Ordovician (Whiterock) calymenid and encrinurid trilobites from the Precordillera of Argentina

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

ABSTRACT-Phacopida from upper Whiterock (Lower Caradoc) horizons in the Las Aguaditas Formation of San Juan Province, Argentina, include the calymenid Platycalymene trapezoidalis (Baldis and Pothe, 1995) and the encrinurids Frencrinuroides edseli new species, Lasaguaditas oweni new genus and species, and Walencrinuroides? new species A. Sulcocalymene Baldis and Pothe, 1995, a purported subgenus of Flexicalymene, is a junior subjective synonym of Platycalymene Shirley, 1936. Platycalymene is now known from Avalonia, Baltica, Laurentia (Scotland), and the Precordillera. Emending the cladistic analysis of Ordovician Encrinurinae by Lesperance and Desbiens (1995), the monophyly and membership of Frencrinuroides and Walencrinuroides are ambiguous. Frencrinuroides edseli is closely related to species from eastern Laurentia. Lisaguaditas new genus is superficially cybelinid-like in some respects but shares several derived characters with the Encrinurinae. All three encrinurinid species in the Las Aguaditas Formation possess two instars in the protaspid period, a general pattern in Ordovician encrinurinid ontogeny.

INTRODUCTION

TRILOBITES FROM the Las Aguaditas Formation in the Precordillera of San Juan, Argentina, were first treated taxonomically by Baldis and Blasco (1975) in a study of the Telephinidae. A number of additional trilobite taxa occurring at the type locality of the Las Aguaditas Formation have been described by Baldis and Pothe (1995) and Baldis, Shergold and Peralta (1995).

In 1993,1994, and 1996, the authors collected and acid-processed limestone from the type section of the Las Aguaditas Formation at Quebrada de Las Aguaditas (see Chatterton et al., 1997, fig. I for locality, and Figure 1 herein for stratigraphic position of samples). These samples have yielded rich trilobite collections of Whiterock (Llanvirn to Early Caradoc) age, with numerous horizons bearing well preserved early growth stages. The present work, documenting the Calymenidae and Encrinuridae, is part of a series of studies on the systematics and ontogeny of Ordovician trilobites from the Precordillera (Chatterton et al., 1997, 1998; Edgecombe et al., 1997). Most Phacopida treated herein occur in strata of the Lower Caradoc Nemagraptus gracilis Zone, which Brussa (1994, 1996) recognizes through 150 metres in the upper part of the Las Aguaditas Formation. A calymenid is documented from the Paraglossograptus tentaculatus Zone/Eoplacognathus suecicus Zone, of Llanvirn age.

Lesperance and Desbiens (1995) recently elaborated a phylogeny for species formerly classified as Encrinuroides Reed, 1931, a paraphyletic group that Edgecombe and Chatterton (1990) critiqued at length. The cladogram produced by Lesperance and Desbiens (1995) led them to recognize a few new genera within the former "Encrinuroides." The discovery of a new species, Frencrinuroides edseli, in the Las Aguaditas Formation permits a test of the robustness of groupings identified by Lesperance and Desbiens. A co-occurring encrinurid in the Las Aguaditas Formation is recognized as a new genus, Lasaguaditas.

One of the few members of the Phacopida identified by Baldis and Pothe (1995), Platycalymene trapezoidalis, is revised on the basis of more complete material and its affinities are reevaluated.

SYSTEMATIC PALEONTOLOGY

Material cited and figured in this work is housed in the collections of the Catedra de Estratigrafia y Geologia Historica, Universidad Nacional de Cordoba (prefixed CEGH-UNC).

Order PHACOPIDA Salter, 1864

Suborder CALYMENINA Swinnerton, 1915

Family CALYMENIDAE Milne Edwards, 1840

Genus PLATYCALYMENE Shirley, 1936

Type species.-Asaphus duplicatus Murchison, 1839, from the Caradoc Series of west Shropshire, England; revised by Hughes (1969). By original designation.

Diagnosis.-See Hughes (1969, p. 83).

Discussion.-Hughes (1969) distinguished Platycalymene Shirley, 1936, from Flexicalymene Shirley, 1936, by three criteria, as follow: a more steeply inclined and convex ("roll-like") anterior border; a trapezoidal, rather than subparabolic, glabellar outline; and more rectangular lateral glabellar lobes. Hughes reiterated Shirley's (1936) observation that the exoskeleton of Platycalymene is more depressed than is that of Flexicalymene and speculated that Platycalymene might be a shaly facies variant of Flexicalymene.

Siveter (1979, p. 373-374) noted that the earliest species of Platycalymene, the lower Llanvirn P. tasgarensis Shirley, 1936, closely resembles certain contemporary and older (Arenig) species of Neseuretus Hicks, 1873. Elsewhere (Siveter, 1977, p. 383), he suggested that the earliest Flexicalymene, F. cambrensis (Salter, 1865), was also "descended, with modification, from a Neseuretus-like ancestor." These hypotheses of separate ancestries of Platycalymene and Flexicalymene from Neseuretus imply that the two descendant genera are less closely related than Hughes (1969) had believed, and also that the subfamily Flexicalymeninae sensu Siveter, 1977, is a polyphyletic group. We observe that Platycalymene trapezoidalis possesses certain primitive features shared with Flexicalymene, and favor Hughes' hypothesis that these genera are more closely related to each other than either is to Neseuretus. The eye ridge that is typically pronounced in Platycalymene is indistinct in P. trapezoidalis and the area of the fixigenae is relatively smaller (the eye is set closer to the axial furrow than in other Platycalymene). Early growth stages described here for P. trapezoidalis are extremely similar to complementary stages in Flexicalymene (Chatterton et al., 1990).