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

The presence of this type of anaprotaspid stage is important in these and some other closely related proetide trilobites, since a distinctly different type of anaprotaspis occurs in some other groups of Proetida. Trilobites belonging to the Proetidae and Tropidocoryphidae (see Chatterton, 1971, pl. 21, figs. 8-11, 17 for Devonoproetus talenti (Chatterton, 1971); Edgecombe et al., 1997, figure 8 for Stenoblepharum; Chatterton and Speyer, 1997, figure 166.5, probable Proetidae) and Dechenellidae (unpublished work by B.D.E.C.) have a distinctive small bulbous anaprotaspis, with three pairs of submarginal spines, and a large lenticular hypostome that extends the entire length, and covers about half of the ventral surface, of the exoskeleton. Work in progress by us on the ontogeny of a new species of Telephina shows that genus to have a smallest protaspis that is more similar to that of the Proetidae than to that of the Dimeropygidae and Aulacopleuridae. The larger protaspid stage of that species shows the following similarities, however, to the anaprotaspides of toernquistd and dimeropygid trilobites, suggesting that the Proetida may still be monophyletic: reentrant posterior margin, sharply turned inward doublure, a single pair of small, sharp, posterior, marginal spines. Unique to telephinid protaspides among Proetida appears to be the presence of a median furrow on the protocranidium. Clearly, "anaprotaspid" stages may prove important in unravelling the relationships among the Proetida.

Metaprotaspid Stages.-At least two metaprotaspid instars were found for three of the four toernquistiid species with preserved early ontogenies from our Argentinian samples. Our morphometric data (Figure 3) do not show clear instars, but the size range of the protaspides, when combined with distinct morphologies, is suggestive of at least two instars. The smaller of the two stages has a weakly defined protopygidium (often only defined by L0 in front of a protopygidial axis); it has a distinct pattern of slightly spinose tubercles; the axis is more distinct than in the earlier anaprotaspid stage, and a shallow anterior border furrow may be distinguishable so that a preglabellar field may be apparent; and the posterior margin is only weakly concave medially or subtransverse. The pair of spines at the back of the "anaprotaspid" stage is no longer visible. Apart from the pattern of tubercles, the sculpture may include granules, which in some species (e.g., Paratoernquistia sanchezae new genus and species) may show signs of linear arrangement subparallel to the margins. The pattern of tubercles, discussed in more detail below, shows similarity to patterns found in some other Proetida (Dimeropygidae in Tripp and Evitt, 1983, text-figs. 2-4, and Chatterton, 1994; Aulacopleuridae in Adrain and Chatterton, 1994, fig. 1; and Hystricuridae in Lee and Chatterton, 1997), and even in trilobites assigned to other orders (Odontopleurida, Whittington, 1956; Lichida, Chatterton, 1971; see also Chatterton and Speyer, 1997). Free cheeks, at this stage, lack genal spines, with the lateral margin of the free cheek continuous and colinear with the margin of the protopygidium.