Protopleurobema: a new genus of freshwater bivalve from the Lower Cretaceous of the Cameros basin (NW Spain)/Protopleurobema: un nuevo genero de bivalvo de agua dulce del Cretacico Inferior de la Cuenca de Cameros (NO de Espana).
1. IntroductionPalacios and Sanchez (1885) defined the species Unio numantinus in a study conducted by the Spanish Geological Map Commission in the provinces of Logrono and Soria. A few years later, Palacios (1890) re-described and figured the species (using the same pictures) in a report on the geology of the province of Soria. The specimen came from the Lower Cretaceous of Villarijo (San Pedro Manrique, province of Soria). Several years later, Mongin (1966: 50-51) cited "Unio" numantinus Palacios and Sanchez (1885) in the discussion of "Unio" cf. porrectus Sowerby and argued that both species from the Spanish Weald facies were very different.
Subsequently, the syntype of this taxon was identified and figured by Delvene (2005, lectotype, Plate 1, Figs. 3a and 3b) according to article 73.2 of the International Code of Zoological Nomenclature (I.C.Z.N.).
Bermudez-Rochas et al. (2006) reported a preliminary study of several sections of the Urbion Group (Lower Cretaceous) identifying the presence and high abundance of "Unio" numantinus coexisting with gastropods of the Viviparidae family in Valdeperillo (La Rioja province), a section very close to Villarijo. This was followed by a brief description by Delvene and Araujo (2008) of the species "Unio" numantinus including the original specimen of Palacios and Sanchez (1885) and specimens recorded in the Valdeperillo section. These authors described its most characteristic features and suggested its possible ascription to a new genus of the order Unionoida, or naiads. Delvene and Araujo (2009) have also recently described other freshwater mussels from this same lithological group (Urbion Group). The abundant species Margaritifera idubedae (Palacios and Sanchez, 1885) recorded in Navajun (Cameros Basin, province of La Rioja, Spain) formed a monospecific bivalve association (Delvene et al., 2006) in a fluvial environment. Another species of Margaritifera appears in lacustrine sediments of the Cameros Basin, and is common in the Weald sediments of England. Specimens of Margaritifera valdensis (Mantell, 1844) have been collected from the Enciso Group at Cornago in La Rioja province (Delvene and Araujo, 2009).
The present study describes a new bivalve association in the Urbion Group pertaining to the Lower Cretaceous of the Cameros Basin, whose main representative is "Unio" numantinus.
2. Geological framework
This paper focuses on the Cameros Basin, in the northwestern Iberian Peninsula (Spain). The studied material originates from the Urbion Group, a lithostratigraphical unit divided into four subunits (A-D) corresponding to different depositional sequences of clearly fluvial characteristics. Bivalves were collected at the Valdeperillo site (Figs. 1, 2), La Rioja province, belonging to the subunit D of this lithological group. According to Mas et al. (2003), this subunit D, also known as Urbion D, is included in a depositional sequence designated number seven and its age is Upper Barremian-Lower Aptian (Lower Cretaceous). It is interpreted as a braided fluvial system progressing to a meandering system eastwards with a welldeveloped floodplain (Barrenechea, 1993).
3. Systematic palaeontology
All the specimens examined are provisionally housed at the Museo Geominero (IGME, Madrid, Spain). The lectotype (Delvene, 2005) belongs to the collections of this museum and is coded by the letters MGM (Museo Geominero), followed by a number and the letter C (Cretaceous). Specimens whose designated codes start with the abbreviation for the outcrop Valdeperillo (VDPR) (La Rioja province) will be deposited at the end of this study in one of La Rioja's palaeontological centres.
Main measurements (height, length, thickness of shell, angle [alpha], convexity) were made on the best-preserved specimens (Fig. 3). All the figured specimens were coated with sublimated ammonium chloride. The asterisk indicates the type species.
Subclass Palaeoheterodonta Newell, 1965 Order Unionoida Stoliczka, 1871 Superfamily Unionoidea Rafinesque, 1820
Genus Protopleurobema gen. nov.
Derivatio nominis
Proto (Greek, [pi][rho][omega][tau]o) = First, Early
Pleurobema = A recent genus of Unionidae described by Rafinesque (1820) from North America.
Type species
Unio numantinus Palacios and Sanchez, 1885
Diagnosis
Shell trigonal, equivalve, inequilateral, gibbose, very tumid and posteriorly elongated to forming a slight wing. Lunule well developed. Hinge very strong. Right valve bears a massive pyramidal cardinal tooth with pronounced longitudinal grooves. Left valve presents two grooved cardinal teeth; anterior tooth flat and sharp and posterior massive. Lateral teeth run from the umbo parallel to the valve edge.
Protopleurobema numantina (Palacios and Sanchez, 1885) Figures 4, 5
*1885 Unio numantinus Palacios and Sanchez: 137, plate 7a, figures 6, 6a-6d
1890 Unio numantinus Palacios and Sanchez Lozano --Palacios: 284, plate 6a, figures 1-5
2005 "Unio" numantinus Palacios and Sanchez - Delvene: 169, plate 1, figures 3a-3b
2008 "Unio" numantinus Palacios and Sanchez - Delvene and Araujo: 37, figures 1a-1b
[FIGURE 1 OMITTED]
Material
Lectotype: Right valve with posterior region broken (MGM1781C), (Fig. 4.1). Type locality: Pena de las Huecas, Villarijo (Soria, Spain).
The lectotype figured by Delvene (2005) (plate 1, figs. 3 a, 3b) is housed at the Museo Geominero (Instituto Geologico y Minero de Espana). This specimen had been figured by Palacios and Sanchez (1885: plate 7a, figs. 6, 6a-c) and Palacios (1890: plate 6a, figs. 1-4) and belongs to the collection Invertebrados y Plantas Fosiles de Espana, which is one of the Museo Geominero's main collections. The other specimen figured by Palacios and Sanchez (1885: plate 7a, fig. 6d) and Palacios (1890: plate 6a, fig. 5), supposedly a left valve, was not found in this museum. If this specimen eventually appears, it will be considered a paralectotype.
[FIGURE 2 OMITTED]
Recently collected specimens: 805 specimens from the Valdeperillo site (La Rioja province), coded VDPR-A1-125 (26 articulated specimens, 46 right valves, 53 left valves) and VDPR-C-1-680 (26 articulated specimens, 322 right valves, 332 left valves) from the levels A y C respectfully.
Description
The material examined differs from any other known fossil bivalve. Shell medium-sized to large (88.2 mm of maximum shell height and 120 mm of maximum shell length), equivalve, inequilateral, gibbose and very tumid. Outline completely trigonal with a very acute prosogyrous umbo. Sculpture of the umbones absent. Lunule well developed, flat and wide, ornamented with smooth striae. Both valves exceptionally convex with a marked anteriormedial shoulder (Fig. 3); this shoulder builds the thickest part of the shell, which attains a thickness of 12.6 mm in the largest specimens. The ligament pit, straight and narrow (Fig. 4.5 a-b), marks the dorsal margin of the valve. Ligament external and opisthodetic. The dorsal margin forms a variable angle coded a (Fig. 3, Table 1) with the anterior margin at the level of the umbo, being a right angle in some specimens. Anterior and dorsal margins joined by a rounded ventral border that continues into a posterior slight wing, very visible in the best-preserved specimens.
[FIGURE 3 OMITTED]
Hinge very strong and distinctive with two plates that diverge from the umbo: the anterior short and wide bearing the cardinals, and the posterior, large and straight, bearing the laterals. Right valve shows a massive pyramidal cardinal tooth with pronounced longitudinal grooves. This tooth is flanked by two deep sockets, the posterior socket being larger and grooved. Besides this socket, the straight lateral tooth, which starts just below the umbo, enlarges as it runs parallel to the posterior border. The left valve presents two grooved cardinal teeth, which fit in the two opposite sockets. Anterior cardinal flat and sharp, height variable (Fig. 5. 4a-c) and joined to the anterior border of the valve. Posterior cardinal massive, pyramidal and similar to the one in the right valve. Sockets between the two cardinals very grooved. Two straight lateral teeth run from the umbo parallel to the valve edge.
Anterior adductor muscle impression very deep and close to the shell margin. In the right valve (Fig. 4. 6a; 5. 2b), the retractor pedal muscle scar, posterior to the adductor scar, is circular; while the protactor one, posterior and mostly ventral to the adductor muscle scar, is oval. Posterior adductor muscle impression unknown.
Discussion
The external and internal morphological features of "Unio" numantinus are obviously not characteristic of the genus Unio as defined by its authors. Thus, in view of the lack of any known genus (recent or fossil) that could accommodate the species, we here define the new genus Protopleurobema.
According to the fossil mussel's trigonal outline, freshwater habitat and above mentioned hinge characters (opisthodetic external ligament, and hinge consisting of two strong cardinal teeth in the left valve and one in the right), Protopleurobema numantina may be included in the order Unionoida. Moreover, the presence of lateral teeth and their shape in these specimens suggest the inclusion of the genus Protopleurobema in the superfamily Unionoidea (Cox et al., 1969). Although this designation seems clear, the number of characters is insufficient for its ascription at the family level, but it could be assigned to the family Unionidae as Pleurobema, since our specimens resemble the members this genus more than those of any other known genus, as we discuss below.
The genera Pachycardia, Hadrodon, and Pleurobema are the only known closely related Mesozoic genera described in the literature:
Pachycardia Hauer (1857) is a European genus of the Middle to Upper Triassic record. Its main similarity to Protopleurobema is the general shape of the shell, which is markedly trigonal; another shared characteristic is the presence of a wide and easily distinguished lunule. However, the hinge in Pachycardia is less strong and massive. In effect, this genus was formerly included in the Cardiniidae family (Heterodonta Subclass) (Cox et al., 1969: N468). Owing to the hinge not really being heterodont, it was transferred to the Palaeoheterodonta Subclass, but its hinge is not as thick and strong as that of the members of the genera in the Unionoidea superfamily bearing cardinal and lateral teeth.
The genus Hadrodon Yen (1952) appears in the Jurassic of North America. Its members exhibit a strong hinge with a massive pyramidal cardinal tooth, which is very similar to the tooth observed in Protopleurobema. Although species of Hadrodon also present a wing-like posterior slope, its conspicuously undulated external surface distinguishes it very clearly from that found in Protopleurobema.
Whitfield (1907: 624) described Unio pyramidatoides, a Pleurobema-like species from the Cretaceous of Montana. Although this is the species that most resembles Protopleurobema numantina, several important features differentiate the two. In U. pyramidatoides, the lunule is quite small or nearly obsolete and the posterior region is straight and has a very pronounced ridge, while P. numantina displays a large patent lunule. The former species also lacks a posterior wing.
[FIGURE 4 OMITTED]
The stratigraphical record of the North Americam genus Pleurobema Rafinesque (1820) spans the Upper Cretaceous to recent times. Of all the known genera, this is the genus that most resembles the new Protopleurobema. The main difference between the two lies in the shape of the shell, which is much more equilateral in Pleurobema than in Protopleurobema. The clearly recognized slight posterior wing makes the outline of the Protopleurobema shell very inequilateral. The hinges are nevertheless very similar in both, with cardinal teeth that are markedly pyramidal.
4. Palaeoecological environment
After detailed sampling of a 40-metre section of subunit D of the Urbion Group, we identified 3 thick levels (A, B and C) of bivalves (Fig. 2). Lithologies varied from red sandstones of very fine grain size to grey marls, in which a greater abundance of well-preserved specimens was observed. Gastropods appeared throughout this section and coexisted with the bivalves in some levels. These were large gastropods belonging to the family Viviparidae that could be ascribed to the genus Viviparus, though their species identification remains unclear. What is clear is that they point to a freshwater environment. As described above, the sedimentologic features of the section also indicate a freshwater environment represented by a fluvial system (Barrenechea, 1993).
The taphonomic characteristics of this bivalve association such as the high number of specimens (805), the presence of both juvenile and adult specimens, and the preserved articulated valves indicate it should be interpreted as a palaeoecological association. The "butterfly position" was relatively frequent among articulated specimens, especially in level A of the outcrop. The bivalve association is almost exclusively comprised of the species Protopleurobema numantina. The level B contains a large number of specimens but it is very difficult to remove them from the matrix.
According to the available literature, the order Unionoida only inhabits freshwater environments. Thus, the presence of this palaeoecological community confirms the freshwater nature of the palaeoenvironment. As are all known naiads, this type of bivalve would have been a filter feeder that lived burrowed in the substrate.
Acknowledgements
This work is a contribution to project CGL2006-10380/ BTE. Photographs were prepared by the Servicio de Fotografia Paleontologica of the Universidad de Zaragoza (Spain). Support was received from the SYNTHESYS Project (http://synthesys.info/) financed by European Community Research Infrastructure Action under the FP6 "Structuring the European Research Area" programme. The authors thank the Direccion de Educacion y Cultura del Gobierno de La Rioja (Spain) for permission to collect and analyze the fossil material. We also thank reviewers A. Bogan and A. Marquez Aliaga for their comments. Ana Burton reviewed the English.
[FIGURE 5 OMITTED]
Received: 12/05/09 / Accepted: 30/06/09
References
Barrenechea J.F. (1993): Evolucion de la mineralogia de arcillas en el transito diagenesis-metamorfismo de bajo grado en el Grupo Urbion (Cretacico inferior) de la cuenca de Los Cameros (Soria - La Rioja). Tesis Doctoral. Universidad Complutense de Madrid. Facultad de Ciencias Geologicas. Madrid, 299 p.
Bermudez-Rochas, D.D., Delvene, G., Hernan, J. (2006): Estudio preliminar del contenido paleontologico del Grupo Urbion (Cretacico Inferior, Cuenca de Cameros, Espana): restos ictiologicos y malacologicos. Boletin Geologico y Minero, 117 (monografico especial): 531-536.
Cox, L.R., Newell, N.D., Branson, C.C., Casey, R., Chavan, A., Coogan, A.H., Dechaseaux, C., Fleming, C.A., Haas, F., Hertlein, L.G., Myra Keen, A., LaRocque, A., McAlester, A.L., Perkins, B.F., Puri, H.S., Smith, L.A., Soot-Ryen, T., Stenzel, H.B., Turner, R.D, Weir, J. (1969): In: R.C. Moore, (ed.). Treatise on invertebrate paleontology, Part N, Mollusca 6, vol.1, Bivalvia. The Geological Society of America and University of Kansas.
Delvene, G. (2005): El material tipo de las especies de "Unio" (Bivalvia) del Cretacico Inferior del Museo Geominero (IGME, Madrid). Boletin Geologico y Minero, 116 (2): 167-172.
Delvene, G., Araujo, R. (2008): A new freshwater bivalve association from the Lower Cretaceous of Spain. Documents des Laboratoires de Geologie Lyon, 164: 37-39.
Delvene, G., Araujo, R. (2009): Early Cretaceous non-marine bivalves from the Cameros and Basque-Cantabrian basins of Spain. Journal of Iberian Geology, 35 (1): 19-34.
Delvene, G., Araujo, R., Bermudez-Rochas, D.D. (2006): Cretaceous Spanish margaritiferids. Organisms Diversity and Evolution 6, Electronic Supplement 16 (1): 24. http:// www.senckenberg.de/odes/06-16.htm.
Hauer, F. R. (1857): Ein Beitrag zur kenntniss der Fauna der Raibler Schichten. Sitzungsberichte der Kaiserlichen Akademie der wissenschaften. Mathematisch-naturwissenschaftliche Classe, 24 (1):537-566.
International Commission on Zoological Nomenclature. (1999): International Code of Zoological Nomenclature. International Trust for Zoological Nomenclature. http://www.iczn.org/iczn/index.jsp.
Mantell, G. A. (1844): On the Unionidae of the river of the country of the Iguanodon. The American journal of Science and Arts, 47 (1): 402-406.
Mas, R., Benito, M.I., Arribas, J., Serrano, A., Guimera, J., Alonso, A., Alonso Azcarate, J. (2003): The Cameros Basin: from late Jurassic-Early Cretaceous extension to Tertiary contractional inversion-implications of hydrocarbon exploration. AAPG International conference and exhibition, Barcelona, Spain. Geological field trip, 11: 1-56.
Mongin, D. (1966) : Description paleontologique de quelques lamellibranches limniques des facies wealdiens d'Espagne. Notas y Comunicaciones del Instituto Geologico y Minero de Espana, 91: 41-60.
Palacios, P. (1890): Descripcion fisica, geologica y agrologica de la provincia de Soria. Memorias de la Comision del mapa geologico de Espana, 284 p.
Palacios, P., Sanchez, R. (1885): La formacion Wealdense en las provincias de Soria y Logrono. Boletin de la Comision del Mapa Geologico de Espana, 12: 1-32.
Rafinesque, C. S. (1820): Monographie des coquilles bivalves fluviatiles de la riviere Ohio. Contenant douze genres et soixante huit especes. Annales Generales des Sciences Physiques. Bruxelles, 287-322 + 2 Plates.
Whitfield, R. P. (1907): Remarks on and descriptions of new fossil Unionidae from the Laramie Clays of Montana. Bulletin American Museum of Natural History 23: 623-628.
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Graciela Delvene (1), Rafael Araujo (2)
(1) Instituto Geologico y Minero de Espana, C/. Rios Rosas 23, 28003 Madrid (Spain). [email protected] (2) Museo Nacional de Ciencias Naturales (CSIC), Jose Gutierrez Abascal 2, 28006Madrid (Spain); [email protected]
Table 1. Measurements of the biometric parameters of the
best-pre-served specimens, expressed in mm.
Tabla 1. Medidas de los parametros biometricos de los ejemplares
mejor conservados, expresadas en mm.
Height Length Thickness Angle Convexity
Specimen (H) (L) shell a
VDPR-C-7 - - 6 85 -
VDPR-C-8 50 - - 80 41.4
VDPR-C-9 - - 4.9 87 -
VDPR-C-10 - - 6.4 78 -
VDPR-C-11 58 - 8.4 79 -
VDPR-C-12 36.4 - - 90 -
VDPR-C-13 49.6 - - - -
VDPR-C-14 - - 7.6 84 -
VDPR-C-15 32 - - 79 -
VDPR-C-16 17.9 - - 78 -
VDPR-C-17 18.2 - - 73 -
VDPR-C-18 39.6 - 6.8 88 -
VDPR-C-19 46.5 - 9 - -
VDPR-C-20 53.9 - 9.6 - -
VDPR-C-21 - - 8.9 75 -
VDPR-C-22 35.8 - - 90 18.7
VDPR-C-23 57 - - 80 37
VDPR-C-24 - - - 65 -
VDPR-C-25 30.4 - - 81 18.3
VDPR-C-26 5.2 86
VDPR-C-27 29.3 +34 - 81 29,6
VDPR-C-28 - - 10.8 86 -
VDPR-C-29 +57.4 +53.5 9.7 75 -
VDPR-C-30 53.6 - 6.8 73 -
VDPR-C-31 27.8 - 5.8 87 -
VDPR-C-32 46.6 - - 77 -
VDPR-C-33 15.3 +9.6 2 75 -
VDPR-C-34 31.4 - - 85 -
VDPR-C-35 51.4 - 6.8 90 -
VDPR-C-36 - - 9.2 75 -
VDPR-C-37 17.6 - 2.4 - -
VDPR-C-38 - - - 82 -
VDPR-C-39 - - - - -
VDPR-C-40 12.2 - - 83 -
VDPR-C-41 19.4 - - 72 -
VDPR-C-42 44 +37 6.8 88 -
VDPR-C-43 46.4 - 6.7 80 -
VDPR-C-44 - - - 78 -
VDPR-C-45 - - - 70 -
VDPR-C-46 41.1 - - 79 -
VDPR-C-47 49 - 12.6 - -
VDPR-C-48 52.5 - - - -
VDPR-C-49 - - - 86 -
VDPR-C-50 - - - 80 -
VDPR-C-51 41.3 57.4 9 - -
VDPR-C-52 - - 9 79 -
VDPR-C-53 42.8 +34.4 9.5 79 -
VDPR-C-54 49.5 +35.5 4.1 - -
VDPR-C-55 - - 5.4 72 -
VDPR-C-56 - - 6 78 -
VDPR-C-57 - - 6.3 78 -
VDPR-C-59 54 +45.5 8 80 -
VDPR-C-60 - - 8.5 77 -
VDPR-A-1 64.2 68.3 - 81 37.7
VDPR-A-4 88.2 120 - - -
VDPR-A-5 64.2 72 - 89 -
VDPR-A-6 54.5 59.8 - 88 -
VDPR-A-7 65.5 82.2 - 83 -
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| Author: | Delvene, Graciela; Araujo, Rafael |
|---|---|
| Publication: | Journal of Iberian Geology |
| Article Type: | Report |
| Date: | Jul 1, 2009 |
| Words: | 3035 |
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