Halmaheramys bellwoodi, Aplin & Flannery & Boeadi & Fabre & Helgen, 2023

Halmaheramys bellwoodi sp. nov.

urn:lsid:zoobank.org:act:394DE26D-8BA9-4088-9849-13DBA0BFF45D

Figs 10–11 View Figure 10 View Figure 11 , Tables 1–2 View Table 1 View Table 2

Holotype: AM F101456 , a fragmentary left dentary with M 1–3 in a moderate state of wear and the basal portion of I 1 in the alveolus .

Paratypes: AM F101454 , a fragmentary right dentary with M 3 in a moderate state of wear ; AM F101455 , a fragmentary left dentary with M 1–2 in a moderate state of wear and the basal portion of I 1 in the alveolus ; AM F101458 , a fragmentary left dentary with I 1 and M 1 in an advanced state of wear ; AM F101461 , a fragmentary right dentary with M 3 in a moderate state of wear and the basal portion of I 1 in the alveolus ; AM F101462 , a fragmentary left dentary with M 3 in a moderate state of wear ; AM F101470 , a fragmentary left dentary with M 2 in a moderate state of wear ; AM F101471 , a right dentary with M 3 ; AM F101472 , an incisor ; AM F162028 , left dentary with M 1–3 in moderate state of wear .

Additional attributed specimens: An additional maxilla and mandible from the type locality figured by Hull et al. (2019: 145), attributed to “ Rattus morotaiensis ”, would appear to represent this species. We presume these specimens are stored at the Australian National University in Canberra.

Type locality and age: Known only from the archaeological deposit in Cave no. 2 behind the village of Daeo, on the southern side of Morotai Island , North Maluku (Maluku Utara) Province, Indonesia. The bulk of the remains are believed to date from terminal Pleistocene to mid-Holocene times ( Flannery et al., 1998; Hull et al., 2019). Hull et al. (2019) noted that “these sole cranial elements of rodent in the [Daeo Cave no. 2] assemblage are from Squares E4–E5 at 10–15 cm, and hence postdate the C14 date of 6463–6194 cal. BP (ANU 9452).”

Diagnosis: Halmaheramys bellwoodi is larger than other Halmaheramys species, with a M 1–3 alveolar length of 8.9 mm, versus a mean of 8.3 mm in H. funderus (8.2–8.4, n = 2), 8.0 mm in H. wallacei (7.1–8.3, n = 4), and 6.4 mm in H. bokimekot (see Table 2 View Table 2 ). Compared with H. bokimekot and H. wallacei , the molar lophids are more laminar, with less distinct cuspids. It is further distinguished from H. funderus by the presence of unicuspid anterior lamina on M 1, weakly indicated labial cusplets on M 1–3, and a more elongate mandibular diastema leading to a less steeply inclined lower incisor, signifying a longer rostrum.

Etymology: This species is named for Professor Peter R. Bellwood of the Australian National University, Canberra, in recognition of his seminal efforts in the study of Moluccan prehistory.

Description: The dentary is represented by eight specimens that collectively illustrate the entire mandibular morphology save for the tip of the coronoid process ( Fig. 11 View Figure 11 ). The most complete specimen is AM F101455 and the following description is based on this specimen unless indicated otherwise. The horizontal ramus is moderately large and robust relative to the size of teeth and shows strongly developed muscular features. The masseteric crest is well defined, relatively straight and terminates 1.5–2 mm behind the mental foramen. The anterior section of the crest, below M 1, is less prominent than the more posterior section below the rear molars and ascending ramus. The mental foramen is in the usual position, below and forward of M 1. The lower incisor is complete and in position in AM F101458 and AM F101470. It is oriented at a shallow angle and terminates level with the occlusal plane and 18.8 mm forward of the M 1. The symphyseal region is moderately robust, reflecting the overall condition of the dentary. The ascending ramus arises level with the midpoint of M 2; it has a straight anterior margin that forms an angle of 135° with the plane of the cheekteeth. Although the tip of the coronoid process is missing, this process clearly rose above the level of the condylar notch. The incisor proximal tubercle is situated below the coronoid process and condylar notch; it is a prominent structure and encloses a deep posterior zygomaticomandibularis fossa. The postalveolar foramen lies 5.3 mm behind the rear of M 3 and just above the occlusal plane of the cheekteeth. The articular condyle is unusually elongate compared with “typical” murines of similar jaw size (e.g., R. norvegicus ) and lies relatively close behind the mandibular foramen. The angular process, complete on AM F101461, is unusually broad and bears a conspicuous masseteric scar on its latero-ventral surface for the insertion of the posterior deep masseter muscle. The inner surface of the angular process bears a broadly concave medial pterygoid fossa that lacks conspicuous internal scarring. The mandibular foramen is located in the usual position, near the front of the internal pterygoid fossa. Complete lower incisors are retained in AM F101458 and AM F101470, with basal fragments embedded in several other dentaries. The complete incisors measure 2.4–2.6 mm in depth and 1.5–1.7 mm in width, and have radii of curvature of 17.0–18.5 mm. The tip of both complete incisors is accuminate rather than chisel-shaped as in most murines, and the occlusal surface is remarkably elongate, measuring 8.7–8.8 mm in length. The enamel is orange, ungrooved, and covers the ventral surface and the lower one-half of the labial surface of the tooth.

At least two examples are available for each of the lower molars, with varying stages of wear represented ( Table 1 View Table 1 ). The M 1–3 crown length is 8.9 mm, and M 1–3 alveolar length ranges from 9.2–9.6 mm. M 1 is longer and subequal in width to M 2. M 3 is shorter and narrower than M 2. Basic molar configurations mirror those described for H. funderus . The following account focuses on key points of difference between the two species. The M 1 is retained in three specimens. These differ from the M 1 of H. funderus in having anterior and posterior laminae of equal width, slightly less bulbous metaconids and entoconids, a more deeply folded anterior lamina, and an anteroconid formed of a single, oval-shaped cusp rather than the paired cusps seen in H. funderus . A small anterolabial cusplet is present on the anterior surface of the protoconid of AM F101456 and AM F101458; this structure is represented by a cingular crest on AM F101455. Posterolabial cusplets are indicated on all specimens by grooves on the anterolabial surface of the hypoconids. The root pattern of M 1 differs from that H. funderus in showing a broadening of the anterior root and its partial fusion with that positioned below the protoconid; the tip of the latter root remains separate ( Fig. 10 View Figure 10 ). Three specimens retain the M 2. Where both M 1 and M 2 are present, the latter tooth is equal in width to the posterior lamina of M 1, rather than broader than M 1 as in H. funderus . The crown is less bulbous than the M 2 of H. funderus and has a more deeply folded anterior lamina. There is no trace of an anterolabial cusp on any specimen. However, one specimen (AM F101456, the holotype) has a posterolabial cusplet defined by a weak groove on the outer surface of the hypoconid. Five specimens retain the M 3. This tooth is substantially shorter than M 2 but only slightly narrower. Compared with the condition in H. funderus , the anterior lamina of M 3 is less deeply folded. A small anterolabial cusp is present onAM F101461 but absent on all other specimens.