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SILURIAN and UPPER ORDOVICIAN ATRYPIDS of the GENERA Plectatrrpa and SPIRIGERINA

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SILURIAN and UPPER ORDOVICIAN ATRYPIDS of the GENERA Plectatrrpa and SPIRIGERINA NORSK GEOLOGISK TIDSSKRIFT 47 SILURIAN AND UPPER ORDOVICIAN ATRYPIDS OF THE GENERA PLECTATRrPA AND SPIRIGERINA BY A. J. BoucoT & J. G. JoHNSON (Division of Geological Sciences, California Institute of Technology, Pasadena, California, U.S.A.) Abstract. The nomenclatural history of Plectatrypa and Spirigerina is traced and the basis for recognition of each taxon as a valid genus is developed. The earliest progenitors of Spirigerina marginalis, which range from the Upper Ordovician into the Lower Llandovery, are separated as a new subgenus Spirigerina (Eospirigerina), based on Atrypa praemarginalis Savage which is recognized in mid-North America and in Gaspe. The closely related S. (Eospi­ rigerina) gaspeensis is identified in Britain and Venezuela in the Lower Llan­ dovery. True Plectatrypa is unknown below the Lower Llandovery; previous citations from the Ordovician are of forms that the writers propose to include in Eospirigerina. A new species, Plectatrypa henningsmoeni, is proposed for a Lower Llandovery form from the Oslo region. Plectatrypa is regarded as an early offshoot of the Atrypinae, off the main line of evolution that resulted in Atrypa in a strict sense and then little differentiated from Eospirigerina, the earliest known member of the Carinatininae. The species and distribution of Plectatrypa and of Spirigerina are listed. Introduction In 1930, ScHUCHERT & CooPER proposed the genus Plectatrypa (1930 p. 278) for a group of sulcate Ordovician and Silurian atrypoids in­ cluding Atrypa imbricata Sowerby and Atrypa marginalis Dalman and their allies. Until about 1960, the name was commonly used for atrypoids related to imbricata and marginalis, with the exception of the work by PouLSEN (1943), who treated Plectatrypa as a synonym of Nalivkinia Bublitschenko. The work of ALEKSEEVA (1960) brought out the fact that the genus Spirigerina D'ORBIGNY (1849 p. 42), 80 A. J. BOUCOT & J. G. JOHNSON although long forgotten, had clear priority over Schuchert and Cooper's genus Plectatrypa, and, in her paper discussing Spirigerina, Alekseeva designat ed Terebratula marginalis Dalman, 1828 as the type species of the genus. This was the first valid designation of a type species for Spirigerina, because d'Orbigny originally included both marginalis and imbricata in his list of species assigned without designating one as type. Because Schuchert and Cooper chose Terebratula imbricata as a type for Plectatrypa and Alekseeva chose T. marginalis as the type of Spirigerina, there remained a subjective decision about possible synonymy of the two, and past usage alone would suggest that the genera are synonymous. According to Alekseeva, Spirigerina differs from Plectatrypa in having l) a straight or nearly straight ventral beak and visible delthyrium, 2) a clearly defined interarea, 3) numerous fine concentric growth lines opposed to lamellose growth lines in Plectatrypa, and 4) dental lamellae. At the time Alekseeva's work came to the writers' attention, work was far progressed on the atrypoids for the brachiopod volume of the Treatise on Invertebrate Paleontology (Moore 1965), and in an addendum to the writers' paper dealing with systematic revision of some of the atrypoids (BOUCOT, J OHNSON & STATON 1964 p. 820) the Plectatrypa-Spirigerina problem was noted, but the writers' ap­ praisal in the aforementioned addendum was based on an assumption (now found false) that finely ribbed specimens from the Silurian of North Greenland illustrated by PouLSEN (1943, Pl. 5, figs. 24--31) as marginalis truly belonged to that species. During the present study, the writers have had the opportunity to examine a number of well­ preserved specimens of Spirigerina marginalis from the type locality, Klinteham, Gotland, and find that we are in dose agreement with Alekseeva's appraisal of the points of distinction between Plectatrypa and Spirigerina. Some species that bear all the essential characteristics of Spirigerina lack dental lamellae; therefore, we would exclude Alekseeva's point 4 as a standard of comparison. Furthermore, a hetter insight into the nature of Ashgillian and early Llandovery age sulcate atrypoids convinces us that they rep­ resent a relatively tight-knit, phylogenetically homogeneous group not easily assignable to Plectatrypa or to Spirigerina when those two genera are diagnosed essentially on the characters of their type species. Plectatrypa has numerous, evenly spaced, frilly growth lamellae, a SILURIAN AND UPPER ORDOVICIAN ATRYPIDS 81 FRASN AS H­ LLAN DOVE RY LUDLOW GEO. EMS . GIVET G ILL WENLOCK SKALA SIEGEN L l M l U EIFEL / l> l l l l l l l"" l l l PRO TRYPA ATRYPA l l l � l:f �-- l )li :.� lj 1 l ond o/liedgenera 1 l � LECTATRYPA )o / I._P l l l l [Tl l 11 l l ! l l l l l l l l l l l l ! �/ l l 1 11 � IEOSPIRIGERINA l SPIRIGERINA l z l\ l l l >� l : \� ___ l .. f.�.0?!�_A_TI_N_A �,.I � l 1 on� o/lied genera � �·----------------------------�--------------�l � Fig. l. Relationship of Plectatrypa and Spirigerina to related atrypacean genera and their stratigraphic ranges in the Ordovician to Lower Devonian. strongly incurved ventral beak lacking a deltidium, and impressed track-like ventral diductor scars. Spirigerina lacks frilly growth lamel­ lae, and. instead, well-preserved specimens exhibit fine concentric filae; its ventral beak is straight and exhibits a delthyrium closed by conjunct deltidial plates. The ventral muscle impressions are faint, roughly trigonal, and neither flabellate nor track-like. Thus defined, on the basis of post-Lower Llandovery representatives, Plectatrypa and Spirigerina appear to exemplify the already fully differentiated subfamilies Atrypinae and Carinatininac, respectively. The frilly growth lamellae and strongly incurved ventral beak of Plectatrypa are important characters of true Atrypa of the Silurian and Devonian. Conversely, the straight ventral beak, the closed delthyrium, and the non-frilly growth lines of Spirigerina are important and characteristic features of most Devonian members of the Carinatininae. Late Ordovician and early Llandovery ribbed, sulcate atrypids are less easy to characterize in terms of the subfamilies Atrypinae or Carinatininae. The growth lines are not frilly as in Plectatrypa, nor 82 A. J. BOUCOT & J. G. JOHNSON are they essentially absent as in Spirigerina. Instead, there are com­ monly a few irregularly spaced imbricate growth lines. The ventral beak is not straight, but is slightly incurved, and lacks deltidial plates. Internally, these early forms lack the track-like impressed diductors of Plectatrypa and instead have a non-impressed trigonal muscle field of Spirigerina type. We believe that this combination of features indi­ cates a close relation with Spirigerina, and we therefore propose Eospirigerina for those late Ordovician and early Llandovery forms. Eospirigerina is the earliest member of the Atrypidae and thus may be the ancestor of Protatrypa and Plectatrypa, which we assign to the Atrypinae-or the latter may have had a common ancestor with Eospirigerina so that Plectatrypa arose as a convergent homeomorph of Eospirigerina. Eospirigerina is thought to have given way to Spiri­ gerina sometime in the middle or early Late Llandovery, and Spiri­ gerina, in turn, is thought to have given rise to Carinatina sometime in the late Silurian (Fig. 1). Systematic Paleontolo�y Suborder ATRYPOIDEA Superfamily ATRYPACEA Family ATRYPIDAE Subfamily CARIN ATININAE Emended diagnosis: Atrypidae with straight or nearly straight ventral beaks and conjunct deltidial plates enclosing the delthyrium. Shape typically subequally biconvex. Growth lines characteristically non­ frilly. Discussion: The genus Spinatrypina RZHONSNITSKAYA (1964 p. 101) may belong to the Carinatininae as it has a straight ventral beak and deltidial plates, but it is exceptional in the subfamily in ha ving frilly( ?) or at !east well-developed imbricate growth lamellae. In the brachiopod volume of the Treatise on Invertebrate Paleontology (MooRE 1965), the writers included the recently proposed genus Mi­ matrypa Struve, 1964, in the Carinatininae, although Struve had proposed the genus as a member of the Palaferellinae, i.e. a genus allied to Gruenewaldtia. COPPER (1964 p. 363), citing the presence of SILURIAN AND UPPER ORDOVICIAN ATRYPIDS 83 ventral muscle platforms in both genera, amplified suggested relations of M imatrypa with the Palaferellinae by suggesting that M imatrypa was in fact derived from Gruenewaldtia in the Middle Devonian. The writers feel that the presence of a ventral platform in these atrypoids has been greatly overemphasized. Our survey of the atrypoids as a whole, in connection with the synoptical presentation in the Treatise, together with the present work convinces us that atrypoid ventral platforms occur in a number of genera that are otherwise unrelated. The structure is well known in certain smooth atrypoids such as Meifodia, Lissatrypa, Lissatrypoidea, and Glassia. It is strongly devel­ oped in some species of Plectatrypa as shown by the specimens illus­ trated in the present paper (Pl. 4, figs. 7, 11). That ventral platforms originated separately in several distinct groups of atrypoids appears undeniable and we therefore recommend reappraisal of the basis of including Mimatrypa and closely related shells in the Palaferellinae. The latter subfamily is exemplified by Gruenewaldtia in which there is developed a strongly convex pedicle valve with an extravagantly incurved ventral beak, and with plate-like platforms in both valves that are elevated on radial septa above the
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