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On the Structure and Affinities of Tristichopterus Alatus, Egerton

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On the Structure and Affinities of Tristichopterus Alatus, Egerton ( 383 ) XVIII.—On the Structure and Affinities of Tristichopterus alatus, Egerton. By RAMSAY H. TRAQUAIR, M.D., F.G.S., Keeper of the Natural History Collections in the Museum of Science and Art, Edinburgh. (Plate XXXII.) (Read April 5, 1875.) Concerning the affinities and systematic position of this very remarkable Devonian fish, there has hitherto prevailed very great uncertainty. The two original specimens, discovered by Mr C. W. PEACH, in the Old Eed Sandstone of John O'Groat's, Caithness, and described by Sir PHILIP EGERTON,* left us in complete ignorance as to the osteology of the head and the dentition, while the evidence they afforded as to the structure of the pectoral fins was by no means so clear as might have been wished for. To quote from Sir PHILIP'S descrip- tion:—"The bones of the head, with the exception of a small fragment of the operculum, are wanting, but the impressions left upon the matrix show that they were sculptured in rather a bold pattern, not unlike the ornament on some of the cranial bones of some of the Holoptychii, and consequently differing in this respect from the corresponding parts in Dipterus. The pectoral fins are very indistinctly seen. They appear to have had a short obtuse lobe forming the base, and extending therefrom a set of numerous fin-rays more elongated than those forming the pectoral fin in Dipterus." To Dipterus, however, in Sir PHILIP EGERTON'S opinion, its affinities pointed, as far as could be gathered from the structure of the body as displayed in the specimens, his description concluding as follows:—" The absence of all evidence as to the dental apparatus of Tristichopterus is much to be regretted. On other points the affinities between this genus and Dipterus are so striking that they cannot be classified in separate families.t Accordingly he assigned to Tristichopterus a place along with Dipterus in the family of " Ccelacanthi," the term being used in its former extended sense, not as now restricted to the peculiar genera Ccelacanthus, Undina, Holophagus, and Macropoma. Professor HUXLEY, at the conclusion of his Essay on the Classification of the Devonian Fishes,^ published in the same Decade of the Geological Survey, makes the following statement regarding the genus in question:—"In the absence of a full knowledge of the head, of the paired fins, and of the dentition, * Deo. Geol. Survey, x. 1861, pp. 51-55, pi. v. t Loc. cit. p. 55. + Dec. Geol. Survey, x. 1861, p. 40. VOL. XXVII. PART III. 5 H 384 DR TRAQUAIR ON THE STRUCTURE AND AFFINITIES it would be hazardous to form any decided opinion as to the affinities of Tristichopterus; I strongly suspect, however, that it will turn out to be the type of a new family allied to the Ctenododipterini and Coelacanthini." How- ever, at page 24 of the same essay, he places it along with Dipterus in PANDER'S family of Ctenododipterini, though with a mark of interrogation. But when, in 1871, Dr GUNTHER* pointed out the close relationship between Dipterus and the recent Ceratodus and Lepidosiren, and the consequent desirability of transferring the first named genus to the group of Dipnoi, the question of course came up as to whether Tristichopterus should also accompany it thither. But to this no satisfactory answer could be given, so long as our knowledge on so many essential points of its structure was deficient. After referring to the manner in which the innumerable fine and closely-placed rays of the fins overlapped " with their proximal ends, the extremities of the inter- spinous bones, as in the Dipnoi," and the peculiar form of the tail, which "represents a most curious intermediate condition between the diphycercal tail of the Sirenidce and the heterocercal of Dipterus," Dr GUNTHER con- cludes :—" Unfortunately, the head and the base of the paired fins are destroyed in the only two specimens known; and it is chiefly the last-named character which prevents me from associating this genus with the Dipnoi." No further description of the structure of Tristichopterus having appeared since the publication of the Tenth Decade, I felt great satisfaction when Mr PEACH communicated to me a number of additional specimens, collected by him in the years 1864^65, and which throw a very great amount of the light desired on those points of its structure previously unknown to us.t These, specimens exhibit in a clear and unmistakable manner the character of the dentition, the structure of the paired fins, and the leading features of the osteology of the head, and completely prove that Tristichopterus has no special affinity either with Dipterus or Coelacanthus. Before, however, passing on to discuss the question of its real affinities and systematic position, I shall first proceed with the description of the new facts disclosed. General Proportions.—One of the specimens, the counterpart of which is in the British Museum, is quite entire, though the axis of the body is so curved as to render the dorsal margin considerably concave, the ventral corre- spondingly convex. The entire length of this specimen, carefully ascertained with a flexible measure, is 10£ inches, of which the head occupies about £th part; the greater depth of the body, just behind the subacutely lobate pectorals, being 2 inches. The general form of the fish is thus rather slender, and the fins are crowded towards the posterior aspect of the body,—the first dorsal commencing 6 inches, and the second 1\ inches from the tip of the snout, * Description of Ceratodus, Phil. Trans. 1871. t These specimens are now in the Museum of Science and Art, Edinburgh. OF TE.ISTICHOPTERTJS ALATUS. 385 while opposite these two dorsals are respectively placed the ventrals and the anal. The lower lobe of the very peculiarly shaped caudal fin commences at 8^ inches from the front, and on the dorsal aspect the rays of the upper lobe begin to be apparent a few lines further back. Another specimen, crushed vertically, and lying on its back,—or more correctly, the counterpart of a specimen in that position, is also very nearly entire; the front and a considerable part of the right side of the head being unfortunately cut off by the edge of the slab, and the extremity of the tail being rather distorted and indistinct. If we add half an inch to complete the head in front, the length of this example would also be about 10 inches; the pectorals arising 2\ inches, the ventrals 6 inches, and the anals 7| inches from the supposed extremity of the snout. These two specimens thus closely correspond with each other, and with the more complete of the two examples figured by Sir PHILIP EGERTON, and show the accuracy with which he allowed for its missing portions. Some of the more fragmentary specimens before me show, however, that the fish must sometimes have attained a considerably larger size, one head measuring, from the tip of the snout to the hinder .margin of the gill cover, no less than 3J inches, which would give over 15 for the entire length of the fish. The Head.— The head was protected above by a cranial "buckler" (Plate XXXII. figs. 1 and 2, C.B.), which in the leading features of its configuration recalls to our minds that of the Saurodipterini, though its external sculpturing is very different. As in that family, it tends to divide across into two portions,— a posterior or parietal, and an anterior or fronto-ethmoidal; here, however, the anterior moiety is the longer, exceeding the other by nearly ^ of its length. The hinder division of this buckler is on the whole quadrate in form, but broader behind than in front, the posterior and wider margin being also somewhat con- cavely excavated. The front portion forms anteriorly a rounded depressed snout, and seems on each side to be excavated to take part in the formation of the upper boundary of the orbit, though this part of the margin is not so clearly defined as might be wished; nor are the nasal openings seen in any of the specimens, which is not strange, taking into account their position in Osteolepis and Diplopterus, so close to the margin of the upper lip. It is not possible to map out the ossifications entering into the composition of this buckler; probably their arrangement would not depart much from that which is to some extent traceable in the Saurodipterini. As far as the anterior portion is concerned, the impression of two distinct frontals, entering largely into its composition, is distinctly seen in the specimen represented in fig. 2, and the presence of a small conical tooth on the labial margin of the snout in another (fig. 1), leads us to conclude that the premaxillary [p. mx) was also here represented. The entire outer surface of the buckler, as indeed of all the external bones of the head, 386 DR TRAQUAIR ON THE STRUCTURE AND AFFINITIES is sculptured with tolerably fine, irregular, angularly contorted, and interrupted rugae, with intervening furrows and pits, the pattern assuming sometimes almost a tubercular aspect. Along the posterior margin of the cranial shield are three plates (s. t, figs. 1 and 2), one mesial, somewhat polygonal in form, and two lateral, each apparently of a triangular shape. These are obviously the representatives of the three plates, which occur in a similar position in Osteolepis, Glyptolcemus, Megalichthys, &c, and of which different interpretations have been given by different authors. In Professor HUXLEY'S description of Glyptolcemus,* the mesial one is marked " supra-occipital," the two lateral, " epiotic." Mr PARKER! has, however, pointed out that they are dermal bones, and not to be considered homologous with those other deeper ossifications of the cranial cartilage.
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