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Reptilia, Diapsida): New Insights from High-Resolution Ct Scanning of the Holotype

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Reptilia, Diapsida): New Insights from High-Resolution Ct Scanning of the Holotype Palaeontologia Electronica http://palaeo-electronica.org THE BRAINCASE OF YOUNGINA CAPENSIS (REPTILIA, DIAPSIDA): NEW INSIGHTS FROM HIGH-RESOLUTION CT SCANNING OF THE HOLOTYPE Nicholas M. Gardner, Casey M. Holliday, and F. Robin O’Keefe Nicholas M. Gardner. Department of Biological Sciences, Marshall University, Huntington, West Virginia. [email protected] Casey M. Holliday. Department of Pathology & Anatomical Sciences, University of Missouri, Columbia, Missouri. [email protected] F. Robin O’Keefe. Department of Biological Sciences, Marshall University, Huntington, West Virginia. [email protected] ABSTRACT Detailed descriptions of braincase anatomy in early diapsid reptiles have been historically rare given the difficulty of accessing this deep portion of the skull, because of poor preservation of the fossils or the inability to remove the surrounding skull roof. Previous descriptions of the braincase of Youngina capensis, a derived stem-diapsid reptile from the Late Permian (250 MYA) of South Africa, have relied on only partially preserved fossils. High resolution X-ray computed tomography (HRXCT) scanning, a new advance in biomedical sciences, has allowed us to examine the reasonably com- plete braincase of the holotype specimen of Youngina capensis for the first time by dig- itally peering through the sandstone matrix that filled the skull postmortem. We present the first detailed 3D visualizations of the braincase and the vestibular system in a Permian diapsid reptile. This new anatomical description is of great comparative and phylogenetic relevance to the study of the structure, function and evolution of the reptil- ian head. KEY WORDS: Youngina capensis, diapsid reptiles, CT scanning, 3D models PE ERRATUM In the paper Gardner et al. (2010), we stated of the collections of the University of Chicago. This that UC 1528, the holotype specimen of Youngoi- was in error, as the specimen is housed at the Field des romeri Olson and Broom 1937, a junior syn- Museum of Natural History (Chicago, IL). Special onym of Youngina capensis Broom 1914, was part thanks to William F. Simpson (FMNH) for facilitat- PE Article Number: 13.3.19A Copyright: Palaeontological Association November 2010 Submission: 14 September 2009. Acceptance: 8 September 2010 Gardner, Nicholas M., Holliday, Casey M. , and O’Keefe, F. Robin, 2010. The Braincase of Youngina capensis (Reptilia, Dipsida): New Insights from High-Resolution CT Scanning of the Holotype Palaeontologia Electronica Vol. 13, Issue 3; 19A:16p; http://palaeo-electronica.org/2010_3/217/index.html GARDNER, HOLLIDAY, & O’KEEFE: BRAINCASE OF YOUNGINA ing the loan of FMNH UC 1528, thereby making Diapsida): New Insights from High-Resolution CT this research possible. *Nicholas M. Gardner, Scanning of the Holotype. Palaeontologica Elec- Casey M. Holliday, and F. Robin O'Keefe, 2010. tronica 13(3); 19A: 16p. The Braincase of Youngina capensis (Reptilia, NOTE IN PROOF Reisz et al. (2010) find a non-diapsid position ing topologies as new data are incorporated. How- for Apsisaurus as a varanopid synapsid, but unfor- ever, our tree was taken from Müller (2003) who tunately, their paper came too late for us to correct noted that the exclusion of Apsisaurus from his Figure 1 by removing it from our tree. We are data set does not affect the rest of the tree topol- aware that our placement in the tree for Apsisaurus ogy in the final analysis. is outdated, this is the unfortunate nature of shift- INTRODUCTION lution to this disputed portion of the amniote phy- logeny, and contribute to a new understanding of Reptiles first appeared in the fossil record dur- the evolutionary history of reptiles and the relation- ing the Late Carboniferous (320-310 MYA) and ships between extant diapsids (Modesto and Sues rapidly diversified into two different lineages, the 2002). parareptiles and the eureptiles (Müller 2003; Figure The Late Permian (250 MYA) diapsid reptile 1). The earliest known examples are Carboniferous Youngina capensis is often regarded as the ‘arche- eureptiles such as Hylonomus (Carroll 1988a) and typal’ basal diapsid (Smith and Evans 1996) and Petrolacosaurus (Reisz 1977; Reisz 1981). Para- recognized as an “ancestral morphotype” (Carroll reptiles and eureptiles further diversified into 1988b) between more primitive taxa such as para- numerous clades, of which only the diapsid eurep- reptiles and captorhinids and modern diapsids tiles survived past the Triassic and into modern (Müller 2003; Figure 1). Its relationships among times (archosaurs, lepidosaurs and turtles). The other diapsids have been disputed. Currie (1981, precise relationships among extant reptile clades 1982) posited that Youngina shared a closer rela- remain a problem for reptile biologists and paleon- tionship with Acerosodontosaurus (Currie 1980), tologists. For example, turtles have become con- Galesphyrus (Carroll 1976), Hovasaurus (Currie sensually accepted among reptile paleontologists 1981), Kenyasaurus (Harris and Carroll 1977), as being diapsids, but it is uncertain whether or not Tangasaurus (Currie 1982), and Thadeosaurus they are part of the crown-diapsid clade (Gregory (Carroll 1981). These taxa collectively were 1946; Ivachnenko 1987; Kordikova 2002; Laurin referred to as the Younginiformes, which were vari- and Reisz 1995; Lee 1997; Lee 2001; Lyson et al. ously allied to lepidosauromorphs (Benton 1985; 2010; Werneburg and Sánchez-Villagra 2009), or if Evans 1988) or as stem-diapsids (Gaffney 1980; they are crown-diapsids, whether or not they are Laurin 1991). Their monophyly was never explicitly closer to archosaurs (Bhullar and Bever 2009; tested and recently Bickelmann et al. (2009) pub- Evans 2009; Hedges and Poling 1999; Zardoya lished the results of their phylogenetic analysis that and Meyer 1998) or lepidosaurs (Bickelmann et al. suggested that these taxa do not form a monophyl- 2009; deBraga and Rieppel 1997; Li et al. 2008; etic relationship with each other to the exclusion of Müller 2003; Rieppel 2002; Rieppel and deBraga other diapsid reptiles, though the relationships 1996; Rieppel and Reisz 1999). Though conflicting between all stem-diapsids were highly unresolved data from molecular studies, soft-tissue morphol- in their topology. Youngina is the most derived ogy and bony morphology provide differing sup- known stem-diapsid to retain two complete fenes- ports for the position of turtles (Lee 2001; Rieppel trae in the temporal region, rather than having 2002), a more detailed understanding of the anat- evolved this condition secondarily (as in derived omy of early diapsids and other morphologically archosauromorphs and sphenodontids and possi- primitive reptiles would provide much-needed reso- bly as in sauropterygians and ichthyopterygians). 2 PALAEO-ELECTRONICA.ORG FIGURE 1. Cladogram demonstrating the evolutionary relationships between Youngina with other reptile taxa (modified from Müller 2003). Dagger denotes extinct taxa. Nodes: A, Amniota; B, Reptilia; C, Eurep- tilia; D, Diapsida; E, Neodiapsida; F, Sauria; G, Lepidosauromorpha; H, Lepidosauria; I, Archosauromor- pha. Thus it appears that the loss of the lower temporal superficial anatomy of the skull, and described it bar occurred in higher stem-diapsids more derived largely in palatal and occipital views. Gow (1975) than Youngina (Müller 2003). Youngina is well provided a preliminary discussion of the braincase known from numerous specimens that were previ- of TM 3603, which was given more attention later ously described as distinct “younginid” or by Evans (1987). Evans sawed this specimen in “younginiform” taxa (Gow 1975). Despite its obvi- half to gain access to the internal aspects of the ously critical position as a stem-diapsid and the neurocranial bones. Her description is currently the existence of multiple specimens, Youngina is in only published, detailed treatment of the braincase need of a thorough re-description. Formal descrip- of Youngina. tion of its anatomy is a crucial precursor to under- While all reptiles (and in fact, all amniotes) standing: 1) its phylogenetic relationships, 2) its ossify the caudoventral portion of the braincase relevance to the interrelationships between other cartilages, ossification patterns differ for the rostral diapsid reptiles and 3) the evolution of the skull in cartilages. Archosauriforms (birds, crocodiles and these reptiles (Bickelmann et al. 2009; Modesto their ancestors) have uniquely ossified the pila and Sues 2002). antotica as the laterosphenoid, which encloses the The first discussion of the braincase of Youn- rostral region of the cavum cranii (Clark et al. gina was carried out by Olsen (1936), who 1993). Turtles ossify the pila antotica adjoining to described UC 1528, the holotype of Youngoides the clinoid process of the basisphenoid, similar to romeri. He was limited to observations of the the condition found in captorhinids, sauroptery- 3 GARDNER, HOLLIDAY, & O’KEEFE: BRAINCASE OF YOUNGINA gians and many other primitive reptiles, though the HRXCT Scanning and Visualization clinoid process is much taller in turtles leaving the Preliminary CT scanning of both specimens in rostral braincase largely open (Rieppel 1993) and the medical CT scanner at SUNY Stoneybrook the pila antotica may have also ossified into an revealed that only the holotype (AMNH 5561) archosauriform-like paired “laterosphenoids” primi- showed enough differentiation between bone and tively in turtles (Proganochelys: Bhullar and Bever matrix to justify the expense of HRXCT scanning. 2009; Kayentachelys:
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