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81 Homology Mapping of the Primitive Archosaurian Reptile Palate on The JO 81 Homology Mapping of the Primitive Archosaurian Reptile Palate on the Palate of Birds* Sam McDowell, Department of Zoology and Physiology, Rutgers University, Newark, New Jersey, 07102. Received October 6, 1978 ABSTRACT: The bone called "palatine" in birds, along with the bone called "pterygoid", together represent the homologue of the rep- tirlian pterygoid. The reptilian pterygoid has become divided by a hinge in birds as part of increased kinesis, just as the nasal bone has been divided into two bones by a hinge in many birds, as part of this same kinesis. In some birds (Phasiani, Anseres) there is a single division of the pterygoid, associated with Pfannenstiel- kinesis (primarily transverse rotation of the quadrate to broaden the gape), but most birds follow this division of the pterygoid with a second division after the nestling stage, this second di- vision being associated with Versluys-kinesis (primarily longitud- inal rotation of the quadrate, lifting the upper jaw when the lower jaw is depressed). The true homologue of the palatine in birds is the "maxillo-palatine" of "maxillary bone". Modern birds have pro- bably lost the true maxillary bone in the process of losing teeth. The parasphenoidal process called "basipterygoid process" in modern birds is not the homologue of the reptilian basipterygoid process (formed from the basitrabecular process with a ventral investment of parasphenoid); the true basipterygoid process is present in birds, however, as a vertical lamina between the orbital and tym- panic cavities, usually forming the lateral wall of a presphenoid air sinus. Because the dermal pterygoid of birds has become almost entirely free of (and ventral to) the endochondral palato-quadrate arcade ("orbital process of the quadrate"), the true basipterygoid process does not approach the pterygoid, but may (e.g. Spheniscidae) make a glancing contact with the orbital process of the quadrate. The pila antotica spuria is a morphologically extracranial support- ing strut for the part of the cranial wall giving origin to the levator pterygoideus muscle and is only one of several extracranial struts in this region that support the levator origin and define foramina for the oculomotor, abducens, trochlear, and various tri- geminal nerves that do not correspond to the primary (dural) for- amina for these nerves. It is possible that the pattern and con- tents of these foramina could be of taxonomic value in birds. ***************************$************************$*$**$****** -This communication is part of an unsolicited, handwritten letter I received from Sam McDowell in November 1976, with instructions to use any of the ideas contained therein as I saw fit. Having subsequently been impressed with the fact that much of systematic ornithology is based on unsubstantiated traditions, I realized McDowell's hypothesis, that the avian homologues of the elements of the reptilian palate were misidentified, to have a better than average chance of being correct. I have therefore submitted the pertinent portions of the text of McDowell's letter as received, with illustrations supplied later by McDowell, in the expectation that if nothing else, avian anatomists will be forced to produce a more rigorous justification for the nomenclature now in use for the avian palate than presently exists. Storrs L. Olson, Smithsonian Institution, Washington, DC 20560 Evolutionary Theory 4: 81-94 (December, 1978) The editors thank R. Carroll and M. Jollie for help in evaluating this paper. <g) 1978, the author 82 s. MCDOWELL At the time when Huxley, Kitchen Parker, Pycraft et al. were forming the traditions for labelling parts of the bird skull, little was known of the details of skull structure of fossil Archo- sauria or even of Crocodilia; the relationships of skeletal elements, bony protruberences, foramina and cavities to soft structures was largely unknown; and no attempt was made to catalogue which points on an archosaurian skull roust enlarge, which must be reduced, and which remain the same in a gradual transformation into the skull of a bird. The names of various structures of the bird skull (e, g, basipterygoid process, palatine bone, maxillopalatine process) reflect this period of imprecision. As more knowledge of the development of the vertebrate skull and of the skull structure of earlier archosaurs accumulated, no attempt was made to bring the naming of bird skull structures into conformity with that of reptiles. This has no serious consequences for most comparisons of birds with other birds, but it does confuse comparison of birds with reptiles and thus makes difficult the determination of what characters in birds are primitive (i.e., retentions of reptilian features) and what characters are spec- ialisations. The braincase and palate of early archosaurs was remarkably similar in details to that of lizards, except for the lack of a Squamatan peculiarity (separation of the vomer from the pterygoid by the palatine), and the contents of the cranial foramina, canals, and grooves can be inferred from those of lizards with at least as much confidence as the contents of the cranial foramina of a dog skull can be inferred from dissection of the head of a cat. This is important, because the only living (and dissectable) archosaurian reptiles, the Crocodilia, have departed more from the primitive archosaurian pattern in attachment of the palate to the braincase than have birds, and less is known about develop- ment of the crocodilian head. Sphaenodon [J.E. Gray's original spelling] is even better for Interpreting the skull of early fossil archosaurs, but adds very little that cannot be learned from lizards. The usual naming of bones in the palate of birds suggests a most remarkable transformation from the reptiles; the pterygoid flange and most of the muscles originating on the pterygoid of reptiles have been transferred to the palatine bone. I believe the "palatine" of birds is merely the anterior portion of the reptilian pterygoid, separated from the posterior part of the pterygoid by a new joint that has developed in the waist of the pterygolds, just behind the pterygoid flanges(Figure 1). The *+**********************$************************************** Figure 1 (to right). Homo1ogles here proposed for palatal struc- tures between (on left) the primitive thecodont archosaur Protero- suohus (after Cruickshank) and (on right) a modern bird, Dlomedea. Abbreciations: bpp, true basipterygoid process, formed from basl- trabecular process of chondrocranium with parasphenoid addition; ept, ectopterygoid ("os lachrymopalatinum" or "os uncinatum" of Dlomedea); iral, intermuscular lamina of pterygoid (pre-pterygoid) of birds, without homologue in Proterosuchus; iof, infraorbltal fenestra; ipth, intra-pterygoid hinge, without homologue in rep- tiles; jug, jugal bone (fused to adjacent bones in birds); lpf, lateral end of pterygoid flange; m, maxilla (lost in modern birds); mpf, medial end of pterygoid flange; opf, orbital process of quad- PALATE OF BIRDS AND REPTILES 83 rate bone; pal, palatine bone ("maxillopalatine" of birds); pttr, middle pterygoid tooth row of Proterpsuchus, represented in Dio- medea by the edge of the nasopharyngial duct opening; qj, quadrato- jugal bone (fused to jugal in birds); qrpt, quadrate raraus of pterygoid bone (post-pterygold of birds, "pterygoid bone" of birds according to other authors); vom, vomer (separate in Proterpsuchus, but partially fused to its fellow In birds and most archosaurs ); vpt, vomerine process of pterygoid; zpp, zygomatic process of palatine, forming lateral border of infraorbital fenestra in Proterpsuchus, fused to jugal in birds. 84 s. MCDOWELL "palatine"-pterygoid joint of birds is functionally linked to the Versluys-kinesis [i shall explain this term below] of the skull that elevates the upper beak when the lower end of the quadrate and palate slide forwards in opening the mouth. The other major joint involved in Versluys-kinesis is across the nasal bones. In most birds, the nasal hinge involves only an histological change in the bone, a transverse zone of fibrous and elastic bone across the nasals; but in parrots, boobies, and some other birds, the nasal bone becomes completely divided into anterior and posterior parts by joint capsule. I suggest that the reptilian pterygoid has been similarly divided into anterior and posterior portions by a joint capsule in birds (the fusion of "palatine" and ptery- goid in some birds, such as the hawfinch, is probably secondary). Even conventional ornithology accepts this for the majority of birds. Pycraft showed (most succinctly in Journal of the Linnean Society, London, Zoology, vol. 28: pp. 3^3-357, pis. 31, 32) that the definitive ^palatine"-pterygoid joint of most "Neognathae" lies entirely within the pterygoid of the nestling and the anterior end of the nestling pterygoid is thus cut off as a "hemipterygoid" to fuse with the palatine. In Anatidae and (according to Pycraft) Phasiani this does not occur, and Pycraft interpreted the "paleo- gnathous" or "dromeognathous" palate as a consequence of either fusion of the hemipterygoid with the vomer (a later interpretation) or failure of the pterygoid to segment into hemipterygoid and de- finitive pterygoid. Jollie (1957, Journal of Morphology, vol. 100, pp. 389-436) showed that in Phasiani (at least Gallus) the condition is not as in Anatidae; rather, the "pterygoid" has its anterior ("hemipterygoid") portion separated off at the inception of ossification from the rest of the pterygoid and is
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