Neurotrichus sp.

Oberg, Danielle E. & Samuels, Joshua X., 2022, Fossil moles from the Gray Fossil Site (Tennessee): Implications for diversification and evolution of North American Talpidae, Palaeontologia Electronica (a 33) 25 (3), pp. 1-39 : 16-19

Neurotrichus sp. Günther, 1880

Figure 8A - D

1858 Urotrichus Baird ; Baird, p. 76

1880 Neurotrichus Günther ; Günther, p. 440.

Referred specimens. ETMNH 4915 - left humerus, missing proximal end; ETMNH 10277 - left humerus, missing proximal end; ETMNH 9713 - left edentulous dentary; ETMNH 9728 - left posterior dentary fragment with m3; ETMNH 16023 - right m3; ETMNH 20737 - right dentary with m1-3; ETMNH 20741 - left dentary with m1-2; ETMNH 20862 - left dentary with m2; ETMNH 26996 - left dentary with m2; ETMNH 30380 - right dentary with m1.

Locality. Gray Fossil Site, TN.

Description. The dentary is thin and gracile with minimal curving along the inferior margin ( Figure 8A - B ; Table 7). The greatest point of curvature is below the talonid basin of m2 and the trigonid of m3. The anterior mental foramen is positioned under the p3 anterior alveolus (ETMNH 9713 and ETMNH 20741). There is a small gap between m3 and the ascending ramus. The angle between the ascending ramus and the horizontal ramus is approximately 90°.

The m1 trigonid is robust and elongate, with an anteriorly directed paraconid. The protoconid is the tallest cusp, and then metaconid, with the paraconid as the shortest cusp. The metaconid and

PALAEO- ELECTRONICA.ORG paraconid have wear on their buccal aspect. The paraconid has a bladed notch along its posterior aspect. In the m1, the talonid is buccolingually broad, and about the same size and height as the trigonid. The cristid obliqua terminates anteriorly (buccally) and is semi-separated by a small notch from the posterior wall of the trigonid. The entoconid and hypoconid are distinct. There is a small posterior accessory cusp (entostylid) on the posterior aspect of the entoconid. There is a strong hypoflexid between the hypoconid and protoconid. There is a well-formed entocristid between the entoconid and the metaconid. The postfossid is relatively deep, but not as deep as in the m2. The cingulids are not particularly prominent in the m1, with the precingulid, ectocingulid, and postcingulid present. and entocingulid absent. There is no mesoconid on any of the lower molars.

In the m2, the trigonid is taller than the talonid and anteroposteriorly smaller ( Figure 8B ). The protoconid is the tallest cusp of the trigonid, then metaconid, and the paraconid is the shortest cusp. The paraconid has a bladed notch along the posterior aspect. The talonid has a strong hypoconid and entoconid. The cristid obliqua is separated from the trigonid by a small notch. There is a tiny posterior accessory cuspid (entostylid) just posterior to the entoconid. There is a strong hypoflexid between the hypoconid and protoconid. The postfossid is very deep. An entocristid is present, but weak. Cingulids are similar to those of the m1, with precingulid, ectocingulid, and postcingulid present, but there is also a weak entocingulid

The m3 is broader anterioposteriorly than buccolingually ( Figure 8B ). The trigonid flares anterioposteriorly (paraconid and metaconid diverge from one another) making it appear very open. The paraconid has a bladed notch along the posterior aspect. The talonid is narrow and very open, and lower and smaller than the trigonid. There is a distinct hypoconid, and evidence for an entocristi in some specimens (not present in ETMNH 20737 but present in ETMNH 16023 and ETMNH 9728). The entoconid is large and has a well-formed posterior accessory cuspid (entostylid) present. There is no hypoconulid.

The humerus is quite gracile ( Figure 8C - D ); it is long and thin with minimal flaring projections on the distal end ( Table 7). The pectoral ridge narrows to a point about halfway down the diaphysis. ETMNH 10277 has a less prominent pectoral ridge present on the diaphysis than ETMNH 4915. The teres tubercle is long, relatively robust, and rather rectangular. In the anterior view, the olecranon fossa has a distinct concave notch at the base towards the center. The olecranon fossa is horseshoe shaped and asymmetrically slanted medially. In the posterior view, the trochlea is relatively dorsoventrally elongate and mediolaterally narrow. There is a groove in between the trochlea and the entepicondylar process along the distal-most aspect of the humerus. This gives the distal end of the humerus an asymmetrical appearance. The medial epicondyle is rather robust and probably had a large process on it. The entepicondylar foramen is visible just above the medial epicondyle and is quite large. The ectepicondylar foramen is very large. The entepicondylar process is a small projection sticking off the medial aspect. The capitulum is very small and oval-shaped.

Discussion. The teeth of shrew moles tend to be brachydont, and the upper molars have a small accessory cuspule (Skoczeń, 1980) that resembles and functions like a hypocone. The lower molars usually have a posterior accessory cuspule, like Desmanini talpids. The size of the accessory cuspule varies depending on the tooth and taxon, but it is quite large in the tribe Urotrichini . All shrew moles have a distinct bladed crest along the posterior aspect of the paraconid in all three lower molars.

Recent morphological analysis by Rzebik-Kowalska (2014) and Sansalone et al. (2016) have yielded recognition of three genera within Neurotrichini , the extant Neurotrichus and fossil Quyania and Rzebikia . The teeth, dentary, and humeri of these taxa show some clear similarities, but have a number of diagnostic differences (Rzebik-Kowalska, 2016; Sansalone et al., 2016). The known teeth and humeri of Rzebikia are similar in size to extant Neurotrichus , but larger than Quyania .

The genus Quyania is defined by having a gracile trigonid, mesoconid on m1, crista obliqua of the m1 terminate anteriorly (buccally) and separated by a small notch from the posterior wall of the trigonid, and anterior mental foramen lying under the p2 (Storch and Qui, 1983; Rzebik-Kowalska, 2014). The genus Rzebikia has a number of features intermediate between the other genera, and is defined, in part, by having reduced cingula on all three lower molars, variably present mesoconids, variable separation of the cristid obliqua from the trigonid via a small notch, variable anterior mental foramen position, and entoconids of both m1 and m2 robust and displaced lingually making the lingual side of the molars concave (Sansalone et al., 2016). In Neurotrichus , the m1 trigonid is more robust and anteroposteriorly elongate, the mesoconid is absent, the cristid obliqua is slightly separated from the trigonid without a notch, lower molar cingula are present, and the anterior mental foramen is under the anterior root of the p3. The upper and lower teeth of Rzebikia and Quyania are wider and lower-crowned than the teeth of Neurotrichus . The orientation of the ascending ramus of the dentary relative to the horizontal ramus also shows differences, with a 90° angle in both Neurotrichus and Rzebikia , and>90° angle in Quyania .

The humeral morphology of Rzebikia polonica resembles that of Quyania chowi , Neurotrichus gibbsii (American shrew mole), Urotrichus talpoides (Japanese shrew mole), and Dymecodon pilirostris (True’s shrew mole). All of these taxa have similar general morphology: a long, gracile humeral diaphysis with enlarged teres tubercle, reduced greater tuberosity, reduced lesser tuberosity that also projects medially, and deep bicipital groove (Sansalone et al., 2016); however, all species in Quyania have a small notch between the pectoral ridge and the teres tubercle (Storch and Qui, 1983), while the extant species do not have this feature. The humerus of R. polonica can be differentiated from that of Q. chowi based on a more robust shaft and a more prominent “scalopine ridge” (Rzebik-Kowalska, 2014; Sansalone et al., 2016).

Comparisons of these taxa by several authors (Storch and Qiu, 1983; Rzebik-Kowalska, 2014; Sansalone et al., 2016) suggest Rzebikia polonica fits in an ancestor - descendant relationship with Q. chowi and could belong to one phyletic lineage of Old World moles, though its affiliation with Neurotrichus or Quyania europaea is still uncertain (Skoczeń, 1980, 1993; Sansalone et al., 2016).

The GFS shrew mole described here has characteristics that clearly indicate it is a member of the Neurotrichini , and has features that allow referral to the genus Neurotrichus . Like both Quyania and Neurotrichus , cingula are present on all lower molars, though they are not strongly developed. The m1 trigonid is elongate with a relatively anteriorly directed paraconid, which is characteristic of Neurotrichus . The lower molars lack mesoconids, like Neurotrichus and some specimens of Rzebikia . The cristid obliqua of the m1 is semi-separate from the trigonid, with only a small notch, similar to some specimens of Rzebikia and Neurotrichus . Entoconids of the lower molars are relatively large, but they are not lingually displaced to yield a concave lingual surface of the tooth, as is typical of Rzebikia . The anterior mental foramen lies below the anterior root of the p3, as in Neurotrichus and some specimens of Rzebikia . The orientation of the ascending ramus in the GFS shrew mole (90° angle to horizontal ramus) is similar to that of both Neurotrichus and Rzebikia .

Based on this combination of characters, the GFS shrew mole is referred to Neurotrichus , though the fragmentary nature of the remains precludes more refined taxonomic assignment. The absence of a complete proximal humerus in the GFS shrew mole sample precludes direct comparison to some characteristics considered diagnostic of the other neurotrichine taxa (Sansalone et al., 2016).

Figures

FIGURE 8. Neurotrichus sp. skeletal material ETMNH 20737, right dentary with m1-3 - A) lingual and B) occlusal views. ETMNH 4915, left humerus - C) posterior and D) anterior views. Scale bars equal 1 mm.