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Ameghiniana

versión impresa ISSN 0002-7014

Ameghiniana vol.46 no.3 Buenos Aires jul./set. 2009

 

NOTA PALEONTOLÓGICA

First record of Malayaspira Kobayashi, 1958 (Mollusca, Gastropoda) in the Lower-Middle Ordovician San Juan Formation, Argentine Precordillera

Verónica Bertero

CICTERRA-CONICET, Centro de Investigaciones Paleobiológicas, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Velez Sarsfield 299, X5000JJC Córdoba, Argentina. CONICET. berteroster@gmail.com

Introduction

The San Juan Formation is a passive margin carbonate succession that crops out extensively in the Precordillera fold belt of Western Argentina. Its age ranges from latest Tremadocian to Darriwilian. This unit contains diverse and well preserved megafossils, mainly brachiopods, sponges, bryozoans and trilobites, which have been the focus of several papers (see Benedetto, 2003 for a review). Despite their relative abundance throughout the San Juan Formation, gastropods have never been studied in detail. The first description of Ordovician Precordilleran gastropods is that of Kayser (1876), who figured and described species of Ophyleta, Maclurites and Murchisonia from the San Juan Formation limestones. Subsequently, these taxa were revised by Kobayashi (1935, 1937). More recently, Carrera (1999) described the opercula of Teiichispira argentina (Kayser) and an unidentified macluritacean from the same formation in several localities of the San Juan province, and Rohr et al. (2001) gave a list of genera and families of gastropods from the same unit.
Some natural casts of gastropod protoconchs were recovered from acid-etched residues from the upper San Juan Formation limestones at the Cerro Viejo locality and around twenty-six species described in several localities (Bertero, 2006, 2007). These preliminary reports are part of the author´s PhD project that involves the taxonomy, diversification patterns and biogeography of the Ordovician gastropods of the Argentine Precordillera.
One of the reasons why the study of San Juan Formation gastropods has been neglected is their preservation almost exclusively as internal molds. As it is known, shell ornament and external features are critical for taxonomic identification at familial, generic and specific levels. In the past few years, intensive sampling of the San Juan Formation at different localities yielded well preserved material, opening promissory perspectives for the knowledge of the Ordovician gastropods from Argentina. The purpose of this paper is to describe and illustrate for the first time the genus Malayaspira Kobayashi, 1958 from the Precordillera terrane and to assess its biogeographic significance.

Locality and stratigraphy

The samples were collected from exposures of the San Juan Formation along the Talacasto River, approximately 3 km west of the Baños de Talacasto locality (figure 1). Fossiliferous beds are located on the western flank of the Talacasto anticline. The Malayaspira material described herein is restricted to some beds of skeletal wackestones within the Niquivilia extensa brachiopod Zone (figure 2), which according to the associated conodonts can be confidently referred to the late Floian O. intermedius Zone and the early Dapingian T. laevis Zone (Benedetto, 2001). Fossil assemblages are dominated by brachiopods (mainly orthides and leptellinids), sponges, trilobites, encrusting receptaculitids, bryozoans and gastropods. This association was referred by Carrera et al. (1999) to the leptellinid-dominated biofacies, which flourished in a middle ramp setting (Cañas, 1999; Carrera, 2001, Cañas and Aguirre, 2005; Benedetto, 2007).


Figure 1. Location map of the studied stratigraphic section in the Ancha creek, Talacasto range (San Juan Formation outcrops in grey) / mapa de ubicación de la sección estudiada en la quebrada Ancha, sierra de Talacasto (afloramientos de la Formación San Juan en gris).


Figure 2. Generalized stratigraphic column of the San Juan Formation showing the ocurrence of Malayaspira aff. rugosa (modified from Carrera, 2001) / columna generalizada de la formación San Juan señalando la ubicación de Malayaspira aff. rugosa (modificado de Carrera, 2001).

Systematic paleontology

The collection is held at the Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba (prefix CEGH-UNC).

Order EUOMPHALINA de Koninck, 1881
Superfamily OPHILETOIDEA Knight, 1956
Family OPHILETIDAE Knight, 1956

Genus Malayaspira Kobayashi, 1958

Type species. Malayaspira rugosa Kobayashi, 1958

Malayaspira aff. rugosa Kobayashi, 1958
Figure 3.1-6

Material. Thirteen incomplete individuals, CEGH-UNC 23189- 23194 and 23594- 23600 from the San Juan Formation, Talacasto river, Niquivilia extensa Zone, upper Floian.- lower Dapingian.


Figure 3. Malayaspira aff. rugosa. 1, CEGH-UNC 23189 Basal view x 1.5. / 2, CEGH-UNC 23190 apical view x 1.5. 3, CEGH-UNC 23190 lateral view x 2.2. 4, CEGH-UNC 23190 apical view showing details of ornamentation x 2. 5, CEGH-UNC 23190 detail of selenizone x 5. 6, CEGH-UNC 23194 cross section x3./ Malayaspira aff. rugosa. 1, CEGH-UNC 23189 vista basal x 1.5. 2, CEGH-UNC 23190 vista apical x 1.5. 3, CEGH-UNC 23190 vista lateral x 2.2. 4, CEGH-UNC 23190 detalle de la ornamentación x 2. 5, CEGH-UNC 23190 detalle de la selenizona x 5. 6, CEGH-UNC 23194 corte transversal x 3.

Description. Moderately large (3.2 cm wide, 1.1 cm high on average), discoidal, widely umbilicate shell with depressed spire and flat base. Three whorls preserved, but impressions on the rock indicates up to five whorls. The rate of whorl expansion is about two times per whorl. Mean pleural angle of 130°. Shell ornamentation consists of strong growth lines which are opistocline at the base, prosocline on the lateral sides and opistocline on the upper side. Collabral growth lines becoming more closely spaced with growth, on the last whorl the space between ribs is about 1 mm. Whorl bases rounded, internal and external sides flattened or slightly concave which converge toward a prominent, flat topped carina bearing the selenizone. Transversal section of the whorls subquadrate to suboval. Aperture shape and character of apex unknown.
Discussion. The San Juan Formation material is referred to Malayaspira on the basis of its nearly flat base, depressed spire, moderate rate of whorl expansion, subquadrate whorl profile, carinate upper side of whorls, and ornament features, among other characters (Kobayashi, 1958, 1959). The Precordilleran specimens are very close to the material of M. rugosa illustrated by Rohr (1994, fig. 7) from the Middle Ordovician of Malaysia, except that the profile of the Argentinean material is less angular than in the Malaysian specimens. However, the lack of complete specimens precludes a more conclusive specific assignment. The species M. hintzei, Rohr, 1994 and M. speciosa (Billings, 1865) differ from the studied specimens in that the whorl is higher than wide, whereas in M. rugosa and the Argentinean form this proportion is reversed. Moreover, in the Argentinean Malayaspira the outer whorl surface curving uniformly downwards, whereas in both M. hintzei and M. speciosa is nearly planar, sloping at an acute angle to lower-outer angulation. M. canningense (Yu, 1993) clearly differs in its cancellate sculpture formed by the intersection of the crossed spiral threads with the growth lines.
Internal molds of Macluritaceans are very common in the San Juan Formation. Rohr (1994) noted some affinities between Malayaspira hintzei and small specimens of Maclurites. However, the internal molds of Maclurites are quite different from those of Malayaspira in having a more elevated spire and a deeper umbilicus. Maclurites avellanadae has a higher shell and consequently a deeper umbilicus. In section, the whorls of the spire are oval. M. sarmienti looks similar to Lesueurilla or Ecculiomphalus, in having a crest on the external margin (Rohr, 1980; Rohr et al., 2001) and the internal side of the spire fall abruptly forming an angle of approximately 45º. M. stelzneri has a larger size in relation to the number of whorls; however its original description is too limited and should be revised.

Paleogeographic significance

The genus Malayaspira (type species M. rugosa) was erected by Kobayashi in 1958 on material from the Lower Ordovician Setul Formation, Langkawi Islands, Malaysian peninsula. After that, at least four other species of Malayaspira have been recognized: M. hintzei Rohr, 1994, from the Whiterockian of Nevada, and M. speciosa (Billings, 1865), from Whiterockian of Newfoundland (Canada). Wagner (2002) also referred to Malayaspira the euomphaloidan Oriostoma cannigense Yü, 1993 from the Darriwilian Gap Creek Formation of Australia. One unpublished species of Malayaspira also occurs in the Upper Ordovician of the Taimyr Peninsula (Siberia) (A. Gubanov, pers. comm., 2008). As figure 4 shows, Malayaspira has been recovered from shallow-water carbonate platforms developed on low-latitude plate margins. Such a widespread, almost pan-Tropical geographic range suggests that Malayaspira had planctotrophyc larvae. According to Nützel and Fryda (2003) and Nützel et al. (2006) free-swimming and plankton-feeding larvae evolved near the Cambrian- Ordovician boundary, during the initial stages of the Ordovician radiation. Dispersion of modern gastropod larvae largely depends on the length of pelagic existence and oceanic current velocity and direction (Scheltema, 1977). Evidence from warm-temperate and tropical gastropod species indicates that planctotrophic forms can be dispersed over long distances, and many species can be transported across the Atlantic Ocean by North and South Equatorial currents, attaining a peri-Equatorial distribution. Another very common Lower Ordovician Precordilleran gastropod is Teiichispira (represented only by opercula), which has been recorded also from coeval beds of Australia (Yü, 1993). In addition, Carrera and Rigby (1999) recognized a similar pan-Tropical pattern (Laurentia, South China, Precordillera) in several Precordilleran sponges. It is also noteworthy the similitude between the pattern of distribution of some Precordilleran rhynchonelliform brachiopods and that of Malayaspira. For example, Aporthophyla is a widespread pan-Equatorial genus recorded in Laurentia, Kazakhstan, Baltica, North China, Malaysia and Thailand. Of the numerous species described, the Precordilleran species A. neumani Benedetto, 1998 is closely related to A. tianjingshanensis Fu, 1982 from the Setul Formation, Langkawi Islands, Malaysia and Thailand (Laurie and Burrett, 1992), the same beds yielding the type material of Malayaspira rugosa. This widespread low-latitudinal dispersion is consistent with the fact that many Lower Paleozoic rhynchonelliform brachiopods have large protegula, indicating a planktotrophic larval stage (Freeman and Lundelius, 2005, 2008). During such a relatively long planktonic period, larvae can be dispersed by ocean currents over long distances. The species M. rugosa was restricted to an oceanic corridor between the eastern Gondwana (and peri-Gondwanan plates) and the Iapetus Ocean. Trilobite evidence indicates that this pattern was established before the Middle Ordovician. During the Upper Cambrian-Tremadocian active larval dispersion into the Andean platforms from Australia, New Zealand and South China is well documented upon the occurrence in the western South American basins of Onychopyge, Amzasskiella, and Australoharpes, among others (Vaccari et al., 2006). According to the paleogeographic model by Benedetto et al. (1999), at that time the Precordillera was not adjacent to Gondwana, but probably it was not far from its margin and received some selected taxa capable of long-term dispersion. In fact, typical peri-Gondwanan brachiopods and trilobites attain the Precordillera during the Middle Ordovician (Benedetto et al., 1999; Benedetto, 1998, 2004). The Lower Ordovician acritarchs display a comparable pattern of geographic dispersion. Similar temperate to warm-water Tremadocian acritarch assemblages have been recorded in Laurentia, North China and Baltica, and their absence in Siberia and Australia is probably due to the lack of detailed studies (Servais et al., 2005). This supports the hypothesis that dispersion of Malayaspira was essentially controlled by oceanic currents.


Figure 4. Early Ordovician paleogeograpic reconstruction showing the distribution of the genus Malayaspira (modified from Benedetto, 2003) / mapa de reconstrucción paleobiogeográfica mostrando la distribución del género Malayaspira (modificado de Benedetto, 2003).

Acknowledgments

I wish to thank T. Sánchez, J.L. Benedetto and M. Carrera (CIPAL, Córdoba University) for many fruitful discussions. Careful reviews by J. Peel and A. Gubanov greatly improved this manuscript. This work was supported by grants from the Consejo Nacional de Investigaciones Científicas y Ténicas (CONICET, PIP 5599) and Agencia Nacional de Promoción Científica y Tecnológica (PICT 16-21857).

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Recibido: 23 de octubre de 2008.
Aceptado: 13 de mayo de 2009.