Bulletin of the Geological Society of Denmark
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Muscle scars in For cellia (Gastropoda; Pleurotomariacea) from the Carboniferous of England JOHN S. PEEL Peel, J. S.: Muscle scars in Porcellia (Gastropoda; Pleurotomariacea) from the Carboniferous of England. DGF Bull. geol. Soc. Denmark, vol. 35, pp. 53-58, Copenhagen, October, 29th, 1986 Two shell retractor muscles are described on an internal mould of Porcellia woodwardi, a pleurotomaria- cean gastropod from the Carboniferous of England. The scars are located at the junction between the um bilical wall, and the apical surface and the basal surface, respectively. Similar positioning of muscle scars in Bellerophon of the same age reflects morphological convergence of the planispiral, anisostrophic Por cellia with the planispiral, but isostrophic Bellerophon. It is concluded that the shape of muscle scars, in detail, can not contribute to solving the question of the systematic position of Bellerophon. John S. Peel, Grønlands Geologiske Undersøgelse, Øster Voldgade 10. DK-1350 København K, Denmark, January 8th, 1986. The bellerophontiform molluscs are a complex of its single pair of circumbilical muscles and the more than fifty isostrophically coiled genera of single pair of shell-attachment muscles of some uncertain systematic position. While traditionally extant, slit-bearing pleurotomariaceans. Other considered to be gastropods, an extended debate bellerophontiform molluscs have been conside exists in the literature as to whether or not this red to be monoplacophorans after comparison position should be maintained, or if the group between their multiple pairs of muscle scars and should be transferred in part, or as an entity, to the discrete pairs of muscle scars in the living the Monoplacophora. The central theme in this monoplacophoran Neopilina Lemche 1957, or its debate concerns the presence or absence of tor immediate, but ancient relatives Pilina Koken sion. Proponents of a monoplacophoran assign 1925 and Tryblidium Lindstrom 1880 (cf. Lemche ment consider the bellerophontiform molluscs to & Wingstrand 1959; Wingstrand 1985). be untorted, with the mouth anterior and the Muscle scars were first described in Bellero anus posterior. In such a case, the earlier coiled phon by Knight (1947) in specimens of Carbonif portion of the shell would be located anteriorly, erous age from North America, although some over the head. If bellerophontiform molluscs are what similar structures were figured, apparently considered to be gastropods, torsion necessitates without comment, more than twenty years earlier that the originally posterior mantle cavity and its in the Ordovician Sinuites ammonoides Koken contained gills and anus be rotated anteriorly, to 1897 by Koken & Perner (1925, pi. 18, fig. 16). lie over the head. Thus, the earlier coiled portion Knight (1947) also described a pair of muscle of the shell would be posterior (cf. Peel 1984, scars in an Ordovician specimen of Sinuites Ko text-fig. 4). ken 1896, considering the scars to be closely The various arguments cited in the debate are comparable to those in Bellerophon. This opin reviewed by Harper & Rollins (1982) and need ion, however, is not now accepted. Only a single not be repeated here. It is relevant, however, to pair of muscle scars is present in Bellerophon, note the central position in the controversy tradi while Sinuites probably has three pairs of muscle tionally occupied by interpretations of muscula scars in a different configuration (Peel 1980, but ture based on preserved muscle scars. In essence, see also Runnegar 1981). Bellerophon de Montfort 1808 was considered by It is now quite widely appreciated that muscle Knight (1947; 1952) to be torted (hence, a gas scar patterns alone do not necessarily provide tropod) on the basis of parallels drawn between definitive evidence as to monoplacophoran or 54 Peel: Muscle scars gastropod affinities (Harper & Rollins 1982; Peel The possibility for confusion between some 1982; Runnegar 1981). The nature and location porcelliform eotomariids and certain relatives of of muscle scars in Bellerophon are to a large ex Bellerophon, e.g., Megalomphala Ulrich & Sco- tent the result of the isostrophic form of the shell field 1897, is compounded by a tendency among and similarly located muscle scars occur in the some of these porcelliform eotomariids to ap isostrophic ammonites and extant Nautilus (Jor proach bilateral symmetry around the plane of dan 1968). However, a serious obstacle to the coiling, i.e., to approximate the isostrophic form satisfactory evaluation of the significance of mus of the bellerophontiform molluscs. In addition, cle scars in Bellerophon is the rarity of descrip porcelliform eotomariids and many bellerophon- tions of muscle scars in relevant, undoubted, fos tiform molluscs possess a well-developed slit, sil gastropods, notably Palaeozoic pleurotomaria generating a peripheral selenizone. In slit-bear ceans. ing bellerophontiform molluscs, however, coiling The present paper describes a single specimen is usually isostrophic at all stages of growth. In of the pleurotomariacean Porcellia woodwardi porcelliform eotomariids, coiling is usually (Sowerby 1829) from the Carboniferous of the clearly anisostrophic at early growth stages, even United Kingdom in which muscle scars are pre conispiral instead of planispiral. In addition, the served. Insofar as the imperfect preservation al planispiral adult often shows asymmetry about lows, the specimen is compared in terms of mus the plane of coiling, i.e., anisostrophy, in terms cle scars with Bellerophon of similar age, and its of whorl tumidity, the location and size of tu contribution to the question of the systematic po bercles, and other details of ornamentation. sition of Bellerophon is evaluated. In Porcellia itself, the selenizone is at mid- whorl and effectively divides the adult shell into two almost identical halves. However, examin Porcelliform pleurotomariaceans ation of earlier whorls in the loosely coiled shell demonstrates a conispiral juvenile stage, permit In most pleurotomariacean gastropods, the con- ting easy recognition of apical and umbilical sur ispiral shell is coiled into the familiar turbinate or faces. The selenizone is also at mid-whorl in Ko- trochoid form characteristic of gastropods in gen kenella Kittl 1891 (Triassic), but anistrophism is eral. This basic shell form is modified during on more pronounced in Talantodiscus where the se togeny in some members of the pleurotomaria lenizone also lies away from the mid-line of the cean family Eotomariidae Wenz 1938 such that whorl. Knight et al. (1960) considered Raphis- coiling becomes planispiral, i.e., lying within a chisma Knight 1936 (Carboniferous) to be a close single plane perpendicular to the axis of coiling. relative of Porcellia but the closed umbilicus and These planispiral eotomariids form a small group sub-sutural selenizone would appear to deny centred around the genus Porcellia Léveillé 1835 close affinity. (Devonian-Carboniferous, ?Permian). The pla nispiral coiling of the porcelliform eotomariids Porcellia woodwardi (Sowerby 1829) has caused them to be assigned to the Cephalo poda (nautiloids and goniatites) and the bellerop- Porcellia woodwardi differs from most species as hontiform molluscs by early workers, but their signed to the genus Porcellia in lacking tubercles assignment to the pleurotomariacean gastropods along the upper and lower surfaces of the whorl has been secure for more than a hundred years (fig. 1). In standard orientation for a conispiral (de Koninck 1883). In the Treatise on Invertebrate gastropod, with apex upward and aperture facing Paleontology, the porcelliform pleurotomaria the viewer, the aperture of the dextrally coiled ceans were afforded full family status as the fam shell lies to the right (fig. IB). Oriented as a bell ily Porcellidae Broili 1924 (Knight et al. 1960). erophontiform mollusc, with the axis of coiling The close similarity between the porcelliform horizontal, the near bilateral symmetry of the pleurotomariaceans and members of the family whorl is evident (fig. ID). It is equally apparent, Eotomariidae was recognised by Batten (1966), however, that the shell merely approaches bilat who consequently abandoned the Porcellidae as eral symmetry (isostrophy); minor details of a separate family. whorl profile and ornamentation betray ist un- Bulletin of the Geological Society of Denmark, . 35 1986 35 Muscle scars in Porcellia woodwardi A single muscle attachment scar is preserved on the lower whorl surface of the specimen from Longnor, immediately adjacent to the fractured adapertural margin of the internal mould (figs 2C, D, F). The scar lies on the basal surface of the whorl, next to its almost perpendicular junc tion with the umbilical wall. The muscle scar is shallowly crescentic with the concave side of the crescent lying on the adumbilical side. The scar extends over more than one eight of a revolution and its length, measured spirally around the whorl, is more than seven times its width (fig. 2D). The muscle attachment scar lies at the adapical termination of a prominent spiral ridge which lies Fig. 1. Porcellia woodwardi (Sowerby 1829), line drawings re along the baso-umbilical shoulder. This ridge is produced from de Koninck (1883) to show general features of deeply channeled along its convex margin, but this Carboniferous planispiral pleurotomariacean, xl. A, up per surface (apical view); B, apertural view in standard gas finer grooves also occur along its crest (fig. 2D).