Revision of the plectorthoid brachiopod Platystrophia dentata (Pander, 1830) from the Middle Ordovician of the East Baltic

Due to the scanty description of Porambonites dentata Pander, 1830 and loss of its single type specimen, the name dentata has been subsequently attributed to various Ordovician to Silurian species of the genus Platystrophia s.l. with two costae in the sulcus and three on the fold. In The Natural History Museum, London, there is a complete shell identified presumably by Christian Pander himself as Spirifer dentatus from Pulkowa. That specimen is selected here as neotype to Platystrophia dentata, i.e. to a species, which on the basis of new material is restricted to the lower Darriwilian in the St Petersburg region. Considering differences in the interior of the dorsal valve, the other species (some with subspecies) of the so-called dentata-group, especially from the stratigraphically younger strata, are discussed and excluded from the material described here of P. dentata.


INTRODUCTION
) described his new species as 'Porambonites dentata n. sp. feiner gerippt. In der breiteren Bucht zwei Längsrippen'. This description was supported by the schematic illustration of a single specimen. However, no data regarding its type locality or age are actually known. The specimen illustrated in Pander's monograph is unfortunately lost like many other Pander types (Jaanusson & Bassett 1993). Specimens with definitive labels or indeed similar to Platystrophia dentata in remnants of Pander's collection in the Mining Museum of the St Petersburg Mining Institute, St Petersburg, Russia (mentioned below as MMI) are also lacking. Thus, it was virtually impossible for subsequent authors to correctly understand the specific concept of this important species, generating considerable confusion. For example, Alikhova (1951, p. 12) noted: 'Usually, the name Platystrophia dentata (Pander) is applied to many different species. The only one reason for that is the number of costae in the sulcus -two and three on the fold.' Such an approach to the specific discrimination of Platystrophia-like brachiopods has been criticized by many authors (see, e.g., Zuykov & Harper 2007). Therefore, P. dentata as well as seven 'allied' taxa, which are considered as varieties or subspecies of the nominate form, require revision (Zuykov 2001).
The main goal of this paper is to clarify the original concept of P. dentata based on a well-preserved complete shell labelled as 'Spirifer dentatus Pander, Silur, Pulkowa' in The Natural History Museum, London (below NHM). It is suggested that this specimen was identified thus after publication of the monograph by von Buch in 1837 and before King's monograph on the Permian Fossils of England in 1850. In the former paper Pander's platystrophiids were referred initially to the genus Spirifer; in the latter they were moved to King's new genus Platystrophia. However, Davidson (1848Davidson ( , 1871, whose large brachiopod collections from the Silurian are deposited in the NHM, described Platystrophia-like taxa under the generic name Orthis. A comparison of the label accompanying the specimen from the NHM by V. Stolbova at the MMI with those written by Pander himself supported probable Pander's implication here. Further, the Pulkowka River ( Fig. 1) was one of the most popular places amongst Ordovician workers in the 19th century with numerous important fossiliferous localities. It was also mapped by Pander (1830, map). Today, these localities along the river are not sufficiently exposed. In terms of the modern stratigraphy of the St Petersburg region, the NHM specimen (as well as Pander's original) of P. dentata may have been collected from a stratigraphic interval within the following four regional stages: Volkhov, Kunda, Aseri and Lasnamägi. The platystrophiids from the first two stages in the East Fig. 1. A, map showing localities of Platystrophia dentata described in this paper: 1, Vilpovitsy quarry; 2, Pulkowka River (locality was destroyed in the middle of the 20th century); 3, Mga River; 4, Putilovo quarry; 5, Volkhov River. B, stratigraphical distribution of P. dentata and other discussed species; black rectangle -data obtained by the authors; white rectangle -data from Alikhova (1951); P. sublimis = P. sublimis rectangularis.
Baltic have been revised by Zuykov (1999), and forms with external (and internal) morphologies similar to those of the specimens illustrated here have not been reported. At the same time, more than 20 specimens newly collected from the upper part of the Simankovo Formation and in the entire Duboviki Formation of the Aseri Stage in the St Petersburg Region (Fig. 1) form a well-defined species, to which the NHM specimen (as well as Pander's lost original) can be assigned; the name Platystrophia dentata is thus applicable to all of these specimens. It is interesting to note that both Öpik (1930, p. 105) and Alikhova (1951, p. 13) suggested that Pander's original specimen was derived from the Aseri Stage. This suggestion, however, has not been supported by any original sources or by further study.
The material illustrated and discussed in the present paper is deposited in the Natural History Museum, London
Discussion. In features of external morphology P. dentata is similar to the majority of platystrophiids in its juvenile growth stages, being clearly different in the internal characters of the dorsal valve. The shell size in P. dentata is always small, as noted already by Verneuil (1845, p. 138). However, without data on the internal morphology, the taxonomic position of the two specimens illustrated in that monograph (see synonymy) cannot be confirmed with any confidence. Öpik (1930, p. 105), nevertheless, suggested an extended stratigraphical distribution for P. dentata, from the Aseri to upper Kukruse stages of North Estonia, but without illustration or discussion of the internal morphology of the oldest specimens. External and internal morphologies of the youngest specimens, as shown in Öpik (1930, pl. 5, figs 51-61, pl. 6, figs 62, 63), are quite different from those of P. dentata described here. For instance, blade-like, divergent brachiophores of the dorsal valve in pl. 5, fig. 56 seem to be similar to those of P. dentata lata Alichova.
The morphology of P. costata and three selected species of the 'dentata-group' from the sequences older and younger than those where P. dentata occurred is shown for comparative purposes in Fig. 4I-Q. In Fig. 2. Serial sections of Platystrophia dentata based on the shell CNIGR 2/13221, Vilpovitsy quarry, Aseri Stage. The distance from the umbo shown in brackets. particular, P. dentata described here differs from P. dentata veimarnensis Alichova (1951), P. d. lata Alichova (1951) and P. d. trapezoidalis Alichova (1951 in the shape of its brachiophores and in the absence of a median septum between posterior adductors. Platystrophia dentata triata Oraspõld (1956) from the Oandu Stage of North Estonia was recently transferred to the genus Neoplatystrophia Zuykov & Harper (2007). Oraspõld (1956, p. 45) reported a number of minor, and probably insignificant, differences between P. d. triata and P. d. evari; both are probably the end members in a morphological range of the same species. The internal features of P. d. dentatoides Oraspõld (1956) andP. d. trigonalis Bondarev (1968) are known on the basis of too limited material for a detailed re-study of these subspecies at present. (Pander, 1830). A-E, complete shell BM 26089, neotype;ventral, dorsal, lateral, anterior and posterior views, Ordovician, Pulkowa River, St Petersburg Region, Russia;F, ventral  The width of the sulcus, rounded radial costae, characters of cardinalia and the presence of a median ridge only between the anterior adductors distinguish P. dentata from specimens of P. ex gr. P. dentata (Pander) described by Zuykov (1999, p. 207, pl. 2, figs 6-9).

Fig. 4. A-H, Platystrophia dentata
Moreover, all other taxa identified by the name P. dentata or P. ex gr. P. dentata listed among the Ordovician and Silurian brachiopods from the Baltic basin and its margins (see Zuykov 2001) cannot be unambiguously confirmed without new material.