Sairadelphys tocantinensis, Oliveira, Édison Vicente, Nova, Patricia Villa, Goin, Francisco J. & Avilla, Leonardo Dos Santos, 2011

Oliveira, Édison Vicente, Nova, Patricia Villa, Goin, Francisco J. & Avilla, Leonardo Dos Santos, 2011, A new hyladelphine marsupial (Didelphimorphia, Didelphidae) from cave deposits of northern Brazil, Zootaxa 3041, pp. 51-62 : 55-60

Sairadelphys tocantinensis sp. nov.

Figs. 2–4

Etymology. tocantinensis after the State of Tocantins, where the paleontological site is located.

Type. DGEO-UFPE 6745, incomplete left maxilla with complete M2–4.

Hypodygm. The type and DGEO-UFPE 6746, an isolated left m1.

Locality and age. Municipality of Aurora de Tocantins, cave of Gruta dos Mouras, Sate of Tocantins, Brazil (12º42´47´´S and 46º24´28´´W); Pleistocene.

Diagnosis and description. Same as for the genus.

Comments. The lower molar assigned to Sairadelphys tocantinensis is similar in morphology to that of Hyladelphys and Marmosa (e.g. M. murina Linnaeus ). However, in comparison with Marmosa we noted that in DGEO- UFPE 6746 the crests are less developed and the cristid oblique is most labially placed in relation to the postprotocristid notch; therefore, the cristid oblique in the new taxon runs less obliquely in relation to the anteroposterior dental axis, a feature that is compatible with the linear centrocrista described for upper molars of Sairadelphys . Another complementary occlusal relationship between the holotype and the assigned lower molar is related to the low protocone in Sairadelphys , which is compatible with lower molars with high entoconid as seen in DGEO- UFPE 6746.

Measurements (mm). DGEO-UFPE 6745: M2 L =1.38, M2 W=1.77; M3 L=1.42, M3 W=1.50; M4 L= 0.73, M4 W=0.97; DEGEO-UFPE 6746: m1 L = 1.68, m1 W=0.93.

Comparisons. Several features of this new marsupial confidently place it in the Didelphimorphia (see below). However, the presence of plesiomorphic features in Sairadelphys tocantinensis , as exemplified by the presence of a linear centrocrista, and subequal paracone and metacone, constitute characters absent in most living didelphimorphians and, therefore, warrants comparisons with fossil and generalized forms such as Caluromyinae , Sparassocynidae , Peradectidae, Wirunodon Goin & Candela , and Microbiotheriidae . A relationship to the latter can be rejected on the basis of several characters: the trigon basin is reduced in Sairadelphys (very wide in Microbiotheriidae ), the protocone is narrow (it is wide in Microbiotheriidae ), and the stylar shelf is wide in relation to the talon (very reduced in Microbiotheriidae ). Sairadelphys tocantinensis differs from the Peradectidae in the absence of cingula, para- and metaconule and StC, and in the preparacrista ending at the anterolabial angle of the tooth (stylar cusp A region). Furthermore, peradectids are significatively older than Sairadelphys , which was found on Pleistocene sediments (see below). The enigmatic marsupial Wirunodon, from the late Paleogene of Santa Rosa, Peru (Goin & Candela 2004) resembles Sairadelphys , with some characters such as the reduction of stylar cusps and of the protocone. However, Sairadelphys differs from Wirunodon in having subequal paracone and metacone, absence of anterior cingulum, and much less developed stylar cusps. Sparassocynid affinities of Sairadelphys can be ruled out on the basis of its absence of cingula and conules (except Hesperocynus Forasiepi, Goin & Martinelli), deep ectoflexus (mainly in M3), absence of cutting crests, reduced postmetacrista, and in that the paracone and the metacone are similar in size.

Although Sairadelphys shows an unusual morphology, a combination of derived features are consistent with its assignment to the Didelphimorphia , among which a transversely well-developed stylar shelf in relation to the talon, and absences of paraconule, anterior cingulum and posterior cingulid. Most of the living didelphimorphians have an incomplete anterior cingulum in the upper molars and do not have a posterior cingulid in the lower molars ( Voss & Jansa 2009). However, most didelphimorphians have a V-shaped centrocrista, (albeit with variations; Goin 1997), and show a clear difference in height and volume between the paracone and the metacone (as seen in Sairadelphys , the paracone and the metacone are subequal in height and volume).

Below we describe some characters (including three synapomorphies) that suggest a close relationship between Sairadelphys tocantinensis and the living Didelphimorphian Hyladelphys kalinowskii , described from the amazon region in French Guiana, Peru, and Brazil ( Hershkovitz 1992, Voss et al. 2001, Jansa & Voss 2005, Astúa 2006). When compared to Hyladelphys, Sairadelphys is differs in having more inflate cusps, strongly reduced stylar cusps, and in lacking cingula.

Cladistics. The phylogenetic analysis resulted in 1093 parsimonious trees of 372 steps [consistency index (CI) = 0.4731, retention index (RI) = 0.7768]. The consensus tree resulting from parsimony analysis of morphological + karyotypic data set is identical to that which has been previously published ( Voss & Jansa 2009: fig. 27), except for the inclusion of Sairadelphys as the sister taxon of Hyladelphys ( Fig. 5 ). This last clade (Hyladelphinae) composes a monophyletic group together with Glironia and caluromyines. The genus Tlacuatzin and the Marmosa group were recovered as a polytomy within the clade reuning hyladelphines +glironiines+caluromyines. Regarding the results of Voss & Jansa (2009) it is interesting to note that the trees resulting from a mixed-model Bayesian analysis of a combined (nonmolecular + molecular) data set recovered glyroniines+caluromyines and Hyladelphys as successive sister groups to remaining didelphids.

The sister taxon relationship between Sairadelphys and Hyladelphys (bootstrap=67%) is supported by three synapomorphies: development of stylar cusps reduced in number and size (character 130, state 0; CI 0.067), M2 subequal to slightly larger than M3 (character 136, state 1; CI 0.333), and presence of well-developed prefossid (character 138, state1; CI 1.000). With regards to Hyladelphys, Jansa & Voss (2005) performed a phylogenetic analysis based on nuclear genes and morphology concluding that this Amazonian marsupial is a didelphid that occupies an internal branch separating the traditionally recognized subfamilies, Caluromyinae and Didelphinae . Later, Voss & Jansa (2009) diagnosed the subfamily Hyladelphinae by their vestigial milk dentition (dP3/dp3 are large, more or less molariform teeth in other opossums) and indicated six characters listed in Jansa & Voss (2005, table 5), which according these authors, support an intermediate position of Hyladelphys between caluromyines and didelphines. Similarly, considering the postcranial morphology in a combined analyses, Hyladelphys was recovered as the sister taxon to didelphines, with Glironia and calurmonyines occupying a most basal position ( Flores 2009). Besides the synapomorphies listed above, we found additional dental characters supporting the clade Hyladelphinae, including relatively low crowned molars, similarly sized paracone and metacone in M3, reduction of stylar cusps in number and size, convex labial faces of paracone and metacone, upper molars not strongly carnassialized (i.e. postmetacristae are only slightly longer than postprotocristae) and postprotocrista are without a carnassial notch.

Size and inferred feeding habits. Sairadelphys tocantinensis was a very small marsupial comparable in dental dimensions to some of the smallest living didelphimorphians such as Hyladelphys and Gracilinanus Gardner & Craighton. Adult Hyladelphys weigh between 13 and 18 g ( Voss et al. 2001), but Sairadelphys is slightly larger than Hyladelphys in terms of dental dimensions. Using upper and lower molars, Gordon (2003) published an estimate of body mass in fossil and living marsupials. In all length and width molar measurements of Didelphidae analyzed in the Gordon (2003) study, Gracilinanus agilis Burmeister and Thylamys elegans Waterhouse are those better comparable to Sairadelphys , with an average weight of approximately 27 g. The body mass for these species using length and area of first molars was estimated between 28 to 37 g for Thylamys elegans and from 30 to 40 g for Gracilinanus agilis . Thus it is probable that Sairadelphys weighed no more than 40 g.

The upper molar morphology of Sairadelphys includes relatively low crowns, reduced M4, absence of shearing cristae and reduction of protocones. These features are suggestive of insectivorous-frugivorous feeding habits. Hyladelphys has more developed posmetacristae than Sairadelphys , which is suggestive of a more insectivorous diet.

Age of material. In absence of radiometric dating for the mammal-bearing deposit, the age of the Aurora de Tocantins fauna is discussed on the basis of the marsupials and associated taxa such as the tayassuid Catagonus Ameghino , the xenarthran dasypodid Propraopus Ameghino and the glyptodontid Pachyarmatherium Downing & White (unpublished data). Although some of these taxa (e.g. Propraopus , Pachyarmatherium ) are frequently found in late Pleistocene deposits in Brazil, other taxa do not permit a very accurate age ( SALMA or epoch) assignment. For example, the Catagonus and Pachyarmatherium genera range from the late Pliocene to late Pleistocene of South and South/North America, respectively ( Porpino et al. 2009, Gasparini et al. 2009). However, the genus Propraopus suggests a Pleistocene age because it ranges from the Ensenadan to Lujanian SALMAs (early to late Pleistocene) ( Cione & Tonni 1999). The marsupial fauna include numeral isolated teeth, incomplete maxillaries and dentaries. In addition to the new taxa described here, the preliminary analysis of the remaining marsupials also suggests the presence of a probable new species of Monodelphis Burnertt. Given that none of the studied marsupial specimens can be assigned confidently to any known living species cited for in Brazil ( Gardner 2008), the marsupial fauna is inconclusive in terms of age. Other mammals recovered from the same cave include numerous undescribed remains of rodents, bats, and indeterminate microvertebrates, currently under study ( Avilla et al. 2010).

Although we regard the fauna of Gruta dos Mouras cave as Pleistocene, we do not rule out the possibility of temporal mixing (“time-averaging”) of Pleistocene and Holocene specimens, and thus there is a chance that the newly described taxon is still a living marsupial in the study area. More field work is necessary for corroboration of this exciting possibility.

Figures

FIGURE 2. Sairadelphys tocantinensis gen. et sp. nov., DGEO-UFPE 6745, type. A, M 2 – 4 in occlusal view; B, M 2 – 4 in linguo-occlusal view. Scale = 1 mm.

FIGURE 3. Lower molar of Sairadelphys tocantinensis gen. et sp. nov., DGEO-UFPE 6746. A, mesial view, B, posterior view. Scale bar = 1 mm.

FIGURE 4. Lower molar of Sairadelphys tocantinensis gen. et sp. nov., DGEO-UFPE 6746. A, lingual view; B, labial view; C, occlusal view; D, labio-occlusal view. Scale bar = 1 mm.

FIGURE 5. Strict consensus tree of 1093 equally most parsimonious trees. The numbers above branches indicating bootstrap support values (> 50 % only; 1000 replicates); numbers below branches represent decay indices.