Evaluating internal versus external characters: Phylogenetic analyses of the Echinoconchidae, Buxtoniinae, and Juresaniinae (Phylum Brachiopoda)

Journal of Paleontology, Jul 2002 by Leighton, Lindsey R, Maples, Christopher G

ABSTRACT-Evolutionary relationships between the Echinoconchidae, Productidae, Buxtoniinae, and Juresaniinae (Phylum Brachiopoda. Order Productida) have been the subject of debate for the better part of a century. The original (Muir-Wood and Williams, 1965) and revised (Brunton et al., 2000) Brachiopoda volumes of the Treatise on Invertebrate Paleontology use markedly different classifications and emphasize different characters. The 1965 Treatise classification for these taxa primarily was based on internal features, especially the cardinal process; the revised Treatise (2000) relied on external ornament and shell shape. Multiple phylogenetic analyses (global parsimony, stratocladistics, nearest-neighbor) of 14 genera, representing all of the relevant subfamilies and outgroups, are in strong agreement that 1) the subfamily Buxtoniinae belongs in the family Echinoconchidae, not to the Productidae; 2) the subfamily Juresaniinae is more closely related to the Echinoconchinae than to the Buxtoniinae; and 3) that internal characters, such as a shafted cardinal process and anterio-medial position of the brachial valve adductor field, provide the best phylogenetic signal, and are synapomorphic for the Echinoconchidae. Jackknifing and Bremer Support corroborate these results.

INTRODUCTION

"This genus [Buxtonia] is related to Pustula but mimics the true Productus."-Dunbar and Condra (1932).

"This species [Buxtonia] is the only completely costate productid in the fauna and sole representative of the Productinae. "-Stainbrook (1947).

THE ABOVE quotes illustrate a long term and ongoing problem in the study of the systematics and evolution of the productidine (suborder Productidina) brachiopods. Over much of the past century, three genera, Buxtonia, Pustula, and Juresania, now expanded to subfamily or family level [Buxtoniinae, Echinoconchidae, and Juresaniinae, respectively (Figs. 1-3)], have been shifted repeatedly within classifications and phylogenies relative to one another (Thomas, 1914; Chao, 1928; Fredericks, 1928; Muir-Wood, 1930; Dunbar and Condra, 1932; Huang. 1932; Stainbrook, 1947: Muir-Wood and Cooper, 1960; Martinez-Chacon, 1979; Lazarev, 1985; Brunton et al., 1995). Martinez-Chacon (1979) placed the genus Juresania in synonymy with Buxtonia. but this suggestion was not accepted by Brunton et al. (1995) and the newly revised Brachiopod volume of the Treatise on Invertebrate Paleontology (Brunton et al., 2000). Two questions are relevant here: 1) are the Buxtoniinae more closely related to the Echinoconchidae or to the Productidae; and 2) are the Juresaniinae derived from members of the Buxtoniinae or from the Echinoconchidae? At first glance, the relationship of these groups to each other might seem to be an obscure problem of little interest to anyone except those who study productidine brachiopods. However, this is an important issue on several levels.

First, the systematic position of these three groups largely has been determined in the past by the relative emphasis placed on internal versus external characters. Debate over the relative phylogenetic value of internal vs. external characters is roughly analogous to the debate in mammalian systematics over the value of dental, cranial, and post-cranial characters (Szalay, 1985; Van Valen, 1993; Sanchez-Villagra and Williams, 1998). Although external characters of many skeletonized invertebrates generally are observed more easily, one might expect internal characters to be more evolutionarily conservative, and therefore to be of greater use in both phylogenetics and systematics. Vermeij and Carlson (2000) addressed this problem with respect to gastropods through the use of phylogenetic analysis, but such an approach has not been undertaken for any brachiopods.

Second, the Buxtoniinae, Echinoconchidae, and Juresaniinae were extremely successful, in terms of abundance, environmental range, and diversity, through much of the Late Paleozoic. These groups share several characters, such as dorsal spines, a shafted cardinal process, and an anterio-medial dorsal adductor muscle field, each of which is of functional interest. Determining how these characters evolved potentially might provide insight into the ecological and evolutionary success of these important brachiopod groups.

Third, the recent revision of the Brachiopod Volume of the Treatise on Invertebrate Paleontology (Brunton et al., 2000), proposes a different classification of the Productidina from the classification of the original Treatise (Muir-Wood and Williams, 1965), which once again raises the debate over the placement of these three higher taxa relative to each other. The original Treatise (Muir-Wood and Williams, 1965), which emphasized internal characters as diagnostic, placed the Juresaniinae within the Buxtoniidae, and suggested that the Buxtoniidae and Echinoconchidae were closely related. In contrast, the revised Treatise (Brunton et al., 2000) placed the Juresaniinae within the Echinoconchidae, and put the Buxtoniinae (reduced to a subfamily) in an entirely different superfamily, primarily on the basis of external features and shell shape (Brunton et al., 1995).