LOXOCEMIDAE

Head, JJ, 2015, Fossil calibration dates for molecular phylogenetic analysis of snakes 1: Serpentes, Alethinophidia, Boidae, Pythonidae, Palaeontologia Electronica 18, pp. 1-17 : 9

LOXOCEMIDAE

Node Calibrated. Divergence between total clades Loxocemidae and Pythonidae .

Fossil Taxon. “ Ogmophis” compactus .

Specimen. PTRM 19378, left quadrate.

Phylogenetic Justification. PTRM 19378 is united with Loxocemidae on the basis of: 1) dorsal head expanded into broad plate; 2) dorsal head lies wholly in saggital plane; 3) well-defined lip present on lateral side of dorsal margin ( Smith, 2013).

Minimum Age. 35.2 Ma.

Soft Maximum Age. Indeterminate.

Age Justification. Justification follows the rationale for Charinidae above.

Discussion. Morphological phylogenetic analyses of snakes have traditionally recovered either monophyly of ( Loxocemus + Xenopeltis ) as the sister-taxon to all other macrostomatans (e.g., Lee and Scanlon, 2002; Scanlon, 2006), or as a paraphyletic grade at the base of Macrostomata ( Tchernov et al., 2000). Molecular phylogenetic analyses recover Loxocemus as the sister taxon of Pythonidae (e.g., Slowinski and Lawson, 2002; Pyron et al., 2013). Based on molecular topologies, PTRM 19378 constrains ( Loxocemidae + Pythonidae ) to no younger than late Paleogene. Smith (2013) associated isolated cranial elements including PTRM 19378 with vertebrae assigned to the vertebral form genus Ogmophis based primarily on relative abundances of elements collected by dry-screening and surface collection from a single locality, PTRM V89002 ( Smith, 2013). As with Calamagras (see above), the diagnosis ( Holman, 2000) used to assign specimens to Ogmophis compactus by Smith (2013) is problematic because it does not include any individual apomorphies or a unique character combination for either the genus or species, following descriptions of other taxa in Holman (2000). Thus, while the generic assignment of Smith (2013) is tentatively accepted for the purposes of identifying PTRM 19378 in snake phylogeny ( Figure 1 ), the taxonomic and systematic ambiguity of “ Ogmophis ”, requires restricting the first occurrence of Loxocemidae to cranial remains described in Smith (2013), and not to stratigraphically older published records of the genus ( Holman, 2000).

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).