Crateromys ballik Ochoa & Mijares & Piper & Reyes & Heaney 2021, new species

Crateromys ballik, new species

Figs. 4D, 5D, 8, 9, and 10; Tables 1 and 3;

Supplementary Data SD2

Holotype.— PNM II-1977-I8-402 right dentary with m1–3; incisor and all posterior processes missing (Figs. 8A and 8B). The m1–3 are moderately worn (Fig. 5D), indicating that the animal was an adult at the time of death.

Type locality.— Dalan Serkot Cave, Barangay San Roque, Peñablanca Municipality, Cagayan Province, Luzon Island, Philippines (Fig. 1), 165 m elevation, 17°39 ′ 52.2 ″ N, 121°49 ′ 12 ″ E, Square 2, Layer 2 (aceramic layer, spit 24, approximately 1.2 m below surface).

Paratypes.— (n = 6; Supplementary Data SD2) Callao Cave: partial maxilla with left and right M1–3 (PNM II-1977-J3-10389; Fig. 10), partial right dentary with incisor fragment and m1–2 (PNM II-1977-J3-11075/2); Minori Cave: partial left dentary with m1–3 (PNM II-1977-J11A-17018), partial left dentary with m1–2 (PNM II-1977-J11A-16692), partial left dentary with m2–3 (PNM II-1977-J11A-896); Eme Cave: partial left dentary with m1–3 (PNM II-1977-G-292).

Age of specimens.— The holotype comes from an aceramic layer (Layer 2) of Dalan Serkot Cave, dated to 7,254–6,995 cal BP (Table 1); specimen 10389 derives from the Late Pleistocene levels of Callao Cave at least ca. 66,700 BP; specimen 11075/2 comes from Layer 11 of Callao which is older than ca. 30,000 cal BP and less than 52,600 BP; specimen 292 derives from an aceramic layer in Eme Cave dated to 3,985–3,702 cal BP; specimens 17018 and 16692 come from aceramic Holocene levels of Minori Cave (prior to the introduction of pottery on Luzon at 4,500–4,000 cal BP); specimen 896 derives from aceramicbearing level from Minori.

Etymology.— The new species name comes from the Dupaningan Agta language of NE Luzon, an adjective meaning “small” (see Reid 1994; Robinson 2011); we use it as an adjective in the nominative singular neuter.

Distribution.— Known from four cave sites in the Callao Cave Complex, Peñablanca Municipality, Cagayan Province (Fig. 1), as noted above.

Diagnosis.— A relatively small cloud rat of the genus Crateromys as diagnosed and defined by Musser and Gordon (1981), Musser et al. (1985), and Musser and Heaney (1992), distinctly smaller and less robust in comparison to the only living congener on Luzon, Cr. schadenbergi (Table 3; Fig. 9). Incisive foramina (if) extend nearly (about 0.5 mm anterior) to the anterior margin of the M1 alveolus (Fig. 10). Palate is narrow, and palatal grooves end about parallel to t1 on M1 (Fig. 10). Dentary is moderately robust relative to molar dimensions (Fig. 9A). Lower incisors proportionately narrow, 1.8 mm at midpoint. Upper toothrow length is 11.45 mm; lower toothrow length ranges from 10.22 to 12.91 mm. M1 length is 4.17 mm; M1 width is 3.27 mm. m1 length ranges from 4.2 to 5.41 mm; m1 width ranges from 2.79 to 3.34 mm (Table 3). Labial cusps (t3 and t6) of M1 and M2 are less prominent relative to t2 and t5. M1 and M 2 t 6 is directly fused with t5, notches are absent between t5 and t6. M1 anteroconid is not connected to the anterolabial– anterolingual loph.

Description and comparisons.— Overall, Cr. ballik is a small member of the genus Crateromys, smaller than Cr. schadenbergi and Cr. heaneyi, similar in size but a bit smaller on average than Cr. paulus, and smaller than Ca. dakal (Tables 2 and 3; Fig. 9). The dentary of Cr. ballik is less robust relative to its molar dimensions compared to Cr. schadenbergi and Cr. heaneyi (Fig. 9A), and less robust in absolute terms than Cr. paulus. The mental foramen of the dentary is situated at or slightly above the apex of the superior and inferior masseteric ridges on Cr. ballik (Fig. 8A), whereas it is placed high above the apex in Cr. paulus, situated on the dorsal margin of the dentary and anterior to m1, and is much lower on Cr. schadenbergi and Cr. heaneyi (see also Musser and Gordon 1981; Gonzales and Kennedy 1996; Reyes et al. 2017).

The lower incisor, as represented by Callao Cave paratype specimen 11072/2, is robust, orange-yellow on the anterior surface. The tip is missing, but at a place estimated to have been the midpoint, it is 1.8 mm wide, and from the broken tip to the anteriormost projection of bone along the medial edge is 3.7 mm. The dorsal wear surface stops ca. 1 mm above the beginning of the dentary bone. It appears that the tip would have extended about to the level of the occlusal surface of the mandibular molars. The lower incisor of Cr. schadenbergi is a slightly paler version of orange-yellow on the anterior surface. At its midpoint, the incisor averages about 2.2 mm wide and about 8.7–10.7 mm long (n = 3) from the tip to the anteriormost projection of bone along the medial edge. The dorsal wear surface ends ca. 2.9 mm above the beginning of the dentary bone along the medial edge, and the tip extends to about the level slightly below the occlusal plane of the molars. The lower incisor of Cr. heaneyi is similar in color to Cr. schadenbergi. At its midpoint, the incisor averages about 2.5 mm wide, which is wider than the generally larger Cr. schadenbergi, but we suggest caution in interpreting this as the two specimens available to us were raised in captivity. Figure 4 in Musser and Gordon (1981) shows the lower incisor of Cr. paulus as having a dorsal wear surface that stops about 2 mm above the anteriormost projection of bone along the medial edge, with the tip ending somewhat below the occlusal plane of the molars.

On our single maxillary of Cr. ballik (Callao Cave specimen 10389; Fig. 10), the incisive foramina (if) extend posteriorly to about 0.5 mm anterior to the anterior margin of the M1 alveolus. In two Cr. heaneyi this distance is 1.8 mm and 3.3 mm, and in six Cr. schadenbergi, the range is 1.6 to 3.3 mm, averaging 2.7 mm. In Cr. ballik 10389, the palatal bridge is narrow and longitudinally scored by deep medial palatal grooves. This trait is also seen in Cr. schadenbergi and appears to be a shared character of Luzon Crateromys, but the grooves in Cr. ballik 10389 end about parallel to t1 (Fig. 10), whereas in Cr. schadenbergi, the grooves flare laterally and become shallow anterior to M1. On other Crateromys species, the palatal bridge is relatively wider and the medial palatal grooves (pg) are shallower and less conspicuous (Musser et al. 1985; Gonzales and Kennedy 1996). The anterior root of the zygomatic arch (za) appears to originate lateral to t3 on M1 (Fig. 10).

The molar morphology of Cr. ballik differs from Batomys species on Luzon based on the traits listed above for the genus Crateromys. In comparison to the fossil Batomys described below, Cr. ballik is significantly larger, has higher crowns, and has a more robust dentary (Figs. 8 and 9A). Overall molar morphology follows that shown in Figs. 2C and 2D, except as noted below. Measurements of individual teeth are provided in Table 3. Among Luzon cloud rats, Cr. ballik is intermediate in size between Cr. schadenbergi and B. cagayanensis n. sp. (Fig. 9). In comparison to Cr. schadenbergi, crown length and width of Cr. ballik are smaller (Table 3; Fig. 9). Apart from absolute size of the teeth and the dentary, Cr. ballik differs from Cr. schadenbergi in its m1 configuration: the anteroconid is not connected to the second loph in Cr. ballik including in specimens with heavily worn teeth (Fig. 5D; Supplementary Data SD2A–D), whereas this is connected in Cr. schadenbergi (Fig. 5E).

In comparison to Cr. heaneyi, crown length and width of Cr. ballik also are smaller (Fig. 9; Table 3). Instead, tooth measurements of Cr. ballik overlap with the range of measurements published for fossil Cr. paulus (Fig. 9; Reyes et al. 2017). M1 and M2 of Cr. ballik are differentiated from Cr. paulus (see figure 6 in Musser and Gordon 1981) and Cr. heaneyi (Fig. 4F) by less prominent labial cusps (t3 and t6). The t 6 in M1 and M2 of Cr. paulus and Cr. heaneyi is separated by a relatively deep notch from t5, and this notch is absent in Cr. ballik. On the m3, the hypoconid and entoconid loph is chevronate on Cr. ballik and Cr. schadenbergi (i.e., both of the Luzon Crateromys), whereas it is fused in a nearly transverse loph in Cr. paulus and in Cr. heaneyi.