Hystricurid trilobite larvae from the Garden City Formation (Lower Ordovician) of Idaho and their phylogenetic implications

Journal of Paleontology, Sep 1997 by Lee, Dong-Chang, Chatterton, Brian D E

Discussion.-This species differs from 'Paraplethopeltis' generectus (Hintze, 1953, plate 7, figures 6-9) in having a more slender posterior fixigenal area, a more distinct eye ridge, and more forwardly based genal spines. Hystricurus? sp. aff. H.? genacurvus (Berg and Ross, 1959, plate 21, figure 21, 23) represents a later ontogenetic stage of this species.

Material.-UA 11048, UA 11047, UA 11050 (protaspides from R5-50.3); UA 11049 (meraspid cranidium from R5-50.3).

'PARAPLETHOPELTIS n. sp. A

Figures 3.6, 3.7, 8.1-8.4, 8.6, 8.10, 8.11

Description.-Two protaspid instars (stages 1 and 2) are recognized, based on groupings on the length versus width plot (Figure 9) and morphologic criteria described below.

Stage 1 protaspides 0.52-0.63 mm long and 0.48-0.55 mm wide, suboval in outline; anterior margin nearly straight and posterior margin re-entrant. Protocranidium trapezoidal and occupies 70-75 percent of shield length. Axis spindle-shaped, with four glabellar lobes and occipital ring (IM-LO); glabellar furrows continuous across axis; L4 twice as long (sag.) as subequal Ll/L2/L3, tapers forwards and reaches anterior cranidial border; Ll/L2/L3 weakly bilobed. Anterior cranidial border furrow shallows distally. Eye ridge slightly convex and gently curves backwards; palpebral lobe located opposite S3, and continuous into eye ridge. Posterior cranidial marginal furrow indistinct and runs only halfway out across shield. Protopygidium semicircular in outline, with one axial ring and terminal piece; interpleural furrows indistinct. Shield covered with large number of small granules.

Stage 2 protaspides 0.76-0.88 mm long and 0.62-0.74 mm wide. Anterior margin slightly indented medially. Glabella straight-sided and forward-tapering. Palpebral lobe strongly arched outwards; eye ridge indistinct. Posterior cranidial border furrow impressed; posterior cranidial marginal furrow distinct and runs transversely before it curves strongly backwards distally. Posterior facial suture strongly divergent laterally and strongly curved posteriorly. Protopygidium about 34 percent (avg.) of shield length, with two axial rings and terminal piece; marginal border gently slopes downwards.

Meraspid cranidium with narrow preglabellar field with median furrow, disappearance of transglabellar furrows, and narrow posterior fixigenal area. Transitory pygidium semicircular in outline; axis gradually tapers posteriorly and bears at least six axial rings; border gently arched downwards. Holaspid cranidium with slender posterior fixigenal area, relatively large and strongly convex palpebral lobe, slightly divergent anterior facial suture, distinct preglabellar median furrow and convex anterior margin.

Discussion.-This species seems new, but the paucity of adult materials does not allow us to name it. Hystricurus sp. B, described by Ross (1951b, plate 10, figure 18, 19, 23, 24, 27, 28) may be conspecific with these materials, even though the immature cranidium has a relatively larger palpebral lobe and a more rounded anterior margin. Hystricurus cf. H. sp. B Ross, 1951b (Dean, 1989, plate 14, figures 9, 12, 15) may represent a later adult stage of this species, even though the cranidium is covered with sparsely distributed smaller tubercles.