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

Discussion.-The protaspis of genus and species indet. 2 is similar to that of stage 0 of Ichangia ichangensis, but the latter is subcircular rather than transversely oval and possesses a more expanded anterior glabellar lobe and two pairs of relatively strong fixigenal spines.

One specimen that had been misidentified previously as protaspis of/. ichangensis (Zhang, 1989, fig. 12B) is a stage 1 instar of this indeterminate trilobite species. These protaspides possess one axial ring posterior to the occipital ring and a pair of marginal spines which vary in length from specimen to specimen. This variation indicates a continued growth of the first pair of spines prior to the appearance of the second pair. These instars with long or short marginal spines may result from more than one protaspid molt stage.

Protaspides of stage 0 of this species are distinguished from those of L ichangensis by their extremely long, paired protomarginal spines. These protaspides conceivably represent an aberrant or special developmental phase of I. ichangensis, rather than another species. However, it is hard to explain why the trilobite would have possessed two kinds of similarly sized protaspid morphologies.

The protaspis with long, paired protomarginal spines may have been adapted to a planktonic mode of life. The paired spines likely disappeared during growth, because no adult trilobite bearing such long marginal spines has been found from the Lower Cambrian. This could suggest a dramatic modification during the early ontogenetic period, a metamorphosis that led to the adults of this species being adapted to another mode of life in different surroundings, rather than co-occurring with these larvae.

Genus and species indeterminate 3

Figure 7.9, 7.10

Material.-2 meraspid hypostomes.

HYPOSTOME

Description.-Anterior wing conical, spine-like, projecting laterally horizontally. Median body moderately inflated; anterior lobe long. Anterior margin transverse. Posterior margin gently curved with 5 pairs of border spines; median three (third to fifth) pairs short; second pair long, gently curved; anterior (first) pair short, extending dorsally. Rostral plate fused, width (trans.) 6 times length (sag.).

Discussion.-The two hypostomes probably belong to one of the other indetermenite redlichds described above, on the basis of the transverse anterior margin and general resemblance to those assigned to Ichangia ichangensis. The pair of long posterior border spines might favor genus and species indet. 2 because of the long spines on the posterior margin of the protaspis of that taxon.

COMPARISON WITH PROTASPIDES OF OTHER GROUPS

Because of generally poor preservation, the smallest of previously described redlichiid protaspides from China provide limited morphological information. Only those somewhat bigger (about 0.7 mm long) of later protaspid stages share some common characters to what is documented here in Ichangia ichangensis Chang, 1957: (1) the anterior border is a thin ridge; (2) the eye ridge is strong; and (3) the facial suture is moved to the dorsal side (Zhang et al., 1980). Protaspides of Palaeolenus lantenoisi Mansuy, 1912 (Yi, 1988) share some common features with that of L ichangensis, such as the expanded anterior glabellar lobe and elevated fixigenal lobe. They probably possess marginal spines and weak eye ridges, but preservation is insufficient to show these.