Australia's first fossil marsupial mole (Notoryctemorphia) resolves controversies about their evolution and palaeoenvironmental origins

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.

Figure 1.

Cusp evolution from tribosphenic to zalambdodont patterns in representative metatherians (top row) and eutherians (bottom row), and dental terminology used in text. Naraboryctes philcreaseri n. gen. and sp. indicates that in zalambdodont metatherians the paracone (red) is suppressed; in eutherians it is the metacone (blue) that is reduced. Upper and lower molars redrawn from: [63] (Gypsonictops); [64] (Potamotelses, Aegialodon, Alphadon); [65] (Widanelfarasia lower); [66] (Potamogale); [67] (Widanelfarasia upper). Abbreviations: A, anterior; B, buccal; end, entoconid; hld, hypoconulid; hyd, hypoconid; me, metacone; med, metaconid; pa, paracone; pad, paraconid; pr, protocone; prd, protoconid; stB, stylar cusp B; stC, stylar cusp C; stD, stylar cusp D.

Figure 2.

Composite phylogeny of therian mammals illustrating the occurrence of dental zalambdodonty within Theria. The topology is based on recent studies including [–,,,,–72]. Taxa that include at least one fully zalambdodont member are indicated by red branches. Extinct taxa are indicated by a dagger. Images (from top to bottom) are: the tenrecoid Tenrec ecaudatus (J. F. Eisenberg; AMS Image Library); an unidentified chrysochlorid Cryptochloris sp. (Wikimedia Commons image; Killer18); the apternodontid Apternodus baladontus (frontispiece [17]: p. 2); the solenodontid Solenodon paradoxus (J. Nuñez-Miño; www.thelastsurvivors.org); the necrolestid Necrolestes patagonensis (N. P. Archer); the yalkaparidontian Yalkaparidon coheni (D. Dunphy); the notoryctid Notoryctes typhlops (M. Gillam). The phylogeny was created using the phylogenetic drawing tool MrEnt 2.0 [73].

Figure 3.

Naraboryctes philcreaseri new genus and species Riversleigh World Heritage Area, northwestern Queensland, Australia; early Miocene. (a–d) QM F23717, holotype, left dentary with i2-m4; (a) buccal view; (b,c) stereopair occlusal view; (d) lingual view of anterior dentition. (e,f) QM F23719, left dentary with i1-p2 m1-2 m4; (e) buccal view; (f) lingual view. (g) QM F23718, left m1. (h) QM F51329, left m3. (i) QM F51330, right m4 (reversed image). (j) QM F54502, left edentulous maxilla. (k) QM F51322, paratype, left premaxilla with I1-4. (l) QM F23716, paratype, partial right maxilla with P1-3 M1. (m) QM F51323, right dP3 (reversed image). (n) QM F51324, left M1. (o) QM F51325, right M2 (reversed image). (p) QM F51327, left M3 (scale bar, 2 mm).

Figure 4.

Comparison of humeri and ulnae of Naraboryctes philcreaseri gen. et sp. nov., Notoryctes typhlops (SAM637) and Antechinus stuartii (UNSWZ465). (a–c) left humerus, anterior view; (a) A. stuartii; (b) N. philcreaseri (QM F54559); (c) N. typhlops. (d–f) left humerus, posterior view; (d) A. stuartii; (e) N. philcreaseri (QM F54559); (f) N. typhlops. (g–i) left ulna, anterior view; (g) A. stuartii; (h) N. philcreaseri (QM F54560); (i) N. typhlops. (j–l) left ulna, posterior view; (j) A. stuartii; (k) N. philcreaseri (QM F54560); (l) N. typhlops (scale bar, 2 mm). Mirror image of right humerus and ulna of A. stuartii and N. typhlops used for comparison. Abbreviations: cpt, capitulum; dpt, delto-pectoral tuberosity; gt, greater tuberosity; h, head; lap, lateral anconeal process; le, lateral epicondyle; lsr, lateral supracondylar ridge; lt, lesser tuberosity; map, medial anconeal process; mc, medial crest; me, medial epicondyle; of, olecranon fossa; op, olecranon process; tn, trochlear notch; tr, trochlea.