Bull. Soc. géol. Fr., 2003, t. 174, no 3, pp. 261-269 Some Cretaceous long-looped terebratulide brachiopods analysed in the light of the diversity observed in the ontogeny of Recent representatives DANIÈLE GASPARD1 Key words. – Terebratulide Brachiopods, Brachial ribbon, Diversity, Ontogeny, Cretaceous, Recent. Abstract. – The shape of the loop is of prime importance in brachiopod terebratulide classification and has been widely discussed and revised [Williams et al., 1996]. In the Rhynchonelliforma, we have focused our attention on the Terebratellidina, long-looped brachiopods with a wide-range diversity. The observation of its internal structure in recent representatives, investigated from the early growth stages, using the Scanning Electron Microscope (S.E.M.), demon- strates the complexity of its growth. Two main growth types of the descending branches are expressed during early on- togeny [Gaspard, 2002], followed in both cases by forward growth in opposite directions, with a brachial ribbon attached or unattached to the median septum. The purpose of such observations is to find in the fossil records similar growth types. An approach from the recent representatives allows a better knowledge of the Cretaceous taxa considered, even if the only way to recognise the loop is by means of transverse serial sections. The study of the recent terebratellidine loop facilitates the establishment of links between species from different geological periods. It helps to determine superfamilial distinctions and to find possible ancestors of recent terebratellidines. Brachiopodes térébratulides crétacés à grande boucle brachiale analysés à la lumière de la diversité exprimée lors de l’ontogenèse de représentants actuels Mots clés. – Brachiopodes térébratulides, Boucle brachiale, Diversité, Ontogenèse, Crétacé, Actuel. Résumé. – La classification des brachiopodes de longue date abusivement basée sur la forme du ruban brachial, support du lophophore, a été maintes fois remise en question puis revue par Williams et al. [1996]. Chez les Rhynchonellifor- mea, notre attention a surtout porté sur les Terebratellidina, brachiopodes dont le brachidium s’exprime avec une très grande diversité de forme. Une observation de cette structure interne chez des formes actuelles, documentée depuis le stade juvénile, avec l’aide du microscope électronique à balayage (M.E.B.), montre la complexité de leur évolution. Celle-ci s’exprime, pr. p., par deux types principaux de croissance d’une partie du brachidium (les branches descendan- tes) lors de l’ontogenèse précoce [Gaspard, 2002]. Ces deux types n’évoluent pas forcément de façon parallèle, dans la mesure où l’on observe des aboutissements croisés, selon que la boucle brachiale reste attachée ou non au septum. Le but de telles observations est de retrouver des cas équivalents dans les archives fossiles. Une étude à partir des brachio- podes actuels permet de mieux connaître la structure brachiale chez les représentants du Crétacé; l’observation de sec- tions sériées transversales est l’étape obligée pour reconnaître cette structure. Ceci aboutit à mieux cerner le niveau de la superfamille et même de reconstituer les lignées ancestrales des térébratulides actuels. INTRODUCTION considered. A terminology with taxonomic names used for the adult lophophore skeletal supports of recent former “Te- The loop shape has always been of prime importance in rebratellacea” was proposed [Beecher, 1895; Thomson, brachiopod terebratulide classifications, a question criti- 1927; Allan, 1940; Elliott, 1953 and Atkins, 1960-1961, cally discussed by Williams and Rowell [in Moore Ed., among others]. It allowed to distinguish stages of the loop 1965]. How to tackle this classification which is still a topi- development of Dallinidae, Laqueidae, Macandreviidae, Te- cal question ? With this aim, Williams et al. [1996] pro- rebratellidae, Megathyrididae, Platiidae, and Kraussinidae; posed a revised supra-ordinal classification. Various while different parts of the loop were described as : crura, biological and morphological features have been taken into ac- crural processes, descending branches, ascending branches, count to distinguish, in the subphylum Rhynchonelliformea, median septum or septal pillar, hood, and lacunae. This ap- among the Terebratulida : the Terebratulidina and Terebra- proach introduced confusion during comparative studies. tellidina Suborders. I will focus on the representatives of the Richardson [1975] proposed another terminology, focusing latter suborder. on the use of descriptive in place of generic adjectives to The brachiopods when assessed by biologists or pa- differentiate stages of the loop development. She did not, laeontologists pose problems, because most often variabili- however pay particular attention to the origin of the descen- ty and diversity of all parts of the shell are not fully ding branches. 1Université de Paris-Sud, Département des Sciences de la Terre, Bât. 509, F- 91405 Orsay cedex. Manuscrit déposé le 13 mai 2002 ; accepté après révision le 5 décembre 2002; Bull. Soc. géol. Fr., 2003, no 3 262 D. GASPARD The early loop ontogeny particularly for recent Occurrences of descending branches growing only from long-looped terebratulide brachiopods, when investigated the crura using the S.E.M., highlights the micromorphology and Investigations using the S.E.M. facilitated studies of the clearly shows similarities [Gaspard, 2003a, observations micromorphology of different parts of the brachidium and from the Walda cruise collections] or differences in deve- their microstructure. Personal observations of dorsal valves lopment. Further observations distinguish two types of (2-5 mm) from species of the recent genera Macandrevia growth with their corresponding skeletal fabrics [Gaspard KING and Ecnomiosa COOPER show similarities in their 2003b]. The observation at this level reflects metamorpho- early loop ontogeny reported in MacKinnon and Gaspard sis of the loop and developmental constraint advocated also [1996]. The descending branches in M. africana COOPER, by Gunji [1987]. Rather than attach too much importance to M. cranium (MÜLLER)andE. inexpectata COOPER grow the origin of the descending branches, it also appears neces- from the crura (pl. I, fig. 1,14) [Gaspard, 2003b, Fig. 1B] to sary to take account of all the brachial components and the join with the anterior part of the septal pillar (pl. I, fig. 5,6). way they grow. This approach emphasises differences be- This later structure evolves from a juvenile condition (pl. I, tween the two main ways of growth and can be used to ex- fig. 1-3, 13) via a blade-like structure (pl. I, fig. 14; amine data of the fossil records in better conditions. Such [Gaspard, 2003b, Fig. 1A], dorsal valve around 4mm), to an comparative studies of Cretaceous and recent specimens im- immature stage with a more elaborate hood (pl. I, fig. 7), prove perception of possible links and ancestors. which is spiny at the anterior part. Later, this hood gives rise to the ascending branches of the brachial loop. The complex process of biomineralization appears in LOOP ONTOGENY IN RECENT TAXA the skeletal fabrics of the brachidium. Incipient crura are fi- brous with a smooth anterior end (pl. I, fig.4). External The early loop ontogeny is illustrated by two main growth (dorsal) surfaces of the descending branches, internal side ways : (1) descending branches growing from the crura, (2) of the hood and the ascending branches are fibrous with a descending branches growing both from the crura and the smooth brachiotest (term proposed by MacKay et al. septal pillar. Further stages will be presented to ensure com- [1994]) at the marginal zone (pl. I, fig. 7-8). The basal part parisons with the fossil taxa. of the septal pillar is fibrous near the valve floor (pl. I, PLATE I Fig.1. – SEM image of a juvenile dorsal valve of Macandrevia africana COOPER from Walda cruise collections (off Angola, 3431 m, station CY 14) (s : median septum) – Valve dorsale juvenile de Macandrevia africana COOPER au M.E.B. Fig. 2. – Juvenile dorsal valve of M. africana (more advanced stage). – Valve dorsale juvénile de M. africana à un stade plus avancé. Fig. 3. – Detail of the septal pillar (sp) at a juvenile stage. – Détail du pilier septal (sp) à l’état juvénile. Fig. 4. – Incipient crus (cr) and fibrous calcite mosaic. – Crura naissant (cr) et mosaïque fibreuse calcitique. Fig. 5. – Ends of descending branches (db) converging to the anterior part of the septum topped by the incipient hood (ih), M. africana.–Extremités des branches descendantes convergeant vers la partie antérieure du septum surmonté par une structure naissante en forme de coiffe. (ih), M. africana. Fig. 6. – Detail of the end of the left descending branch on the previous figure. – Détail de l’extrémité de la branche descendante gauche sur la figure pré- cédente. Fig. 7. – Immature dorsal valve of M. africana showing the descending branches (db) and the beginning opened hood (h) with anterior projecting spines (white arrows). The SEM inclined view reveals the narrow waist at the basal part of the septal pillar (sp) – Valve dorsale immature de M. africana révélant les branches descendantes (db) et l’évasement naissant au sommet du septum avec des épines projetées antérieurement (flêches blanches). La vue in- clinée, au MEB, révèle l’étranglement basal du pilier septal. Fig. 8. – Detailed upper part of the hood on figure 7, revealing the incipient ascending branches (ab) fibrous internally, while their external surfaceandthe internal