notes on acanthodian sharks

XOT’ES OK ACAKTHODIBK SHARKS.

BY

BASHFORD DEAN. From the Zoological Laboratory of Columbia University.

WITH 86 FIGURES.

The following notes relating to Acantliodian material preserved in several European museums were brought tcgcther several years ago as an incident to another study, but they have now been transcribed for publication, since they deal with several unnoted, or little noted, features of these ancient sharks.

The Acanthodians are known to have constituted the earliest line of sharks which underwent a period of evolutional prosperity. First known from the late Silurian, they gave rise t o a series of highly modified forms in the Devonian (represented by a t least three families and upward of twenty species), and became extinct by the close of the Permian. The earliest of the remaining groups of sharks, on the other hand, do not appear before the Devonian ancl were indeed sparingly represented during the maximum epoch of the Acanthodian; but by the coal times they had evolxwl characters which, as we hare reaqon t o belieTe, placed them under more favorable conditions for survival and thus enabled them t o supersede their more specialized Acanthodjari neighbors.

Dentition.-The dental strnctures of Acanthodians have received but little comment. It ~vas known that while in many genera, Acanthodes, Parexus, Diplacanthus, Climatius, and Cheiracanthns, teeth are rudimentary (or even lacking), in other f o r m they are exceedingly conspicuous. Tli~is, in Acanthodopsis, according to A. X. Woodward, tliere are “ a few large, latcrallv compressed, triangular teeth ”; in Tschnacan- thus, “ a few large conical teeth, the interspaces betiveen these teeth being occupied lsy a close series of minute cusps, all apparently in firm connection with a membrane bone in both jam.” But no details of dentition hare been given, in spite of the fact that they are of no little importance in comparing Scanthodians \Ti th other early sharks. Accordingly, nre are led to give the accompanying figures, 1-11, o-it11 com~ne~~ts .

There arc present i a Ischnacanthus gmcil is . cf. Fig. 1 ( f rom specimen

A ~ I I E R I C ~ Y J O L I : X ~ ~ L O F A\ATO\IT.-TOL 1-11 16

210 Xotes on dcanthodian Sharks

in the Edinburgh Collection), teeth arranged in a graded series with stouter ones at tlie side and with longer and narrower ones in front (similar, accordingly, to the conditions in Cladoselache ancl iiiany modern sharks) ; the spaces between thc teeth, however, appear soiiiewliat irregular (artifact 3) . I n Fig. 2, in a similar speci inen. the heavier teeth show lateral cusps, distinctly cladodont in type. O n each side of the jaw there can be counted about a dozen marginal teeth. There is also shown in this specinien (noted for the first time in Acanthodians) a second or successional row of teeth, sue; these are distinctly smaller than the fnnctional teeth, and cannot be niistakcn for the elements of the opposite'half of the manclible. I n Fig. 3 (British JIusemi, P. 6996) is shown a niandihle of Ischnacanthus which illustrates strikingly the shape and proniinence of the lateral teeth. I n Fig. 4 (Edinbnrgli) there appear again the prominent lateral teeth, but with indications that their bases were somewhat as in other sharlm-not fused (as Sniith Wood~vard and others belieTed) with the " membranous calcifications " of hoth jaws. The separateness of the teeth is also indicated in similar specimens ( I . gracilis, in Edinburgh), shown in Figs. 5-8; in tlie first of these the bases of the separate teeth are particularly well shown, inclucling the intcrlap of the bases, which here occurs very much after the fashion of certain modern sharks.' Interesting, also, is tlie arrangement of iiiiiior cusps on either side of the major one, which suggests clearly the teeth of Cladodonts. Strikingly Clndodont, morcover, are the Tarions fortiis of teeth occurring in different regions of the mouth. Tl i~s , the form of tooth shown in Fig. 9 occurs more nearly sppl ipsea l than the forni in Fig. 6, a i d this in turn precedes that of Fig. ?; in Fig. 8 is a detail of a favorably preserved series of lateral teeth. I n the matter of tlie quc- cessional series of teeth no inore definite result could be obtained than that shown in Fig. 2 ; i. e., indicating a rcplaccnicnt of teeth, and shom- ing in a inore inarkccl way than in Cladodonts or in modern shark^, a difference in size between the old and the new t ~ e t l i . ~ The gron tli of the new teeth, ivc therefore infer, was conditioned quite clifferently tliaii in the dental fold of later sliarlrs. X single specimen (Edinburgh), Fig. 10, confirms the foregoing account of tlentaI sncccssion ; it preSerms, it is

Cf. especially Figs. 6 and 8. There is accordingly a suggestion that in this order of sharks a dental

trench and a successional series of teeth had not been perfectly attained; in the event of the loss of its large teeth, such an Acanthodian as I. graczits would evidently have to depend upon the small teeth in the second row. That these teeth could then increase in size and finally equal the large teeth remains an open question; but it is clear that teeth could not be replaced as reaclliy as in a modern shark.

Eashford Dean 211

true, only a fragment of the jaw, showing in visceral aspect the region of its articular margin. Here it presents a series of enlarged denticles which pass from the condition of shagrcen tubercles into the inargiiial teeth; but it will be seen that as the snbtiiarginal clenticles pass forward and outward toTarc1 the rim of the jaw, they become reduced t o a single successional row, as already noted in Fig. 2 .

In the genus Scanthodopsis a reinarkably perfect dentition is shown in a specimen in the British 3lnseum (Fig. 11). I n each ramns there was present a row of a dozen (possibly there were several more) teeth which increase in size as they pass toward the rcgion of the syniphysis. They are certainly stouter in this form than in Ischnacanthus, and the synipliyseal teeth n ere as conspicuous, or even more conspicuom, than the similarly situatecl teeth in a inodern Oclontaspid shark. Xoreover, that they were here functionally important seems evident from the thick- ness of the anterior reach of the iiiecBelian cartilage.

From the evidence of dentition, accordingly, Acanthodians, in certain of the genera at least, resemble Devonian Cladodonts, and from this structural standpoint, there is suggested a closer kinship between the

3That the Acanthodians were in a general sense planktonophagous, as Dollo has recently suggested, '06, on the ground that in several genera teeth have not been described, is by no means evident. It is certainly possible, in view of the evidence that a widely diversified evolution took place within this group, that some of its members were specialized to secure plankton, just as were members of other optimum groups, euselachians, ganoids, teleosts. It is, however, clearly safer to conclude, as has hitherto been done, that the Acanthodians in which the dentition was reduced (or rudimentary) were forms whose diet was restricted to small or minute organisms. To specify that this food was plankton, and that the structures of the fish were typically planktonophilous, carries one beyond the limits of evidence. Nor does it seem safe to assume, as Dollo has cione, that those Acanthodians which were not of this form were developed with an abyssal type of dentition, after the fashion of Stomias, Aulops, etc., seems equally wide of the mark. The present notes indicate a rather normal type of selachian dentition, primitive, we infer, i n as much as the mouth-invading shagreen seems to have gone no farther than the mouth rim,-there being no evidence of a wide successional series of teeth as in euselachii. And we can assume, accordingly, that in the Acanthodian, as in other fishes, Characinids for example, large teeth predicate large prey and rapacious habits. But not necessarily bathybial. There is evidence on the other hand that Acanthodians could not have inhabited deep water, for the rule is fixed that bathybial forms, on account of the mechani- cal conditions of their habitat, are extremely defective in hard structures, scales, bones, and spines. The stout-walled spines and dense shagreen of Acanthodians should alone have precluded the assumption that they were deepwater forms.

212 Kotes on Acanthodian Sharks

X l ~ z i l l and Bmix7iinl Arches.--To discus tlir complicated structure of the skull and branchial arches o€ Acantliodians is hardly the part of the present paper. I have had, hoverer, thanks to the courtesy of Professor Jaekel, the privilege of examining the extraordinary a i d beautifully prepared series of the Permian species, A. b~-onizi , in the Berlin Xusenm. And I am able to confirm the essential Mai l s given for this species, i. e., the subdivision of the pterygo-quadrate and nieckelian cartilages, as shown in Jaekel’s Fig. 1, in Zei tschr. deutscli. geolog. Gesell., gg, or Reis’s Fig. 1, Schmalbe’s Morph. Arbeiten, Tol. VI.‘ One certainly finds liitle reason to dissent from the interpretation of these elements in terms of the succeeding brancliial arches, for, in view for example of the conditions in the young Chiniaeroid, n e have clearly the grouncls for con- cluding that the inandibular arch of primitive elasinobranchs was proli- ably a structure segmented like a branchial arch. Thus, in Acanthodes byonni (Fig. 12) , the epibranchia! element is represented in the mandibular arch by the eleinent q, the pliargngo-branchial by p t , tlie cerato-brancliial by na, tlie basi-brancl?ial by m’, and the hypo-branchial by c. What the anterior element, u, represents is by no means a j prolsable. I aiii not convinced that Jaeliel’s conclusion is a just one in regarding this as tlie “ primary maxilla,” nor indeed his transcendental views as to the hoinologies of such parts of tlie mandibular arch as an “ infra dental,” or an “ articular,” in his endeavor to demonstrate homo- logies between subdivisions of the Acanthodian mandible and the clerinal bones of the jaw of higher forins.’ As regards the dermal sub-iiieckelian element, nz“, one can also hardly subscribe unreservedly to the I-iew that it represents the definite spleiiial of higher foriiis, for there is the obvious possibility, in lriew always of the fact that there are here no other deriiial bones associated with the mandibular arch, that this eleinent may be sui gefzeiis, without, however, goiiig to the extreme view of Reis that it functioned as an extra-mandibular spine, vliich churned up the lsottolil and aided in securing the hidden food of the fish.

This T iew of Reis, it may be aclded parenthetically, is founclecl in part upon his assnniption that this c l e m n l sub-mcclielian element bore a series of appendages sonien hat as branehiostegal rays. I have, hovm er, a snspicion-for a final judginent one should of course compare Eeih’s, material-that the present appen(1ances belonpecl in reality to the under- lying liyoideaii arch : they are beautifnlly shon n. for esaiiiple. 111 the

4Cf. the present Fig. 12 . ‘SB. Gesell. naturforsch. Fr. Berlin, 1903, 1). 134 ef seq.

Eashford Dean 213

Berlin specimen from nhich Pig. 12 ~ v a s prepared. In an? eveiit they indicate that the ceratohyal region more perfectly gill-bearing than in modern ~ha r l i s .~

That the nianclibular arch in ;Iccirzt7~odes bi onni sho~vs segmentation inxist, accordingly, be admitted ; and that its elements indicate honiologies with the braiichial arches. On the other hand, nnfortunately, these honiologies cannot be accepted as final, for the following reason : The condition of the Perniian species, -1. broniai is not confirined b) the con- rlition in seieral of the Lower Devonian forms. I refer particularly to s ~ e h a specimen, for e~aiiiple, as that of IscI~nncanthus gl-ncilis ' (Fig. 13), in nhich no separate elements can be distinguished; and this is true, also, in a well-preser\ ed jaw arch of G7~eil-acant7~zis mzidzisoni (Edinburgh) . For it is obi ious that if the earlier dcanthodians show no trace of these elements, the condition in the much later forms may, like Cope's '' elements " in the crania of Senacanths, be interpreted as artifact.

As far as the nriter is anare the roofing of the Acanthodian skull with cleriiial elements has never been described in detail. Its interest, however, is patent, in as iiiuch CIS it represents an early, if not the earliest, form of strengthening the brain capsule in the gnatliostonie series. In certain genera it undoubtedly forms an effective shield for the brain, although, morphologically spealmg, it must be regarded as but a paral- lelism of the dernial head shield of the higher fishes. For it consists not of a series of plates, each formed of fused shagreen elements, as one hnows it i n the ontogen? of recent fishes, but of a series of single, although greatly enlarged shagreen elements. Tlins we see, for example, in Cliiizutizis sczitiger' (Fig. N), that the broad head roof is protected with dernial plates, numerous (a score or more), arranged irregu- larly, not closely oposed to one another, and clearly not to be regarded as the honiolognes of parietals, frontals, pre- and postorbitals, etc. From their shape and radial ornamentation, they are obviously to be compared u-ith the enlarged dcriiial clenticlcs of many other elasmobranchs, e. g., rays.3 It is evicleiit, furthermore, froin a comparison of the head-roofing

Dollo's recent suggestion, '06, thar. Acanthodians are plankton-eating forms, is recalled by these laminse-if they be interpreted as branchiostegal, and not as traces of gill filaments. ' Powrie Collection, No. 258, Edinburgh.

OCf. in this regard (Fig. 15) an enlarged dermal denticle from the head- Brit. Mus. 35,908.

roof of Climatizcs reticulatus in the Edinburgh Museum.

214 Kotcs on Acanthocliaii Sharks

plates v-ith the other ileriiial defenses in Acanthodians, that there exists an erolational range in the shape, size, aiid character of these structures; from the small deiiticles of the trunk of the fish we map trace transitional forins to the enlarged denticles which margin the structures of tlie lateral line, to those n.hich constitute the dermal head- roof, aiicl finallg, to thosc which margin the eye; the last sometimes seven or eight in number, instead of the four usually described. That the dermal denticles of the truiili- may in themselves be subject to an evolntioiial range of forms is also e1 ident ; in certain species and genera these denticles may acquire the quadrate iiiargiiis (Figs. 33 ancl 34), which snggest tlie bony plates of ganoids (fishes with wliich Agassiz and others associated them), others, on the contrary (Fig. 36). may imbri- cate their margins to such a degree as to suggest superficially the scales of teleosts. Noreorer, prominent contour lines may be defended mth rows of enlarged dermal denticles (already noted by Smith Wooda-ard) to such a degree as to suggest the analogous scales of sturgeons or siluroids. T~LIS, in the specinicii (C'liiiinfius scutiger), pictnred in Fig. 14, enlarged denticles, shown a t r s, form a ridge between the head ancl the dorsal fin. I n such a ridge as many as ten successire elements may be counted."

In some forms they may be small and tubercular, in others large and tubercular; in still other fornis they niay be flattened and closely compacted.12

VertebraZ Column.-The vertebral axis of Acanthodians is knon-n to retain its notochordal condition and the arrangement of neural and hzeiiiP1 spines has already been figured by several authors, lzeis notably. Speci- mens, however, are rare in which these axial relations may be determined, and 1 might refer in this regard to a specimen of Ischnucu~zthus gracilis (Fig. 16) . Here the neural and h a n d arches appear as distinct (meta- meral ?) elcnients ; as in Cladoselachus there are no interneurals, ancl there is no evidence that the arches approached closely to the surface of the body (i. e., arc not connected with tlie bases of fins). A remarkable condition, on the other hancl, is shomn in a specimen of Isch~~crcaizth~s gracilis (Fig. 1'7) , (counterpart of Poivric's type, S o . 251, Edinburgh Collection), in which appears a series of superficial elements, 11, in front

A final word regarding the head-roofing denticles.

io Specimens in Brit. Mus., P. 6955-56.

l2 Cf. Euthacanthus macnicoli, Edinburgh. l3 No. 334 in the Edinburgh collection.

Cf. specimens of Parexus recurvus and falcntus. in Edinburgh.

Eashford Dean 215

of and in the iicighborhood of the dorsal fins. They are distiiictly separate from the neural arches and arc apparently nietameral ; their interest is obvious from the standpoint of fin morphology, for they can he interpreted as the mdiments of the hasalia of a niore continuous type of unpaired fin.

Fins mcl Girdles.-The fins of Acanthodians are beyond peradventure of a lateral fin-fold type, and as snch they have been given a prominent pIace in the much-discussed problem of the origin of the yertebrate limbs, From this standpoint it is clear, either that the liiiib strnctnre of Acanthoclians and other primitive sharks innst be redncible to a common plan, or that the curious spine supported Scanthodian webs must ham arisen sui gene&. The latter view is clifficult to accept, since it is conceded unanimously that the paired limbs of all other vertebrates are homologous, and it seeiiis, therefore, if only from purely ti priori gronncls, illogical to ass~iiiie that Acanthoclians, Jvliich arc sharks in so inany details of structnre, could not have had fins based essentially npon shark-like structures. The Tien,, iiioreonx-, of the homology of the fins of Acanthodians ancl sharks is supported by the evidence of Cladowlachian sharks whose fin structures are in important regards intermediate in type. In both forms the paired fins functioned as balancing organs, rather than as paddles, and in Cladoselacliians there is a coiicentration of the supporting elements, radials, in the anterior rim of the fin which, I have maintained, served as the ancestral condition of the spine of the Acanthodian.lZ This YieJv is supported by the following facts: The caudal fin of Acantho- dians shows the radials in the process of concentration in the anterior rini of the fin. There is no spine present, although the anterior fin margin is encrusted and stiffened by shagreen. Such a condition has already been noted by various writers; it is admirably seen in one of Professor Jaekel’s beautifully prepared specimens of Acanthodes bronni, Jrhich I have figured herewith (Fig. 2.5). And similar conditions are knon-n in other genera. I n Pni.esus faZcutus,lG for example, the hypural lohe of the candal fin is strengthened by shagreen to such a degree as to

I4Cf. 1894, Jour. Morph., Vol. IX, pp. 98-111; 1896, Anat. Anz., Vol. XI, pp. 677-679; 1896, Nat. Science, Vol. VIII, pp. 245-253.

Is In this evolution the dermal elements played an important part, encrust- ing and strengthening the anterior r im of the fin, a process which caused or was accompanied by a reduction in the radials.

lo British Museum, P. 130.

216 Sotes on Acanthodian Sharks

suggest a spine-like fin support (Fig. 20). Indeed, in such a form as Diplacantlaus tentiistl-icittis (Fig. 22) such a condition is actually attained. And the dermal elements arrange themselves in the series of rows characteristic of Aeantliodian spines, while the hinder radials become obsolescent. If, accordingly, s ~ x h a conditioii be compared with that shown in the second dorsal fin of Eutlzacanthus nzacnicoli (Fig. 2 3 ) ) we cannot fail to note the close correspondence; the fin spine is made up of radial components in which dermal tubercles are concentrated, and these even appear behind the fin spine strengthening a series of delicate rays which are evidently comparable with the obsolescent rays shown in the caudal fin of Fig. 22. A second dorsal spine of an Scanthodian is thus the homologue of the concentrated radials in a caudal fin. And i t is easj to see how such a condition mould readily become highly specialized in forms in which dermal structures present a wide range in their scheme of evolution. I n such a broad spine, for example, as that shown in Fig. 26, Clinaatius +eticulatus,'J the broad striation in its dermal crust still testifies to the component radial rle- ments,-the basal elements, or a t least some of them, forming the region b. I n Fig. 24, Climatius uncinntus, a inore highly modified type, there can still be seen traces of the component radials in a spine which has become curved. I n other spines (Fig. 1 9 ) the dermal elements have completely obscured the core of radial elements over which primitively they have been laid down.

Admitting, then, that the fin spines of Acanthodians are founded upon the concentrated radials of a Cladoselachian fin, the puzzle of the paired fins of the Aeanthodians becomes greatly simplified. And for a further study of the matter, I believe that an important hint is given 11s in the behavior of the dermal elements as contributing to the formation of fin spines. For if dermal elements play so important a part in strengthening the outer supports of fins, may they not also have pressed deeply into the integument and strengthened the basals? For it is these elements which have contributed no little confusion in the studies of the dcanthodian paired limbs. Without attempting to reconcile the clifler- ences which occur in the plan of the shoulder girdle and spines of varions Acanthodians, I may point out that there is evidence not merely of a pair

Edinburgh Museum. '"Brit. Mus., P. 35,907. IJBrit. Mus., P. 1343.

Basliford Dean ’’17

of “ inter-clavicnlar ” or “ claviculoid ” membrane hones in certain genera, but that the basal region of a fin may be strengthened with many dermal elements. This is wc11 shown in a specimen of Pcwezus falcatus in the British Jlusenni, P. 130 (Fig. 21), of mliich an enlarged fignre of the base of a pectoral fin is shown in the present Fig. 2 7 ; here as many as half a dozen dermal elements may hc counted along tlie mesial margin of the basal plate, and it is even possible that the entire flat distal surface of this plate was streng thened by dermal elenients. Accordingly, the shoulder girdle of Acanthodians, like tlie girdle of a teleost (siluroid, for example) is to be treated as inclrtding a dermal complex. Our present linowledge does not warrant us in attempting to homologize its elenients with those in the higher forms (as, nevertheless, Jaekel has clone in identifying “ suprascapular,” “ scapular,” “ coracoid,” and ” clei- thral ” elements), since such structures can only be compared Ti-ith tlie conditions in the higher vertebrates when it has been slioxn that the Acanthodians have closer phyletic relationships to the higher forms. Until this can be demonstrated the structural elements of the Scantho- dians must evidently be interpreted in ternis of their nearest Icinclrrd ; i. e., fossil and recent sharks. The safest conclusions in our present stage of linowledge are, T believe, these: That we are to regard the proximal portion of tlie shoulder girdle, tlie region of sg iii Figs. 15, 19, 21 (elements u, b, c, of Jaekel) as equivalent to the proximal portion of the shoulder girdle in sharks; the distal portion becomes a region of concentrated fin-supporting elements, fusecl hasals, a portion of the basal parts of tlie radials, with as an important if not a niaxiniiini coni- ponent a dermal complex, e. g., tlie region d in Fig. 19 or the more discrete elements in Fig. 27. With regard to the homology of the cartilaginous pieces which Jaeliel and Reis describe in A. bionni, as fin- web-supporting elements placed immediately behind the pectoral spine, one may, I conclude, regard them as tlie remains of tlie series of baso- radial elements. The elements of this series had in general, as has been noted, fused a t the base of the pectoral fin spine; bnt it is by no means improbable that in certain forms a number of the posterior elements remained discrete, retaining more or less accurately their primitive func- tion. That tliere is the greatest range in the degree of concentration of fin-suporting structures in the Acanthodians all will, I think, agree who have examined these structures in many genera.

The supports of tlie ventral fins, so important in the general discussion of the origin of tlic paired limhs, have, as far as I am amare, never been described. Interesting, accordingly, is the s~xciiiien of Diplncn1zflr71.~

218 Kotes on Acanthodian Sharks

striutzis in the British lIuseiini, 1’. 36,5S2, cf. 1’. 1151, ci (Fig. 19),x-liich shon-s a distinct pclvic girdle, 2-1. At the base of the fin spine the enlarged supporting inass, T + b, represents, we infer, tlie fused baso- radial elements. At d appears a pair of distinct plates, clcrinal apparently in origin. IYliether tiicy arc d a t e d to the much discussed series of “ fin spines passing between the pectoral and pelvic fins ” is not clear.

Sense O1.~U72S.-~ilr knowleclge of the end organs of Acanthodians appears to be confined to a fern notes on the lateral line and on the large eyes protected by “ four dermal plates.” As to the former struetiire, Smith Woodward remarks 2o that “ a single lateral line occurs high on each flank, niarkccl not lip any tubular or other excavation of the scales, but by tlie ridge-like displacement of tJro series, betn-een which the organ originally estencled. The snpposeed evidence of additional sensory canals appears to the present writer to he clue to a misinterpretation of the dis- placed dorsal and rcntral ridges, which exliibi t no nieclian series of scales.” Reis, in this connection,’l has made the interesting observation that the lateral lines are joined by an occipital conmiissure yery much as in recent clasmobranchs and teleostoines ; and he refers also to the presence of a nieclian canal in the forehead wliich he compares to (or parallels with ?) the rostro-median canal of Chinizra ; lie figures, finally, a supraorbital canal and refers to the absence of a suborbital branch arising from the iiiain canal in that region. Reis’s figure representing these conditions *’ is of especial importance. To the foregoing notes upon sensory structures I map add: The “additional sensory canals,” in spite of the skeptical coininent of Smith Woodward, are beautifully seen in several specimens in the British Nusenm and in the Edinburgh Collection. I refer particularly to Glinintius gl-uizdis and Ischiaucunthus qi-acilis 23 (Figs. 30, 31, Edinburgh, and 32, this after S o . 241, Pomie Collection). I n these particular forms the branching of the lateral line, 1 Z, in tlie ncighhorhood of the dorsal spine is, indeed, so. coininon an occurrence that one niiglit even maintain that it was the normal condition. It has not, howerer, been observecl to occur on both sides in the same indi- ridnal; nor is it definite that it occurs always on the same side, although a close examination has conrincetl me that i t occurs on the right side in most eases. The m o s t reinarkahl~ preserved sensory structures in hcanthodians are probahly s h o m in thc qwciiuens of 9. b ~ o n i i i (from

2o Cat. Foss. Fishes, Vol. 11, p. 5. 21 Rlorph. Arb. Schwalbe, VI, pp. 195-196. ”Op. cit.. p. 195.

London, P. 6974.

Basliforcl Dean 219

Lcbach) in the Berlin Xuseuni, for tlie inspection of irhich I am greatly indebted to Professor Jaelxl. For they shon- not only the lateral line hit its many branches, the sensory canals of the head, and, most interesting of all, the anclitory organs aiicl the T i m of the nasal calxule. Thus, in the specimen shox-n in Fig. 'is, one readily distinguishes '' tlie canals of right and left sides, 17, 11, from which a series of vertical branches arise very much as in recent selachians, cf. especially in the young specimmf (20-30 em.) of Chla~ii~closelachus ; the lateral lines apparently draw to- gether in the occiput am1 from this region ]mss forxyard, giving off various branches, a supraorbital canal, s u p ~ c ~ o; there, also, appear a saborbital canal, sub 0, an auditory capsnle, cizi, of extraordinary size, and traces of sensorg canals, 6 r s, i n the branchial region, again a concli- tion which suggests the iiioclern sliark ( Chlamydoselachus) . These conditions are shown to eren better advantage in a specimen from the same collection shon-n in Fig. 29. (I) The position of tlie suborbital canal, for it is seen to arise (contrast Reis's note given above) from the main canal passing to the trunk, i. e., in tlie position usual in fishes. (11) The great size of the sensory branches passing to the region of the gills. (111) The structures of the auditory organ, the ntricnlus, atr, and, prominently sliomn, a vertical simicircular canal, 'CSG. ( I T ) The upper iiiargin of the nasal capsnle, nus, whicli indi- cates accurately the position of this organ.

I n the manner of the protection of the lateral line the Acanthodians shorn a range of evolutional characters ; in EzitlzacanthtLs e1egans the canal is marked onlg by a ridge-like prominence of marginal scales (Figs. 34 and 34a) ; in Acnnthodes bronni the marginal scales become enlarged and prosalient (Fig. 35) ; in Euthacanthiis gracilis they niay completely orer- lap and enclose the sensory canal (Fig. 33). It is finally observed that a great range in the iiianner of protecting the sensory structures is found a t different regions in the same indiriclnal.

Especially noteTyorthy are here :

Relationskip' of t h e Acanthodbizs.--It has been pointed out in the foregoing notes that the hcantliodians agree in a number of regards with the Claclosclachian sharl;s, and that from this euidence we conclude that they are more closely related to these forms than has hitherto been gen- erally accepted. Dental characters, structures of fins candal, unpaired and paired, vcrtchral axis, cren the mode of protection of the eye, are

% I n A. bronni the shagreen denticles are reduced in size; those only are large which protect the sensory canals, hence the clearness with which the arrangement of the canals may be followed.

220 Notes oil dcantliodian Sharlts

distinctly cladoselacliian in type. Taxonomically, tlicrefore, Cladose- lacliians and Acanthodiani should he inore cloqelv associated than slioulcl, for example, Cladoselachians and Senacanths, or Alcanthodiaiis and Xena- canths. We are thus led to suggest again ’’ that a qioup such as clefiiiccl by Pleuropterggii (sharks with fins of fin-fold t y e ) , would justly in- clude both Cladoselachians and Acanthodians. This group, however, might more accurately be regarclccl as of superor(lina1 rather than of ordinal rank, and ordinal rank ~vould thus remain in tlie groups Cladoselachia ancl ,lcaiithotlia.

The question as to which groups, Acanthodian or Cladoselachid has retained the inore primitive character is indicated in the accompanying table (u,p.221). I n the great majority of structures one can only conclude that the Acantliodians have passed through a stage in evolution which is best represented by the Cladoselachian. I n one regard only (apart from the matter of size) does the latter appear the more specialized, i. e., in the unsegmented character of the niandilsular arch (ZI, however, supra).

If these things be true, it may next be queried -vrliy is it that the more specialized group is known from an earlier horizon? For certainly the appearance of the Bcanthodians in the upper Silurian (as against tlie Cladoselachian in the upper Devonian) is in general evidence of the greater primitiveness of their group. And this is in truth a question which can be answered only by the time-morn appeal to the defectiveness of the palteontological record, noting especially in this regard that the soft structures of the Cladoselachians would be less apt to be preserved than the hard structures of the Acanthodians. We may, however, safely predict that from the earliest Acanthodian horizon there will be dis- coTTerec1 forms which will represent the ancestors of all of the early groups of sharks. And we niay predict with almost the saiiie degree of security that these forms will be found to picture the Cladoselachian in essential characters. For the L2canthodians, as we at present know them: are obviously too specialized to have represented the aiicestors of the line of Cladoselachians.

The canses of the extinction of the Acanthodians can, I believe, be suggested with a fair degree of probability. It is evident, of course, that the fin characters of this group premise great capability on the part of these fishes to dart forward, i. e., in clircct lines, a t y e of inovcment especially valuahle when a definite lrintl of food is to be secured. On the

‘j Cf. J. Morph., Val. IX, pp. 110-111. 2c Cf. the accompanying table.

CO

NTR

AST

ED

CH

AR

AC

TE

RS O

F

Sm

alle

r an

d m

ore

unif

orm

in

patt

ern

thro

ugho

ut

all r

egio

ns.

Pra

ctic

ally

unor

nam

ente

d.

Mor

c gr

adua

l tra

nsit

ion

from

sha

grec

n de

ntic

les

to

scle

roti

c pl

ates

and

to

tcct

h.

On

ante

rior

fin

m

argi

ns t

he

dcnt

icle

s be

com

e la

rge

and

con-

cc

ntra

ted,

bu

t do

not p

rodu

cc s

pine

s.

Den

ti-

cles

no

t sp

ecia

lize

d on

th

c m

argi

ns o

f la

tera

l li

nes.

Ilr(

rrl.

Arc

hes

sim

ilar

, app

aren

tly,

to

Not

idan

ids.

,,~

tl.~

eh

l ,rl

c,~

izlm

7L

, N

otoc

hord

al.

Nei

iral

and

htr

mal

ele

men

ts h

eavy

, no

t lon

gand

notc

onne

cted

wit

h un

pair

edfi

ns.

~

130t

h pr

esen

t: b

ut s

mal

l in

size

. T

he p

elvi

c el

e-

men

ts s

ingl

c, n

ot c

onjo

ined

(on

the

cvid

encc

of

a n

ew s

peci

men

of

C. k

egle

ri).

G

LI tll

ta.

,

Fin

s.

Sen

sory

str

uct

ure

s.

Sup

port

ing basal

and

rad

ial

elem

ents

arr

ange

d si

mil

arly

in

pai

red

and

unpa

ired

fin

s, c

once

n-

trat

ing

ante

rior

war

d.

Not

wel

l kn

own.

E

yes

smal

lcr (

rela

tive

ly) t

han

in A

cant

hodi

ans

and

less

per

fect

ly p

rote

cted

w

ith

derm

al p

late

s.

Lat

eral

line

inco

nspi

cu-

ous,

ope

n, a

ppar

entl

y un

bran

ched

.

AC

AN

TH

OD

IAN

S.

Lar

ge o

f si

ze, v

arie

d in

pat

tern

, so

met

imes

hig

hly

orna

men

ted.

In

ce

rtai

n re

gion

s (h

ead

roof

) in

divi

dual

sc

ales

bec

ome

grea

tly

enla

rged

and

fu

ncti

on a

s m

emhr

anc

bone

s.

In g

ener

al s

harp

co

ntra

st b

etw

een

shag

reen

den

ticl

es, t

eeth

and

scl

erot

ic p

late

s.

Hig

h sp

,eci

aliz

atio

n of

den

ticl

cs m

argi

ning

sen

sory

str

uctu

res.

E

xtra

ordi

nary

dev

elop

men

t of

spi

nes,

whi

ch m

ay b

e re

gard

ed

mor

phol

ogic

ally

as

coll

ecti

ons

of

dent

icle

s en

larg

ed a

nd c

on-

cres

ed o

n an

teri

or f

in m

argi

ns (

as su

gges

ted

in t

he

cond

itio

n in

th

e ca

udal

s of

Aca

ntho

dian

s an

d in

tho

pai

red

fins

of

Cla

dosc

- Ia

che)

, or

as h

ugc

sing

le d

enti

cles

.

w P.! m

Man

dibu

lar

arch

(A

cant

hode

sr s

egm

ente

d li

ke a

bra

nchi

al a

rch.

2

0

Not

ocho

rdal

. N

eura

l an

d hr

omal

clc

mon

ts sh

orte

r and

mor

e de

lica

te

than

in C

lado

sela

chia

n.

Y

i,

m 6

Bot

h pr

esen

t, b

ut t

hei

r str

uct

ure

obs

cure

d by

th

e in

vasi

on o

f de

rmal

el

emen

ts.

Pel

vic

elem

ent

on e

ach

side

as

in C

lado

scla

che.

bu

t lo

nger

and

mor

e va

ried

in

outl

ine.

In

Aca

ntho

des

“seg

men

ts”

(art

ifac

t? p

rese

nt i

n p

ccto

ral

arch

, h

ut

in o

ther

gen

era

thes

e ar

e ab

sent

.

Spi

nes

supp

orti

ng p

aire

d an

d un

pair

ed f

ins.

C

auda

l pr

ovid

ed w

ith

radi

als

as i

n C

lado

sela

che.

T

race

s of

ra

dial

s in

unp

aire

d an

d pa

ired

fin

s.

Eye

s an

d au

dito

ry o

rgan

s of

lar

ge s

ize.

Mar

ked

bran

chin

g of

lat

eral

li

ne s

yste

m.

Usu

ally

bu

t fo

ur

(lar

ge)

derm

al p

late

s pr

otec

ting

ey

e.

222 Notes on -1canthoilian Sharks

other hand, it is very probablc that they could not have altered quickly their direction, speed, or plane of nioveinent, and thev would thus be placed at a serious disadvantage n-licn competing with ganoids, dipnoans, even with other sharks, for the capture of varied foriiis of’preg. So, also, the teeth of Xcanthodians indicate a smaller range for functional adaptation than the dentition of other sharks. Finally, the elaborate specializations of the integunientarg defenses of Acanthodians lead in the direction of enlarging individual dermal cusps, instead of that of fusing the bases of the cusps. And the result mas an unfavorable one. For the limit of the size of the individual cusp could soon be reached, while that of the fused basal plate (a true nieiiibranc bone) was greater and more plastic. Even the most perfectly arinorecl dcanthodian, therefore, could not compete in this regard with the conteniporary ganoids.

DESCRIPTION O F FIGURES.

FIGS. 1-11, Teeth of Acanthodians. Figures of Ischnacanthus, excepting Fig. 11, which is of Acanthodopsis warbi.

FIG. 12. Jaw and branchial arches of Acnnthocles bronni. 13. Jaw arches of Ischnacanthus gracilis. 15. Detail of a shagreen denticle from the head-roof of Glimatitis reticulatus. 16. Neural and hzmal arches of Ischnacanthus yracilis. 17. Neural arches and dorsal fin, ibib. 19. Pectoral and pelvic girdles and spines of Diplacanthus striatus.

14. Head-roof of Climatius scutiger.

18. Pectoral spines and girdle of Climatius reticulatzis.

FIGS. 20-27. Fins and related structures in Acanthodians. 20. Parezus falcatus. 21. Ib id . 22. Caudal of Diplaccinthzis teizuistriatus. 23. Second dorsal of Eut7aacanthus nzacnicoli. 24. Dorsal of Clinzatius unciizatus. 25. Caudal of Acanthodes broiani. 26. Detached spine of Cliiizatius reticulatzis. 27. Shoulder girdle (half) of Climalizis falcatus, showing dermal elements.

FIGS. 28-36. Sensory structures in Acanthodians. 28, 29. Acanthodes bronni. 30. Clinzatius grandis. 31, 32. Ischnacaizthus gracilis. 33. Climatius. Detail of lateral line at a point anterior to second dorsal fin. 34. Euthacanthus ele- gnizs. Lateral line. 34a. Detail of a shagreen denticle margining lateral line. 35. Acairthodes bronni. Detail of lateral line. 36. Eiithacnnthus macizi- coli. Detail of enlarged scales of ventral kody-wall.

2

5 3

11 4

20 -a/

notes on acanthodian sharks

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