Megalomys camerhogne, Mistretta & Giovas & Weksler & Turvey, 2021

Mistretta, Brittany A., Giovas, Christina M., Weksler, Marcelo & Turvey, Samuel T., 2021, Extinct insular oryzomyine rice rats (Rodentia: Sigmodontinae) from the Grenada Bank, southern Caribbean, Zootaxa 4951 (3), pp. 434-460 : 443-445

Megalomys camerhogne sp. nov.

Holotype. GNM A1 -E-10-F1.S2, left mandible with m1 ( Figure 5a–c , Figure 6a ).

Type locality. Pearls, pre-Columbian archaeological site (associated AMS dates, 400–1650 CE); Unit E (100–110 centimetres below datum), dated to Saladoid–Barrancoid period (200–750 CE); Saint Andrew, Grenada (12.14573, -61.613847) .

Hypodigm. Other studied material from Pearls: three edentulous right mandibles ( GNM A1-E-10-F1.S1, S7, S8); two isolated m1s ( GNM A1-E-10-F1.S28, GNM A1-E-10-F1.S 30); one isolated incisor ( GNM A1-E-10- F1.S24); one right humerus ( UF 164); two left radii ( GNM A1-E-10-F1.S15, S16); two right radii ( GNM A1-E-10- F1.S20, S29); one right ulna ( GNM A1-E-10-F1.S17); one left femur ( GNM A1-E-10-F1.S27); three right tibiae ( UF 164, GNM A1-E-10-F1.S18, S23); two right calcanea ( GNM A1-E-10-F1.S21, S22); one left ilium ( GNM A1- E-10-F1.S19); one right ilium ( UF 164); two caudal vertebrae ( GNM A1-E-10-F1.S25, S26) ( Figure 5 d-f).

Distribution. Known from the Holocene of Grenada. Only known so far from archaeological contexts at Pearls (Saladoid–Barrancoid to Suazan–Troumassoid periods, 400–1650 CE).

Etymology. From the Indigenous Caribbean (Arawakan) name for Grenada, in recognition of the island’s people and their heritage; neuter noun.

Diagnosis. Megalomys camerhogne sp. nov. can be distinguished from other described Megalomys species by the following combination of characters: m1 has four alveolar roots, m2 has three alveolar roots, and m3 has two alveolar roots; capsular process of lower incisor alveolus well-developed as conspicuous swollen projection; largest lower toothrow length and body mass estimate for any Megalomys species; posterior margin of mandibular symphysis extends posteroventrally beyond margin of ramus as prominent point; entepicondylar foramen and supratrochlear foramen both absent on humerus ( Table 1 ).

Description. Known only from incomplete skeletal material. Character numbering and character state coding from Weksler (2006).

Mandible: Robust, broad and high. Coronoid process gently falciform, raised slightly above maximum height of broad, squared condylar process. Sigmoid notch rounded, deep and wide. Mandibular foramen large and elongated; at base of condylar process.Angular process very broad, with gently curved outline and well-defined inferior masseteric ridge reaching distal margin; masseteric fossa broad and deep. Capsular process of lower incisor alveolus welldeveloped as conspicuous swollen projection (character 45: state 2). Mental foramen opens laterally to diastema, at body of mandible (character 44: state 0). Superior and inferior masseteric ridges converge anteriorly as open chevron, joining at their anteriormost point (character 46: state 0). Confluence of masseteric crest horizontally level with mental foramen, and vertically aligned with anterior portion of m1 (character 47: state 0). Posterior margin of mandibular symphysis extends posteroventrally beyond margin of ramus as prominent point.

Lower dentition: Molars bunodont and brachydont (character 54: state 0). Labial flexi closed off by cingula (character 55: state 0). First lower molar (m1) with two large roots and two accessory central roots (character 50: state 2). Anteroconid undivided; containing anteromedian fossettid (character 70: state 1). Anterolabial cingulum present (character 71: state 1); protoflexid narrowly compressed labially, with bifurcated apex, and obliquely mesiolinguad, almost reaching anteromedian fossettid. Hypoflexid transverse, narrow and deep. Small, low ectostylid present; ectolophid absent (character 72: state 0). Metaflexid short and narrow. Mesolophid and mesostylid present, connected to entoconid by lingual cingulum (character 73: state 0). Posteroflexid long and outlines entoconid. Metaconid and entoconid subquadrate, protoconid and hypoconid subtriangular.

Second and third lower molars (m2 and m3) not preserved in available material, but root condition can be diagnosed from alveolar configuration: m2 with three alveolar roots (character 51: state 1), and m3 with two alveolar roots.

Postcrania: Available appendicular skeletal elements large. Entepicondylar foramen of humerus absent (character 82: state 1). Supratrochlear foramen of humerus absent (character 83: state 0). Femur has well-developed flattened lateral crest that extends almost halfway along diaphysis. Calcaneum shows gap between proximal edge of short trochlear process and posterior articular facet (character 84: state 1).

Craniodental measurements given in Table 2 . Mean body mass estimated as 348 g ( Table 5 ).

Remarks. The large Grenada rice rat can be assigned confidently to Megalomys based on the following characters: mental foramen lateral to diastema; lower incisor alveolus with well-developed capsular process; superior and inferior masseteric ridges join at their anteriormost point as an open chevron; confluence of masseteric crest horizontally level with mental foramen and vertically aligned with anterior portion of m1. In comparison to other rice rat genera from the main island chain of the Lesser Antilles, it differs from Pennatomys in possessing superior and inferior masseteric ridges that join at their anteriormost point as an open chevron (the anterior portion of these ridges are conjoined as a single ridge in Pennatomys ) and in lacking a supratrochlear foramen on the humerus, and it differs from Antillomys in possessing a greater number of roots on m1 and m2 (only two roots are present on both of these teeth in Antillomys ).

The Grenada Megalomys differs from all previously described Megalomys species in both morphological characteristics and available measurements. It differs morphologically in possessing four roots on m1 (three roots on m 1 in M. audreyae ); in possessing three roots on m2 and two roots on m3 (three roots on both m2 and m 3 in M. desmarestii and M. luciae ; two roots on both m2 and m 3 in ‘ M.’ curazensis and M. georginae ), and in lacking both an entepicondylar foramen and a supratrochlear foramen on the humerus (foramina absent in M. desmarestii ; present in ‘ M.’ curazensis and M. georginae ; condition unknown in M. luciae ). It is also the largest known representative of Megalomys , with the greatest available measurement for lower molar toothrow length in any other Megalomys species ( M. desmarestii , 10.35 mm; Turvey et al. 2012) corresponding to the lower end of the available measurement range for M. camerhogne (10.35–10.67 mm; Table 2 ).

Figures

FIGURE 5. Megalomys camerhogne sp. nov. from Pearls, Grenada: a–c, GNM A1-E-10-F1.S2, left mandible with m1 (holotype); d–f, GNM A1-E-10-F1.S1, edentulous right mandible. Left scale bar = 2 mm, right scale bar = 5 mm.

FIGURE 6. Comparison of m1 morphology between Megalomys species from the Lesser Antilles: a, Megalomys camerhogne sp. nov., GNM A1-E-10-F1.S2, left m1 (holotype); b, Megalomys desmarestii, NHMUK 55.12.24.201, right m1. Scale bars = 1 mm.