phylogenetic relationships and classification of didelphid marsupials ...

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12 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 322 mation. 1 In effect, such information comes from commonly available materials that can be examined without dissection or other special preparations. Therefore, microscopic features (e.g., those of the spermatozoa; Temple-Smith, 1987) and visceral characters (e.g., of the digestive tract; Santori et al., 2004) are not reviewed below, nor are osteological traits that require X-ray computed tomography, serial sectioning, or destructive methods for their study. Size and External Features Most opossums are externally unremarkable mammals with pointed muzzles, large rhinaria, well-developed vibrissae, prominent eyes, membranous ears, nonspinous pelage, subequal limbs, pentadactyl feet, and naked tails. In many of these respects they resemble other plesiomorphic marsupials (e.g., Dromiciops and dasyurids) as well as certain unspecialized placentals (e.g., solenodontids, rice tenrecs, gymnures, and tree shrews). Closer inspection, however, reveals numerous distinctive and phylogenetically informative details of didelphid external morphology. SIZE: Didelphids are small to mediumsized mammals. The smallest Recent species is probably Chacodelphys formosa, the young adult holotype of which had a head-andbody length of 68 mm and probably weighed about 10 g (Voss et al., 2004a). By contrast, the largest living opossum, Didelphis virginiana, can measure almost 500 mm in headand-body length and weigh more than 3000 g (Hamilton, 1958). Most didelphids, however, range in head-and-body length from about 100 to 300 mm and weigh between about 20 and 500 g (table 4). Although some authors have recognized ‘‘large’’, ‘‘medium’’, and ‘‘small’’ opossums, taxonomic assignments to discrete size categories are often arbitrary and sometimes misleading. For example, Reig et al. (1987: character 25) scored Metachirus as ‘‘large’’ and Caluromys as ‘‘medium’’ in their phylogenetic analysis, but linear measurements and weights that we compiled suggest that these taxa are indistinguishable in size. More taxonomically 1 An important exception is postcranial skeletal morphology, the topic of an independent study (Flores, 2009). comprehensive compilations of morphometric data will probably document a continuum of didelphid size distributions. By comparison, Old World marsupials include some species that are smaller and others that are much larger than any didelphid. The smallest living Australian species, for example, is said to be Planigale ingrami, with an adult weight of only about 4 g, and the largest is Macropus rufus, males of which are said to weigh as much as 85 kg (Dawson et al., 1989). The extinct Pleistocene species Diprotodon optatum, however, may have weighed almost 2800 kg (Wroe et al., 2004). RHINARIUM AND MOUTH: The rhinarium, a prominent pad of naked glandular skin surrounding the nostrils, is divided by a median crease or sulcus (sulcus medianus; Ade, 1999) that extends from between the nares to the upper lip in all examined didelphids. The part of the rhinarium that borders the upper lip (pars supralabialis) is broad, and its ventral margin is notched by one or two distinct grooves on each side of the median sulcus (fig. 1). Two ventrolateral grooves are present on each side in most examined taxa, but only a single groove is present in Chironectes, Didelphis, Lestodelphys, Lutreolina, Metachirus, Monodelphis, Philander, and Thylamys pallidior. Dasyurids and Dromiciops have rhinaria that are essentially similar in gross morphology to those of didelphids, with a pars supralabialis that makes broad contact with the upper lip; however, only a single ventrolateral groove is present in these taxa (Pocock, 1926: figs. 26– 28). By contrast, the supralabial part of the rhinarium is reduced to a narrow philtrum in caenolestids and peramelids, such that the groove-bearing ventral rhinarial margin of other marsupials is effectively absent. The mouth is large in all didelphids, with a posterior angle (angulis oris; Brown, 1971) that extends posteriorly to a point below the eye; the upper and lower oral margins are smooth or irregularly wrinkled and anatomically featureless. Most other marsupials have anatomically featureless oral margins like those of didelphids, but caenolestids are uniquely provided with reciprocating fleshy lappets of unknown function on the upper and lower lips (Osgood, 1921: pl. 2; Bublitz,
2009 VOSS AND JANSA: DIDELPHID MARSUPIALS 13 TABLE 4 External Measurements (mm) and Weights (g) of Exemplar Didelphid Species a Caluromys (Caluromys) philander e Caluromys (Mallodelphys) lanatus f Caluromysiops irrupta g Chironectes minimus h Cryptonanus unduaviensis i Didelphis marsupialis e Glironia venusta j Gracilinanus agilis k Hyladelphys kalinowskii e Lestodelphys halli l Lutreolina crassicaudata m Marmosa (Marmosa) murina n Marmosa (Micoureus) regina f Marmosops noctivagus f Marmosops pinheiroi e Metachirus nudicaudatus e Monodelphis emiliae f Philander opossum e Thylamys karimii o Thylamys macrurus o Tlacuatzin canescens p 1987: fig. 4; Patterson and Gallardo, 1987: fig. 3). FACIAL VIBRISSAE AND MARKINGS: Didelphid facial vibrissae are grouped into discrete tracts that are easily homologized with those described and illustrated by Pocock (1914), N b Head and Body c Tail d Weight 7 261 (224–279) 390 (373–410) 330 (220–390) 8 278 (270–296) 422 (400–446) 412 (349–500) 1 260 310 496 6 289 (276–307) 348 (316–362) 605 (520–700) 8 105 (97–121) 122 (112–135) 25 (15–40) 9 419 (405–446) 434 (366–497) 1351 (1025–1700) 2 194 (188–201) 208 (201–215) 130 (129–130) 11 98 (86–109) 137 (121–162) 25 (18–34) 3 77 (76–78) 111 (107–113) 16 (13–18) 1 132 88 76 11 295 (241–342) 283 (242–336) 530 (300–800) 13 133 (118–152) 173 (156–195) 51 (35–80) 41 180 (142–209) 262 (238–294) 118 (70–164) 16 138 (118–155) 183 (154–202) 51 (30–70) 11 103 (94–121) 149 (137–156) 27 (21–33) 9 262 (249–287) 345 (326–370) 380 (260–480) 6 107 (97–113) 50 (45–53) 30 (20–38) 11 301 (264–346) 306 (280–333) 549 (380–695) 33 104 (78–129) 80 (69–106) 28 (16–43) 6 112 (101–126) 144 (136–153) 39 (30–55) 7 137 (126–149) 137 (131–145) — (38–60) a Tabulated statistics are the sample mean (rounded to the nearest whole unit) and the observed range (in parentheses) of measurements and weights recorded from dentally mature specimens; male and female data were combined to increase sample size despite apparent sexual dimorphism in some species. Most genera are represented by a single exemplar species, but several genera with recognized subgenera or that include taxa differing conspicuously in size or body:tail ratios are represented by additional species. b Sample size. c Obtained by subtracting length of tail from total length following the standard American protocol. d Basal flexure to fleshy tip. e French Guianan specimens measured by Voss et al. (2001). f Measurements and weights from western Brazilian specimens (Patton et al., 2000). g Measurements and weight of AMNH 208101; because this was a zoo specimen that may have been obese, the weight datum is suspect. h From Paraguayan specimens (UMMZ 126289, 134022, 134023, 134025, 134559, 134560). i From Bolivian specimens measured by Voss et al. (2005). j Measurements and weight of MMD 607 (collected near Iquitos, Peru; to be deposited in the Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos, Lima; M.M. Díaz, personal commun.) and INPA 5237 (collected near Mirassol d’Oeste, Mato Grosso, Brazil; Santos Filho et al., 2007). k From Paraguayan specimens (UMMZ 124675, 126104, 133998–134006). l Measurements and weight from MVZ 173727. m From Paraguayan specimens (UMMZ 126109–126111, 126113, 134010, 134011, 134017–134021). n From Surinamese specimens measured by Voss et al. (2001). o From Carmignotto and Monfort (2006). p External measurements from Oaxacan specimens unaccompanied by weight data (AMNH 3111/2433, 3111/2434, 3114/2437, 148969, 149104, 165651, 165653); range of weights from Zarza et al. (2003). Lyne (1959), and Brown (1971), whose terminology is followed here. All of the taxa we examined (including representative species from every genus) exhibit well-developed mystacial, submental, interramal, superciliary (supraorbital), and genal vibrissae. La-
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