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

Halmaheramys funderus sp. nov.

urn:lsid:zoobank.org:act:91C4586B-FC9F-4CDF-8239-02B75871E45A

Figs 7–8 View Figure 7 View Figure 8 , 9D View Figure 9 , Tables 1–2 View Table 1 View Table 2

Holotype: AM F101459 , a fragmentary left maxilla with M 1–3 in a moderate state of wear and preserving portions of the palatal side of the maxillary bone .

Paratypes: Among murine rodents, the upper molar row is almost always slightly longer than the associated lower molar row. The following lower jaws are associated with the holotype on the basis of cheektooth size and overall morphology: AM F101457 , a fragmentary right dentary with M 1–3 in a moderate state of wear and the basal portion of I 1 in the alveolus ; AM F101463 , a fragmentary right dentary with M 1–3 in a moderate state of wear; AM F101464, a fragmentary right dentary with M 1 in an advanced state of wear.

Type locality and age: Known only from the archaeological deposit in Cave no. 2 behind Daeo village, 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).

Diagnosis: Halmaheramys funderus is smaller than H. bellwoodi sp. nov. of Morotai, and is further distinguished from that species by the presence of a bilobed anterior lamina on M 1, more extreme reduction of anterolabial cusps on M 2–3 as well as by its less proodont lower incisor and less elongate mandibular diastema, leading to more steeply inclined lower incisor (together signifying a shorter rostrum).

Halmaheramys funderus of Morotai (M 1–3 crown length 8.8 mm) is distinguished from H. bokimekot of Halmahera by its much larger size (M 1–3 crown length 6.4 – 6.8 mm in H. bokimekot , n = 6), and from H. wallacei of Obi and Bisa by its somewhat larger size (M 1–3 crown length 7.8 – 8.4 mm in H. wallacei , n = 4). It further differs from both species in that all of its molar lophs are more laminar, more inclined antero-posteriorly, with less distinct cusps. The anterior loph of M 1 in particular is highly laminar relative to the extant species, the t3 being shifted well anteriorly and poorly defined. The labial cusp t4 both on M 1 and M 2 is antero-posteriorly developed with a distinct anterior inflection not seen in the two extant species of Halmaheramys . Compared to H. wallacei and H. bokimekot , the lower molars lack posterolabial cusplets, and the posterior cingular is smaller on M 1 and M 2.

Etymology: The species name is Latin for “to fuse”, in reference to the fusion of molar cusps into transverse laminae in the molars.

Description. The maxilla is known only for the holotype ( Figs 7–9 View Figure 7 View Figure 8 View Figure 9 ). This specimen retains part of its palatal side as well as all three molars at an early to moderate stage of occlusal wear. Although the anterior portion of the palatal lamina has also suffered damage, a small section of the premaxillary suture is preserved alongside the postero-lateral margin of the incisive foramen. This is positioned 4.4 mm forward of the anterior root of M 1. The midline and palatine sutures are also partially preserved; the latter runs very close alongside the lingual roots of the posterior molars and swings medially to join the midline suture level with the posterior end of M 1. The posterior end of the fragment preserves a small portion of the palatine suture, situated 1.5 mm behind the rear of M 3.

The maxilla of H. funderus is relatively gracile in construction. The alveolar portion of the maxilla is shallow, especially above the posterior molars, and the transition from the labial surface to the orbital surface is gently rounded rather than sharply angular as it is in species of Rattus and many other murines (see below for detailed comparisons).

The palatal lamina is thin and only slightly thickened along the midline suture. The palatine sulcus (that carries blood vessels and nerves between the posterior palatal and incisive foramina) is broad and well-defined posteriorly but it shallows and fades anteriorly, and is indistinct forward of cusp t1 of M 1. The incisive foramen penetrates 4.1 mm behind the premaxillary suture; its lateral margin is arcuate and it terminates in a broad V-shaped broad point, 1.1 mm forward of the anterior root of M 1. The position of the posterior palatal foramen is indeterminate.

Although the malar process of the maxilla is damaged, it is clear that the zygomatic plate was relatively narrow and gracile in form. The postero-medial border of the zygomatic plate is weakly developed; its anterior edge starts anterolateral of the forward root of M 1 and its posterior edge ends midway along the border of the incisive foramina.

The position of the maxillo-palatine suture relative to the molar row gives some indication of the degree of elongation of the palatal bridge. As indicated above, this suture lies 1.5 mm behind the molar row in H. funderus . In a specimen of R. halmaheraensis (AM M.23653) of approximately equal size it is located 1.9 mm behind the molar row.

A good impression of palatal dimensions and morphology in H. funderus is obtained by reflecting an image of the holotype along the midline suture ( Fig. 8 View Figure 8 ). This reveals a relatively narrow palate dominated by proportionally large molars set in weakly divergent rows. The incisive foramina are wide and bowed, and probably measured around 3.6 mm in combined width. The palatal width measured at the midloph of M 1 is 4.1 mm to the lingual side of the molars, 9.3 mm to the labial side of the molars. The relatively large molars of H. funderus are evident in comparison with the palate of a more typically proportioned rat such as Rattus rattus ( Fig. 8 View Figure 8 ).

The holotype retains all three molars in a state of slight to moderate wear ( Table 1 View Table 1 ; Fig. 9 View Figure 9 ). The first molar is considerably longer but only slightly wider than the second molar; the second molar is considerably longer but is considerably wider than the third. All molars show a moderate degree of longitudinal overlap.

The M 1 has five roots, one positioned anteriorly and two on each side of the tooth. The anterior root, supporting cusps t2 and t3, exceeds all others in bulk. The posterolabial root, supporting cusps t8 and t9, is next largest. The centrolabial, centrolingual, and posterolingual roots are subequal in size and support cusps t6, t1, and t4, respectively. The posterolabial root is positioned slightly behind the level of the posterolingual root.

The M 1 crown is relatively elongate and narrow, and is lamellate rather than cuspidate in form. The enamel is smooth where contact with food bolus has abraded the surface but coarsely punctate in more protected areas ( Fig. 9 View Figure 9 ). The cusp pattern is simple and follows the general murine pattern, with three primary cusps in each of the anterior and central laminae and two in the narrower, posterior lamina.

The anterior lamina is broadly arcuate with each of the labial (t3) and lingual (t1) cusps strongly united with the central cusp (t2). However, its occlusal surface is distinctly asymmetric, giving the impression that cusp t1 is positioned closer to cusp t2 than is cusp t3. Cusps t1 and t3 are positioned at a similar level relative to the front of the tooth; both cusps are rounded posteriorly, without accessory ridging.

The second lamina is slightly more cuspidate due to the presence on the anterior surface of the lamina of broad grooves between the central cusp (t5) and each of a labial cusp (t6) and a lingual cusp (t4). Cusps t4 and t6 are both slightly posterior to t5. Cusp t6 is subequal in size to t5 and rounded in occlusal shape. Cusp t4 is more elongate and angular in form due to a posterior extension, a distinctive feature. A slight indentation of the lingual margin of this structure suggests the presence of a partially discrete cusp (t4b) situated posterior to cusp t4 (see accounts of M 2, below).

The third lamina consists of a rounded central cusp (t8) and a smaller, rounded labial cusp (t9). These cusps are broadly united but clearly defined by a broad groove on the anterior surface of the lamina. Cusp t8 is slightly larger than cusp t5. Cusp t9 is smaller than cusp t6 but subequal to cusps t1 and t3. A weak enamel ridge ascends the lingual surface of cusp t8; it meets the posterior tip of cusp t4. A slight flexure of the posterior margin of the tooth between cusps t8 and t9 suggests a remnant of a posterior cingulum. The posterior surface of the tooth is indeed weakly grooved in this position.

The pattern of wear on M 1 and M 2 is stepped, wherein the occlusal surfaces of the laminae are angled relative to the overall occlusal plane of the molars.

The M 2 is shorter and slightly narrower than the M 1. It is supported by four separate roots, two on each side of the crown. The M 2 is shield-shaped in outline, tapering posteriorly; it is slightly longer than wide. The cusp pattern follows the typical murine pattern, cusps t1 and t3 representing isolated elements of the anterior lamina, followed by complete second and third laminae that mirror the construction seen in M 1. Cusp t1 of M 2 is a well-defined, sub-rounded cusp that lies on a common occlusal plane with cusp t8 of M 1; its anterior surface projects well forward of the anterior surface of cusp t5. In contrast, cusp t3 is a small structure that is closely adpressed to the anterolabial face of t5; it lies above the occlusal plane of the tooth. The central lamina on M 2 is broadly arcuate and almost symmetrical in structure. Cusp t5 is rounded but separated from the flanking cusps by broad grooves. Cusp t6 is similar in size to the equivalent cusp on M 1 but is more angular due to the presence of a weak posterior ridge. The lingual end of the anterior lamina is complex in form consisting of a small anterior cusp (t4a), that is broadly united to cusp t5, and a larger posterior cusp (t4b) that is adpressed against cusp t4 but separated from it by a deep lingual fissure and retains a complete enamel rim at occlusal level ( Fig. 9 View Figure 9 ). It is unclear if this structure reflects an unusual ridging, or is a discrete cusp, which would generally be identified as t7 by its position. However, it is clearly not the same as the structure labelled t7 in various other Asian murines such as species of Chiropodomys and Lenothrix (in these taxa cusp t7 is associated with the posterior lamina; Misonne, 1969; Musser, 1979; Musser & Newcomb, 1983); for purposes of discussion, we will refer to this unusual structure in H. funderus as an “accessory lingual cusp”.

The posterior lamina on M 2 is dominated by a cusp t8 that is narrower but slightly longer than its serial homologue on M 1. Cusp t9 is well-defined but considerably smaller than on M 1. The anterolingual ridge on t8 is weakly developed.

The M 3 is considerably shorter and narrower than the M 2. Three roots are present, two anteriorly and one supporting the posterior lamina of the tooth. Cusp t1 on M 3 is very similar in size, shape and relations to this cusp on M 1. Cusp t3 on M 3 is represented by a tiny tubercle above the occlusal surface. The central lamina on M 3 is an irregular structure tentatively made up of a relatively small central cusp t5, a narrow cusp t4 that extends lingually from t5, and a hook-like posterolabial extension from t5 that presumably represents t6. The posterior lamina of M 3 consists of a single rounded cusp that is sharply divided from cusp t4 but linked to the putative cusp t6 by a high enamel ridge that encloses a small posterolabial fosette.

The three dentaries represent different individuals with cheektooth wear ranging from moderate to advanced stages. All are damaged but collectively lack only the tip of the coronoid process, and details of the condylar and angular processes ( Fig. 7 View Figure 7 ). The horizontal ramus is small and lightly built relative to the size of teeth but shows developed muscular features involving deep masseter layers. Indeed, the masseteric crest is well developed, relatively straight and starting 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 absent or broken on all specimens. However, the orientation of the alveolus suggests a relatively steeply angled incisor and correspondingly short diastema. The symphyseal region is relatively slender, reflecting the overall gracility of the dentary. The ascending ramus rises to the level of the M 2 anterior lamina; it has a straight anterior margin that forms an angle of 140 o with the plane of the cheekteeth. Although the tip of the coronoid process is missing, the coronoid clearly rose above the level of the condylar notch (retained on one specimen). The incisor tubercle is situated below the coronoid process and condylar notch. The lower incisor is represented by a basal fragment embedded in AM F101457. This shows a tooth that is approximately 1.1 mm in width and slightly higher than wide. The enamel is orange, ungrooved and covers the ventral surface and the lower one-third of the labial surface of the tooth.

Two dentaries retain all three lower molars in a moderate state of wear ( Table 1 View Table 1 , Fig. 7 View Figure 7 ). The third specimen retains M 1 in a more advanced state of wear. The M 1–3 crown lengths are 8.2 and 8.4 mm, and M 1–3 alveolar length ranges from 8.4–8.6 mm. M 1 is longer and slightly narrower than M 2. M 3 is shorter and narrower than M 2. All molars are laminate and relatively brachyodont, with forwardly inclined laminae. As for the upper molars, the enamel is finely punctate apart from on thegotic facets. The pattern of wear on M 1 and M 2 is stepped, matching that observed in the upper molars.

The M 1 retains the usual murine arrangement of cusps, with an anterior group of cusps (collectively, the anteroconid), four primary cusps arranged in two more or less transverse laminae, and a distinct posterior cingulum. The tooth is relatively broad and chunky; it is broadest across the posterior lamina, narrowing to the front.All cusps are weakly bulbous towards the crown base. The pattern of roots below M 1 follows the pattern observed in Rattus and related genera, with one large circular root situated beneath the anterior cluster of cusps; two small circular roots are positioned on either side and near the rear of the second lamina; and one large, oval-shaped root is located below the posterior lamina. The central roots lie closer to the posterior than the anterior root. The anteroconid of M 1 is a short, broad structure that consists of two rounded cusps, positioned side by side and separated by a deep anterior groove. The anterolingual cuspid is slightly larger in occlusal area than the anterolabial cuspid (greater size disparity in AM F101463 than in AM F101457). The anteroconid is tightly adpressed against the second lamina such that the anterolabial and anterolingual flexids each penetrate less than one-quarter of the way across the tooth. The anterior lamina of M 1 is considerably broader than the anteroconid, which is a common characteristic of all Halmaheramys species. It is dumbbell shaped in occlusal outline, with concave anterior and posterior occlusal margins. The labial protoconid and lingual metaconid are sub-rectangular in occlusal outline and broadly united. The approximate boundary between the two cuspids is evident only from the concave posterior surface of the lamina; this suggests a slightly greater occlusal area for the protoconid over the metaconid. As in most Halmaheramys specimens, there is no trace of an accessory labial cusplet associated with either the anterior or middle laminae (but see AM M.24389). The posterior lamina of M 1 is slightly broader than the anterior lamina. It repeats the basic structure of the anterior lamina but with a straighter anterior occlusal margin. The labial and lingual cuspids (the hypoconid and entoconid, respectively) are subequal in occlusal area. There is no trace of a posterolabial cusplet associated with the posterior lamina. The posterior cingulum is a broad, oval shaped cusp. It is positioned low and centrally at the rear of the tooth and, unlike the primary cusps, is vertical rather than forward sloping. The occlusal surface of the posterior cingulum lies in the same occlusal plane as the anterior lamina of M 2.

The M 2 is square in basal outline and noticeably wider than the posterior lamina of M 1. The crown is more bulbous than M 1, particularly so on the labial side of the tooth. Five roots support the crown, two beneath the anterolabial corner of the tooth, and one beneath each of the three other corners. The divided anterolabial root is an unusual feature among murines. The arrangement of primary cusps on the M 2 is similar to that observed on the two laminae of M 1. The anterior lamina is notable for its breadth, being the widest element in the entire toothrow. The posterior lamina is substantially narrower and is surpassed in width by both the anterior and posterior laminae of M 1. Both laminae on M 2 have broadly concave posterior surfaces. The posterior cingulum of M 2 replicates the structure on M 1. Small but distinct anterolabial cusps are present on both examples of M 2. On the less-worn M 2 of AM F101457 this structure is represented by a tiny tubercle, attached to the anterior face and lying below the occlusal surface of the protoconid. On AM F101463 a small but distinct cusp is present in the same position. In this specimen it possesses a separate, functional dentine basin. However, further wear would soon see this basin merge into the anterolabial end of the anterior lamina. There is no trace of a posterolabial cusplet associated with the posterior lamina.

The M 3 is substantially shorter and narrower than M 2. The crown is lower than either anterior tooth. The anterior lamina on M 3 is more strongly folded than on the preceding tooth, reflecting a slight posterior expansion of the protoconid. The posterior lamina is a simple D-shaped structure, flattened anteriorly and concave posteriorly. A small anterolabial cusp is present on AM F101463, situated just below the occlusal surface. This cusp is absent on AM F101457. There is no trace of a posterolabial cusplet associated with the posterior lamina.