Tlacuatzin, VOSS & JANSA, 2003

VOSS, ROBERT S. & JANSA, SHARON A., 2003, Phylogenetic Studies On Didelphid Marsupials Ii. Nonmolecular Data And New Irbp Sequences: Separate And Combined Analyses Of Didelphine Relationships With Denser Taxon Sampling, Bulletin of the American Museum of Natural History 2003 (276), pp. 1-82 : 61-65

Tlacuatzin , new genus Figures 23 , 24

TYPE SPECIES: Didelphis (Micoureus) canescens Allen, 1893 , subsequently transferred to Marmosa by Allen (1897) and thereafter known as Marmosa canescens throughout the 20th century mammalogical literature (e.g., by Tate, 1933; Gardner, 1993).

GEOGRAPHIC DISTRIBUTION: Apparently endemic to Mexico, where it occurs in seasonally dry (deciduous) tropical 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). Armstrong and Jones (1971) implied that T. canescens occurs in Guatemala, but we have not examined specimens from that country, nor have we seen any published references to vouchered Guatemalan records.

CONTENTS: Only a single valid species is currently recognized following Tate (1933) and Gardner (1993), who treated gaumeri Osgood (1913) , insularis Merriam (1898), oaxacae Merriam (1897), and sinaloae Allen (1898) as synonyms or subspecies of canescens . However, substantial geographic variation has been noted by authors (e.g., Wilson, 1991), and sufficient material (amounting to several hundred specimens in U.S. and Mexican museums) is now available for a long­overdue critical revision of these nominal taxa. Among them, the status of the Yucatecan population ( gaumeri ) merits particular attention due to its zoogeographically unusual disjunction from supposedly conspecific western Mexican forms (see Remarks, below).

ETYMOLOGY: From the Nahuatl (Aztec) word for ‘‘opossum’’ ( Karttunen, 1983), which is still in colloquial use as a singular masculine noun (e.g., by Villa­Ramírez, 1991).

DIAGNOSIS AND COMPARISONS: Small didelphines that can be distinguished from all oth­ er members of the subfamily by nonmolecular characters analyzed in this report, among which the following are salient points of morphological comparison. Rhinarium with two ventrolateral grooves; dark ocular mask present; supraocular spots absent; gular gland absent; dorsal fur unpatterned (unicolored grayish or reddish­gray), with short, inconspicuous guard hairs; manual digits III and IV subequal; large adult males with well­developed lateral carpal tubercles but not medial carpal tubercles; plantar surface of hind foot naked from heel to toes; pedal digit IV longer than other pedal digits; marsupium absent; tail essentially naked (without a conspicuously furred base), covered by epidermal scales in annular series except for distal prehensile surface, not incrassate. Rostral process of premaxillae absent; nasals conspicuously wider posteriorly than anteriorly; very large, flattened, wing­like postorbital processes present in mature adults; sagittal crest absent; parietal and alisphenoid in contact on lateral braincase; no lateral petrosal exposure through fenestra between parietal and squamosal; maxillopalatine fenestrae long (usually extending from the level of P3 to M3); palatine fenestrae absent; maxillary fenestrae present, often confluent with maxillopalatine openings; posterolateral palatal foramina posterior to M4 protocones; posterior palate with prominent lateral corners, the internal choanae constricted behind; transverse canal foramen present; secondary foramen ovale absent; ectotympanic suspension direct; fenestra cochleae exposed; paroccipital process of exoccipital small, round­ ed, adnate to posterior aspect of petrosal; dorsal margin of foramen magnum formed by supraocciptal and exoccipitals. Crowns of I2–I5 symmetrically rhomboidal and subequal (not increasing in size from front to back); P2 and P3 subequal in height; P3 without an anterior cutting edge; upper molars strongly dilambdodont; distinct ectoflexus present on M2 and M3 (much deeper on the latter tooth); anterior cingulum complete on M3. Distinct lingual cusp present on i1– i4; c1 an erect, dorsally recurved tooth (not procumbent and premolariform); p2 much taller than p3; dp3 with incomplete (bicuspid) trigonid; entoconid a tall sharp cusp, much higher than hypoconulid on m1, m2, and sometimes m3.

Although Marmosa and Tlacuatzin are superficially similar, T. canescens differs from all other species currently referred to Marmosa (sensu Gardner, 1993) in several nonmolecular characters that we coded for phylogenetic analysis, including: caudal scales in unambiguously annular series (character 23), absence of a premaxillary rostral process (character 29), possession of maxillary palatal vacuities (character 40), and 2 n = 22 chromosomes. Two additional comparisons also provide diagnostic criteria, but these were not coded for phylogenetic analysis due to morphological intermediates observed among other didelphine taxa. The first concerns the morphology of I2–I5, which have rhomboidal crowns that increase in breadth from front to back in Marmosa , such that I2 is visibly narrower than I 5 in lateral view. The crowns of I2–I5 are also rhomboidal in Tlacuatzin , but in that taxon they do not increase in breadth from front to back, and the crowns of I2 and I5 appear subequal in lateral view. The second comparison involves the lower canine, which is a procumbent tooth with a flattened, blade­like apex and (occasionally) a small posterior accessory cusp in Marmosa . By contrast, c1 is an erect, simple, recurved­conical tooth in Tlacuatzin . Additional character­state differences (scored in appendix 5) distinguish Tlacuatzin canescens from Marmosa murina (the type species of Marmosa ), but these are not consistently useful for generic diagnosis.

Tlacuatzin canescens resembles Marmosa andersoni (the type species of Stegomarmosa Pine, 1972 ) in having large postorbital processes ( Pine, 1972: fig. 1), but these taxa are otherwise dissimilar. Based on our examination of the Peruvian type specimen (FMNH 84252), M. andersoni differs from T. canescens by having tail scales in spiral series; a long rostral process of the premaxillae; large palatine fenestrae; no maxillary fenestrae; upper incisor crowns that increase in breadth from I2 to I5; and a procumbent, apically flattened c1. Unfortunately, the karyotype of M. andersoni is unknown. Based on these and other character data, we see no evidence for a close relationship between T. canescens and M. andersoni , and we concur with the current treatment of Stegomarmosa as a synonym or subgenus of Marmosa .

REMARKS: Tlacuatzin canescens is part of a distinctive fauna that inhabits dry tropical forests in western Mexico. Recent syntheses of distributional data have documented the impressive diversity of western Mexican dry forests, to which at least 25 mammalian species are endemic (Ceballos, 1995; Ceballos and García, 1995; Ceballos et al., 1998). A different fauna inhabits the dry tropical forests of the Yucatan Peninsula, however, where T. canescens also occurs. The Yucatecan dry forest fauna is less easily defined than that of western Mexico because dry forest on the Yucatan Peninsula grades into more mesic vegetation formations, but most Yucatecan endemics are primarily dry­forest species (e.g., Heteromys gaumeri , Otonyctomys hatti , Mazama pandora ; see Reid, 1997; Medellín et al., 1998).

Tlacuatzin is one of seven mammalian genera endemic to Mesoamerican dry forests, the other six consisting of a shrew ( Megasorex ), one bat ( Musonycteris ), and four rodents ( Hodomys , Osgoodomys , Otonyctomys , Xenomys ). Without exception, all of these genera are currently considered to be monotypic ( Wilson and Reeder, 1993). Whereas five are endemic to western Mexico ( Megasorex , Musonycteris , Hodomys , Osgoodomys , Xenomys ) and one is endemic to the Yucatan Peninsula ( Otonyctomys ), only Tlacuatzin has a disjunct distribution in both dry­forest regions. In fact, Tlacuatzin appears to be one of only two mammalian taxa (of any rank) endemic to Middle American dry forests that occurs both in western Mexico and the Yucatan, a biogeographic anomaly that begs critical attention. 13

Figures

Fig. 23. Skull of Tlacuatzin canescens, a composite drawing based on USNM 125659 and 511261.

Fig. 24. Tlacuatzin canescens, photographed by Gerardo Ceballos in March 1995 at the Chamela­Cuixmala Biosphere Reserve, Jalisco, Mexico. Specimens from southern populations (especially topotypical material from Oaxaca) are markedly grayer than this individual.