Metachirus Burmeister, 1854

Voss, RS & Jansa, SA, 2009, Phylogenetic Relationships And Classification Of Didelphid Marsupials, An Extant Radiation Of New World Metatherian Mammals, Bulletin of the American Museum of Natural History 2009 (322), pp. 1-177 : 110-113

Metachirus Burmeister, 1854 Figure 44

CONTENTS: nudicaudatus E. Geoffroy, 1803 (including antioquiae J.A. Allen, 1916; colombianus J.A. Allen, 1900; bolivianus J.A. Allen, 1901; dentaneus Goldman, 1912; imbutus Thomas, 1923; infuscus Thomas, 1923; modestus Thomas, 1923; myosuros Temminck, 1824; personatus Miranda-Ribeiro, 1936; phaeurus Thomas, 1901; and tschudii J.A. Allen, 1900).

MORPHOLOGICAL DESCRIPTION: Combined length of adult head and body ca. 250– 300 mm; adult weight ca. 250–500 g. Rhinarium with one ventrolateral groove on each side of median sulcus; dark circumocular mask present, continuous with dark coronal fur; pale supraocular spot present; dark midrostral stripe absent; throat gland present in adult males. Dorsal pelage unpatterned, usually some shade of grayish or reddish brown; dorsal underfur gray; dorsal guard hairs short and inconspicuous; ventral fur self-whitish, -cream, or -buffy. Manus mesaxonic (dIII. dIV); manual claws shorter than fleshy apical pads of digits; dermatoglyph-bearing manual plantar pads present; central palmar epithelium smooth or sparsely tuberculate; carpal tubercles absent. Pedal digits unwebbed; dIV longer than other pedal digits; plantar surface of heel naked. Pouch absent; mammae 4–1–4 5 9, all abdominalinguinal; cloaca present. Tail longer than combined length of head and body, slender and muscular (not incrassate), unfurred except at base; naked caudal integument bicolored basally (brownish or grayish dorsally, paler ventrally), gradually becoming all pale distally; caudal scales in both annular and spiral series (neither pattern predominating), each scale with three subequal bristlelike hairs emerging from distal margin; ventral caudal surface not externally modified for prehension.

Premaxillary rostral process absent. Nasals long, extending anteriorly above or beyond I1 (concealing nasal orifice from dorsal view), and conspicuously widened posteriorly near maxillary-frontal suture. Maxillary turbinals elaborately branched. Two lacrimal foramina laterally exposed on each side on or just anterior to orbital margin. Interorbital region very broad, with distinctly beaded supraorbital margins; postorbital processes absent. Left and right frontals and parietals separated by persistent median sutures. Frontal and squamosal in contact on lateral braincase (no parietal-alisphenoid contact). Sagittal crest absent or weakly developed just anterior to occiput. Petrosal not exposed laterally through fenestra in parietal-squamosal suture (fenestra absent). Parietal-mastoid contact normally present (interparietal usually does not contact squamosal).

Maxillopalatine fenestrae present; palatine and maxillary fenestrae absent; posterolateral palatal foramina small, not extending anteriorly between M4 protocones; posterior palatal morphology conforms to Didelphis morphotype (with well-developed lateral corners, the choanae constricted behind). Maxillary and alisphenoid usually not in contact on floor of orbit (separated by palatine in most examined specimens). Transverse canal foramen present. Alisphenoid tympanic process laterally compressed (not globular), with anteromedial process enclosing extracranial course of mandibular nerve (secondary foramen ovale present), and not in contact with rostral tympanic process of petrosal. Anteri- or limb of ectotympanic suspended directly from basicranium. Stapes triangular with large obturator foramen. Fenestra cochleae usually exposed, not or incompletely concealed by rostral and caudal tympanic processes of petrosal. Paroccipital process large, erect, projecting ventrally (not adnate to petrosal). Dorsal margin of foramen magnum bordered by exoccipitals only, incisura occipitalis absent.

Two mental foramina present on lateral surface of each hemimandible; angular process acute and strongly inflected.

Unworn crowns of I2–I5 symmetrically rhomboidal (‘‘premolariform’’), with subequal anterior and posterior cutting edges, and increasing in length (mesiodistal dimension) from I2 to I5. Upper canine (C1) alveolus in premaxillary-maxillary suture; C1 simple, without accessory cusps. First upper premolar (P1) smaller than posterior premolars but well formed and not vestigial; second and third upper premolars (P2 and P3) subequal in height; P3 with posterior cutting edge only; upper milk premolar (dP3) large and molariform. Molars highly carnassialized (postmetacristae much longer than postprotocristae); relative widths M1, M2, M3. M4 or M1, M2, M3, M4; centrocrista strongly inflected labially on M1–M3; ectoflexus usually shallow on M1 and M2 but distinct on M3; anterolabial cingulum and preprotocrista usually discontinuous (anterior cingulum usually incomplete)28 on M3; postprotocrista without carnassial notch. Third upper premolar (P3) and M4 erupt simultaneously.

Lower incisors (i1–i4) with distinct lingual cusps. Lower canine (c1) procumbent, with flattened bladelike anterior margin, but usually without a distinct posterior accessory cusp. Second lower premolar (p2) taller than p3; lower milk premolar (dp3) trigonid complete (tricuspid). Hypoconid labially salient on m3; hypoconulid twinned with entoconid on m1–m3; entoconid much taller than hypoconulid on m1–m3.

DISTRIBUTION: Metachirus is widely distributed in humid lowland and lower montane forests (usually below 2000 m) from Chiapas, Mexico ( Medellín et al., 1992) to northern Argentina ( Flores et al., 2007). Published distribution maps (e.g., Reid,

28 Because this character is normally quite constant within species, it is noteworthy that one specimen from Paracou, French Guiana (AMNH 266451), has a complete anterior cingulum whereas others from the same locality (e.g., AMNH 267362) have an incomplete anterior cingulum.

1997; Gardner and Dagosto, 2008) indicate several range disjunctions, but some of these may be artifacts of inadequate collecting effort in suitable habitats.

REMARKS: Although only a single species of Metachirus is currently regarded as valid, recent mtDNA sequencing studies have revealed deep molecular divergence (.10% at the mitochondrial cytochrome b locus; Patton et al., 2000; Patton and Costa, 2003) among supposedly conspecific geographic populations, and it seems likely that several of the names currently listed as junior synonyms of M. nudicaudatus will be resurrected as valid species by future revisionary studies.