Heterodon Latreille, 1801

Jacisin Iii, John J. & Lawing, A. Michelle, 2024, Fossil snakes of the Penny Creek Local Fauna from Webster County, Nebraska, USA, and the first record of snakes from the Early Clarendonian (12.5 - 12 Ma) of North America, Palaeontologia Electronica (a 2) 27 (1), pp. 1-42 : 19

Heterodon Latreille in Sonnini and Latreille, 1801 or Paleoheterodon Holman, 1964 † sp. indet.

Figure 9

Material. UNSM 139985 (12 pre-cloacal trunk vertebrae).

Description. In dorsal view, the vertebrae are nearly square to slightly longer than wide. The anterior edge of zygosphene is convex, and the prezygapophyseal facets, when preserved, are ovoid in shape. The prezygapophyseal accessory processes are well-developed and the tips are moderately pointed to obtuse. Epizygapophyseal spines are absent. The postzygapophyseal accessory processes are ovoid in shape.

In anterior view, both articular facets are visible on the synapophyses. The cotyle is mostly round, but sometimes slightly taller than wide. The neural arch is depressed. The neural canal is similar in shape to a ventrally restricted semi-cylinder, even somewhat squarish, and similar in size to the cotyle. The cotyle is somewhat dorso-ventrally compressed, with well-excavated pits on either side.

In lateral view, the neural spine is longer than it is tall, somewhat depressed, and is typically more undercut posteriorly than anteriorly. In posterior view, the shape of the condyle is a dorso-ventrally compressed oval, and similar overall to the cotyle.

In ventral view, the hemal keel generally is weak, wide, and oblong, and very depressed to flat. In some specimens, the hemal keel is slightly more distinct, and is slightly constricted between the synapophyses, indicating a more anterior trunk position for these vertebrae. The subcentral ridges are indistinct. The centrum itself is longer than it is wide.

Remarks. The somewhat more vaulted nature of the neural arch in the specimens from Wt 13B when compared to extant Heterodon vertebrae (a morphological difference also shared with Farancia , as stated by Head et al., 2016) would previously have suggested that these fossils belong to or are comparable to Heterodon (or Paleoheterodon) tiheni , a species that is more easily determined based on skull morphology, rather than vertebral morphology ( Holman, 1964; Holman, 2000; Parmley and Hunter, 2010). However, Parmley and Hunter (2010) determined that this neural arch character showed considerable and overlapping variation in specimens assigned to Paleoheterodon and Heterodon , and that vertebral characters are not sufficient to differentiate between the two taxa. Unfortunately, we lack reproducible or statistical data on both the variation and the degree of overlap between these taxa, as both Holman (1977; 2000) and Parmley and Hunter (2010) included only personal observations on the topic. Furthermore, these vertebrae lack the apomorphies of the fossil species Heterodon meadi , which exhibits a zygosphenal groove and anteroposteriorly directed sculpturing on the neural arch, but lacks skull elements to clarify its association with Heterodon /Paleoheterodon ( Jurestovsky, 2021). Similarly, the unnamed but potentially different taxon from the Sappa Creek Fauna in Kansas also lacks cranial elements ( Holman et al., 2011). Because we presently do not have enough information regarding both vertebral and skull characters to confidently differentiate these taxa, and because these taxa overlap temporally (albeit in younger deposits; Parmley and Hunter, 2010), we describe it here as Heterodon or Paleoheterodon of an indeterminate species. Future work using quantitative and morphometric methods with isolated skeletal elements may potentially help untangle this taxonomic issue.

Figures

FIGURE 9. Middle trunk vertebra of Heterodon cf. Heterodon (Paleoheterodon) tiheni from the Penny Creek local fauna. From top left: dorsal, ventral, anterior, posterior, and lateral views (anterior to the left). Scale bar equals 1 mm.