Ontogenies of some Ordovician Telephinidae from Argentina, and larval patterns in the Proetida (Trilobita)

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

ABSTRACT-The ontogenies of three new species of Telephinidae, Telephina calandria, Telephina chingolo, and Telephina problematica are described from Arenig-Caradoc strata in the Argentine Precordillera, and compared with the larval stages of some other Proetida, including other telephinids.

New findings reveal 1 ) a radical metamorphosis in the ontogenies of these Telephina species late in the meraspid period, not previously described among Trilobita; and 2) distinctive hypostomes of Telephinidae containing long, thin anterolaterally and dorsally splayed anterior wings. Early ontogenies of different species currently assigned to the genus Telephina fall into at least two distinct morphological and life history groups, and hypostomes (if correctly assigned in previous works) vary significantly. The three new species strengthen the hypothesis of a phylogenetic connection between Oopsites and Telephina.

Three morphological types of protaspid instars are described for proetide trilobites. Two are anaprotaspides, and the third is a metaprotaspis. They always occur in the same sequence in the ontogeny, but no cases are known of all three types in the same species. These larval types are important for understanding the systematics and life cycles of the Proetida. Benthic/pelagic transitions identify four life history patterns among the the Proetida. The best larval synapomorphy for the Proetida is the distinctive metaprotaspid larval type, which is absent in very few proetides (some Telephinidae), perhaps as a result of heterochronic displacement of this stage into the meraspid period.

INTRODUCTION

THE MONOPHYLETIC or polyphyletic origin of the trilobite order Proetida (Fortey and Owens, 1975, 1979; Bergstrom, 1977; Chatterton, 1980; Fortey, 1990) has been a source of debate. Some of the most important arguments in favor of the Proetida as a natural group are based on similarities of small growth stages. In this work, we describe excellently preserved ontogenies of new proetide trilobites belonging to the Family Telephinidae, and review the available evidence of life cycle patterns within the Proetida.

This work is the fifth of a series on the systematics of Middle to Late Ordovician (Llanvirn to Caradoc) trilobites from localities of the Las Aguaditas Formation near Jachal in the Argentine Precordillera. Earlier works by us on the trilobites of this region include Chatterton et al. (1997, 1998), and Edgecombe et al. (1997, 1998). Other works on the trilobites from the type locality of the Las Aguaditas Formation are by Baldis and Blasco (1974), Baldis and P6the de Baldis (1995), and Baldis et al. (1995). A locality map, showing the location of the Quebrada de Las Aguaditas, is included in Chatterton et al. (1997, fig. 1). The location of this section, was determined by a Garmin 12 XL GPS unit (location of part of section at Las Aguaditas North bearing Llanvirn Telephina chingolo new species is 3018'18.li"S, 68o49'15.9"W; upper Caradoc part of same section is at 3018'2.8"S, 68o49'23.9"W).

One new species (Telephina calandria n. sp.) was obtained from the upper part (over 20 m below the top, horizon RGE-3) of the San Juan Formation. This species occurs in a section along the Rio Gualcamayo (RGE = Rio Gualcamayo East) in southwestern La Rioja Province (latitude 2945'29.4"S, longitude 6839'08"W. Determined by a Garmin 12XL GPS unit). The Las Aguaditas Formation is exposed in the northern part of the San Juan Province Precordillera, in the eastern flank of the Cordon de Los Blanquitos. The section at Quebrada de Las Aguaditas is located 12 km southwest of San Jose de J*chal. It 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. The Las Aguaditas Formation reaches 260-300 m in thickness, and is composed of a rhythmic succession of limestones and calcareous mudstones with intercalated breccias. A stratigraphic column for the Las Aguaditas Formation at the type section was provided by Chatterton et al. (1997, fig. 1). Astini (1995) considered this succession to have been deposited in a carbonate margin to foreslope environment, which developed in the northwestern margin of the central Precordillera.

The age of the Las Aguaditas Formation is based on a sequence of graptolites described by Brussa (1994, 1996). He has recognized the Paraglossograptus tentaculatus Zone through the lowest 45 m of the unit at Quebrada de Las Aguaditas, associated with conodonts of the Eoplacognathus suecicus Zone (Llanvim). Telephina chingolo n. sp. is a rare component of the faunas in several horizons towards the top of this interval of Llanvim age.

After a hiatus, the Nemagraptus gracilis Zone extends through 150 m of this formation at the Las Aguaditas section. Telephina problematica n. sp. occurs in these Caradoc gracilis Zone strata within the middle to upper parts of that interval. All of the specimens of this species that we recovered were from two prominent breccia units that contain a mixed fauna that is mostly allochthonous and from slightly shallower water facies than the autochthonous trilobite fauna that occurs in the sub- and superjacent beds. We presume that T. problematica, one of the more abundant species in the lower and more massive of the two breccia units, is an allochthonous component because we have not found it in the non-brecciated autochthonous beds.