Contr. Tert. Quatern. Geol. 27(2-3) 1 pi. Leiden, September 1990 93 A new species of mobulid ray (Elasmobranchii, Mobulidae) from the Oligocene of Belgium Taco J. Bor SLIEDRECHT, THE NETHERLANDS Taco A of mobulid from the of — Contr. Tert Bor, J. new species ray (Elasmobranchii, Mobulidae) Oligocene Belgium. Quatern. Geol., 27(2-3): 93-97, 1 pi. Leiden, September 1990. A new mobulid species, Plinthicus kruibekensis, from the Oligocene (Rupelian) Boom Clay Formation of Belgium is described. Key words — Chondrichthyes, Mobulidae, Oligocene, Rupelian, Belgium. T.J. Bor, P.A. de Genestetstraat 102, 3362 TH Sliedrecht, The Netherlands Contents (Late Palaeocene - Middle Eocene), Burnhamia Palaeocene - Middle Eomobula Introduction 93 (Late Eocene), (Late p. Eocene - Middle Manta Miocene - 94 Eocene), (Early Systematic part p. Recent), Mobula (Late Eocene - Recent) and Plin- Acknowledgements p. 96 thicus Miocene - Middle How- References 96 (Early Miocene). p. ever, mobulid teeth have not yet been described Introduction from the Oligocene. Pfeil (1981) described two The family Mobulidae (devil rays) includes the mobulid species from the Latdorfian Schonecker largest representatives of living batoid fishes. Some Fischschiefer of southern Germany, but the Lat- Manta birostris dorfian species, e.g. (Donndorff, 1798), may is now incorporated into the Eocene while least 6.7 and in attain a disc width of at m a weight the Oligocene comprises only the Rupelian and of Chattian excess 1,400 kg (Bigelow & Schroeder, 1953). (Cowie & Bassett, 1989). Nowadays, devil rays occur worldwide in tropical In 1984, a tooth of an undescribed Plinthicus and subtropical seas quite commonly, more rarely species was collected by the author from the sieve in continental residue of sediment from the Boom warm-temperate waters; over a sample Clay shelves well around distant islands. Formation as as larger (Oligocene, Rupelian) of Kruibeke Their mode of life is and It is the earliest known pelagic they are highly near Antwerp, Belgium. oc- migratory, swimming often in small schools close currence of this genus and first evidence of its the surface. Unlike all other mobulids in the North Sea Basin. The elasmo- to rays, presence filter-feedon zooplankton (fish fry, crustacean lar- branch fauna from the Boom Clay Formation in vae and small squids) by means of a specialised Belgium is well known and has been the subject of branchial sieve In the several Leriche Steurbaut apparatus. living mobulids, papers, e.g. (1910), & small and teeth wide of Herman and It is dominated numerous display a range (1978) Nolf(1986). by variation with regard to theirmorphology, which is small teeth ofSqualus (72%) and Raja (24%); larger about of of their teeth of e. p. Carcharias and brought by a process regression are extremely rare repre- dentition which is in than of of no longer use. sent no more 4% the total number Fossil mobulid teeth have been described from elasmobranch teeth. The extreme rarity of mobu- the of North Africa and lid teeth be due the considerablefall in Tertiary Europe, America, may to tem- Asia. Archaeomanta the The family includes the genera perature at Latdorfian-Rupelian boundary. 94 ferred to and van den Bosch SYSTEMATIC PART Vandenberghe (1978) & Janssen (in press). Class Chondrichthyes — of the Boom For- Type horizon Upper part Clay Subclass Elasmobranchii mation, Putte Member. Sediment sample from Superorder Batomorphii bed number 56, which coincides with septaria Order Myliobatiformes S6 level S60, formerly referred to as (Vanden- Family Mobulidae berghe, 1978; Vandenberghe & Laga, 1986). Age: Genus Plinthicus Cope, 1869 Oligocene, Rupelian. — — 1869. The tooth is but of Type species Plinthicus stenodon Cope, Description very fragile, per- — known fect of abrasion Generic diagnosis Extinct mobulid ray on- preservation, showing no signs The dentition is similar due functional trans- ly from isolated teeth. to to usage or 'post-mortem' whatsoever. The tooth is 3.0 5.4 that of Rhinoptera with hexagonal teeth which port mm high, decrease in width wide and 1.6 mm regularly from the symphysial mm long. be The is in outer- towards the lateral rows. The teeth may quite crown very high, compressed is inner In to 25 mm wide. The crown com- direction and mesio-distally. large, up high, expanded view the has pressed in outer-inner direction, expanded mesio- inner and outer crown a subrectangu- the mesial than the distal lar the basal side the distally, higher on on outline; being rectilinear, oc- the occlusal clusal side the mesial and distal part, strongly sloping inwards, placing slightly concave, somewhat backwards with the basal sides The crown is surface respect to crown semi-elliptical. face, and shows pronounced enameloid laminae asymmetrical, being slightly higher on the mesial inner and faces. The than the distal The mesial and on the outer polyaulacorhizid on part. elliptical half thick root is low and poorly developed. distal crown extensions are about as as — of the between the Discussion The Plinthicus is known. the middle crown occlusal genus poorly part and basal In lateral the first The only species described hitherto is the type faces. view, crown is and then towards species, P. stenodon Cope, 1869. It common in slopes outwards curves strongly the occlusal face back- the Miocene of Maryland and North Carolina, the inner side, thus placing in the Miocene of southern wards with the basal face. The U.S.A., and rare respect to crown oc- outline with France (Cappetta, 1970, 1987). clusal surface has a clearly hexagonal Plinthicus is included in the family Mobulidae, sigmoidal inner and outer sides, the inner edge since the general tooth morphology (especially the being sharp, the outer edge rounded and slightly face. The flat and high, compressed crown and polyaulacorhizid overhanging the outer crown face is times wide root) resembles that of certain extant Mobula smooth occlusal 2.6 as as deep. It shows a number of low, broad and species. In addition, the degree of wear of the teeth irregular in direction. The in- is negligible, which would suggest a diet of soft folds that run an outer-inner of thus of life ner crown face shows an prey, and a pelagic way (Cappetta, anastomosing pattern vertical with intermediate 1970, 1987). very pronounced ridges of On the outer face grooves varying depth. crown kruibekensis similar of is Plinthicus nov. sp a pattern anastomosing ridges pres- inner face which is PI. 1, Figs 1-5. ent, in contrast to the crown covered with large and blunt spine-like outgrowths Derivation — This is named after the of size and The inner rim is of name species varying shape. crown in extended and the The type locality Kruibeke northern Belgium. smooth, overhangs root. — rim is smooth and Material A single tooth, the holotype, collected outer crown slightly convex. by the author in 1984. It is deposited in the collec- The basal face of the crown has a hexagonal out- tions of the Nationaal Natuurhistorisch Museum line with subrectilinear inner and outer sides. The Minéra- crown/root lies in in the (formerly Rijksmuseum van Geologie en junction a depression basal surface of the logie), Leiden, The Netherlands. Registration crown. number RGM 177 500. The polyaulacorhizid root is low and poorly lobes Type locality — "Gralex" clay pit at Kruibeke, developed. The irregularly shaped root are broad and inter- situated SW of the city of Antwerp, Belgium. separated by rather deep parallel in and of Coordinates: = = 208.5. For des- median section x 146.5, y a grooves, U-shaped vary- width and The mesialmost and distal- cription of the section exposed the reader is re- ing depth. 95 lobes fused. The known and the chances of further most root are partly foramina, specimens being being variable in size and shape, are randomly discovered in the near future are slight. scattered of in over the root; the larger foramina occur 4. The specimen is importance representing in the intermedian grooves. the stratigraphically oldest species of Plinthicus and — The Plinthicus mobulid described. Differential diagnosis genus was only Oligocene yet until The P. monotypic now. type species, steno- ACKNOWLEDGEMENTS described and don, was by Cope (1869: 116-117), for the first time Hussakof The electron made figured by (1908: 33, scanning micrographs were by and Dr M. den with the fig. 10); a good description figure were given van Boogaard Jeol JSM-840A also by Cappetta (1987: 177-178, fig. 148G-I). SEM unit of the Nationaal Natuurhistorisch The tooth of P. kruibekensis described here Museum, Leiden; his assistance is gratefully resembles those of P. stenodon in general mor- acknowledged. but differs in the phology, following respects: I wish to thankMr M. van den Bosch (Winters- wijk, The Netherlands), Dr H. Cappetta (Mont- Plinthicus kruibekensis Plinthicus stenodon pellier, France), Dr J. Herman (Brussels, Bel- gium), Mr D.C. Hovestadt and Mrs M. Hove- - first in - inwards and crown slopes out- crown slopes stadt-Euler (Terneuzen, The Netherlands) for ward direction and then finally curves upwards items of literature SEM on mobulids, photographs curves inwards of teeth of living Mobula species and constructive - occlusal face flat - occlusal face hollow comments. - anastomosing enameloid - parallel enameloid faces of I thank the editors of this laminae on crown faces of laminae on crown Finally periodical, varying height. equal height. Messrs A.W. Janssen and J.W.M. Jagt, for critically reading the manuscript and correcting There literature the the are no data as to presence or English. absence of dignathic heterodonty in Plinthicus. The REFERENCES possibility that the differences mentioned above of different tooth are partly an expression mor- Bigelow, H.B., & W.C. Schroeder, 1953. Fishes of the North Atlantic. skates in the and lower cannot be rul- western Sawfishes, guitarfishes, phology upper jaws and — Mem. Sears Found. Mar.