Ordovician pliomerid and prosopiscid trilobites from Argentina

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

Second protaspid stage, Pbl. (Fig. 2.5-2.8, 2.10-2.12).-Differs front first protaspis stage by: length 0.4-0.43 mm, width 0.46-0.48; position of maximum width at infs rather than afs; preglabellar and axial furrows deeper@ anterior protocranidial border developed medially; frontal lobe wider than more posterior parts of glabella; longitudinal median glabellar furrow indistinct; axial segmentation includes distinct Ll and LO; afs and nifs stouter, more triangular, posterior marginal furrow on protocranidium distinct; lateral cranidial border defined by distinct furrow; librigena extends back nearly midway between afs and mfs; one protopygidial axial ring and one pair of short, posteromedially directed marginal spines, a few weak terrace ridges on doublure.

Hypostome slightly wider than long. widest at spine at shoulder; length about 45 percent length (sag.) of protaspis. Anterior lobe of middle body of inverted triangular outline; middle furrow well impressed; anterior border furrow weak. Lateral margin running straight back behind small anterior wing. Posterior border very short (sag.), gently lengthened abaxially. Three pairs of marginal spines, largest behind midlength (exsag.) of hypostome, directed laterally and slightly posteriorly; two smaller, denticulate pairs on posterolateral margin, second pair about equally distant to first and third@ margin weakly convex backward between third spine pair.

Third protaspid stage, Pb2 (Fig. 2.9, 2.13-2.15, 2.17-2.21).Differs from second protaspis stage by: length 0.54-0.56 turn, width 0.59-0.63 nun, deeper impression of axial furrow and posterior protocranidial border furrow; lateral border furrow well marked along protocranidium, continuous with posterior border furrow; posterior protocranidial margin transverse (rather than posterolaterally oriented); two well defined protopygidial axial rings; three pairs of protopygidial marginal spines.

Connective sutures strongly convergent posteriorly, rostral plate trapezoidal, narrow posteriorly. Hypostome very similar to preceding stage.

Etymology. --Greek, wanderer, for the geographic extension provided by this species, and for the globetrotting Barry Webby, who clarified the morphology of Plionerina.

Types.-Holotype pygidium CEGH-UNC 14945 (Fig, 1.151. 18), from horizon LAS-B, Las Aguaditas Formation, Quebrada de Las Aguaditas (sec. Las Aguaditas north of Chatterton et al., 1998, fig. 2): Nemagraptus gracilis Zone (Lower Caradoc). Paratypes CEGH-UNC 14933-14944, 14946-14969, 1497514991. from type locality and stratum.

Occurrence.-Horizons LAS-B and LAS-T-Br, section Las Aguaditas north-, Nemagraptus gracilis Zone (Lower Caradoc).

Discussion.-The incomplete knowledge of most species of Pliomerina discussed above means that whilst P. peripata can be diagnosed by a unique combination of character states, it is not currently possible to place the new species in a sound phylogenetic context. The abaxial elongation of L3 (such that it is considerably longer than L2) provides a distinction from P. prima, P. rigida. P. speciosa and P. tolenensis. The elongated, rather inflated L3 is considered apomorphic by comparison with other pliomerid genera, and is shared by P. austrina, P. fupingensis, P. martelli antetus, P. sulcifrons, and P. yaoxianensis. The steep angle of inclination of S3 in P. peripata permits its distinction from most other species, and the proximal positioning of the palpebral lobe is unique. The form of the pygidial terminal piece may be a phylogenetically informative character. P. peripata resembles the Australian P. austrina and P. prima in the large, conical/triangular terminal piece, whereas other species (such as P. fupingensis and P. yaoxianensis) have a narrower, more fib-like terminal piece. Pliomerina peripata is, however, distinguished from P. austrina (Eastonian, New South Wales) by the stronger convergence of the axial furrow against the terminal piece, with the tips of the terminal pleural ribs nearly touching. The Precordilleran species differs from both Australian species in the shallowing of the axial furrow against the terminal piece.