Zygodontomys brevicauda ( Allen & Chapman, 1893 )

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 : 445-450

Zygodontomys brevicauda ( Allen & Chapman, 1893)

Distribution. Recorded from the Grenada Bank from Troumassan–Troumassoid and Suazan–Troumassoid horizons (associated AMS dates, 600–1400 CE) at Sabazan, Carriacou, and Saladoid–Barrancoid to Suazan–Troumassoid horizons (associated AMS dates, 400–1650 CE) at Pearls, Grenada.

Material. Sabazan, Carriacou: left maxilla with M1–M3 ( SFU 0707 CSZ 000036-M-84), left maxilla with M3 ( SFU 0707 CSZ 000133-M-248), juvenile right maxilla with M3 ( SFU 0707 CSZ 000024-M-249), two zygomatic processes ( SFU 0707 CSZ 000036-M-82, 0707 CSZ 000101-M-136), three isolated M1s ( SFU 03 CAR 000235-M- 211, 0707 CSZ 000027-M-219, 0707 CSZ 000199-M-225), isolated M2 ( SFU 0707 CSZ 000215-M-134-B), isolated M3 ( SFU 0707 CSZ 000101-M-135), three upper incisors ( SFU 03 CAR 000235-M-205, 0707 CSZ 000164-M-234, 0707 CSZ 000241-M-235), left mandible with attached m1-m2 and associated m3 ( SFU 03 CAR 000235-M-191), right mandible with m1-m2 ( SFU 0707 CSZ 000085-M-41-A), right mandible with m3 ( SFU 0707 CSZ 000133-M- 247), right mandible fragment with m1 ( SFU 03 CAR 000235-M-213), three isolated m1s ( SFU 0707 CSZ 000086- M-100, 0707 CSZ 000114-M-106, 0707 CSZ 000042-M-215), four isolated m2s ( SFU 0707 CSZ 000215-M-134-A, 03 CAR 000235-M-198, 03 CAR 000235-M-208, 0707 CSZ 000207-M-230), two edentulous right mandibles ( SFU 0707 CSZ 000063-M-67-A1, 0707 CSZ 000063-M-67-A2), anterior fragment of left mandible ( SFU 03 CAR 000235- M-199), two posterior mandibular bodies ( SFU 03 CAR 000235-M-194, 0707 CSZ 000101-M-233), two distal humeri ( SFU 0707 CSZ 000086-M-98, 03 CAR 000235-M-203), five calcanea ( SFU 0707 CSZ 000064-M92, 0707 CSZ 000101-M-102, 0707 CSZ 000114-M-109, 0707 CSZ 000127-M-116, 03 CAR 000235-M-209) ( Figure 7 ). Pearls, Grenada: one right mandible with m1-m3 ( GNM A1-E-10-F1.S3), four left edentulous mandibles ( GNM A1-E-10-F1.S4, S5, S6, S9), two left femora ( GNM A1-E-10-F1.S11, S12), two right femora ( UF 164, GNM A1- E-10-F1.S10); one unsided femur ( GNM A1-E-10-F1.S14) ( Figure 8 ).

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

Cranium: Zygomatic plate broad, creating zygomatic notch (character 28: state 1). Posterior margin of zygomatic plate apparently situated anterior to alveolus of M1, but maxillary material fragmentary and broken in adult individuals so difficult to confirm (character 28: state 0?). Posterior margins of incisive foramina conspicuously projecting between procingula of M1 (character 31: state 0). Bony palate with shallow lateral longitudinally oriented troughs (character 33: state 0).

Upper dentition: Enamel band of upper incisors without distinct labial bevel (character 53: state 0). Molars bunodont and brachydont (character 54: state 0). Labial flexi closed off by cingula (character 55: state 0). Maxillary toothrows apparently parallel (character 56: state 0). M1 with three main roots; labial accessory root absent; small central accessory rootlet present (character 49: state 0). Flexi of M1 and M2 do not interpenetrate (character 57: state 0). Anterocone of M1 undivided, anteromedian flexus and internal fold of procingulum absent (character 58: state 2). Anteroloph and anteroflexus absent on M1 (character 59: state 3). Protostyle absent on M1 (character 60: state 0). Protocone and paracone of M1 forming single transversely oriented dentine basin with no enamel connection (character 61: state 2). Mesolophs absent on M1 and M2 (character 62: state 2). Median mure centrally located on M1, connected to both protocone and paracone (character 63: state 0). No unworn adult M2 or M3 available, so specific characters of unworn occlusal surfaces of these teeth not diagnosable.

Mandible: Coronoid process low, falciform and sharply curved backward; raised slightly above maximum height of evenly rounded condylar process. Sigmoid notch rounded, shallow and wide. Mandibular foramen narrow and elongated. Angular process damaged in all available specimens; with broad, deep masseteric fossa. Capsular process well-developed as conspicuous swelling slightly posteroventral to coronoid process (character 45: state 2). Ramus quite shallow. Mental foramen opens dorsolabially on diastema anterior to m1 (character 44: state 1). Superior and inferior masseteric ridges converge anteriorly and fuse as a single robust masseteric crest below m1 (character 46: state 1). Anterior edge of masseteric crest below anterior portion of m1 (character 47: state 0).

Lower dentition: m1 with two roots in total; labial and lingual accessory roots absent (character 50: state 0). m1 with undivided anteroconid and anteromedian fossettid present (character 70: state 1), and with anterolabial cingulum present (character 71: state 1). Ectolophid and ectostylid absent on m1 (character 72: state 0). Mesolophids absent on m1 and m2 (character 73: state 2). m2 and m3 with three roots, sometimes also with small central accessory rootlet between two anterior roots (character 51: state 1). Anterolabial cingulum present on m2 (character 74: state 0). Small metaflexid present, defining small anterolophid, on several m2 and m 3 specimens (character 75: state 1). Anterolabial margin of m3 with shelflike cingulum (character 76: state 0). Posteroflexid absent on m3 (character 77: state 2).

Postcrania: Entepicondylar foramen of humerus absent (character 82: state 1). Supratrochlear foramen of humerus present (character 83: state 1). Calcaneum shows gap between proximal edge of short trochlear process and posterior articular facet (character 84: state 1).

Craniodental measurements given in Tables 3–4. Body mass estimated as 63.4 g (Carriacou population), and 62.2 g ( Grenada population) ( Table 5 ).

Remarks. The small Grenada Bank oryzomyine is assigned to Zygodontomys because it shows the characteristic tetralophodont occlusal molar morphology of this genus (all other Caribbean endemic oryzomyine taxa show strongly pentalophodont molars), and the presence of the following diagnostic apomorphic character states for the genus, as recovered in phylogenetic analysis: M1 procingulum without anteroloph; mesolophs and mesolophids not present on molars; paracone-protocone with basined connection; posterior margins of incisive foramina projecting conspicuously between first molars. Specimens from Carriacou and Grenada show similar morphological characters and are closely comparable in toothrow measurements, and so are interpreted as representing the same taxon.

Zygodontomys is one of the most abundant mammal genera in mainland Central America and northern South America, where it is widely distributed across open vegetation formations (non-forest savanna-type habitats) ( Voss 1991; Bonvicino et al. 2009). The genus has also been recorded on several continental-shelf islands in the southern Caribbean. It occurs today on Trinidad, Tobago and some of their satellite islets, which are only ~ 125 km from Grenada today at their closest point, where it also inhabits lowland and montane rainforest ( Allen & Chapman 1893; Voss 1991). It is also known from the Late Quaternary fossil record of Aruba ( Hooijer 1967).

Seventeen nominal extant taxa have variously been assigned to Zygodontomys ( Cabrera 1961; Hershkovitz 1962; Honacki et al. 1982; Reig 1986), but only two species were recognized in the comprehensive morphological review by Voss (1991) and followed by subsequent authors: Z. brevicauda ( Allen and Chapman, 1893) , which occurs from southwest Costa Rica to northern Amazonia, and Z. brunneus Thomas, 1898 , which is restricted to Andean intermontane valleys in western Colombia ( Pardiñas et al. 2017). Extant insular populations from Trinidad, Tobago, and nearby continental-shelf islets represent Z. brevicauda , the type specimen of which was collected in Trinidad ( Allen & Chapman 1893; Voss 1991; Pardiñas et al. 2017), although some of these populations have also previously been assigned to the now-unrecognized taxa Akodon frustrator , Z. brevicauda tobagi , and Z. brevicauda soldadoensis ( Allen & Chapman 1897; Thomas 1900; Goodwin 1962, 1965). Late Quaternary material from Aruba was also assigned to Z. cf. brevicauda ( Hooijer 1967) . However, molecular phylogenetic analysis indicates that populations traditionally assigned to Z. brevicauda across northern Amazonia and continental-shelf Pacific islands show deep middle-upper Pleistocene divergences and probably represent multiple undescribed species ( Bonvicino et al. 2009; González et al. 2010).

The two currently recognized Zygodontomys species differ in size and in molar lingual profile and occlusal architecture ( Voss 1991). Where both species occur sympatrically, Z. brevicauda is much smaller than Z. brunneus , although some other populations of Z. brevicauda , in particular extant insular populations, are also relatively large and can exhibit comparable craniodental dimensions to Z. brunneus ( Figure 9 ). Upper molars of Z. brunneus are hypsodont, with narrow re-entrant folds extending two-thirds to three-quarters of the depth of the unworn crown in lingual view. Upper and lower molars in Z. brunneus also show an oblique arrangement of folds and crests, with anterior and median mures and murids not colinear along the tooth midline but instead disrupted by small anterior deflections of the lingual reentrant folds and by corresponding medial deflections of the interior labial folds. In contrast, upper molars of Z. brevicauda are low-crowned, with wide re-entrant folds usually extending only half the depth of the unworn crown in lingual view. Upper and lower molars in Z. brevicauda have colinear anterior and median mures and murids with principal cusps positioned symmetrically in opposite labial-lingual pairs, with M1 exhibiting labial re-entrant folds that bend abruptly around the anteromedial margin of the paracone and metacone, and lingual re-entrant folds that run straight from the lingual margin to the tooth midline ( Voss 1991).

Zygodontomys material from both Carriacou and Grenada cannot be differentiated morphologically from extant populations of Z. brevicauda on the basis of diagnostic molar lingual profile and occlusal architecture. Dental measurements for specimens from the Grenada Bank (occlusal upper toothrow length, M1 width) are larger than comparative measurement series available for most mainland populations of Z. brevicauda , but fall within the upper end of existing size variation seen in living insular populations, especially compared with the geographically nearby extant populations on Trinidad and Tobago which are also morphometrically divergent and substantially larger than adjacent mainland populations ( Voss 1991; Figure 9 ). No comparative measurements are available for Late Quaternary Zygodontomys material from Aruba, but this material also matches Z. brevicauda in molar morphology ( Hooijer 1967).

Three subspecies are currently recognized within Z. brevicauda : Z. b. brevicauda , Z. b. cherriei (Allen, 1895), and Z. b. microtinus (Thomas, 1894). These subspecies are differentiated largely by population-level frequencies of qualitative cranial traits related to stapedial arterial circulation and fenestration of the mesopterygoid fossa ( Voss 1991), characters that are not preserved in available Zygodontomys material from the Grenada Bank. However, the subspecies also differ in some cladistic character states recognised by Weksler (2006). The Grenada Bank Zygodontomys shares two synapomorphies with Z. b. cherriei to the exclusion of Z. b. brevicauda : (i) capsular process well-developed as conspicuous swelling slightly posteroventral to coronoid process (character 45: state 2); (ii) superior and inferior masseteric ridges converge anteriorly and fuse as single robust masseteric crest below m1 (character 46: state 1).

Figures

FIGURE 7. Zygodontomys brevicauda from Sabazan, Carriacou: a, SFU 03CAR000235-M-211, right M1; b, SFU 0707CSZ000199-M-225, right M1; c, SFU 0707CSZ000024-M-249, juvenile right maxilla with M2; d, SFU 0707CSZ000133- M-248, left maxilla with M3; e, SFU 0707CSZ000036-M-84, left maxilla with worn M1-M3; f, SFU 0707CSZ000086-M-100, left m1; g, SFU 0707CSZ000114-M-106, right m1; h, o, SFU 03CAR000235-M-191, left maxilla with attached m1-m2 and associated m3; i, SFU 0707CSZ000042-M-215, left m1; j, t, u, SFU 0707CSZ000063-M-67-A1, edentulous right mandible; k, r, s, SFU 0707CSZ000063-M-67-A2, edentulous right mandible; l, SFU 03CAR000235-M-194, posterior body of left mandible; m, SFU 03CAR000235-M-208, left m2; n, SFU 0707CSZ000207-M-230, right m2; p, SFU 03CAR000235-M-198, right m2; q, SFU 0707CSZ000215-M-134-A, right m2. Scale bars = 1 mm; upper scale bar for teeth, lower scale bar for maxillae and mandibles.

FIGURE 8. Zygodontomys brevicauda from Pearls, Grenada: a–b, GNM A1-E-10-F1.S3, right mandible with m1-m3; c–e, GNM A1-E-10-F1.S4, edentulous left mandible. Scale bars = 1 mm.

FIGURE 9. Comparison of measurements (means and ranges, in millimetres) for zooarchaeological Zygodontomys specimens from the Grenada Bank, and for extant Zygodontomys populations from mainland Central America and northern South America and Central and South American continental shelf islands, including all currently recognized species and subspecies. A, occlusal length of maxillary molar toothrow; B, breadth of M1 across the protocone-paracone cusp pair. Data for non-Grenada Bank Zygodontomys populations from Voss (1991).