Abstract
Fossils of a marsupial mole (Marsupialia, Notoryctemorphia, Notoryctidae) are described from early Miocene deposits in the Riversleigh World Heritage Area, northwestern Queensland, Australia. These represent the first unequivocal fossil record of the order Notoryctemorphia, the two living species of which are among the world's most specialized and bizarre mammals, but which are also convergent on certain fossorial placental mammals (most notably chrysochlorid golden moles). The fossil remains are genuinely ‘transitional', documenting an intermediate stage in the acquisition of a number of specializations and showing that one of these—the dental morphology known as zalambdodonty—was acquired via a different evolutionary pathway than in placentals. They, thus, document a clear case of evolutionary convergence (rather than parallelism) between only distantly related and geographically isolated mammalian lineages—marsupial moles on the island continent of Australia and placental moles on most other, at least intermittently connected continents. In contrast to earlier presumptions about a relationship between the highly specialized body form of the blind, earless, burrowing marsupial moles and desert habitats, it is now clear that archaic burrowing marsupial moles were adapted to and probably originated in wet forest palaeoenvironments, preadapting them to movement through drier soils in the xeric environments of Australia that developed during the Neogene.
Highlights
Notoryctemorphia, the marsupial moles, is the least diverse but most extraordinarily distinct of the four orders of living Australian marsupials
Most recent studies have supported a close relationship between Notoryctemorphia and Dasyuromorphia (Australian carnivorous marsupials [5]), Peramelemorphia or both [8,9,10,11,12,13]
The hypertrophied olecranon process of the ulna, which enables powerful extension of the forelimb during digging, while decidedly hypertrophied and notoryctidlike in N. philcreaseri and a synapomorphy for this family, is not quite as large nor as strongly curved medially as it is in species of Notoryctes
Summary
Notoryctemorphia, the marsupial moles, is the least diverse but most extraordinarily distinct of the four orders of living Australian marsupials. Notoryctes is unique in exhibiting a distinctive molar morphology termed zalambdodonty [14,15]. Modern notoryctids are confined to deserts in Australia and exhibit numerous anatomical specializations that appear exceptionally well suited to burrowing through desert sands These include a conical skull, extreme modifications of the axial and appendicular skeleton (in particular, enormous enlargement of the muscle attachment sites on the fore- and hindlimbs) and soft tissue features such as a ‘nasal shield’, a lack of eyes and external ears, and a tubular body shape [21]. It has long been assumed that notoryctids evolved these specializations in a desert environment. As well as zalambdodont molars, Notoryctes and chrysochlorids share similar fossorial specializations of the skeleton [19,26] and closely resemble each other in terms of external appearance (figure 2). Case denotes upper (e.g. M2) and lower (e.g. m2) teeth
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