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The Skull of the Upper Cretaceous Snake Dinilysia Patagonica Smith-Woodward, 1901, and Its Phylogenetic Position Revisited

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The Skull of the Upper Cretaceous Snake Dinilysia Patagonica Smith-Woodward, 1901, and Its Phylogenetic Position Revisited Zoological Journal of the Linnean Society, 2012, 164, 194–238. With 24 figures The skull of the Upper Cretaceous snake Dinilysia patagonica Smith-Woodward, 1901, and its phylogenetic position revisited HUSSAM ZAHER1* and CARLOS AGUSTÍN SCANFERLA2 1Museu de Zoologia da Universidade de São Paulo, Avenida Nazaré 481, Ipiranga, 04263-000, São Paulo, SP, Brasil 2Laboratorio de Anatomía Comparada y Evolución de los Vertebrados. Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Av. Angel Gallardo 470 (1405), Buenos Aires, Argentina Received 23 April 2010; revised 5 April 2011; accepted for publication 18 April 2011 The cranial anatomy of Dinilysia patagonica, a terrestrial snake from the Upper Cretaceous of Argentina, is redescribed and illustrated, based on high-resolution X-ray computed tomography and better preparations made on previously known specimens, including the holotype. Previously unreported characters reinforce the intriguing mosaic nature of the skull of Dinilysia, with a suite of plesiomorphic and apomorphic characters with respect to extant snakes. Newly recognized plesiomorphies are the absence of the medial vertical flange of the nasal, lateral position of the prefrontal, lizard-like contact between vomer and palatine, floor of the recessus scalae tympani formed by the basioccipital, posterolateral corners of the basisphenoid strongly ventrolaterally projected, and absence of a medial parietal pillar separating the telencephalon and mesencephalon, amongst others. We also reinterpreted the structures forming the otic region of Dinilysia, confirming the presence of a crista circumfenes- tralis, which represents an important derived ophidian synapomorphy. Both plesiomorphic and apomorphic traits of Dinilysia are treated in detail and illustrated accordingly. Results of a phylogenetic analysis support a basal position of Dinilysia, as the sister-taxon to all extant snakes. The fossil taxa Yurlunggur, Haasiophis, Eupodophis, Pachyrhachis, and Wonambi appear as derived snakes nested within the extant clade Alethinophidia, as stem-taxa to the crown-clade Macrostomata. The hypothesis of a sister-group relationship between Dinilysia and Najash rionegrina, as suggested by some authors, is rejected by the results of our analysis. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164, 194–238. doi: 10.1111/j.1096-3642.2011.00755.x ADDITIONAL KEYWORDS: cranial anatomy – phylogeny – Serpentes – Squamata – systematics. INTRODUCTION by Smith-Woodward (1901) and then in detail by Estes, Frazzetta & Williams (1970) and Frazzetta The cranial anatomy of the Upper Cretaceous snake (1970). Despite its completeness and importance, Dinilysia patagonica Woodward, 1901, was until several important details of the cranial anatomy of recently known only from an almost complete skull D. patagonica are still poorly understood. Not surpris- pertaining to the holotype. It still represents one of ingly, its phylogenetic affinities are also a matter of the best-preserved snake skulls known so far, and has controversial debate. Smith-Woodward (1901), Romer been illustrated in several textbooks (e.g. Parker & (1956), and Estes et al. (1970) considered Dinilysia to Grandison, 1977; Carroll, 1988) and described briefly be closely related to the booids. However, Estes, Frazzetta & Williams (1970) also clearly acknowl- edged the presence of plesiomorphic lizard-like fea- *Corresponding author. E-mail: [email protected] tures in Dinilysia, and argued that ‘if Dinilysia is 194 © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164, 194–238 CRANIAL ANATOMY OF DINILYSIA PATAGONICA 195 related to boids and aniliids, it must belong rather far lary complex, and incomplete lower jaws (MACN-RN down in the ancestry of the booid complex’, and that 1013; Fig. 3); a fragmentary skull corresponding to the ‘in its lizardlike aspects, it must be emphasized, braincase with its right quadrate and quadrate ramus Dinilysia does not show special affinities to any group of the pterygoid (MACN-RN 1014; Fig. 2); an articu- of lizards’. Rage (1977, 1982) briefly commented on lated specimen comprising a partially preserved skull some of the character evidence put forward by Estes and almost complete presacral vertebral column et al. (1970), concluding that Dinilysia represents the (MACN-RN 976); a badly preserved basicranium with sister group of macrostomatan snakes rather than the disarticulated frontals (MACN-RN 1015); an incom- most basal snake known. Rieppel (1979c, 1988) also plete skull comprising a braincase with its right quad- regarded Dinilysia as a more derived snake, placing it rate articulated to the incomplete jaw and first as the sister group of the clade Alethinophidia. More presacral vertebrae preserved (MPCA-PV 527); a recently, Dinilysia has been included in several phy- partial skull with fragmentary jaws and first presacral logenetic analyses, appearing either as the sister vertebrae (MUCPv 38); two partial skulls of juvenile group of the clade Alethinophidia (Zaher, 1998; Cald- individuals (MACN-N 104, 71-VII-29-1). MLP 26-410, well, 1999; Rieppel & Zaher, 2000; Tchernov et al., MACN-N 104, MLP 71-VII-29-1, and MACN-RN 1014 2000; Apesteguía & Zaher, 2006; Wilson et al., 2010) were subjected to new mechanical preparations in or as the sister group to all extant snakes order to reveal new and noteworthy information on the (Scolecophidia + Alethinophidia) (Scanlon & Lee, vomeropalatine contact and otico-occipital region, 2000; Scanlon, 2006). respectively. In the last few years, Caldwell & Albino (2002), The specimens MACN-RN 1013, MACN-RN 1014, and Budney, Caldwell & Albino (2006), and Caldwell & MUCPv 38 were scanned at the High-Resolution X-ray Calvo (2008) have described important new skull Computed Tomography (HRXCT) Facility at The Univer- material of D. patagonica. We independently revised sity of Texas at Austin, USA, using a FeinFocus micro- the same material and found significant differences focal X-ray source operating at 200 kV and 0.09 mA in all between our observations and their descriptions. The specimens. Slice thickness corresponded to two lines in a skull of the holotype of D. patagonica (MLP 26–410) CCD (charge-coupled-device) image intensifier imaging as well as MACN-RN 1014 were further prepared for system, with a source-to-object distance (180 mm in a better visualization of the palatal, rostral, and otico- MACN-RN 1013; 135 mm in MACN-RN 1014; 175 mm occipital complexes. The holotype is the only specimen in MUCPv 38). For each slice, 1400 views were taken that bears a preserved and articulated vomero- with three samples (MACN-RN 1013, MUCPv 38) and palatine contact (Fig. 1), whereas MACN-RN 1014 four samples (MACN-RN 1014) per view. The fields of retains an almost complete and undistorted braincase image reconstructions were 59 mm (MACN-RN 1013), (Fig. 2). Three-dimensional reconstructions based on 44 mm (MACN-RN 1014), and 57 mm (MUCPv 38), high-resolution X-ray images were also available for and an image reconstruction offset of 5000 was the skulls MACN-RN 1013 and 1014. As the new employed with a reconstruction scale of 2500 material of D. patagonica will have important conse- (MACN-RN 1013), 3000 (MACN-RN 1014), and 3700 quences for the present debate on the origin and (MUCPv 38), respectively. The data sets of HRXCT higher-level phylogeny of snakes, we found it relevant slices (833 for MACN-RN 1013; 505 for MACN-RN to describe and illustrate in more detail these new 1014; 570 for MUCPv 38) were taken along the coronal observations. (transverse) axis of the skull. Each slice image was gathered at 1024 ¥ 1024 pixels resolution, resulting in an in-plane resolution of 12.2 mm per pixel. The data MATERIAL AND METHODS sets were resliced along the other two orthogonal axes Descriptions presented herein are based on the analysis (horizontal and sagittal) and three-dimensional visu- of previously known specimens of D. patagonica depos- alizations were produced using VGStudio MAXw 1.2 ited in the following institutions (institutional abbrevia- (Volume Graphics, Heidelberg, Germany). Density tions in parentheses): Museo Argentino de Ciencias contrast between bone and matrix was sufficient to Naturales ‘Bernardino Rivadavia’, Buenos Aires permit digital ‘preparation’ of the specimen, rendering (MACN); Museo de La Plata (MLP); Museo Provincial the matrix completely transparent. ‘Carlos Ameghino’, Cipoletti (MPCA), and Museo de la Universidad del Comahue, Neuquen (MUCPv). Speci- DESCRIPTION mens analysed are as follows: the holotype (MLP 26-410; Fig. 1), an almost complete skull with a fragmentary THE DERMATOCRANIUM nasal region, mostly complete braincase and incom- Premaxilla plete lower jaws; a partial skull comprising an almost The premaxilla is not preserved in any of the speci- complete braincase, the right nasal and palatomaxil- mens. However, some authors (e.g. Frazzetta, 1970) © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164, 194–238 196 H. ZAHER and C. A. SCANFERLA Figure 1. Holotype of Dinilysia patagonica (MLP 26–410). Skull in dorsal (A), ventral (B), right lateral (C), and posterior (D) views; fragmentary right dentary in lingual (E) and medial (F) views; right compound bone in dorsal view (G) and left compound bone in dorsolateral view (H). have suggested that the premaxilla of D. patagonica medial surface of the right nasal that is not available was toothed and only loosely connected (not articu- in the holotype (see Fig. 1). As described by Estes lated) with the anterior end of the maxilla. Whether et al. (1970), the nasals are paired elements that the premaxilla was toothed or not cannot be ascer- together form a triangular structure in dorsal view. tained with the evidence at hand. However, a loose This is because of the dorsal laminae that taper connection between the transverse process of the pre- distinctly anteriorly, but broaden posteriorly where maxilla and the maxilla is suggested by the rounded they meet the prefrontals posterolaterally and the and smooth surface of the anterior tip of the maxilla frontals posteriorly. In both the holotype and MACN- in MACN-RN 1013 (Fig. 9).
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