Iberirhynchiidae, a new family of ancistrorhynchoid rhynchonellids (Brachiopoda) from the Middle Ordovician to the Lower Devonian of Euroamerica and Gondwana.

Abstract

Extended Abstract: The type species of the genus Iberirhynchia Drot & Westbroek, 1966, I. santaluciensis Drot & Westbroek, 1966 is a small, subtriangular, dorsibiconvex, completely costate rhynchonellid, with ventral vertical dental plates and a dorsal cup, supported on a median partition, covered anteriorly by a connectivum. After their morphological characters Iberirhynchia has usually been included in the Family Trigonirhynchiidae Schmidt, 1965 (Superfamily Rhynchotrematoidea Schuchert, 1913). However the Iberirhynchia median dorsal partition structure departs clearly from that of Trigonirhynchia Cooper, 1942 and related genera. These last forms have a true median septum provided by an axial median granular calcite sheet (mediotest) which during further ontogeny became moderately thicker by apposition of secondary shell lamellae on both sides of the septum. On the contrary, the Iberirhynchia median dorsal partition is in fact a rather thick swelling of the bottom shell originated by differential secretion of secondary shell from the floor of the valve upwards (pseudoseptum). The throughlike internal dorsal structure formed by the merging of both the crural plates and pseudoseptum was named by Rozman (1969) (fide Havlíček & Štorch, 1990) a pseudoseptalium and its occurrence was used to characterize the Subfamily Rostricellulinae against other trigonirhynchiid genera provided by a conventional median septum and septalium. The pseudoseptum becomes also evident in the shell exterior by the occurrence of a marked dorsal median umbonal depression that extends distally sometimes as far as to reach the frontal commissure. There is a great anatomical difference between the drastic folding of the dorsal outer epithelium along the plane of symmetry that gives rise to the trigonirhynchiid median septum and the simple apposition of secondary shell lamellae that originates a pseudoseptum. Havlíček & Štorch (1990) and García-Alcalde (1998) considered long ago the occurrence of a pseudoseptalium as important as to reassigning several trigonirhynchiid taxa to the Family Oligorhynchiidae Cooper, 1956, belonging to another different Superfamily unit, the Ancistrorhynchoidea Cooper, 1956. Recent systematic studies by Savage have limited the concept of Oligorhynchiidae to very small, paucicostate, elongated rinchonellids, lacking of septalial or pseudoseptalial dorsal structures although with a tendency to develop a pseudoseptum in the floor of the dorsal valve. In these conditions, all the forms with pseudoseptate structures are considered provisionally as belonging to the Superfamily Ancistrorhynchoidea, although without ruling out that the pseudoseptate forms with a complete, usually covered pseudoseptalium lacking of a cardinal process could be grouped in a new superfamily unit. Iberirhynchia is proposed herein as the type of the new Family Iberirhynchiidae that combines features usual in Trigonirhynchiidae Schmidt, 1965 with the occurrence of a well developed, usually covered pseudoseptalium. To that family and to the new Subfamily Iberirhynchiinae would belong, besides the type genus, at least Tectogonotoechia García-Alcalde, 1998, Ancillotoechia Havlíček, 1959, Aratoechia Havlíček, 1982 and Microsphaeridiorhynchus Sartenaer, 1970. These genera are characterized by the occurrence of a complet, covered pseudoseptalium, vertical dental plates and simple radial ornamentation. Myrmirhynx Havlíček, 1982 and Sufetirhynchia Havlíček 1982, integrate another new Subfamily Myrmirhynxinae, close to Iberirhynchiinae but with divided radial ornamentation. The Iberirhynchiidae would be completed by the subfamilies Rostricellulinae Rozman, 1969, lacking of connectivum, and Virginiatinae Amsden, 1974, with a partial cover plate, that lacks sometimes. According both the ages and the paleogeographical distribution of the iberhynchiid genera, the Family Iberirhynchiidae started at the middle Ordovician with a basal trunk constituted by the cosmopolitan genus Rostricellula Ulrich & Cooper, 1942 and the other upper Ordovician-lower Silurian Rostricellulinae. Virginiatinae Amsden, 1974 could in turn derivate from the Rostricellulinae along the upper Ordovician. Later on, along the Silurian appeared both the Iberirhynchiinae nov subfam. (Wenlock-upper Emsian), and the Myrmirhynxinae nov. subfam. (Wenlock-Lochkovian). Ancillotoechia, a Silurian to lowermost Devonian, cosmopolitan genus, was probably the ancestral, more generalized Iberirhynchiinae form. In the other hand, Myrmirhynxinae formed a marginal, specialized branch, of uncertain origin, only known in Bohemia. The Silurian Family Niorhynicidae Savage, 1996, with complet, covered or not, pseudoseptalium, but lacking of dental plates could be either a sister group of Iberihynchiidae among the Ancistrorhynchoidea or form with them a new superfamily unit. Iberirhynchia (upper Emsian) is the younger iberhynchiid genus, but its origin and relationships with other Iberirhynchiinae are not well known at the present. Iberirhynchia santaluciensis is redescribed from: a) new, numerous possible topotypic specimens, coming from the Pico Aguasalio region, León province (N Spain); b) several specimens from different localities in Asturias and León provinces; and c) a small original sample that we have got on loan from the Rijksmuseum van Geologie en Mineralogie of Leiden, Nederland. In this redescription several problems concerning the type material, locus typicus and stratum typicum are dealt with and solved as far as possible. Ontogenetic and paleoecological observations are also given. Shells show an allometric growth with a proportionally greater increase of width and thickness against length with the age. Neanic shells are thin, small, ventribiconvex, longer than wide or as long as wide, with sharp commissures, faintly inclined beak, acute apical angles (40-60º) and inconspicuous ventral sinus and dorsal fold; the transition to the ephebic stage is marked usually by a strong growth lamella that indicates a more or less prolonged growth stopping. The ephebic shells are dorsibiconvex, gradually thicker and clearly wider than long, with suberect beaks and apical angles greater than 60º, well developed ventral sinus and dorsal fold and verticalized lateral and anterior margins. Gerontic stage begins also after a marked growth stopping; gerontic shells are very thick and transverse, with high and vertical lateral sides that meet at the commissure in angles close to 180º, and apical angles between 70-90º or more. In general, epizoans and repair damage signals are not abundant indicating a not too rough environment and a rather stable sea bottom. Until to now, I. santaluciensis is the sole known Iberirhynchia species. Bohemian forms named Iberirhynchia nargis Havlíček & Kukal, 1990, and Iberirhynchia sp. Havlíček 1992, are true trigonirhynchiids, with conventional, not covered septalium.