Riolestes capricornicus, Goin & Candela & Abello & Oliveira, 2009

Goin, Francisco J., Candela, Adriana M., Abello, M. Alejandra & Oliveira, Edison V., 2009, Earliest South American paucituberculatans and their significance in the understanding of ‘ pseudodiprotodont’ marsupial radiations, Zoological Journal of the Linnean Society 155 (4), pp. 867-884 : 874-875

RIOLESTES CAPRICORNICUS SP. NOV. ( FIG. 3 )

Etymology: In reference to the geographic placement of the fossil locality, near the Southern Tropic of Capricorn.

Locality, horizon, and age: São José de Itaboraí , State of Rio de Janeiro, Brazil (22°44′51″S, 42°51′21″W); Itaboraí Basin, Late Palaeocene (Itaboraian Age) .

Type: MCN-PV 1790 ( Fig. 3 ), an isolated m1.

Hypodigm: The type only.

Measurements: See Table 1. Diagnosis: Differs from the remaining Paucituberculata in the following combination of characters: large size, m1 with reduced paraconid, mesially placed; persistence of the paracristid notch, vestigial anterobasal cingulum, trigonid basin absent; metaconid well displaced posteriorly.

Description: The most distinctive features of this molar are its proportionally large size, reduced paraconid, high protoconid, backwardly placed metaconid, absent trigonid basin, labially salient hypoconid (i.e. laterally expanded), and wide talonid basin. The paraconid is poorly developed, much less than the metaconid; its anterior face is flat. The anterobasal cingulum, although vestigial, runs at a short distance below and labial to the paraconid. The metaconid is moderately developed and is set close to the protoconid but quite posterior to it. Thus, the metacristid (postprotocristid + postmetacristid) is short, high, and in occlusal view is obliquely placed with respect to the dental axis. The paracristid (preprotocristid + postparacristid) is sharp, trenchant, and runs steeply downwards, almost subvertical, from the protoconid to the paraconid. The paracristid notch is placed just posterior to the paraconid. The talonid is much wider than the trigonid; its basin is well bound by the pre-entocristid, the cristid oblique, and the posthypocristid. The hypoconid and the entoconid are subequal in height, the latter being laterally compressed; the hypoconulid is located posterolingually relative to the entoconid, and is projected backwards. The cristid obliqua reaches anteriorly up to a point below the notch of the metacristid; it is not parallel to the labial face of the tooth but instead narrows the talonid basin anteriorly. There is no postcingulum. Below the crown, the anterior root is smaller than the posterior one. The cristid obliqua is worn on its anterodorsal edge, as well as the posthypocristid near the hypoconulid and the entoconid at its labial face.

Comments: Riolestes capricornicus can be considered a marsupial on the basis of the pairing of the entoconid and the hypoconulid. Its most distinctive features agree well with those expected for a member of the Paucituberculata : labially projected hypoconid and laterally compressed entoconid (see below). An additional feature, the metaconid clearly posterior to the protoconid, occurs in the first molar in most Paucituberculata – except in Stilotherium , Phonocdromus , and Pichipilus . Finally, the paraconid of m1 is reduced in all paucituberculatans, as in the type of Riolestes capricornicus .

Although not frequent among more modern marsupials, the location of the metaconid somewhat posterior with respect to the protoconid appears not only in the m1 but also in the dp3s of several different lineages, including some very generalized forms such as Alphadon . Cifelli (1994), referring to several deciduous molars tentatively referred to Alphadon perexiguus , pointed out that ‘... the trigonid is broadly open lingually, with the paraconid and metaconid more anteriorly and posteriorly placed, respectively, than on the permanent molars. The paraconid and metaconid are weakly developed (...) As usual among marsupial dP3s, the cristid obliqua extends anterolingually as a crest to the tip of the metaconid, resembling the distal metacristid (...) seen in permanent molars of primitive Tribosphenida’ ( Cifelli, 1994: 124; Cifelli & de Muizon, 1998). The posterior placement of the metaconid with respect to the protoconid is also verified in the permanent molars of some Early Palaeocene South American taxa, such as the polydolopimorphian Roberthofftetteria nationalgeographica. Among the most primitive Sparassodonta , such as some Hathliacynidae , the reduction of the metaconid in m1 occurs together with its posterior placement in the trigonid. In some Microbiotheriidae , such as Pachybiotherium acclinum , Eomicrobiotherium gaudry , and Microbiotherium gallegosense , the posteriorly ‘shifted’ metaconid in m1 occurs together with the mesial ‘shift’ of the paraconid, as in Riolestes . Some early Didelphimorphia , such as Itaboraidelphys camposi , also show a slight posterior displacement of the metaconid. Finally, among the Paucituberculata Caenolestidae and, especially, Palaeothentidae , the m1 metaconid also shows a displacement similar to that of Riolestes . In short, the posterior displacement of the metaconid is not exclusive to the marsupial dp3 but also occurs in several permanent molars in various lineages, noticeably in the m1 of most Paucituberculata . Luckett & Hong (2000) demonstrated that the dp3 is vestigial and extremely reduced in paucituberculatans, which is obviously not the case in MCN-PV 1790. Finally, the positions and relative sizes of the trigonid cusps in Riolestes capricornicus match well those of the m1 of paucituberculatans.

Riolestes capricornicus and Bardalestes sp. from Las Flores are contemporary paucituberculatans. Differences between them include the following features: Riolestes capricornicus triples Bardalestes sp. in size; its type specimen differs from the lower molar referred to Bardalestes sp. in that there is a less salient hypoconid, the entoconid is comparatively smaller, the hypoconulid is less proximate to the entoconid (in Bardalestes it is almost immediately posterior to the entoconid), it has a shallower talonid basin, and it lacks a posterobasal cingulum. All these differences warrant a generic distinction between these taxa.

Figures

Figure 3. Riolestes capricornicus gen. et sp. nov. Occlusal view of specimen MCN-PV 1790, an isolated m1.