AMEGHINIANA Revista de la Asociación Paleontológica Argentina Tomo VII Setiembre de 1970 N~ 3
THE LOWER CRETACEOUS BIVALVE \NPPbmS[[N IN PATAGONIA AND ITS PALEOGEOGRAPHIC SIGNIFICANCE FOR CONTINENTAL DRIFT
L i5 L5 c K_O\RYa]O S mzp K5 C. \SMMK\NS9
ABSTRACT: The subgenus Q· ¡tmqm /Wuyqá~»á·q~z0 Bonarelli in Bonarelli and Nágera (1921) is considered to be a junior subjective synonym of Wmoo~ qxxm Etheridge (1892). Wmoo~ qxxm n~zm·qxxuu (Leanza, 1963) u» redescribed, and shown to be similar to Wmoo~ qxxm uzoé·émám Waterhouse 1959 from New Zealand (Aptian? or Albian?) and M. ·~owñ~~pqz»u» (Etheridge ) írcrn Australia (Albian). The age of the Patagonian species u» probably Aptian. This genus u» important paleogeographically. By restoring the three regions with Wmoo~ qxxm to their probable lower Cretaceous positions, based on paleomagnetic, faunal snd structural analyses, uá is shown that Wmoo~ qáxm inhabited a restricted paleolatitudinal belt, close to 50° S to 60° S. It is now found over a much wider spread of latitudes, ranging from nearly 10° S to 50" S.
RESUMEN: Se considera al subgénero Q· ¡nmqm /Wuyqá~»á·q~z0 Bonarelli /uz Bonarelli y Ná- gera, 1921) como un sinónimo de Wmoo~ qxxm Etheridge (1892). Se redes cribe Wmoo~ qxxm 774 zm·qxxuu (Leanza, 1963), señalando su similitud con M. uzoé·ímám Waterhouse, 1959, de Nueva Zelandia (Aptiano? o Albiano?) y M. ·~owñ~~pqz»u» (Etheridge) de Australia (Albiano). La edad de la especie patagónica es probablemente aptiana. Este género es importante paleo- gecgráficamente Situando las tres regiones con Wmoo~ qxxm en sus probables posiciones en el Cretácico inferior, de acuerdo con datos paleomagnéticos, faunísticos y estructurales, se observa que' Wmoo~ qáxm habitó un faja paleolatitudinal restringida, ubicada entre 500 S y 60° S. En la actualidad se la encuentra en sedimentos distribuidos en una gama más amplia de latitudes, desde casi los 10° S a 50° S.
INTRODUCTION SYSTEMATIC DESCRIPTION
Family Yde_YWSNKO Ichikawa 1958 Wmoo~ qxxm is a short-ranged Lower Cre- (nom. transl. Cox 1961, p . .592) taceous bivalve genus, first discovered in Australia, and later found in two other Gen-rs Etheridge 1892 parts of the Southern Hemisphere, South America and New Zealand. TYPE SPECIES: (by original designation) Kíu4 The paleogeographic implications, ta- oéxm nm·wx u Moore 1870. xonomy and systematics of the South SYNONYM:Wuyqá~»á·q~z Bonarelli 1921, type American species are discussed in this species Kíuoéxm o~·nuqz»u»Moore 1870. paper. DIAGNOSIS:Medium-sized to large acline or prosocline shells with submedium or anteriorly • Departrnent of GeoJogy, University of Toronto, Ca- placed umbones, .1ft valve moderately to strong- Iy convex, right; V\J.lveflat, or weakly convex or nada. concave, posterior'; wings obtuse, may be poorly o Departrnent of Geology, McMasler University, Ca- defined or absent from right valve, anterior left nada. ear small, right anterior ear small and thick, -- 281 - usually with narrow interior subauricular notch, word. The ornament is costate (or costellate ), left byssaI gape small to Iarge, Costae of two and the inner surface of the shell is smooth at to four orders, subspinose 'in some species, maturity, though not of course in early growth growth lines prominent. Ligament area on plate stages. In our experience costae frequently affect in left valve, on posterior rim in right valve, the entire shell in juvenile stages, and so are both with opisthocline resilifer, large anterior like small plicae. left tooth and right socket, posterior adductor large submedian in each valve, pallial line pit- ted. a a (Leanza 1963) DrSCUSSION: Wuyqá~»á·q~z with type Pl. 1, figs. 1-10 species Kíuoéxm o~·nuqz»u» Moore 1870 8C98 Q· ¡tmqm /Wuyqá~»á·q~z n. subgen.) was proposed by Bonare11i (in Bonarelli cfr. o~·nuqz»u» (Moore}-Bonarelli, p. 21, and Nágera, 1921) in the course of des- pl. 2, figs. 5a-b, 6. cribing Cretaceous fossils from Patago- 8C:B Q· ¡tmqm cfr. o~·nuqz»u» (Moore l-Piar- nitzky, p. 74, pl. 5, figs. 24, 25. nia. Bonarelli was apparently unaware 8C?: Wuyqá~»á·q~z n~zm·qxxuun. sp., Leanza that Etheridge (1892) had included Kíu4 p. 222. oéxm o~·nuqz»u» in his genus Wmoo~ qxxm3 8C?A Wuyqá~»á·q~z n~zm·qxxuuLeanza, p. 64, based on an allied species Kíuoéxm nm·w4 pl. 1, figs. 1-6. x u Moore. The two species differ con si- derably in shape, K5 nm·wx u being much NEOTYPE: The type material of the larger with a less convex left valve and species 5W5n~zm·qxxuuis amongst that des- less incurved umbo. It may eventually be cribed by Bonarelli (in Bonarelli and Ná- decided that such incurved species form gera, 1921), if we are to give priori ty to the first designation (Leanza, 1963, p. a distinct lineage of generic or subgeneric rank. But present evidence suggests that 222; 1967b, p. 63) over later designa- such "gryphaeation" recurred several ti- tions of another specimen( Leanza, 1967b, mes, possibly from different stocks, so pp. 64,65, pl. 1, figs. 1,2). Unfortuna- that placement of a11incurved Wmoo~ qxxm tely, the two specimens figured by Bona- into Wuyqá~»á·q~z would obscure rather relli seem to be lost. A neotype should than clarify phylogeny. be chosen from topotype material. We therefore designate as neotype, specimen Leanza (1963, 1967b) correctly sho- MLP 11220, here figured on pl. 1, figs. wed that affinities of Wuyqá~»á·q~z lay 1, 3. with Yó á~ym rather than Q· ¡tmqm3 and Cox (1969, p. L346) and Day (1969, DIAG OS1S:Medium-sized Wmoo~ qxxm3left val- p. 159) synonymized Wuyqá~»á·q~z with ve convex with incurved umbo and wide byssal Wmoo~ qxxm5 notch, right valve concave, costae moderately numerous (12-14 primaries), very fine and well- The Liassic species Q· ¡tmqm »á·umám spaced, secondaries inconspicuous. Resilifers high- Phillipi (in Behrendsen 1891-2, p. 394, ly opisthocline under the umbo, notch in front pl. 22, figs. 5-6) was included in Wuyq4 of right hinge obscure. á~»á·q~z by Bonarelli. The specimens are MATERIAL: Three specimens with valves pcorly known, but the right valve does conjoined and seven left valves coIlected not appear to be like that of Wmoo~ qxxm5 by A. C. Riccardi (MLP 11220-11229) and a composite mould of a left valve (MLP 2831) Ncr is the Callovian species Q· ¡tmqm collected by DI. J. Frenguelli, aIl kept at Museo zqn·m»oqz»u»5 Meek and Hayden 1862 (d. de La Plata, Argentina. Imlay, 1948, p. 18; 1967, p. 8]) conge- LOCALITIES:Upper part of a small creek, west neric, though a1so referred to Wuyqá~»4 of Arroyo Calafate, south-east of Bahía de la á·q~z by BonareIli. Lancha .;E S, 72° 10' W), Lago San Martín, Santa Cruz, Argentina, in the upper part of the MORPHOLOGY:Speden (1968, 1969) has des- shaly Rio Mayer Formation, with Ku~x~oq·m»m·4 cribed the shell of Wmoo~0.qxxmas "plicate". It sqzáuzéy (Bonarelli ): must be stressed that the valves of Wmoo~ qxxm This locality matches the one located on the are not plica te in the normal sense of the Geological Map of Bonarelli & Nágera (1921), - 282- PLATE 1
2 1
4
5 6 7
Wmoo~ qxxm n~z~·qááu»(Lcanza ): 1,3. Externa' nnd inter na l view of neol~'pe, ~ILP 11220; left val ve ; 2,4. External and internal vi ew of left val ve ~I LP 11221 ; 5. RiglJt aspect of M LP 2831 ; 6. Right aspect of !\ILP 11222, right exterior worn ; 7. Lateral view of le(t val ve MLP 11223; 8, 10. lnl erna l ami ext.er- nal aspects of rig-ht val ve MLP 11222; 9. Right uspcct of MLP 11225 .. \1I spccimens save MLP 2831 from z part Á smal l creek west Á Ar-royo Sx xÁx °6 iU ÁRxá x ~° x bx záx6 bxÓ ix Ma rt¡n , eoll. A. C. Hiccar-di . J\ILP 2831 from probably same locality , eoll. J. Frenguelli.AlI specimens natural ixe,
- 283- irnmediately to the north of "Puesto Martín", One of the specimens described tJy Leanza in the "Serie arcilloso-m argos a ... ", close to the (1967b, pl. 1, fig. 6) carne frorn the same area contact with the 'Cretaceo". The same locality, and horizon, probably, has yielded one of the specimens DISTRIBUTION: In Patagonia this species, or here described (MLP 2831), and those described forms referred to Q· ¡nmqm cf. o~·nuqz»u» by Bonarelli (in Bonarelli & Nágera, 1921 under (Moore ), has been mentioned frorn San Martín Q· ¡nmqm /Wuyqá~»á·q~z n. subgen.) cfr. o~·4 Lake, in Viedma. Lake (north and east shore.. nuqz»u» (Moore ) (p. 21, pl. 2, figs., 5a-b, 6). Leanza, 1967b) and a borehole in Punta Norte, The latter specimens are missing from the col- 8 km north of Puerto Coig, Atlantíc Ocean, ap- leetion of Bonarelli and Nágera which is de- praximately 51° S (Feruglio, 1949, T. 1, p. 179). posited in the Instituto Nacional de Geología y Minería (Buenos Aires, Argentina). DIMENSIONS (in mm):
Muxtruurn ° Óy Umboual Umbo fl'0111 i °zé ° Leug th XzéÓá 'Yi