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

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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 con- siderable, 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 com- plete 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 figures the 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 to- gether in the middle line by a ligament (Fig. B). A some- what 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 sup- porting 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 (basiptery- gium) 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.
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