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

Description.-All protaspid specimens are considered to belong to one instar. Protaspid shields 0.54-0.66 mm long and 0.48-0.59 mm wide, suboval in outline. Cranidium 60-66 percent of shield length and trapezoidal in outline. Glabella parallelsided or slightly barrel-shaped and tapers forwards; three pairs of glabellar furrows short, faintly represented by non-granulated patches, and directed inwards and backwards. Preglabellar field short (sag.), with indistinct preglabellar furrow. Three pairs of inner series tubercles developed in regular spacing; first pair (from anterior) opposite S3, second slightly behind level of S2, and third opposite L1. Occipital ring rectangular and as wide as maximum glabellar width (tr.). Posterior border tubercle present. Anterior facial suture slightly convergent anteriorly and posterior one gently divergent posteriorly. Protopygidium semicircular in outline. Protopygidial axis gently tapers backwards and bears two to three rings and terminal piece; rachial series tubercles on anterior two rings. Pleural field gently convex upwards; two pleural ribs with shallow pleural furrows; first interpleural furrow deep and reaches pygidial margin. Shield covered with small granules.

Meraspid cranidium differs from protocranidium in showing four pairs of glabellar (rachial series) tubercles, more deeply incised preglabellar median furrow, more strongly divergent posterior facial suture, and increment of pair of tubercles in inner series and palpebral lobes. Holaspid cranidium with strongly forward-tapering glabella; fine terrace lines on anterior border; dispersed tubercles on glabella and fixigenal field.

Discussion.-This Hystricurus species appears to be new, but the paucity of material (in particular, absence of correctly associated pygidia) precludes its formal recognition. The holaspid cranidium is distinguished from other Hystricurus species in having a strongly forward-tapering glabella and distally broadening posterior border. Hystricurus sp. C (Ross, 195 lb, plate 10, figure 22) is similar to this species, but differs in having two pairs of glabellar furrows, a smooth skeletal surface, and a forwardly divergent anterior facial suture. Hystricurus sp. L (Terrell, 1973, plate 2, figure 7) is greatly similar to this species, except for the forwardly-curved posterior border. The larvae of this species differ from those of Hystricurus paragenalatus in the smaller size of tubercles, development of rachial series on protopygidium and tubercle on posterior cranidial border, and dispersed granules on glabella.

Material.-UA 10999, UA 10998, UA 11003, UA 11000 (protaspides from R5-50.3); UA 11001, UA 11002, UA 11005 (meraspid cranidia from R5-50.3); UA 11004 (meraspid cranidium from RS-76.4); UA 11006 (holaspid cranidium from R576.4).

HYSTRICURUS? sp. A

Figures 3.2, 5.8, 5.13

Description.-Early protaspid stage about 0.61 mm long and 0.48 mm wide, suboval in outline. Glabella parallel-sided and tapers forwards. Preglabellar field relatively long (sag.), with median furrow. Three pairs of inner series tubercles regularly spaced; tubercle developed on distal end of posterior cranidial border. Occipital ring rectangular, with node. Protopygidium semicircular in outline; axis tapers backwards and bears one to two rings and terminal piece; rachial series tubercles on anterior rings. Two pairs of outer series along pygidial margin.