Ordovician gastropods from Argentina

ROHR, D. M., M. BERESI, AND E. L. YOCHELSON

Journal of the Czech Geologica Society

The Czech Geological Society

Lugar: Czech; Año: 2001 vol. 46 p. 3 - 4

1210-8197

Journal of the Czech Geological Society 4613-4(2001) 213                                                                                                    Ordovicli gastropodi z Argentiny (3 figs) Ordovician gastropods from Argentina (1 text-fig, 2 photos) DAVID M. ROHR1 - MATILDE S. BERESI2 - ELLIS L. YOCHELSON3 1Sul Ross State University, Alpine, TX 79832 2CRICYT-IANIGLA: Unidad de Paleontologia,  Av. A. Ruiz Leal, s/n,  Parque General San Martín (5500) Mendoza, ARGENTINA 3Museum of Natural History, MRC 121, Smithsonian Institution, Washington, D.C. 20560 Ordovician (Arenig to Lower Llanvirn) gastropods are summarized from the San Juan Formation, in the San Juan Precordillera of  Argentina.  Preservation is poor, but at least fourteen genera representing bellerophontoids, macluritoids, euomphaloids, pleurotomaroids, murchisonoids, and an onychochiloid are present.  The San Juan Formation is the only known Ordovician occurrences of  the superfamilies Macluritoidea and Onychochiloidea in South America.                                 Key Words:  Ordovician, Gastropoda, Argentina Ordovician gastropods are poorly known from South America.  Jell and others (1983) reported Peelerophon from Argentina and discussed the belleophontids illustrated by Kobayashi (1935, 1937) and Harrington (1938).  The most diverse gastropod collection was made from the Precordillera  of western Argentina, by Kayser (1876, 1925).  The current report is based on 109 specimens from the San Juan Formation, in the San Juan Precordillera, collected by Beresi (1986). The Precordillera of Argentina (Fig. 1) is a Paleozoic sedimentary sequence with a thick Cambro-Ordovician carbonate platform  succession. The San Juan Formation (Arenig to Lower Llanvirn) is the carbonate unit  of this succession characterized by shallow subtidal fossiliferous limestones. The formation shows a diverse fauna of brachiopods, trilobites, nautiloids, sponges, gastropods, pelmatozoans, calcareous algae and conodonts. The specimens of gastropods are not well preserved, and are mostly internal molds from limestone.  Solution of much of the shell material must has occurred during early diagenesis because little evidence of the shells is seen in sawn sections.  In spite of this, we attempt to identify the specimens because of the paleobiogeographic significance of the collection.  At least fourteen genera are present, and the diversity is comparable to North American occurrences of the same age.  Taken together, the tentative identifications would be consistent with an age range of late-Early Ordovician to Middle Ordovician.  The number of endemic taxa is uncertain, because the specimens are assigned to the most likely common genera.  Internal molds of some of the more abundant specimens are tentatively assigned to the common and cosmopolitan Lophospira and Hormotoma.  Slowly expanding, low-spired shells appear similar to Helicotoma (Kayser, 1876, pl. 4. Figs. 7 and 7a).   Lenticular forms are likely to be Liospira or Pararaphistoma.  A single large lenticular shell with very rapid expansion appears to be a new genus.  Holopeiform molds may be Holopea or thick-shelled Trochonema. Internal molds of Maclurites were illustrated by Kayser (1876, pl. 4), and similar specimens are found in this collection.  They appear rounded and have a concave base because the shell material is missing.  In sawn specimens some have the characteristic sub-pentagonal whorl of Maclurites (late Early Ordovician - Late Ordovician), but at least one macluritid specimen has a vertically elongated profile like Teiichispira (latest Early Ordovician).  This is the only formation in South America from which the cosmpolitan Maclurites is known (see Rohr, 1978). Larger specimens superficially resembling Maclurites are euomphaloids.  The internal molds suggest the presence of a crest at the outer margin which is characteristic of Ecculiomphalus and Leuseurilla (Kayser, 1876, figs. 4, 4a-c). Two types of bellerophontiform shells are present in small numbers.  One has a very open umbilicus like Megalomphala and the other is more involute like Sinuites.   A single specimen of an onychochiloidean is readily identified by its unique coiling (Fig. 2). The most typical character of many onychochiloidean is that whorl is elliptical in axial cross section. Based on the axial cross section, it may belong to a new genus close to group of Mimospira, Ferrogyra, and Laeogyra (Fryda, person commun.).  This is the only known occurrence of this superfamily in South America (Frýda and Rohr. 1999). This occurrence the gastropod fauna from the San Juan Formation, can provide new elements for paleoenvironmental and paleobiogeographical interpretations and correlations between the Precordillera terrane and Laurentia.