J. Li,rrr.Soc. (ZOOZ.), 47, 311, pp. 69-81 With 4 plates und 3 text-Jiyures Prirrtrd in &xit Britain Ocfobc r , 1967

Ateleaspis tessellata Traquair, a non-cornuate cephalaspid from the Upper Silurian of Scotland

B r A L E S AN 1) li:R IC I TCH TE

nepurttnpnt o j Geolo!yy. I ' t i i'i:ersit,y qf Shcfield

.-I telecrspis tessellritrr Traquair, a nun-cornuwte r.eplialq)id from the U. Silurian of Scotland, is re-examined on the evidence of new material from I I i c original localities. The species is found 1 o be less 'primitive' thari was previously tliorighl in t Irat the pectoral fins are separated from tile trunk posteriorly by a dist,inc 1 firis are developed essentially as i i i -Arercispis robustus Kiaer. The on is described for the first time. .-Itckcispis and A4crrtrspis are so close in _. respects that retention of separate genera is

has priority. Revised diagnoses quair and 4 . robtistus (Kiaer), are

,given and their relationships with other n o i i - ( w r n u a t p crphalaspids discussed.

TABLE OF ('OXTENTS

Introduction . Material and Methotis .

Head-shield . Lateral and dorsal sensory fields Orbits arid associated structures Sensory canal system . . Pentral side of the head . Trunk . Ventro-lateral ridge . Endoskeleton . Unpaired fins arid caudal fin Paired fins

Morphology .

Relationships . Diagnosis I~cktioi\-ledgements . References .

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Ihl'ROLlL1CTIOK

Rare and mostly fragmentary remains of a primitive cephalaspid, Atekaspis tessellata Traqiiair, have been recovered from maiiy exposures in the Lesrnahagow and Hagshaw Hills Silurian inliers in Lanarkshire and Ayrslin e % Scotland, in association with anaspids (Bii keiLia, Lasaizius). thelodonts (l'helutltit5, Lanarkia) and eurypterids. Fragments of the same forms have also been recorded from a Silurian inlier in the Pentland Hills (Rlykura & Smith. 1962). The vertebrate horizon. first described by Peach & Horne (1899) as the Downtonian Fish Bed, ha.; inore iwxmtly been suspected to be considerably older and a date between U Wenlock-ill Lutllon is now favoured (cj' discussion in Walton, 1965: 194-200; Rolfe & Fritz, 19titi 159-164).

Since Traquair's original accounts (189!3 834-837, 1905. 883-886) Ateleaspis has been re-examined in detail by Heintz (1939. 81;-91) 111 tlic light of his findings on the much better-preserved material of Aceraspi, iohic +trs Kiacr and Hirella ( = Micraspis) gracilis (Kiaer) from the U. Ludloa of Rudstangm, Ringerike, S. Norway. Stensih (1927 : 26-29; 1932: 150-151) did not have the opportnnit3 o f examining the Ateleaspas material and

70 ALEXANDER RITCHIE used Traquair’s description. Heintz’s review of the species was hampered by the small quantity of material then available which, despite some later discoveries, did not exceed 20 pieces and many of these were fragments. Despite this he was able to demonstrate that Ateleaspis was, in most respects, a typical cephalaspid which closely resembled Aceraspis, Hirella and Hem,icyclaspis, especially the first of these. It appeared that the pectoral fins, or flaps, of Ateleaspis were similar’to those of Aceraspis but that they were not nearly so differentiated in that they lacked a ‘sinus’ between the fin and the trunk (Heinte, 1939, fig. 31). This ‘primitive’ stage of differentiation was considered by Heintz (1939: 101, fig. 35) and Westoll (1945, fig. 7 ; 1958: 186, fig. 3) to represent one of the earliest stages in the development of paired pectoral fins in the non-cornuate Osteostraci.

In recent years additional material of Ateleaspis tessellata Traquair has been discovered at several horizons in the Dippal Burn and Slot Burn Formations of the Lesmahagow inlier and in the Fish Bed Formation of the Hagshaw Hills inlier-the stratigraphy of the inliers has been revised by Jennings (1961) and Rolfe (1961) and a summary is available in Walton (1965: 194-200, fig. 5.16). This material, supplemented by other specimens unearthed in museum collections, allows a fuller understanding of this interesting genus, one of the oldest known cephalaspids. It has become apparent that the material available to Traquair and to Heintz gave a misleading impression of several aspects of the morph- ology-especially concerning the nature of the pectoral and median fins. It now seems certain that the pectoral fins of Ateleaspis tessellata Traquair are scarcely more ‘primitive ’ than those of Aceraspis robustus Kiaer and that the dorsal fins of the two forms are also remarkably alike. The differences which remain are probably only of specific importance and the two should really be included in the same genus. If this is accepted Ateleaspis has priority and the genus Aceraspis, which has been widely figured and discussed, must unfortunately be suppressed.

To facilitate comparison with Aceraspis robustus Kiaer, which will henceforth be referred to as Ateleaspis robustus (Kiaer), this account of ,4teleaspis tessellata Traquair follows the layout adopted by Heintz (1939: 9-70) in his description of the R’orwegian species.

MATERIAL AND METHODS

-4s Heintz (1939: 94) has pointed out, the specimen figured by Traquair, G.S.E. 5992 (1899, pl. 4, fig. 6) must be chosen as the lectotype. All the specimens figured by Traquair (1899, 1905) are preserved in the Geological Survey, Scottish Office (G.S.E.), as are those figured by Heintz (1939, pl. 30). Others are in the Royal Scottish Museum (R.S.M.), British Museum (Nat. Hist.), and the Museum of Comparative Zoology, Harvard. A fine specimen from the collectioi~ of Prof. F. H. Stewart, Grant Institute of Geology, Univ. of Edinburgh is figured here (Pl. 3, fig. 3, F.R. 1378). I n addition to the specimens figured in this paper (which have been presented to the Royal Scottish Museum) the writer has several others which are less complete.

The best material has come from the well-known localities a t Slot Burn (N.G.R. 680321) and Birkenhead Burn (763359) both in the Lesmahagow inlier and from Shiel Burn, Monks Water (777291) and Smithy Burn (789631) in the Hagshaw Hills inlier. Although Ateleaspis tessellata Traquair has been recovered from most of the numerous exposures of these later fish beds throughout the two inliers i t is always one of the rarest elements in the vertebrate fauna.

Many of the specimens have been prepared mechanically, chiefly by means of a Burgess Vibrotool modified to take gramophone needles, but in others, including the almost complete individual (R.S.M. 1966.48.1), from which most of the new information has come (Pl. 1, figs 1, 2; Text-fig. l a , b), the bone had weathered to a brownish powder. This powder was carefully brushed and washed away to reveal an extremely detailed mould with the dorsal surface exposed on one side and the ventral surface exposed on the

Ateleaspis tcssellata Traguair a b

Test-fig. 1 AteZecispis tessellnttr Traquair. Part anti counterpart of R.S.M. 1966.48.1 from Slot Burn, Seggholm, Ayrshire, showing (a) the dorsal surface of the head-shield and right flank of the trunk and (b) t)he ventral si~rfac~e of the head-shield and trunk, arid left flank. Natural size. (rf. P1. 1, figs 1 to 3; P1. 2, figs 1. 2 ; 1’1. 3, figs 1, 2) . See Text-fig. 3 for abbrevi- d o n s used.

counterpart. Latex casts taken from these have been coated with ammonium chloride and used for the illustrations.

MORPHOLOUY

Ateleaspis tessellata Traquair is a moderately large cephalaspid (160 to 200 mm), rather larger than A. robustus (Kiaer) (160 to 180 mtn) and slightly smaller than Hemicyclaspis

72 ALEXANDER RITCHIE

a b

Text-fig. 2. Atelerrspis tesselltrtcz Traquair. Reconstruction of the head-shield and front part of the trunk, seen (a) from the dorsal and (b) from the ventral side. Based mainly on R.S.M. 1966.48.1 (Text-fig. 1, PI. 2). x 16.

(200 to 210 mm). The available material indicates a rather greater size range than is com- monly found in cephalaspids, which are now thought to have acquired their dermal skeleton only a t a rather late stage in growth. This may reflect some peculiarity in the nature of the exoskeletal units of the head-shield and in the timing of their final fusion with the deposition of the basal layer (cf. Parrington, 1958: 127).

IIend-sh ield

The completely flathened nature of the material makes i t difficult to estimate accurately the proportions of the head-shield in the original state but allowance has been made for this in the reconstruction (Text-fig. 2a, b). Westoll encountered a similar problem with regard to Hemiteleaspis (1945: 343), uhich is now regarded as a Hemicyelaspis. The head-shield in ,4. tessellata is comparatively large and approximately as long as wide.

PL-4TE 1 Atelertspis tessellata Traquair

Figs 1, 2. Part and counterpart of R.S.M. 1966.48.1 from Slot Burn, Seggholm, Ayrshire. Natural size (cf. Text-fig. la,b). Fig. 3. Detail of antero-ventral lobe ( a d ) of the caudal fin of specimen in Fig. 2.

Latex casts whitened with ammonium chloride.

"I

Ateleaspis t cssellata Traquair 73

The head : body ratio is approximately I ‘ 4 , comparable ith tlie proportions found in A . roblcvtus and in IIemicycZaspis (Hemtz, 1939. 9) .

The dorsal surface of the head-shield is covered \I ith small, close-set, rounded tubercles 11 hich increase markedly in size around the orbital and naso-hypophysial openings and along tlie anterior and lateral margins. In most spxw”ns the polygonal ‘ tesserae ’ noted by Traquair (1899: 834, pl. 4. figs 9, l o ) ai’e clearly visible. Honever, Stensio (1932, figs 5,6,‘i) has demonstrated that the groox cs Iwtween the polygonal exoskeletal units are due partly to the venous ring sinuses antl partly to the interareal inucoiis grooves. In the flattened material of Atelenspis i t is scarcely surpi%ing that the exoskeleton has tended to fragment into polygonal units along what intist have been incipient h ies of weakness. Tlie nature of the basal layer remains uiiknov 11 Sevcrtlieless, the polygonal areas are generally distinct over most of the dorsal burface m t l not merely over the lateral and dorsal sensory fields as in .4 . robustusand in Ifir(lln (Heintz. 1939, pl. 1 ; pl 1. fig. 1 ; pl. 24, figs 3. 4). The individual units in A . r r f u arc riiiiisiially small in size and the new material does riot support Heintz’s ob ion ( 1 !EN 88) that ‘in the lateral and rnedian electi tc fields the polygonal area 1s aypaiwtl j bigger than in the liead elsewhere’. The iinits are. in fact. so uniform in size that it is frequently difficult to distinguish the bound- aries of the lateral aiitl median sensory iic~lds.

Whereas in -4 teasellafa tlie head-shieltl c)riianir~ritatiori is of small, rounded tubercles throiighout, with a fairly abrupt transition a t the. anterior scales of the zoiial antl inter- zonal regions (PI. 4, fig. 3 ) , i n A . robusfii.\ only tlic region in front of tlie line of the eyes hears rounded tubercles ; behiiicl tlie ( ) I hits the tubercles become elongate antero- posteriorly and grade imperceptibly into the zonal and ~nterzorial scalcs.

The nature of the head-shield rim also s h o ~ s intcwsting differences in the two forms. In .1 robustus ‘the ventral and clorsal w i ( l s of the rim are strongly hculptured n i th loiigitutlinal ridges of varying hrearltli ‘I’lic. 1 1 1 1 1 i q ~ratiirally divided into tliree parts, one rostra1 aiid two laterals, by imaiis of soiiiv shoi t I itlgrh and tubercles placed transversely on it ’ (Heintz. 1930 12. pl. 1 ; 111 2 , figs 6. 5 . 8: 111 8, fig 3; figs 1, 8, 9). In A . te.sxellatc6 the rini is not composed of long. continuoiis ~xlges hiit of several rows ot sniooth, enlarged tubercles 1% IiicIi range from short, ovatc, to vlorigatc structures running parallel t o the margin. There is some evidence to shou that i lit, larger, and presumably olcler, individuals have these incipient marginal ridges bthtt tleveloped. The marginal tubercles on the ventral rim are larger a i d inorc variahlr ttiari those on the dorsal surface (PI. 2, figs 1, 2 ; Text-figs 1, 2). It nonld appear that A . / l lal(r represents an early stage in the process of elongation and fusion of the marginal t i i \ ~ ( ~ c I e s of the head-shield into continuous ridges and a solid rim such as \ve see i n - 1 r o h ~ c n t c r s ; the basic structure IS iievertheless the same.

Lnt e ral a I 1 tJ dorsn 1 5 P I / hOI y jirlds

Tlie Iateral sensory field. ( Is f ) in t P lltrttr are large antl broad M itli a length : breadtli ratio of approxiinately 5 . l They are tliiii n i t i t r than those of A . robusfua (6 : 1); Henti- cycZa~p.pis and most other cephalaspitls Iiavcb CT cn Iiighei* ratios of 8 : 1 or more The lateral seiisory fields are best seen in the speciiiirws ticiiicd by Traquair (1905. pl. 2 , fig. 9, G S.E. 5900) and Heintz (1939. pl. 30. figs 2, 3) ”tie i i e ~ material does not show them a t all clearly; even \\here tlie entire clorsal siirface is preserved as an exter~ial mould (PI. 2 , fig. 1 ) the margins of the fields cannot h i (Iihtiiigiiished Anteriorly the lateral sensory fields are separated 137 a distance appou~niately (qua1 to their arerage breadth ;

1’1 ATL 2 .4 t e lempts t c & w l l ( i t ( t T~aqi ia i r

Pig.; 1, 2. Head >hield aiid front part of the trunk of R S hZ 1!)66.48.1 iecn in d o r d VIEW (1) and ventral view ( 2 ) ((f. Text-fig. ln ,b for interpretation nirt l Text fig. 2a,b for reconstruction).

74 ALEXANDER RITCHIE posteriorly they terminate against the pectoral fin scale-covering along a clearly defined, oblique line which meets the lateral margin almost on a level with the posterior margin of the orbits. The polygonal units over bhe fields are not noticeably larger or smaller than those over the rest of the head-shield.

The same specimens which display the lateral sensory fields remain the best also for evidence on the shape of the dorsal sensory field (ds f ) (Heintz, 1939, pl. 30, figs 2,3) . They indicate it to be broadly ovate with the widest point slightly nearer the posterior margin ; the length :breadth ratio is around 2.0 to 2.5/1.0. Anteriorly the dorsal sensory field connects with the pineal plate along most of its length, while the posterior margin appears to be bluntly rounded. As in the lateral fields the units are similar in size, shape and ornamentation to those over the rest of the head-shield. The dorsal sensory field in A . robustus, which is unusually narrow (4 : l ) , posteriorly pointed and with a narrow anterior margin, is bordered marginally by larger tubercles and covered by plates resembling ‘scales’ with an ornamentation of longitudinal ridges (Heintz, 1939, pl. 2, fig. 4 ; pl. 4,

The poorly differentiated condition of the lateral and dorsal fields in A . tessellata is possibly partly due to the nature of preservation but more probably reflects a more primitive stage of development than we find in A . robustus.

fig. 1).

Orbits and associated structures

The orbits in A . tessellata (Pl. 2, fig. 1 ; Text-figs 1 a, 2a) are proportionately larger and more nearly rounded than those in A. robustus (Heintz, 1939, pls. 1 , 3 ) . They are situated just forward of the midline of the head-shield and connected by the rather narrow, pineal plate. The lateral margins of the orbits are composed of elongate tubercles and ridges (Heintz, 1939, pl. 30, fig. 3) (Pl. 2, fig. 1 ) . The sclerotic ossification has been observed in several specimens and was probably similar in structure to that in A . robustus (Heintz, 1939, pl. 3, figs 1 t o 6 ; figs 4 to 7 ) where it was shown, by means of serial sections, to be a hollow ball with an elongate, laterally-directed aperture. The upper surface of the ossification in A . tessellata is covered with close-set, rounded tubercles-not elongate as was suggested by Heintz (1939: 89, fig. 33)-and thus differs from A . robustus (Heintz, 1939: pl. 3, fig. 2).

The naso-hypophysial foramen is narrow, elongate and constricted in the middle ; the smooth margin rises posteriorly above the level of the surrounding tubercles (Pl. 2, fig. 1) .

Sensory canal system

The pattern of the sensory canals on the dorsal surface of the head-shield (which was poorly preserved in the original specimens) can be clearly seen in R.S.M. 1966.48.1 (Pl. 2, fig. 1 ; Text-fig. 1 a), confirming Heintz’s suspicions that it was developed essentially as in A . robustus (1939, figs 1 , 2 ) .

The infra-orbital sensory canal (ifc, P1. 2, fig. 1 ; Text-figs la, 2a, 3) runs from the postero-lateral corner of the orbit in an almost straight line antero-laterally to the lateral sensory field where it first curves sharply inwards and then backwards to terminate

PLATE 3 Ateleaspis tesselkta Traquair

Fig. 1. Detail of branchial aperturesin R.S.M. 1966.48.1 (cf. Text-fig. 1 b). Same magnification as Fig. 2. Fig. 2. Detail of oral fenestra and lateral oral fields in R.S.M. 1966.48.1. Fig. 3. Trunk and caudal fin, F.R. 1378, from Birkenhead Burn, Lanarkshire. Natural size. Fig. 4. Caudal fin from Birkenhead Burn, Lanarkshire. x 14.

Figs 1, 2. Specimen whitened with ammonium chloride; Figs 3, 4. Specimen photographed under alcohol. al, apical lobe of the caudal fin; D1 and D2, anterior and posterior dorsal fins.

Ateleaspis tessellata Traquuir 75

pointing in the general direction of the naso-hypophysial foramen. The corresponding canal in A . robustus swings outwards well clear of the orbit and a t its anterior end points more medially than posteriorly (Heintz. 1939, pl. 1, fig. 1).

The lateral canal ( lc P1. 2 , fig. 1 ; Text-figs 1 a , 2a, 3) can be traced from a point near the junction of the zonal area and the pectoral fin anteriorly to where i t terminates level with, but lateral to, the postero-lateral orbital margin. A transverse canal in two parts (cmml, cmrnz, P1. 2 , fig. 1 ; Text-fig. 1 a) conntcts the posterior end of the dorsal sensory field to the lateral canal. Another transvcrsc canal (rrnm) which meets the lateral margin of the dorsal sensory field a short distance behind the orbits, is most clearly seen in G.S.E. T320a (Heintz, 1939, pl. 30, fig. 3) on both hides of the field.

The full extent of the canal system in A . tPssellata (Text-figs 2a, 3) thus differs only in minor details from that in A. robustus (Heintz, 1939, figs 1 ,2) .

J7entrd s i d p of fhc head

The original material of A . tessellata prc)ved very uninformative concerning the nature of the ventral scale covering, both on the head and the trunk. Heintz (1939: 90) found indications that ‘the ventral visceral exosheleton M as formed of minute scales, similar to Arpraspis, but not of larger plates as in Mu-raspis (now Hirella) or Hemicyclaspis.’ No evidence of the oral or branchial apertures could be detected.

Because of the nature of its preservation and preparation R.S.M. 1966.48.1 displays an exceedingly detailed external mould of the ventral exoskeleton (Pl. 2 , fig. 2 ; P1. 3, figs 1 , 2 ; Text-fig. l b ) . The oralo-brancliial fenestra is covered with minute, rounded tlenticles which are very regular in sizc. arid are, in places, arranged in rows. During burial the left pectoral fin has been doublcd i1ntlt.r the head, obscuring part of the left side of the ventral surface. Just inside thc rim ( P I 3 . fig. 2 ; Text-fig. 1 b). in antero-lateral positions, are two areas of coarse tubercles ccliich compare closely with the lateral oral fields present in 9. robustus (Heintz, 1939, pl. 8, figs 1, 2 ; pl. 9, fig. 3 ; fig. 8). Separating these lateral oral fields anteriorly and Imrtlcrinp them posteriorly is a scale-free area, roughly subtriangular in shape, with rat her indeterminate margins ; this must represent the oral fenestra which Heintz suggested may liave been partly closed with soft tissue. Along the broad posterior margin the small tlrnticles are arranged in oblique, intersecting rows and the midline is clearly visible. Similar denticles cover the rest of the ventraI surface, becoming less regularly arranged posteriorly and laterally. There is a distinct long,narrow, curving strip (Pl. 2 , fig 2 ; PI, 3 . fig. 1 ; Text-figs 1 b, 2b) , free of scale-cover- ing, just inside the right ventral rim which. u itliout doubt, represents the right bran- cliial apertures; these mere not well preserved in A . robustus (Heintz, 1939, pl. 7 ) .

Apart from some difference in the shape of the oral feiiestra the ventral exoskeleton of the head-shield in -4. tessellata is developed esscntially as in A . robustus.

Tr. u I / k

The anterior portion of the trunk is incolporated in the posterior portion of the head- shield and the trunk squamation continues anteriorly merging with the dorsal surface of

1’r.a~~;: 4

Atelcnspis /c .sxclLi /a Trnquair Fig. 1. R.S.M. 1966.48.2 from Smithy Burn, Hagshaw Hills, Lannrkshire. Fra.gment of the anterior t.runk with anterior dorsal fin (Ul) and ventral scales. Antero-ventrally to Z>1 is a structure interpreted here as possible remains of an axial skeleton (q f . €1. 77). Fig. 2. Detail of ventral scales in R.R.M. 1966.48.2. Fig. 3. Specimen from Birkenhead Burn, Lan:wkshirc, with disart’iculated head-shield and anterior part, of t,runk, t,o show junction between tubercdiited head-shiuld and spinous t,runk scales. x 4h.

Fig. 1. Specimen photographed under alcohol ; Figs 2,3. Specimen whitened with ammonium chloride.

76 ALEXANDER RITCHIE

the shield. In the median interzonal region some six segments can be distinguished ; in the lateral zone fields a further two segments are probably present a t the anterior end. Laterally the zonal scales merge into the exoskeleton of the pectoral fin.

In general the trunk of A . tessellata is constructed as in A . robustus, triangular in cross- section with slightly convex sides and a more or less flat base. The undivided lateral scales are directed vertically for most of their length with the dorsal part flexed sharply

Text-fig. 3. Ateleaspis tessellata Traquair. Reconstruction in lateral view. Details of t,runk and dorsal fins based mainly on F.R. 1378 (PI. 3, fig. 3). x 8 approx. Abbreviations used: a d , antero-ventral lobe (of caudal fin) ; bu, branchial apertures; cmm, cm,ril, cmwt3, transverse sensory canals; D1, Dz, anterior and posterior dorsal fins; dsf, dorsal sensory field; ifc, infra- orbital sensory canal; Zc, lateral sensory canal; Us, rls, left and right lateral scales of the trunk; Zof, lateral oral field; Zsf, lateral sensory fields; Lpec, r.pec, left andright pectoral fins; ndb, main lobe (of caudal fin); nhf, naso-hypophysial foramen; of, oral fenestra; o, orbit; sclo, sclerotic ossification ; wZr, ventro-lateral ridge; z'rs, ventro-lateral scales; ws, ventral scales.

forwards a t an oblique angle to the body axis (PI. 1, fig. 1 ; PI. 3, fig. 3; Text-figs l a , 3). The total number of the scales from the zonal area to where they merge into the axial scales of the caudal fin is around 65, approximately the same as in A. robustus.

The outer surface of the lateral scales (PI. 4, fig. 3) bears a coarse ornament of posteriorly directed spines, which vary considerably in size, with the larger spines situated along the posterior margin projecting over the scale behind. In a few disarticulated individuals the inner surfaces of the scales can be seen and, in one instance, the internal openings of the ascending canals were clearly visible.

The ventro-lateral scales, which Heintz was unable to distinguish with certainty (1939 : 92), are well preserved on several of the new specimens (Pl. 1, fig. 2; Text-figs 1 b, 3). They are perhaps slightly longer than those of A. robustus but are otherwise similar.

Ventro-lateral ridge

The ventro-lateral ridge (v lr) , also unknown previously in A . tpsnellata, is particularly well exposed in R.S.M. 1966.48.1. (PI. 1, fig. 2; Text-figs 1 b, 2b) where it consists of a double row of prominent, smooth, oval tubercles. It is impossible to determine whether these are the thickened ventral ends of the ventro-lateral scales as in A . robustus (Heintz, 1939: 48, fig. 14) or whether they are enlarged and modified marginal scales of the ven- tral covering. Posteriorly the ventro-lateral ridges converge just in front of the anterior ventral lobe (awl) of the caudal fin; anteriorly they diminish level with the base of the pectoral fin.

The area between the ventro-lateral ridges is occupied by minute scales slightly larger than those over the oralo-branchial fenestra (ws, P1. 1, fig. 2 ; Text-fig. l b ) . They are arranged in oblique rows, as in A. robustus (Heintz, 1939, pls 16, 17), but they are not fused along the margins into ventro-marginal plates (vpm) such as are presented in the Norwegian form. The ventral scales are very well preserved in a disarticulated individual from Smithy Burn, Hagshaw Hills, R.S.M. 1966.48.2 (Pl. 4, figs 1 ,2) which reveals them to be rhomboidal in shape with rounded corners and a smooth, flat, outer surface.

Bteleaspis tcssellata Traquair 77

In R.S.M. 1966.48.1 (Pl. 1, fig. 2; Text-fig. l b ) this ventral scale covering has broken away in places revealing the smooth inner surface of the right, lateral, trunk scales.

ETIdoskrlPto~?

As Heintz has deriionst8ratetl. the endosltelcton in =L . tohastus, Hirella and Hemicyclaspis was very poorly developed and -4. te.rsella>trt is no exception. Only in G.S.E. 320a (Heintz, 1939, pl. 30, fig. 3) have bhe tubes leading t,o tlw lateral sensory fields been observed, and then only over the fields t~hemselves. An infilling, perhaps representing one of the otic capsules, is visible immediately hehind tjlic. right’ orbit’ in R.S.M. 1966.48.1 (Pl. 2, figs 1 , 2).

A peculiar struct,ure which niay possibly rq)rcsent part of an axial skeleton is present in tlie fragmentary Smithy Burn specimen nwntioiied above (Pl. 4, fig. 1 ) . This is anarrow strip of pale-coloured material, 8 nini x 3 mm. which is clearly segnieiit)ed (segments 1.2 nini wide), lying against tlie smooth, inner surface of the right latJeral scales just forward of the anterior dorsal fin ( I l l ) . Tliv strip \\.its revealed by removal of the overlying left, lat’eral scales, there having been no infilt.rut ion of sediment into the body cavity prior to burial. The segmentation of the strip is not merely an inipression of the inner surface of the lateral scales since it crosses these obliquely ; this must rule out the pos- sibility that it, represents gut contents. 1 t) is suggest,ed here that these are the remains of slightly calcified vertebral elements n.liic.h part,l?; surrounded an unconstricted noto- chortl-a feature not previously recorded i n t>he Osteostraci. If this interpretation is correct then Ateleaspis (and presumably other. ost,rostracans) was, in this respect, more advanced than the living cyclostomes.

tTnpnired$fi,is cold c.aitdrcl$fit

Although Heintz found difficulty in dcterinining tlie exact shape of the dorsal fins in the material a t his disposal he was able to she\\ that .I. t~asellata, like A . robustus, possessed two nell-developed dorsal fins. In the excellent Noraegian material he was able to reconstruct, with the aid of serial sections. hoth of’the dorsal fins in detail (Heintz 1939, pl. 12. figs 2, 4 ; pl. 13, figs 1 , 2 ; figs 2, 19. 20. 2 2 ) The anterior fin in A . robustus is low nit11 a concave posterior margin antl the posterior fin is much larger with a well- developed fin membrane and a convex rear rnargin Distinctive fulcral scales are present along the dorsal margins of both fins. In Heintz’s reconstruction of A . terselZatu (1939, fig. 31) the anterior and posterior dorsal fins &ere depicted (in outline only) with convex and concave posterior margins respectively, the reverse of that in A . robustus.

Several of the new A . tesseZZata individuals have one or both dorsal fins preserved, the best being that discovered by Prof. F. H Stenart a t Rirkenhead Burn, Lesmahagow inlier. l n this specimen, F.R. 1378 (PI 3, fig 3 ) . tlie head-shield is missing and the trunk is sem in lateral riea . Although it is imnrc~tiratrly obvious that the dorsal fins of A . tessellata and .4. robustus are alike in position antl structure, closer examination reveals irnportant differences in detail

The anterior dorsal fin of A tesspllata (U1. 1’1 1, fig. 1 ; 1’1. 3, fig. 3 ; P1.4. fig. 1 ; Text-figs l a . 3) is relatively short and high, rising moie abruptly from the median dorsal crest than its counterpart in A . robustus. Thti sitles are covered in small, spine-covered scales which grade into the trunk scales; tlie dorsal marginal scales are not differentiated into special fulcral scales ( ( f . Heintz. 1939, pl. 12, figs 2, 4; text-fig. 20). The posterior margin of the fin is concave. The posterior dorsal fin ( 0 2 ) is longer and higher than the anterior dorsal fin (01) but the point where it rises from the dorsal crest is difficult to determine. The scale-covering. in row s. Iwconics more minute towards the posterior margin and it is not difficult to recognize the same basic structure antl shape which Heintz found in the 0 2 of A . robustus (1939. p1 1 3 . figs 1. 2 ; fig. 22) ; even the delicate fin

78 ALEXANDER RITCHIE

membrane has been preserved (Pl. 3, fig. 3). The posterior margin is concave above and convex below. Specialized fulcra1 scales are not developed.

The caudal fin of A . tessellata (Traquair, 1905, pl. 3, fig. 2; Heintz, 1939, pl. 30, fig. 1) (PI. 3, figs 3, 4) appears to be somewhat shorter and deeper than that of A . robustus but how much of this is due to different types of compression is difficult to ascer- tain. Below the rear margin of the posterior dorsal fin the lateral trunk scales fragment and merge into the axial scales of the caudal fin as minute rhombic units, arranged in radiating rows. The terminal portion of the fin curves slightly downwards and, in a t least one individual (Pl. 3, fig. 3) , appears to taper to a point, lacking the rounded apical lobe present in A . robustus (Heintz, 1939, al; pl. 13, fig. 2; fig. 24a). The dorsal mar- ginal scales in A . tessellata are not differentiated as in A . robustus (Heintz, 1939, pl.

The peculiar antero-ventral lobe of the caudal fin (awl) is clearly visible in R.S.M. 1966.48.1 (Pl. 1, figs 2 , 3 ; Text-fig. l b ) where it is pressed flat against the caudal fin membrane and covered with rather coarse tubercles. It is thus similar in structure and shape to that in A . robustus and other cephalaspids (Heintz, 1939, figs 24a, b, 25;-d).

20, fig. 2 ) .

Paired fins

The pectoral fins of A . tessellata have been one of the least well-known aspects and the nature of their preservation in the original material led Traquair (1905, pl. 3, fig. 1; fig. 3) and Heintz (1939, fig. 31) to depict them as primitive, undifferentiated, postero- lateral lobes with a rounded outline and no evidence of a ‘sinus’ between the fin and the trunk. This in turn has resulted in A . tessellata being depicted as the most primi- tive member in a series to show the gradual differentiation of the fins in non-cornuate cephalaspids (Heintz, 1939, fig. 35; Westoll, 1945, fig. 7 ; 1958, fig. 3)

To date, only one specimen of A . tessellata displays a complete and undistorted pectoral fin (Pl. 1 , figs 1, 2; P1. 2, figs 1 , 2 ; Text-figs 1, 2). The right fin is seen spread out in apparently natural position u hile tlie left has been doubled under the head and pressed against the ventral exoskeleton. The lateral rim of the head-shield, composed both dorsally and ventrally of coarse, elongate tubercles, continues smoothly into the antero- lateral margin of the fin, which thus lacks both cornua and pectoral sinus. The inner margin connects, along most of its length, with the scales of the zonal area ; only posteriorly is the fin separated from the trunk by a distinct, but not very deep, ‘sinus’, leaving the distal portion of the fin as a freely-projecting, rounded flap.

The junction, on the dorsal surface, between the plateso f the lateral sensory field and tlie more elongate, obliquely directed scales of the fin base IS quite distinct; the scale- covering over the fin is composed of smaller and smaller elements distally. On the ventral surface the oral-branchial exoskeleton passes without noticeable break into that of the fin covering.

It is now clear that the paired fins in A . tessellata are scarcely, if a t all, more ‘primitive’ or undifferentiated than those of A . robustus and it is even possible, with hindsight, to recognize in the specimen, G.S.E. 5990, figured by Traquair (1899, pl. 4, fig. 6) and Heintz (1939, fig. 32), one of the varied positions in which the similar fin of A . robustus was occasionally preserved (Heintz, 1939, fig. 18c). In both forms, therefore, the fin forms an extension of the lateral rim of the head-shield, is still ‘fused’ to the body over a considerable part of its length and has a distinct ‘sinus’ between the posterior, free portion of tlie fin and the trunk. In Hirella (Micraspis) and He?nicyclaspis, where a similar non- cornuate condition exists, the sinus is long and the fin more or less isolated along its inner margin ; these two may still be considered as more ‘advanced’ forms. The undifferentiated form of the pectoral fin thought to have been present in A . tessellata must surelyhave been passed through a t some point in the evolutionary history, still awaiting discovery in

Ateleaspis tessellata Traquair 79

yet older sediments. Ateleaspis tessellatu Traquair. although one of the oldest knon n cephalaspids, is still a highly developed form 15 ith a long history behind it.

As demonstrated above A. tessellata T i q i m i r and A. robustus (Kiaer) must now be included in the same genus, Ateleaspis ( .4r p r u <pis having been suppressed), and indeed there is little or nothing in the structiirt, of the Scottish bpecies which might preclude it from being on, or a t least very near, tlie direct ancestral line of the ISoiwegian form, which occurs in rather later sediments.

REIA'I'IONSILIPS

Heintz (1939 : 96) erected the subfamily Heniicyclaspinae to include Ateleaspis, Aceraspis, Micraspis, Hemicycla.spi.s ant1 this was accepted by Robertson (1953) and West,oll (1945). Denison (1951 : 186) revived thc family Ateleaspidae Traquair (1899) to comprise Witaaspis, Ateleaspis, d c e m s y i s , Miut.a.r.pis, Hemiteleaspis and Hemicyclaspis, and considered that Saarenmaspi.s was closc to the common ancestor of the family (1951 : 176) : Fowler (1947) and Westoll (1958 : 186) tJiought that Ateleaspididae was, etymo- logically, the more correct form. Wingsjij ( 1962 : 559) put Bteleaspis, Aceraspis, Hirella (Mioaspis) and Heniicyclaspi.s in the subfamily Ateleaspidinae.

In his most recent accounts of the Ost,eostraci Stensiii (1958: 176-820; 1964: 96-151) has proposed a subdivision into three oi,tlers~Ortliobraiicliiata, Oligobranchiat'a and ~ectaspiformes-largely on the basis of' t>lie st'ructure of tlie mouth and the branchial apparat.us. Hirella (Nicraspis) was placed in the Ortliobrancliiat~a, Aceraspis and Hemi- cycbspis (together wit'h tlie new genus Pnttei~rzspis) in the Oligobrancliiata (suborder Aceraspida) and Ateleaspis was relegated t o Ost,costraci lncert. Sed.' Obruchev (1964) retained the same orders but distributetl the various forms differently ; the Hemicyclas- piditlae (Hemicyclaspis and Hirella) are in thr Ortliobranchiata (p. 102) and the Ateleas- pididae ( Witaaspis, Ateleaspis, Aceruspi.s. and ? Z'uvaspis) in the Oligobranchiata (p. 99).

As demonstrated above AteZeaspis and ..lcercrsp,is are alike in so many details that they must now be considered to belong to t'he sanw grnus, and since Ateleaspis Traquair (1899) has priority Aceraspis Kiaer (191 1) must be suppressed. Concerning the relationships of the other non-cornuate cephalaspids nient'iond above some reservations may be expressed on the validity of Stensio's criteria for his new classification. Although the branchial chamber is known in considerable detail in many of the Osteostraci (as a result of Stensio's work) in Ateleaspis tessellata, A. robustus, Hirplla and Hemicyclaspis, all of which had a poorly ossified endoskeleton, it is riot \\.ell known. The placing of Hirella in a separate order from the others-which it rescni blcs in so niariy respectas--would appear to be based on the structure of the mouth. This i s a tire of tlie osteostracans which is known in detail in relatively few genera (since t h e oralo-branrhial cover is seldom preserved), a feature which is so subject to modification fi,r different modes of life and feeding that it cannot be considered a reliable charact.er for classification.

It is suggested here that the family At'eleaspididae be retained to include the following genera which appear to form a nat,ural group- Witaaspis, Ateleaspis (A . tessellata, A . robustus), Hirella spp. and Hemicyclaspis sppp. Hemiteleaspis heintzi West,oll (1945) is now generally accepted as a Hemicyclaspis, while l ' ~ ~ ~ v u s p i s Obruchev (1956) should probably remain incertae sedis until more and better preserved material is available.

DIAGNOSIS

The following diagnosis of Ateleaspis and its species is based on, and modified after, that of Heintz (1939 : 97-98).

Family Ateleaspididae Genus Ateleaspis Traquair

Relatively large osteostracans with the pectoral fins more or less clearly differentiated, forming the immediate prolongation of tlie postero-lateral corner of the cephalic shield,

80 ALEXANDER RITCHIE

covered with very minute scales, the free posterior lobe of the fin short, pectoral sinus absent. The cephalic shield moderately long, incorporating a number of the body scales, a postero-lateral crest completely absent. The lateral sensory fields long, the median sensory field relatively long, ovate. The ventral visceral exoskeleton consists of a great number of minute, Thelodus-like scales, arranged in regular rows. The mouth opening, situated in front of the oralo-branchial fenestra, is limited anteriorly by the ventral rim and laterally by fields of larger tubercles (lateral-oral fields). The endoskeleton poorly ossified.

The first dorsal fin developed as a more or less fin-like skin-fold; no membrane is developed. The fin begins somewhat behind the posterior limit of the cephalic shield. The second dorsal fin large with a membrane developed. The lateral scales of the trunk not divided into two parts. The ventral side of the body covered with regularly arranged, minute scales.

Ateleaspis tessellata Traquair. U. Wenlock-L. Ludlon of Lanarkshire, Ayrshire and

Ateleaspis robustus (Kiaer). U. Ludlow of Rudstangen, Ringerike, S. Norway.

Two species are known :

Peeblesshire, Scotland.

Ateleaspis tessellata Traquair

Diagnosis as for genus with the following additions : the lateral sensory fields long and rather broad, the median field moderately long, tapering posteriorly and connected anteriorly to the pineal plate along its length. Ornamentation on dorsal surface of head- shield of rounded tubercles throughout. Lateral rim of the head coniposed of rather small, ovate tubercles on the dorsal surface and of larger, more elongate units ventrally. Ventral rim forms anterior limit of mouth opening. The first dorsal fin, rising rather sharply from the dorsal surface, is entirely covered with small, spinoiis scales. Both dorsal and caudal fins lack specialized ridge-scales. Ventral scales of thc trunk small, rhombic and arranged in rows; proportionately larger than in A . robustus and not fused marginally. Ventro- lateral ridge developed as a double row of rounded, ovate tubercles. Apical lobe of the caudal fin with pointed termination.

Lectotype and type species (Trayuair, 1899, pl. 4, fig. 6) is a specimen in the Insti- tute of Geological Sciences, Edinburgh, formerly the Geological Survey, Scottish Office (no. G.S.E. 5992).

Atelpaspis robustus (Kiaer)

Diagnosis as for genus mith the following additions : the lateral sensory fields long, broadest anteriorly, the median field long, relatively narrow, tapering posteriorly and connected to the pineal plate only over the middle third of its length. Ornamentation on dorsal surface of the head-shield of rounded tubercles anterior to the orbits and of low spines behind this line (except along the margins of the dorsal sensory field). Lateral rim of the head composed both dorsally and ventrally of continuous ridges broken only antero-laterally by patches of small tubercles. Teeth-like tubercles developed on median part of the ventral rim anterior to the mouth opening. The first dorsal fin rises gradually from the mid-dorsal crest ; both dorsal fins and the caudal fin have specialized ridge scales well-devcloped. Ventro-lateral ridge developed as thickened ridges on the lower margin of the ventro-lateral scales. Ventral surface of the trunk covered with regularly arranged, minute scales in rows; along the margins these are partly coalesced into larger plates. Apical lobe of the caudal fin with rounded termination.

Holotype is no. E. 1084 in the Palaeontologisk Museum, Oslo (Heintz, 1939, pl. 13).

Ateleaspis t cssellata Traquair s1

ACKNOWLEDGEMENTS

This paper is dedicated to Dr E. I. White, British Museum (Natural History), in appre- ciation of his help and advice, aiid for the loan of material during the present research. The work was begun in the Grant Institute of Gcology. Univ. of Edinburgh under a D.S.I.R. Research Grant. Thanks are also due to Dr G . Y . Craig and Prof. F. H. Stewart of the above department for advice and encouragement and to the latter for the use of his matcrial. Additional material has been borrowcd from the Royal Scottish Museum, Edinburgh, the Institute of Geological Scienccs, Edinburgh, and the Kelvingrove Museum, Glasgow, and thanks are extended to Dr C' 2). Waterston, Mr R. Wilson and lllr C. E. Palmar of these institutions rcspectively for their help. Finally. the coinparison of the Scottish and Korwegian forms has been grcatly aided by the invaluable assistance of Prof. A. Heintz, Palacontologisli Museum, Oslo, who, during my two visits to the Museum, provided every facility for study and allonc~l nic to borrow many of Iiiaer's finest spec- imcm for extended cxamination.

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