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Homology of Facial Structures in Extant Archosaurs (Birds and Crocodilians), with Special Reference to Paranasal Pneumaticity and Nasal Conchae LAWRENCE M

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Homology of Facial Structures in Extant Archosaurs (Birds and Crocodilians), with Special Reference to Paranasal Pneumaticity and Nasal Conchae LAWRENCE M JOURNAL OF MORPHOLOGY 225269-327 (1995) Homology of Facial Structures in Extant Archosaurs (Birds and Crocodilians), With Special Reference to Paranasal Pneumaticity and Nasal Conchae LAWRENCE M. WITMER Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205; Department ofdnatomy, New York College of Osteopathic Medicine, New York Institute of Technology, Old Westbury, New York 11568 ABSTRACT Homology of virtually all major components of facial anatomy is assessed in Archosauria in order to address the function of the antorbital cavity, an enigmatic structure that is diagnostic for the group. Proposed functions center on its being a housing for a gland, a muscle, or a paranasal air sinus. Homology is approached in the context of the Extant Phylogenetic Bracket method of reconstructing unpreserved aspects of extinct organisms. Facial anatomy and its ontogeny was studied in extant archosaurs (birds and crocodilians)to determine the osteological correlates of each soft-tissue compo- nent; resemblances between birds and crocodilians comprised the similarity test of homology. The congruence test of homology involved surveying phyloge- netically relevant fossil archosaurs for these bony signatures. The facial anatomy of extant birds and crocodilians is examined in detail to provide background and to discover those apomorphic aspects that contribute to the divergent specialization of these two groups and thus obscure homologies. Birds apomor- phically show enlarged eyeballs, expanded nasal vestibules, and reduced maxil- lae, whereas crocodilian faces are dorsoventrally flattened (due to nasal rota- tion) and elongated. Most facial attributes of archosaurs are demonstrably homologous and in fact characterize much more inclusive groups. Special emphasis has been placed on the nasal conchae and paranasal air sinuses. Within Amniota, the following conchal structures are homologous, and all others are neomorphs: avian caudal concha, crocodilian concha + preconcha, Sphenodon caudal concha, squamate concha, and probably the mammalian crista semicircularis. The avian antorbital paranasal air sinus is homologous with the crocodilian caviconchal sinus; the maxillary sinus of placental mam- mals is not homologous with the archosaurian paranasal sinus. With regard to the function of the antorbital cavity, archosaurs possess homologous nasal glands, dorsal pterygoideus muscles, and paranasal air sinuses, but the osteo- logical correlates of only the paranasal sinus involve the antorbital fenestrae and fossae. Thus, the antorbital cavity is best interpreted as principally a pneumatic structure. o 1995 Wiley-Liss, Inc. The age-old image of a tiny plover calmly vertebrates living today, during the Mesozoic gleaning the leeches from within the gaping Era extinct archosaurs (i.e., nonavian dino- mouth of a crocodile hardly suggests the no- saurs, pterosaurs, and a variety of early tion of any kinship between these two very forms) radiated into virtually all habitats and different vertebrates. However, among mod- by all measures were the dominant terres- ern vertebrates, birds and crocodilians are trial vertebrates. As a result of this radiation, indeed sister taxa, representing the only sur- viving clades of Archosauria. Although ex- Address reprint requests to Lawrence M.Witmer, Department tant archosaurs, with their 10,000 species, of Biological Sciences and College of Osteopathic Medicine, Ohio remain the most diverse group of terrestrial University,Athens, OH 45701. email: [email protected] o 1995 WILEY-LISS,INC 270 L.M. WITMER archosaurs present an extraordinary diver- called the antorbital fenestra and cavity, re- sity of skull morphology. The pattern of ar- spectively (Witmer, '94). The antorbital cav- chosaur phylogeny among extinct as well as ity (defined below) is ubiquitous in at least living members is beginning to be unraveled the basal members of all clades of archosaurs (Gauthier, '86; Sereno, '86, '91; Gauthier et and is a synapomorphy of a slightly more al., '88a; Benton and Clark, '88; Cracraft, inclusive group (Archosauriformes; Fig. 1; '88; Norell, '89; Novas, '92; Clark et al., '93; Gauthier et al., '88a; Benton and Clark, '88). Parrish, '93), and we are now in a good In some archosaurs (e.g., some theropod dino- position to understand the evolution of archo- saurs) the antorbital cavity is very promi- saur craniofacial adaptation. nent, occupying somewhat more than half An important component of the diversity the total skull length, whereas in others (e.g., in skull morphology in archosaurs pertains some ornithischian dinosaurs) it is all but to the facial skeleton, in particular to an lost (see Witmer, in press). Ironically, the opening and space in the side of the snout function (and hence soft-tissue relations) of Abbreviations a o cav cavitas antorbitalis nldu ductus nasolacrimalis a o fos fossa antorbitalis n 1 du ost ostium nasale, ductus nasolacrimalis a o sin sinus antorbitalis n 1gl fos fossa glandulae nasolacrimalis a o sin ost ostium, sinus antorbitalis npdu ductus nasopharyngeus acc cav cavitas accessorius nar apertura nasi ossea (= naris) ad co aditus conchae nas 0s nasale antorb sin sinus antorbitalis nas gl glandula nasalis ap cavico rec apertura recessus caviconchalis nas gl du ductus glandulae nasalis apnldu apertura ductus nasolacrimalis nas gl gr groove for glandula nasalis atr tur atrioturbinal nas tur nasoturbinal caud co concha nasalis caudalis neurovas neurovasculature caudolat rec recessus caudolateralis neurovas sp neurovascular space cav co cavum conchae olf bulb rec recessus bulbus olfactorius cavico rec recessus caviconchalis ophth N n. ophthalmicus cavico sin sinus caviconcbalis orb orbita cavico sin ost ostium, sinus caviconchalis Pal 0s palatinum cec rec recessus caeci pal bulge bulge of 0s palatinum ch choana pal pr rnax processus palatinus, 0s maxillare CNP cavum nasi proprium pal sin sinus palatinus CN Vi n. ophthalmicus Pare cartilago paranasalis co concha nasalis Pmx 0s premaxillare cr sem crista semicircularis pmx div diverticulum premaxillare eth tur ethmoturbinal pn for foramen pneumaticurn ex a o fen fenestra antorbitalis externa PO vest rec recessus postvestibularis ex add m. adductor mandibulae externus PO vest sin sinus postvestibularis ex co rec recessus extraconchalis postco postconcha fen nar fenestra narina postco cav cavitas postconchalis font a o fonticulus antorbitalis preco preconcha for epiph foramen epiphaniale preco rec recessus preconchalis fr 0s frontale Prf 0s prefrontale in a o fen fenestra antorbitalis interna prf rec recessus prefrontalis jo organum vomeronasale ( = Jacobson's prim ch choana prima (= primary choana) organ) PtC cartilago parietotectalis jug 0s jugale pter 0s pterygoideum jug bar arcus jugalis pter dor m. pterygoideus, pars dorsalis lac 0s lacrimale Ram lat nas ramus lateralis nasi. n. oDhthalmicus lac cav cavitas lacrimalis Ram med nas ramus medialis nasi, n. opbthalmicus la div diverticulum lacrimale rec du np recessus ducti nasopharyngei lam orb lamina orbitonasalis roof nas cap roof of cartilaginous nasal capsule lat Gr laterale Grenzfalte root co root of concha LTR lamina transversalis rostralis ros co concha nasalis rostralis mand mandibula scl sclera mand N n. mandibularis sec ch choana secundaria (= secondary choana) max 0s maxillare SeP septum male max N n. maxillaris suborb fen fenestra suborbitalis rnax sin ost ostium sinus maxillaris sep sulc sulcus septalis max tur maxilloturbinal tect nas tectum nasi mes 0s mesethmoidale vest vestibulum nasi mid co concha nasalis media vom vomer musc fos fossa muscularis, 0spalatinum FACIAL HOMOLOGIES IN ARCHOSAURS 271 a o cav lac Comparison of birds and crocodilians within the context of both amniote and archo- saur phylogeny also provides the opportunity to study a striking example of divergent spe- cialization superimposed upon a shared, in- herited ground plan. Crocodilians have a “pri- mordial” look about them and are commonly portrayed as living fossils little changed since their origin over 200 million years ago (Romer, ’66).Indeed, they retain manyprimi- tive features such as the presence of teeth “\ and most of the original complement of skull max pter Pal bones. However, modern crocodilians exhibit numerous apomorphies in all regions of the Fig. 1. Eupurkeria capensis, a basal archosauriform skull (Langston, ’73) and particularly in the from the Triassic of South Africa, in left lateral view, face, probably in association with skull flat- showing the antorbital cavity within the lateral aspect of the snout. Modified after Ewer (’65) and Witmer (’87). tening and formation of a long nasopharyn- geal duct. On the other hand, birds, despite being highly modified for flight, show some aspects of skull morphology that are actually the cavity is enigmatic and has been a matter primitive in comparison to crocodilians. For of some dispute (see Walker, ’61; Ewer, ’65; example, although birds apomorphically have Osmolska, ’85; Witmer, ’87; see Witmer, in press), the three major hypotheses being that lost several skull bones in association with the cavity housed 1) a gland, 2) a portion of the evolution of cranial kinesis, they primi- the jaw musculature, or 3) a paranasal air tively retain an external antorbital fenestra. sinus. The glandular hypothesis never has In fact, this mosaic pattern is a good illustra- had many adherents, whereas the muscular tion of why characters, not taxa, should be hypothesis has been by far the most popular
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