Ontogeny and systematics of Toernquistiidae (Trilobita, Proetida) from the Ordovician of the Argentine Precordillera

Journal of Paleontology, Mar 1998 by Chatterton, Brian D E, Edgecombe, Gregory D, Waisfeld, Beatriz G, Vaccari, Norberto E

ABSTRACT-The following new Toernquistdae from Whiterockian (Llanvirn to lower Caradoc) strata of the Precordillera, western Argentina, are described: Chomatopyge canasi new species, Lasarchopyge benedettoi new genus and species, Lasarchopyge correae new genus and species, Lasarchopyge new species A, and Paratoernquistia sanchezae new genus and species. Phylogenetic analysis suggests that the Toernquistdae Hupe, 1953 (including Chomatopyge, Lasarchopyge, Mesotaphraspis, Paratoernquistia, and Toernquistia) is closely related to but distinct from the Dimeropygidae Hupe, 1953 (including Ischyrotoma, Ischyrophyma and Dimeropyge). It is difficult to find unique synapomorphies that unite toernquistds and dimeropygids into a monophyletic group that excludes other proetides (e.g., hystricurids), although details of their ontogenies are similar. Several species of Toernquistdae are known from the Argentine Precordillera (one species previously described), and other species of this family have been described from Australia, Baltica, China, Kazakhstan and Laurentia. Members of the Dimeropygidae are known only from Baltica and Laurentia.

INTRODUCTION

THIS WORK is the third of a series on the ontogeny and phylogeny of Ordovician (Whiterockian; Llanvirn to early Caradoc) trilobites from near San Jose de Jachal, San Juan Province in the Argentine Precordillera (Chatterton et al., 1997; Edgecombe et al., 1997). It is based on excellently silicified growth series of trilobites from horizons within the Las Aguaditas Formation at the Quebrada de Las Aguaditas and equivalent age strata at two sections nearby, at La Tuna (Llanvirn) and Mogotes Azules (Caradoc; Figure 1; see also Chatterton et al., 1997, figures 1 and 2). The object of this work is to describe new taxa, including their ontogenies, and to consider their relationships (phylogenetic and biogeographic).

The Las Aguaditas Formation is exposed in the northern part of the San Juan Province Precordillera, Argentina, in the eastern flank of the Cordon de Los Blanquitos. One of the localities (two sections at Quebrada de Las Aguaditas) is located 12 km southwest of San Jose de Jachal (Figures 1 and 2). In this locality, the type section of the Las Aguaditas Formation overlies, in paraconcordance, the carbonate San Juan Formation, and is overlain by the Lower Silurian La Chilca Formation, from which it is separated by an erosional disconformity (Figure 2). The Las Aguaditas Formation reaches 260-300 m in thickness, and is composed of a rhythmic succession of calcareous and marly beds with intercalated breccias. Astini (1995) has interpreted this succession as having been deposited in a carbonate margin to foreslope environment that developed in the northwestern margin of the central Precordillera.

The age of the Las Aguaditas Formation at the type section has been established on the basis of graptolites by Brussa (1994, 1996). He has recognized the Paraglossograptus tentaculatus Zone through the lowest 45 m of the unit, associated with conodonts of the Eoplacognathus suecicus Zone (Llanvirn). Two new Llanvirn toernquistd species, Chomatopyge canasi and Lasarchopyge correae, were found in the lower part of the Las Aguaditas Formation in the type section.

After a hiatus (located just above prominent horizon c in the section at Las Aguaditas north, see Figure 2, column on left side of figure), the lower Caradoc Nemagraptus gracilis Zone occurs through 150 m of this formation at the type section. Paratoernquistia sanchezae new genus and species and Lasarchopyge benedettoi new genus and species were obtained from the middle to upper parts of that interval in sections on both the north and south sides of Quebrada de Las Aguaditas; and Lasarchopyge new species A was obtained from the lower part of that zone on the north side of the Quebrada de Las Aguaditas.

The specimens collected from La Tuna come from a section on the south side of Quebrada de La Tuna (Sierra de La Trampa) located 42 km south of San Jose de Jachal. The outcrops of the upper member of the San Juan Formation (Espisua, 1968; Astini, 1994) are a 58 m thick succession of nodular limestones conformably overlying the lower member of the San Juan Formation on a hardground surface. This succession is overlain by Lower Silurian conglomerates of the La Chilca Formation. Albanesi and Astini (1994) recognized the Eoplacognathus suecicus Zone (Llanvirn) in this succession, which may be correlated with the lower 45 m of the Las Aguaditas Formation at its type section. The age of the rocks at La Tuna may also be established on the basis of trilobites. Trilobite species occurring in the Llanvirn part of the section at Las Aguaditas occur through most of the upper member of the San Juan Formation at La Tuna, including species of: Ampyx, Annamitella, Carolinites, Mendolaspis, Nileus, Remopleurides, and Shumardia; and Chomatopyge canasi new species and Lasarchopyge correae new genus and species.

Paratoernquistia sanchezae new genus and species occurs in Caradoc limestones at Mogotes Azules. These silicified trilobites from this section come from the upper part of the Las Aguaditas Formation (32 m above a prominent breccia unit) in the Cerro Blanco, northern part of the Sierra de Mogotes Azules, 20 km southwest of San Jose de Jachal. In this area, the Las Aguaditas Formation lies in paraconcordance on top of the limestones of the San Juan Formation, and is overlain unconformably by the Lower Silurian La Chilca Formation (Furque, 1975). The specimens of P. sanchezae from Mogotes Azules are most similar to specimens from near the top of the range of that species in the type locality, suggesting a correlation with the upper part of the type section at Las Aguaditas. Similar (or the same) species of the following genera are also shared between this horizon at Mogotes Azules and the upper, Caradoc, part of the Las Aguaditas formation at the type section: Ceratocara, Incaia, Porterfieldia, Raphioampyx, Shumardia, Stenoblepharum, and Telephina.