Early Cambrian trilobite larvae and ontogeny of Ichangia ichangensis Chang, 1957 (Protolenidae) from Henan, China

Journal of Paleontology, Jan 1999 by Zhang, Xi-Guang, Pratt, Brian R

Genus and species indeterminate 1 Figures 3.3, 3.4, 6.1-6.10

Trilobite larva, ZHANG, 1989, p. 28, fig. 12C.

Material.-Of 40 shields, 24 are early protaspides, which bear no axial ring behind the occipital ring. Subdivision of three early protaspid instars (POa, POb, and POc) is based on size, development of sagittal glabellar furrow, and addition of third pair of fixigenal spines between anterior and posterior fixigenal spines. 16 larger shields are assigned to protaspid Stage 2 (P2), because their protopygidia bear two axial rings and two pairs of protomarginal spines. Neither protaspis of Stage 1, which is assumed to have been characterized by one pygidial axial ring and pair of protomarginal spines, nor meraspis or holaspis of this species was found.

PROTASPID PERIOD

STAGE 0a (Figs. 3.3, 6.1, 6.2)

Description.-Shield subcircular in outline and moderately convex, about 0.35 mm long (excluding posterior fixigenal spines), and 0.40 mm wide (excluding anterior fixigenal spines). Glabella with five lobes poorly defined by shallow axial furrows, crossed by transverse furrows. Anterior glabellar lobe slightly expanded extending to short anterior border, with pair of deeply impressed fossulae on each side. Posteriormost axial lobe is occipital ring located close to posterior margin. Sagittal glabellar furrow very shallow. Pair of faint eye ridges connected to palpebral lobes. Posterolateral border narrow, posterolateral border furrow shallow. Anterior pair of fixigenal spines short, situated at lateral border; posterior pair of fixigenal spines strong, at posterior extremity of the posterior border.

STAGE 0b (Figs. 3.4, 6.3, 6.4)

Description.-Shield transversely oval in outline, 0.38 mm long (excluding posterior fixigenal spines) and 0.43 mm wide (excluding anterior fixigenal spines). Axis relatively wide, defined by well incised axial furrows. Sagittal glabellar furrow moderately impressed, dividing three central axial rings (LI to L3) into paired lobes. Eye ridge moderately well defined. Fossulae present. Paired fixigenal spines markedly longer than in stage 0a.

STAGE 0C (Fig. 6.5, 6.6)

Description.-Shield circular in outline, 0.57 mm long (excluding anterior fixigenal spines) and 0.65 mm wide (excluding anterior fixigenal spines). Sagittal glabellar furrow running from rear part of anterior glabellar lobe to occipital furrow. Eye ridge and palpebral lobe prominent. Librigenae located on anterior lateral border. Paired mid-fixigenal spines located between anterior and posterior fixigenal spines.

STAGE 2 (Fig. 6.7-6.10)

Description.-Shield subcircular in outline, 0.62-0.75 mm long (excluding posterior fixigenal spines), and 0.64-0.80 mm wide (excluding anterior fixigenal spines). Occipital ring strongly convex, behind which are two axial rings. Two pairs of protomarginal spines located between posterior fixigenal spines, first pair possessing branched extremities. Sagittal glabellar furrow faint. Prosopon fine network with fine granules.

Discussion.-Protaspides assigned to genus and species indet. 1 differ from those of Ichangia ichangensis because they have a sagittal glabellar furrow, a wider axis, three pairs of fixigenal spines, and branched protomarginal spines. This feature cannot be related to tails with short marginal spines assigned to L ichangensis (Fig. 6.13-6.15). Instars assigned to protaspid degree 0 (Oa, Ob, and Oc) are based on discontinuous morphological differentiation of their exoskeletons. Without an axial ring added behind the occipital ring, ecdysis events did not always coocur with the increase in number of axial rings during this protaspid stage.