On the Pelvic Girdle and Pin of Eusthenopteron. by Edwin S

PELVIC GIRDLE AND D'lN . OF EUSTHKNOPfERON. 311

On the Pelvic Girdle and Pin of Eusthenopteron. By Edwin S. Goodrich, M.A., Fellow of Mertoa College, Oxford.

With Plate 16.

1 THROUGH the kindness of Mr. A. Smith Woodward, I have recently had the opportunity of looking through the fossil fish acquired by the British Museum since the Cata- logue was published. Amongst these was found a specimen of Eusthenopteron foordi, Whit., showing the endo- skeleton of the pelvic girdle and fin, of which I here give a description. The interest attaching to this fossil is considerable, since, of all the numerous extinct fish usually included in the group " Crossopterygii," it is the first and only one in which the parts of the skeleton of the pelvic girdle and its fin have been found complete and in their natural relations.2 The specimen (P. 6794) of which both the slab and the counterslab have been preserved, comes from the Upper Devonian of Canada. In it can be made out the skeleton of the pelvic girdle and fin of the right side, in a fairly complete and well-preserved condition, as represented in PI. 16, fig. 1, natural size. 1 To Mr. Smith Woodward I am also indebted for constant help when working in his Department. a The skeleton of the pelvic fin of Megalichthys has to some extent been made known by Cope, Miall, and Wellburn (2, 5, and 9), and the essential structure of that of Eusthenopteron has been briefly described by Traquair (7). VOL. 45, FART 2.—NEW SKKIES. Y 312 EDWIN S. GOODttlCH.

It will be seen at once that the skeleton of this fin closely resembles that of the pectoral fin of the same species already described and figured by Whiteaves and Smith Woodward (10 and 12).1 In the pelvic fin (figs. 1 andF) we find an axis consisting of three segments or mesomeres, and three pre- axial endo-skeletal rays or parameres. Of these the first two and largest are borne by the first and second mesomeres respectively, whilst the last is in a rudimentary condition,

FIG. A.—Ventral view of the pelvic girdle and fins of Heptanclius cine- reus, Gm.; in this and the succeeding figuresth e complete skeleton is exposed on one side only. For the letteiing see the Explanation of Plate 16. being represented by a small rounded piece at the distal end of the outer branch of the slightly bifurcated terminal mesomere. A similar semi-lunar piece fits on to the axial branch of the third mesomere. The whole skeleton of the fin is formed, then, of an axis consisting of three large segments, and a small terminal piece, and of two well- 1 This resemblance was pointed out by Traquair (Ji), who writes "A veiy similar arrangement is found in the pelvic fin . . . ; here J find at least two mesomeres, each bearing a paramere, there being, I think, aiso a piobabilily of the presence of a third or disial mesomere." PELVIC GIRDLE AND FrN OF EUSTHENOPTISRON. 313 defined pre-axial rays, and probably a vestigial third ray. The chief difference between the pectoral and the pelvic fin is, that whereas in the former there are post-axial expansions on the first, third, and fourth axial segments, in the pelvic fin no such post-axial process is visible in our specimen (figs. 1 and 2). The pelvic fin projects from the body as a free lobe of considerable size, and is covered with scales similar to those on the trunk. Round this lobe the extensive web of the fin is supported by jointed bony dermal fin rays. On the pre-

FlG. B.—Ventral view of the pelvic girdle and fins of Cliimeera monstrosa, L. axial side the dermal rays are, as usual, stronger than on the post-axial side. The pre-axial edge of the fin is straighter aud considerably longer than the post-axial, so that the fin is not symmetrical about its skeletal axis either internally or externally. The pelvic girdle is represented on the right side by a somewhat triangular elongated bone. It is pointed in front, and widens out behind into a broad plate. The outer edge is almost straight, whilst the inner edge is sharply curved where the bone widens out. To the posterior edge is articulated the axis of the pelvic fin. The whole girdle consists of two such bones, which in the living- animal no 314 EDWIN S. GOODRICH. doubt lay parallel to the ventral surface, with their sharp ends pointing forwards and converging towards the middle line. The posterior expansions would also extend towards the mid-ventral line (fig. F). The structure of the skeleton of the pelvic girdle and fin is of great importance as a taxonomic character. But before attempting to discuss the value of these organs in deciding the position occupied by Eusthenopteron in the scheme of classification, it will be well to_ briefly compare the various types of pelvic supports in the Fish series.

FIG. C.—Ventral view of the pelvic girdle and fins of Acipenser sturio, L. Amongst the Elasmobranchs we find embedded in the body-wall a median-ventral cartilaginous plate, to which the pelvic fins are attached by a moveable joint (Fig. A). In the Holocephali there is on each side an elongated cartilage supporting the pelvic fin; these cartilages are joined together in the middle line by a ligament (Fig. B). A somewhat similar, but shorter, pair of cartilages is found in the extinct Pleuracanthidee (Fig. D). A specimen in the Oxford Museum (Fig. E) shows particularly well their ligamentous union in the middle Hue. Coming now to the Teleostomi, PELVIC GIRDLE AND FIN OF EUSTHKNOPTELtON. 315 we find in the Chondrostei somewhat ill-defined, more or less triangular, cartilaginous plates stretching from the base of the pelvic fins towards the middle line (Fig. C). A mi a lepidosteus and all the ''Teleostei," in fact all

FIG. I).—Ventral view of the pelvic girdle and fin of a female Pleura- canthus Oelbergensis, Fr. (from Fritsch). the Actinopterygii, possess paired bony ventral plates supporting the pelvic fins. These plates may be joined together in front by a median cartilage, as in Gadus (Fig. J), or they

FIG. E.—Ventral view of the pelvic girdle of Pleuracanthus Gandryi, Brogn. may be merely united by ligament (Figs. H, I). In the Dipnoi, on the contrary, the pelvic support is represented by a median cartilage with two diverging branches, to which are articulated the fins (Fig. K). Of the so-called Crossopterygii, 316 EDWIN S. GOODBIOH. the only forms in which the pelvic girdles are thoroughly well known are the Ccelacanthidge and the Polypteridas.1 In both these families the pelvic bones closely resemble those of the Actinopterygii (Figs. G, L). Concerning the morphology of these pelvic supports there is considerable confusion. Whilst the older anatomists be- lieved them, I think quite rightly, to be homologous, and the representatives of the pelvic girdle of other fish, some modern authors would have us believe that they are of quite different

;pIGi Y.—Ventral view of the pelvic girdle and fins of Eusthenopteron Foordi, Whit., restored. nature in the various orders of Pisces. They hold that whilst, for instance, in the Selachii, Holocephali, and Dipnoi, a true pelvic girdle is present, the supports in the Crossop- terygii and Actinopterygii are, on the contrary, derived from the fin skeleton itself. Let us see what difficulties such views lead us into. Whether we hold, with the advocates of the fin-fold theory, 1 Paired bony plates seem to have been present in Megalichthys (5, 9), and Specimens 21,547 and P. 6513 of the British Museum Collection show traces of similar structures in Osteolepis and Glyptolepis. PELVIC GIRDLE AND FIN OF BtJSTHBNOPTBBON. 317 that the pelvic girdle originated as an ingrowth of the base of the primitive fin skeleton, or whether, following Gegenbaur, we consider it to have been derived from a gill arch, it will be admitted that the girdle was primarily differentiated as a right and a left support and fulcrum for the fin, and as a point of attachment for the muscles whereby the fin is moved. The girdle plate must have been from the first

FIG. G.—Ventral view ol the pelvic girdle and fins of Polypterus bichir, Geoffr. embedded in the ventral body-wall from which sprang the free lobe of the pelvic fin. Such appears to have been the structure of the paired pelvic girdle of the Pleuracanthidae (Fritsch [3] and Figs. D and E), and such it is essentially at the present day in the Holo- cephali (Fig. B). The development of the pelvic girdle in the Selachii (Balfour [1], Mollier [6]) warrants the view that the median cartilage there found has been formed by the fusion of two originally separate halves. Presumably in this way has also originated the median cartilage of the Dipnoi. 318 EDWIN S. G00D1U0H. It is not until we reach the Teleostomes that difficulties arise. Davidoff1 held that the girdle proper is represented in the Bony G-auoids and Teleosts by the cartilage at the anterior ends of the long pelvic bones, which themselves would be homologous with the metapterygium (basipterygium) of the Selachian fin. Weidersheim (11), considering the bones as metapterygial, believes the girdle to be appearing in Polypterus as small

EIG. H.—Ventral view of the pelvic girdle and fins of Amia calva, Bon. (Partly from Davidoff.) paired or median cartilages (Fig. G) at the tip of the pelvic bones. A somewhat similar little piece of cartilage, occasion- ally found at the anterior extremity of the pelvic supports of the Chondrostei, is supposed to have the same significance. Kautenfeld (8), on the other hand, compares the basal support in the Ganoids to the propterygium of the Selachii. Wiedersheim's view, that the minute paired or median 1 Davidoff, " Beitrage z. vergl. Anat. der Hinteren Gliedmasse der Fisclie." 1 Morpb. Jahrb..' vols. v, vi, and ix. PELVIC GIliDIiE AND FIN OF EUSTHKNOPTERON. 319 cartilages often found in front of basal supports, represent the first origin of the girdle, has not met with much favour for many reasons. More especially the obvious objection may be urged against it, that the pelvic girdle is already fully developed in more primitive forms (Elasmobranchs). Gegenbaur (4), whilst adopting the theory of the homology of the basal supports with the metapterygium of the Selachian fin, considers that the small anterior cartilages of the Ganoids represent the last vestiges of the pelvic girdle, which has

Fio. I.—Ventral view of the pelvic girdle and fins of Lepidosteus osseus, L. (Partly from Davidoff.) undergone degeneration, and may have entirely disappeared in other Teleostomes. Now Gegenbaur's view seems to be no less open to objection than Wiedersheim's. For if we are to believe that the girdle has disappeared and been functionally replaced by bones derived from an ingrowth of the already differentiated fin skeleton, we may well ask, what plausible reason can be given for the substitution in the place of the girdle supports of these new structures of very similar shape, and of perhaps VOL. 45, PANT 2. NEW SKR1ES. Z 320 EDWIN S. GOODK1CH. even larger size ? During this important change what has become of the muscles attached to the girdle for the moving of the fin ? Have they disappeared also, and been replaced by others, or have they shifted their base of attachment on to the basals ? What evidence is there that the moveable joint, where the base of the fin skeleton articulates with the girdle, firmly embedded in the body-wall, has not always been where it now is, but has been carried forwards at the tip of the basal bones and lost its primitive function ? What evidence is there that this primary articulation between the moveable fin skeleton and the fixed pelvic girdle, has been

FIG. J.—Ventral view of the pelvic girdle and fins of Gad us morrhua, L. replaced by a new joint between two different regions of the fin skeleton itself ? Moreover, is it credible that such a fundamental alteration in the relations of the internal skeleton should have taken place without a corresponding change in external shape ? Here the evidence afforded by the structure of Eusthenop- teron may be called in. On the one hand there seems to be no reasonable doubt that the pelvic bones of this fish are homologous with those (so-called basals) of Polypterus, Coelacanthus, or Amia (Figs. G, L, and H). On the other hand, it will, I think, be allowed that the moveable joint whereby PELVIC GIRDLE AND FIN OF EUSTHENOPTERON. 321 the segmented axis of the fin of Eusthenopteron articulates with the pelvic bone is strictly homologous with the corresponding articulation at the base of the pelvic fin of Ceratodus or Pleuracanthus.1 We are, then, inevitably led to the con- clusion that the pelvic supports, whether paired or unpaired,

FIG. K.—Ventral view of the pelvic girdle and fins of Ceratodus Porsteri, Kr. (Partly from Davidoff.) are homologous throughout the fish series. These structures are similarly situated, fulfil the same functions, and are, as far as we know, developed in the same way in all fish. In 1 The persistence of the same articulation between the girdle and fin skeleton is obvious in the case of the pectoral limb of fishes. 322 UDW1N S. G00DK10H. some cases, however, as in Dipnoi and Teleosts, they are well differentiated; in other cases,as in the Chondostrei, they are ill-defined, and probably in a more or less degenerate condition, not clearly marked off from the true fin skeleton. To conclude, we may briefly mention the evidence afforded by the structure of the skeleton of the pelvic fin as to the systematic position of Eusthenopteron. Unfortunately we know hardly anything about the structure of the fin skeleton of other extinct " Crossopterygii." But from our know- ledge of the Dipnoi, it may be inferred with some degree of certainty that the skeleton of the elongated lobed fins of such forms as Grlyptolepis and Osteolepis was built on the

FIG. L.—Ventral view of the pelvic girdle and fins of Holophagus gulo, Huxley. biserial archipterygial plan (distichopterygia). It is there- fore of considerable interest to note that although in the shape of the outline of the fin-web, and in the disposition and structure of the dei'mal rays, the pelvic limb of Eus- thenopteron approximates to that of the more highly specialised Teleostbmes (Actinopterygii) ; yet its internal skeleton is probably to be interpreted as a modification of the biserial archipterygium, with a distinct axis, in which the post-axial endo-skeletal rays have been lost. Further, the skeleton of the pectoral and of the pelvic fin of Eusthe- nopteron still exhibit that close resemblance to each other which is so marked a characteristic of the Dipnoan fins, and presumably also of the more primitive forms from which they have been derived. In contrast to this we find in Polypterus and the Actinop- PELVIC GIRDLE AND FIN OF EUSTHENOPTERON. 323 terygii a great and increasing modification in structure of the fins. All trace of an axis is soon lost in the pelvic limb, whilst at the same time the pelvic girdle bones in these fish and the Coelacanthidze, assume that peculiar elongated and flattened shape, widening out in front, which is so characteristic. Finally it may be pointed out that", whilst Eusthenopteron is undoubtedly closely allied to the Rhizodontidte, judging from the skeleton of the pelvic fin, it appears to be very far removed from Polypterus, which probably belougs to the Actinopterygian line of development.

LIST OP REFERENCES.

1. BALFOUB., F.—' Comparative Embryology,' vol. li. 2. COPE, E. D.—' Pioc. W. S. Nat. Museum,' vol. xiv. 3. FEJTSCII, A.— ' Fauna der Gaskohle,' Prague, vol. ii, 1889. 4. GEGENBAITR. C.—'Vergl. Anatomie der Wirbelthiere,' vol. i, Leipzig, 1S98. 5. MIALL, L. C.—'Quart Journ. Geol. Soc.,' vol. xl, p. 347. 6. MOLLIEJI, S.—"Die paaiigen Extremiliiten," 'Anat. Hefte,' vol. i, 1893. 7. TRAQUAIK, 11. H.—"Devonian Fishes of Canada," 'Geol. Mag.,' vol. vii, 1S90. 8. RAUTENFELD, E. V.—" Uebcr das Skelet, der liintercn Glicdmassen," 'Inaug. Dissert. Dorpar,' 18S2. 9. WELLBUKN, E. D.—" On the Genus Megalichtliys," ' Proc. Yorks. Geol. and Polyt. Society,' vol. xiv, 1900. 10. WHITEAVES, J. F.—"The Forsil Fishes of the Devonian Rocks of Canada," ' Trans. Roy. Soc. Canada,' vol. iv, 1887, and vol. vi, 1888. 11. WIEDERSHEIM, R.—'•' Das Gliedmassenskelet der Wiibeltlncre," Jena, 1892. 12. WooDWAiii), A. S.—'Eiitish Museum Catalogue of Fossil Fishes,' pt. 2, 1891.

VOL. 45, PAKT 2.—NEW SERIES. A A 324 EDWIN S. GOODRICH.

EXPLANATION OF PLATE 16,

Illustrating Mr. Edwin S. Goodrich's paper " On the Pelvic Girdle and Fin of Eiistlienopteron."

LIST OF REFERENCE LETTERS IN PLATE 16 AND TEXT-FIGURES A—L. a. Axial mesomere. b. Basipterygium; basal mesomero in fig. 2. c. Cartilage, d.r. Dermal fin ray. I. p. Lateral process. »/. c. Median cartilage, p. Pelvic cartilage, p.f. Pelvic fin. pr. r. Preaxial endoskeletal ray. p.s. Posterior expansion, pt.r. Postaxial endoskeletal ray. s. Scale, x. Poblaxial process. PIG. i.—Outer view of the right half of the pelvic girdle and of the right fin of Eusthenopteron Foordi (Brit. Museum, No. P. 6794). FIG. 2.—Outer view of the left pectoral fin of Eusthenopteron Foonii (Brit. Museum, 679b), copied fiom A. Smith Woodward.