= See *. £ eSeTaeet ‘ dy Dy AO Te A LM ts wn K mali 4A ey y vary ad bP EU RS 0 ae mathe YL) ALP ) bey ade YE? aor ke my ‘ ARMM hha 8c wey Seah v6 ate”, x) i ease 5 ied bErasiBi Abt ate Oh BERN LY Ah ay ach ty Ve, omar alth 4 i. Brice (aes fe o t fons india, Pages p. » AK ay ook be ha be VY 4 He Oe aE wip! Pe Bide y te we Wht AD hr ataergnyay tM y okie TP ae) ¥ Pally Te oe ’ 4 A Cae a) DET ie pt bl hehe beta d: BA8. bes ‘ WEP Rib ID all badd ! i wb $ ie: Wilit ahve eaTy nea Ae fpd , u ont ’ Dae gee aaa rit Af ‘es 24; bey Rae bi nt fi ra! AONE het RU see ice SRP SRS ie rs Fa het frail Ry th SN ete ALAA bois gt HSER Boece eke hams SUE Me ‘i me aie Aaa is Rie erash mi hee j aa : aires ‘as syead iin ART en ca Ue a Bieta Pie tee sprees { Botti hi ghindedtin teeta) that pt rate ear at et et Havercee ys aa IE Pospag 2 i ‘ Ys pit ial ts ee wa , ee Pree Atoka ust Ata ny ieee +S) Fae st de sie he pied ¢ T se rds 4) Wadi ‘ sie Ue He weaves ish diate) PRM eps ihe LP Pah oe rt tid bay! td ‘ % Rivera We Lrpinay x Ayer Hike say she r ea ata oneal ay ” ieee sae ie Cab Seer MP. ZOOL LIBRA RY FEB 23 \973 HARVARD | UNIVERSITY POSTILLA PEABODY MUSEUM YALE UNIVERSITY NUMBER 157 19 JULY 1972 NEW EVIDENCE ON THE EVOLUTION OF THE PAIRED FINS OF RHIPIDISTIA AND THE ORIGIN OF THE TETRAPOD LIMB, WITH DESCRIPTION OF A NEW GENUS OF OSTEOLEPIDAE KEITH STEWART THOMSON POSTILLA Published by the Peabody Museum of Natural History, Yale University Postilla includes results of original research on systematic, evolution- ary, morphological, and ecological biology, including paleontology. Syntheses and other theoretical papers based on research are also welcomed. Postilla is intended primarily for papers by the staff of the Peabody Museum or on research using material in this Museum. Editors: Zelda Edelson. Elizabeth G. ‘Veiiman, Elise K. Kenney. Postilla is published at frequent but irregular intervals. Manuscripts, orders for publications, and all correspondence concerning publications should be directed to: Publications Office Peabody Museum of Natural History New Haven, Conn., 06520, U.S.A. Lists of the publications of the Museum are available from the above office. These include Postilla, Bulletin, Discovery, and special publica- tions. Postilla and the Bulletin are available in exchange for relevant publications of other scientific institutions anywhere in thé world. Inquiries regarding back numbers of the discontinued journal, Bulletin of the Bingham Oceanographic Collection, should be directed to: Walter J. Johnson, Inc. 111 Fifth Avenue New York, N.Y. 10003. NEW EVIDENCE ON THE EVOLUTION OF THE PAIRED FINS OF RHIPIDISTIA AND THE ORIGIN OF THE TETRAPOD LIMB, WITH DESCRIPTION OF A NEW GENUS OF OSTEOLEPIDAE KEITH STEWART THOMSON Department of Biology and Peabody Museum of Natural History Yale University, New Haven, Connecticut 06520 (Received April 25, 1972) ABSTRACT A specimen of Sterropterygion brandei, gen. et sp. nov., a rhipidistian from the Upper Devonian of Pennsylvania, shows for the first time the detailed internal structure of the pectoral and pelvic fins and girdles in a member of the Family Osteolepidae. The structure conforms to the general pattern once thought to be directly antecedent to that of tetrapods but which now must also be considered an ancient feature of rhipidistian fishes. It is contended that the known Rhipidistia could not support their own weight during ter- restrial locomotion through fin action alone and a scheme of evolution is proposed according to which the paired fins of osteolepids and tristicopterids evolved with a dual function: in locomotion and support of lung ventilation. POSTILLA 157: 7p. 19 JULY 1972 2 POSTILLA 157 INTRODUCTION In studies of the fish-tetrapod transition, great attention has been paid to the paired fins and girdles of the ancestral group of fishes — the Devonian Rhip- idistia. Andrews and Westoll (1970a,b) have recently summarized our knowledge of the postcranial skeleton of Rhipidistia and contributed greatly to our understanding of the early evolution of the pentadactyl limb. How- ever, all such studies have had to depend heavily upon a single fish — the lowermost Upper Devonian Eusthenopteron foordi Whiteaves (Family Tristicopteridae, in the classification used by Thomson, 1969) and com- parative materials have been scarce. The recent discovery of a well-preserved member of the rhipidistian Family Osteolepidae (Figs. 1-5), showing for the first time both the pectoral and pelvic fins, with girdles, provides an im- portant source of new data. The specimen with which the present notice is concerned was collected by Mr. Scott Brande at a highway cut outcropping of an Upper Devonian forma- tion in Lycoming County, Pennsylvania. The present preliminary account will be followed at a later date by a full description and analysis. The new osteolepid fish represents a new genus and species. A brief taxonomic diagnosis is necessary. CLASS OSTEICHTHYES ORDER CROSSOPTERYGII FAMILY OSTEOLEPIDAE Genus Sterropterygion nov. TYPE SPECIES. Sterropterygion brandei Thomson, nov. PRELIMINARY DIAGNOSIS. Rhipidistian with dermal bones and rhomboid scales bearing complete external covering of enameloid and dentine typical of the Osteolepidae. Estimated total length 380 mm. First dorsal fin inserted slightly behind the level of the pelvic insertion, and second dorsal in front of, or level with, the anal. Number of dorsolateral scale rows in front of first FIG. 1. Reconstruction of the general features of Sterropterygion brandei gen. et sp. nov., Holotype. PAIRED FIN EVOLUTION 3 dorsal, second dorsal and pelvic fins: 41, 53, 50 respectively. Tenth dorso- lateral scale row with 6 scales above and 5 below the lateral line scale. Dorsomedian scale row count: anterior division 41 + 1, median division 6; length of anterior division approximately 4.7 times longer than the estimated length of the ethmoidal division of the skull roof. HORIZON AND LOCALITY. The Upper Devonian Susquehanna Group, probably Catskill Formation, of Northern Lycoming County, Pennsylvania. DERIVATIO NOMINIS: from the Greek for “‘firm-—fin.” Sterropterygion brandei sp. nov. HOLOTYPE and sole known specimen: YPM 6721, incomplete fish. DIAGNOSIS AND OCCURRENCE. As for the genus above. DERIVATIO NOMINIS: after the discoverer, Mr. Scott Brande. DISCUSSION The right pectoral fin of Sterropterygion brandei (Fig. 4) is almost complete. The internal fin skeleton consists of a stout humerus with a large entepicon- dyle, a long thin radius, and a short ulna supporting four more distal elements. The ventral surface of the humerus is marked by a prominent row of projections marking the insertion of the ventral flexor musculature and M. pectoralis. The dorsal side bears only moderately developed processes for the insertion of the deltoid muscles. The pectoral fin differs from that of Eusthenopteron in that the lobed part occupies just slightly less than half (as opposed to approximately one third) the total length of the fin. In the in- ternal skeleton, the new fish is unique in its short and broad humerus with a massive entepicondylar platform, the specialization of the ventral humeral ridge into a series of large discrete processes for muscle insertion, the slender and elongate radius, and the great breadth of the postaxial elements articu- lated with the ulna. The pelvic structures of Sterropterygion brandei are shown in Figure 5. The internal fin skeleton differs from that of Eusthenopteron in being broader and shorter, particularly in the distal elements. The pelvic fin lobe occupies half the total length of the fin (as opposed to one third in Eusthenopteron). The osteolepid pelvic girdle previously has been completely unknown; that of Sterropterygion brandei consists of a pair of arch-shaped elements with long anterior iliac processes coming close together in the midline. It is similar to that of Eusthenopteron, differing only in the slightly greater breadth of the dorsal public process. 4 POSTILLA 157 The discovery of Sterropterygion brandei reveals the remarkable fact that there are no fundamental points of difference in pectoral or pelvic structure between two families of Rhipidistia, the Osteolepidae and Tristicopteridae, that have been distinct since least the Middle Devonian. (This confirms the observation of Andrews and Westoll, 1970b, based on fragmentary pectoral material of the Carboniferous osteolepid Megalichthys.) Moreover, there is evidence that the osteolepids range back to the Lower Devonian (undescribed material from China reported to be in the process of description, several years ago, by Mi-man Chang). From the earliest fossil record of the Rhipidistia there seem to have been two distinct patterns of paired fin development. The holoptychoids (Porolepidae and Holoptychidae) had a monoseriate pattern similar to that of lungfish and coelacanths and it is extremely unlikely that they gave rise to any tetrapods (Schultze, 1970). The biseriate condition seen in osteolepids and tristicopterids is equally ancient and must be considered a primitive, rather than derived, character in these lines. Thus, the biseriate condition of the paired fins, which shows great similarity to that of the Amphibia in the proximal elements (but not the distal elements) must be primarily an adaption for life in water. It is not a specialization of the immediate tetrapod ancestors and it is extremely unlikely that this condition evolved in response to selection involving specific factors of semiterrestrial existence. The following interpretation of the evolution of the tetrapod limb is sug- gested by this new evidence. The paired fins of Rhipidistia seem to be adapted primarily for slow swimming movements, much as in the modern lobe-finned chondrostean Polypterus. Presumably, at an early point in their history, the Rhipidistia began to make excursions overland. The possible environmental and adaptive contents have been reviewed by Thomson (1969) and Andrews and Westoll (1970a). But it is extremely doubtful that the pectoral and pelvic limbs were sufficiently developed that these fish could “walk,” that is, move through the actions of the paired fins alone. Furthermore, in all known Rhipidistia there was a large fin web that would have been a severe hindrance to attempts at rapid terrestrial movement, primarily by means of limb movements with the fin sharply flexed ventrally to raise the trunk from the ground. It has been pointed out previously (Thomson, 1969) that, in their ex- perimental movements overland, the prime new mechanical problem facing the lobe-finned fishes was not locomotion but jung ventilation. Locomotion was always possible by lateral body undulation, but lung ventilation required that the anterior trunk (at least) be raised up so that the weight of the body did not crush the lungs. While the pectoral limbs of Rhipidistia do not seem to have been strong enough to contribute greatly to active terrestrial locomo- tion, they were probably strong enough to lift up the anterior trunk when the animal was at rest. Thus the paired limbs of Rhipidistia may have been subject to two separate selective pressures. First, the fins were used as relatively immobile props in terrestrial locomotion. The mode of progression PAIRED FIN EVOLUTION 5) was based on the normal sinuous movement of the body, but the trunk musculature could also apply force around the fixed point where each paired fin touched the ground. In this way a lever effect was produced and also the opposite fin was raised off the ground as it was being swung forward. Any addition to this action that could be made by the muscles and skeleton of the fins themselves would be favored. Since this type of locomotion involves a net raising of the trunk, lung ventilation would be facilitated. When the fish was at rest, however, the only means for raising the anterior trunk would be through the direct action of the fins themselves. This would require con- siderably less strength and mobility than a similar action during active locomotion and perhaps was possible from an early stage in rhipidistian evolution. Any increase in this capacity would be favored, but it is worth noting that the latter function applies more to the pectoral than pelvic fins and this may be a partial explanation of their differential development toward the pentadactyl condition. Only significantly later (Late Devonian) came reduction of the fin web and further modification of the internal fin skeleton (including development of the carpus and tarsus) allowing greater mobility and more extensive weight support. The robust development of the paired fins in Sterropterygion brandei may indicate that it belongs to an advanced stage in this sequence, but it remains essentially a fish and one must expect that the final stages will be found in animals that were more amphibian than piscine in overall organization. ACKNOWLEDGMENTS I am deeply grateful to Mr. Scott Brande for his careful collection of the fish and its donation to the Peabody Museum of Natural History. Professor Z. D. Bowen, University of Rochester, also recognized the importance of the specimen and kindly brought it to my attention. The study would not have been possible without the extraordinarily delicate and painstaking preparation by Mr. Peter Whybrow. The photographs were taken by Mr. A. H. Coleman and the study supported by National Science Foundation Grant GB 28823 X. LITERATURE CITED Andrews, S. M. and T. S. Westoll. 1970a. The postcranial skeleton of Eusthen- opteron foordi Whiteaves. Trans. Roy. Soc. Edinb. 68: 207-329. 1970b. The postcranial skeleton of rhipidistian fishes excluding Eusthen- opteron. Trans. Roy. Soc. Edinb. 68: 391-489. Schultze, H-P. 1970. Folded teeth and the monophyletic origin of tetrapods. Novitates 2408: 1-10. Thomson, K. S. 1969. The biology of lobe-finned fishes. Biol. Rev. 44: 91-154. 6 POSTILLA 157 FIG. 2. General view of the right flank of Sterropterygion brandei gen. et sp. nov., Holotype. «0.55. FIG. 3. Detail of the right pectoral fin of Sterropterygion brandei gen. et sp. nov., Holotype. x 1.2. PAIRED FIN EVOLUTION q/ FIG. 4. The pectoral fin of Sterropterygion brandei gen. et sp. nov., Holotype. Scales removed to show internal elements, ventral. x 1.0. Mii | i METRICIICM — 2! [i as FIG. 5. The pelvic fin and girdle of Sterropterygion brandei gen. et sp. nov., Holo- type. Prepared specimen showing both girdles and the left pelvic fin. x 1.0. ale i _ = REVIEW STYLE FORM TITLE ABSTRACT NOMENCLATURE ILLUSTRATIONS FOOTNOTES TABLES REFERENCES AUTHOR'S COPIES PROOF COPYRIGHT INFORMATION FOR AUTHORS The Publications Committee of the Peabody Museum of Natural History reviews and approves manuscripts for publication. Papers will be published in approximately the order in which they are accepted; delays may result if manuscript or illustrations are not in proper form. To facilitate review, the original and one carbon or xerox copy of the typescript and figures should be submitted. The author should keep a copy. Authors of biological papers should follow the Style Manual for Biological Journals, Second Edition (Amer. Inst. Biol. Sci.). Authors of paleontological manuscripts may choose to follow the Sugges- tions to Authors of the Reports of the U.S. Geological Survey, Fifth Edition (U.S. Govt. Printing Office). Maximum size is 80 printed pages including illustrations (= about 100 manuscript pages including illustrations). Manuscripts must be typewritten, with wide margins, on one side of good quality 8% x 11” paper. Double space everything. Do not underline anything except genera and species. The editors reserve the right to adjust style and form for conformity. Should be precise and short. Title should include pertinent key words which will facilitate computerized listings. Names of new taxa are not to be given in the title. The paper must begin with an abstract. Authors must submit com- pleted BioAbstract forms; these can be obtained from the Postilla editors in advance of submission of the manuscripts. Follow the International Codes of Zoological and Botanical Nomen- clature. Must be planned for reduction to 4%4 x 7” (to allow for running head and two-line caption). If illustration must go sideways on page, reduction should be to 4 x 714”. All illustrations should be called “Figures” and numbered in arabic, with letters for parts within one page. It is the author’s responsibility to see that illustra- tions are properly lettered and mounted. Captions should be typed double-spaced on a separate page. Should not be used, with rare exceptions. If unavoidable, type double-spaced on a separate page. Should be numbered in arabic. Each must be typed on a separate page. Horizontal rules should be drawn lightly in pencil; vertical rules must not be used. Tables are expensiye to set and correct; cost may be lowered and errors prevented if author submits tables typed with electric typewriter for photographic reproduction. The style manuals mentioned above must be followed for form and for abbreviations of periodicals. Double space. Each author receives 50 free copies of his Postilla. Additional copies may be ordered at cost by author when he returns galley proof. All copies have covers. Author receives galley proof and manuscript for checking printer’s errors, but extensive revision cannot be made on the galley proof. Corrected galley proof and manuscript must be returned to editors within seven days. Any issue of Postilla will be copyrighted by Peabody Museum of Natural History only if its author specifically requests it. fi Acme Bookbinding Co., Inc. 300 Summer Street. Boston, Mass. 02210 AVANTE 044 066 305 2 He Pyle Sort eran a gly Poe Pir bein Pah a 4 ae f eke rad eta rs Gt SPER ij ‘ RE y 5 ey daat APN RAN Way ak eT hy OF Vase ae, alt We heer ama a it rettich INCA ee Wve eeea ets Ura § ays PIER (PEE I pay ee iat ne feae y one Ate aha BAG a re aA Wy a Hoag 4 zoe el ae