. sp.) y cuatro especies de pínnidos de las cuales una es nueva (Plesiopinna pampeana n. sp.). Todos los ejemplares provienen de las formaciones Jagüel (Maastrichtiano) y Roca (Maastrichtiano/Daniano) expuestas en las provincias de Río Negro, La Pampa y Mendoza, Argentina. Gregariella amara es la única especie que se registra en el Maastrichtiano y Daniano.

Key words. Bivalvia; Mytilids; Pinnids; Cretaceous/Paleogene boundary; Argentina.

Palabras clave. Bivalvia; Mytílidos; Pínnidos; Límite Cretácico/Paleógeno; Argentina.

Introduction

Upper Cretaceous and Paleogene marine faunas from Patagonia are known since the end of the nineteenth century and early part of the twentieth century, particularly from Northern Patagonia (Burckhardt, 1900, 1901; Ihering, 1903, 1907, 1914; Böhm, 1903; Fritzsche, 1919; Weaver, 1931; and references therein). However, most of the species were based on insufficient material, whether because the available specimens were few or else poorly preserved, a fact that hinders taxonomic, paleobiogeographic, and biostratigraphic inferences. Although true that the material is generally poor, extensive collections during the last few decades have allowed a better understanding of the taxonomic composition of the fauna and of its paleobiogeographic relationships (Camacho, 1970; Casadío, 1998; Casadío et al., 2005; Griffin et al., 2005; and references therein). Micropaleontological studies - mainly on ostracods and foraminifers - were published by Bertels (1968, 1969, 1970), Concheyro and Villa (1996), and Malumián and Caramés (1995). On this occasion we describe seven taxa collected at different localities of the Neuquén Basin and belonging in the Order Mytiloida, a group which had remained until now largely unnoticed in the Cretaceous/Paleogene rocks of the area. The material described comes from beds of well constrained Maastrichtian and Danian age; at some of the localities the K/P boundary is exposed. Therefore, in addition to aiding in the elucidation of the paleobiogeographic relationships of the fauna, these taxa also become valuable tools in understanding the faunal events occurring at the K/P boundary.

Geological setting

During the Cretaceous-Paleogene transition, wide areas of northern Patagonia were covered by a shallow sea developed as a consequence of relative sea level changes related to subsidence along the Atlantic margin.
In the Neuquén Basin, in northwestern Patagonia, the K/P boundary is recorded within rocks included in the Malargüe Group, which includes from bottom to top the Loncoche, Allen, Jagüel, Roca, Pircala and El Carrizo Formations. These units are well exposed in southern Mendoza, northeastern Neuquén, western La Pampa and northern Río Negro. Because these rocks provide an excellent opportunity to study the changes operated in mid-latitude continental and marine ecosystems of the southern hemisphere at the end of the Cretaceous, they have been lately subject to renewed interest (Casadío et al., 2005).
The Cretaceous - Paleogene boundary coincides with an important change in the composition of the marine invertebrate assemblages, consisting in an increase in the number of species from low latitudes, and a decrease of the endemic taxa with austral affinities. This modification in the faunal composition was coeval with an increase of the relative sea level and the beginning of carbonatic sedimentation (Feldmann et al., 1995; Casadío et al., 2005).
The studied fossils come from beds referred to the Jagüel and Roca formations exposed in southern Mendoza, western La Pampa, and northern Río Negro (figures 1 and 2). The Maastrichtian Jagüel Formation comprises mainly siltstone, and transitionally grades into the limestones of the Roca Formation, which has been referred to the Maastrichtian-Danian (Casadío et al., 1998; Parras et al., 1998). Overlying the Roca Formation are evaporitic deposits of the El Carrizo Formation, while in the Andean area it gradually gives way to the continental sandstone of the Pircala Formation.

Figure 1. Map showing fossil localities / mapa mostrando las localidades fosilíferas. PVL, Portezuelo Vega del Loro; LA, Laguna Amarga; ALR, Arroyo las Ramaditas; PLB, Puesto La Bebida; AB. Arroyo Brea; AL, Arroyo Loncoche; CB, Cerro Butaló; RC. Ranquil-Có; BAP, Bordo Alto del Payún; LM, Liu Malal; SA, Salitral de La Amarga; BB, Barda Baya; CBA, Cerros Bayos; GR, General Roca.

Figure 2. Stratigraphic sections at localities studied, showing lithology and fossil-bearing beds; vertical scale in meters / secciones estratigráficas de las localidades estudiadas, mostrando la litología y los niveles fosilíferos; escala vertical en metros.

The specimens studied are housed in the Departamento de Ciencias Naturales of the Universidad Nacional de La Pampa (GHUNLPam), Muséum National d'Histoire Naturelle, Paris (MNHN-R), and Naturhistoriska Riksmuseet, Stockholm (SMNH).

Systematic paleontology

Order MYTILOIDA Férussac, 1822
Superfamily MYTILOIDEA Rafinesque, 1815
Family MYTILIDAE Rafinesque, 1815

Subfamily MYTILINAE Rafinesque, 1815

Genus Septifer Recluz, 1848

Type species. Mytilus bilocularis Linné, 1758; subsequent designation by Stoliczka, 1871.

Remarks. According to Soot-Ryen (1955), Septifer is easily separated from the similar Hormomya Mörch, 1853 (type Mytilus exustus Linné, 1758), by the presence of a deck or septum for the insertion of the anterior adductor muscle. Except for differences in the interior muscle insertion patterns, Septifer and Hormomya share the same kind of ornamentation on the outer shell surface, especially on the ventral area, where the unilateral furcating ribs are readily distinct from the bifurcating pattern seen in the otherwise externally very similar Brachidontes Swainson, 1840 (type Modiola sulcata Lamarck, 1819).
Septifer is known to occur in rocks as early as Triassic, and is nowadays widespread in tropical and subtropical seas (Soot-Ryen, 1955; Beu and Maxwell, 1990). Recent species of Septifer inhabit upper subtidal zones attached to rocks or shell debris (Selin and Latypov, 2006).
Several taxa mentioned in the geological literature as Mytilus Linné, 1758 (type Mytilus edulis Linné, 1758), may probably belong better in Septifer, such as the occurrences reported by Bodenbender (1892), who recorded Mytilus cuvieri Matheron, 1843, in the limestones of the Roca Formation exposed in Arroyo Pequenco, west of Malargüe. Similarly, Weaver (1927, p. 430; 1931, p. 89) mentioned Mytilus sp. from the same unit exposed southeast of Cerro Auca Ma- huida (province of Neuquén) and along the eastern limb of the Malargüe syncline (province of Mendoza); this record is also probably referable to Septifer.

Septifer mayai n. sp. Figures 3.1-3

Figure 3. 1-3, Septifer mayai n. sp. 1, right valve, holotype / valva derecha, holotipo, GHUNLPam 808; 2, right valve / valva derecha, GHUNLPam 809; 3, ventral view / vista ventral, GHUNLPam 806. 4, Mytilaster ? edseli n. sp. left valve, holotype / valva izquierda, holotipo, GHUNLPam 10143. 5-8, Gregariella amara n. sp. 5, right valve / valva derecha, GHUNLPam 593. 6, left valve / valva izquierda, GHUNLPam 586. 7, right valve, holotype / valva derecha, holotipo, GHUNLPam 592. 8, right valve / valva derecha, GHUNLPam 606. Scale bar 1 cm / escala gráfica 1 cm.

Diagnosis. Average-sized Septifer, with very strong postumbonal keel and 35 to 45 simple, flat, quadratesectioned, smooth ribs separated by deeply incised interspaces showing scaly ornamentation produced by commarginal growth lines.
Localities. Barda Baya (36º 54' S, 67º 55' W) and Salitral de La Amarga (36º 33' S, 68º 9' W), both in the province of La Pampa; Liu Malal (36º 45' S, 68º 43' W), Bordo Alto del Payún (36º 31' S, 69º 27 W), Ranquil Có (36º 12' S, 69º 30' W), Puesto La Bebida (35º 26' S, 69º 44' W), and Portezuelo Vega del Loro (34º 44' S, 69º 39' W), all in the province of Mendoza.
Stratigraphic range. All the material comes from the late Maastrichtian Jagüel Formation and the Maastrichtian part of the Roca Formation.

Material. Thirteen variably preserved specimens, generally as calcite replacements of original shell. GHUNLPam 806-809; 1177- 1178; 10764; 10803; 15192-15193; 16049; 16582; 17170.
Holotype. Specimen GHUNLPam 808; all other specimens are paratypes.

Type locality. Salitral de La Amarga (36º 33' S, 68º 9' W), province of La Pampa.
Description. Shell mytiliform, small (largest specimen 42 mm long and 16 mm high); umbones terminal, sharp; dorsal and posterior margins merged into a wide uninterrupted curve that ends abruptly at junction with ventral margin; anteroventral margin concave; area next to posterior margin flat; ventral area of shell enclosed by postumbonal ridge conspicuously concave, deep; greatest shell width at central area near ventral margin; byssal slit well developed; shell surface covered by 35 to 45 radial flat, smooth and quadrate-sectioned radial ribs; ribs generally simple but occasionally bifurcating near dorsal and posterior margins; intercalated radial ribs appearing sometimes after growth stops marked by commarginal lines; intercostal spaces narrower than ribs, deeply incised, with scaly ornamentation (visible on well preserved specimens) resulting from the fine commarginal growth lines.
Measurements. Holotype GHUNLPam 808, length: 23 mm, height: 13.5 mm, width: 11 mm.
Remarks. Septifer mayai n. sp. is very similar to the material described by Dartevelle and Freneix (1957, p. 49) as Brachidontes ? charmesi (Péron, 1890) from the Cenomanian- Senonian of Bulu-Zambi in Congo (pl. 6, fig. 8) and the Senonian of Logbatjeck in Cameroon (pl. 6, fig. 9). Questionably placed in Brachidontes, the species described by Péron probably fits better in Septifer (Dartevelle and Freneix, 1957). Examination of the African specimens (MNHN-R 51397), which were collected from Upper Cretaceous rocks in Cameroon, allowed recognition of the following differences: smaller size, less concave ventral margin and radial ribs bifurcating at about half their length from the umbones.
Age. Late Maastrichtian. Zone CC26 (late Chron 30N - Chron 29R).
Etymology. After Domingo "Cocho" Maya, in recognition of his help during the field work in the Barda Baya area, province of La Pampa.

Genus Mytilaster Monterosato, 1883

Type species. Mytilus lineatus Gmelin, 1791; original designation.

Remarks. The type species of Mytilaster is very similar in outline and ornamentation to the specimens from the Roca Formation, although the presence of dysodont teeth behind the ligament could not be observed in our material because no interiors are available. Therefore, generic placement is subject to confirmation. Fossil records of the genus are not common and probably need further taxonomic work in order to confirm them. It is known from the Miocene of Poland (Brzozowska, 2004), and Recent species of the genus, such as Mytilaster lineatus and Mytilaster minumus (Poli, 1795), inhabit brackish waters of the eastern Mediterranean, Black, and Caspian seas.

Mytilaster ? edseli n. sp. Figure 3.4

Diagnosis. Small Mytilaster ? with very strong postumbonal keel; anterior flank almost vertical and completely smooth except for commarginal growth lines; posterior area covered by c. 65 narrow and rounded ribs separated by shallow interspaces about one third the width of ribs.
Localities. Arroyo Loncoche (35º 41' S, 69º 40' W), Arroyo Brea (35º 31' S, 69º 43' W), and Arroyo Las Ramaditas (34º 54' S, 69º 32' W), all in the province of Mendoza.
Stratigraphic range. All the material comes from the Maastrichtian part of the Roca Formation.

Material. Six specimens, variably preserved. GHUNLPam 10035, 10143-10144; 15093-15094; 15097.
Holotype. GHUNLPam 10143; the rest of the specimens are paratypes.

Type locality. Arroyo Brea (35º 31' S, 69º 43' W), province of Mendoza.
Description. Shell small, reaching up to 28 mm in the largest specimen, but generally around 15-18 mm, mytiliform, with terminal umbones and a slight anteroventral lobe; strong postumbonal ridge; anterior part of dorsal margin straight, joining the almost straight to very slightly convex posterior part of dorsal margin at shell mid-length forming an angle of c. 110º; ventral margin slightly concave and posteroventral end rounded; greatest width of shell at about one third of height; outer shell surface divided into two distinct areas: anterior area smooth and only carrying fine commarginal growth lines, posterior area beginning just in front of postumbonal ridge and covered by c. 65 fine rounded ribs with shallow interspaces that are never more than one third the width of ribs; commarginal ornamentation of very fine growth lines and occasionally a few growth ledges.
Measurements. Holotype GHUNPam 10143, length: 18 mm, height: 10 mm.
Remarks. As stated above, the similarity in outline and ornamentation of this species with Mytilus lineatus is striking. However, further material may prove that the generic placement lies elsewhere. A species described by Stoliczka (1871, p. 379-380, pl. 23, fig, 4, 6, and 7) as Modiola (Brachydontes) radiatula Stoliczka, 1871, from the Arrialoor Group near Comarapolliam, southern India, is quite similar to our material. It shows the same shell shape and ornamentation pattern as Mytilaster ? edseli n. sp. and probably belongs in Mytilaster too, as suggested by the anterior lobe and radial ornamentation, but the radial ribs appear to be weaker and more numerous, and the commarginal growth lines are stronger. In addition, in the Indian species the postumbonal ridge is weaker.
Age. Late Maastrichtian. Zone CC26 (late Chron 30N - Chron 29R).
Etymology. This species is named after Edsel Brussa, paleontologist at the Universidad Nacional de La Pampa, Argentina.

Subfamily CRENELLINAE Adams and Adams, 1857

Genus Gregariella Monterosato, 1883

Type species. Modiolus sulcatus Risso, 1826 (= Modiola opifex Say, 1825); by monotypy.

Remarks. The genus Gregariella includes a number of species closely resembling Musculus Röding, 1798 (type Mytilus discors Linné, 1758). The two genera differ mainly in the pattern of ornamentation. While both share valves with two radially ribbed areas at each end and a smooth central area, in Gregariella the first few ribs of the posterior area do not reach the ventral margin, which they do in Musculus. In addition, the postumbonal ridge is notably more marked in Gregariella, which produces strongly keeled shells. This contrasts shells of Musculus, which are generally flatter as the postumbonal keel is poorly developed or else it is entirely missing. Gregariella has been described from rocks dated as far back as Eocene (Beu, 2004).

Gregariella amara n. sp. Figure 3.5-8

Diagnosis. Gregariella with well defined postumbonal ridge; anterior area with 7-10 ribs and posterior area with about 65 to 80, less crowded below postumbonal ridge (18-20) than on dorsal area; rib section gently convex.
Localities. Cerros Bayos (37º 40' S, 67º 30' W) and Salitral de La Amarga (36º 33' S, 68º 9' W), province of La Pampa; General Roca (39º 00' S, 67º 32' W), province of Río Negro; Arroyo Brea (35º 31' S, 69º 43' W), province of Mendoza.
Stratigraphic range. This species occurs within the topmost beds of the late Maastrichtian Jagüel Formation, and also in the overlying Maastrichtian- Danian Roca Formation.

Material. Forty moderately well preserved specimens, most of them as internal moulds but partially covered by calcite replacements of the original shell. GHUNLPam 586-616, 4894; 4924-4925; 5226-5227; 10141-10142; 10157-10158.
Holotype. Specimen GHUNLPam 592 is designated holotype; the rest of the specimens are paratypes.

Type locality. Salitral de La Amarga (36º 33' S, 68º 9' W), province of La Pampa.
Description. Shell small (ranging between 7.5 and 25 mm long, but most of them about 18 mm), subovate, umbones sub-terminal; anteroventral margin straight to gently convex at mid-length; anterior dorsal margin straight and ascendant; dorsal posterior margin convex; posterior margin rounded; greatest shell width at mid-length of shell; well defined postumbonal ridge; shell ornamentation of fine radial ribs covering anterior and posterior areas of shell; anterior area with 7-10 ribs and posterior area with about 65 to 80, less crowded below postumbonal ridge (18-20) than on dorsal area; ribs on most dorsal part of the shell deflected upwards to meet dorsal margin at a nearly straight angle; few intercalated ribs present; central area smooth; rib section gently convex; first few ribs of posterior area not reaching ventral margin; intercostal spaces shallow, as wide as ribs.
Measurements. Holotype GHUNLPam 592, length: 16.6 mm, height: 8.6 mm, width: 7.0 mm.
Remarks. Gregariella amara n. sp. appears to be one of the earliest members of the genus in the Western Hemisphere. In southern South America, the earliest representative known up to now is Modiola andina Ortmann, 1902 (p. 122, pl. 24, fig. 4; Frassinetti and Covacevich, 1999, p. 21, pl. 3, fig. 2-3) from the late Oligocene-early Miocene Centinela Formation exposed just South of Lake Pueyrredón, province of Santa Cruz, Argentina. Ortmann's species is very similar and probably belongs in Gregariella too, but it appears to be slightly larger, with a sharper portumbonal keel, and the radial ribs are less numerous and restricted to the posterior end only. Another probable member of this genus is Modiola aprilis Feruglio, 1935 (p. 67, pl. 1, fig. 5.; Feruglio, 1936, p. 211, pl. 21, fig. 7; Camacho, 1992, p. 37, tab. 1), from the Danian Salamanca Formation, a unit exposed along the Atlantic coast of central Patagonia just north of Comodoro Rivadavia. However, identification of Feruglio's material is at least problematic. The specimens were not available for study and his original description is too imprecise to warrant any certainty in the taxonomic placement of his material, however much the general shape resembles other taxa from approximately coeval deposits in this part of South America as is the case of Gregariella amara. New collections from the Salamanca Formation may eventually help to clarify the status of Feruglio's species.
According to the original description, Brachidontes bifurcatus Popenoe, 1937 (p. 383, pl. 46, fig. 2) from the Upper Cretaceous Williams Formation in Southern California can be placed in Gregariella, as it shows the same pattern of ornamentation, although the hinge details remain unknown. While similar to the Patagonian species, the ribs are wider, fewer and more strongly marked. The shell is also significantly larger, reaching up to twice the length.
Modiolus kirkerensis Clark, 1918 (p. 133, pl. 9, fig. 8; Moore, 1983, p. A68, pl. 17, fig. 9) from the Miocene San Ramon Sandstone in central California resembles the Patagonian species in sculpture, although the anterior part of the shell is rather poorly preserved and thus no traces of the sculpture are preserved there. However, the shell is much larger in the Californian species. Moore (1983, p. A68) placed Clark's species doubtfully in Brachidontes. Such a placement seems to be incorrect but only further study will allow a correct generic assignment.
Modiolus pontotocensis del Valle and Medina, 1980 (p. 53-54, pl. 2, fig. 3-5) from the Upper Cretaceous López de Bertodano Formation exposed at Cape Lamb on Seymour Island (Antarctica), is similar to Gregariella amara in its shell outline, relatively low umbonal ridge, and radial ornamentation. However, the ribs are weaker and less distinct than in the northern Patagonian species and the shells are considerably larger, reaching a length of 42 mm.
Modiolus cf. M. pontotocensis del Valle and Medina sensu Zinsmeister and Macellari, 1988 (p. 263-264, Fig. 8.14 -15), from unit 9 (late Maastrichtian) of the López de Bertodano Formation, has more evenly convex shells. The ornamentation is not apparent in the available figures, but the authors mention the presence of radial ribs, albeit not whether they are all over the surface or restricted to the anterior and posterior areas only.
Another Antarctic species that resembles Gregariella amara -at least superficially- is Arcuatula sootryeni Stilwell and Zinsmeister, 1992 (p. 58, pl. 3, figs c, e, and g), from the Eocene La Meseta Formation exposed in Seymour Island. The radial ribs and the general shell outline -similar to Gregariella amara - suggest it may fit in Gregariella.
An earliest Eocene specimen described by Tessier (1952, p. 317, pl. 19, fig. 5-6) as Musculus elegans (Sowerby, 1812) and collected in Kade-Baldji, Senegal, appear to be quite similar to Gregariella amara. They differ in the shell proportions, as the African specimen seems to be higher and with a wider anterior end. The ornamentation is also different, as the specimen described by Tessier has 12 ribs on the anterior area and those on the posterior are fewer, stronger and the intercostals spaces are wider.
Age. Late Maastrichtian-Danian. Zone CC26 to Zone NP4 (late Chron 30N - Chron 26R).
Etymology. Latin for "amarga" (bitter), after Salitral de La Amarga, the type locality.

Superfamily PINNOIDEA Leach, 1819
Family PINNIDAE Leach, 1819

Genus Pinna Linné, 1758

Type species. Pinna rudis Linné, 1758; subsequent designation by Children, 1823.

Remarks. According to Turner and Rosewater (1958), Rosewater (1961), Packard and Jones (1965), and Cox and Hertlein (1969), Pinna is characterized by equivalve wedge-shaped shells, anteriorly placed umbones, medial ridge, radial ribs, interior nacreous layer divided into two distinct lobes by a longitudinal groove, and the posterior adductor muscle scar placed within the posterior nacreous lobe. Species of Pinna are generally distinguished from each other based on the general shell outline, apical angle, strength of the medial ridge, and characteristic ornamentation of the outer shell surface. The genus is known from rocks as early as Carboniferous and fossil species show a cosmopolitan distribution. Recent representatives of this group of suspension feeding bivalves are restricted to tropical and subtropical shallow seas. Extant species live with the anterior end buried in a soft substrate and attached to particles of sediment by means of well developed byssal threads (see Yonge, 1953; Stanley, 1970; Moore, 1983; Butler et al., 1993; Richardson et al., 1999, for further details on the life mode of pinnids).

Pinna sp. aff. P. cretacea (Schlotheim, 1813) Figure 4. 1

Figure 4. 1, Pinna sp. aff. P. cretacea Schlotheim, right valve of slightly compressed specimen / valva derecha de un especimen algo comprimido, GHUNLPam 15314. 2, Pinna sp., right valve / valva derecha, GHUNLPam 2913. 3-6, Plesiopinna pampeana n. sp. 3, posterior view of shell broken perpendicular to hinge, showing flattened dorsal area, holotype / vista posterior de la conchilla mostrando el area dorsal aplanada, holotipo, GHUNLPam 810. 4, posterior view of obliquely broken shell, showing flattened dorsal area / vista posterior de la conchilla mostrando el area dorsal aplanada, GHUNLPam 811. 5, left valve / valva izquierda, GHUNLPam 811. 6, right valve, holotype / valva derecha, holotipo, GHUNLPam 810. 7, Atrina sp., right valve / valva derecha, GHUNLPam 17308. Scale bar 1 cm / escala gráfica 1 cm.

Localities. Cerro Butaló (35º 50' S, 69º 40' W), Arroyo Brea (35º 31' S, 69º 43' W), Arroyo Las Ramaditas (34º 54' S, 69º 32' W), and Laguna Amarga (34º 46' S, 69º 33' W), all in the province of Mendoza.
Stratigraphic range. All specimens come from the Roca Formation, which is late Maastrichtian at all four localities.

Material. Five specimens, internal moulds of anterior ends showing fragments of replaced shell. GHUNLPam 10138, 10459, 15314, and 15091-15092.

Description. Preserved shell anterior wedge-shaped; margins straight; apical angle of c. 40º; medial ridge well defined; dorsal flank ornamented with c. 10 strong, slightly wavy radial ribs; ventral flank with curved ribs meeting medial ridge at an angle of 25º; shell thin, reaching up to 1.1 mm.
Remarks. The available specimens are very fragmentary for any accurate comparison. However, shell outline, radial ribs, median ridge, and curved ventral ribs suggest that they are close to Pinna cretacea (Schlotheim, 1813). This species of Pinna is very common in Upper Cretaceous rocks from localities spreading over most of the globe and seems to have been cosmopolitan at that time (Seeling and Bengston, 2003).
Age. Late Maastrichtian. Zone CC26 (late Chron 30N-Chron 29R).

Pinna sp. Figure 4. 2

Locality. Cerros Bayos (37º 40' S, 67º 30' W), province of La Pampa.
Stratigraphic range. All the specimens come from the Roca Formation, that at this locality is Danian.

Material. Six internal moulds, fragmented and very weathered, but exhibiting vestiges of the original shell. GHUNLPam 1950- 1951, 2913-2914 and 4917-4918.

Description. Shell triangular, elongate, with terminal umbones; anterior and posterior margins of anterior part of the shell meeting at umbones at an angle of c. 35º; straight indistinct ridge running from anterior end towards posterior margin, rendering a flat rhomboidal shell section; outer surface with at least seven straight, low, and thin radial ribs separated by wide and shallow intercostal spaces on the dorsal flank, and numerous curved ribs on the ventral flank that intercept the medial ridge.
Measurements. GHUNLPam 2913, height: 51 mm, width: 28 mm.
Remarks. Because of the poor preservation of the available specimens, the interior of the shells is not visible. Thus, it remains unclear whether the nacreous layer was or not divided by a groove. However, the narrow shell outline, the laterally rather flat section, low apical angle, and very shallowly concave ventral margin suggest that - while different from P. aff. P. cretacea mentioned previously - these specimens fit in Pinna. Also because of the fragmentary nature of the available material, naming a new species appears premature. Better specimens, though, may confirm that it is in fact a distinct Danian species from northern Patagonia.
The specimens are similar to Pinna anderssoni Wilckens, 1910 (SMNH 1610-1625). However, the following differences were noticed: the ventral flank of the shells of P. anderssoni has five straight radial ribs parallel to the medial ridge that are not seen in Pinna sp.; the curved ribs near the ventral margin in specimens of P. anderssoni do not intercept the medial ridge as they do in Pinna sp.; and the anterior angle is slightly smaller in P. anderssoni.
Pinna sp. is also similar to P. morenoi Wilckens, 1907 (p. 117-118, pl. 5, fig. 2a-b). These two species can be separated by the smaller size of Pinna sp., its relatively more inflated shell, its finer ribs and greater development of the medial keel.
Contrarily, P. sobrali Zinsmeister, 1984 (p. 1510, fig. 5G-5H; Stilwell and Zinsmeister, 1992, p. 59-60. pl. 3, fig. j-k), has a less clearly rhomboidal section and fewer radial ribs, besides being an apparently larger shell.
Differences are greater with P. freneixae Zinsmeister and Macellari, 1988, from the Upper Cretaceous beds of the López de Bertodano Formation (units 5 through 9), in Seymour Island, Antarctica. The Antarctic species has shells with a narrower apical angle, stronger radial ribs, and a straight ventral margin.
Comparison with other species referred to Pinna is rather fruitless until the taxonomy of the Upper Cretaceous and Paleogene members of this group from the Southern Hemisphere is more clearly understood. A revision of all the nominal fossil species of Pinna, together with enhanced stratigraphic data about them, is sure to reveal many cases of synonymy and should certainly aid in the understanding of the paleobiogeographic relationships within this group of bivalves.
Age. Danian. Zone NP1 (Chron 29R-Chron 29N).

Genus Plesiopinna Amano, 1956

Type species. Plesiopinna atriniformis Amano, 1956; original designation.

Remarks. The genus Plesiopinna was introduced by Amano (1956, p. 71, pl. 1, fig.1-5) for material collected in the Albian-Cenomanian Gosyonoura Group exposed at Shishijima, Japan. This peculiar shell is subtrigonal in outline and two are the characters that separate it from other members of this family. One of these is the strongly curved medial ridge, that runs from the umbones to the posteroventral position, describing a well defined curve that renders the anteroventral part of the concave shell. The other typical feature of this genus is the very wide and flat saillike posterior area. No traces of the interior are known. Therefore, its affinities with other pinnids remain uncertain. It has been classed as a subgenus of Pinna, but the general shape reminds more of some species of Atrina Gray, 1847. Until the interior is studied, this can not be resolved. Besides the type species, other material included in this genus is Plesiopinna sp. described by Cobban (1977, p. 14, pl. 1, fig. 1, 3)from the Oak Canyon Member of the Cenomanian Dakota Sandstone of New Mexico. This specimen also clearly shows the two shell features described for the type species.

Plesiopinna pampeana n. sp. Figure 4.3-6

Diagnosis. Shell trigonal to ham-shaped; tearshaped section; anterior ventral margin concave; dorsal margin straight; postumbonal keel very well defined and clearly differentiated from the rest of the shell surface, which is rather flat; ridge curved, describing an arch following ventral margin from umbonal area to posterior ventral sector of shell margin; shell ornamentation of 12 strong and flat radial ribs on dorsal flank, separated by widening intercostal spaces, which at posterior margin reach up to three times the width of ribs.
Locality. Barda Baya (36º 54' S, 67º 55'W), province of La Pampa.
Stratigraphic range. The material comes from the late Maastrichtian Jagüel Formation.

Material. Three moderately well preserved specimens, with replaced shells. GHUNLPam 810 - 811 and 4825.
Holotype. Specimen GHUNLPam 810; the other specimens are paratypes.

Type locality. Barda Baya (36º 54' S, 67º 55'W), province of La Pampa.
Description. Trigonal ham-shaped shell; available specimens broken, but seemingly large and very inflated (preserved part of largest specimen, reaching 140 mm long and 82 mm wide and a width/height ratio of 1.3 measured at 55 mm from the presumed anterior end); umbones terminal, but broken in the three known specimens; dorsal margin straight; ventral margin sigmoid, concave near the anterior end; posterior margin not preserved, but growth lines showing it was gently rounded; maximum shell convexity marked by a rounded and curved keel that runs from the umbones to the posterior ventral margin, dividing the shell surface into two distinct flanks; dorsal flank flat, with twelve relatively wide, radial ribs separated by wide intercostal spaces; both widen towards the margin, intercostals more than ribs; rib section very slightly convex; straight commarginal growth lines on dorsal flank visible in intercostal spaces; commarginal ribs intercepting median ridge at an angle of c. 30º to 35º. Shell thick (2.7 mm at most inflated part of postumbonal ridge), decreasing towards dorsal margin, where it is very thin.
Measurements. Holotype GHUNLPam 810, length: 113 mm; width: 82 mm.
Remarks. These remarkable shells clearly belong in the Pinnidae, and fit best in Plesiopinna. Generic assignment is supported by the most striking feature of these shells, the curved median ridge that coupled with the convex ventral flank and much flatter dorsal flank produce the distinct tear shaped section. Although the placement in this genus is relatively easy on account of these characters, further studies including a phylogenetic analysis among the Pinnidae (extant and fossil) may prove otherwise. None of the specimens shows the interior nacreous shell layer, rendering it difficult to establish the presence or absence of any bilobation or the shape and position of the adductor muscle scar.
The species from La Pampa can be distinguished from the type species by its larger shell, its flatter posterior area and its ornamentation of radial ribs covering the entire posterior flank. However, the holotype is an internal mould with all but a few fragments of the shell missing. The radial sculpture, while visible, is not well preserved enough to compare with our material. The postumbonal ridge is not quite as strong as in Plesiopinna pampeana n. sp. nor is the posterior area as flat as in our material.
Plesiopinna sp. from the Oak Canyon Member of the Dakota Sandstone in New Mexico (Cobban, 1977, p. 14, pl. 1, fig. 1, 3) resembles our species more closely in shell outline and in the large and flat posterior area and strongly curved postumbonal ridge. However, the ornamentation is different in the two species and allows easy separation. While the dorsal part of the posterior flank of Plesiopinna sp. only shows fine commarginal growth lines, in P. pampeana this area is covered by radial ribs. In the North American species, the radial ribs are restricted to the ventral part of the dorsal flank and they are more numerous and densely packed. The commarginal anteroventral ribs are weaker and also far more restricted than in P. pampeana.
South American species of Pinnids superficially similar to Plesiopinna pampeana can be readily differentiated as none of them show the straight dorsal margin, flat posterior area and strongly curved and very inflated umbonal ridge. Such is the case of Pinna tumida Philippi, 1899 (p. 50-51, pl. 25, fig. 1) from the Cretaceous of Chile, Atrina reginamaris (Maury, 1930, p. 190, pl.7, fig. 3) from the Creataceous of northeastern Brazil, and A. rioturbiensis Griffin, 1991 (p. 128-129, fig. 5.1-5.3), from the Eocene of southwestern Patagonia.
Another species that resembles Plesiopinna pampeana is Atrina triangularis Zinsmeister (1983, p. 1286, fig. 1H) from the Paleocene of the Simi Hills in California. It also has a broad triangular shape, but the ridge is not as strongly marked and only very slightly curved, while the ribs are narrower. The interior is unknown too.
The striking shell-form of this pinnid may be related to a life habit slightly different than the one generally observed in modern Pinnids, i.e., vertically buried in soft bottoms and byssally attached to particles of sediment (Yonge, 1953; Stanley, 1970, 1972). The large relatively flat wing-like posterior area and the strongly inflated and curved postumbonal ridge suggest that - while still byssaly attached as supported by the byssal gape - it could have lived in a not quite as vertical position (i.e., with the ligament almost perpendicular to the substrate surface), with the straight hinge line inclined at an angle to the bottom and thus with the anteroventral area within the sediment and the posterodorsal expanded area above it. This mode of life is suggested for other groups of bivalves with analogous shell outlines such as Bakevellids and Pteriids (Aberhan and Muster, 1997). It is also the case of Bakevellia pintadae Damborenea, 1987 (p. 126-129, p. l 1, fig. 1-5; text fig. 4), from the Lower Jurassic of the Neuquén Basin, and Isognomon lusitanicum (Sharpe, 1849), I. praemytiloides Arkell, 1933 and I. volaticum Zakharov, 1966, from Jurassic localities in the Northern Hemisphere (Fürsich, 1980). Likewise, Bakevellids from the Lower Cretaceous of the Neuquén Basin probably had a similar habit (Lazo, 2003).
Age. Late Maastrichtian. Zone CC26 (late Chron 30 N-Chron 29R).
Etymology. Pampeana for La Pampa from were the type specimen was collected.

Genus Atrina Gray, 1842

Type species. Pinna nigra Dillwyn, 1817; subsequent monotypy, Gray, 1847.
Remarks. The distinguishing character that separates Pinna from Atrina, i.e., the bilobation of the inner prismatic layer, is not always preserved in fossil pinnids. However, as discussed by other authors (Rosewater, 1961; Zinsmeister, 1984; Griffin, 1991), the ham-shape of the shell and the rhomboidal shell section are generally better developed in members of Atrina, therefore allowing generic placement of fossil material lacking other key characters.

Atrina ? sp. Figure 4.7

Locality. Liu Malal (36º 45' S, 68º 43'W), province of Mendoza.
Stratigraphic range. Roca Formation, that at this locality is Danian.

Material. One specimen, an internal mould. GHUNLPam 17308.

Remarks. The only available specimen probably referable to Atrina is a poorly preserved internal mould that nevertheless shows the strong, straight postumbonal ridge typical of most species of this genus. It somehow resembles A. rioturbiensis Griffin, 1991 (p. 128-129, fig. 5.1-5.3) from the Eocene Río Turbio Formation, in southwestern Patagonia, but further comparisons will be possible only with better specimens in which an adequate suite of characters can be observed. Pinna tumida Philippi, 1899 (p. 50-51, pl. 25, fig. 1) from the Cretaceous of Chile, and Atrina reginamaris (Maury, 1930, p. 190, pl. 7, fig. 3) from the Creataceous of northeastern Brazil also show an overall similarity in shell shape and section.
Age. Danian. Zone NP1-Zone NP2 (late Chron 29RChron 29N).

Conclusions

Although known from the earliest years of the nineteenth century, the fossil invertebrate faunas from the Maastrichtian and Paleocene of the Roca and Jagüel formations have remained poorly understood. This is in part due to the fact that the preservation is generally poor, and intensive collection is necessary to assemble enough specimens (other than the extremely common oysters with calcitic shells) in which key characters are present that may allow proper taxonomic placement. Some of the material described herein - while imperfectly preserved - in general shows enough characters to allow positive identifications. The new species described are all related to taxa already known from other parts of the world in rocks of the same or similar age. Such is the case of the mytilids, in which very similar forms occur in Africa, Europe, India, and North America.
However, the exact relationship among these taxa must remain as yet uncertain, at least until better and more abundant material is collected from a wider range of localities.
The presence of Septifer in Maastrichtian rocks of western La Pampa and southern Mendoza suggests that during this time there were hard substrates available nearby, as the specimens studied are generally bivalved and relatively unworn. The location of this kind of substrate has yet to be identified. Contrarily, the rest of the fauna described consists of soft bottom dwellers, a life mode consistent with the lithology of the bearing rocks. Pinna cretacea is a cosmopolitan species and the taxon found in the Roca Formation is very close to it. On the other hand, Plesiopinna seems to be a more restricted genus and the specimens described are the only ones known so far from South America.
The relationship between the distribution of this group of bivalves and water temperature is still unclear, although it appears possible that the presence of Pinna sp. aff. P. cretacea in the area is reflecting the cosmopolitan distribution of this taxon in accordance with a global increase (at 65.5 Ma) in surface and intermediate water temperatures of approximately 3-4 ºC (Barrera and Savin, 1999).
Of interest is the probable presence of Mytilaster, a genus that nowadays inhabits brackish environments in the Northern Hemisphere. Its occurrence in beds of the Roca Formation needs to be properly assessed in view of the normal marine character of the rest of the bivalve taxa accompanying it. A survey of the stratigraphic distribution of the material described shows that the only Maastrichtian species that survived into the Danian was Gregariella amara suggesting that the environmental changes taking place at this crucial time were important enough to ensure a significant impact within the Mytiloida.

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Recibido: 22 de noviembre de 2006.
Aceptado: 27 de noviembre de 2007.