The systematic position of Bethia serraticulma
This appendix provides an extended comparison between Bethia and other rhynchonelliformeans; the classifications and diagnoses of the revised Treatise on Invertebrate Paleontology (Williams et al. 2000a) are followed where possible.
Class Strophomenata
The pedicle in Bethia emerges from an open delthyrium in the ventral valve. There is no pseudodeltidium; instead, incipient deltidial plates are present. All strophomenate brachiopods, with the exception of the clitambonitidines (see below), possess a pseudodeltidium; the pedicle foramen, when present, is invariably apical or supra-apical (Williams et al. 2000b). These differences in structure are fundamental, and they strongly imply that Bethia is not a strophomenide, productide, orthotetide, or billingsellid billingsellide.
The suborder Clitambonitidina was provisionally placed within the strophomenate Order Billingsellida by Rubel & Wright (2000); however, unlike other strophomenates, clitambonitidines lack a pseudodeltidium and apical/supra-apical foramen; their pedicle emerged though a delthyrium, either open or partially closed by deltidial plates. As noted by Rubel & Wright (2000), they may by derived from an orthid ancestor, and hence belong in the Rhynchonellata, with which they share this posterior architecture. Clitambonitidines are best diagnosed by the presence of a spondylium, which cannot be determined in Bethia. They arestrophic, have well defined ventral interareas, and typically also have a closed notothyrium in their dorsal interareas. Most genera have pointed cardinal extremities, and several are concavoconvex. Marked concentric ornament combined with weak radial ornament also occurs in some polytoechioid genera (e.g. Peritritoechia, Platytoechia). Mantle canals in the group are typically pinnate, but saccate in at least the ventral valves of some genera (Wright & Rubel 1996). A clitambonitidine affinity for Bethia is thus plausible but, as the group is not otherwise known outside the Ordovician, we do not consider this the most parsimonious option.
Minor rhynchonelliformean classes
The classes Chileata, Obolellata, and Kutorginata are restricted to the Cambrian, with the exception of the dictyonellidine chileates. These groups all have apical or supra-apical perforations or foramina (Williams et al. 2000a), and no genus from these classes resembles Bethia in either shell form or ornament.
Class Rhynchonellata
Nine rhynchonellate orders were recognised by Williams et al. (2000a). They are united primarily by the absence of a pseudodeltidium and presence of a pedicle that emerged posteriorly through the delthyrium; this is the configuration seen in Bethia, which is hence likely to be a rhynchonellate.
Of the extant orders, the Thecideida originated in the Mesozoic, and are not discussed further. Members of both the Rhynchonellida and Terebratulida are almost exclusively astrophic and biconvex, and lack interareas or pointed cardinal extremities. Both orders display a relatively narrow range of morphologies, and no rhynchonellide or terebratulide genera resemble Bethia in either shape or ornament. Further, the terebratulides originated after the Silurian, and possess distinctive lophophore supports that are absent in Bethia. Within the Order Pentamerida, from which the extant rhynchonellates arose (see Carlson et al. 2002), only the porambonitoids and stricklandioids are strophic; no genus in either of these groups has an outline, pedicle opening or ornament comparable to Bethia. Further, all pentamerids have a dorsal valve that is convex, in most cases strongly so.
The Orders Atrypida, Athyridida and Spiriferida are united by the presence of spiralia, calcareous supports for a spirolophous lophophore. These structures are not apparent in Bethia and, although the lophophore is immature, the nature of its flexure is not suggestive of a spirolophous adult morphology. Concavoconvex strophic shells with relatively pointed cardinal extremities occur within the atrypide Superfamily Davidsonioidea (see Copper 2002). The genus Prodavidsonia is the most closely comparable to Bethia but has a unisulcate rather than uniplicate folding, a much lower ventral valve, and lacks strong growth lamellae. Within the athyridides (see Alvarez & Rong 2002), the diverse Family Athyrididae includes rare genera that are concavoconvex (e.g. Galeatathyris) or strophic with pointed cardinal extremities (e.g. Comelicania); many also have strong growth lamellae and lack radial ornament (e.g. Lamellothyris). However, even in relatively immature specimens, both athyridides and atrypides normally have a small pedicle foramen and a fused deltidium (the atrypide attached to Bethia provides an example); hence the pedicle structures of Bethia are not suggestive of these groups. The Spiriferida (= Spiriferidina sensu Boucot et al. 1965) typically possessed a large open delthyrium in a well developed interarea, and a strophic shell with well developed alae. However spirifides are exclusively biconvex and radially ornamented and/or folded. Hence we conclude that Bethia is very unlikely to belong in any of these three orders.
The Order Protorthidia (see Williams and Harper 2000a) contains strophic forms that possess a large open delthyrium, and in some cases (e.g. Crossiskenidium) a strong concentric ornament. Protorthids, however, invariably possess an open notothyrium, and their dorsal valves are also almost exclusively convex; only the Cambrian genus Loperia (which is only questionably referred to the order by Williams and Harper 2000a) is resupinate. We do not exclude a protorthid affinity for Bethia, but do not consider it likely.
The order Orthida (see Williams and Harper 2000b) embraces a wide variety of morphologies. Orthides are exclusively strophic, typically with their greatest width anterior to the hinge-line; they also normally possess a large open delthyrium in a well defined and triangular apsacline interarea. Deltidial plates, though not common, developed several times in unrelated genera (e.g. Barbarorthis, Trematorthis, Phragmophora). Pointed cardinal extremities are common, and saccate dorsal vascular systems occur. Most orthides have an open notothyrium, but several genera of hesperonomiids, plaesiomyids and productorthids possess a notothyrium closed by chilidial-like structures. Concavoconvex shells are rare, but occur at least in some productorthids (e.g. Nicollela, Whittardia), hesperonomiids (e.g. Hesperonomia) and other genera (e.g. Cyrtonotella, some Dicoelosia,). Folding is typically uniplicate, but all harknessellids and saukrodictyids are unisulcate, as are some enteletids and dalmanellids. The vast majority of orthides have strong radial and weak concentric ornament, but the former is greatly reduced or absent in a few genera (e.g. Leptoskelidion, Murrinyinella), and strong growth lamellae occur in the glyptorthids and some productorthids.Hence although Bethia is far removed from a ‘typical’ orthide, its morphology does fit within the broad range of the group., and we consider this to be the most parsimonious placement for the new genus. Bethia is clearly differentiated from all other orthid genera by its distinctive ornament, and by its combination of a unisulcate concavoconvex shell, deltidial plates and closed notothyrium. However the new genus remains taxonomically problematic, and we defer a full analysis of its position within the Orthida until such time as further information becomes available.
References:
Alvarez, F. and Rong, J. 2002. Athyridida. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 4, 1475-1614 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Boucot, A. J., Johnson, J. G., Pitrat, C. W. and Staton, R. D. 1965. Spiriferida. in Treatise on Invertebrate Paleontology (ed. Moore, R. C.) Part H, Vol. 2, 632-728. (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Carlson, S. J., Boucot, A. J., Rong, J. and Blodgett, R. B. 2002. Pentamerida. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 4, 921-1026. (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Copper, P. 2002. Atrypida. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 4, 1377-1474 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Rubel, M. and Wright, A. D. 2000. Clitambonitidina. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 3, 692-708 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Williams, A., Carlson, S. J. and Brunton, C. H. C. 2000a. Brachiopod Classification. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 2, 1-27 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Williams, A., Brunton, C. H. C. and Cocks, L. R. M. 2000b. Strophomenata. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 2, 215-216 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Williams, A., and Harper, D. A. T. 2000a. Protorthida. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 3, 709-714 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Williams, A., and Harper, D. A. T. 2000b. Orthida. inTreatise on Invertebrate Paleontology (ed. Kaesler, R. L.) Part H, Revised, Vol. 3, 714-846 (Geological Society of America, Boulder and The University of Kansas, Lawrence).
Wright, A. D. and Rubel, M. 1996. A review of the morphological features affecting the classification of clitambonitidine brachiopods. Palaeontology39, 53-75.
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