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PolydolopsPolydolops was a scansorial, omnivorous marsupial from the Palaeocene, Eocene and Oligocene. Fossils have been found in Chile, Argentina and Antarctica.
Polydolops was a member of the Polydolopidae and belonged to the Polydolopoidea, which was part of the Polydolopimorphia. Poydolopids were among the earliest-described South American fossil marsupials and are moderately common elements of most early Cenozoic faunas over 30 million years, but are rarer than placentals in most Cenozoic South American assemblages. Polydolopids reported from Antarctica (Antarctodolops, Eurydolops) may be closely related to particular Polydolops species (5,9), which would render Polydolops paraphyletic or mean that the Antarctic taxa should be assigned to Polydolops (2). Eurydolops is known from one tooth (considered P3 by Case et al. (5)). It has been considered to be a polydolopine, but the shape and morphology of the available tooth has many resemblances to the P2 of Epidolops, which could be an epidolopine rather than polydolopine. Eudolops tetragonus has an elongate P2 and slightly elongate P3, but these are not as elongate as in P. mckennai.The primitive condition for molar shape in polydolopids seems to be squared (as in most polydolopines) or broader than long (as most marked in the polydolopine proximal outgroup Epidolops). While the elongate molars in Eudolops hernandezi make inferences about the ancestral molar shape in polydolopids and polydolopines a bit problematic, Eudolops is distinct from Polydolops and the roughly squared or relatively broad molars in all other polydolopids (including other species of Eudolops). P. mckennai has very elongate molars and seems to have many fewer buccal and stylar cusps on the upper molars than do other species of Polydolops, which may be used to exclude it from a Polydolops clade. As the cheek teeth of P. mckennai have similarities with other species assigned to Polydolops and most of these lack other skull material is lacking in most, Flynn and Wyss (1) chose to ally the new species with other species of Polydolops. Details of the anterior teeth from Polydolops mckennai (1) and P. abanicoi (2) strengthened homology inferences for the anterior teeth in polydolopids. P. mckennai indicates that Polydolops has one, or perhaps two, upper incisor, while Epidolops had three (6). As in Epidolops, the incisor(s) were not on the anterior rim of the snout, but were set more posteriorly on the buccal rim of the palate, closely appressed to several other teeth. The lateral development of a battery of anterior upper teeth [incisor(s), canine, P1] seem congruent with the greatly enlarged, procumbent lower canine and associated great reduction or loss of all lower incisors. The specimen had a large, vertically oriented upper canine and had P1, contradicting the inference that Polydolops lacked P1 and p1 were both absent in Polydolops (6), but agreeing with a small, anterior p1 in P. abanicoi (2) and perhaps P. clavius (2,7) and similar P1/p1 in Epidolops. This suggests that the ancestral condition for polydolopids and polydolopines was a small P1/1, located far anteriorly on the ramus or maxilla (near the canine and separated from P2/2 by a long diastema) (1). Epidolops and polydolopines had very large upper and lower canines; procumbent lower canines and other anterior teeth; great reduction or loss of upper and lower incisors; reduction of P1/p1 and a long P1/p1–P2/p2 diastema separating the anterior dental battery from the cheek-tooth row. These support the hypothesis of polydolopid monophyly (1). Goin and Candela (8) suggested that Polydolopidae may not be monophyletic, as it is difficult to derive the highly modified lower first molar of polydolopines from a hypothesized epidolopine (Epidolops) ancestry. Epidolops represents a lineage that diverged from the other polydolopids (Polydolopinae) at least as early as the Itaboraian SALMA. Epidolops ameghinoi represents the earliest polydolopid with different aspects of molar morphology and is the proximal outgroup of the polydolopines, rather than an ancestor. Therefore, one should not necessarily expect to be able to derive every feature seen in polydolopines from the condition in Epidolops, but rather from a more generalized condition that marked the common ancestor of both lineages. Epidolops differs from polydolopines in its cheek teeth and cranium (6). The skull of Epidolops ameghinoi seems more robust than in P. mckennai, with a very broad rostrum and massive zygomatic arches. P. mckennai has a proportionately narrower snout and larger braincase. Flynn and Wyss (1) regard Epidolops and polydolopines as constituting a monophyletic group (see 2).