Pinnocaris, , Engeser and Riedel, 1996

Peel, John S., 2004, Pinnocaris and the origin of scaphopods, Acta Palaeontologica Polonica 49 (4), pp. 543-550 : 546-547

Pinnocaris and scaphopods

As oriented in the model of scaphopod origins proposed by Runnegar and Pojeta (1974) and Pojeta and Runnegar (1976, 1979), it is evident from the present comparison of ontogenies that the larval shells of Pinnocaris and Antalis entalis have opposite coiling directions ( Fig. 4 ). Pinnocaris is clearly exogastric while the sense of coiling in A. entalis is endogastric, although the tubular adult subsequently becomes exogastric. Thus, the arguments of Morris (1979, 1990) and Engeser and Riedel (1996) concerning the unsuitability of Pinnocaris as an ancestor to scaphopods are supported, but from the more fundamental perspective of shell coiling in the earliest growth stage rather than their analyses of post−larval shell growth vectors.

Morris (1979, 1990) and Engeser and Riedel (1996) sought the origin of scaphopods within the conocardioid rostroconchs (Middle Ordovician to Permian) rather than amongst the Ribeirioidea ( Pojeta and Runnegar 1976). Morris (1990) pointed out similarity in shell form between elongate Carboniferous conocardioids and scaphopods, but noted that this must reflect convergence since the appearance of scaphopods predates these unusually elongate conocardioids. Engeser and Riedel (1996) argued for a close relationship of scaphopods with the conocardiid genera Conocardium Bronn, 1835 (Devonian–Carboniferous) or Arceodomus Pojeta and Runnegar, 1976 (Carboniferous–Permian). They concluded that scaphopods probably arose during the Devonian since suitable ancestral conocardiids are not described from earlier strata, although Yochelson (2002) and Yochelson and Holland (in press) rejected the Devonian record of scaphopods. Thus, Engeser and Riedel (1996) discounted the Ordovician Rhytiodentalium kentuckyensis as a scaphopod on the grounds that they were unable to identify a suitable ancestor within the confines of their hypothesis. Rhytiodentalium was described by Pojeta and Runnegar (1979) from silica replicas from the Lexington Limestone of Kentucky, U.S.A. While the nature of the elongate, slightly curved and slowly expanding tube is obscure, Pojeta and Runnegar (1979) have demonstrated that Rhytiodentalium has both the form and size of many scaphopods.

Engeser and Riedel (1996) assumed that the rostroconchian protoconch was cup−shaped, although this is clearly not the case in Pinnocaris . Neither is it the case in the conocardioid Hippocardia ? of Pojeta and Runnegar (1976: pl. 47: 13–15; see also Runnegar 1978: pl. 1: 2, 3 and Pojeta 1987: fig. 14.70K). The endogastrically coiled protoconch in this Carboniferous specimen overhangs the tubular rostrum which Engeser and Riedel (1996) consider to be homologous to the fumarium in scaphopods. Similar protoconchs are discernible in other Carboniferous conocardioids and have been illustrated most recently by Rogalla and Amler (2003: pl. 4: 7, 8, 13, 14), contrasting with the exogastrically coiled protoconch of Pinnocaris . When considered in connection with the developmental studies of Antalis entalis presented by Wanninger and Haszprunar (2001), it is this underlying coiling pattern of the conocardioid protoconch which provides the strongest support for their thesis that the “ Scaphopoda evolved by ontogenetic predisplacement of a conocardiid adult shell into the larval ontogeny” ( Engeser and Riedel 1996: 125).

Figures

Fig. 4. Comparison of the location and coiling direction of protoconchs in Pinnocaris (clockwise expansion of the shell) and scaphopods (anti−clockwise expansion). Straight black arrows indicate growth vectors. Note that the overall coiling pattern of the adult scaphopod is exogastric.