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 nerve and jugular canal; trf/os, anterior establish and promote taxonomic stability. An alternative, and part of trigeminofacialis opening/possible eyestalk insertion preferred, course of action is to erect a new genus for the Bears- site; vmr, ventrolateral mandibular ridge; von, ventral otic den species, and take advantage of the unusually detailed con- notch. dition of the specimens to introduce a more precisely defined taxon to the existing list of early chondrichthyan genera. Pos- SYSTEMATIC PALEONTOLOGY sible synonymy with Stethacanthus can be tested elsewhere, Class CHONDRICHTHYES Huxley, 1880 following revision of material referred to S. altonensis and/or Order SYMMORIDA Zangerl, 1981 the discovery of new specimens, but this is beyond the scope (sensu Zangerl, 1990) of the current work. Family STETHACANTHIDAE Lund, 1974 (sensu Zangerl, 1990) METHODS Genus AKMONISTION, gen. nov. Bearsden material was prepared using an S. S. White indus- Remarks For the purposes of the present work, the Stetha- trial airbrasive unit, with sodium bicarbonate as the abrasive canthidae will be treated as monophyletic, despite indications agent mixed minimally with a low-pressure airstream of less from recent analyses that this taxon may be paraphyletic rela- than 1.0 kg/cm2. Fine preparation was completed using mounted tive to the Holocephali (Coates and Sequeira, 2000). tungsten carbide needles. Fragile specimens were reinforced Type Species Akmonistion zangerli, sp. nov. with a thin coating of methacrylate resin consolidant dissolved Generic Diagnosis In the context of relevant hypotheses of in acetone. Enhanced contrast photography of HM V8246, com- early chondrichthyan interrelationships, Akmonistion may be di- pleted by Dr. Keith Ingham of the Hunterian Museum, Univer- agnosed as follows (sources: Schaeffer, 1981; Zangerl, 1981, sity of Glasgow, was achieved by immersing the specimen in 1990; Young, 1982; Maisey, 1984; Lund, 1985a, b, 1986; Wil- methanol. All specimen drawings were completed using Zeiss liams, 1985; Gaudin, 1991; Janvier, 1996; Coates and Sequeira, Stemi SV6 and Wilde M10 binocular microscopes with camera 1998, 2001). lucida attachment. Symplesiomorphies at various levels: persistent otico-occip- ital fissure; endolymphatic fossa divided from fissure by pos- ABBREVIATIONS terior tectum; jaws amphistylic; palatoquadrate otic process Institutional AMNH, American Museum of Natural His- with simple, convex, posterodorsal rim; hyoid arch articulates tory, New York; CMNH, Cleveland Museum of Natural His- with lateral otic process; pectoral fin metapterygial plate artic- tory, Cleveland, Ohio, USA; FMNH, Field Museum of Natural ulates with 5ϩ distal radials; paired and median fins plesodic; History, Chicago; U.S.A.; HM, Hunterian Museum, Glasgow mandibular teeth with up to three cusplet pairs flanking primary University, Glasgow; MCZ, Museum of Comparative Zoology, cusp. Harvard University, Cambridge, Massachusetts, U.S.A.; Possible synapomorphies with symmoriids and stethacan- NMMNH, New Mexico Museum of Natural History, Albu- thids: narrow suborbital shelf; hyomandibula short and sub- querque; NMS, National Museums of Scotland, Edinburgh; crescentic; enamel and orthodentine discontinuous between UMZC, University Museum of Zoology, Cambridge Univer- neighbouring tooth cusps; oropharynx lined with single and sity, Cambridge, England. compound buccopharyngeal denticles; scales single-crowned, Anatomical ac.cart, accessory cartilage; aep, anterior eth- non-growing, and restricted to particular body regions; single, moid process; a.h.rad, anterior hypochordal radial; apr, ante- anacanthous, radial-supported dorsal fin at pelvic level, with rior process; art.c, articular crest; artp, articular facet for pal- delta-shaped cartilage; pectoral fin with trailing ‘whip’; caudal atoquadrate; ax.crt, axial cartilage; ax.rad, axial radial; bbr, axis steeply upturned; hypochordal lobe supported by elongate, basibranchial; bp, basal plate; br, brush; cbr, ceratobranchial; distally splayed, radials. chy, ceratohyal; cor, coracoid; cr, cartilaginous branchial ray; Synapomorphies with stethacanthids: basicranium encloses d.crt, delta cartilage; dlof, dorsolateral otic fossa; d.rad, distal Y-shaped canal enclosing division of dorsal into lateral aortae; 440 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001 whorls of fused, 3-cusp, tooth units occur close to level of jaw The lithology of the formation indicates that it was deposited articulation; pectoral level subtriangular osteodentinous spine under variable conditions of salinity with seasonal periodicity. with distinct anterior shoulder seated on globular calcified car- All of the Bearsden chondrichthyans except for Tristychius de- tilage baseplate; baseplate continuous with brush consisting of rive from the finely laminated marine shales of the Shrimp elongate calcified rods; lateral line scales C-shaped; large, cra- Member, now known from several other localities in the west- nial cap and brush apex scales include morphologies referrable ern Midland Valley of Scotland (Clark, 1989). The Shrimp to Lambdodus hamulus and Cladodus pattersoni. Member bears evidence (enterospirae 87Sr/86Sr ratios; trace el- Autapomorphies of genus and species: neurocranium with ement and rare earth analyses: Clark, 1989) of a sequentially broad supraorbital shelf; jaw margins scalloped with 6–7 re- marine and non-marine environment, subject to seasonal fluc- cesses for tooth files along each ramus; teeth on upper and tuations. lower jaws aligned in precise, crown-to-crown, opposition with Diagnosis As for genus. no dental interdigitation; level of jaw articulation ventral to Etymology The species is named ‘‘zangerli’’ after Rainer tooth row level; vertebral column of 90ϩ precaudal neural arch- Zangerl, in recognition of his major contributions to pa- es includes ca. 14 subrectangular cervical arches and ca. 9 laeoichthyology. small, trapezoidal, pre-caudal arches; spine histology includes outer layer of acellular bone; bone layer extends onto leading DESCRIPTION edges of baseplate and brush; male brush length reaches around 160% of neurocranium length; scales with longest crowns flank Neurocranium The neurocranium has been described in posterolateral third of brush apical platform; pectoral whip in- detail elsewhere (Coates and Sequeira, 1998); for a diagram- cludes 22ϩ axial cartilages; caudal neural and supraneural matic summary/reconstruction see Figure 2. Key features in- spines extended distally and leaf-shaped. clude: an otico-occipital region of about the same length as the Other characters of uncertain polarity: neurocranium with sphenethmoid division; short but distinct lateral otic processes; short otico-occipital division; hyoid and gill arches with elon- a short, broad, endolymphatic fossa; a persistent otico-occipital gate cartilaginous rays; male pelvic claspers with distal com- fissure; Y-shaped basicranial canals indicating origin of the lat- ponents consisting of non-prismatic calcified cartilage; fifth epi- eral aortae anterior to occipital level; very broad supraorbital branchial antero-posteriorly broader than those of first to fourth shelves; and minimal suborbital shelves limited to small, eth- gill arches. moidally located outgrowths. The dorsal aspect differs from Etymology From the ancient Greek ‘akmon’ for anvil, and those of Stethacanthus altonensis (Lund, 1974, 1985a) and cf. ‘istion’ for sail; these terms refer to the shape and location of S. productus (Lund, 1985a) in the following details: the post- the spine-brush complex. orbital processes extend out further from the main body of the neurocranium; the supraorbital shelves are far more extensive; AKMONISTION ZANGERLI, sp. nov. the endolymphatic fossa is laterally broad and well defined; the (Figs. 1–4, 5A–C, F–J, 6–16) span of the olfactory capsules is narrower. Mandibular Arch The mandibular arch is preserved in 3 Holotype HMV8246 (Figs. 1B, C, 3A, 6, 7B, 8A, 9A, 10A, specimens: HM V8246, UCMZ GN1047 and NMS 1981. 11B, 12G, 14), a skeletally complete male specimen, approxi- 63.23A (Fig. 3). In each case the left palatoquadrate and meck- mately 620 mm long, preserved in lateral view; used as the elian cartilage are flattened and exposed in lateral view. In source of body proportions in the reconstruction of Stethacan- HMV8246 and UCMZ GN1047 parts of the right side of the thus altonensis presented in Zangerl (1984). mandibular arch are exposed, but insufficiently well to add sig- Referred Specimens NMS 1981.63.22C, NMS 1981.63.23A, nificantly to morphological description. and UMZC GN1047, the second most complete skeleton (Fig. The palatoquadrate resembles those of other stethacanthids 1A). (Lund, 1985a, b, 1986a; Williams, 1985), Cobelodus (Zangerl Locality and Horizon Specimens of Akmonistion zangerli, and Case, 1976), xenacanths, and living notidanids (Hotton, sp. nov. were collected by S. P. Wood (Wood, 1982) from the 1952), plus other amphistylic Paleozoic chondrichthyans. The Manse Burn Formation of the Bearsden locality, near Glasgow, expanded posterior of the palatoquadrate, the otic process (Fig. Scotland, during the summers of 1981 and 1982. Assistance 3, op), is of about the same length as the suborbital palatine was provided by the Hunterian Museum, University of Glas- ramus (Fig. 3, pr). The ventral edge of the palatoquadrate is gow, and the Nature Conservancy Council. The type locality is gently sigmoid in profile: concave below the otic process and the Manse Burn, Ordnance Survey Grid reference NS convex below the palatine ramus. The posterodorsal edge of the 529427329-NS 53057325 (Clark, 1989). The Manse Burn For- otic process is strongly convex, somewhat thickened, and forms mation lies within the Pendleian (Serpukhovian) E1 Zone of the a distinct lip which appears to have been folded ventrally post Lower Carboniferous, based on spore, conodont and goniatite mortem (NMS 1981.63.23A). There is no short, vertical process analysis. The Manse Burn Formation consists of shales from posterior to the quadrate region as in Stethacanthus altonensis the Top Hosie Limestone Marine Band to the base of the first (Lund, 1985a) and Orestiacanthus (Lund, 1984), or elevated thick sandstone (Clark, 1989); this complex is further divided facet or crest, cf. Tamiobatis vetustus (Williams, 1998). into six members based on their lithology. There is an approx- The otic articular fossa (Fig. 3, oaf) is restricted to the lateral imate correspondence between the Shrimp Member, Posidonia surface, and resembles the V-shaped articular fossa of Stetha- Member, Nodular Shale Member, Platey Shale Member, Be- canthus altonensis (Williams, 1985:118), although there are sig- twixt Member, and Lingula Member, as listed by Clark, and nificant differences. In the Bearsden species this fossa is an- those beds listed by Wood (1982). teroposteriorly narrower, and the anterior rim is continuous with In addition to Akmonistion, the chondrichthyan component of the anterodorsal angle of the otic process. In S. altonensis the the Bearsden fauna includes Denaea, Tristychius arcuatus posterior edge of the fossa extends to the anterodorsal angle, (from the non-marine shale beds), Deltoptychius (Dick et al., and the leading fossa edge terminates in front of the main body 1986) and an isolated head attributed (doubtfully in our view) of the otic process, creating a distinct step in the otic process to Symmorium. Other vertebrates from this fauna include at outline, as shown in Williams’ (1985) and Lund’s (1985a) de- least eight actinopterygian species, rhizodont and osteolepid scriptions. Other Paleozoic chondrichthyan palatoquadrates fragments, a coelacanth skull, and Acanthodes specimens showing a similar, laterally directed surface for articulation with (Coates, 1988, 1993, 1998). the postorbital process include cladoselachians (Maisey, 1989a; COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 441

FIGURE 1. Akmonistion zangerli, gen. et sp. nov. A, UCMZ GN1047, drawing of complete specimen; B, HMV8246, drawing of complete specimen; C, HMV8246, photographed in methanol (courtesy of Dr. Keith Ingham, Hunterian Museum, University of Glasgow). Scale bars equal 40 mm. 442 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

FIGURE 2. Akmonistion zangerli, reconstruction of neurocranium from Coates and Sequeira (1998), in A, ventral, B, dorsal, C, lateral, and D, posterior, aspects (reproduced with permission of the Royal So- ciety of Edinburgh). Scale bar equals 10 mm.

pers. obs. CMNH 8207), Cobelodus (Zangerl and Case, 1976), and Symmorium and Denaea (Williams, 1985). The quadrate articular surface is not well preserved. The best example (from UCMZ GN1047) includes an outer, rounded, rim bordering a transverse slot in the quadrate base. This feature is not attributed easily to the presence of a condyle, and it may reflect the simpler (and perhaps specialized) mandibular hinge joint which Maisey (1989a) described in a cladoselachian (al- though Williams (pers. comm.) interprets this as a preserva- tional artifact). The palatine ramus is proportioned similarly to that of Ste- thacanthus cf. S. altonensis (Lund, 1985a), and is much nar- rower, dorsoventrally, than those of ctenacanths (Williams, 1998) and xenacanths (Hotton, 1952). The anterior end is ex- panded dorsally (cf. Cobelodus, Zangerl and Case, 1976) to produce a flange (Fig. 3A, fl) which is assumed to have artic- ulated with the ethmoid processes of the neurocranial internasal FIGURE 3. Akmonistion zangerli, visceral skeleton. A, HMV8246, plate (Fig. 2, inp) (Coates and Sequeira, 1998). The labial (lat- mandibular, hyoid, and branchial arches; B, UCMZ GN1047, mandib- eral) surface close to the ventral margin bears a series of at ular, hyoid, and fragmented branchial arches; C, NMS 1981.63.23A, least six, near-hemispherical, humps. Corresponding concavities mandibular and hyoid arches. Scale bars equal 20 mm. (‘‘scalloping,’’ Maisey, 1989a) on the lingual surface accom- modated individual tooth files (families). Below each bulge the ventral palatoquadrate edge is reinforced, forming a short shelf, divided from neighbouring shelves by shallow, acute, grooves. towards the point of jaw articulation. This contrasts with other These shelves probably functioned as tooth platforms; so far, Stethacanthus species (Lund, 1985a), cladoselachians (Maisey, no equivalent shelves have been identified in comparable spe- 1989a), and Cobelodus (Zangerl and Case, 1976), in which the cies. Tooth file positions are arranged so that the fifth or sixth biting margin is either level with or slightly below the mandib- (from anterior) lies below the otic articular fossa. The posteri- ular articulation. Other Meckel’s cartilages which possess a ormost shelf is the shortest; approximately 1/3 to 1/2 the an- similar angle between the biting margin and the rim flanking teroposterior length of the largest. Each shelf is in precise 1:1 the zone of adductor muscle insertion include those of Damo- register with an equivalent shelf on the mandibular dorsal edge. cles (Lund, 1986) and Falcatus (Lund, 1985b, where this angle This contrasts with conditions in taxa such as hybodontids is described as a ‘‘coronoid process’’). (Maisey, 1983:fig. 18), where the off-set arrangements of teeth A prominent rod-like ridge parallels the ventral mandibular in upper and lower jaws result in dental interdigitation when edge in each of the three known examples of Meckel’s cartilage the jaws are closed. (Fig. 3, vmr). Comparison with three-dimensionally preserved As in the palatoquadrate (HM V8246), only the lateral sur- material (cf. Maisey, 1989a:fig. 2; Orthacanthus casts and orig- face of Meckel’s cartilage is exposed. This, too, bears a series inal material, pers. obs.) shows that this ridge forms the ven- of six or seven humps and platforms associated with tooth file trolateral mandibular angle. Ridge thickness confers ridgidity positions (uncertainty over tooth file number results from un- to the entire jaw, and the lateral prominence of this structure certainty about presence of a shelf/platform at the anteriormost probably provided a good insertion area for adductor muscu- end of the mandible). Posterior to the tooth-bearing margin, lature. The flange ventral to this ridge is assumed to have been which occupies no more than two thirds of the mandibular dor- directed medially in life. There is no retroarticular process (cf. sal edge, the upper rim of Meckel’s cartilage slopes ventrally Tristychius, Dick, 1978; Orthacanthus, Hotton, 1952) or flange COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 443 for the ceratohyal (cf. Falcatus, Lund, 1985b). Like the pala- (Lund, 1985a, b), of which Falcatus shows similarly shaped toquadrate, the articular region is insufficiently well preserved pharyngobranchials. These slender cartilages are quite unlike to identify articular surfaces. the more substantial pharyngobranchials of Tristychius (Dick, There are no traces of labial cartilages. Such cartilages are 1978), Hybodus, and recent sharks such as Heterodontus (Mais- present in extant holocephalans and neoselachians, and, given ey, 1983:fig. 9). Conditions in Acanthodes (Miles, 1973; Gar- the size and quality of preservation of HM V8246, it seems diner, 1984) indicate that slender, posteriorly directed pharyn- likely that evidence of such structures would have been pre- gobranchials are plesiomorphic. served (cf. Tristychius, Dick, 1978; Hybodus, Maisey, 1982, The anteriormost four epibranchials are simple rods, but the 1983). fifth has a broader, more complex outline (Fig. 3A, ebrV). The Hyoid Arch and Gill Arches All three specimens include distinct morphology of the fifth gill arch resembles the pattern the left hyoid arch, but only UCMZ GN1047 preserves those of Tristychius (Dick, 1978), in which the fifth ceratobranchial of both sides (Fig. 3). Each hyomandibula (epihyal of Zangerl is broader than the preceding four; similarly distinct fifth arches and Case, 1976; Maisey, 1989a) is flattened laterally, moder- are present in many Recent sharks. Ceratobranchial 5 of Ak- ately arched and expanded anterodorsally (Fig. 3, hy). The de- monistion is insufficiently well known for evidence of special- gree of curvature parallels that of the posterodorsal edge of the isation comparable to the epibranchial condition. palatoquadrate otic process; the overall shape is a half-crescent, The hyoid and gill arches bear elongate branchial rays, the and unlike the more linear epibranchials. When articulated with extent of which is displayed most clearly in HM V8246 (Fig. the neurocranium, it appears that little of the hyomandibula 3A, cr). As noted by Allis (1923) and others, such elongate would have projected behind the palatoquadrate otic process rays support septa extending beyond the lateralmost limit of (cf. HM V8246). A shallow groove runs parallel to the poster- primary gill lamellae. It is noteworthy that while these slender odorsal edge of the hyomandibula; a similar groove may be and delicate gill rays are well preserved, there are no traces of present in cladoselachians, but the significance of this feature extrabranchial cartilages whatsoever. The distribution and is unclear. In Stethacanthus the hyomandibula is around half length of these rays matches those of cladoselachians (Maisey, the length of the ceratohyal. This resembles the cladoselachian 1989a) and Chlamydoselachus (Allis, 1923), but are quite un- condition (Maisey, 1989a), but it contrasts with Cobelodus like Tristychius in which a dense, elongate array is associated (Zangerl and Case, 1976) in which the hyomandibula is pro- with the hyoid arch while only short branchial rays extend from portionately longer. There is no evidence of a separate (or the gill arches (Dick, 1978). fused) pharyngohyal (cf. discussions in Hotton, 1952; Zangerl Mandibular and Pharyngeal Dentition Scalloping and and Case, 1976; Maisey, 1984, 1989a). associated tooth platforms along the margins of the palatoquad- The hyomandibula-ceratohyal articulation lay mesial and rate and Meckel’s cartilage indicate the presence of at least six, slightly posterior to the articulation between palatoquadrate and and probably seven, well spaced tooth files per jaw ramus. In- meckelian cartilages. As Dick (1978) comments on Tristychius, dividual teeth resemble those of Gutturensis neilsoni (Sequeira the entire articulation was probably padded with a thick con- and Coates, 2000), as well as teeth attributed to Stethacanthus nective tissue layer also serving as the origin for suspensory (Williams, 1985; Zidek, 1993). Williams (1985) identified such ligaments to the mandibular arch. The form of the hyoid artic- teeth as corresponding to the form taxon Cladodus exilis St. ulation is better preserved than that of the mandibular arch. John and Worthen (1875). Most have a large central cusp From HM V8246 it is fairly clear that it consists of a simple flanked by two pairs of lateral cusplets (terminology after Cap- convex surface on the distal end of the hyomandibula and a petta, 1987) (Fig. 4). Each central cusp is recurved lingually correspondingly concave ceratohyal proximal surface. There is and slightly sigmoid in lateral aspect. Smaller teeth may have no interhyal; the possible interhyal identified in a cladoselachi- only a single cusplet pair (Fig. 4C), while some of the largest an specimen (Maisey, 1989a) appears to be the broken proximal teeth have a third pair adjacent to the the central cusp base (Fig. end of the ceratohyal. The ceratohyal in Akmonistion zangerli 4F). Marginal cusplets are taller than proximal cusplets, and are (Fig. 3, chy) bears a well marked fossa in the lateral surface, between one quarter and one fifth the height of the central cusp just below the articulation with the hyomandibula. Otherwise (unlike Ohio and Oklahoma material and C. exilis, in which the elongate ceratohyal is a slender, curved rod of near uniform marginal cusplets are half the height of the central cusp). All thickness throughout its length. cusps bear fine, closely packed, cristae (unlike the more widely A subtriangular cartilage lies anterior to the ceratohyal and spaced cristae of the Oklahoma teeth), and a more pronounced medial to the two halves of the lower jaw in HM V8246. This cutting edge divides labial from lingual surfaces. Where the cartilage is probably the anteriormost (first) basibranchial (Fig. central cusp broadens towards the base, further cristae interca- 3A, bbr). A similarly shaped and located first basibranchial late between those extending from cusp base to apex (also un- occurs in the Bear Gulch Stethacanthus material (Lund, 1985b: like Ohio material and C. exilis, Williams, 1985). Cusplet cris- fig. 6A). Thus far, no calcified basibranchials are known from tae are fewer and more widely spaced than those of the central cladoselachians, although a series of irregularly shaped basi- cusp, and at their bases they display a degree of twist, the ro- branchials is associated with gill arches in Cobelodus (Zangerl tation of which is mirrored on either side of the central cusp. and Case, 1976) and a variety are associated with the partial Cusps consist of orthodentine, coated with what appears to branchial skeletons of specimens attributed to Phoebodus (Wil- be a thin enameloid monolayer. Lund (1985b) reported the pres- liams, 1985; identification challenged by Ginter, 1998, who di- ence of an enameloid layer on the Bear Gulch Stethacanthus agnoses Phoebodus as only a tooth-form taxon). teeth, and that the fluted profile of the external (enameloid) There appear to be five gill arches present, the least disturbed surface is not reflected at the subjacent enameloid-orthodentine series of which is displayed in HM V8246 (Fig. 3A). Preserved junction. This description matches conditions revealed by bro- parts of the arches consist mostly of elongate, slender epibran- ken cusp surfaces in Akmonistion. It is also noteworthy that in chials (Fig. 3, ebr) and slightly longer, curved, ceratobranchials Akmonistion, each cusp is histologically separate from its neigh- (Fig. 3, cbr). No hypobranchials have been identified. The three bor. Cusp enameloid and orthodentine in the largest teeth is anteriormost epibranchials in HM V8246 each articulate with a separated from the root by a short neck region, consisting of slender, posteriorly directed, pharyngobranchial (Fig. 3, pbr). osteodentine/acellular bone identical to that of the root. In general, the condition of these arches resembles that of cla- Tooth roots (bases) are sub-elliptical in basal and apical doselachians (Maisey, 1989a). Close comparison may also be views, and almost biconvex in labial view, with cusps posi- made with gill arch skeletons of the Bear Gulch stethacanthids tioned along the labial margin. Lateral margins are reflexed dor- 444 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

cally youngest, member of the series. The fused roots of these units form a tightly curled base which turns through almost 360 degrees, so that the largest tooth is beginning to over-grow the smallest. Comparable tooth whorls attributed to Stethacanthus are described from Ohio (Williams, 1985:pl. 9, fig. 6) and Oklahoma (Zidek, 1993:fig. 1B) (Fig. 5D, E). Both of these are interpreted as symphysial (cf. edestid chondrichthyans, Zangerl, 1981), consist of up to five monocuspid teeth, and show no appreciable changes in tooth size. In fact, none of these whorls is preserved in near-articulation with a supporting skeletal structure, and their orientation in life is unclear. The whorls in UCMZ GN1047 are preserved below the rear half of the lower jaw, and it seems equally likely that they may have been located at the front of the gill skeleton, as may be the case for the smaller whorls known in Falcatus (Lund, 1985a; Zidek, 1993). The buccopharyngeal region in HM V8246 and UCMZ GN1047 includes a dense mat of buccopharyngeal denticles (Fig. 5F–J). These seem to have lined the entire mouth and pharynx, extending back from the mandibular margin to the pharyngoesophageal boundary. However, within this region there is no evidence of any more precisely regionalized pattern of the kinds documented in neoselachians (Nelson, 1970). Nei- ther is there any evidence indicating that different varieties of compound denticle predominated in particular regions of the oropharynx. Buccopharyngeal denticle shapes are varied, and range from single-cusped cones to single-rowed (Fig. 5H, ‘‘Stemmatias simplex,’’ Williams, 1985), and, more rarely, double-rowed (Fig. 5I, ‘‘S. bicristus,’’ Williams, 1985), compound forms. From external appearance, the histology of these denticles is identical to that of the mandibular teeth. Each cusp is slightly recurved, bears cristae and lateral carinae, is closely appressed to its neighbor, and, if not solitary, is a member of a size-graded series. The largest may be more than triple the linear dimen- sions of the smallest, and the root and cusp of a larger member FIGURE 4. Akmonistion zangerli, UCMZ GN1047, mandibular teeth, is recessed to partly envelop the base of a preceding, smaller, all drawn to the same scale (note size difference between specimens E member. From the variety of forms preserved, there is every and C). A, labio-basal view; B, labial view; C and D in profile; E and reason to believe that, unlike the squamation, this buccopha- F in labial view; G in lingual view. Scale bar equals 2 mm. ryngeal mat consisted of growing denticles; i.e., they were not formed in a single morphogenetic event (Reif, 1985). Instead, newer, larger, cusps were added to each compound denticle throughout its lifetime. sally beyond the marginal cusps, and the labial margin projects The root of each denticle is pierced by a row of marginal aborally as a near-rectangular tongue beneath the central cusp. foramina adjacent to the cusp base. This feature seems to be Unlike the Ohio Stethacanthus (Williams, 1985), in which this absent from the otherwise similar denticles from the Oklahoma tongue extends to the level of proximal cusplets, tongue width Stethacanthus (Zidek, 1993:fig. 2). The maximum number of is equal to or less than central cusp width. The labial surface denticle units in any single compound series is around eight. is otherwise marked by a curved row of marginal foramina Vertebral Column Almost the entire vertebral column is bordering the central cusp base. The convex orolingual surface preserved in HM V8246 (Fig. 6). Preservation of this quality bears a central protuberance (button) of about the same width is exceptional for a Paleozoic chondrichthyan of this size. Su- as the central cusp (Fig. 4G), from which it is divided by a perficially, the column resembles that of Cobelodus. The no- trough. When teeth are articulated in-file, this trough accom- tochord is unconstricted, and while there are traces of peri- modates the tongue of the following tooth. Numerous small chordal calcification, these are confined to the tail. Most of the foramina pierce the orolingual surface, predominantly around 90ϩ pre-caudal neural arches (basidorsals or neurapophyses) the margins and cusp bases, and three, larger, vascular canals consist of slender rods of prismatic calcified cartilage. Left and enter the lingual face of the aforementioned protuberence. The right sides of each arch are fused dorsally in at least the anterior aboral root surface is concave and mostly featureless, except third to half of the vertebral series. All arches are preserved in for the rectangular tongue projection and a thickened, raised, close association with anterior and posterior neighbours, but lingual rim (Fig. 4A). As such, the root is anaulacorhizous. there are no signs of zygapophyseal surfaces or other intercon- Small foramina are distributed mostly adjacent to the lingual nections. As in Cobelodus (Zangerl and Case, 1976), the ver- rim, while three or four larger foramina lie central to the shal- tebral series can be subdivided into approximately four regions: low concavity. cervical (occiput to pectoral level), thoracic (pectoral to pelvic Three examples of tooth whorls with fused roots are present levels), peduncular (pelvic to caudal), and caudal (included on UCMZ GN1047 (Fig. 5A–C). Each subunit of a whorl re- within the tail description). sembles a three-cusped tooth (a central cusp flanked on each The cervical region extends from vertebra 1 to about 14 (Fig. side by a cusplet). The most complete specimen (Fig. 5B) con- 6, pct.lv marks cervical–thoracic boundary), and compares sists of at least seven tooth units, the smallest and earliest of more closely to the 15 cervical arches in Cobelodus (Zangerl which is less than half the size of the largest, and ontogeneti- and Case, 1976) than to the 9 or 10 in Falcatus (Lund, 1985b) COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 445

FIGURE 5. Tooth whorls and buccopharyngeal denticles. A–C, Akmonistion zangerli UCMZ GN1047, tooth whorls with fused bases: D and E attributed to Stethacanthus; D, redrawn from Zidek, 1993:fig. 1B, and E, from Williams, 1985:pl. 9, fig. 6. F–I, Akmonistion zangerli, UCMZ GN1047, buccopharyngeal denticles: all match form-taxon Stemmatias simplex except specimen I, matching S. bicristus; J, Akmonistion zangerli, NMS 1981.63.23A, buccopharyngeal denticle. Scale bars equal 1 mm.

or the 6 or 7 in Damocles (Lund, 1986). All cervical neural accommodates the ventralmost extent of the dorsal spine base- arches are subrectangular in lateral view; each is anteroposte- plate (Fig. 15B; cf. Falcatus, Lund, 1985b:fig. 6B; Tristychius, riorly broad and, like the rest of the vertebrae, sloped posteri- Dick, 1978:text-fig. 26). orly. Likewise, all cervical arches have a large foramen which The thoracic region is the most uniform of the vertebral col- probably enclosed one of the segmental nerve roots. Lund umn, and includes neural arches from around vertebra 15 to (1985b) suggests that the anteroposterior breadth of cervical about 55 (Fig. 6, plv.lv marks thoracic–penduncular transition). arches is caused by incorporation of dorsal intercalaries or ar- All are anteroposteriorly narrow, closely packed, have a short, cualia, but conditions in the new material fail to shed further posteriorly directed spine, and most exhibit a foramen for a light on this hypothesis. Each cervical neural arch apex is segmental nerve root. The thoracic series in HM V8246 curves drawn into a short prong; in arches 1–6 this is directed anteri- towards the posteroventral edge of the spine-brush complex and orly, and in 7ϩ this is directed posteriorly. A gradual depression the proximal radials (basals) of the pelvic-level dorsal fin. This in total arch height flanking this switch in prong orientation curvature probably results from post mortem distortion follow-

FIGURE 6. Akmonistion zangerli, HMV8246. Complete, pre-caudal, vertebral column; anterior to left of figure. Scale bar equals 20 mm. 446 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

FIGURE 7. Akmonistion zangerli, spine and brush complexes. A, UCMZ GN1047; B, HMV8246; C, reconstruction, note that long-crowned scales are oriented laterally (see Fig. 15A); anterior to left of figure. Scale bars equal 30 mm.

ing contraction of ligaments and muscles anchoring dorsal out- Spine-Brush Complex and Dorsal Fin The Bearsden ma- growths. terial includes several examples of the so-called ‘‘spine-brush’’ The peduncular region is slightly longer than the thoracic, (Zangerl, 1981) complex (Fig. 7). In HM V8246 the entire and extends from vertebra 56 to around 89 or 90, the origin of structure appears to have moved very little relative to the axial the tail (Fig. 6, cau.lv). Such proportions are closer to Clado- skeleton post mortem (Fig. 1B, C). The spine and basal plate selache (after Zangerl, 1981) than Cobelodus or Symmorium, are situated above the branchial region, while the brush projects in which the (reconstructed) peduncle is shorter than the tho- above the pectoral girdle. racic region. In comparison with thoracic arches, those of the Coates et al. (1998) analyzed the histology of the spine, peduncle are inclined more posteriorly and diminish in size to- brush, and basal plate, and reached the following conclusions: wards the root of the tail. Around five of the posteriormost (1), the spine (Fig. 7, sp) consists of osteonal dentine surround- segments of the penduncle bear haemal arches (perhaps includ- ed by acellular bone and lacks any enamel-like surface tissue; ing basiventrals and haemapophyses). Each of these is pre- (2), the brush (Fig. 7, br) and basal plate (Fig. 7, bp) consist served as a slender cartilage rod, but the distal parts of the of non-prismatic globular calcified cartilage, the peripheral re- anteriormost four are missing or damaged. It is therefore un- gions of which include a meshwork of crystal fibre bundles; known if these formed a fused arch, and/or extended to artic- and (3), a thin, acellular bone layer of variable thickness coats ulate with the expanded, anteriormost radial of the hypochordal the leading edge and base of the brush, and edges of the basal lobe of the tail (Fig. 6, a.h.rad). plate. These results complement and extend upon previous in- COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 447 terpretations of spine-brush histology (Zangerl, 1984; Williams, 1985). The spine of A. zangerli resembles those attributed to S. al- tonensis (cf. Williams, 1985; Lund, 1984), and, to a lesser ex- tent, S. productus (cf. Lund, 1984). All specimens of the spine of A. zangerli are crushed laterally. The overall form is of a right-angled triangle with a concave hypotenuse facing anter- odorsally; the external surface is completely unornamented, and there is a prominent ‘‘horizontal shoulder’’ (cf. Lund, 1974:168; Williams, 1985:121). Just below the spine apex, a slight ridge passes from the anterior surface in a posteroventral direction across the lateral surface towards the rear edge. The posterior edge, most easily seen in UCMZ GN 1047 (Fig. 7A) is rounded, and there is no evidence of a vertical median ridge flanked by sulci, cf. S. altonensis (Williams, 1985; Lund, 1984:fig. 1). Un- like certain examples attributed to S. altonensis (Williams, 1985), no small denticles are associated with the anterior sur- face of the spine. Horizontal sections through a spine (NMS 1981.63.24) show that the gross morphology of the internal cavity and walls resembles that of a specimen attributed to S. FIGURE 8. Akmonistion zangerli, dorsal fins. A, HMV8246; B, depressus, with no indication of ontogenetic development from UCMZ GN1047; anterior to left of figure. Scale bars equal 20 mm. separate shaft and mantle regions (Zangerl, 1984). An arc of large canals, assumed to have enclosed a nutritive vascular sup- ply, lies about half way between inner and outer surfaces of the spine wall. bles the female. In Akmonistion, scale bases are packed closely The topographic relation of the spine to the cartilaginous bas- together, forming a continuous mosaic of rows aligned diago- al plate in A. zangerli is as described for S. altonensis (Wil- nally relative to the anteroposterior midline (reconstruction liams, 1985). However, as reported by Coates et al. (1998), the shown in Fig. 15A). Anteriorly, and at the posterior extremity acellular bone of the spine is continuous with that of the keel- of the platform, these rows are narrow and the scales small. shaped basal plate, where it forms an anterior carina, and the The largest scales are situated laterally, along the posterior third globular calcified cartilage of the basal plate extends into the of the platform edge. Counting straight across, from platform cavity of the spine. Furthermore, although the basal plate and side to side, there may be as many as nine (small) scales an- spine are separated from the base of the brush complex in all teriorly, and as few as four posteriorly. Scale morphologies are Bearsden specimens, in each case this division lies in a different described in the squamation section. position and results from post mortem breakage. In life, the Finally, in both well preserved examples of the brush com- acellular bone of the basal plate and spine was continuous with plex, a slender rod of what appears to be acellular bone lies that of the brush, and these three units must have formed a within or projects from the anterior third of the brush apex (Fig. single, rigid, unit. There is no evidence of the condylar artic- 7, r). The functional and structural significance of this structure ulation between basal plate and brush described by Lund (1974) is obscure, but it corresponds topographically to the ‘‘dorsal as present in S. altonensis. rod’’ supported by the spine in Damocles (Lund, 1986). Con- Brush structure has been described and discussed repeatedly junction (Patterson, 1982) of rod and brush indicates that for- (Lund, 1974; Zangerl, 1981, 1984; Wood, 1982; Williams, mer is not a direct homologue of the latter (cf. Lund, 1986). 1985; Zidek, 1993; Coates et al., 1998). In A. zangerli, both The dorsal fin (Fig. 8) is situated at pelvic level, in a position principal specimens of which are male, the brush expands dor- identical to that of the single dorsal fin in anacanthous sym- sally to produce a denticle-bearing platform of around 160% of moriids (Zangerl, 1981) and the second dorsal fin in genera neurocranial length. The ‘‘fibres’’ of the brush are hollow rods such as Goodrichthys (Moy-Thomas, 1936), Tristychius (Dick, (Williams, 1985) consisting of globular calcified cartilage 1978), and Onychoselache (Dick and Maisey, 1981). As in all (Coates et al., 1998). These rods are superficial to an interior Stethacanthus specimens (Lund, 1974; Zangerl, 1981; Williams, which is not resolvable into separate units. In anterior, leading 1985), both examples of dorsal fins from A. zangerli are dis- edge, and ventral regions, rods merge into a consolidated mass rupted: in HM V8246 proximal radials are obscured by the of bone-covered calcified cartilage. Anteriormost rods are gen- vertebral column and the posterior of the fin is scattered; in erally better preserved than those that are close to the trailing UMCZ GN1047 the fin radials are clustered but oriented ran- edge. Irregularly shaped blocks of material resembling bone- domly. Conditions in Cobelodus and Symmorium (Zangerl and covered calcified cartilage are located posteroventrally to all Case, 1976; Zangerl, 1981; Williams, 1985) probably provide brush specimens (Fig. 7, ac.cart). Like similarly positioned the closest comparison. structures in Damocles and Falcatus, these are interpreted as In HM V8246, the dorsal fin includes at least 21 radials di- accessory supports for the complex (cf. Lund, 1985b, 1986). vided into proximal and distal series. This compares closely The brush platform of Akmonistion is laterally narrow and with the estimated 24 radials in S. altonensis (Lund, 1974; Wil- bears an array of prominent, long-crowned scales. In a male S. liams, 1985). The fin outline was probably subtriangular (radial altonensis from Bear Gulch (MV 2830) there are up to nine length increases from the first to at least the fourteenth from scale rows across the brush anterior, tapering to four or five the leading edge); there is no direct evidence for the short, across the rear, and the posterior median scales bear the longest posterior, proximal extension included in Zangerl’s (1984) res- crowns (Lund, 1974). Alternatively, the probable female from toration. Proximal radials (basals) (Fig. 8, p.rad) consist of lat- the Logan Quarry Shale (FMNH PF2207) has up to seven rows erally flattened cylinders (probably caused by post mortem across the brush and the posterior lateral crowns are longest compression), and each articulates with a single distal radial. (Williams, 1985:pl. 10). Compared with these, scale number Distal radials (Fig. 8, d.rad) are much longer than the proximal and distribution in a male Akmonistion brush resembles that of series, and neither branch nor articulate with any further, more the male S. altonensis, while crown length distribution resem- distal series. Most of the distal radials of UCMZ GN1047 have 448 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

FIGURE 9. Akmonistion zangerli, caudal fin. A, HMV8246; B, reconstruction; anterior to left of figure. Scale bar equals 30 mm.

rotated so that anterior or posterior surfaces are visible. These condition of the hypochordal lobe in a specimen of Cobelodus show that the distal radials (unlike the cylindrical proximal se- aculeatus (NMMNH P-19182) from the Kinney Quarry, New ries) are calcified so that the proximal, dorsal and ventral sur- Mexico (Zidek, 1992:fig. 3A). faces form a slender triangular frame, while the central region The transition between peduncle and caudal regions occurs is uncalcified and thus remains hollow. This morphology is at about vertebral segment 91. The anterior caudal boundary is characteristic of distal radials in all fins of A. zangerli. identified by the anteriormost vertebra supporting a hypochord- A delta-shaped cartilage (Fig. 8, d.crt), the homologue of al radial. This lies seven or eight segments in front of the nar- Zangerl and Case’s (1976) ‘‘‘V’-shaped element,’’ lies posterior row region in which the vertebral column (and notochord) turns to the fin in UMCZ GN1047. The cartilage apex points dorsally dorsally to form the upper lobe of the tail. The region of hy- in this disrupted example, but in less disturbed Cobelodus and pochordal outgrowth thus extends anteriorly relative to epi- Symmorium specimens it points caudally, with the plate orient- chordal outgrowth. The abbreviated heterocercal tails of Fal- ed sagittally. Corroboration for this orientation comes from Fal- catus (Lund, 1985b) and Damocles (Lund, 1986) are thus dif- catus (Lund, 1985b:fig. 13), where incompletely separated dor- ferent in that dorsal and ventral skeletal outgrowth levels co- sal fin radials are continuous posteriorly with a triangular car- incide. tilage plate sharing distinct characteristics with symmoriid delta Neural arches show a significant change in morphology at cartilages (Zangerl, 1981). In all examples the near triangular the level of the anteriormost point of hypochordal outgrowth. outline is acute, and the shortest side, facing anteriorly, is con- The spindle-shapes of the thoracic and penduncle regions are cave. The arms of the ‘V’ consist of the thickly mineralized replaced by a series of eight or nine low, rhombic, plates (Fig. dorsal and ventral plate edges. In A. zangerli the center of the 9A, na). As in more anterior arches, each of these includes a cartilage is uncalcified; anteriorly there appears to have been a segmental nerve foramen. A horizontally directed rod lies just calcified bar connecting dorsal and ventral edges, but this is above this series in HM V8246, but this seems to be a displaced broken in UMCZ GN1047. Foramina perforate this bar dorsally hypochordal radial rather than epineural structure. No neural and ventrally. arch series similar to these is known in other early chondri- The Caudal Fin and Axial Skeleton The heterocercal tail chthyans, although this may result from incomplete preserva- is well preserved in HM V8246 (Fig. 9A) and resembles the tion of otherwise similar tails. superficially symmetrical, semilunate tails described in Sym- From the point of chordal upturn, at around the hundredth morium, Denaea, and Cobelodus (Zangerl, 1981; Williams, vertebral segment, neural arches either articulate distally with 1985). As in most of the well preserved specimens of these supraneural radials (Fig. 9A, sn.rad) or extend as lanceolate genera, the elongate hypochordal radials are grouped as a nar- neural spines (Fig. 9A, n.sp). The preserved, prismatic calcified row and distally acute ventral caudal lobe, with the posterior- layer of these cartilages is notably thinner than elswhere in the most radials separated from the caudal haemal spines. However, endoskeleton. At least 23 radials and spines precede the pos- unlike other restorations, the tail of A. zangerli is restored so terodorsal caudal apex. Superficially, the broad spines resemble that all radials articulate closely with proximal endoskeletal those of Cladoselache, and corroborate observations of simi- structures. As a result, the radials of the ventral caudal lobe are larly expanded epichordal supports in Stethacanthus (Lund, splayed distally, the apparent surface area of the ventral lobe is 1974; Williams, 1985) and Stethacanthulus (Zangerl, 1990), un- increased, and the near-symmetry of the total caudal outline is like the slender epichordal rays of Cobelodus, Denaea, and lost (Fig. 9B). The restored hypochordal skeleton thus resem- Symmorium. When reconstructed in A. zangerli it becomes ap- bles more closely those of Falcatus (Lund, 1985b) and Dam- parent that the expanded apices overlapped and stiffened the ocles (Lund, 1986). Furthermore, Symmorium, Cobelodus, and dorsal leading edge of the caudal fin. Denaea probably had a similarly splayed arrangement, unlike The anteriormost hypochordal radial (Fig. 9A, a.h.rad) forms current restorations (Zangerl, 1981), but rather similar to the a subtriangular cut-water that articulated proximally with two COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 449

FIGURE 10. Akmonistion zangerli, pectoral girdle and fin. A, HMV8246; B, UCMZ GN1047; C, reconstruction; anterior to left of figure. Scale bars equal 20 mm.

or more haemal arches. Posterior to this the haemal arches and quality of preservation, it is not currently possible to distinguish hypochordal radials (Fig. 9A, h.rad) extend to form the ventral pre-ural from ural caudal zones. Caudal haemal arches and lobe of the tail. Around twelve closely spaced radials contribute spines 23–31 are clearly defined, but more posterior arches are to the leading edge, and about eight, more widely spaced, to incompletely separated from each other, and form a ragged- the trailing edge. Proximally, the haemal arches extend in edged ribbon of prismatic cartilage. length for the anteriomost five of the caudal region, then de- The space occupied by the caudal stretch of notochord is crease through to the fourteenth. Haemal arches nine to fourteen almost undistorted and mostly empty. However, it includes a are proximodistally short and resemble those described as ‘boo- chain of small calcifications which are interpreted as having merang shaped’ in Symmorium (Williams, 1985). Eleventh and formed within the perichordal sheath (Fig. 9A, pc). Finally, it twelfth arches have the most squat profiles, are situated at a is noteworthy that at the point of caudal dorsal upturn, there transformation in the proximal curvature of these arches, and appears to be a significant loss of segmental register between may bear foramina (but these might equally be artifactual de- neural and haemal arches. Given the quality of preservation, pressions cleared incompletely of matrix). From the fifteenth this is unlikely to be artifactual. caudal haemal arch and more posteriorly, haemal spines (Fig. Pectoral Girdle Details of pectoral girdle morphology are 9A, h.sp) are extended, and articulate with radials reaching the displayed most clearly on UCMZ GN1047 (Fig. 10B), although trailing hypochordal perimeter. More anterior arches articulate in-life orientation is preserved better on HM V8246 (Fig. 10A). mostly with a series of short proximal radials. Despite the high Each half of the girdle consists of a tall scapulocoracoid and a 450 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001 subtriangular procoracoid cartilage connecting with the anter- oventral surface of the coracoid region. The scapular process is a near-parallel sided cartilage sheet, the dorsal apex of which expands into anterodorsal and poster- odorsal processes (Fig. 10, sc, apr, ppr). From HM V8246 it appears that the long axis of the scapular process slopes pos- teriorly relative to the axial skeleton. The scapular process base is thickened mediolaterally and expanded in a mostly posterior direction to form and support the articular surface for the pec- toral fin radials. A large foramen (Fig. 10, dzf) perforates the cartilage just above the articular area. This probably enclosed the diazonal nerve and brachial artery. The articular surface consists of a well formed, robust, rounded crest (Fig. 10, art.c). Oriented horizontally, the crest extends anteriorly from the pos- teriormost prominence for almost three quarters of the total sca- pulocoracoid width. The posterior end of the articular crest is expanded to form a condylar surface at the articulation with the metapterygium (Fig. 10, mpt. c). The narrow cartilage surface immediately ventral to this crest is smooth and featureless. The medial face of the scapulocoracoid is exposed on UCMZ GN1047. The articular region is penetrated dorsally by the slot- shaped diazonal nerve foramen. As in Cobelodus (Zangerl and Case, 1976), the medial surface of the scapular process is dom- inated by a dorsoventrally oriented trough, the posterior edge of which forms a prominent, anteriorly convex, curved ridge. The coracoid region (Fig. 10, cor) is convex anteriorly and concave posteriorly, and, in life, curved medially. The procor- acoid cartilage (Fig. 10, pcor) is assumed to be homologous with the similarly positioned cartilage plates in xenacanths and the ‘claw-shaped elements’ of Cobelodus, Symmorium and De- naea. However, unlike these, the procoracoids are not directed posteriorly, and appear to have moved little from their former articulated position in HM V8246. Pectoral Fin Pectoral fins are preserved most completely in HMV8246 (Fig. 10A), where those of left and right sides are superpositioned. General fin morphology resembles those of Cobelodus (Zangerl and Case, 1976) and Denaea (Williams, FIGURE 11. Akmonistion zangerli, pelvic girdle, fin and clasper. A, 1985). Eight or nine short proximal radials articulate with the UCMZ GN1047, anterior to right of figure; B, HMV8246, anterior to girdle directly; rather fewer than the thirteen in Stethacanthus left of figure; C, reconstruction, anterior to right of figure. Scale bars altonensis (Zangerl, 1981). In UCMZ GN1047 these consist of equal 20 mm. anteroposteriorly flattened cylinders, slightly expanded at prox- imal and distal articular surfaces. Each proximal radial articu- lates with a single distal radial, except the anteriormost, which may terminate at the fin leading edge. The fifteen or so distal HM V8246 a band of tiny skin denticles beyond the posterior radials are much longer than the proximal series. Distal radials distal radials (Fig. 10A) provides some indication of fin size neither branch nor articulate with any further radials. (employed to predict fin proportions in the reconstruction The posteriormost proximal endoskeletal fin support, usually shown in Fig. 16). Despite the otherwise exceptional preser- identified as the metapterygium (Fig. 10, mpt), is a broad, near- vation of this specimen, there are no traces of ceratotrichia. triangular plate of about the same anteroposterior length as the Pelvic Girdle The pelvic girdle consists of simple sheets scapulocoracoid articular ridge. The proximal metapterygial ar- of prismatic cartilage, each of which is a subtriangular, rounded, ticular surface is concave and faces anteriorly, towards the con- plate (Fig. 11, p.pl). The most completely preserved examples dyle on the rear of the scapulocoracoid. The distal metaptery- are present on UMCZ GN1047. The anterior edge of each pel- gial edge bears a stepped series of six articular facets for distal vic plate is slightly concave, and the posterior is convex. The radials. These facets are not separated by deep grooves, as in girdle halves are reconstructed as lying horizontally in the ven- Stethacanthus altonensis (Zangerl, 1981, 1984), Symmorium tral body-wall musculature. The lateral edge, articulating with (Williams, 1985), and Falcatus (Lund, 1985b). The posterior the proximal radials of the fin, bears separate articular facets. metapterygial surface articulates with the anteriormost of a se- There is no suggestion of any symphysial union between left ries of about 22 axial radials (Fig. 10, ax.rad) forming a whip- and right pelvic plates. Five diazonal nerve foramina (Fig. 11, like pectoral fin extension. The anteriomost three axial radials dzf) pierce the pelvic plate adjacent to the lateral edge, and are shorter than more posterior members of the series; this re- these are accompanied by a larger and slightly more postero- sembles conditions in the similar pectoral whips of Cobelodus medially situated foramen. A similarly large foramen is present and several other primitive chondrichthyans. Unlike Gutturensis in the pelvic plates of Cobelodus and Denaea (Williams, 1985), neilsoni (Sequeira and Coates, 2000), no distal radials articulate but is lacking from the Bear Gulch Stethacanthus altonensis with these anterior cartilages. From the barely disturbed skel- (Lund, 1984:fig. 9). eton of HM V8246 it appears that the complete whip was al- Pelvic Fin Pelvic fins of male specimens include eleven or most as long, proportionally, as that of Denaea (Williams, twelve proximal radials articulating directly with the girdle 1985). (Fig. 11). In HM V8246 the posteriormost, premetapterygial, The complete distal extent of the pectoral fin is uncertain. In proximal radial bifurcates and articulates with two distal radials. COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 451

Distal radials are most completely preserved in HM V8246, and, like those elsewhere, are unbranched. Two distal radials probably articulated with the metapterygium (Fig. 11B, mpt), which includes a large right-angled notch accommodating the most proximal of a series of around seven axial or ‘basal’ car- tilages (Fig. 11, ax.crt). Each of these articulates with further short, broad, radials; two per segment for the larger members of the series. Axial cartilages, the myxopterygium and terminal cartilages seem to consist of globular calcified cartilage like that of the spine-brush complex (Coates et al., 1998), rather than prismatic cartilage. The rod shaped myxopterygium (Fig. 11, mxpt) is gently curved, and nearly the same length as the pelvic fin and axial cartilages combined. All myxopterygia show transverse frac- tures suggesting incomplete division into or construction from a segmented pattern (cf. segmented myxopterygia in Falcatus, Lund, 1985b). However, the irregularity of fracture distribution within and between specimens indicates that these apparent seg- ments may be preservational artifacts. The myxopterygial core was unmineralized, and the rod is preserved as a flattened cyl- inder broken along the dorsomedial edge. Calcification is thick- est distally and ventromesially, while proximally and dorsolat- erally mineralization consists of no more than small, isolated, calcified ‘islands.’ A broad, shallow, groove (Fig. 11A, grv) passes across the dorsolateral myxopterygial surface in a prox- FIGURE 12. Akmonistion zangerli, scales from the cranial cap, lateral imoventral to distodorsal direction. This groove is undescribed line, and flank region. A–C: cranial cap scales from UCMZ GN1047, in other fossil claspers. Comparison with clasper anatomy in anterior to right of figure; single cranial cap scale from UCMZ GN1047 Chlamydoselachus (Smith, 1937) suggests that the groove may in D, lateral view (anterior to right), E, basal view (anterior to top), and reflect the location of an erector/expansor muscle, or the site of F, apical view (anterior to top); G, lateral line scales from HM V8246, a venous sinus extending from the iliac vein. anterior to left of figure; H, flank scales from UCMZ GN1047. A–F, scale bar equals 2 mm; G and H, scale bar equals 1 mm. Terminal clasper parts include at least two and probably three or more cartilages (Fig. 11, t.crt). Unlike Cobelodus and De- naea (Williams, 1985), none of these are continuous with the myxopterygium. In both preserved clasper pairs, the most prominent terminal cartilage is ovoid with a convex surface bone, that characterise examples from cranial cap and brush marked by a shallow groove, oriented anteroposteriorly. This apical regions. convex unit is faced by a similarly shaped concave cartilage, Most details of the cranial cap scales and brush apical scales with fragments of a third protruding from between these valve- match the description of corresponding material attributed to S. like structures in UCMZ GN1047. At least one smaller cartilage altonensis by Williams (1985), and thus include forms resem- lies between the proximal surfaces of the terminal cartilages bling Lambdodus hamulus (St. John and Worthen, 1875; Figs. and the distal end of the myxopterygium. The concave terminal 12B, C, 13E) and Cladodus pattersoni (Newberry, 1889; Figs. cartilage resembles a proportionally larger concave component 12D–F, 13A–D). These scales are an order of magnitude larger in Falcatus. It is noteworthy that there are no plates consisting than lateral line and other body scales, and the crowns have of or supporting acute scales, as in Damocles (Lund, 1986) or distinct carinae close to the apex. primitive holocephalans (Lund, 1982). The cranial cap scale pavement has disintegrated in all Bears- Scales Although Akmonistion and other stethacanthids den specimens. AMNH 1734, attributed to S. altonensis and could be described as predominantly scale-less, this would figured by Zangerl (1981) and Williams (1985), thus remains overlook the fact that they are among the most remarkably het- the best source of information about scale distribution in a erosquamous chondrichthyans discovered thus far. In fact, Ak- closely comparable region. Smaller scales are situated anteriorly monistion retains four distinct scale regions: (1), the cranial cap and laterally, larger scales medially and posteriorly, and all scales (Fig. 12A–F); (2), the brush apical scales (Fig. 13); (3), crowns are directed posteriorly (pers. obs., MIC). Individual an area of miniscule flank scales (Fig. 12H); and (4), lateral cranial cap scales are well preserved in HM V8246, UCMZ line scales (Fig. 12G). All scales are single crowned and are GN1047, and NMS 1981.63.23A. Scales resembling Lambdo- therefore inferred to be non-growing, i.e., formed in a single dus hamulus (Fig. 12B, C) have a smooth, recurved, crown, morphogenetic event (Reif, 1985). Each scale could also be arching back over an extended, quadrangular, base with a described as consisting of a single odontode (Ørvig, 1977). This coarse, fluted and ridged surface. A scale with an apparently is strikingly dissimilar to the ‘polyodontode’ (Ørvig, 1977) simpler, more equilateral base (Fig. 12D–F), therefore resem- scales of genera such as Tamiobatis (Williams, 1998) and Di- bling Cladodus pattersoni, has been removed from the matrix. plodoselache (Dick, 1981), in which growth occurred by the This reveals a grooved underside with a pair of large, centrally apparent welding of newly formed scales to pre-existing ex- placed, basal canal foramina (Fig. 12E). Smaller foramina, as- amples (Reif, 1978, 1985). The scales of Akmonistion might sumed to open into neck canals, are clustered above and below also be described as ‘placoid,’ implying that they match tradi- a central prominence on the posterior surface. In basal and api- tional or more precise descriptions of neoselachian scales (e.g., cal views, the modified square outline (regarded by Reif, 1985, Romer and Parsons, 1977:158; Reif, 1985:16). But Akmonistion as the general and possibly plesiomorphic condition) of the scales lack a key placoid feature: no crown has been demon- scale base indicates the way in which it overlapped anterior and strated to consist of anything other than dentine: an enamel and/ underlay posterior neighbours (assuming a posteriorly directed or enameloid cap appears to be absent. Otherwise, these scales crown). This indicates that such scales were interarticulated are noteworthy for the massive bases, consisting of acellular within predominantly anteroposteriorly directed series. No neck 452 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

FIGURE 13. Akmonistion zangerli, scales from the brush apex of UCMZ GN1047. A, long-crowned, posterolateral scale; dorsal view; B, midline scale, lateral view, cf. Cladodus pattersoni; C, scale base of long-crowned, posterolateral scale in dorsal view; D, profile of long-crowned, posterolateral scale in anteroposterior aspect, ventrally directed surface to left of figure; E, scale from anterior of brush apex, cf. Lambdodus hamulus, lateral view, anterior to right. F, patch of associated scales from anterior of brush apex. All scale bars equal 2 mm; cross-hatching indicates broken crown surface.

canal foramina appear on the anterior surface, but smaller, nu- (1984) describes general, if well spaced, covering of similar tritive foramina are clustered around the crown base. scales in the stethacanthid Orestiacanthus. Lateral line scales (Fig. 12G) are preserved only in HM Brush apical scales, like those of the cranial cap, also include V8246, where a short row lies between the dorsal extremities forms comparable to Lambdodus hamulus (St. John and Wor- of the neural arches and the ventral edge of the spine basal then, 1875) (Fig. 13E) and Cladodus pattersoni (Newberry, plate. The scales are remarkably small relative to total body 1889) (Fig. 13B), but the morphological variety is more diverse. size, measuring around 0.5 mm across. Each scale is triradiate, Crowns are directed posteriorly, except for those situated along with a single cusp and a crescent-shaped base. As in the cranial the posterolateral margin which face laterally (Figs. 7, 15). cap and spine-brush scales, the crown is smooth and curved. Scale size varies considerably from anterior to posterior of the The scale base consists of a pale material and the crown of brush apex, the overall shape of which is slender and stream- darker material; a collar region where the crown meets the scale lined. From HM V8246 and UCMZ GN1047, the rear third of base is translucent, revealing dentine tubules. Two or more ca- the brush apex seems to have been no more than four scales nals penetrate the scale base below the crown. Pairs of these across. The smallest scales, some 2 mm long, are clustered scales are arranged so that the crescentic bases meet, and create around and over the leading edge (Fig. 13F) forming an anterior a C-shaped incomplete ring around the course of the sensory buckler, whereas the largest scales have crowns which are about canal. Again, this resembles conditions in Orestiacanthus 20 mm long (equivalent to almost one third of total brush (Lund, 1984). height), overhanging the posterior third of the brush lateral sur- The miniscule body scales appear to be vestigial, Petrodus- face (Fig. 13A, D). Posterior scales closer to the midline have like, buttons, about 0.2 mm across with a subcircular base and crowns which are no longer than 5mm. Bases of the largest minimal, apical, blister of dentine (Fig. 12H). These scales are scales (Fig. 13A, C, D) have a concave apical surface with known only from a patch covering the flank above the pelvic prolonged buttresses flanking, but not ventral to, the crown plate in UCMZ GN1047. Airbrasive preparation has removed base. The dorsoventrally deep sides of these scale bases are much of their surface detail, and it is uncertain if they were crenelated, and adjacent bases seem to have interdigitated, con- present elsewhere. Williams (1985) reports the presence of sim- ferring greater rigidity to the total platform surface. Each of the ilar denticles in the orbit region of AMNH 1734, while Lund longest scale crowns emerges dorsally from its base, but bends sideways and extends outwards with a slight sinusoidal curve (Fig. 13D). Gut Trace and Cololite An area of debris posterior to the branchial skeleton and pectoral girdle in HMV8246 probably represents remains of stomach/foregut contents. The extent of this region is shown in Figure 1C (light shaded with a dark border). Debris includes small fragments (around 1–2 mm across) of arthropod cuticle and actinopterygian scales (pers. comm., Neil Clark; pers. obs., MIC.). A second region of pre- served gut contents lies directly above the pelvic plates and ventral to the dorsal fin. This squat bolus (Fig. 14) measures FIGURE 14. Akmonistion zangerli, cololite. Scale bar equals 5 mm. about 15 mm along its longest axis. The posterior end is blunt, COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 453 two or three deep incisions and several minor striae indicate its resulted from a serial-replacement ontogeny resembling that of spiral form, and it resembles many isolated coprolites found in chondrichthyan (and other primitive gnathostome) dentitions. the fish-bearing beds of the Manse Burn Formation. However, The phylogenetic position of Akmonistion and other stetha- the position of this particular example indicates that it had not canthids among the Chondrichthyes is a moot point. According been expelled from the intestinal tract prior to fossilisation, to Zangerl (1990), the family Stethacanthidae includes Stetha- hence use of the term cololite instead of coprolite (Hunt, 1992). canthus (Newberry), Orestiacanthus (Lund), Stethacanthulus (Zangerl), and Bethacanthulus (Zangerl). Falcatus (Lund) and DISCUSSION Damocles (Lund) are placed in a separate family, the Falcatidae (Zangerl, 1990). As noted in the introduction, Akmonistion The skeletal reconstruction of Akmonistion (Fig. 15) and its clearly fits within Zangerl’s (1990) definition of the Stethacan- restoration in life (Fig. 16) illustrate the unusual proportions of thidae rather than the Falcatidae, but these families can be unit- this species. There is no close analogue of the tail morphology ed at a more inclusive level by the presence of a specialized among Recent chondrichthyans. High aspect ratio tails of extant pectoral level dorsal spine without any closely associated pec- pelagic genera (e.g., Rhincodon, Isurus, Lamna, Carcharhinus) toral level dorsal fin (in males, at least). share the presence of a steeply upturned caudal lobe, but none In an earlier scheme, Lund (1985a) grouped Orestiacanthus possesses the larger, and endoskeletally supported, hypochordal with the Falcatus-like genera on the basis of a shared, sagitally lobe. The unconsolidated vertebral column suggests a degree of compressed, spine morphology, while Stethacanthus clustered anguiliform axial flexibility like that of Recent lampreys, and with the symmoriids because of shared features of the pelvic the apparently small pectorals and pelvics suggest paired fin plate and metapterygium. More recently, Lund and Grogan proportions similar to those of modern pseudotriakid and scy- (1997:fig. 6; Node 48) found several synapomorphies to unite liorhinid sharks (pers. comm., M. Gottfried). Hydrodynamic Falcatus and Stethacanthus as the Stethacanthidae, which their performance must have been affected by the spine and brush analysis placed at the apex of a Paleozoic shark clade extending complex. The structure probably generated considerable drag, from the base of the elasmobranch stem-lineage (Lund and Gro- and the broad lateral surface area and anterior position of this gan, 1997:fig. 6; elasmobranchs distal to Node 57). Synapo- keel-like outgrowth would reduce yaw to a minimum. Median morphies of the Stethacanthidae sensu Lund and Grogan (1997) fin distribution is configured inappropriately for sudden accel- include the presence of scales restricted to specialized areas eration, and, given the size of the spine and brush complex, so (ethmo-rostral, supraorbital, and dorsal fin-crest zones); ring- too is sustained, high speed, pursuit of agile prey. Akmonistion shaped lateral line scales; a sexually dimorphic first dorsal fin is thus envisaged as a slow, steady, swimmer, suited for oppor- with an unornamented triangular spine; the first dorsal fin de- tunistic scavenging and preying on benthic invertebrates. This veloping at puberty; an elongate pectoral post-metapterygial contrasts strongly with the inferred swimming and prey capture axis; and absence of an anal fin and pelvic basipterygium. How- modes of the Devonian genus Cladoselache (Williams, 1990). ever, Lund and Grogan’s analysis focused on chimaeriform in- The function of the spine and brush complex remains ob- terrelationships and they included few non-holocephalan chon- scure, although it is now clear that in Akmonistion, at least, it drichthyans. Most notably, Cladoselache and the symmoriids was rigid, immobile, and unlikely to have served as an acces- (sensu Zangerl, 1981) are not included, although these taxa ex- sory clasper and/or a jaw-like threat display (see discussions in hibit many of the hypothesized stethacanthid apomorphies. It is Zangerl, 1984; Lund, 1985b; Williams, 1985; Coates et al., also relevant to the present work that they characterized the 1998). No Recent chondrichthyan possesses a similar range and brush complex as a dorsal fin, an interpretation that we, like distribution of specialized scales, although otherwise reduced Williams (1985), reject. Here, the brush complex is character- squamations with scatterings or discrete patches of enlarged, ized as a specialized basal plate outgrowth, and is thus mutually thorn-like scales, occur in numerous skates and rays (most no- exclusive of radial and ray supported fins (Coates et al., 1998; tably the bowmouth guitarfish, Rhina ancylostoma) as well as Coates and Sequeira, 2001). the bramble shark, Echinorhinus (Reif, 1985). Importantly, Lund and Grogan’s (1997) cladogram resembles The unusual squamation of Echinorhinus has special rele- several other hypotheses of primitive chondrichthyan phyloge- vance for Reif’s (1980, 1985) model of chondrichthyan self- ny which place stethacanthids as stemgroup elasmobranchs. organizing scale patterns. This model hypothesizes that each These include examples by Young (1982:text-fig. 9B), Gaudin (placoid) scale is surrounded by an inhibitory field preventing (1991:fig. 2), Coates and Sequeira (1998:fig. 9), and Maisey new scales forming in close proximity to existing scales. Echi- (2001:fig. 2). Two major alternative positions for stethacanthids norhinus, however, breaks the general rule (and apparent neo- have also been proposed, either as stem-group holocephalans selachian synapomorphy) of regular scale spacing, because its (Janvier, 1996:fig. 4.39, left hand side) or as stem-group chon- large, thorn-like, scales can occur with bases fused or welded drichthyans (Janvier, 1996:fig. 4.39, right hand side; this resem- together (Reif, 1985:pl. 15). This implies absence or suppres- bles Maisey’s (1984) hypothesis of symmoriids and Cladose- sion of the hypothesized inhibitory field. Resemblance to con- lache as stem-group chondrichthyans). The remainder of this ditions in primitive chondrichthyans, including xenacanths discussion summarizes the results of an ongoing reevaluation (Hampe, 1997) as well as the cranial cap and brush apex scales of all three alternatives. of stethacanthids, suggests that such suppression has been a New data from Akmonistion has contributed to a data matrix repeated phenomenon in chondrichthyan phylogeny. (appendices 1–3) of 86 non-additive binary coded characters The largest scales of stethacanthids have been compared to (Pleijel, 1995) for an in-group including 19 chondrichthyan teeth, as if they shared a specialized developmental relationship genera and 3 non-chondrichthyans (Coates and Sequeira, 2001). like that proposed for the teeth and extra-oral dermal denticles Using PAUP 3.1.1 (Swofford, 1993; since replicated using in Denticeps clupeoides (Sire et al., 1998). This suggestion is PAUP 4.0b3, Swofford, 1999) with heuristic search options, rejected for the following reasons: morphology, because com- one tree was found of 161 steps (Fig. 17A). The primary feature parisons between Figures 4, 5, 12, and 13 yield numerous dif- of this result is that stethacanthids and symmoriids emerge as ferences clustering scales and teeth of Akmonistion into distinct stem-group holocephalans (cf. Janvier’s [1996] novel conjuc- sets; histology, because microscopical inspection of broken sur- ture). Tree support statistics are detailed in the figure caption, faces indicates that enamel/enameloid is absent from scales but it is noteworthy that Bremer support values (Bremer, 1994) whereas it forms the outermost layer of tooth crowns; and on- are low: at four extra steps all resolution is lost above node D togeny, because no evidence suggests that the largest scales (Fig. 17) in a strict consensus of the resultant 2064 trees. 454 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

FIGURE 15. Akmonistion zangerli, reconstruction in A, dorsal, and B, lateral, aspects. Scale bar equals 50 mm. Pectoral fins in A extended horizontally; in B flexed ventrally; elongate gill rays shown extending from hyoid arch but omitted from gill arches. COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 455

FIGURE 16. Akmonistion zangerli, reconstruction in life. Extent of gill covers based upon branchial ray length and comparable conditions in Chlamydoselachus (Smith, 1937); likewise tooth families around jaw margins, which, based upon preserved size range (Fig. 5), probably retained worn teeth displaced externally relative to the functional position (cf. Gutturensis, Sequeira and Coates, 2000). Pectoral fins flexed ventrally as in Figure 15. Scale bar equals 50 mm.

Preliminary investigation of the data set relocated stethacan- thids and symmoriids as a monophyletic clade of stem-group elasmobranchs, leaving only genera above node K (holoce- phalans) branching from node D. This transformation, resem- bling the tree topologies proposed by Young (1982) and others, increased tree length by only 4 steps (MacCLADE v. 3.05, Maddison and Maddison, 1993). Although this suboptimal so- lution is one of the many trees of 165 steps or less, a near- identical branching sequence was found after reanalysis of char- acters re-weighted according to data from all 2064 trees. This tree, shown in Figure 17B, is near-identical to the described transformation, except for the more basal position of Cladose- lache. Alternatively, when symmoriids and stethacanthids are shifted to a stem-group chondrichthyan position, tree length in- creases to 169 steps. This configuration has not been investi- gated further. Characters uniting Akmonistion with the holocephalans at node H (Fig. 17A) include the following: 4, ring- or C-shaped sensory canal scales; 6, an anteriorly concave spine shape in lateral view (reversed to absent at node K); 13, a calcified an- terior dorsal fin basal plate; 50, calcified perichordal centra. Characters at node E uniting symmoriids with stethacanthids plus holocephalans include the following: 2, scales reduced or absent; 8, a spineless pelvic level dorsal fin; 15, a delta-shaped cartilage at pelvic level (reversed to absent at node K); 19, a lunate tail (reversed to absent at node I); 27, a broad pectoral fin insertion (reversed to absent at node I); 52, a laterally di- rected fossa on the palatoquadrate otic process (reversed to ab- sent at node K); 58, a semicrescent-shaped hyomandibula (un- known in genera included above node K). The hypothesis of symmoriids and stethacanthids as stem- holocephalans therefore depends upon a series of character states unknown and implied state reversals in the apical group of Harpagofututor, Helodus and Ischyodus. In the alternative and arguably more conventional tree topology (Fig. 17B), char- acters 15, 19, 27, 52, and 58 perform significantly better as synapomorphies uniting the monophyletic clade of stethacan- FIGURE 17. Cladograms, adapted from Coates and Sequeira (in thids and symmoriids. However, the character set joining this press). A, single shortest tree, 161 steps, consistency index: 0.53; B, clade with elasmobranchs above the divergence from holoce- single tree resulting from character reweighting (rescaled consistency index and best fit options) using 2,064 trees saved at a maximum of phalans is less robust. These include characters 65, emergence 165 steps. Underlined figures: Bremer support values; double figures: of the glossopharyngeal nerve through the metotic fissure (re- 50% majority rule values for nodes in set of 2,064 trees saved at max- versed in Falcatus, Damocles, and at node R); 73, dorsomedi- imum of 165 steps. Box encloses holocephalan/chimaeroid taxa. ally directed endolymphatic ducts (unknown in included fossil 456 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001 holocephalans); 76, hyoid articulation at the posterolateral angle Transactions of the Royal Society of Edinburgh: Earth Sciences 89: of the otic capsule (reversed in Falcatus and Damocles; ple- 63–85. siomorphic holocephalan condition unknown); 79, palatoquad- ———, and ——— 2001. Early sharks and primitive gnathostome in- rate articulates on the rear of the postorbital process (plesio- terrelationships; pp. 241–262 in P. E. Ahlberg (ed.), Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny and De- morphic holocephalan condition unknown); and 86, presence of velopment. Taylor and Francis, for the Systematics Association, a precerebral fontanelle (possible homology with the holoce- London. phalan ethmoid canal is uncertain; once again the plesiomorphic ———, ———, I. J. Sansom, and M. M. Smith. 1998. Spines and holocephalan condition is unknown). The character state distri- tissues of ancient sharks. Nature 396:729–730. bution for this branching sequence (Fig. 17B) is clearly not Denison, R. H. 1979. Acanthodii. H.-P. Schultze (ed.), Handbook of more satisfactory than that of the shortest tree (Fig. 17A). Paleoichthyology, 5. Gustav Fischer Verlag, Stuttgart, New York, In conclusion, available evidence indicates that stethacan- 62 pp. thids are crowngroup chondrichthyans rather than members of Dick, J. R. F. 1978. On the Carboniferous shark Tristychius arcuatus the stem lineage. Attempts to more fully resolve early chondri- Agassiz from Scotland. Transactions of the Royal Society of Ed- inburgh 70:63–109. chthyan interrelationships will need to include more taxa and ——— 1981. Diplodoselachi woodi gen. et sp. nov., an early Carbon- characters. The present matrix lacks characters describing the iferous shark from the Midland Valley of Scotland. Transactions of dentition, and characters describing the squamation are mini- the Royal Society of Edinburgh, Earth Sciences 72:99–103. mal. Primitive holocephalan conditions may be informed by the ———, M. I. Coates, and W. D. I. Rolfe. 1986. Fossil sharks. Geology addition of taxa such as iniopterygians and eugeneodontids (re- Today 2:82–84. viewed in Zangerl, 1981; Janvier, 1996), and redescriptions of ———, and J. G. Maisey. 1981. The Scottish Lower Carboniferous genera such as Pucapampella (Maisey, 2001) and Antarctilam- shark Onychoselache traquairi. Palaeontology 23:363–374. na (Young, 1982; existing referred material may include more Gardiner, B. G. 1984. The relationships of the palaeoniscid fishes, a than one taxon, and neurocranial specimens require compre- review based on new specimens of Mimia and Moythomasia from the Upper Devonian of Western Australia. Bulletin of the British hensive reinterpretation, pers obs. MIC) may yet shed light on Museum (Natural History), Geology 37:173–428. conditions preceding the basal divergence of modern chondri- Ginter, M. 1998. Taxonomic problems with Carboniferous ‘‘Cladodont- chthyan clades. level’’ sharks’ teeth. Ichthyolith Issues Special Publication 4:14– 16. ACKNOWLEDGMENTS Goujet, D. 1984. Les poissons Placodermes du Spitsberg. Arthrodires Dolichothoraci de la Formation de Wood Bay (De´vonien infe´rieur). We thank the staffs of the Hunterian Museum, University of Cahiers de Pale´ontologie, Centre national de la Reche`rche scien- Glasgow, the Zoology Museum, University of Cambridge, the tifique, Paris, 284 pp. National Museum of Scotland, Edinburgh, the Natural History Gross, W. 1937. Das Kopfskelett von Cladodus wildungensis Jaekel. 1. Museum, London, the American Museum of Natural History, Teil. Endocranium und Palatoquadratum. Senckenbergiana 19:80– New York, the Cleveland Museum of Natural History, The 107. Field Museum, Chicago, and the Carnegie Museum of Natural Hampe, O. 1997. Zur funktionellen Deutung des Dorsalstachels und de History, Pittsburgh, for access to collections, loan of specimens, Placoidschuppen der Xenacanthida (Chondrichthyes: Elasmobran- chii; Unterperm). Neues Jahrbuch fu¨r Geologie und Pala¨ontologie, and permission to prepare selected material. Particular thanks Abhandlungen 206:29–51. are due to Dr. J. A. Clack, University of Cambridge, for allow- Harris, J. 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Courier Forschung- APPENDIX 1 sinstitut Senckenberg 78:1–255. Ride, W. D. L., H. G. Cogger, C. Dupuis, O. Kraus, A. Minelli, F. C. Characters and Character-States Used in the Thompson, and P. K. Tubbs. 1999. International Code of Zoological Phylogenetic Analysis Nomenclature. 4th ed. The International Trust for Zoological No- menclature 1999, The Natural History Museum, London, 306 pp. 1. Prismatic cartilage. Absent (0); present (1). Romer, A. S., and T. S. Parsons. 1977. The Vertebrate Body. 5th ed. W. 2. Body mostly scale-less. Absent (0); present (1). B. Saunders Company, Philadelphia, 624 pp. 3. Lateral line passes through scales. Absent (0); present (1). Schaeffer, B. 1981. The xenacanth shark neurocranium, with comments 4. Ring or C-shaped scales enclosing sensory canals. Absent (0); pre- on elasmobranch monophyly. Bulletin of the American Museum of sent (1). Natural History 169:1–66. 5. Dorsal fin spine consists mostly of vascularised osteodentine, with Schaumberg, G. 1982. Hopleacanthus richelsdorfensis n. g. n.sp., ein no clear division into two or three discrete layers as in most elas- Euselachier aus dem permischen Kupferschiefer von Hessen (W- mobranch examples. Absent (0); present (1). Deutschland). Pala¨ontologische Zeitschrift 56:235–257. 6. Physonemid spine shape (anteriorly concave profile in lateral view; 458 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 21, NO. 3, 2001

posterior opening proximodistally extensive). Absent (0); present 52. Laterally directed otic articular fossa on palatoquadrate. Absent (0); (1). present (1). 7. Spine precedes dorsal fins. Absent (0); present (1). 53. Quadrate condyle. Absent (0); present (1). 8. Spine present in only anteriormost dorsal fin position; pelvic level 54. Double condylar-glenoid mandibular joint. Absent (0); present (1). dorsal fin spineless. Absent (0); present (1). 55. Labial cartilages. Absent (0); present (1). 9. Cephalic spines. Absent (0); present (1). 56. Holostyly. Absent (0); present (1). 10. Two dorsal fins. Absent (0); present (1). 57. Hyoid rays elongate, supporting opercular flap. Absent (0); present 11. Second or single dorsal fin opposite pelvis. Absent (0); present (1). (1). 12. Single dorsal fin elongate, extending from near-pectoral level to 58. Hyomandibula semicrescent shaped. Absent (0); present (1). anal or pre-anal fin level. Absent (0); present (1). 59. Interhyal. Absent (0); present (1). 13. Anterior dorsal fin base plate calcified. Absent (0); present (1). 60. Hypobranchial orientation. Medially or anteriorly (0); largely pos- 14. Posterior dorsal fin baseplate calcified. Absent (0); present (1). teriorly (1). 15. Posterior dorsal fin with delta-shaped cartilage. Absent (0); present 61. Basibranchials 1 and 2. In contact or close apposition (0); separated (1). by a gap (1). 16. Anal fin. Absent (0); present (1). 62. Large posteriorly projecting basibranchial copula. Absent (0); pre- 17. Anal fin supported by radials external to body wall. Absent (0); sent (1). present (1). 63. Persistent metotic/otico-occipital fissure. Absent (0); present (1). 18. Anal fin with double base-plate. Absent (0); present (1). 64. Ventral cranial fissure. Absent (0); present (1). 19. Caudal axis upturned steeply, supporting high aspect ratio (lunate) 65. Glossopharyngeal nerve exits through metotic fissure. Absent (0); heterocercal tail; elongate hypochordal radials unsegmented or seg- present (1). mented only proximally. Absent (0); present (1). 66. Glossopharyngeal nerve foramen situated posteroventral to otic 20. Horizontal or near-horizontal notochordal caudal extremity. Absent capsule and anterior to metotic fissure. Absent (0); present (1). (0); present (1). 67. Glossopharyngeal nerve foramen exits dorsally, posterior to otic 21. Caudal neural and/or supraneural spines extended. Absent (0); pre- capsule. Absent (0); present (1). sent (1). 68. Canal for lateral dorsal aorta within basicranial cartilage Absent 22. Ventral lobe of tail supported by hypochordal radials extending (0); present (1). beyond level of body wall. Absent (0); present (1). 69. Canal for lateral dorsal aorta long. Absent (0); present (1). 23. Scapular blade. Absent (0); present (1). 70. Otico-occipital proportions: greater than length of ethmo-orbital 24. Scapular anterodorsal and posterodorsal processes. Absent (0); pre- portion (0); equal to or less than ethmo-orbital portion (1). sent (1). 71. Posterior tectum. Absent (0); present (1). 25. Procoracoid directed posteriorly. Absent (0); present (1). 72. Occipital unit wedged between rear of otic capsules. Absent (0); 26. Supra-articular pectoral foramina: numerous (0); single or absent present (1). (1). 73. Endolymphatic ducts directed dorsally and fossae close to midline. 27. Pectoral articular surface: narrow/short (stenobasal) (0); broad/long Absent (0); present (1). (eurybasal) (1). 74. Endolymphatic ducts exit into slot-shaped median fossa. Absent 28. Pectoral articular surface (and fin) elevated. Absent (0); present (1). (0); present (1). 29. Paired fin radials barely extend beyond level of body wall. Absent 75. Dorsal ridge posterior grades smoothly into occipital roof, with no (0); present (1). horizontal crests. Absent (0); present (1). 30. Dibasal pectoral fin endoskeleton. Absent (0); present (1). 76. Hyoid articular area on posterolateral angle of otic capsule. Absent 31. Tribasal pectoral fin endoskeleton. Absent (0); present (1). (0); present (1). 32. Anteriormost proximal radial (propterygium) broad. Absent (0); 77. Prominent lateral otic process. Absent (0); present (1). present (1). 78. Lateral commissure expanded anteroposteriorly. Absent (0); present 33. Anteriormost distal pectoral radial largest of series. Absent (0); pre- (1). sent (1). 79. Articulation for palatoquadrate on rear of postorbital process. Ab- 34. Middle of three proximal radials (mesopterygium) articulates with sent (0); present (1). 3ϩ distal radials. Absent (0); present (1). 80. Positions of foramina for nerves II, III and IV in orbit. Nerve fo- 35. Posteriormost radial (metapterygium) broad and articulates directly ramen II situated mid-orbit; III and IV posterior to II (0); nerve with 5ϩ distal radials. Absent (0); present (1). foramen II situated mid-orbit, IV anterior to II (1). 36. Proximal articular facet of metapterygium directed anteriorly. Ab- 81. Myodome for superior oblique muscle situated anterodorsally. Ab- sent (0); present (1). sent (0); present (1). 37. Metapterygium connects with distinct series of distal radials artic- 82. Broad suborbital shelf. Absent (0); present (1). 83. Suborbital shelf expanded anterolaterally. Absent (0); present (1). ulating proximodistally to form a short or long axis. Absent (0); 84. Palatobasal process. Absent (0); present (1). present (1). 85. Antorbital process. Absent (0); present (1). 38. Axial radials articulate with pre- and post-axial radials. Absent (0); 86. Precerebral fontanelle. Absent (0); present (1). present (1). 39. Pelvic plate semicircular with anterolateral concavity. Absent (0); present (1). APPENDIX 2 40. Pelvic girdle with narrow anteromedial process and single diazonal foramen. Absent (0); present (1). Data Matrix of Early Chondrichthyans and Outgroups 41. Fused puboischiadic bar. Absent (0); present (1). Character-state distributions for taxa obtained from original obser- 42. Anteriormost pelvic radial broader than posterior, pre-metapterygial vations and/or the following literature sources: Norselaspis, Janvier, members of series. Absent (0); present (1). 1996; Kujdanowiaspis, Stensio¨, 1963, Goujet, 1984; Acanthodes, Miles, 43. Four or fewer radials articulate directly with pelvic girdle. Absent 1968, 1973, Denison, 1979, Coates, 1994; Mimia, Gardiner, 1984; Di- (0); present (1). plodoselache, Dick, 1981; Orthacanthus, Hotton, 1952, Schaeffer, 1981, 44. Claspers in males. Absent (0); present (1). Heidtke, 1982; ctenacanth (Tamiobatis), Moy-Thomas, 1936, Zangerl, 45. Myxopterygial claspers. Absent (0); present (1). 1981, Williams, 1998; ‘‘C.’’ wildungensis, ‘‘C.’’ hassiacus, Stensio¨, 46. Claspers with clawed terminus. Absent (0); present (1). 1937, Gross, 1937, Schaeffer, 1981; Hopleacanthus, Schaumberg, 1982; 47. Claspers with ‘toothed’ plate. Absent (0); present (1). Tristychius, Dick, 1978; Onychoselache, Dick and Maisey, 1981; Ham- 48. Fused anterior vertebral arches (synarcual) at anchorage of dorsal iltonichthys, Maisey, 1989b; Hybodus, Maisey, 1982, 1983; Cladose- spine. Absent (0); present (1). lache, Harris, 1938, Zangerl, 1981, Maisey, 1989a, Williams, 1998; 49. Calcified ribs. Absent (0); present (1). Cobelodus, Zangerl and Case, 1976; Denaea, Zangerl, 1981, Williams, 50. Calcified perichordal ‘centra’ or rings. Absent (0); present (1). 1985; Stethacanthus, Zangerl, 1981, 1984, Coates and Sequeira, 1998; 51. Palatoquadrate otic process expanded with an anterodorsal angle. Falcatus, Lund, 1985b; Damocles, Lund, 1986a; Harpagofututor, Lund, Absent (0); present (1). 1982; Helodus, Patterson, 1965; Ischyodus, Patterson, 1965. COATES AND SEQUEIRA—NEW SCOTTISH CHONDRICHTHYAN 459

APPENDIX 3 Character-taxon Matrix used in Phylogenetic Analysis.

Characters 1 2 3 4 5 6 7 8 8 Taxon 1 0 0 0 0 0 0 0 0 6 Norselaspis 00000000000000000000000000000????????0000000000?0000000?000???00000000?00000000?00000? Kujdanowias. 0000?0000000000000000000000000?000000000000100000000100000?000000000000000000100010000 Acanthodes 00100010000000010000011001001010000000?????0000000101100?110001101000100??00001??0010? Mimia 00100010000000010000000001001000000000000000000000101100001000110100010000000000100100 Cladoselache 100010110110000000101111011000000010100000000000001100?011000??????111?????1??1????00? Cobelodus 110000010010001000100111111000000011101000011100001111?0?1000010100111?????1001??0000? Denaea 1100000100100010001001111?10000000111010000111000011???0?1????10100??1??100??01?10000? Akmonistion 1101111101101010001011110110000000111000000110000111???011000010100111101001001?10000? Falcatus 11011111011010100001111101000000001110000001100001111??011000000000111????00001?100001 Damocles 110111110010101000011111010100000011??000011101001111??0?1????00000??1?????0001?1???0? Harpagofututor 1101000100?000000001111101010000101110000011101001001??11?????00000111???000000?1???00 Helodus 100110110110100000010?100100010010110000001110?10?001??1??????00000001????0000001???00 Ischyodus 1101101101101000000111110100010010110000001110?101001??1??????00000001????00000?1???00 Ctenacanth 10000010011011011000?11?0??0001000111100000110000010???0??????101001101111011110111001 ‘‘C.’’ wild. 1?????????????????????????????????????????????????101??0??????10100110??110111101110?1 ‘‘C.’’ hass. 1?????????????????????????????????????????????????101??0???????0??0111?11??111101110?? Hopleacanthus 10000010?110???11?000?1001000010001?100?0?11100?01??1??0??????00000??1?????1111??10011 Tristychius 10000010011011011000001101000011010011010111100011001?1010011100000001?111101001110011 Onychoselache 1000001011101101100001100100001101101001011110001000???00?????00000??1????11110?????11 Hamilton. 100000101110?1011100001001000011011000011111100010001??000011100001101011111110??10011 Hybodus. 1000001011101101110001100100001101100000110110001000111000011100001101?111111101110011 Diplodosel. 1000101?00?10001100011110100001000111?0000011000001011?0?00???????????????????1???0??? Orthacanthus 1000101?00?1000110011111010000100001110000011010101011?0100??0101001101111011111110001