SHELL STRUCTURE AND ITS BEARING ON THE PHYLOGENY OF LATE ORDOVICIAN-EARLY SILURIAN STROPHOMENOID BRACHIOPODS FROM ANTICOSTI ISLAND, QUEBEC

Journal of Paleontology, Mar 2004 by Dewing, Keith

ABSTRACT-Four shell types are recognized from strophomenide brachiopods from Anticosti Island based on their fibrous or laminar character and on the type of taleolae. The shell types consistently co-vary with the types of cardinal process and style of socket plates. Treating shell structure as a conservative, non-reversing character implies that there are four groups of strophomenide brachiopod. The laminar-shell strophomenoids probably originated two separate times; one origination giving rise to two groups within the Strophomenida and one origination producing the Orthotetida. This matches the Treatise classification with only minor variation. The fourth group, that with fibrous shell and trilobed cardinal process and which includes the Plectambonitoidea, does not clearly fit into the Strophomenata and may be more closely related to the Clitambonitoidea. A limited cladistic analysis supports the idea that shell structure should be an important factor in establishing evolutionary kinship and points to ways of optimizing the Treatise classification.

INTRODUCTION

THE STROPHEMENATA arose in the Early Ordovician and diversified to include such well-known Paleozoic genera as Strophomena, Leplaena, Productus, and Chonetes. The group is united by its unusual shell structure (laminar rather than fibrous as in most brachiopod clades), in having a bilobed cardinal process, a strophic hingeline, and a supra-apical foramen. However, there remains a great deal of difficulty in establishing evolutionary kinship in the Strophomenata; the ancestor can only be guessed at as a Late Cambrian laminar-shelled brachiopod (like Billingsella), the shells are morphologically simple, and many taxonomic features are demonstrably homoplastic due to convergence. The new Treatise (Williams, Carlson, and Brunton, 2000) rectifies many of the problems with previous classification schemes. Unlike many previous attempts at classification (e.g., Williams, 1956, 1968a; Pope, 1976; Manakov, 1979), however, the Treatise treats shell structure as homoplastic and hence not of primary importance in classification. This paper examines this assumption by looking at the microstructural features of Late Ordovician shells from Anticosti Island, then using multiple microstructural characters to recognize four types of strophomenoid shell structure. Assuming shell structure is a character that is conservative, irreversible and which provides a strong phylogenetic signal helps point to ways of optimi/ing the Treatise classification.

Williams, Brunton, and Cocks (2000) concluded that the laminar shell structure is homoplastic, having evolved from fibrous-shelled plectambonitoids in the Early Ordovician strophomenides (Williams, 1970), and again in some of the Siluro-Devonian chonetids families (Brunton, 1972). Neither of these cases is supported by solid evidence. It is now known that the oldest laminar-shelled strophomenides are older than the oldest plectambonitoids (Rong et al., 1999) so that the laminar-shelled strophomenides probably derived directly from a laminar-shelled Billingsellide. The origin of the chonetidines is similarily in doubt; Racheboeuf and Copper (1986) and Dewing (1999), in their studies on the oldest chonetid, present evidence for a strophomenide ancestor. There is no firm evidence to suggest that laminar shell structure in the Ordovician or Early Silurian members of the Strophomenata developed more than once.

A wide range of strophomenoid genera from across the Ordovician-Silurian boundary on Anticosti Island, Quebec, were serially sectioned to investigate the articulation surfaces (Fig. 1; Dewing, 1999, 2001). A byproduct of this descriptive taxonomic work was a chance to look at the shell microstructure in great detail. The microstructural features delimit four types of shell structure. This division in shell structure also seems to match the distribution of other morphological characters LiSeFuI in classification (e.g., socket plates, cardinal process).

SHELL STRUCTURE OF BRACHIOPODS

Strophomenoid shell structure has been described by Williams (1956, 1968a, 1968b, 1970, 1971, 1990, 1997), Williams and Rowell (1965), Williams and Brunton (1993), Brunton (1972) and Manakov (1979). Williams has demonstrated that the shells of most articulated brachiopod groups have an outermost organic periostracum; a thin, primary layer of granular or acicular, low-magnesium calcite crystals; and a secondary layer, which in most articulate brachiopods consists of long calcite fibres, each with a crescentic inner face and pointed outer face, surrounded by an organic sheath. The secondary layer forms the bulk of the brachiopod shell, including the cardinal process and lophophore supports.

This succession of layers is typical of all articulated orders except the strophomenides, productides and orthotetides. Strophomenides typically lack a well-preserved primary layer, and most species have pseudopunctae-small, inwardly pointed inflections in the secondary layer, like a series of stacked cones, anteriorly inclined and at a high angle to the surface of the shell. These pseudopunctae may be comprised solely of bent laminae of the secondary layer, or they may be cored by a granular calcite rod, called a taleola (Williams, 1997).