Journal of Vertebrate Paleontology 21(3):438–459, September 2001 ᭧ 2001 by the Society of Vertebrate Paleontology A NEW STETHACANTHID CHONDRICHTHYAN FROM THE LOWER CARBONIFEROUS OF BEARSDEN, SCOTLAND M. I. COATES*1 and S. E. K. SEQUEIRA*2 Department of Biology, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom ABSTRACT—Exceptionally complete material of a new stethacanthid chondrichthyan, Akmonistion zangerli, gen. et sp. nov., formerly attributed to the ill-defined genera Cladodus and Stethacanthus, is described from the Manse Burn Formation (Serpukhovian, Lower Carboniferous) of Bearsden, Scotland. Distinctive features of A. zangerli include a neurocranium with broad supraorbital shelves; a short otico-occipital division with persistent fissure and Y-shaped basicranial canal; scalloped jaw margins for 6–7 tooth files along each ramus; a pectoral-level, osteodentinous dorsal spine with an outer layer of acellular bone extending onto a brush-complex of up to 160% of neurocranial length; a heterosquamous condition ranging from minute, button-shaped, flank scales to the extraordinarily long-crowned scales of the brush apex; and a sharply up-turned caudal axis associated with a broad hypochordal lobe. The functional implications of this anatomy are discussed briefly. The rudimentary mineralization of the axial skeleton and small size of the paired fins (relative to most neoselachian proportions) are contrasted with the massive, keel-like, spine and brush complex: Akmonistion zangerli was unsuited for sudden acceleration and sustained high-speed pursuit of prey. Cladistic analysis places Akmonistion and other stethacanthid genera in close relation to the symmoriids. These taxa are located within the basal radiation of the chondrichthyan crowngroup, but more detailed affinities are uncertain. They may represent a plesion series on the holocephalan stem lineage, or a discrete clade branching from the base of the elas- mobranch lineage. INTRODUCTION lished photograph (Wood, 1982:fig. 2) was used to refine a sub- sequent restoration (Zangerl, 1984:fig. 1). However, while Zan- Stethacanthus is one of the most widely known of Paleozoic gerl (1984) referred to HMV8246 as ‘‘cf. Stethacanthus,’’ chondrichthyan genera, mostly because of its unusual spine and Wood (1982) identified the specimen as either S. altonensis St. ‘brush’ complex (Lund, 1974, 1985a; Zangerl, 1981, 1984; Wil- liams, 1985; Coates et al., 1998). Unfortunately, this promi- John and Worthen (1875) or Cladodus neilsoni Traquair (1898), nence among early sharks (sensu lato) is not matched by de- and suggested that these species are synonymous. Neither of tailed knowledge of its skeletal anatomy or a clearly defined the reviews by Williams (1985) or Lund (1985a) refer to the taxonomic diagnosis. The several species of Stethacanthus Bearsden material. Since then, Coates and Sequeira (1998) erected in the late nineteenth century were based upon isolated completed a detailed comparative description of the Bearsden spines (Newberry, 1889), and the first associated skeletal re- stethacanthid neurocranium, and, most recently, histological mains, from the Mississippian of Montana (Lund, 1974, 1985a) analysis of the spine and ‘brush’ complex revealed a remark- and the Devonian and Mississippian of Ohio (Zangerl, 1981; ably well preserved, and so far unique, combination of skeletal Williams, 1985), were undescribed until a century or so later. tissues (Coates et al., 1998). Lund (1974, 1985a) and Williams (1985) compiled the most The hypothesized synonymy of Cladodus neilsoni and the detailed reviews of the genus, and it is noteworthy that Williams Bearsden stethacanthid has now been rejected, following rede- chose to retain many specimens within the type species, Ste- scription of the single specimen of C. neilsoni, NMS (National thacanthus altonensis (St. John and Worthen, 1875), pending Museums of Scotland, Edinburgh) 1911.62.52 (Sequeira and the discovery of additional material to resolve questions of tax- Coates, 2000). Cladodus neilsoni has been removed from Cla- onomic diversity, sexual dimorphism, and sampling bias. There- dodus (a nomen dubium, Chorn and Whetstone, 1978), distin- fore, it is worth emphasizing that in the following systematic guished from HMV8246 on the basis of differences in neuro- description, comparisons with all material described as Stetha- cranial, pectoral fin and branchial arch morphology, and placed canthus altonensis should be treated with caution. Further re- within a new genus, Gutturensis Sequeira and Coates (2000). ports of material attributed to Stethacanthus have extended its Key remaining questions about the Bearsden stethacanthid range to the Mississippian of Oklahoma (Zidek, 1993) and the therefore concern its relationship to the species S. altonensis, Lower Tournaisian of Central Russia (Lebedev, 1996). the genus Stethacanthus, and the Family Stethacanthidae Lund In 1982, Wood announced the discovery of an Upper Car- (1974). boniferous (basal Namurian/Serpukhovian) fish fauna from The specific diagnosis for S. altonensis is rudimentary (Wil- Bearsden, Scotland. His report included a photograph of HM liams, 1985) because the lectotype is an isolated spine, desig- (Hunterian Museum, Glasgow University) V8246 (Fig. 1C), nated by Lund (1974): FMNH (Field Museum of Natural His- one of the most complete Paleozoic chondrichthyan specimens tory, Chicago) UC27404 (St. John and Worthen, 1875:pl. 19, ever discovered and a principal subject of the present article. fig. 1). The generic diagnosis (Williams, 1985) is similarly min- Significantly, this individual resembles Zangerl’s (1981:fig. 81) imal, and refers to no more than spine shape, histology, the reconstruction of S. altonensis, and information from the pub- presence of a brush with specialized apical scales, and the as- sociation of these with a ‘‘medium sized cladodont shark’’ (as- * Current Address: 1, Department of Organismal Biology and Anat- sumed to mean any Paleozoic chondrichthyan with multicusped omy, the University of Chicago, 1027 East 57th Street, Chicago, Illinois teeth with a lingual torus). The family level is defined a little 60637-1508; 2, Department of Biology, Birkbeck College, University more clearly following Zangerl’s (1990) revision, in which the of London, Malet Street, London WC1E 7HX, United Kingdom. Stethacanthidae includes symmoriids with ‘‘neural arch ele- 438 COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 439 ments enhanced in the neck,’’ as well as presence of the dis- radial; dzf, diazonal foramen; ebr, epibranchial; ebrV, fifth epi- tinctive spine and brush. branchial; endf, endolymphatic fossa; fcda, foramen/canal for In practice, the taxonomic level at which these spine mor- dorsal aorta; fehy, foramen for efferent hyoidean artery; fhyp, phologies are diagnostic is uncertain. Spines of this shape are hypophyseal/internal carotid foramen; fl, flange; flda, foramen present in specimens attributed to S. altonensis (Lund, 1974, for lateral dorsal aorta; fm, foramen magnum; foa, foramen for 1985a; Williams, 1985) as well as the Bearsden species. But orbital branch of external carotid; fosn, foramen for occipitos- the Bearsden species differs from S. altonensis in terms of the pinal nerve; fpal, foramen for palatine nerve or branch of or- structure and shape of the spinebrush complex, the arrangement bital artery; grv, groove; gica, groove for internal carotid; of radials in the tail and paired fins, and, most significantly, the gr.opt, optic groove; ha, haemal arch; h.rad, hypochordal ra- gross structure of the neurocranium. Consequently, the Bears- dial; hsp, haemal spine; hy, hyomandibula; inp, internasal den species is easily diagnosed as a new taxon, but are these plate; jc, jugular canal; lof, lateral otic fossa; lop, lateral otic differences sufficient to support the erection of a new genus? process; lor, lateral otic ridge; mc, Meckel’s cartilage; mdr, To an extent, the choice between new species or new genus and median dorsal ridge; mpt, metapterygium; mpt.c, metaptery- species is arbitrary. Analyses of early chondrichthyans are like- gial condyle; mxpt, mixopterygium; na, neural arch; nc, nasal ly to treat S. altonensis and the Bearsden species as distinct capsule; nsp, neural spine; oaf, otic articular fossa; occ, occip- entities irrespective of decisions about taxonomic badging, but ital cotylus; ocr, occipital crest; oof, otico-occipital/metotic fis- it is also likely that such analyses will generate trees in which sure; op, otic process; pbr, pharyngobranchial; pc, perichordal these species emerge as sistergroups. Therefore, inclusion of calcification; p.cor, procoracoid; pct.lv, pectoral level; pep, the Bearsden species within Stethacanthus might be the sim- posterior ethmoid process; plv.lv, pelvic level; pof, preoccipital plest solution, but this would also, necessarily, increase the lev- fossa; pop, postorbital process; p.pl, pelvic plate; ppr, posterior el of polymorphism within the genus. And the end result would process; pq, palatoquadrate; pr, palatine ramus; p.rad, proxi- achieve much the same as the addition of new species to ill- mal radial; pro, preorbital process; psc, posterior semicircular defined genera such as Cladodus or Ctenacanthus. We regard canal; r, calcified rod; rad, radial; sc, scapula; sn.rad, supra- this as inconsistent with the aim of the International Code of neural radial; sp, spine; t.crt, terminal cartilage; trf.jc, foramen Zoological Nomenclature (Ride et al., 1999), which seeks to for trigeminofacialis