phylogenetic relationships and classification of didelphid marsupials ...

110 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
and subequal 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) present, 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
moderately carnassialized (postmetacristae
longer than postprotocristae); relative
widths usually M1 , M2 , M3 , M4;
centrocrista strongly inflected labially on
M1–M3; ectoflexus usually absent on M1,
shallow on M2, deep and distinct on M3;
anterolabial cingulum continuous with preprotocrista
(complete anterior cingulum present)
on M3; postprotocrista without carnassial
notch. Last upper tooth to erupt is P3.
Lower incisors (i1–i4) with distinct lingual
cusps. Lower canine (c1) erect, acutely
pointed, and simple (without a posterior
accessory cusp). Second lower premolar (p2)
taller than p3; lower milk premolar (dp3)
with incomplete (usually bicuspid) trigonid.
Hypoconid labially salient on m3; hypoconulid
twinned with entoconid on m1–m3;
entoconid taller than hypoconulid on m1–
m2, usually subequal in height to hypoconulid
on m3.
DISTRIBUTION: Tlacuatzin is apparently
endemic to Mexico, where it occurs in
tropical dry forests from Sonora southward
(principally along the Pacific littoral and
adjacent slopes of the coastal cordilleras) to
Oaxaca and Chiapas; isolated populations
also occur in the northern part of the
Yucatan Peninsula and on the Tres Marías
Islands (Hall, 1981; Wilson, 1991; Reid, 1997;
Zarza et al., 2003).
Tribe Metachirini Hershkovitz, 1992
CONTENTS: Metachirus.
DIAGNOSIS: Metachirini is uniquely distinguished
from other members of the subfamily
Didelphinae (indeed, from all other didelphids)
by contact between the frontal and
squamosal on the lateral surface of the
braincase.
REMARKS: Although Metachirus could
logically be referred to the Didelphini, a
new name would then be needed for the clade
now awkwardly referred to in the literature
as the ‘‘large 2n 5 22 opossums’’ (Chironectes,
Didelphis, Lutreolina, Philander). Because
many morphological characters unambiguously
diagnose the latter group, whereas
only a few diagnose the group that includes
the large 2n 5 22 opossums plus Metachirus
(see appendix 5), the present arrangement
seems preferable. Additionally, because the
branch leading to Metachirus is a long one
(e.g., in fig. 33), we expect that fossils will
eventually be found to occupy it, and that
such discoveries will minimally disrupt the
suprageneric nomenclature if a tribe is
already available to accommodate them.
The name Metachirini was credited by
Gardner and Dagosto (2008) to Reig et al.
(1985), but no family-group name based on
Metachirus fulfills the technical criteria for
availability (ICZN, 1999: Article 13) prior to
Hershkovitz (1992b), who effectively diagnosed
Metachirinae in a key. By the Principle
of Coordination (ICZN, 1999: Article 36),
Metachirini is available with the same
authorship and date.
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