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An External Mandibular Fenestra and Other Archosauriform Character States in Basal Pterosaurs

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An External Mandibular Fenestra and Other Archosauriform Character States in Basal Pterosaurs Palaeodiversity 3: 225–233; Stuttgart 30 December 2010. 225 An external mandibular fenestra and other archosauriform character states in basal pterosaurs STERLING J. NESBITT & DAV I D W. E. HONE Abstract Pterosauria, a successful clade of extinct fl ying vertebrates, possesses a radical body plan that offers few clues about their origin and closest relatives. Whereas most researchers hypothesize an origin within Archosauria as the sister-group to Dinosauromorpha, others favor a position among non-archosauriform archosauromorphs. Here we present evidence that supports a placement within Archosauriformes: the presence of an external mandibular fe- nestra in two basal pterosaur taxa, Dimorphodon macronyx and a specimen referred to Eudimorphodon cf. ranzii (= ‘Seefeld Eudimorphodon’; BSP 1994 I 51). Furthermore, the arrangement of the mandibular bones surrounding the mandibular fenestra and the presence of a posterior process of the dentary that laterally overlaps the angular in the mandible of Dimorphodon and BSP 1994 I 51 are identical to those of Erythrosuchus, Euparkeria, and Archo- sauria. When mapped on a cladogram, presence or absence of an external mandibular fenestra in basal pterosaurs possibly indicates that the feature is primitive for Pterosauria but later lost. The presence of an external mandibu- lar fenestra, along with morphological evidence elsewhere in the body of pterosaurs (serrated teeth, antorbital fossa present, fourth trochanter on the femur present), supports a placement of Pterosauria within Archosauriformes and is consistent with a position within Archosauria. K e y w o r d s : Pterosauria, archosaur, dinosauromorph, phylogeny. Zusammenfassung Der Bauplan der Pterosaurier liefert nur wenige Hinweise auf den Ursprung und die nächsten Verwandten die- ser erfolgreichen Gruppe ausgestorbener fl iegender Wirbeltiere. Während die meisten Forscher einen Ursprung in- nerhalb der Archosaurier als Schwestergruppe der Dinosauromorpha annehmen, favorisieren andere eine Stellung innerhalb nicht-archosauriformer Archosauromorpha. Wir präsentieren hier Hinweise, die eine Stellung innerhalb der Archosauriformen stützen: Dies ist zum einen die Existenz eines externen Mandibularfensters bei zwei basalen Pterosaurier-Taxa, Dimorphodon macronyx und einem als Eudimorphodon cf. ranzii (= ‘Seefeld Eudimorphodon’; BSP 1994 I 51) bezeichnetem Exemplar. Zum anderen entsprechen die Anordnung der Knochen, die das Mandibu- larfenster umgrenzen, sowie die Existenz eines hinteren Fortsatzes des Dentales, der das Angulare in der Mandi- bel von Dimorphodon und BSP 1994 I 51 überlappt, der Situation bei Erythrosuchus, Euparkeria und den Archo- sauria. Die Verteilung der Merkmalszustände des externen Mandibularfensters deutet darauf hin, dass die Existenz dieses Fensters ein ursprüngliches Merkmal der Pterosaurier ist, das später verloren gegangen ist. Die Existenz ei- nes externen Mandibularfensters sowie weitere morphologische Merkmale an anderen Körperteilen der Pterosau- rier (Zähne mit gezackten Schneiden, antorbitale Grube vorhanden, vierter Trochanter am Femur vorhanden) un- terstützen deren Stellung innerhalb der Archosauriformes und stehen im Einklang mit einer Position innerhalb der Archosauria. Contents 1. Introduction .........................................................................................................................................................225 2. Materials and methods ........................................................................................................................................226 3. Description ..........................................................................................................................................................227 4. Character distribution of the mandibular fenestra within Pterosauria ..............................................................228 5. Discussion ............................................................................................................................................................229 6. References ........................................................................................................................................................... 231 1. Introduction taxa have been named from the Triassic (DALLA VECCHIA 2003) and recent revisions of specimens once referred Despite over 200 years of research on the anatomy and to Eudimorphodon have turned out to be new lineages relationships of pterosaurs, their origins and the identity ( DALLA VECCHIA 2009a), revealing a radiation of early pte- of their closest relatives remain an outstanding problem. rosaurs unparalleled in their evolutionary history. Many Pterosaurs fi rst appear in the fossil record in the Late Tri- Triassic pterosaur specimens are represented by superb- assic (Norian) from localities across Europe and Green- ly preserved and articulated material (although general- land (WILD 1978; JENKINS et al. 2001; DALLA VECCHIA 2003; ly compressed into two dimensions). Yet all Triassic pter- BARRETT et al. 2008). At least six different species-level osaurs, despite being the earliest members of Pterosauria, 226 PALAEODIVERSITY 3, 2010 share a divergent morphology of the skull, axial column, Institutional abbreviations forelimbs, and hindlimbs that characterizes Pterosauria as BMNH/NHM Natural History Museum, London, United King- a unique group of vertebrates. Hence, there is a paucity dom of morphological clues linking pterosaurs to their closest BNM Bündner Naturmuseum of Chur, Grisons, Swit- reptilian relatives. zerland BP Bernard Price Institute for Palaeontological Re- Within Archosauromorpha (taxa closer to Aves than to search, Johannesburg, South Africa Squamata), Pterosauria has been linked with ‘prolacerti- BSP Bayerische Staatssammlung für Paläontologie forms’ (WILD 1978, 1983, 1984; PETERS 2000), a likely para- und historische Geologie, München, Germany phyletic or polyphyletic clade (MÜLLER 2003); the enigmatic MCSNB Museo Civico di Scienze Naturali di Bergamo, Italy drepanosaurids (RENESTO & BINELLI 2006); the non-archo- MFSN Museo Friulano di Storia Naturale, Udine, Italy saurian archosauriforms (BENNETT 1996); and nested with- MGUH Geological Museum, University of Copenhagen, in crown-group Archosauria as the sister-taxon of Dino- Denmark sauromorpha (PADIAN 1984; GAUTHIER 1986; SERENO 1991a; PVSJ Division of Paleontology of the Museo de Cien- JUUL 1994; BENNETT 1996; BENTON 1999; IRMIS et al. 2007; cias Naturales de la Universidad Nacional de San NESBITT in press). The archosaurian hypothesis is support- Juan, Argentina ed by numerical phylogenetic analyses, though typically SC Italian State collections, Italy SMNS Staatliches Museum für Naturkunde Stuttgart, with few cladistic characters supporting the node that joins Germany the pterosaurs to other clades. However, a number of char- acter states (e. g., the lack of serrated teeth and a fourth Acknowledgments ENNETT ETERS trochanter) in Pterosauria are cited (B 1996; P Thanks to SANDRA CHAPMAN, LORNA STEEL and PAUL BARRETT 2000) as being incompatible with archosaurian ancestry. for access to material at the BMNH, OLIVER RAUHUT for access One of the most commonly cited contradictions to material at the BSP and ANDREAS MATZKE at the SMNS. We in the archosaurian-Pterosauria hypothesis for ptero- also wish to thank GEORGE JANSSEN of the BSPG for the pho- saur origins is the apparent absence of an external man- tographs used in fi gure 1F. Discussions with KEVIN PADIAN, W ILLIAM PARKER, PETER WELLNHOFER, DREW EDDY, A. J. DEBEE, dibular fenestra (EMF) in pterosaurs (WILD 1978, 1983, CLINT BOYD, ADAM SMITH, and ZHIHENG LI helped improve the 1984; BENTON 1990; BENNETT 1996; PETERS 2000). This fe- manuscript. Finally we thank ROSS ELGIN and FABIO DALLA nestra has long been used to diagnose Erythrosuchus + V ECCHIA for the comparative photographs. crowngroup Archosauria (e. g., BENTON 1985; JUUL 1994; B ENNETT 1996) and persists in a majority of clades of Ar- chosauria. RICHARD OWEN (1870) tentatively identifi ed an 2. Materials and methods EMF in Dimorphodon stating that the mandible had a “va- cuity, if it be natural and not due to the abrasion of the thin We studied the remains of two basal pterosaurs, the outer wall” (OWEN 1870: 59) and this was later followed Early Jurassic Dimorphodon macronyx (BMNH 41212, by the independent reconstruction of ARTHABER (1919). 43486, R 1035) and a Triassic pterosaur specimen referred ROMER (1956) also noted a mandibular fenestra in Dimor- to Eudimorphodon cf. ranzii (BSP 1994 I 51; WELLNHOFER phodon, but his reconstruction was copied from ARTHABER 2003). Of the Dimorphodon specimens with mandibles, (1919). Later descriptions and reconstructions of Dimor- the posterior portions of the mandibles of BMNH 41212 phodon simply show no EMF (e. g., fi g. 9 of YOUNG 1964; and BMNH 43486 are exposed whereas the posterior por- fi g. 2 of WELLNHOFFER 1978; fi g. 6 of PADIAN 1983a; fi g. 3 tions of the mandibles of BMNH R 1035 are covered by of KELLNER 2003; fi g. 8 of UNWIN 2003) with the exception forelimb elements. of PADIAN (1983a, fi g. 29; 1983b; 1984) or state that ptero- BSP 1994 I 51 (= ‘Seefeld Eudimorphodon’) was origi- saurs lack one (e. g., BENNETT 1996). Thus, pterosaurs are nally assigned to Eudimorphodon cf. ranzii by WELLNHOFER widely regarded as lacking an EMF – a key character for (2003). However, DALLA VECCHIA (2009a) noted that BSP diagnosing Erythrosuchus + Archosauria. Nevertheless, a 1994 I 51 did not share any apomorphies of Eudimorpho-
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