Didelphidae Gray, 1821

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 : 86-88

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; uninflect- ed 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.

Didelphids have often been compared with dasyurids, but the literature contains no adequate statement of the many characters that distinguish these superficially similar yet highly divergent clades. Among the external and craniodental features treated in this report, dasyurids consistently differ from didelphids by their small, nonopposable hallux; lack of a grooming claw on pedal digit II; a nonprehensile tail (provided with a terminal tuft of hairs that is never present in opossums); anteriorly truncated and medially notched nasals; a small interparietal that, when present in juvenile skulls, is suturally distinct from the supraoccipital; incomplete posterolateral palatal foramina; an undivided vomer that extends posteriorly to underlie the presphenoid within the mesopterygoid fossa; well-developed squamosal epitympanic sinuses; four upper and three lower incisors; a distinct posterior lobe on the unworn cutting edge of i3; vestigial milk premolars; and a distinct posterior cingulid on the lower molars.

Originally described as a didelphid by Thomas (1894) and long maintained in that family by subsequent authors (e.g., Simpson, 1945), Dromiciops differs from opossums by its basicaudal cloaca; a foramen rotundum recessed in a common vestibule with the sphenorbital fissure; frontal-squamosal contact; a large interparietal that is suturally distinct from the supraoccipital; an undivided vomer that underlies the presphenoid within the mesopterygoid fossa; left and right pterygoid bones that contact one another in the midline; keeled basisphenoid; a concealed ectotympanic; a specialized malleus (with a short uninflected neck, no orbicular apophysis, and no lamina); lack of a paroccipital process; a discontinuous stylar shelf on M1; an unstaggered i2 alveolus; and a vestigial anterior cingulid on the lower molars (of which m1 lacks a hypoconulid notch).

Caenolestids, peramelids, and other marsupials are sufficiently distinct from opossums that explicit comparisons would be pointless here. Some stem metatherians (e.g., herpetotheriids) that are strikingly similar to didelphids in most respects (including ear morphology; Gabbert, 1998) differ from didelphids by having a distinct posterior cingulid on the lower molars in addition to the diagnostic endocranial and postcranial features described by Sánchez-Villagra et al. (2007).23

REMARKS: A wide range of fossil taxa have at one time or another been regarded as didelphids (e.g., by Simpson, 1935, 1945; Clemens, 1979; Marshall, 1981; McKenna and Bell, 1997), but phylogenetic analyses suggest that most of the extinct forms once thought to be closely related to Recent opossums (e.g., † Alphadon , † Andinodelphys , † Glasbius , † Herpetotherium , † Jaskhadelphys , † Pediomys ) are stem metatherians and not members of the crown group Marsupialia ( Rougier et al., 1998; Wible et al., 2001; Luo et al., 2003; Sánchez-Villagra et al., 2007). Herein we explicitly restrict Didelphidae to living didelphimorphians, their most recent common ancestor, and all of its descendants.

Even more restrictive concepts of Didelphidae have been proposed, but none is widely accepted. Hershkovitz (1992b), for example, used Didelphidae to include just the large opossums with 2 n 5 22 chromosomes ( Chironectes , Didelphis , Lutreolina , Philan- der), whereas Kirsch and Palma (1995) excluded Glironia , Caluromys , and Caluromysiops from the family. Because Didelphidae in any of these applications (ours, Hershkovitz’s, or Kirsch and Palma’s) is monophyletic, the choice among them must be justified by other criteria. In our view, the name Didelphidae and its colloquial equivalent (‘‘didelphids’’) are so deeply entrenched in the literature as referring to all Recent opossums that more restrictive applications would serve no adequate compensatory purpose.

Although didelphid monophyly is impressively supported by nucleotide sequence data

23 As noted above, the posterior cingulid is absent in didelphids, caenolestids, microbiotherians, and peramelids, so it optimizes as an unambiguous marsupial synapomorphy on all metatherian topologies in which these groups are recovered as basal clades (e.g., Meredith et al. 2008). Because most Cretaceous and early Tertiary metatherians are known only from teeth, this trait provides a potentially useful criterion for distinguishing stem taxa from members of the crown clade. For example, several of the Paleocene Brazilian fossils classified as didelphids by Marshall (1987) are described by that author as having posterior cingulids and might plausibly be regarded on that basis as stem metatherians. By contrast, Marsupialia (in the crown-group sense) does not seem to be diagnosable by any upper molar trait.

(e.g., Jansa and Voss, 2000; Amrine-Madsen et al., 2003; Meredith et al., 2008), unambiguous morphological synapomorphies of the family remain to be confidently identified. Because our analyses did not include any nonmarsupial outgroup, the position of the marsupial root node was not determined, and the phylogenetic interpretation of characterstate transformations on the branch separating didelphids from nondidelphid marsupials is correspondingly equivocal. If, as most recent analyses suggest, didelphids are the basalmost branch of Marsupialia ( Nilsson et al., 2004; Amrine-Madsen et al., 2003; Horovitz and Sánchez-Villagra, 2003; Asher et al., 2004; Sánchez-Villagra et al., 2007; Meredith et al., 2008; Beck, 2008), then such transformations might be didelphid synapomorphies, or they could be synapomorphies of the unnamed clade that includes caenolestids and Australidelphia.

Comparisons with stem metatherians that are believed to be close outgroups to Marsupialia , including † Mayulestes (see Muizon, 1998), † Pucadelphys (see Marshall et al., 1995), and † Herpetotherium (see Gabbert, 1998; Sánchez-Villagra et al., 2007) suggest that most of the craniodental traits by which didelphids differ from other marsupials are plesiomorphic, but one exception merits comment. Bone homologies on the posterodorsal braincase have received little attention in the literature, but they appear to be phylogenetically informative at many taxonomic levels within Metatheria. In particular, the presence of a large undivided interparietal bone that is wedged between the parietals anteriorly and fused to the supraoccipital posteriorly may be a didelphid synapomorphy. According to Muizon (1998: 38, fig. 6), paired interparietal ossifications are fused to the parietals in † Mayulestes , but the basis for this interpretation (which has no analog among living metatherians) is not explained, and it seems equally possible that the interparietal(s) is(are) absent in this taxon. In † Pucadelphys , paired interparietals (‘‘postparietals’’) are suturally distinct from the parietals and from the supraoccipital ( Marshall et al., 1995: 50, fig. 12). The occiput of † Herpetotherium has not previously been described, but in a well-preserved specimen that we examined (127684 in the AMNH vertebrate paleontology catalog) there is no bone wedged between the parietals and the supraoccipital, which share a suture just behind the lambdoid crest. Therefore, the available evidence suggests that the didelphid condition is unique.

Other alleged morphological synapomorphies of didelphids include features of the spermatozoa ( Temple-Smith, 1987), postcranial skeleton ( Horovitz and Sánchez-Villagra, 2003), and petrosal ( Ladevèze, 2007). However, the phylogenetic interpretation of such traits is compromised by sparse ingroup sampling. Spermatozooa, for example, have not been studied from many genera (e.g., Caluromysiops , Glironia , Hyladelphys , Lutreolina , Marmosops , Thylamys ), and published phylogenetic analyses of postcranial and petrosal characters have not included basal didelphids (analyzed didelphid terminal taxa in both of the osteological studies cited above belong to the subfamily Didelphinae ). Hopefully, future studies will help fill in many of these taxonomic gaps and contribute to a better assessment of anatomical character support for didelphid monophyly.

Only Recent didelphids are formally classified and described below, but several South American Neogene genera represented by well-preserved cranial material († Hyperdidelphys , † Thylatheridium , † Thylophorops ) are clearly members of the didelphimorph crown clade. Pending a phylogenetic analysis of their relationships with living forms, we follow current paleontological judgments of taxonomic affinity in suggesting where these fossils belong. By contrast, most other fossil didelphids are only represented by dental fragments from which few characters can be scored, and their relationships to extant taxa are correspondingly ambiguous.