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

The Argentinian species may be distinguished from F. capitonis by the possession of: less prominent abaxial part of L1; a shorter (exsag.) postocular fixigena; the absence of an eye ridge (distinct in F. capitonis; Shaw, 1974, pl. 19, fig. 3) a denticulate hypostomal margin; a less inflated middle body (compare Fig. 6.20, 6.21 and Shaw, 1974, pl. 10, fig. 5), and; only six pairs of pygidial pleural ribs (vs. seven or eight).

Distinction from Frencrinuroides torulatus may be made based upon: the absence of tubercles on the adaxial part of the cranidial anterior border; weaker development of LI; less robust glabellar tubercles; a more back-turned genal spine; LO and the pygidial axial rings lack a pronounced row of small tubercles; the librigenal field lacks a few larger tubercles, and; the presence of strong hypostomal marginal denticle pairs. The latter feature is shared between F. edseli and Prophysemataspis, but is interpreted as convergent in most trees based on the fit of other characters. Comparison of early growth stages of F. edseli and F. torulatus (Evitt and Tripp, 1977, pl. 2, figs. 2-4) indicates that a very strong divergence of the posterior fixigenal spine in meraspides may be a shared derived character of Frencrinuroides. The degree of expression of the torulus in meraspides (bloated in torulatus; weak in edseli) is noteworthy, and provides a striking specific differentium.

Ontogeny.-Early growth stages, including two protaspid stages, are available for this species. The assignment of protaspides to F. edseli and the co-occurring encrinurid Lasaguaditas oweni is straightforward, and most easily determined by the divergence of the posterior fixigenal spine. Protaspides with a long, very strongly divergent posterior fixigenal spine (Fig. 9.49.9) can be compared with obvious Frencrinuroides meraspides that retain strong divergence of the long spine (Fig. 9.10-9.14, 9.19). The posterior fixigenal spine in protaspides and meraspides assigned to L oweni (Fig. 10.11-10.13, 10.15, 10.16) is shorter and much more posteriorly directed. The assignment of early-degree transitory pygidia to one or the other encrinurid is more ambiguous (most specimens being referred to the more common species, F. edseli). Later transitory pygidia can be distinguished by the pair of closely spaced axial tubercles on some rings of Frencrinuroides (Fig. 10.3) vs. unpaired sagittal spines in Lasaguaditas oweni (Fig. 10.18, 10.21).

Protaspides.-The morphology of two protaspid instars in Frencrinuroides edseli shows very close correspondences to the two stages described by Edgecombe et al. (1988) for all species of Ordovician encrinurinids with a record of early ontogeny. These include Physemataspis insularis (Shaw,1968; Edgecombe et al., 1988), Prophysemataspis neuter (Evitt and Tripp, 1977), and Walencrinuroides rarus (Walcott, 1877; Chatterton, 1980). This supports the view that early encrinurinids (as well as primitive staurocephalinids and advanced cybelinids) have a fixed ontogenetic groundplan of two protaspid instars.