phylogenetic relationships and classification of didelphid marsupials ...

52 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322
Fig. 21. Occlusal views of left upper molar
rows illustrating taxonomic differences in relative
widths of teeth (not drawn to the same scale). A,
Caluromysiops irrupta (FMNH 84426); B, Caluromys
philander (AMNH 267334); C, Glironia
venusta (AMNH 71395); D, Marmosa regina
(MVZ 190333); E, Marmosops impavidus (MUSM
13284); F, Lestodelphys halli (MMNH 15708).
have usually been called ‘‘predilambdodont’’
(e.g., by Reig et al., 1987). Unfortunately,
neither descriptor applies unambiguously to
some didelphids (Goin, 1997), and certain
taxa have been coded with contradictory
character states in different phylogenetic
datasets (e.g., Didelphis; see Reig et al.,
1987 [character 1]; Wroe et al., 2000 [character
10]; Wible et al., 2001 [character 31]). Our
observations agree with Johanson’s (1996),
that the shape (linearity versus labial inflection)
of this crest is correlated with its
occlusal relief (apical height above the trigon
basin), and we recognize an intermediate
condition for taxa that do not conform with
either traditionally recognized morphotype.
Among didelphids, only Caluromysiops
has a truly linear centrocrista on M1–M3.
In this taxon, the apex of the centrocrista is
essentially level with the floor of the trigon
basin, clearly conforming to the predilambdodont
condition defined by Johanson
(1996). By contrast, the centrocrista is
strongly inflected labially (buccally)—and
therefore distinctly ‘-shaped—in Chacodelphys,
Cryptonanus, Gracilinanus, Lestodelphys,
Marmosa, Marmosops, Metachirus,
Monodelphis, Thylamys, and Tlacuatzin. The
apex of the crest is elevated well above the
trigon floor in these taxa, which are unambiguously
dilambdodont sensu Johanson
(1996). The intermediate condition occurs in
the seven remaining genera (Caluromys,
Chironectes, Didelphis, Glironia, Hyladelphys,
Lutreolina, Philander) in which the centrocrista
has a weak labial inflection with a
slightly elevated apex; these taxa can appropriately
be described as weakly dilambdodont.
In many didelphids—Caluromys, Caluromysiops,
Glironia, Gracilinanus, Hyladelphys,
Marmosa, Tlacuatzin, and some species of
Cryptonanus and Marmosops—the preprotocrista
passes labially around the base of the
paracone to join with the anterolabial cingulum.
This results in the formation of a
continuous shelf along the anterior margin
of the tooth (fig. 21A–D), and taxa possessing
this feature are sometimes said to have a
‘‘complete’’ anterior cingulum (Archer,
1976b: 3) or to exhibit ‘‘double rank prevallum-postvallid
shearing’’ (Cifelli, 1993:
213). In the alternative morphology—exhibited
by Chacodelphys, Chironectes, Didelphis,
Lestodelphys, Lutreolina, Metachirus, Monodelphis,
Philander, Thylamys, and other
species of Cryptonanus and Marmosops—the
preprotocrista extends only to a point at or