Palaeontologia Electronica palaeo-electronica.org Fossil calibration dates for molecular phylogenetic analysis of snakes 2: Caenophidia, Colubroidea, Elapoidea, Colubridae Jason J. Head, Kristin Mahlow, and Johannes Müller ABSTRACT Caenophidia, the snake clade that includes the highest species richness, morpho- logical diversity, and ecological breadth within Serpentes, has been extensively studied with respect to molecular phylogenetic systematics. The majority of the caenophidian fossil record, though dense, has been taxonomically defined on the basis of general anatomical similarity or shared geographic provenance with extant reference taxa. As a result, historical patterns of diversification within the clade are poorly constrained due to a paucity of reliable fossil calibration dates. Here we provide 10 fossil calibration dates for phylogenetic analysis of caenophidian relationships. Calibration points include apomorphy-based systematic justifications based on cranial and precloacal vertebral elements and precise dates for hard minimum divergence timings. Calibrated nodes are for Caenophidia, Acrochordus, Elapoidea, Colubridae, and constituent sub- clades. Hard minimum divergence timings range from late Cretaceous to Miocene. The spatial and temporal distribution of reliable first occurrence of colubroid taxa suggests late Paleogene intercontinental dispersals between Asia, North America, and Africa, followed by rapid diversification and subsequent dispersals into all non-Polar conti- nents by the early Neogene. Jason J. Head. Department of Zoology and University Museum of Zoology, Downing St., University of Cambridge, CB2 3EJ, England [email protected] Kristin Mahlow. Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstr. 43, D-10115 Berlin, Germany [email protected] Johannes Müller. Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstr. 43, D-10115 Berlin, Germany [email protected] Keywords: Serpentes; fossil; calibration; Caenophidia; Colubroidea; Elapoidea; Colubridae Submission: 13 January 2016 Acceptance: 16 May 2016 PE Article Number: 19.2.9FC Copyright: Palaeontological Association June 2016 Submission: 13 January 2016. Acceptance: 16 May 2016 Head, Jason J., Mahlow, Kristin, and Müller, Johannes. 2016. Fossil calibration dates for molecular phylogenetic analysis of snakes 2: Caenophidia, Colubroidea, Elapoidea, Colubridae. Palaeontologia Electronica 19.2.2FC: 1-21 palaeo-electronica.org/content/fc-9 Calibrations published in the Fossil Calibration Series are accessioned into the Fossil Calibration Database (www.fossilcalibra- tions.org). The Database is a dynamic tool for finding up-to-date calibrations, and calibration data will be updated and annotated as interpretations change. In contrast, the Fossil Calibration papers are a permanent published record of the information on which the cal- ibrations were originally based. Please refer to the Database for the latest data. HEAD, MAHLOW, & MÜLLER: CAENOPHIDIAN CALIBRATIONS INTRODUCTION Szyndlar and Schleich, 1993; Szyndlar and Rage, 1999; Holman, 2000 [and references therein]; Iva- The vast majority of extant snake diversity is nov, 2000, 2002; Ivanov and Böhme, 2011), as well contained in Caenophidia, a clade consisting of > as records from South America, Africa, and Asia 2500 species (Vidal et al., 2009). Caenophidia was (e.g., Albino, 1996; Albino and Montalvo, 2006; originally erected by Hoffstetter (1939) to include Rage, 1973, 1976, 2003; Rage and Ginsburg, snakes more derived than “henophidian grade” 1997; Rage and Danilov, 2008; Head, 2005, Head taxa, considered at the time to include boids and et al., 2006; Head et al., 2007; Head and Bell, anilioids. Morphological studies recognized a close 2008). The published record has been used to cali- relationship between relationship between Colu- brate multiple caenophidian molecular phylogenies broidea, itself consisting of viperids, colubrids, ela- (e.g., Noonan and Chippindale, 2006; Burbrink and pids, homalopsids, and other “derived snakes”, and Lawson, 2007; Wüster et al., 2007, 2008; Alfaro et Acrochordus, the genus of file snakes, wart al., 2008; Sanders and Lee, 2008; Sanders et al., snakes, and elephant trunk snakes (e.g., Bellairs 2010; Kelly et al., 2009; Lukoschek et al. 2012; and Underwood, 1951), with explicit morphological Pyron and Burbrink, 2012), with different fossils phylogenetic analyses recovering a sister-taxon employed to produce highly disparate divergence relationship between the two (e.g., Rieppel, 1988; estimates (Lukoschek et al., 2012). Kluge, 1991; Cundall et al., 1993; Lee and Scan- There are multiple problems with calibrating lon, 2000; Tchernov et al., 2000). molecular phylogenies using the published litera- Numerous molecular phylogenetic studies of ture on fossil snakes (Head, 2015), especially for caenophidians have subsequently been con- caenophidians: The absence of discrete apomor- ducted. Research foci associated with these analy- phies in vertebral morphology for most taxa limits ses include molecular divergence timing estimates, testable, reproducible taxonomic hypotheses from biogeographic histories, diversification patterns, isolated fossils. Additionally, considerable intraco- and the evolution of toxic venoms, making them lumnar, individual, and ontogenetic variation exists the most intensely studied squamate clade with in vertebral morphology, and very few studies have respect to molecular phylogenetics (e.g., Keogh, attempted to examine the influence of these factors 1998; Burbrink et al., 2000; Gravlund et al., 2001; in taxonomic assignments (Szyndlar, 2012 [but see Burbrink, 2002; Vidal, 2002; Fry et al., 2003; Kelly Szyndlar, 1984 and LaDuke, 1991a for analyses of et al., 2003; Keogh et al., 2003; Nagy et al., 2003; intracolumnar and individual variation]). Pinou et al., 2004; Lawson et al., 2005; Lukoschek Because of these potential limitations, identity and Keogh, 2006; Burbrink and Lawson, 2007; of snake fossils has traditionally relied on a gener- Vidal et al., 2007; Fry et al., 2008; Sanders et al., alized “vertebral form taxonomy” that uses overall, 2008; Hedges et al., 2009; Kelly et al., 2009; Pyron often proportional, shape to recognize taxa and and Burbrink, 2009a,b,c; Zaher et al., 2009; Bur- implicitly does not follow a phylogenetic taxonomy, brink and Pyron, 2010; Sanders et al., 2010; Vidal despite using the same taxon names (see Szyndlar et al., 2010; Murphy et al., 2011; Pyron et al., et al., [2008] and Szyndlar, [2012] for discussion). 2013a,b). The majority of studies have recovered Other approaches include general similarity/ the same sister-taxon relationship between Acro- “gestalt” approaches (see Hecht and LaDuke, 1997 chordus and taxa traditionally included in Colu- for discussion), unsupported authority statements, broidea (but see Pyron et al., 2013a for an and the geographic provenance of extant taxa alternate hypothesis of sister-taxon relationships of (e.g., Holman, 2000[and references therein]), Acrochordus). which results in circularity in reconstructing tempo- The caenophidian fossil record has been ral and spatial histories of extant clades (Bell et al., more thoroughly documented than for any other 2004; Bell et al., 2010). The caenophidian record squamate clade, with a few Late Cretaceous to does not lack utility, however. Diagnostic vertebral early Paleogene records primarily, but not exclu- characters are observable (e.g., Hoffstetter and sively, from southern continental landmasses (e.g., Gasc, 1969; Slowinski, 1994; Sheil and Grant, Rage, 1975; Rage, 2008; Rage et al., 1992; Rage 2001, Head, 2005), and cranial remains are pre- and Werner, 1999; Rage et al., 2008; Rage et al., served for several key taxa. 2013; Parmley and Holman, 2003; Head et al., Here we provide fossil calibrations for multiple 2005) and hundreds of records from the late Paleo- caenophidian clades based on observable apo- gene to Holocene of mainly North America and morphies. We restrict calibrations to taxa repre- Europe (e.g., Rage, 1984[and references therein]; sented by either diagnostic cranial remains or Szyndlar, 1984, 1985, 1987, 1991a, 1991b, 2012; 2 PALAEO-ELECTRONICA.ORG vertebrae that can be diagnosed by discrete char- Russellophiidae on the basis of brief, ven- acters or unique character combinations. Using trolaterally angled articular facets of the this approach, we are only able to recognize 10 prezygapophyses and compressed prezy- extant caenophidian taxa relative to the many doz- gapophyseal buttresses that form a verti- ens recognized by the aforementioned methods, cal ridge, as well as more ambiguously by but our calibrations are testable observations that an elevated posterior neural arch (Rage allow analysis of systematic, ecological, and bio- and Werner, 1999). Characters that unite geographic hypotheses of extant caenophidians Russellophiidae, including Krebsophis, because their phylogenetic justifications are inde- with Caenophidia include a highly elongate pendent of the histories of those taxa. There are centrum, relatively small, circular cotyle two published comprehensive molecular phyloge- and condyle, and well-developed subcen- nies of Caenophidia that can be used as scaffolds tral paralymphatic channels defining the for node calibration (Pyron et al., 2013a, b). We lateral margins of a distinct haemel keel. use the topology of Pyron et al. (2013a) because it Minimum Age. 66 Ma, latest Maastrich- includes greater