Tuesday, May 31, 2016

[PaleoMammalogy • 2016] Malleodectidae fam. nov. • A New Family of Bizarre Durophagous Carnivorous Marsupials from Miocene Deposits in the Riversleigh World Heritage Area, northwestern Queensland, Australia

Malleodectes mirabilis using its massive, ball-peen-like P3 to break into what were perhaps one of this unique Miocene marsupial’s favourite meals — Riversleigh escargots. 
Illustration by Peter Schouten. DOI: 10.1038/srep26911

A new specimen of the bizarrely specialised Malleodectes mirabilis from middle Miocene deposits in the Riversleigh World Heritage Area provides the first and only information about the molar dentition of this strange group of extinct marsupials. Apart from striking autapomorphies such as the enormous P3, other dental features such as stylar cusp D being larger than B suggest it belongs in the Order Dasyuromorphia. Phylogenetic analysis of 62 craniodental characters places Malleodectes within Dasyuromorphia albeit with weak support and without indication of specific relationships to any of the three established families (Dasyuridae, Myrmecobiidae and Thylacinidae). Accordingly we have allocated Malleodectes to the new family, Malleodectidae. Some features suggest potential links to previously named dasyuromorphians from Riversleigh (e.g., Ganbulanyi) but these are too poorly known to test this possibility. Although the original interpretation of a steeply declining molar row in Malleodectes can be rejected, it continues to seem likely that malleodectids specialised on snails but probably also consumed a wider range of prey items including small vertebrates. Whatever their actual diet, malleodectids appear to have filled a niche in Australia’s rainforests that has not been occupied by any other mammal group anywhere in the world from the Miocene onwards.

Systematic Palaeontology

Subclass: Marsupialia Illiger, 1811
Order: Dasyuromorphia Gill, 1872

Family Malleodectidae nov.

Included genera: Malleodectes Arena et al. (2011)

Familial diagnosis: Medium-sized (~1 kg; see below), durophagous, carnivorously-adapted marsupials that differ from all others in the following combination of features: large, caniniform, laterally compressed C1; narrow, premolariform P1 adpressed against the base of C1; asymmetric P2 with wide, diamond-shaped (in occlusal view) posterior region, posteriorly-sloping crown and low, narrow, attenuated anterior region; uniquely (among known dasyuromorphians) large dP3 (similar in size to M1) with three cusps and a functional postmetacrista; enormous, subrounded, dome-shaped, essentially unicuspid, four-rooted P3 that is wider and longer than M1 and M2 (and probably M3), with (M. moenia) or without (M. mirabilis) a tiny cuspule near the posterior edge of the crown; M1 relatively (compared to M2) hypsodont, longer and wider than M2, with StB and StD directly buccal to the paracone and metacone respectively, StD taller than StB, a deep vertical fissure on the buccal flank of the crown, no anterior ectoloph crest, StE present on posterior ectoloph ridge, poorly-developed straight (M1) centrocrista and no posterior cingulum; M2 more conventionally dasyuromorphian-like with v-shaped centrocrista but with conules better developed than in most undoubted dasyuromorphians.

Etymology: The family name derives from the type genus Malleodectes.

Materials: In addition to specimens noted by Arena et al. (2011) we describe here QM F57925, juvenile cranial material including fragmentary left nasal and? frontal bones and a left maxilla with C1, P1, dP3, P3 crown (unerupted), M1–2, alveoli for P2 and M3. QM F57925 is from AL90 Site, a middle Miocene deposit that has been radiometrically dated as 14.64 ± 0.47 Ma old, and which contains a fauna correlating with mid- to late- Riversleigh Faunal Zone C17,18,19,27. AL90 has been interpreted to be a cave deposit, the original entrance of which acted as a natural pit-fall trap27,28,29.

Estimated body mass: Using the “dasyuromorphian-only” dataset of Myers30, the most accurate regression equation that can be used to calculate body mass for Malleodectes mirabilis is the occlusal area of M2. This gives an estimated body mass (including the smearing estimate) of 896 g.

Description of QM F57925, Malleodectes mirabilis
QM F57925 is identified here as Malleodectes mirabilis and is differentiated from M. moenia based on the following features of P3: absolutely smaller; less rounded; lower-crowned; and lacking a posterior cuspule19. Comparison of P3 with that of the holotype of M. mirabilis (QM F50847) and that which Wroe12 originally referred to Ganbulanyi djadjinguli but which Arena et al. (2011) identified as M. moenia, was enabled through digital extraction of the unerupted P3 from micro-CT images (Fig. 3). All P3s referred here to species of Malleodectes are compared in Fig. 4. P3 of QM F57925 is 5.4 mm wide, 6.7 mm long and most closely approximates P3 in M. mirabilis (5.6 wide; 6.5 mm long) rather than the larger M. moenia (6.5 mm wide; 7.1 mm long).

Malleodectes mirabilis using its massive, ball-peen-like P3 to break into what were perhaps one of this unique Miocene marsupial’s favourite meals — Riversleigh escargots.
Illustration by Peter Schouten.  DOI: 10.1038/srep26911

Although malleodectids are only known on the basis of partial upper dentitions, they clearly represent one of the most distinctive groups of marsupials yet discovered. Because they are known from such limited material and because of the many autapomorphic features they exhibit, relatively few of the preserved features clarify their phylogenetic relationships. The features that are available (most obviously, the larger size of stylar cusp D relative to stylar cusp B on M1–2), suggest, albeit tentatively, that malleodectids are dasyuromorphians. Our phylogenetic analysis confirms this assessment, placing Malleodectes within Dasyuromorphia in a polytomy that also includes dasyurids, thylacinids, the fossil forms Barinya and Mutpuracinus, and the sole known myrmecobiid Myrmecobius fasciatus. In terms of dental function, the well-developed molar dentition of Malleodectes suggests that it ate more diverse foods than just snails. The blades on the molars as well as the wear on anterior teeth suggest that small vertebrates were also part of the malleodectid diet. In combination, the large but laterally compressed C1, laterally compressed, delicate P1, hypertrophied, hammer-like P3 and tribosphenic molars with oblique shearing blades suggest that they occupied a niche in Australia’s Miocene rainforests that no other known mammalian group has managed to occupy since.

M. Archer, S. J. Hand, K. H. Black, R. M. D. Beck, D. A. Arena, L. A. B. Wilson, S. Kealy and T.-t. Hung. 2016. A New Family of Bizarre Durophagous Carnivorous Marsupials from Miocene Deposits in the Riversleigh World Heritage Area, northwestern Queensland. Scientific Reports. 6, Article number: 26911. DOI: 10.1038/srep26911

Remains of bizarre group of extinct snail-eating Australian marsupials d... http://bit.ly/1WPDgSa via @UNSWnews @EurekAlertAAAS

Derrick A. Arena, Michael Archer, Henk Godthelp, Suzanne J. Hand and Scott Hocknull. 2011. Hammer-toothed ‘marsupial skinks' from the Australian Cenozoic. Proc. Roy. Soc. B. 278, 3529–3533.  DOI:  10.1098/rspb.2011.0486

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