ANILIIDAE

Head, JJ, 2015, Fossil calibration dates for molecular phylogenetic analysis of snakes 1: Serpentes, Alethinophidia, Boidae, Pythonidae, Palaeontologia Electronica 18, pp. 1-17 : 6

ANILIIDAE

Node Calibrated. Divergence between total clades Aniliidae and Tropidophiidae .

Fossil Taxon. Australophis anilioides Gómez et al., 2008 .

Specimen. MML-PV181, precloacal vertebra.

Phylogenetic Justification. Assignment to Aniliidae Stejneger, 1907 sensu MacDiarmid et al. (1999) is based on the following apomorphic character combination: low neural arch does not rise much above zygapophyseal plane, neural spine reduced, elongate prezygapophyses that are elongate and high-angled, concave posterior margin of neural arch ( Goméz et al., 2008).

Minimum Age. 72.1 Ma (Campanian-Maastrichtian boundary).

Soft Maximum Age. Indeterminate.

Age Justification. The Allen Formation is dated as late Campanian-Early Maastrichtian based on its stratigraphic position overlying the Anacleto Formation and underlying the Jagüel Formation ( Gómez et al., 2008). The Anacleto Formation is dated to late Campanian based on magnetostratigraphic correlation to Chron C33R (83.5-79.5 Ma) ( Dingus et al., 2000), whereas the Jagüel Formation is dated as mid-Maastrichtian to early Danian based on foraminiferal biostratigraphy ( Leanza et al., 2004; Goméz et al., 2008). The horizon within the Allen Formation that produced Australophis specimens is measured at 48 meters below the K- Pg boundary and is considered no younger than early Maastrichtian ( Goméz et al., 2008). The exact minimum age provided is for the Campanian- Maastrichtian boundary, which is likely to be slightly older than the actual age of the type specimen of Australophis .

Discussion. Molecular phylogenetic analyses consistently recover monophyly of Anilius with Tropidophiinae as the sister taxon to all other alethinophidians (e.g., Wilcox et al., 2002; Gower et al., 2005; Pyron et al., 2013), despite morphological support for tropidophiines being deeply nested macrostomatans and Anilius being nested within a monophyletic Anilioidea as sister taxon to South Asian Cylindrophis , Anomochilus , and Uropeltinae (e.g., Lee and Scanlon, 2002; Wilson et al., 2010; Gauthier et al., 2012). If future molecular studies corroborate morphological phylogenetic hypotheses then Australophis represents the oldest occurrence of crown group Anilioidea.

The fossil record of aniliids includes specimens from the Late Cretaceous through Cenozoic of South America and the Paleogene of North America (e.g., Hecht, 1959; Hoffstetter and Rage, 1977; Hecht and LaDuke, 1997; Rage, 1998; Head et al., 2006; Hsiou et al., 2010). Among named taxa, Australophis was considered morphologically most similar to extant Anilius and Paleogene Hoffstetterella from Brazil ( Goméz et al., 2008), however, the taxon shares a thickened zygosphene with Colombophis from the Miocene of Colombia and Venezuela ( Hoffstetter and Rage, 1977; Head et al., 2006; Hsiou et al., 2010). No explicit phylogenetic topology yet exists for Australophis relative to other aniliids.

Constraining the first occurrence of Tropidophiidae is especially problematic because the composition of the clade is controversial in addition to the aforementioned discrepancies between molecular and morphological phylogenetic hypotheses of the taxon within Alethinophidia. Fossil taxa have been assigned to Tropidophiidae based on size and general shape (e.g., Dunnophis Hecht, 1959 ; Falseryx Szyndlar and Rage, 2003 ). Attempts to devise morphological diagnoses for recognizing fossils have been based on an explicit use of a monophyletic Tropidophiidae consisting of extant (( Tropidophis + Trachyboa ) + Ungaliophiinae), following McDowell (1987). Both Molecular and morphological phylogenies have proposed paraphyly of this clade (e.g., Wilcox et al., 2002; Scanlon, 2006), however, and only a single distinct vertebral apomorphy, the presence of anteroposteriorly elongate, squared off ventral hypapophyses in precloacal vertebrae, occurs in Tropidophiidae = ( Tropidophis + Trachyboa ). The oldest record of the clade based on this character is Szyndlaria aureomontensis from the middle Eocene of Lissieu, France ( Augé and Rage, 2010). Lissieu is considered to correspond to the late Lutetian stage ( Augé and Rage, 2010), which is no younger than 41.2 Ma ( Vandenberghe et al., 2012). The late Lutetian is here used as the minimum first occurrence of crown Tropidophiidae ( Figure 1 ).

Figures

FIGURE 1. Time-calibrated phylogeny of Serpentes based on Pyron et al. (2013) and Reynolds et al. (2014). Numbers represent calibrated nodes in text. Fossil taxa are the calibration points for their respective nodes. Thickened bars represent well-sampled, taxonomically resolved stratigraphic distributions for the sister taxa to Pan-Serpentes and snake lineages whose first occurrences are determined by older sister taxa. Thin bars represent estimated stratigraphic distributions for taxa with poorly resolved fossil records, or no published fossil record (names in grey). “Scolecophidia” represents a paraphyletic grade with respect to Alethinophidia (see text).