interpretation of the toothplates of chimaeroid fishes

zoological Journal of the Linnean Society ( I 992), 106: 33-61. With 1 I figures

Interpretation of the toothplates of chirnaeroid fishes

COLIN PATTERSON, F.L.S.

Departmen! of Palaeontology, Brilish Museum (Natural History), London S W7 5BD

Received Auyusl 1991, arcepledJor publication NoNovember 1991

I t has bren argued that the toothplates of chimaeroid fishes exhibit a mode of growth that is I'undamentally diffrrent from that of other rhondrirhthyans. Chimaeroid toothplates are supposed to be slnlodonl, growing from the basal surface, whereas other rhondrichthyan dentitions are [vodonl, growing from the lingual towards the labial surfare of the jaw. l h a t idea is shown to be mistaken, because chimaeroid toothplates grow from the lingual surface, like other chondrichthyan dentitions. 'Ihe mistake rcsulted from confusion about the nomenclature of toothplate surfaces, and on the rhoirr of Chimuera as a Recent model. Callorhnchus is a more appropriate model, since it is shown to exhibit a primitive toothplate conformation, with the labial and symphysial margins of the orclusal surface bounded by a descending lamina whirh is applied to the margin of the jaw cartilage and grows basally throughout life. The descending lamina is well developed in toothplates of thc extinct chimaeroid genera Ischyodus, Pachymylus and Rrachymvlus, but is murh reduced in all Recent genera other than Callor/yzchus. A basally-growing descending lamina also bounds the labial and symphysial margins of the principal toothplates in the Mesozoir myriacanthoids and Squaloraja. The toothplates of the Palaeozoic 'cochliodonts' are reviewed; amongst them, the chondrenchelyids are the only forms with a basally growing desrending lamina. So far as the dentition and its mode of growth are concerned, the rlosest Palaeozoic relatives of chimaeroids seem to be the chondrenchelyids. The only statodont (basally growing) toothplates found in the course of this work are those of ptyctodont placoderms, which are therefore unlikely to be related to any chondrichthyans. Statodonty in its original sensr (failure to shed teeth) is shown to be widesprcad and possibly primitive in chondrichthyans. Cochliodont and chimaeroid toothplates grow in a logarithmic spiral. loothplates of primitive chirnaeroid type, with basally growing marginal descending laminae, can develop only when the constant angle of the spiral is small (less than about 35"), and when the oral surface of the jaw grows to the same logarithmic spiral.

KEY WORDS: Holocephali - Bradyodonti - statodonty ~ lyodonty - dentition - palaeontology.

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . 34 Statodont and lyodont growth . . . . . . . . . . . . . . . . 35

Variation in the desrending lamina in rhimaeroid toothplates . . . . . . . . 42 Comparison of gross structure, orientation and growth of toothplates in chimaeroids,

Orientation and nomenclature ofchimaeroid toothplates . . . . . . . . . 37

ptyctodonts, myriacanthoids and Spaloraja . . . . . . . . . . . . 45

Myriacanthoids . . . . . . . . . . . . . . . . . . 45 Ptyctodonts . . . . . . . . . . . . . . . . . . . 45

Spaloraja . . . . . . . . . . . . . . . . . . . . 49 Conclusions . . . . . . . . . . . . . . . . . . . 49

Comparison with cochliodonts . . . . . . . . . . . . . . . . 49 Cochliodus and Streblodus . . . . . . . . . . . . . . . . 50

Poecilodus, Delladus, Sandalodus, Enniskillcn . . . . . . . . . . . . 54 Deltoptychius and Menaspis . . . . . . . . . . . . . . . . 53

33 0024--4082/92/090033 + 29 $08.00/0 0 1992 The Linnean Society of London

34 C. PATTERSON

Helodus and Psephodus . . . . . . . . . . . . . . . . . 54 Chondrenchelyids. . . . . . . . . . . . . . . . . . 55

Problems of relative growth . . . . . . . . . . . . . . . . 56 Summary and conclusions . . . . . . . . . . . . . . . . . 58 Acknowledgements . . . . . . . . . . . . . . . . . . 59 References . . . . . . . . . . . . . . . . . . . . 59

INTRODUCTION

Reif (1982: 341), Bendix-Almgreen (1983) and Brvig (1985: 73) have argued for a sharp distinction between the statodont mode of growth in chimaeroid toothplates and the lyodont mode in Palaeozoic cochliodonts (and the early Mesozoic myriacanthoids; Brvig, 1985: 74), concluding that a transformation from one mode to the other is impossible, and consequently that neither cochliodonts nor myriacanthoids can be closely related (‘ancestral’) to chimaeroids. On the other hand, Lund (1988: 201) has proposed that “tooth plate relationships.are the same in cochliodonts, chimaeroids . . . and in most elasmobranchs, with new material added to the plate . . . at the lingual border”. The purpose of this paper is to support Lund’s view and to document the fact that there is no real difference between the mode of toothplate growth in chimaeroids and that inferred in myriacanthoids, cochliodonts and the Mesozoic chimaeriform Squaloraja.

The chimaeroids (Chimaeroidei, Holocephali) are a small group of chondrichthyan fishes comprising six extant genera (Callorhynchus, Chimaera, Hydrolagus, Rhinochimaera, Harrioua, Neoharriotta) and about ten extinct nominal genera (Ganodus, Eomanodon, Brachymylus, Pachymylus, Ischyodus, Edaphodon, Elasmodectes, Elasmodus, Amylodon) . The fossil record of chimaeroids mainly comprises isolated toothplates, and extends back to the Lower Jurassic (Ward & Duffin, 1989).

The relationships of chimaeroids have long been controversial. During the last 30 years there have been two principal competing theories. The first is that chimaeroids are immediately related to the early Jurassic Squaloraja and the late Triassic-early Jurassic myriacanthoids (about nine genera, listed by Duffin, 1984), and with them are related to Palaeozoic ‘bradyodont’ chondrichthyans such as Cochliodus, Deltoppchius and Menaspis (Patterson, 1965, 1968; Lund, 1977a,b, 1986, 1988; Zangerl, 1981; Maisey, 1986). The second theory is that chimaeroids are more closely related to ptyctodont arthrodires than to any Palaeozoic chondrichthyans (Brvig, 1960, 1962, 197 1 , 1980, 1985; Bendix-Almgreen, 1968, 197 1 , 1983; Jarvik, 1980). One difference between these two hypotheses concerns the distribution in time of the putative fossil relatives of chimaeroids: in the first theory, the fossil record of holocephalans is virtually continuous back to the Devonian, whereas in the second there is a notable gap between the Mesozoic chimaeroids and the Devonian ptyctodonts. In a proposal that seemed partially to bridge this gap, Lund (1977a,b, 1986, 1988) described the early Carboniferous (Namurian) Echinochimaera as either a chimaeroid (1977a), or the nearest relative of chimaeroids (1977b, 1986, 1988). Although Brvig (1985: 76; “Echinochimaera, a real member of the chimaerids”) appears to endorse this interpretation, it is without foundation, in my opinion (also Maisey, 1986: 221). Reasons for that assertion are touched on below (p. 49), but since the toothplates of Echinochimaera are still poorly understood (Lund, 1977b, 1988; Brvig, 1985) it is not really germane to the present

CHIMAEROID TOOTHPLATES 35

discussion. It should also be mentioned that the only well-known Permian form with cochliodont toothplates, the late Permian Menaspis, has been regarded as related to arthrodires by Bendix-Almgreen ( 197 1 ; related to rhenanids) and Ortlam (1985; related to arctolepids). I also regard these interpretations as mistaken, but will not justify the opinion here; Menaspis has normal cochliodont toothplates (p. 53; also Wiegelt, 1930: pl. 4, fig. 1, pl. 5, fig. 2; Bendix-Almgreen, 197 1 : fig. 5; Zangerl, 198 1 : figs 46-48), which Ortlam identified as part of an arthrodire shoulder-girdle of arctolepid pattern.

The specimens described and illustrated in this paper are all in the British Museum (Natural History). The Recent chimaeroids are in the collections of the Department of Zoology, and the fossils are in the Department of Palaeontology. Fossils are referred to by register numbers, preceded by “P.” or without prefix.

STATODONT AND LYODONT GROWTH

Bendix-Almgreen ( 1983) differentiated the growth pattern of the toothplates in chimaeroids and in cochliodonts by the terms statodont and lyodont (Fig. 1) . Brvig (1976: 83; 1980: 221) had earlier taken up the terms stadodont and lyodont from Jaekel (1901), and like Jaekel he used the first to characterize chimaeroid toothplates and the latter to characterize the dentition of sharks, in which teeth are shed and replaced thoughout life. According to Bendix-Almgreen’s interpretation ( 1983: 18, Fig. 1 here), a mandibular toothplace of Chimaera “grows throughout life continuously in the basal direction” and “forms and grows continuously by fusion in the vertical direction between consecutive generations of dental material (dg,, dg,-dg,), each occupying a hori<onlal position and developing basally to the preceding one”. The cochliodont toothplate (Fig. 1C) on the other hand “is a laterally growing structure, increasing in size by periodic apposition at the lingual side of successive generations of dental material (dgy, dg,) . . . which occupy positions beside each other in a linguo-labially extending series across the jaw exactly as the teeth of the transverse tooth rows of the jaws of both selachians (Fig. 1C: d, d,-d,) and batoids”. Reif (1982:341) made the same point more briefly, differentiating “tooth plate transport” as “ ‘vertical’ ( = basal-apical) with respect to the axis of the dental lamina” in chimaeroids, and as “ ‘lateral’ ( = lingual-labial)” in bradyodonts. Brvig ( 1985) elaborated or refined these distinctions, listing the following five differences between the toothplates of cochliodonts and chimaeroids:

( 1 ) Lyodonty (transverse growth) in cochliodonts versus statodonty (basal growth) in chimaeroids, correlated with the presence of a basal layer of compact hard tissue in cochliodonts and of an open basal part in chimaeroids.

(2) Superficial part consisting in cochliodonts of coronal hard tissue which is not divided into separate columns and terminates growth when the crown reaches its final depth, but consisting in chimaeroids of ascending columns of pleromin which continue to increase in depth thoughout life.

(3) Hypermineralized coronal hard tissue in cochliodonts is always vascular, and hypermineralization is inferred to depend on epidermal cells, whereas in chimaeroids the hypermineralized tissue (pleromin) may be vascular or compact and hypermineralization depends on mesenchyme cells.

36 C. PATTERSON

Figure 1. Bendix-Almgreen’s (1983: fig. 8) interpretation of statodont growth in chimaeroids (A) and lyodont growth in cochliodonts and sharks (B,C). A. Chimaera monsfrosa L., left lower jaw and toothplate, the toothplate drawn schematically and partly in transparency from stereoscopic radiographs. B. Cochliodont left posterior lower toothplate (Dclfodus concha (Trautschold), Upper Carboniferous, U.S.S.R.) and section of lower jaw (based on fleurodus a&is Salter). C. Shark, portion of lower jaw of CarcharhinusJalciJooris (Miiller & Henle). All in oblique lingual view. B and C emphasize the essentially shark-like (lyodont) mode of growth of the cochliodont toothplate, “a laferally growing structure, increasing in size by periodic apposition at the lingual side of successive generations of dental material (dgy, dg,) . . . which occupy positions beside each other (cf. arrow- heads) in a linguo-labially extending series across the jaw . . . these tooth plates, as they grew periodically also changrdposifion (large arrow) towards the side of the jaw: periodic growth and change in position across the jaw were part of the same rhythmic process by which these tooth plates retained maximum biting and grinding efficiency while their undiscardable older and worn parts became inroiled at the labial side of the jaw” [italics original]. In contrast, the Chimaera toothplate (A) “being of the sfafodont kind, grows throughout life continuously in the basal direction. The growth takes place along the entire basal site (gr. 2.) where new dental material continues to form and becomes added, effectuating the tooth plate’s age-correlated increase in size and compensating also for the constant loss of dental material superficially from abrasion on the biting area (ba) . . . this kind of tooth plate forms and grows continuously by fusion in the uerfical direction between consecutive generations of dental material (dg,, dg,-dg,) each occupying a horizonfal position and drveloping basally to the preceding one (cf. arrow-heads)’’ [italics original]. Abbreviations: ba, biting area; com, compartments occupied by prepleromin in ontogenetically youngest part of chimaeroid toothplate; d, d , ,, successive teeth in a tooth family in the shark jaw; dg, dg“ ‘, successive generations of dental material in the chimaeroid and cochliodont toothplate; Jl., grooves in the cochliodont toothplate interpreted as fusion lines between generations of dental material; g r z , basal growth zone in the chimaeroid toothplate; I.co.pk, m.co.pk, laterad and medial columns of pleromin in the chimaeroid toothplate; Lgr, s.l.gr, primary and secondary growth lines in the chimaeroid toothplate. From Bendix-Almgreen ( 1983) through the author’s generosity.


(4) Vascular canals in the coronal hard tissue are surrounded by denteons and are never arranged in rows in cochliodonts, whereas in vascular pleromin of chimaeroids the concentric systems surrounding the vascular canals may resemble osteons, and the vascular canals may be arranged in rows.

(5) Fibre-images in the hypermineralized coronal hard tissue are mostly interwoven in cochliodonts, whereas in chimaeroid vascular pleromin they tend to radiate from the denteons or osteons.

Brvig (1985) remarked that the toothplates of myriacanthoids resemble those of cochliodonts in the structure of the coronal hard tissue and basal compact layer, and that they lack continuously growing pleromin columns of chimaeroid type. The implication is that myriacanthoids have lyodont toothplates. In the early Jurassic Squuloruju the toothplates are clearly of this type (Patterson, in preparation).

ORIENTATION AND NOMENCLATURE OF CHIMAEROID TOOTHPLATES

I shall argue that the supposed gross structural differences between statodont (chimaeroid) and lyodont (cochliodont, myriacanthoid, squalorajoid) toothplates are largely a consequence of confusion or imprecision in orientating and naming the surfaces of chimaeroid toothplates. I n the preceding section on lyodont and statodont growth, Bendix-Almgreen’s ( 1983) characterization of the lyodont (cochliodont) mode of growth includes the terms “lateral”, “lingual”, and “labial”, whereas his characterization of the statodont (chimaeroid) mode includes the terms “horizontal”, “vertical”, and “basal”. Similarly, Brvig’s ( 1985) differentiation of cochliodont and chimaeroid toothplates includes (under item ( 1 ) above) the terms “transverse” and “basal”. The meaning of some of these terms is equivocal. In my opinion, the least ambiguous notation for the surfaces of teeth or toothplates is that adopted by mammalogists: mesial (or symphysial, towards the symphysis), distal (towards the jaw articulation), lingual (towards the tongue), labial (or buccal, towards the lip), and occlusal (or apical, the biting surface). There seems to be no generally accepted term for the surface opposite to the occlusal, which is in contact with the jaw and bears the roots in mammals; alveolar (in mammals) and basal are candidates, and the latter is used here. This system of nomenclature is easily adapted to most fishes (e.g. Cappetta, 1987, on elasmobranchs, who uses basal for the surface in contact with the jaw).

The nomenclature used for the various surfaces of the toothplates in chimaeroids has been varied and so inconsistent as to be chaotic. The sample in

TABLE 1 . Terms used for four surfaces of chimaeroid toothplates by various authors

Surface

Occlusal Lingual Labial Basal

Newton, 1878 Inner, oral, lower Posterior - Outer, upper Woodward, 1891 Inner, oral -. Upper, oral External, superior Ward & McNamara, 1977 Outer, oral - Oral Inner

Almgreen, 1983 Medial Basal Biting area Lateral ~ ~ inner, lateral outer, medial

Brvig, 1985 Medial Basal Superficial -

38 C. PATTERSON

distal

Figure 2. Terminology ol‘ toothpldte surfaces in myriacanthoids (A) and chimaeroids (B). A. Dentition of the early Jurassic myriacanthid Halonodon warneri Duffin, after Duffin (1984: figs 1.2). Anterior and posterior upper toothplates and mandibular toothplate, all from the right side in occlusal view, showing the terminology used by Duffin and that recommended here (Duffin’s terms are in brackets, where the two differ). B. Dentition of Recent Cullorhynchus rnilii Bory, after specimens shown in Figs 3,4. Anterior and posterior upper toothplates and mandibular toothplate, all from the right side in occlusal view, with the terminology recommended here. The broken line across the occlusal surface shows the margin of the wear surface in the specimen in Fig. 3.

Table 1 illustrates some of the problems. Duffin (1984; Fig. 2A here) used d mammalian^' terminology (lingual, labial, occlusal, basal, mesial/symphysial, distal) for myriacanthoid toothplates, and since a cochliodont toothplate is homologous with one or more files of teeth in a shark dentition (Fig. lB,C) this terminology should be as applicable in cochliodonts as it is in sharks. However, unlike mammalian or shark teeth, which are essentially rectangular in occlusal view and so have four margins (lingual, labial, symphysial, distal), the toothplates of cochliodonts, myriacanthoids and chimaeroids are essentially triangular in occlusal view (Figs 2-4, 7-10) and so have three margins. In myriacanthoids, Duffin’s (1984) nomenclature for those margins is shown in Fig. 2A. There are two inconsistencies and one real problem with that nomenclature. The first inconsistency is that in the mandibular toothplate Duffin named the three sides of the triangular occlusal surface the symphysial, labial and lingual margins, whereas in the upper toothplates these three sides were called the symphysial, labial and posterior margins; there is no reason to use different terms for upper and lower dentitions, and in the posterior upper toothplate I recommend “lingual” for Duffin’s “posterior” (this usage is justified later, and the problem of the anterior upper toothplate is discussed below). Second, Duffin was inconsistent in naming the angle or extremity at the mesial end of the labial margin of the toothplates, labelling it “anterior extremity” in the mandibular and posterior upper toothplates, and “proximal extremity” in the anterior upper toothplate. Since he called the opposing angle (at the other end of the labial margin) the “distal extremity”, consistency demands that the mesial angle or extremity be so named. The one problem with Duffin’s


Figure 3. Upper (A,B) and lower (C ,D) toothplatcs, in occlusal view, as shown in dried skulls of Recent Callor/ynchus milii Bory (A,C) and Chimaera rnonstrosa L. (B,D); x 1.6.

nomenclature concerns the anterior upper toothplate. As Fig. 2A shows, I believe that the margins he named “symphysial” and “posterior” are respectively the lingual and distal margins. The reasons for this are explained in the section on myriacanthoid toothplates, on p. 47. It is desirable that the “mammalian” nomenclature should be extended to chimaeroids, and the terms I recommend are shown in Fig. 2B. They are justified in detail below.

Chimaeroids have three pairs of toothplates, two in the upper jaw (here called anterior and posterior upper plates) and one in the lower (here called mandibular) (Fig. 3) . These toothplates are of two distinctive types, exemplified in Figs 3 and 4 by Recent Callorhynchus and Chimaera.

In Callorhynchus the basal surface of the toothplate (Fig. 4F,H), which is identified by its contact or apposition with the jaw cartilage in life (Fig. 5 ) , is easily distinguished from the symphysial surface, which is in contact or proximity with its antimere during life (Fig. 3A,C), and from the “outer” (labial) surface, which in life is partially exposed (towards the occlusal surface) and partially covered by the inner dental epithelium. The basal surface in Callorhynchus (Fig. 4F,H) is perforated by numerous vascular canals mesiolabially, whereas towards the open growth surface i t is composed of a smooth, dense tissue marked by weak growth lines parallel to the open growth margin. In the central part of the basal surface, the vascular canals enter the hard tissue obliquely, from the direction of

40 C. PATTERSON


the open growth surface, whereas towards the symphysial and labial margins of the basal surface the vascular canals enter more vertically, or less obliquely. This basal surface is separated from the symphysial and labial surfaces by a sharp rim which I shall call the descending lamina (d l , Fig. 4F,H; ldl, sdl, Fig. 5). The labial descending lamina of the mandibular tooth plate fits against the labial margin of Meckel’s cartilage (Fig. 5B), while the symphysial descending lamina of the mandibular toothplate and the entire descending lamina of upper toothplates (Fig. 5A) fit against the margins of bolsters or humps on the jaw cartilages. In toothplates of Cullorhynchus the descending lamina is deepest towards the front of the jaw (mesially), where its edge passes from the symphysial to the labial surface through a smooth curve in the posterior upper toothplate (Fig. 4F) and through an angle in the mandibular plate (Fig. 4H). The descending lamina decreases in height as it approaches the open growth margin of the toothplate and disappears just in front of that margin, so that the part of the basal surface closest to the growth margin is hardly distinct from the labial and symphysial surfaces. The outer face of the descending lamina, which forms the symphysial and the labial surfaces of the toothplate, is composed of a dense, smooth tissue which bears growth lines parallel to the free (basal) margin of the lamina, and which is glossier towards the occlusal surface than towards the free margin. The growth lines on the descending lamina show that i t grows basally throughout life. The inner surface of the descending lamina (towards the supporting cartilage) is vascular.

On the occlusal surface of the toothplates in Cullorhynchus (Figs. 2B, 3A,C, 4A,C) the wear surface occupies about two-thirds of the surface anteriorly, whereas the unworn posterior one-third, near the open growth margin, bears growth lines parallel to that margin and consists of glossy, compact tissue. In dried toothplates of Cullorhynchus the open growing margin (Fig. 4F,H) appears as a mesh of trabecular hard tissue surrounded by a thin shell of the dense tissue covering the occlusal, labial, basal and symphysial surfaces of the plate. This growth zone is morphologically posterior in life position (Fig. 3A,C), but in tooth terminology (Fig. 2) it is the lingual surface, since it is not in contact with the jaw (basal surface) and is separated from the symphysial (mesial) and labial surfaces by distinct angles.

In Cullorhynchus embryos, Schauinsland’s (1903: pl. 21, figs 151-156; Fig. 5 here) sections show that in the posterior upper plate the symphysial and labial descending laminae (Schauinsland’s “Klammerartige Fortsatze”) are well developed anteriorly (Fig. 5A), but more posteriorly (his fig. 153) only the labial descending lamina is present. In the mandibular plate (Fig. 5B), only the labial descending lamina is present in Schauinsland’s section through the middle of a plate. Kemp’s (1984: figs 2A,B, 5 ) illustrations of sections through toothplates of a 75 nim Cullorhynchus embryo do not show descending laminae.

The second type of chimaeroid toothplate is exemplified by Chimaera (Figs 3B,D, 4B,D,E,G). The principal difference from the toothplates of Cullorhynchus is that there is no descending lamina, so that the basal and labial surfaces of the

Figure 4. Toothplates of Recent Callorhynchur milii Bory (A,C,F,H) and Chimaera monsfrosu L. (B,D,E,G), all x 2. Left upper posterior (A,F) and right mandibular (C, H) plates of Cullorhynchus milii alongside right upper posterior (B,E) and left mandibular (D,G) plates of Chimaera monrfrosu in occlusal (A-D) and basal (E-H) views. dl, Descending lamina.

42 C. PAII‘ERSON

toothplate are not separable in isolated toothplates (Fig. 4E,G). In dried skulls (Fig. 3B,D) of Chimaera the labial surface is that outer surface of the toothplate exposed above the margin of the jaw cartilage, and the basal surface is the surface in contact with the jaw cartilage. But in isolated toothplates (Fig. 4E,G) these surfaces are continuous: both are composed of the same dense, glossy, hard tissue and are marked by growth lines parallel to the open (lingual) growth surface. The only trace of a descending lamina in Chimaera toothplates is a t the symphysial edge of the labial/basal surface, where there is a narrow pocket, larger in the posterior upper toothplate (Fig. 4E) than in the mandibular (Fig. 4G), containing spongy vascular hard tissue like that in the mesiolabial part of the basal surface of the toothplates of Callorhynchus. In other respects the toothplates of Chimaera and Callorhychus are similar, except that the pattern of tritors differs and the wear surface on the occlusal face is much less extensive in Chimaera, occupying about one-third of the occlusal surface compared with two- thirds in Callorhynchus (Fig. 3); that is, Chimaera has a more sectorial dentition than Callorhynchus, where the extensive wear surfaces imply a crushing habit.

VARIATION IN THE DESCENDING LAMINA IN CHIMAEROID TOOTHPLAI‘ES

Toothplates of Callorhynchus type, with an extensive descending lamina separating the basal surface from the labial and symphysial surfaces, occur in the extinct genera Brachymylus (Ward & McNamara, 1977: pl. 66; Fig. 6A,B here), Pachymylus (Woodward, 1892: pl. 3, fig. l a ) and Zschyodus (Newton, 1878: pl. 9, figs 13, 14; pl. 10, fig. 2; pl. 12, fig 6; Philippi, 1897; pl. 2; Fig. 6E,G here). Toothplates of Chimaera type, with no descending lamina or an insignificant one, occur in the living genera Hydrolagus and Harrioiia ((drvig, 1985: figs 1, 2) and probably also in Rhinochimaera and Neoharriotia, and in the extinct genera Edaphodon (Newton, 1878: pl. 1, figs 3, 5, 6, 8, 10; pl 2, fig. 2; pl. 6, fig. 6; Woodward, 191 1: pl. 40, figs 4, 5; pl. 41, fig. 2; Fig. 6F,H,J here), Elasmodectes, Elasmodus and Amylodon. Newton (1878:6) distinguished the two types of toothplate as “distinct bony layer upon exterior near oral margin” (the Callorhynchus type, with a descending lamina separating the basal and labial surfaces) and “no external bony layer” (the Chimaera type, with no obvious distinction between labial and basal surfaces). Woodward ( 1891 :53) described the Callorhynchus type as having “external thickening along the oral border”, a condition he ascribed to Ganodus, Zschyodus and Callorhynchus; and he described the Chimaera type as having “no external thickening along the oral border”, or (p. 91) “with no well-defined thickening upon the outer aspect immediately below the oral margin”, a condition ascribed to Edaphodon, Elasmodecies, Elasmodus and Chimaera.

In the Middle Jurassic Ganodus, Woodward (1891: pl. 1 , figs 11 , 12) illustrated mandibular plates showing a clear distinction between a glossy descending lamina and a vascular basal surface, but according to D. J. Ward (personal communication [cf. Jaekel, 1901 :545]) most of the mandibular toothplates determined as Ganodus are really Ischyodus, and the condition of the descending lamina is not yet properly known in Ganodus. In these Middle Jurassic mandibular toothplates (Fig. 6C,D) the descending lamina and basal surface are in the same plane and are separated by a narrow crevice, which may extend along the whole labial margin (Fig. 6C) or may be evident only beneath the


' Mc Figure 5. A, B, Transverse sections through toothpla tes of Cullorhynchus embryos, after Schauinsland (1903: pl. 21, figs 152, 155). A is through the anterior part of the right posterior upper plate of a 90 mm embryo, x 38; B is through the middle part of a right mandibular plate of a 95 mm embryo, x 23. At these stages the toothplates are covered by epithelium (epi) and are not yet functional. In the upper plate, A, the labial (Idl) and symphysial (sdl) descending laminae are both well developed at this level, and clasp the margins of a hump on the palatoquadrate cartilage (pq). I n the lower plate, B, only the labial descending lamina (Idl) is developed at this level. It lies close to the labial margin of Meckel's cartilage (Mc). Note that the basal surfaces of the toothplates, adjacent to the supporting cartilage, are closed OK by hard tissue (stippled). The pleromin (pi) of the triton is developing in vascular spaces (vs) within the trabecular hard tissue of the toothplates. Abbreviations: epi, epithelium; ldl, labial descending lamina; Mc, Meckel's cartilage; p l , pleromic hard tissue; p 9 , palatoquadrate; sdl, symphysial descending lamina; us, vascular space in toothplate.

more distal part of the labial surface (Fig. 6D). These are typical patterns in Ischyodus mandibular plates, as shown in Fig. 6E (where the descending lamina is distinct only distally) and by Philippi's (1897: pl. 2, fig. 1 ) illustration of Ischyodus sueuicus, where the descending lamina extends along the whole labial margin. In posterior upper plates of Ischyodus (Fig. 6G; Philippi, 1897: pl. 2, fig. 2), the labial descending lamina and basal surface are separated by a similar narrow crevice, whereas mesially there is a deep pocket, occupied in life by the jaw cartilage and containing vascular hard tissue, as in Callorhynchus. In posterior upper plates of Eduphodon (Fig. 6HJ; Newton, 1878: pl. 1, figs 5, 6) there is no distinction between the basal and labial surfaces, but the mesial part of the basal surface shows a pocket for the jaw cartilage separated from the symphysial surface by a descending lamina. There is not much difference between the form of the basal/labial surface in these upper toothplates of Edaphodon (Fig. 6H,J) and those of Chimaera (Fig. 4E). Similarly, mandibular toothplates of Edaphodon (Fig. 6F; Newton, 1878: pl. 1, fig. 3, pl. 2, fig. 2) have the descending lamina restricted to a narrow pocket on the most mesial 20% of the labial face and are not very different in this feature from mandibular plates of Chimaera (Fig. 4G), where the descending lamina occupies only the most mesial 5% of the labial face.

44 C. PATTERSON

Figure 6. Mandibular (A,C,D,E,F) and posterior upper (B,G,HJ) toothplates of fossil chimaeroids, the mandibular plates in “ventrolateral” view (showing the labial and basal surfaces) and the upper plates in “donolateral” view (showing the labial and basal surfaces), A,B, Left mandibular and posterior upper toothplates of the same individual of Brachymylus alfidens Woodward, Callovian, Oxford Clay, Peterborough (P.57041a,b), x 0.5; C,D, right (C) and left (D) mandibular toothplates of “Canodus’’, Bathonian, Stonesfield Slate, Stonesfield, Oxon. (P.599, P.5108), x 2; E, left mandibular plate of Ischyodus bcaumonfi Egerton, Callovian, Oxford Clay, Peterborough (P.6895), x 0.75; F, left mandibular plate of Edaphodon scdgwicki Agassiz, Campanian, Upper Chalk, Norwich (P.414), ~ 0 . 3 3 ; G, left posterior upper plate of Ischyodus bcaumonfi Egerton, Kimmeridgian, Kimmeridge Clay, Weymouth (43283), x 0.75; H,J, right posterior upper plates of Edaphodon agassizi (Buckland) (H), Upper Cretaceous, Chalk, Lewes (P.14213), x 0.75, and E . lcptognafhus Agassiz u), Lutetian, Earnley Fm, Bracklesham Bay, Hants (25720), x 0.67.


COMPARISON OF GROSS STRUCTURE, ORIENTATION AND GROWTH OF TOOTHPLATES IN CHIMAEROIDS, PTYCTODONTS, MYRIACANTHOIDS AND SQUALORAJA

Given first that there are two types of chimaeroid toothplates, exemplified among Recent forms of Callorhynchus and Chimaera (Fig. 4), and second that the differences between these two types in the form of the descending laminae are partially bridged by extinct genera (“Ganodus”, Ischyodus, Eduphodon; Fig. 6) it is important to discover which of the two types is primitive. Since I know of no cladistic or phylogenetic analysis of chimaeroid genera to which one might turn, and since nothing useful is known from ontogeny, outgroup comparison is the only available way of polarizing the toothplate features. As mentioned in the introduction, two very different outgroups for the chimaeroids have advocates. One is the early Jurassic Spaloraja and the late Triassic-early Jurassic myriacanthoids (Patterson, 1965, 1968), the other is the Devonian ptyctodont arthrodires (Brvig, 1960, 1962, 197 1, 1980, 1985; Bendix-Almgreen, 1968, 197 1, 1983; Jarvik, 1980).

Ptyctodonts

In ptyctodonts, there is a single pair of toothplates in each jaw. The best preserved ptyctodont toothplates accessible to me are those of acid-prepared specimens of Ctenurella and Campbellodus from the late Devonian (Frasnian) Gogo Formation of Western Australia (Miles & Young, 1977; Forey & Gardiner, 1986); in Ctenurella these toothplates are associated with jaw bones. The toothplates of these two genera have labial and lingual descending laminae (lateral and mesial lamellae of Miles & Young, 1977:150, 178) that fitted in grooves in the supporting bones (Miles & Young, 1977: figs 24, 25, 27, 28). I t is therefore clear that the open growth surface that lies between the descending laminae in ptyctodonts is a true basal surface, opposed to the supporting bone and cartilage, and is not comparable to the growth surface in chimaeroids, which is the lingual surface.

My riacan thoids

In myriacanthoids, there are two or three pairs of toothplates in the upper jaw and one pair and a median symphysial toothplate in the lower. Myriacanthoid toothplates have descending laminae, best known in the posterior upper and mandibular plates of Myriacanlhus (Fig. 7). In the posterior upper toothplate (Fig. 7B,C) there is a stout labial descending lamina occupying almost the entire labial margin. It is deepest mesially, at the labial side of the contact with the preceding upper toothplate, and decreases in height distally, fading away at the most distal end of the labial margin, as in Cullorhynchus. Mesially, where the posterior upper toothplate contacts the preceding upper toothplate, the descending lamina decreases in height rapidly after it passes from the labial to the symphysial margin, so that there is only a short symphysial descending lamina, bounding a deep pocket in the basal surface of the mesial end of the toothplate. The inner (towards the jaw cartilage) surface of the labial descending lamina is vascular, as it is in Callorhynchus, but in contrast to the latter, where the labial face of the descending lamina is smooth and glossy, the labial face of the

46 C. PATTERSON

Figure 7. Toothplates and jaws of Myriacanthus paradoxus Agassiz from the Lower Lias (Sinernurian), Lyme Regis, Dorset. A, B, P.4664; A in ventral view (cf. Woodward, 1891: pl. 2, fig. I ) , showing the labial descending lamina (Idl) on the mandibular toothplates; B, the left posterior upper toothplate (Ipu in A) in basal (dorsal) view, exposed in an excavation on the back of the block, showing the labial (Idl) and symphysial (sdl) descending laminae. C, P.151 (cf. Woodward, 1891: pl. 2, fig. Z), dentition in dorsal view showing the labial descending laminae (Idl) on the mandibular and posterior upper toothplates. A,C, x 0.6; B x 0.75. Abbreviations: apu, anterior (of three) pair of upper toothplates; Idl, labial descending lamina; Ipu, left posterior upper toothplate; mpu, middle of the three pairs of upper toothplates; stp, lower symphysial toothplate.


lamina in Myriacanthus is sparsely vascularized. The basal surface of the posterior upper toothplate in Myriacanthus is vascular, but vascularization seems to be age- or size-related, since the vascular area differs in extent in the three available specimens. In the smallest, P. 10130 (Woodward, 1906: pl. I) , vascularization of the basal surface is confined to the mesiolabial quadrant, but in P.4664 (Fig. 7B), a larger individual, and P.151 (Fig. 7C), the largest of the three, the entire basal surface is vascular, except for an area at the mesiolingual corner and a narrow strip along the lingual margin. Towards the lingual margin of this basal surface the vascular canals enter obliquely, running into the hard tissue at right angles to the nearest part of the margin, whereas towards the labial and symphysial margins the vascular canals enter more vertically or less obliquely, as in Cullorhynchus. In Halonodon, Duffin’s (1984: fig. 4D) photograph of the basal surfke of a posterior upper plate shows the same conformation as in My? iacanthus.

In the mandibular toothplate of Myriacanthus (Fig. 7A,C) there is a deep labial descending lamina which decreases in height distally, as in the upper plate and in Cullorhynchus. There is a short but deep symphysial descending lamina, grooved by its contact with the median symphysial toothplate and decreasing rapidly in height towards the lingual margin. The external face of the labial descending lamina is sparsely vascular, and the external surface of the symphysial descending lamina is coarsely vascular in the groove for the symphysial toothplate. The inner faces of the descending laminae and the basal surface of the mandibular toothplate are accessible in only one specimen (P.477; Woodward, 1891: pl. 2, fig. 3, now partially acid prepared). In that specimen the inner faces of the descending laminae and the entire basal surface are vascular, but the vascular canals in the basal surface are sparser in the mesiolingual and labiodistal quadrants. In Halonodon mandibular toothplates there are labial and symphysial descending laminae (Duffin, 1984: 60, “labial and lingual root wall”) like those in Myriacanthus.

Myriacanthus has two pairs of anterior upper toothplates (up., mpu, Fig. 7A,C). As mentioned above, there is a problem in identifying the margins of these and in matching their nomenclature between myriacanthoids and chimaeroids. Figure 2 shows that the principal toothplates (posterior upper, mandibular) of myriacanthoids and chimaeroids are readily comparable, but the anterior upper toothplate is very different in the two. That of chimaeroids resembles the posterior upper toothplate, in growing from a morphologically posterior lingual margin and in having a long symphysial margin which meets its antimere in the midline (Fig. 3A,B). In myriacanthoid anterior upper toothplates, the orientation of the rows of tritors shows that they grew from the midline, from a morphologically medial margin which meets or approximates its antimere (Figs. 2A, 7A,C). Like cochliodont and chimaeroid toothplates and the other toothplates of myriacanthoids, these anterior upper toothplates are essentially triangular. Like the toothplates of cochliodonts (Fig. IB), they grew in “lyodont” fashion, across the jaw from lingual to labial. Their morphologically medial growth margin (Duffin’s (1984) “symphysial” margin, Fig. 2A) is therefore the lingual margin, and their morphologically posterior margin (so named by Duffin (1984), Fig. 2A), which contacts the symphysial margin of the succeeding toothplate (Fig. 7A,B), must be the distal margin. There remains the problem of what name to give to the third side of the triangular occlusal surface.

4a C. PATTERSON

Duffin (1984) named it the labial margin and I follow him since, as Fig. 7A shows, that margin forms the labial edge of the upper dentition. This means that the symphysial border of myriacanthoid anterior upper toothplates is represented only by the mesial angle (Fig. 2A), Duffin’s (1984) “proximal extremity”. Thus, in comparing the toothplate margins of chimaeroids and Myriacanthoids (Figs 2, 3, 7), the principal toothplates (mandibular, posterior upper) differ only in the relative lengths of the symphysial and lingual margins, with the symphysial about twice as long as the lingual in chimaeroids and the lingual about twice as long as the symphysial in Myriacanthoids (Fig. 2) . The anterior upper toothplate of chimaeroids has the same proportions as the other toothplates, with the symphysial margin about twice as long as the lingual, but in Myriacanthoids anterior upper plates the symphysial margin is represented only by the mesial angle of the toothplate. The anterior upper toothplates of Myriacanthus are best known in the two specimens shown in Fig. 7. These imply that there was a well-developed descending lamina along the distal and labial margins and that the descending lamina is deepest at the oldest corner of the toothplate, where the distal and labial margins meet; Halonodon has the same configuration of the upper anterior toothplate (Duffin, 1984:63).

In Metopacanthus, P.1158 shows that the posterior upper toothplate had a labial descending lamina similar to that of Myriacanthus; the lamina is shown in the thin section illustrated by Patterson (1968: fig. 15); P. 1158 also shows that the inner face of the labial descending lamina is vascular, whereas the thin section just cited, P.1158 and 43050 all suggest that the labial face of the descending lamina is not vascular, a difference from Myriacanthus and a resemblance to Callorhynchus. The basal surface of the plate is vascular and, as in Myriacanthus, vascularization seems to be age- or size-related, since in the two available specimens the smaller (43050) has only the mesiolabial one-third of the surface vascularized, whereas P. 1 158 has the entire surface vascularized except for a zone bordering the lingual margin and an area mesiolingually. In the mandibular toothplate of Metopacanthus, P.3099 (Duffin, 1983b: pl. 1, fig. 3) shows the symphysial descending lamina well, and the rather similar mandibular plate named Alethodontus by Duffin (1983b: pl. 1, fig. 2) shows the labial descending lamina. The basal surface of the mandibular toothplate is not accessible in Metopacanthus.

In the Rhaetic Agkistracanthus, Duffin & Furrer (1981: pl. 2, fig. lb,c) illustrated a mandibular toothplate showing a strong labial descending lamina (“labial root wall”) and an incomplete (broken) symphysial descending lamina. The upper toothplates of Agkistracanthus illustrated by Duffin & Furrer (1981: pl. 2, figs 2, 3) are strongly inrolled labiolingually, but there are labial and symphysial descending laminae (Duffin & Furrer’s “root wall” and “end wall”). In all the toothplates of Agkistracanthus the basal surface is vascular labially, with the vascular canals entering from the direction of the nearest lingual margin, as in Myriacanthus. Finally, in Acanthorhina (Duffin, 1983a) the posterior upper and mandibular toothplates have labial descending laminae, apparently similar to those in Myriacanthus. Thus, all myriacanthoids have labial and symphysial descending laminae on the mandibular and posterior upper toothplates, and genera in which the basal surface is known have vascular canals entering the medial face of the labial lamina and all but the lingual part of the basal surface. The labial face of the labial descending lamina is sparsely vascular in


Myriacanthus but appears not to be vascular in Metopacanthus. Growth lines are not visible on myriacanthoid descending laminae, but the Callorhynchus-like pattern, with the deepest parts of the lamina at the oldest angle of the toothplate (the mesial angle in the principal toothplates, the labio-distal angle in anterior upper toothplates), indicates that the lamina grew basally throughout life.

Squaloraja

In Squaloraja there are two upper pairs of toothplates and one pair on the lower jaw, as in chimaeroids, but the long (mesio-distally) and narrow (labio- lingually) toothplates are very different in shape from those of chimaeroids or myriacanthoids. In the principal toothplates (posterior upper, mandibular) of Squaloraja there is a strong descending lamina extending along the labial and symphysial margins and decreasing in depth distally and lingually, as in myriacanthoids and Callorhynchus (Patterson, in preparation). In Squaloraja the basal surface of the toothplates and the surfaces of the descending laminae appear not to be vascularized in the available specimens.

Conclusions

From these comparisons, two principal conclusions emerge. First, so far as the gross structure, orientation and mode of growth of the toothplates are concerned, the only satisfactory immediate outgroup for the chimaeroids is the myriacanthoids and/or Squaloraja; ptyctodont arthrodires had statodont (basally growing) toothplates which are not comparable with those of chimaeroids. Second, because labial and symphysial descending laminae are universal in myriacanthoids and Squaloraja, the condition in Callorhynchus and the fossil chimaeroid genera Brachymylus and Pachymylus, where the labial and symphysial descending laminae are well developed, is primitive for chimaeroids.

COMPARISON WITH COCHLIODONTS

The “cochliodonts” are traditionally a heterogeneous assemblage of nominal genera, almost all based only on isolated toothplates or teeth. They are typically Carboniferous, although there are a few late Devonian and Permian records. In this broad sense, cochliodonts are unlikely to be a natural group. Lund (1986, 1990) has recently published brief accounts of several new “cochliodonts” known by articulated skeletons from the Namurian Bear Gulch Limestone, Montana, referring to them as ( 1986) “cochliodont 1 ” or “CO 1 ”, “cochliodont 3” and “C04” or (1990) “COCHI-9”. The first of these fishes (“COI”) resembles Echinochimaera (Lund, 197713, 1988) in various features (e.g. structure of the two dorsal fins; structure of paired fins), including several of those that Lund (197713: 2 14) had earlier cited as unique to chimaeroids and Echinochimaera (e.g. “basic structure of the synarcuum, first dorsal fin and spine, the pectoral girdle, pelvic girdle, fin, and clasper”). Doubtless for this reason, in Lund’s recent cladogram (1986: fig. 4) “CO1” and “C04” emerge as the sister-group to his Chimaeriformes (Chimaeroidei + Echinochimaeroidei) . However, “CO 1 ” also resembles menaspoids (Patterson, 1965; Menaspis and Deltopochius) in several features which are unknown in chimaeroids or in Squaloraja: presence of

50 C. PATTERSON

(1 ) occipital and (2) mandibular spines, presence of dermal plates on (3) the rear of the palatoquadrate and (4) the supraorbital region, presence of (5) anterior tooth series or families (rather than toothplates), absence of (6) the frontal clasper, absence of (7) polyspondylous chordacentra and of (8) bony crescents surrounding the sensory canals. Echinochimaera shares six of these eight features: ( I ) , (4), (5) (Lund, 1988:198), (6), (7) and (8). Among myriacanthoids, Myriacanthus shares ( I ) , (2), (3) and (4) of the eight, whereas Metopacanthus shares only (2 ) . I therefore see Echinochimaera as less closely related to the chimaeroids than is Squaloraja or the myriacanthoids, and as part of the “cochliodonts” in the broad sense of including the menaspoids. In this broad sense of fishes with toothplates placed by Woodward ( 1889) in the Cochliodontidae, “cochliodonts” will also include the chondrenchelyids (Chondrenchelys, Harpago fututor, PlaQxystrodus, ?Solenodus; Lund, 1982) and Zangerl’s ( 198 1 ) families Psephodontidae (Psephodus, ? Jimpohlia) and Deltodontidae (? Deltodus only).

Cochliodus and Streblodus

The type-species of the type cochliodont genus, Cochliodus contortus (Agassiz), is based on a lower jaw carrying two pairs of toothplates (Patterson, 1968: fig. 12), and in front of the more mesial of those there is a labio-lingually orientated hump or bolster of calcified cartilage (also shown in other more or less intact mandibles, e.g. Fig. 8A-C), which must have carried a third toothplate, assumed by Zangerl ( 198 1 : fig. 2 1 D) to be a series of Helodus-like teeth. Davis ( 1883: 422) noticed that in Cochliodus contortus “the earlier or outside convolutions [of the toothplate] became embedded in the cartilaginous mass of the jaw”. Zangerl (1981: fig. 21D) took up this point in making a reconstructed section through a lower jaw of Cochliodus contortus showing the toothplate curving through about one-and-a-quarter turns (450”), with the yost labial three-quarters of that spiral (the oldest of first-formed part of the toothplate) embedded in the cartilage of the jaw. The accuracy of these observations is confirmed by several BMNH specimens of Cochliodus contortus. In the two principal lower toothplates of C. contortus the exposed surface of the toothplate, when in position on the jaw, occupies just over half a revolution ( 180”), and the most labial or ontogenetically oldest part of the toothplate, which may occupy three-quarters of a revolution, is enclosed within the calcified cartilage of the jaw (Fig. 8). Zangerl (1981: 18, 42) used this feature to distinguish the Cochliodontidae from his Deltodontidae and the Menaspidae, and interpreted it as a consequence of the toothplate growing faster than the jaw. The question of toothplate growth is discussed in a separate section below. The toothplates of Cochliodus contortus have a crown of “tubular dentine” underlain by spongy osteodentine and a basal layer of dense lamellar tissue (Patterson, 1968: fig. 14; Brvig, 1985: 73; Fig. 8D,E here) whose basal surface is not vascularized. Both the coronal hard tissue and the basal lamellar tissue increase in thickness lingually, and on the lingual margin of the toothplate the lamellar tissue extends well beyond the open (unfinished) growth margin of the crown (Fig. 8C). In Cochliodus mandibular toothplates there is nothing comparable with the labial and symphysial descending laminae of chimaeroid and myriacanthoid toothplates.

In reconstructing the dentition of Cochliodus contortus, Zangerl ( 198 1 : fig. 2 1 C) , following St John & Worthen (1883) and Patterson (1968: 195), assumed that


Figure 8. Lower toothplates and jaw of Cochliodus contortus (Agassiz) from the Lower Carbonilerous Limestone (Visean), Armagh, N. Ireland. A-C, A lower jaw of Cochliodur contortur, P.62620, in dorsal (A), left anterolateral (B) and right posterolateral (C) views, x 1.6. The two principal left toothplates are preserved, the more posterior one incomplete. B shows how the oldest part of the toothplates is enrolled in the calcified cartilage of the jaw; C shows how the lingual margin of the lamellar tissue extends well beyond the open (unfinished) growth margin of the crown of the toothplate. D, E, Isolated right posterior mandibular toothplate of C. confortus, P.62625, in basal (D) and mesial (E) views, x 1.6. There is no symphysial or labial des'cending lamina.

the posterior or principal upper toothplates are those named Streblodus oblongus (Portlock). There is one specimen in which Streblodus oblongus toothplates are associated with cartilage (Patterson, 1968: fig. 13) and this confirms that they are from an upper jaw. In this upper dentition there was one pair of principal toothplates (Streblodus oblongus) with two whorls or tooth-families in front, of which the more distal (the only one preserved) consists of Helodus-like tooth crowns on a common base. The Streblodus oblongus (principal upper) toothplates of this jaw show, as Davis (1883) noted, the same growth pattern as Cochliodus contortus, with the most lingual or ontogenetically oldest part of the plate embedded in the cartilage of the palatoquadrate, and, as in C. contortus, the exposed surface of the toothplate occupies just over half a revolution ( 180"), with the most labial part, which may occupy almost an entire revolution (360"; Fig. 9C), enclosed within the jaw. Streblodus oblongus toothplates have the basal lamellar layer extending lingually beyond the open growth surface of the crown tissue, as do Cochliodus lower toothplates, but in contrast to the latter, in S. oblongus the lamellar tissue also extends beyond the crown labio-distally, so that there is a form of labial lamina along the distal part of the labial border (bottom of Fig. 9A). Also in contrast to Cochliodus lower toothplates, in S. oblongus toothplates there is a rudimentary symphysial descending lamina, with the lamellar tissue extending basally in a low ridge along the margin of the toothplate adjacent to the whorl of Helodus-like teeth that precedes it (Fig, 9A,C). But although Streblodus oblongus seems to resemble myriacanthoids and

52 C. PATTERSON

Figure. 9. Toothplates of cochliodonts from the Lower Carboniferous Limestone of Annag,,, N. Ireland (A-K, Visean) and Black Rock, Bristol (L, M, Tournaisian). A-C, Isolated right posterior upper toothplate of Slrcblodus oblongus (Portlock), P.62629, in occlusal (A), basal (B) and mesial (C) views; A, B, x 1.5; C, x 2. D-F, isolated right mandibular toothplate of Dclloptychius acutus (M’Coy), P.2437, in occlusal (D), basal (E) and mesial (F) views, x 2. G , H, Isolated toothplate of Poecilodus

joneszi (Portlock), P.2462, in occlusal (C) and basal (H) views, x 2 . J, K, Isolated toothplate of Dcltodus sublaeuis (M’Coy), P.2444, in occlusal u) and basal (K) views, x2. L, M, Isolated toothplate of Enniskilln ( = Eufomodus) connexus (Davis), P.48457, in occlusal (L) and basal (M) views, x 2.


chimaeroids in having some form of labial and symphysial descending laminae, the resemblance is merely superficial because in Streblodus the labial lamina is deepest distally and the symphysial lamina is deepest lingually, whereas in myriacanthoids and chimaeroids the descending laminae are deepest at the mesial angle of the toothplate, where they meet. This difference is clearly related to mode of growth. In Callorlynchus and Myriacanthus, for example, the descending lamina grew basally throughout life, and for that reason both the symphysial and labial laminae are deepest at the mesial angle of the toothplate, the oldest or first-formed part. In Streblodus oblongus, however, the rudimentary symphysial and labial descending laminae did not grow, but were laid down at the same time as the adjoining part of the coronal tissue; they are therefore shallowest at the mesial angle of the toothplate, the oldest or first-formed part, and deepest adjacent to the lingual growth surface, where the youngest but largest part of the toothplate is laid down.

Deltoptychius and Menaspis

In Deltoptychius (Patterson, 1965, and in preparation) the lower dentition comprises one pair of toothplates and the upper contains one principal pair (“Streblodus colei”) with two or three whorls of Helodus- or Pleuroplax-like toothplates in front of them, each on a common base. The lower and principal upper toothplates of Deltoptychius show the same mode of growth as Cochliodus, with the most labial (oldest) part of the toothplate enrolled in the jaw (seen in P.20146-49, the articulated dentition of Delptoptychius armigerus (Traquair) described by Moy-Thomas, 1936a and Patterson, 1965). The mandibular toothplate of Deltoptychius (Fig. 9D-F) differs from the principal mandibular toothplate of Cochliodus (Fig. 8) in having a pronounced descending lamina along the symphysial margin (cf. Figs 8E, 9F), composed partly of coronal hard tissue (towards the occlusal surface) and partially (most basally) of lamellar tissue only. But this descending lamina seems to be merely a consequence of the form of the lower dentition in Deltoptychius, with only a single pair of toothplates that met in the midline, and the descending lamina was clearly laid down at the same time as the adjoining part of the toothplate and did not grow basally during life. The principal upper toothplate of Delloptychius (“Streblodus colei”) has rudimentary, non-growing labial and symphysial descending laminae like those in Streblodus oblongus, described above.

In Menaspis there is a single pair of principal toothplates in each jaw; there may have been a whorl of Helodus-like teeth mesially in the upper jaw (Patterson, 1968: fig. 7; Bendix-Almgreen, 1971: fig. 5), and one specimen suggested to Zangerl ( 198 1 : fig. 47B) that there is a lower symphysial tooth. Zangerl ( 198 1 : 42) inferred that the oldest portions of the principal toothplates of Menaspis were not enrolled into the jaw; since he drew the same inference about Deltoptychius and Deltodus and was mistaken in both cases (see above on Deltoptychius and below on Deltodus), he might also have been mistaken about Menaspis. The best information on Menaspis toothplate morphology available to me is a series of stereoscopic radiographs of the Halle specimen (Patterson, 1968: figs 2, 6), supplemented by Zangerl’s (1981: fig. 46) stereoradiographs of the upper dentition in Schaumberg’s specimen. In the radiographs of the Halle specimen, the mesiolabial corner of the mandibular toothplates stands out as a radio-

54 C. PATTERSON

opaque area, even in the relatively poor quality of published versions (Patterson, 1968: fig. 6; Ortlam, 1985: pls 2, 3; note that Ortlam interprets the exposed dorsal surface of the fossil as ventral, and so reverses his radiographs), and in the orginal radiographs it can clearly be seen, especially in the left mandibular toothplate, that this opacity is caused by the oldest part of the toothplate being rolled inwards beneath the occlusal surface, just as in Delloptychius, Deltodus and Cochliodus. Opacity in the radiographs also implies that the mandibular toothplate of Menaspis had a symphysial descending lamina like that in Deltoptychius. In the upper toothplates of Menaspis, interpretation is not so easy. The outline and orientation of the upper toothplates are congruent between the Halle (Patterson, 1968: figs 2, 6), Schaumberg’s (Zangerl, 1981: fig. 46), and other specimens (Weigelt, 1930: pl. 4, fig. 1, pl. 5, fig. 2); Zangerl (1981: fig. 47A) restored the Schaumberg specimen as having the middle part of the labial margin rolled basally through about 120”, but both the shape of the toothplate and the orientation of linguo-labial striations in radiographs of the Halle specimen are such that the oldest part of the toothplate is likely to have been more mesially placed on the labial margin than in Zangerl’s restoration, though apparently not at the mesiolabial corner, as it is in the lower toothplate and in Deltoptychius upper toothplates.

Poecilodus, Deltodus, Sandalodus, Enniskillen

Among other genera of cochliodonts (sensu lato; type-species are named and were examined except in Sandalodus, where lack of material forced me to rely on the second species named), Poecilodus (P. jonesii (Portlock), Fig. 9G,H) and Deltodus (D. sublaevis (M’Coy), Fig. 9J,K) resemble Streblodus in having the oldest part of the toothplate enrolled into the jaw and in having rudimentary, non- growing labial and symphysial descending laminae. Sandalodus ( S . parvulus Newberry and Worthen; S. morrisi Davis) mandibular toothplates apparently agreed with those of Deltoptychius, having a pronounced but non-growing symphysial descending lamina mainly composed of coronal tissue. Sandalodus principal upper toothplates are broadly similar to Streblodus, but less tightly enrolled. Enniskillen ( = Eutomodus, E . convexus (Davis), Fig. 9L,M) toothplates are very thick, with a plane symphysial surface and no symphysial descending lamina. They are less strongly curved labio-lingually than Deltodus toothplates (cE Fig. llD,E), and the oldest portion of the toothplate might not have been enrolled into the jaw. There is something resembling a labial descending lamina (Woodward’s “extension of the root beyond’’ the crown; 1889: 191), an area lacking coronal hard tissue along the labial margin, but as Fig. 9L shows, this area is more extensive distally than mesially, and so, like the similar structures in other cochliodont toothplates, it was not basally growing and is not a true descending lamina.

Helodus and Psephodus

Helodus ( type-species H. simplex Agassiz; Moy-Thomas, 1936b) and Psephodus (type-species P. magnus (Portlock); Traquair, 1885) are both known by associated dentitions showing that at least some of the rather numerous tooth families consisted of separate teeth. In these genera there is no evidence that


Figure 10. Isolated posterior left upper (A, C, P.2475d) and right mandibular (B, D, P.2475) toothplates of Pfalyxysfrodus sfriatus (M’Coy) in occlusal (A, B) and basal (C, D) views, x 4 . Both from the Lower Carboniferous Limestone (Visean), Armagh, N. Ireland. In A and B the labial margin is towards the top of the photograph. In A the lingual growth surface is to the right, and in B i t is to the lower right. In C the labial descending lamina is towards the bottom of the photograph, and both i t and the shallower symphysial descending lamina are broken away mesially ( to the left). In D the mesial (deepest) part of the labial descending lamina is crushed on to the basal surface of the toothplate at bottom left, and the middle part of the lamina is broken away. The mesial (deepest) part of the symphysial descending lamina is crushed outwards in this toothplate, and is best seen at the left of B.

teeth were inrolled into the jaw and there is nothing resembling a descending lamina. I t is not known whether any teeth were shed in Helodus or Psephodus, but since both genera have at least one entire tooth family represented by a toothplate (“Pleurodus ajinis” in Helodus; Psephodus magnus), shedding is unlikely.

Chondrenchely ids

Platyxystrodus ( = Xystrodus, type-species P. striatus (M’Coy); Fig. 10) differs markedly from all the preceding genera, most obviously in having the denteons of the “tubular dentine” arranged in rows parallel to the lingual growth surface (Fig. IOA,B). Platyxystrodus striatus was based by M’Coy (1855: pl. 31, fig. 27) on toothplates of two types, one flat and with a relatively narrow lingual margin (M’Coy’s fig. 27, 27a; Fig. lOA,C here), the other broader and with “a twisted appearance” (Woodward, 1889: 193; M’Coy’s fig. 27b; Fig. lOB,D here). Davis (1883) described the two forms as separate species: P . striatus for the broader form and P. angustus for the narrower, although he admitted that the latter “may be the teeth of the upper jaw of [ P . ] striatus”. Woodward (1889) took that view, synonymizing P . angustus with P . striatus, and St John & Worthen (1883), in describing several American species, also assigned the narrow teeth to the upper jaw and the broad ones to the lower. The validity of these interpretations is supported by the arrangement of the toothplates in the chondrenchelyid Harpagofututor (Lund, 1982: figs 2, 4), where the toothplates show the same

56 C. PATTERSON

pattern of parallel rows of denteons as in Plutyxystrodus and the principal upper toothplates are narrow and elongate in comparison to the principal mandibulars. In Hurpagofututor, Lund (1982: 944) found that the principal mandibular toothplate “extends down the mesial surface of the lower jaw to the midline”, that at “the anterior end of each plate the occlusal surface is highest mesially, where it is considerably elevated above the lamellar lateral margin”, and that “growth is from the rear [ = lingual] edge of the plate and, to a lesser extent, along the mesial edge as well”. Plutyxystrodus is difficult to study because of the small size, rarity and indifferent preservation of toothplates, but it seems to match Hurpugofututor in these features. Figure 10 shows that the principal upper and lower toothplates of Plutyxystrrodus had labial and symphysial descending laminae, and that these are deepest at the mesial angle of the toothplate and shallowest near the lingual growth surface, implying (as Lund inferred for the symphysial lamina in Hurpagofututor) that they grew basally throughout life, like the descending laminae in myriacanthoids, Squuloruju and chimaeroids. In both jaws, the external (symphysial) face of the symphysial lamina is vascular, but its internal (labial) face and both faces of the labial lamina are not vascular. Chondrenchelyids therefore had a pattern of principal toothplate growth differing from that in other “cochliodonts” but resembling that in myriacanthoids, Squuloruju and chimaeroids. Anterior upper and lower toothplates of chondrenchelyids are well-known only in Hurpugofututor (Lund, 1982: pl. 2, figs 2, 4, 5). They show a striking resemblance to the anterior upper toothplates of myriacanthoids, in particular to those of Myriacanthus (Woodward, 1891: pl. 2, figs 1, 2u; Fig. 7 here) and Halonodon (Duffin, 1984: figs 2, 3D), in possessing linguo-labial rows of tritors and in growing from the midline rather than from a morphologically posterior lingual margin like the principal toothplates. The rows of denteons parallel to the lingual growth surface in the principal toothplates of Hurpugofututor and Plutyxystrodus are a feature shared with various chimaeroids ((arvig, 1985: 63, 74, figs 6-8).

PROBLEMS OF RELATIVE GROWTH

Symphysial (mesial) views of toothplates of Cochliodus (Fig. 8E) and Streblodus (Fig. 9C) show that their section and mode of growth is a logarithmic or equiangular spiral, like the molluscan shell or the tusks and horns of mammals (Thompson, 191 7). Amongst chondrichthyan dentitions, the logarithmic spiral is most obviously exemplified by symphysial tooth-whorls of the Permian edestoid Helicoprion (Bendix-Almgreen, 1966; Fig. 11A here), which may turn through up to 3.75 volutions. The form of a logarithmic spiral is determined by its constant angle (the angle between a polar radius and a tangent to the spiral at any point) or by the ratio of the breadth of one whorl to its predecessor or successor. In Helicoprion ferrieri I estimate the latter ratio as about 2.3 on the basis of Fig. 11 A, giving a constant angle of about 82”. In Streblodus oblongus (Fig. 11B) and Cochliodus contortus (Fig. 1 lC), the ratio between whorls is respectively roughly 4.3 and 5.6, giving constant angles of about 75” and 77”. In less tightly coiled cochliodont toothplates the constant angle is more difficult to estimate (Blake, 1878 and Thompson, 1917 give methods), but Deltodus, Deltoptychius and Poecilodus appear to have virtually the same curvature, and in Deltodus (Fig. 1 1 D) I estimate the constant angle as about 70°, and the ratio between whorls as


c D/-

Figure 1 1 . The logarithmic spiral of growth in: A, symphysial tooth whorl of the edestoid Helicoprion ferrieri Hay, after Bendix-Almgreen (1966: fig. 12); B, posterior upper toothplate of Streblodus oblongus (Portlock), after P.62629; C, mandibular toothplate of Cochliodus contorfus (Agassiz), after P.62625; D, toothplate of Dcltodus sublueuis (M’Coy), after P.2444; E, toothplate of Enniskillcn conucxus, after P.48457; F, C, presumed mandibular (F) and posterior upper (C) toothplates of PIu&qJstrodus striotus (M’Coy), after P.2475, P.24754 H, mandibular and posterior upper toothplates of Callorhynchus milii Bory (both toothplates have the same spiral).

about 10 (though I do not have great confidence in the accuracy of those estimates). In Enniskillen (Fig. 11E) the curvature is shallower, with a constant angle of perhaps about 65” and consequently a ratio between whorls of about 20. The chondrenchelyid Plutyxystrodus (Fig. 1 lF,G) has a more shallow curve in the lower toothplate, with a constant angle below 50”, and an even shallower one in the upper, with a constant angle of about 30”, approaching the very weak curvature of Cullorhynchus toothplates (Fig. 1 1 H) .

The gentle curvature of the upper toothplate of Plutyxyslrodus and the toothplates of Cullorhynchus is associated with a low constant angle that “causes the spiral to dilate with such immense rapidity that, so to speak, ‘it never comes round”’ (Thompson, 1917: 535). I t is in toothplates of this general form that we find basally growing labial and symphysial descending laminae. If the descending laminae are to retain their positions on the margins of the jaw cartilages, it is evident that the jaw surface must be growing on the same logarithmic spiral as the toothplate, so that there is a constant relation between toothplate and jaw. Zangerl (1983) pointed out the necessity of this relation, and how the enrolling of cochliodont toothplates in the jaw cartilage is a consequence of the toothplate growing faster than the jaw. However, Zangerl (1983: 18)

58 C. PATTERSON

believed that in chimaeroids the toothplates grew faster than the jaw, and that the excess of tooth growth was eliminated by wear. This is conceivably true of the more derived chimaeroids, like Chimaera, in which the descending laminae are obsolete, but i t cannot be true of Callorhynchus or of the myriacanthoids, with their well-developed descending laminae.

SUMMARY AND CONCLUSIONS

The proposition (Fig. 1 ) that chimaeroids are “statodont”, with toothplates that grow by a fundamentally different mode from those of sharks or cochliodonts (“lyodont”), seems to be mistaken. Chimaeroids are statodont in the sense that they do not shed teeth or toothplates in life (the original meaning of the term statodont in Jaekel, 1901), but this property is widespread amongst chondrichthyans, being shared by myriacanthoids, Squaloruja, cochliodonts sensu lato, psammodonts (Patterson, in preparation), iniopterygians (Zangerl & Case, 1973), desmiodontids (Zangerl, 198 1 : fig. 66), Eugeneodontida (Zangerl, 198 1 ) , petalodonts (Janussa, Petalorhynchus) and various other elasmobranchs. Among the latter are some of the most specialized batoids such as Aetobatus, in which the teeth are so closely imbricated that the oldest ones are worn away rather than shed, and also some of the most primitive sharks such as Cladoseluche clarki and Ctenacanthus from the late Devonian Cleveland Shale of Ohio. In Cladoselache clarki, Claypole (1895: pl. 1, pl. 2, fig. 1 ) illustrated BMNH P.9273 and P.9272; both specimens show tooth families in which up to six successively smaller teeth are found labial to the erect functional tooth. The most labial (oldest) tooth visible in these families is less than one-quarter the size of the functional tooth, and is separated from the oldest tooth in the neighbouring family by a gap equal to more than five times its mesio-distal length. These observations imply that Cladoselache retained all the members of a tooth family, packed in beneath the labial border of the functional tooth, like the petalodont Janussa (Zangerl, 198 1 : fig. 20). Cleveland Shale Ctenacanthus show a similar pattern of tooth retention (M. E. Williams, personal communication). Statodonty in the sense of tooth retention is thus so widespread in chondrichthyans that it must be regarded either as a primitive feature (as Jaekel argued, 1901: 557), or as one that has developed independently many times. If Cladoseluche is the sister of all other chondrichthyans (Maisey, 1986: fig. 6), the first of these interpretations may be preferable. Whether statodonty is primitive for a more extensive group of gnathostomes (it occurs in acanthodians; Reif, 1982: 342) is an open question.

The second aspect of statodonty emphasized by Reif (1982: 341), Bendix-Almgreen (1983) and Brvig (1985: 73) is the mode of growth of chimaeroid toothplates, said to be from the basal surface. I have tried to show that this interpretation of chimaeroid toothplates is mistaken, and in particular that the mistake depends on inappropriate terminology of toothplate surfaces, itself in part dependent on choice of an inappropriate Recent model. The model chosen by Bendix-Almgreen was Chimaera, a Recent genus in which the labial and basal surfaces of the toothplates are hardly distinguishable (Fig. 4E,G). A more appropriate model would be Callorhynchus (Fig.4F,H), in which the basal and labial surfaces are sharply separated by a descending lamina, a condition which is evidently primitive for chimaeroids (p. 49). Toothplates of


Callorhynchus type, which also occur in the Jurassic chimaeroids Pachymylus and Brachymylus and in myriacanthoids and Squaloraja, show that the growth surface is lingual, not basal. Chimaeroid toothplates therefore grow from the lingual surface, like the dentition of sharks and cochliodonts. The only toothplates growing from the basal surface that I have encountered in the course of this work are those of ptyctodont placoderms; these are truly statodont in the sense of Bendix-Almgreen (1983) and Orvig (1985), and I therefore agree with Reif (1982: 340) that chimaeroid toothplates “have nothing to do with the gnathalia of any placoderm”.

The presence of labial and symphysial descending laminae relates the toothplates of primitive chimaeroids to those of Squaloraja and myriacanthoids. Beyond these, the only toothplates that I have found with similar features are those of the chondrenchelyids (Fig. lo), which also share other features (rows of denteons parallel with the growth margin; pattern of tritors on the anterior upper toothplates) with certain Mesozoic and Cainozoic holocephalans. So far as the dentition and its mode of growth are concerned, the closest Palaeozoic relatives of the chimaeroids, myriacanthoids and squalorajoids seem to be the chondrenchelyids.

ACKNOWLEDGEMENTS

I am most grateful to Svend Bendix-Almgreen for allowing me to use his original of Fig. 1; to Chris Duffin, David Ward and Mike Williams for unpublished information; and to Barbara Stahl for comments on a draft. The photographs are by the Photo Unit, British Museum (Natural History).

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