Heterodon Latreille 1801

Genus HETERODON Latreille, 1801 (in Sonnini and Latreille, 1801) or PALEOHETERODON Holman, 1964 † Diagnosis. The trunk vertebrae of Heterodon and Paleoheterodon can exhibit a very depressed to slightly more vaulted neural arch (though relatively depressed overall compared to other dipsadids), a narrow neural spine that is longer than it is high, and a wide but flattened hemal keel (Holman, 2000). The vertebrae are slightly more long than wide, including the centrum, but still somewhat squarish appearance (Holman, 2000). The zygpophyseal articular facets are ovoid, and the prezygapophyseal accessory processes are moderately well-developed, end in somewhat obtuse points, and extend just beyond the prezygapophyseal articular facets (Holman, 2000; Mead and Steadman, 2017; Jurestovsky, 2021). The condyle and cotyle are mostly round but may be slightly dorso-ventrally compressed and flattened on the ventral side; both are similar in size to the neural canal (Holman, 2000). The synapophyses are divided into distinct diapophyses and parapophyses. Subcentral ridges are present adjacent to the hemal keel but are not prominent, the epizygapophyseal spines are absent, and hypapophyses are absent from post-cervical vertebrae (Holman, 2000). The zygosphene is variably crenate/convex or concave (Holman, 2000). Remarks. Though it is possible to identify trunk vertebrae to genus for Heterodon or possibly Paleoheterodon, individual vertebrae of this group are difficult to distinguish at the species level. They share most diagnostic features with the genus Farancia, but the vertebrae of Farancia generally exhibit greater anteroposterior compression (leading to a laterally wider appearance overall; Jurestovsky, 2021) a more vaulted neural arch, and a neural spine that is more deeply undercut both anteriorly and posteriorly than in Heterodon (Holman, 2000). Vertebrae associated with specimens previously assigned to Paleoheterodon may show the same differences from Farancia as Heterodon, except in the neural arch, which overlaps with both Farancia and Heterodon in how depressed/vaulted the shape of that region is (Holman, 2000; Parmley and Hunter, 2010; Head et al., 2016). Farancia and the Heterodon /Paleoheterodon group are both identifiable within Dipsadidae in possessing comparatively flattened and depressed neural arches (Head et al., 2016), longer than high neural spines (Jurestovsky, 2021) and wide, relatively flat hemal keels (Holman, 2000, Jurestovsky, 2021).