phylogenetic relationships and classification of didelphid marsupials ...

86 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
incomplete taxon sampling (Horovitz, 1999);
this notion provides some justification for
naming monotypic higher taxa to accommodate
future discoveries of new forms. Point
(5) acknowledges the necessity for widely
accepted rules to govern the use of technical
names.
Because detailed generic synonymies have
recently been published by Gardner (2008),
we do not provide them here. However, a
chronological list of available names with
information about type species and current
status (table 15) contains most of the information
needed to understand current binomial
usage. Under the indented heading
‘‘Contents’’ we list all of the valid lower-level
taxa included by each higher-level taxon; for
example, the genera referred to a tribe, or the
species referred to a genus. In lists of
congeneric species, those currently regarded
as valid are in boldface, followed by an
alphabetic list of available junior synonyms
in parentheses; however, names of doubtful
application (e.g., brasiliensis Liais, 1872,
which might be a junior synonym of more
than one currently recognized species of
Didelphis; see Cerqueira and Tribe, 2008)
are not included.
Under ‘‘Diagnosis’’ we list unique combinations
of traits that distinguish members of
suprageneric taxa from other marsupials,
whether or not such traits are unambiguously
assignable by parsimony as relevant synapomorphies
(listed in appendix 5). Under
‘‘Morphological description’’ (for genera)
we list all phenotypic descriptors that apply
to included species; traits of particular
importance for identification are indicated
by italics. Ranges of metrical traits (e.g., 170–
210 mm) are approximations that match or
slightly exceed known minimal and maximal
adult values that we believe to be reliable
based on published and unpublished sources.
Except as noted otherwise, all descriptions of
cranial traits are based on adult specimens,
and all dental descriptions are based on
unworn teeth.
Under ‘‘Remarks’’ we discuss evidence for
monophyly and other issues that relate to
taxon recognition and rank. Although ranks
are biologically arbitrary, they affect spelling
and information retrieval in the prevailing
Linnaean system, and they are regulated by
widely accepted rules of biological nomenclature.
The lack of species-level revisionary
studies for most genera is also discussed
under this heading.
Family Didelphidae Gray, 1821
CONTENTS: Glironiinae, Caluromyinae,
Hyladelphinae, and Didelphinae.
DIAGNOSIS: Didelphids can be distinguished
unambiguously from other marsupials
by their soft (nonspinous) body pelage;
five subequal claw-bearing manual digits; five
separate pedal digits (loosely connected by
webbing in Chironectes), of which the hallux
(dI) is large and opposable and the second
(dII) bears an asymmetrical grooming claw;
inguinal cloaca; prehensile tail (secondarily
lacking external evidence of prehensility in
some taxa); long nasal bones (extending
anteriorly beyond the facial processes of the
premaxillae and posteriorly between the
lacrimals); unfenestrated rostrum; uninflected
maxillary-jugal suture; distinct foramen
rotundum (not confluent or sharing a common
vestibule with the sphenorbital fissure);
alisphenoid-parietal contact (except in Metachirus);
large interparietal fused to the
supraoccipital; complete posterolateral palatal
foramina; deeply divided vomer that does
not conceal the presphenoid or extend
posteriorly into the mesopterygoid fossa;
lack of midline contact between the left and
right pterygoids; unkeeled basisphenoid;
well-developed tympanic process of the alisphenoid;
well-developed rostral tympanic
process of the petrosal; lack of a squamosal
epitympanic sinus; laterally exposed ectotympanic;
unspecialized malleus (with long
sharply inflected neck, small orbicular
apophysis, and well-developed lamina); distinct
mastoid and paroccipital processes;
unreduced dental complement of 50 teeth;
nongliriform incisors; large milk premolars
(except in Hyladelphys); tribosphenic upper
molars, each with a continuous stylar shelf,
uninterrupted centrocrista, and reduced or
absent para- and metaconules; and tribosphenic
lower molars, each with a welldeveloped
anterior cingulid (notched for the
hypoconulid of the preceding tooth), notched
paracristids, and without any trace of a
posterior cingulid.