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

ABSTRACT-The Lower Cambrian Shuigoukou Formation of Xichuan, Henan province, China, yielded phosphatized instars belonging to the protolenid Ichangia ichangensis Chang, 1957, and two undetermined taxa. With relatively simple body plans, the three larval forms resemble each other to some extent, and protaspides of I. ichangensis show general similarities to those of the ellipsocephalid Palaeolenus lantenoisi Mansuy. However, their early ontogenetic processes exhibit subtle yet distinct differences. One group of unassigned protaspides possesses a sagittal glabellar furrow and protomarginal spines with branching extremities. The other unassigned protaspides bear especially long genal and protomarginal spines. The detailed preservation of these larvae reveals delicate structural variations that help provide a framework for evaluating relationships among Early Cambrian trilobites.

INTRODUCTION

ONE OF the puzzles in trilobite studies is the abrupt appearance in the Early Cambrian of two partially overlapping trilobite provinces (or realms) defined by primitive redlichiid and olenellid trilobites. Although these families displayed quite a high early diversity, they are thought to comprise a monophyletic group (Fortey and Whittington, 1989), yet this assumption lacks direct paleontological evidence because the fossil record tells us little about their prior evolutionary history. Documentation of trilobite larvae is vital because the larvae demonstrate phylogenetic links between taxa that are often morphologically disparate as adults, or point to different evolutionary histories of convergent groups. Clearly, then, study of the ontogenies of these Early Cambrian species will provide critical data concerning their origins and phylogenetic relationships.

The first appearance of articulation in the shield, via a transverse joint, has been considered as distinguishing the meraspis from the protaspis (Whittington, 1959), with the subdivision of meraspid growth stages corresponding to the number of thoracic segments (Barrande, 1852; Raw, 1925). The episodic release of thoracic segments in most cases corresponds to the increase in the number of axial rings of the transitory pygidium, although a few trilobites may form two or more thoracic segments after a single molt (Whittington, 1957, 1959; Feist, 1970; Pabian and Fagerstrom, 1972). Moreover, the segmentation of the axis posterior to the occipital ring in the protaspis, i.e., in the protopygidium, has also been employed as a criterion in dividing discrete growth stages, or ecdysis events, of the protaspid period (Snajdr, 1981; Tripp and Evitt, 1983). This may be suitable for description of many trilobite protaspides, but may not be applicable to all. For example, protaspid instars of genus and species indet. 2 described here display rather large variations in size and morphology prior to the segmentation of their protopygidia. Nevertheless, subtle developmental variation within the protaspid stages may be the harbinger of dramatic morphological differences in subsequent instars.

Dilute (5 percent) acetic acid dissolution of about 20 kg of thin-bedded micritic limestone from the Lower Cambrian Shuigoukou Formation from the Laozizhai section in Xichuan, Henan Province, China (Fig. 1), yielded abundant delicately phosphatized fossils, including bradoriid ostracodes (Zhang, 1987), the eodiscid trilobite Neocobboldia chinlinica Lee, 1963 (Zhang, 1989), as well as lingulate brachiopods, sponge spicules, hyoliths, some enigmatic taxa, and the protolenid trilobite Ichangia ichangensis Chang, 1957, which is widely distributed in southwestern China (Zhou and Yuan, 1980). Because the Xichuan fossil assemblage was found immediately above the late Qiongzhusian Sinodiscus Zone and below Palaeolenus, it is probably from the latest Atdabanian or early Botomian (Fig. 2).

In previous studies, only separate holaspid cranidia and pygidia of L ichangensis were described (Chang, 1957; Zhang et al., 1980). However, articulated trilobites are rarely found in acid-etched residues, and disaggregated exoskeletons coming from different taxa are mixed. This makes identification more complex, especially for the smallest trilobite protaspides with simple and similar body plans. Only by examining ontogenetic variations of certain traits can the links be made between these larvae and their corresponding later instars. On this basis, immature instars of I. ichangensis co-occurring with their nonphosphatized adults are recognized, permitting a more detailed description of their growth history, although the assignment of transitory pygidia and hypostomes to this taxon is tentative. We also describe early instars of two other co-occurring trilobites that are of uncertain identity because no adult trilobites of other taxa have been recovered from the assemblage. Furthermore, a group of hypostomes, although well preserved, is also of uncertain affinity.

SYSTEMATIC PALEONTOLOGY

The terms used here generally follow Palmer (1957), Harrington (1959), Edgecombe et al. (1988), Speyer and Chatterton (1989), Chatterton et al. (1990), Fortey (1990), and Lee and Chatterton (1996, 1997). However, we believe that the four glabellar lobes and one occipital ring are distinct even in the earliest protaspid of the three trilobite taxa, such that there is no reason to infer an earlier instar with fewer glabellar lobes. The dorsal shield of a larval trilobite without visible segmentation posterior to the occipital ring is considered as a stage 0 protaspis. A stage 1 protaspis bears one axial ring posterior to the occipital ring (with or without a pair of protomarginal spines), but no transverse joint. Meraspid degrees are defined by the progressive release of thoracic segments, one by one. Because of their relatively small size, most of the disarticulated cranidia and pygidia in the collection are considered to be meraspides. Only the few bigger specimens are tentatively assigned to the holaspid period.