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

As compared to trilobite dorsal exoskeletons, hypostomes are very rare in our collection. Hypostomes of closely related taxa are commonly indistinguishable, but major differences are preserved in some higher level groups or of different geological ages (Whittington, 1988). Hypostomes described here are somewhat similar to those of olenellids (Palmer,1957) in the presence of the convex anterior lobe of the median body and the posterior marginal spines. Parallel ecological influences may have been responsible for their superficial resemblance.

Fortey (1990) proposed that conterminant is the primitive hypostomal attachment from which natant evolved. Among many ptychparioid trilobites, such as Ehmaniella, Crassifimbra, and Sao, the hypostomal attachment condition changed from conterminant to natant with the lengthening of the preglabellar field during ontogeny. Both types of meraspid hypostomes described here are fused to a rostral plate (Fig. 7.9, 7.15). However, for some larger hypostomes of I. ichangensis, there is no rostral plate attached (Fig. 7.11). Since the width of the cephalic doublure is approximately the same as the cephalic border, and the sagittal length of the rostral plate is equal to that of the cephalic doublure, which appeared in the meraspid period, the hypostome of 1. ichangensis became natant with the lengthening of the preglabellar field during the late meraspid and holaspid stages.

The protaspis of Olenellus gilberti bears a pair of prominent intergenal spines (Palmer, 1957), which are comparable to the long protomarginal spines on genus and species indet. 2, and may indicate adaptation to a planktic lifestyle. However, they are not homologous, since in O. gilberti they are always fixed to the cephalon as intergenal spines, whereas in the Redlichda they formed the first pair of marginal spines with the appearance of the transverse joint. The development of these spines in the olenellids separates them from all other trilobite groups. The redlichiid protaspides described here more closely resemble generalized ptychoparioid counterparts (Palmer, 1958; Hu, 1971). Their morphological differences could imply that the olenellid and redlichiid-ptychoparioid clades diverged prior to the skeletonization of early trilobites.

A cephalon composed of fused anterior segments is one of the apomorphies shared by arthropods, and each body segment may bear a pair of jointed appendages. The first larval stage of crustaceans, the nauplius, possesses three pairs of appendages but shows no external signs of body segments. The cephalon of trilobite larvae is varied in its segmentation: most earliest protaspides have five glabellar lobes (like that of I. ichangensis). but four lobes are also known. For example, protocephala of the eodiscids Neocobboldia chinlinica (Zhang, 1989), Shizhudiscus longquanensis S. G. Zhang, 1980 (Zhang and Clarkson, 1993), and Tsunyidiscus longispinus S. G. Zhang, 1980 (Zhang, 1998), contain three glabellar lobes and one occipital ring. These glabellar lobes are assumed to be primary segments formed from the mesodermal germ band before hatching, and the proliferation of new segments is controlled apparently by genetic system(s) rather than any particular body segment. Like other arthropods, newly hatched trilobite larvae may consist of a certain number of primary segments, and a pygidial growth zone (rather than a specific lobe) gives birth to additional segments. This may be supported by the likelihood that the occipital (or axial) ring would never reduce in size with the growth of a new axial ring posterior to it.