FIRST CARBONIFEROUS PROTASPID LARVAE (TRILOBITA)

Journal of Paleontology, Jul 2005 by Lerosey-Aubril, Rudy, Feist, Raimund

ABSTRACT-The recovery of well-preserved silicified larvae from the early Tournaisian of Montagne Noire, southern France, allows the first description of the early ontogeny of proetoid trilobites, survivors of the end-Devonian Hangenberg extinction event. The fauna comprises various taxa that can only partly and tentatively be assigned to the genera Liobolina, Diacoryphe, and Pedinocoryphe without specific attribution. The close resemblance of these larvae to previously described proetoid larvae from the Ordovician and Devonian emphasizes the ontogenetic homogeneity of this superfamily. Moreover, it supports the view that the Proetida is composed of two clades: the Aulacopleuroidea/Bathyuroidea group and the Proetoidea. The Carboniferous anaprotaspides exhibit an unusually wide size-range that may provide evidence of the acquisition of an extended planktonic period in the early ontogeny of at least some Carboniferous proetoids. This change in the ontogenetic strategy may have enabled proetoid trilobites to survive during the end-Devonian biocrises.

INTRODUCTION

DURING THE second half of the twentieth century, the study of trilobite ontogeny attracted the interest of many paleontologists, thanks to discoveries of well-preserved silicified specimens. Studies of such material have had a wide impact on trilobite systematics, investigation of evolutionary mechanisms, and understanding of selectivity in periods of biological crises. Nowadays, it is common to use larval characters in cladistic analysis, whatever the taxonomic level. Thus, larval features were used both to assess (Fortey and Owens, 1975) and to diagnose (Fortey, 1990) the order Proetida. Likewise, comparisons of growth series permit elucidation of phylogenetic relationships at lower taxonomic levels such as suborders (e.g., Chatterton et al., 1990), families (e.g., Waisfeld et al., 2001), or subfamilies (e.g., Edgecombe et al., 1988).

Ontogenetic traits are also applied to investigations on modalities and mechanisms of evolution. For example, trilobites have often been used to argue that heterochrony is a major evolutionary mechanism (e.g., McNamara, 1986; Feist, 1995). More recently, the use of geometric morphometry in describing ontogenetic changes permitted on the one hand demonstration of the role of heterochrony in the evolution of some phacopids (Crônier et al., 1998) and, on the other, demonstration that heterochrony is not the unique mechanism explaining modifications of ontogenetic trajectories in trilobites (Webster et al., 2001).

Although the two first kinds of applications have been extensively explored, only a single study has dealt with trilobite ontogeny during a period of biological crisis. Speyer and Chatterton (1989) demonstrated that the protaspid trilobite larvae can be separated into two groups on the basis of disparate morphologies. Thus, they distinguished a nonadult-like protaspis, probably planktonic, from an adultlike protaspis, most likely benthic. When a nonadult-like protaspid stage is present, it is supposed that a metamorphosis separated it from the subsequent adultlike one(s). Chatterton and Speyer (1989) characterized different life-history strategies on the basis of the protaspid types. This allowed them to study the selectivity of the End Ordovician crisis for trilobites and demonstrated that "the life history patterns predisposed individual taxa to survival or extinction." This work exemplified that the study of larval stages can be important for constraining the evolutionary histories of trilobites.

The Late Devonian biocrises are considered among the most important instants of global reduction in biomass that affected the marine life on Earth (Raup and Sepkoski, 1982). They are characterized by two major anoxic events related to complex eustatic perturbations. The Upper Kellwasser Event severely affected outer platform benthic communities shortly before the Frasnian/Famennian boundary. Among trilobites three of the five orders present in the Late Frasnian were exterminated simultaneously at the base of the event (Feist, 1991). The surviving proetids and phacopids subsequently diversified until the onset of the latest Famennian Hangenberg Event. Though both crises are generally considered similar in the nature and importance of extrinsic causes, the latter permitted only the proetids to survive into the Carboniferous as the sole remaining trilobite order.

Because no protaspid larvae younger than Middle Devonian have been reported to date, it has been impossible to evaluate the potential role of the life-history strategy in these patterns of extinction. So far, the Carboniferous trilobite ontogenies were only known to include meraspid and holaspid specimens (e.g., Osmolska, 1962, 1968, 1973; Griming, 1986; Yuan and Xiang, 1998). Here we report on the discovery of the first post-Hangenberg protaspides, recovered from early Lower Carboniferous rocks.

STRATIGRAPHY, MATERIAL, AND METHODS

Silicified larval stages were recovered from early Lower Carboniferous beds exposed in the truck roadcut running along the southeastern foothill of Puech de la Suque near Saint-Nazaire-de-Ladarez village, Hérault, France (Fig. 1). In this outcrop the Latest Devonian Hangenberg shale level is overlain by grey-beige, nodular, well-bedded calcilutites that extend across the Devonian/Carboniferous boundary into the Early Tournaisian sulcata through sandbergi Zones where they are overlain by argillaceous deposits with limestone nodules, shales, and carbon-rich radiolarian cherts of sandbergi- and crenulata-Zone age. Silicified trilobites occur together with ostracods (Lethiers and Feist, 1991) in limestone nodules (Bed 27, Fig. 1) attributed (Klapper, personal commun., 2003) to the sandbergi Zone (Lower Tournaisian).