Ordovician pliomerid and prosopiscid trilobites from Argentina

Journal of Paleontology, Nov 1999 by Edgecombe, Gregory D, Chatterton, Brian D E, Waisfeld, Beatriz G, Vaccari, Norberto E

ABSTRACT - Pliomerina Chugaeva, 1958, a characteristic element of the peri-Gondwanan Eakosovopeltis-Pliomerina Province, occurs in Llanvirn and Caradoc strata of the Argentine Precordillera. Three protaspid instars are present in the ontogeny of the Early Caradoc Pliomerina peripata new species. A species possibly representing Phomerella is a rare element in Early Caradoc strata of the Las Aguaditas Formation. Prosopiscus is represented by a Llanvirn species with a first record of protaspid larvae for the enigmatic Family Prosopiscidae. Early growth stages support closest relations of prosopiscids to Phacopina. The presence of Pliomerina and Prosopiscus, along with the cheirurid Pateraspis, demonstrates a peri-Gondwanan influence on the Precordilleran fauna by the Llanvim, and supports the proximity of the Precordillera terrane to the South American margin by that time.

SILICIFIED TRILOBITES from Whiterock (Llanvim)-Lower Caradoc strata in the Precordillera of San Juan, Argentina, have been the subject of several recent studies. These include works on Odontopleuridae (Chatterton et al., 1997), Proetida (Edgecombe et al., 1997; Chatterton et al., 1998, 1999), Calymenidae and Encrinuridae (Edgecombe et al., 1998), and Cheiruridae (Edgecombe et al., 1999). Here we document the Pliomeridae and Prosopiscidae in the Precordilleran silicified faunas.

Most of the material described in this work originates from Llanvirn and Lower Caradoc strata of the Las Aguaditas Formation at its type locality, Quebrada de Las Aguaditas, 12 krn southwest of San Jos6 de Jachal. Some pliomerid material is derived from the Upper Member of the San Juan Formation (Llanvirn) at La Tuna, 42 krn south of San Jos6 de Jachal. Locality maps, stratigraphic columns, and biostratigraphic data for these sections are provided by Chatterton et al. (1998, figs. 1, 2). Stratigraphic horizons at section Las Aguaditas north are abbreviated with the prefix LA and lettered/numbered following Chatterton et al. (1998, fig. 2).

BIOGEOGRAPHIC SIGNIFICANCE

Systematic data for pliomerids and prosopiscids reinforce patterns detected from other trilobite groups in the Precordillera. Two general patterns recur, with Llanvirn-Early Caradoc trilobites being either Laurentian or peri-Gondwanan endemics. Examples of gracilis Zone (Early Caradoc) taxa in the Las Aguaditas Formation with sister species in eastern North America are Ceratocara (Chatterton et al., 1997), Stenoblepharun (Edgecombe et al., 1997), and Frencrinuroides (Edgecombe et al., 1998). A new species of Ceraurinella (Edgecombe et al., 1999) and Telephina problematica (Chatterton et al., 1999) appear to conform to this pattern of Laurentian affinities. Llanvirn Macrogrammus described by Edgecombe et al. (1999) is closely related to an undescribed species from the Table Cove Formation, western Newfoundland (Adrain, personal commun.) and M. jacquelinae Fortey, 1980, from Spitsbergen.

In contrast, several species of Llanvirn-Early Caradoc Phacopida in the Precordillera represent genera that are entirely confined to blocks situated on or near the margin of Gondwana. Pliomerina is known from the North and South China terranes, Kazakhstan, Turkey, and Australia. Its total stratigraphic range is mid Arenig to Early Ashgill (Zhou et al., 1982), and its distribution in part defines an Eokosovopeltis-Pliomerina Province (Webby, 1987). In the Precordillera, Pliomerina is present in the Llanvirn and Early Caradoc. Pateraspis is represented by Llanvirn species in Armorica, Avalonia, Kazakhstan, and China (RAbano, 1989), as well as the Precordillera (Edgecombe et al., 1999). Prosopiscus occurs in North and South China, Tarim, the Himalaya, and Australia, ranging from the Late Arenig through the Early Ashgill (Zhou and Dean, 1989). Its occurrence in the Precordillera is confined to the Llanvirn.

More biogeographically ambiguous are genera with relatively widespread distributions for which sister species relationships have not been identified with confidence. Examples of these from the Early Caradoc Phacopida are Platycalymene and Nieszkowskia. Platycalymene is elsewhere known from the Laurentian margin (Girvan), Avalonia, and Baltica. Nieszkowskia occurs in Baltica, Laurentia, and the Uralian region, although Edgecombe et al. (1999) favor a Laurentian sister group for the Precordilleran species.

The two predominant patterns of biogeographic affinity shown by the Las Aguaditas trilobites can be accommodated by a terrane model that predicts the Precordillera was rifted from the eastern margin of Laurentia in the Cambrian, and drifted (Astini et al., 1995) until its accretion to the margin of Gondwana in the Ashgill. Benedetto et al. (in press) present evidence in support of this history, and critique rival interpretations that consider the Precordillera to have been attached to Laurentia until the Middle Ordovician and transferred to Gondwana by a Laurentia/Gondwana collision (Dalziel, 1997). Llanvirn Phacopida of peri-Gondwanan aspect (Pliomerina, Pateraspis, Prosopiscus) support the positioning of the Precordillera near the margin of Gondwana by that time (the pre-accretion stage of Benedetto et al.). Compared to other faunal groups surveyed by Benedetto et al. (in press), the Phacopida provide a relatively early indication of Gondwanan influence on the Precordilleran biota. The Laurentian aspect of the Caradoc trilobite faunas does not necessarily represent a relict of Early Ordovician proximity of these areas, as this would imply long, unsampled ghost lineages. Benedetto et al. (in press) considered that the proximity of eastern Laurentia and northwestern South America in the Upper Ordovician would have permitted faunal exchange along the continental margins.