LUNGFISHES, TETRAPODS, PALEONTOLOGY, AND PLESIOMORPHY DONN E. ROSEN Curator, Department of Ichthyology American Museum of Natural History Adjunct Professor, City University of New York PETER L. FOREY Principal Scientific Officer, Department of Palaeontology British Museum (Natural History) BRIAN G. GARDINER Reader in Zoology, SirJohn Atkins Laboratories Queen Elizabeth College, London COLIN PATTERSON Research Associate, Department of Ichthyology American Museum of Natural History Senior Principal Scientyifc Officer, Department of Palaeontology British Museum (Natural History) BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY VOLUME 167: ARTICLE 4 NEW YORK: 1981 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Volume 167, article 4, pages 159-276, figures 1-62, tables 1,2 Issued February 26, 1981 Price: $6.80 a copy ISSN 0003-0090 Copyright © American Museum of Natural History 1981 CONTENTS Abstract ........................................ 163 Introduction ...................... ........................ 163 Historical Survey ...................... ........................ 166 Choana, Nostrils, and Snout .............................................. 178 (A) Initial Comparisons and Inferences .......................................... 178 (B) Nasal Capsule ............. ................................. 182 (C) Choana and Nostril in Dipnoans ............................................ 184 (D) Choana and Nostril in Rhipidistians ........................................ 187 (E) Choana and Nostril in Tetrapods ........................................... 195 Nasolacrimal Duct, Labial Cavity, Rostral Organ, and Jacobson's Organ .... ......... 196 Paired Fins and Girdles ...................... ........................ 202 (A) Paired Fin Skeleton .................... .......................... 202 (B) Pectoral Girdle .............. ................................ 216 Dermal Bones of the Skull ....................................................... 221 Palate and Jaw Suspension .............................................. 227 (A) Historical ......................................................... 227 (B) Dermal Bones of Palate .............................................. 227 (C) Palatoquadrate .............. ................................ 232 (D) Relation Between Palate and Braincase .................. ................... 233 Hyoid and Gill Arches ...................... ........................ 235 Ribs and Vertebrae ............. ................................. 242 (A) Ribs ....................................... 242 (B) Interpretation of Gnathostome Vertebrae ................ .................... 246 (C) Heterospondyly in Caturus furcatus, and Vertebral Homologies ..... .......... 249 (D) Vertebrae of Rhipidistians, Lungfishes, and Tetrapods .......... .............. 249 Dermal Skeleton ...................... ........................ 252 (A) Cosmine ......................................................... 252 (B) Folded Teeth ............. ................................. 254 Synapomorphy Scheme ...................... ........................ 255 Conclusions ...................................... 262 Acknowledgments ......................................................... 264 Literature Cited ...................... ........................ 264 ABSTRACT We conclude that the internal (excurrent) nos- hypothesize, in agreement with most nineteenth- tril of Recent lungfishes is a true choana, as and many twentieth-century biologists, and in dis- judged by its comparison with (1) the internal nos- agreement with the current paleontological view, tril of a Devonian lungfish species which opens that lungfishes are the sister group of tetrapods, through the bony palate internal to an arcade of and further that actinistians are the sister group maxillary and premaxillary teeth; (2) the choana of those two, and that Eusthenopteron is the sister of the Devonian ichthyostegid amphibians, and group of those three. We also conclude that the (3) nostril development in Recent urodeles. The characters used formerly to link Eusthenopteron idea that lungfishes might therefore be the sister with tetrapods either (1) are primitive for all bony group of tetrapods is compared with the compet- fishes (including cladistians and actinopterygians) ing, deeply entrenched theory that rhipidistian or for living gnathostomes (including chondrich- fishes and eusthenopterids in particular include thyans); (2) are convergent with those of several the ancestor of tetrapods. Our own theory, de- groups of gnathostomes; (3) only justify the inclu- rived from study of Recent and fossil material, sion of Eusthenopteron in a group with actinis- and an analysis of literature spanning 140 years, tians, dipnoans and tetrapods; or (4) are spurious. is framed in the context of a classification of the We attribute the century of confusion about the main groups of fossil and living gnathostomes: structure and position of lungfishes to the tradi- acanthodians, chondrichthyans, cladistians, ac- tional paleontological preoccupation with the tinopterygians, rhipidistians, actinistians, dip- search for ancestors, to the interpretation of Eus- noans, and tetrapods. In formulating our proposal thenopteron in the light of tetrapods and the re- we have reviewed the anatomy of the nasal cap- ciprocal interpretation of fossil amphibians in the sule, nostrils and related structures, paired fins light of Eusthenopteron, and to the paleontologi- and their girdles, dermal bones of the skull, palate cal predilection for using plesiomorphous char- and jaw suspension, hyoid and gill arches, ribs acters to formulate schemes of relationships. and vertebrae, and scale and tooth structure. We "The convictions of our palaeontological colleagues are very real to them, and under the drive of these convictions they have quite honestly contended for their theories. The colour-blind man sees the scarlet robe and the green lawn the same colour, to him they are the same colour, but he is wrong." Kesteven, 1950, p. 99 INTRODUCTION Miles's (1977) descriptions and illustra- ture of the two outermost lip folds discussed tions of the head of the Devonian lungfish, by Allis, our attention was drawn to a large Griphognathus, caused the independent re- opening in the outermost fold of Neocerat- alization in London (BGG) and New York odus near the corner of the mouth that was (DER) that the internal nostrils of lungfishes first described by Gunther (1871, p. 515): are true choanae. This conviction meant to "At the angle of the mouth, and hidden be- us that the only synapomorphy of Miles's low a duplicature of the skin, there is an (1977, p. 316) Choanata (tetrapods and rhip- opening wide enough to admit an ordinary idistians; cf. Gaffney, 1979b, p. 93) is also quill (pl. XXX, fig. 2, a); it leads into a spa- present in lungfishes. It also meant to us that cious cavity (b), irregular in shape, clothed Allis's (1919, 1932a, 1932b) arguments for with a mucous membrane, and containing rejecting the internal lungfish naris as a coagulated mucous in which an immense choana, based partly on the interpretation of number of mucous corpuscles are deposited. certain folds of soft tissue as primary, sec- This cavity is separated from the cavity of ondary, and tertiary upper lips, might be the mouth by the membrana mucosa only, faulty. With the idea of reviewing the struc- and there is no direct communication be- 163 164 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 167 tween them; a branch cavity runs forward tians, and osteolepiforms in particular, are into the interior of the upper lip." the closest relatives, or direct ancestors, of The extent of this rostral structure, which tetrapods. This doctrine is so pervasive that Gunther called the labial cavity, is from the a researcher seeking the primitive tetrapod anterior wall of the orbit forward to the an- condition of some structure turns automati- terolateral part of the nasal capsule (fig. 18). cally to the rhipidistians, usually to Eusthe- It is subtriangular, narrow anteriorly near the nopteron, forgetting that Eusthenopteron has nasal capsule and wider posteriorly in front been interpreted in the light of tetrapods and of the orbit where the internal wall of the that tetrapods have been interpreted in the cavity is thrown into a series of folds. The reciprocal light of Eusthenopteron. Our as- opening of the labial cavity to the outside is sault on the "rhipidistian barrier" begins through a short and broad tube that leads with an account of the history of opinion on directly to the pore in the outermost acces- lungfishes, tetrapods, rhipidistians and other sory lip (fig. 19). Visual examination of the crossopterygian fishes. We believe that this same region of the snout in premetamorphic account shows that current beliefs about Neoceratodus of about 2 cm. in total length rhipidistians have grown up among a thicket (prior to development of pelvic fins) revealed of preconceptions and wishful thinking, es- neither a pore nor a labial cavity, but, in pecially the belief that processes can be dem- larger young, serial sections show these onstrated in the fossil record. To give one structures (fig. 20). The presence in larger classic example, Watson, in his Croonian young and adults of an apparently mucous- lecture on amphibian origins, wrote (1926, p. secreting organ extending between the nasal 189) "It is possible to view the problem as capsule and the orbit and its opening to the one of purely formal morphology, the estab- outside on the side of the