Eunectes

Head, JJ, 2015, Fossil calibration dates for molecular phylogenetic analysis of snakes 1: Serpentes, Alethinophidia, Boidae, Pythonidae, Palaeontologia Electronica 18, pp. 1-17 : 8

EUNECTES

Node Calibrated. Divergence between total clade Epicrates and Eunectes

Fossil Taxon. Eunectes stirtoni Hoffstetter and Rage, 1977 .

Specimen. MNHN, VIV 7, right prootic.

Phylogenetic Justification. Eunectes stirtoni was erected and united with Eunectes within Boinae on the basis of diagnostic morphology including a shortened posterior margin behind the posterior trigeminal nerve foramen; an incomplete anterior trigeminal nerve foramen, a prominent, long “prootic process” ( Tchernov et al., 2000) that forms the lateral edge of a narrow, well defined canal for the Vidian nerve ( Hoffstetter and Rage, 1977).

Minimum Age. 12.375 Ma.

Soft Maximum Age. Indeterminate.

Age Justification. The type specimen of E. stirtoni was recovered from the Fish Bed of the Baraya member of the lower Villavieja Formation in the upper Magdalena River valley in central Colombia ( Hoffstetter and Rage, 1977). 40 Ar/ 39 Ar dating of pumices from the underlying Monkey Beds and overlying San Francisco St. Beds of the Cerro Colorado Member of the Villavieja Formation constrains the ages of the Fish Bed to between 12.649 ± 0.258 and 12.111 ± 0.259 Ma ( Flynn et al., 1997). The Fish Bed directly overlies the Monkey Beds, and the minimum age estimate for the Monkey Beds, 12.486 ± 0.111 Ma ( Flynn et al., 1997), is used here as the minimum estimate for the Eunectes - Epicrates divergence.

Discussion. Morphology of the prootic and an associated basisphenoid referred to Eunectes stirtoni are very similar in general shape and discrete characters to extant E. murinus ( Hoffstetter and Rage, 1977) . Other fossils referred to Eunectes have been recovered from the middlelate Miocene of periequatoral South America, including La Venta in Colombia ( Hoffstetter and Rage, 1977; Hecht and LaDuke, 1997), Venezuela ( Head et al., 2006; Hsiou and Albino, 2010), and Brazil ( Hsiou and Albino, 2009, 2010). These records consist of isolated or associated precloacal vertebrae, and referral to genus is based on overall similarity instead of discrete apomorphies. The Villavieja Formation record of Eunectes provides a minimum divergence timing for both ( Eunectes + Epicrates ) and ( Chilabothrus +( Epicrates + Eunectes )) ( Reynolds et al., 2013). If E. stirtoni is most closely related to E. murinus , then the record described here minimally constrains divergence timings within the genus. Neogene fossil vertebrae have been compared with Epicrates ( Hsiou and Albino, 2010) , but this record cannot differentiate between that taxon and the newly recognized Chilabothrus, ( Reynolds et al., 2013) and is not considered for calibration.