Neonatrix magna Holman, 1982

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 : 22-24

Neonatrix magna Holman, 1982 †

Figure 11

Material. UNSM 139987 (8 pre-cloacal trunk vertebrae).

Description. The diagnosis is based on that of Holman (2000) but is modified where noted. The vertebrae are mainly distinguished by (1) a very poorly developed hypapophysis that typically terminates near the posterior part of the centrum; (2) ventral portion of the hypapophysis beveled and ending well anterior to the condyle; (3) vertebral size relatively large (at least relative to other North American Neonatrix at> 5 mm); (4) vertebrae nearly the same width and length; and (5) neural spine near same height and length.

In anterior view, the cotyle is round and similar in size to the neural canal, which is an inverted Ushape with slightly convex lateral sides. The zygosphene is dorsally straight. The prezygapophyses are slightly tilted dorsally. Short excavations with single, centered paracotylar foramen border each side of the cotyle. We observe that the synapophyses that are missing from previous descriptions are moderately well developed and differentiated.

In dorsal view, the centrum’s length and width are similar. The prezygapophyses are ovoid. The neural spine is somewhat thick and slightly overhangs posteriorly when preserved. Epizygapophyseal spines are absent. The zygosphene is slightly convex anteriorly. The diapophyses are produced laterally enough to be seen from the dorsal orientation.

In lateral view, the vertebra is only slightly elongate. The neural spines are not completely preserved but appear as though they are similar in height and length. There is a slight overhang on the posterior border of the neural spine. The hypapophysis is beveled ventrally, about as tall as it is long, and ends anterior to the condyle, but appears to have rounded rather than the pointed tips observed by Holman (1982; 2000).

In posterior view, the neural arch is significantly vaulted. We observed that the condyle is mostly round and slightly compressed, rather than truly oval, and is similar in size to the neural canal. The hypapophyses strongly projects ventrally and is clearly visible below the condyle.

In ventral view, the hypapophysis is somewhat narrow, ends clearly anterior to the anterior end of the condyle, and is beveled on the ventral surface. The subcentral ridges are well developed.

Remarks. Neonatrix magna has the shortest relative length and widest appearance, the highest neural spine, and most anteriorly terminating hypapophyses of any species of Neonatrix in North America or Europe. It also has somewhat less strongly developed subcentral ridges and deep subcentral groups than N. elongata or N. infera .

Several of our observations contribute to or differ slightly from previous descriptions of the species. We observe that the synapophyses are moderately developed. The hypapophyses of our specimens have rounded rather than pointed tips, and the condyle, while slightly depressed, is better described as round rather than oval. Comparison with specimens of N. elongata and examination of intracolumnar differences in vertebral morphology in extant natricid snakes suggest that the hypapophysis does change shape along the column; however, the differences in the hypapophysis alone between the two species described from fossils seems to be greater than generally seen in other natricids, except between anterior or middle trunk vertebrae and precloacal vertebrae. Given other morphological differences discussed above and the observation that N. magna is not smaller but would appear to have a morphology more similar to the smaller posterior trunk and precloacal vertebrae, we do not believe that these vertebrae represent the same species. Nonetheless, little is known about the intracolumnar variation of Neonatrix at this time, as preserved elements are mainly trunk vertebrae; this would seem to warrant a more complete investigation, as also suggested by Jasinski and Moscato (2017).

Figures

FIGURE 11. Trunk vertebra of Neonatrix magna 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.