Premaxillary crest variation within the Wukongopteridae (Reptilia, Pterosauria) and comments on cranial structures in pterosaurs

Anais da Academia Brasileira de Ciências (2017) 89(1): 119-130 (Annals of the Brazilian Academy of Sciences) Printed version ISSN 0001-3765 / Online version ISSN 1678-2690 http://dx.doi.org/10.1590/0001-3765201720160742 www.scielo.br/aabc Premaxillary crest variation within the Wukongopteridae (Reptilia, Pterosauria) and comments on cranial structures in pterosaurs XIN CHENG1,2, SHUNXING JIANG1, XIAOLIN WANG1,3 and ALEXANDER W.A. KELLNER2 1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, 100044, Beijing, China 2 Laboratory of Systematics and Taphonomy of Fossil Vertebrates, Department of Geology and Paleontology, Museu Nacional/ Universidade Federal do Rio de Janeiro, Quinta da Boa Vista, s/n, São Cristóvão, 20940-040 Rio de Janeiro, RJ, Brazil 3 University of Chinese Academy of Sciences, 100049, Beijing, China Manuscript received on October 28, 2016; accepted for publication on January 9, 2017 ABSTRACT Cranial crests show considerable variation within the Pterosauria, a group of flying reptiles that developed powered flight. This includes the Wukongopteridae, a clade of non-pterodactyloids, where the presence or absence of such head structures, allied with variation in the pelvic canal, have been regarded as evidence for sexual dimorphism. Here we discuss the cranial crest variation within wukongopterids and briefly report on a new specimen (IVPP V 17957). We also show that there is no significant variation in the anatomy of the pelvis of crested and crestless specimens. We further revisit the discussion regarding the function of cranial structures in pterosaurs and argue that they cannot be dismissed a priori as a valuable tool for species recognition. Key words: Wukongopteridae, Pterosauria, cranial crest, sexual dimorphism, China. INTRODUCTION Cranial crests are present in nearly all pterosaur groups, from species found in Triassic deposits (e.g., Dalla Vecchia 2014) to the last taxa recorded from Late Cretaceous strata (e.g., Kellner and Langston 1996). The various cranial crests have been widely used for taxonomic purposes (e.g., Wellnhofer 1991, Kellner 2003, Andres et al. 2014) and there are several bones involved in those structures, such as the premaxillae, frontals, parietals, supraoccipital (e.g., Young 1964, Campos and Kellner 1985, Kellner and Campos 2002a, Correspondence to: Xiaolin Wang E-mail: Wang and Zhou 2003, Wang et al. 2012, 2014a), and the dentaries (e.g., Wellnhofer 1987, Kellner et al. 2013, Wang et al. 2014b). Cranial crests are more frequent in pterodactyloid pterosaurs, but some are found also in non-pterodactyloids (Czerkas and Ji 2002, Dalla Vecchia 2009, Stecher 2008). Most recently several specimens of the nonpterodactyloid clade Wukongopteridae were discovered in Middle to Late Jurassic Tiaojishan Formation in China (Zhou and Wang 2010, Sullivan et al. 2014). These specimens have called attention due to the particular array of primitive and derived features that placed them as closely related to the derived Pterodactyloidea, although their actual phylogenetical position is still disputed (Wang et An Acad Bras Cienc (2017) 89 (1) 120 XIN CHENG, SHUNXING JIANG, XIAOLIN WANG and ALEXANDER W.A. KELLNER al. 2009, 2010, Lü et al. 2010, Andres et al. 2014). There is also some discussion about their diversity, with some authors considering them monospecific (Lü et al. 2010, 2011a) while others regard them as more diverse (Wang et al. 2010, Lü et al. 2011b). Among the anatomical features used to establish the wukongopterid diversity is the premaxillary crest. In some specimens, the crest is absent (Wang et al. 2010, Lü et al. 2011a) while in others it varies in shape and extension (Wang et al. 2010, Lü et al. 2011b, Cheng et al. 2016). More recently, a specimen with an associated egg, that lacked a crest, was reported (Lü et al. 2011a), raising the possibility that the presence and absence of such structure might indicate sexual dimorphism in pterosaurs. The main purpose of this paper is to review and discuss the differences of the cranial crests found in the Wukongopteridae and revisit the debate concerning these head structures. We also briefly report on a new wukongopterid specimen (IVPP V 17957) that shows a premaxillary crest different from all others within this group. Institutional Abbreviations: HGM-Henan Geological Museum, Zhengzhou, Henan Province, China; IVPP-Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; YH-Yizhou Museum, Yixian, Liaoning Province, China; ZMNHZhejiang Museum of Natural History, Hangzhou, Zhejiang Province, China DESCRIPTION AND COMPARISONS Up to now, eight specimens referable to the Wukongopteridae have been published, all coming from the Middle to Upper Jurassic Tiaojishan Formation (northeast China). They have been considered to represent three genera, Wukongopterus, Darwinopterus, and Kunpengopterus (Wang et al. 2009, 2010, 2015, Lü et al. 2010, 2011a, b), or remained indeterminate (Cheng et al. 2016). Two additional An Acad Bras Cienc (2017) 89 (1) individuals were also referred to this clade. One is the holotype of Changchengopterus pani, that lacks a skull (Lü 2009) and has been considered a putative wukongopterid (Wang et al. 2009). However, this very small and apparently ontogenetic young animal has comparatively shorter cervical vertebrae compared to other wukongopterids and therefore its exact phylogenetical position remains to be established. The second specimen was described by Zhou and Schoch (2011) and consists of a fairly complete skeleton lacking the skull. Based on the elongation of the cervical vertebrae, along with a long tail and developed fifth toe, it can confidently be assigned to the Wukongopteridae. Recently, a new specimen (IVPP V 17957) was discovered from the same region and horizon as the holotypes of Wukongopterus, Kunpengopterus and Darwinopterus (Wang et al. 2009, 2010, Lü et al. 2010, 2011b). IVPP V 17957 consists of a partial skeleton with incomplete skull that can be regarded as a wukongopterid pterosaur based on the following characters: confluent nasoantorbital fenestra, quadrate inclined backwards, elongated cervical vertebrae, reduced cervical ribs, length of the wing metacarpal about half the length of the first wing phalange, elongated tail enclosed by rod-like bony extensions made by the zygapophyses. This new specimen shows a premaxillary crest that differs from previous wukongopterids and therefore is included in the present paper. A full description of this material will be done elsewhere. All wukongopterid specimens with complete skull can be divided into two main types, either having or lacking a premaxillary crest. The holotype of Kunpengopterus sinensis (IVPP V 16047) and ZMNH M8802/IVPP V 18043, an individual that was associated with eggs, lack an ossified (...truncated)