Formation of cardinal process in Ordovician strophomenids

ISSN 00310301, Paleontological Journal, 2012, Vol. 46, No. 12, pp. 1362–1374. © Pleiades Publishing, Ltd., 2012. Formation of Cardinal Process in Ordovician Strophomenids A. A. Madison Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia email: Received May 13, 2011 Abstract—The cardinalia of strophomenids Lynnica fragilis gen. et sp. nov. and Sowerbyella (Sowerbyella) liliifera Öpik, 1930 from the Ordovician of the Leningrad Region are described in detail for different devel opmental stages. The study has revealed that the cardinal process of S. liliifera is bilobed unifid and the car dinal process of L. fragilis is bilobed trifid. L. fragilis gen. et sp. nov. is described. Keywords: strophomenids, ontogeny, Ordovician, Leningrad Region DOI: 10.1134/S0031030112120039 In the last issue of Treatise on Invertebrate Paleon tology, Brachiopoda (2000) the order Strophomenida Öpik, 1934 was divided into superfamilies Stro phomenoidea and Plectambonitoidea based on the peculiarities of the cardinal process structure. The classification of strophomenids was elaborated by Rong and Cocks (Rong and Cocks, 1994; Cocks and Rong, 1989; 2000), who restudied all known by that moment material on strophomenids. The superfami lies were distinguished based on the cardinal process structure: the strophomenoids have a “bifid” cardinal process and the plectambonitoids have a unifid (ridge shaped) or “trifid” cardinal process. However, there is certain confusion with terminology accepted in the first volume of Treatise… (Williams, Brunton, and MacKinnon, 1997; Williams and Brunton, 1997) and the way Rong and Cocks used terms in the description of strophomenids in the second volume of Treatise… (Cocks and Rong, 1989; 2000). According to the sec tions on musculature and glossary of terms in the first volume, the shaft of cardinal process divided into two parts is termed “bilobed.” “The diversity of form results from differences in the extent of growth of the shaft and from the growth or absence of the ridges sep arating the muscle bases medially or enclosing them laterally. To differentiate the main bilobed structure from these ridges, the latter have been designated by the suffix fid. A single median ridge on the cardinal process, normally separating the muscles, is termed unifid, a pair of lateral bounding ridges is termed bifid, a median ridge plus paired bounding ridges is termed trifid, and paired median ridges plus paired lateral bounding ridges is termed quadrifid” (Williams, Brun ton, and MacKinnon, 1997, pp. 394–395). The quadrifid, bilobed cardinal process in the course of evolution or ontogeny could transform into trifid, bilobed by the fusion of median lobes, as in some pro ductids. Herein after the terms accepted in the first volume will be used. In these terms the superfamily Strophomenoidea has bilobed cardinal process (shaft divided into two parts; bounding ridges two, four or absent), while the superfamily Plectambonitoidea has unifid ridge shaped cardinal process or trifid cardinal process (one median and two lateral bounding ridges). The cardinal process of plectambonitoids is believed to originate by developing of bounding ridges around diductor imprints on the notothyrial platform and the following raising of notothyrial platform with myophores and ridges (Williams, Brunton, and MacKinnon, 1997, p. 395). The overhung notothyrial platform bearing bounded myophores and having small cavity below is termed trifid “undercut” cardinal process. The evolutionary scheme of strophomenids assumed by Cocks and Rong (1989) is slightly differ ent. First plectambonitoids inherited from billingsel lids simple ridgeshaped cardinal process, then it split into two lobes and the bilobed cardinal process of stro phomenoids formed. The cardinal process of plectam bonitoids was complicated by developing of two ridges laterally of the unifid cardinal process. At the next developmental stages median, the highest and the most massive ridge started growing anteriorly. It lost connection with the valve floor and was supported only by lateral ridges; by this the “trifid undercut” type of cardinal process of plectambonitoids formed (Cocks and Rong, 1989) (Fig. 1). Hence the develop ment of cardinal process of Plectambonitoidea should be as follows: first in the ontogeny appears simple uni fid ridge (as in billingsellinidins) and then two small additional lobes appear laterally of it. We share the opinion of Rong and Cocks (Cocks and Rong, 1989, 2000; Rong and Cocks, 1994) that the classification of strophomenids should be based on 1362 FORMATION OF CARDINAL PROCESS IN ORDOVICIAN STROPHOMENIDS 1363 Fig. 1. Assumed scheme of cardinalia development in the superfamily Plectambonitoidea (composed after the data of Cocks and Rong, 1989). the endoskeleton of the dorsal valve, namely, type of cardinalia, bema and platform must be considered as the features of the family rank. However, features used in the taxonomy must be termed and determined with maximum accuracy. As an example, different authors described the cardinal process of Anechophragma Neuman, 1976 (in some papers this genus was named Ujukella Andreev, 1993) as simple undercut, trifid sim ple or trifid undercut and consequently variously determined its systematic position; however, the study of ontogeny of its type species revealed that this genus lacks cardinal process and has only long curved socket ridges, which accrete and form a plate closing the pedicle opening (Madison, 2013). In this instance the stereotype ideas prevailed: the strophomenids with complicated endoskeleton and lacking signs of sec ondary simplification must have cardinal process, high central area of the accreted socket ridges resembles cardinal process, therefore it is the cardinal process. But it was not considered that the narrow pointed plate located parallel to the posterior margin is absolutely uncomfortable for the muscle attachment and, more over, it lacks any signs of myophores. Rong and Cocks were the first who tried to revise the order Strophomenida in the modern sense and created new classification based on cardinalia type. They reviewed all known by that moment stro phomenids but the collections they studied consisted PALEONTOLOGICAL JOURNAL Vol. 46 No. 12 mainly of mature specimens (in some cases only liter ary data were used), which were not always wellpre served and the authors could not verify their classifica tion with data on the ontogeny. Our collection of stro phomenids consists only of the Middle–Upper Ordovician strophomenids of the Baltic Region, but some specimens are very wellpreserved, represent early developmental stages of several species and are suitable for construction of ontogenetic series. In the present paper the specimens of juvenile stro phomenids of two genera are described; according to the modern classification the former genus has simp (...truncated)