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

The San Juan Formation is a thick, predominantly shelf limestone unit exposed over a wide area of western San Juan and La Rioja provinces in the western Precordillera. In the area where our trilobites were found, it is overlain by either the Las Aguaditas Formation (as at the Las Aguaditas sections) or thinbedded limestones and shales of the transition beds in the base of the shaly Gualcamayo Formation (as at the Rio Gualcamayo section). Invertebrate fossils are generally abundant in the San Juan Formation, including sponges, bryozoans, brachiopods, trilobites, and ostracodes. The section containing Telephina calandria is considered to be of latest Ibexian (Arenig) age, based on the conodont faunas (Oepikodus evae Zone, Hnicken and Sarmiento, 1982). In the section where T. calandria was collected, southwest of the town of Guandacol, the San Juan Formation consists of bioclastic and intraclastic wackestones and mudstones with a few thin layers of grainstones. This association of sediments and fossils is interpreted to have been deposited in an outer ramp environment (Canas, in press).

The larvae (protaspides) and small meraspides of one of the trilobites described here (Telephina problematica) were first found in abundance in a massive interval of carbonate breccia in section Las Aguaditas South (LAS-B in Edgecombe et al., 1998, fig. 1). Our first determination was that these larvae are small telephinid trilobites because of their large eyes and the shape of their glabellas. However, the comparative rarity of mature telephinid trilobites in this interval where small growth stages are abundant, and the comparative paucity of small growth stages in a higher breccia interval in the same section (LAS-T Br., see Edgecombe et al., 1998, fig. 1) where adult stages are very common, then caused us to revise this view, and consider them to belong to different taxa. Also, including all of these stages in one species requires a radical metamorphosis late in the meraspid period, a feature not previously described in trilobites. In addition, the larvae and early ontogenetic stages of other telephinids in our possession (see below and Fig. 1) are surprisingly dissimilar to the small growth stages of the species described here.

A very recent (August, 1998) discovery of silicified Arenig faunas in the San Juan Formation forced us to revise our views again on the association of these growth stages. In one particular bed (RGE-3), very large numbers of larval to adult stages of Telephina calandria again occur. These clearly show that a radical metamorphosis occurs in the late meraspid period of this species. The juvenile stages of this species are strikingly similar to the juvenile stages of T. problematica, and the changes taking place at the metamorphosis late in the meraspid period are also very similar (see below). The third telephinid species described here, Telephina chingolo n. sp., from Llanvirn strata of the Las Aguaditas Formation at the type section, is less abundant and well preserved. It appears, however, to undergo similar ontogenetic changes. Thus, the three new species whose ontogenies are described here have very similar growth stages, and apparently all underwent a radical metamorphosis at a stage late in the meraspid period. This metamorphosis was presumably associated with a radical change in life mode. The larvae are adultlike in the sense of Speyer and Chatterton (1989) and Chatterton and Speyer (1989, 1990, 1997), and were presumably associated with a benthic life mode. The form of the adult and post-metamorphic growth stages are typical of those of pelagic trilobites (see Fortey, 1985). Thus we suggest that this radical late metamorphosis was associated with a change from a benthic to a pelagic life mode.