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

Librigenal field tall, bearing many scattered, small and moderate sized tubercles as well as abundant pits; surface between pits and tubercles covered with fine, dense granules; eye socle lacking upper or lower furrows; lateral border furrow gently narrowing posteriorly, abruptly effacing just in front of posterior section of facial suture; lateral border nontuberculate but bearing abundant coarse granules; ventrolateral margin approximately straight in external view; anterior furrow moderately deep; precranidial lobe about 75 percent length (exsag.) of field, bearing abundant small and moderate sized tubercles with intervening fine granulation.

Hypostomal length and width subequal (length 97-107 percent of width); maximum width aross anterior wing at about onethird length of hypostome; anterior margin with gentle anterolateral bulge; middle body very slightly longer than wide, gently convex along much of length (sag.) with moderate slope towards extremities; rhynchos prominently tapering forwards, lateral margins rather subdued; macula weakly inflated; middle body and posterior border covered with coarse granules; posterior border 25-29 percent length of hypostome; posteromedian margin approximately transverse but sinuous, with pair of blunt angulations separated by a median convexity; margin concave between two pairs of posterolateral angulations; posterolateral margin gently concave between shoulder at posterior wing and first pair of marginal angulations; doublure long (sag.), with anteromedian projection extending to posterior border furrow.

Pygidium usually slightly wider than long, length 85-100 percent of width; axial furrow narrow, moderately deep, strongly shallowed behind juncture of fifth interrib furrow; axis 43-50 percent of pygidial width anteriorly, gently convex (sag., tr.); up to 16 axial rings, first nine or ten set off by laterally prominent ring furrows, posterior ring furrows weakly incised; first three or four ring furrows well-impressed medially; two congruent segments; one to four rings bearing paired tubercles closely spaced near sagittal line, one or more pairs touching each other in some specimens, tubercles weakly defined in large holaspides; row of coarse granules across anterior rings in some specimens; 62 ribs; ribs at least moderately directed rearwards proximally, directed straight posteriorly distal to fulcrum; first three pleural tips free, weakly turned out, blunt or weakly pointed; ribs densely granulate but lacking tubercles in large specimens; doublure gently lengthening posteriorly, coarsely granulate, with weakly concave inner margins, lacking median embayment.

Discussion.-Most minimal length cladograms and Adams consensus identify Frencrinuroides capitonis (Frederickson, 1964) and F. torulatus (Evitt and Tripp, 1977) as closely allied to F. edseli. Shaw (1974, text-fig. 6) noted close correlation of the units bearing the first two species, in the Bromide Formation (Oklahoma) and lower Edinburg Formation (Virginia). Including information from F. edseli, this Frencrinuroides species group is restricted to the upper Whiterock/Mohawk interval (Caradoc). The encrinurids thus suggest another instance of an upper Whiterock species from the Precordillera nested within an eastern Laurentian group, a pattern detected for the odontopleurid Ceratocara (Chatterton et al., 1997) and the tropidocoryphid Stenoblepharum (Edgecombe et al., 1997).