Herds Overhead: Nimbadon lavarackorum (Diprotodontidae), Heavyweight Marsupial Herbivores in the Miocene Forests of Australia

Abstract

The marsupial family Diprotodontidae (Diprotodontia, Vombatiformes) is a group of extinct large-bodied (60–2500 kg) wombat-like herbivores that were common and geographically widespread in Cenozoic fossil deposits of Australia and New Guinea. Typically they are regarded to be gregarious, terrestrial quadrupeds and have been likened in body form among placental groups to sheep, rhinoceros and hippopotami. Arguably, one of the best represented species is the zygomaturine diprotodontid Nimbadon lavarackorum which is known from exceptionally well-preserved cranial and postcranial material from the middle Miocene cave deposit AL90, in the Riversleigh World Heritage Area, northwestern Queensland. Here we describe and functionally analyse the appendicular skeleton of Nimbadon lavarackorum and reveal a far more unique lifestyle for this plesiomorphic and smallest of diprotodontids. Striking similarities are evident between the skeleton of Nimbadon and that of the extant arboreal koala Phascolarctos cinereus, including the powerfully built forelimbs, highly mobile shoulder and elbow joints, proportionately large manus and pes (both with a semi-opposable digit I) and exceedingly large, recurved and laterally compressed claws. Combined with the unique (among australidelphians) proportionately shortened hindlimbs of Nimbadon, these features suggest adept climbing ability, probable suspensory behaviour, and an arboreal lifestyle. At approximately 70 kg, Nimbadon is the largest herbivorous mammal to have occupied the forest canopies of Australia - an ecological niche that is no longer occupied in any Australian ecosystem and one that further expands the already significant niche diversity displayed by marsupials during the Cenozoic.

Middle Miocene origins for tough-browse dietary specialisations in the koala (Marsupialia, Phascolarctidae) evolutionary tree: description of a new genus and species from the Riversleigh World Heritage Area

“…The morphologically derived nature of Stelakoala agrees with the suggested middle Miocene (Faunal Zone C) age for the Jim's Jaw Site Type Locality which was previously based on lithostratigraphic relationships and biocorrelation of its contained fauna (Creaser, 1997;Travouillon et al, 2006;Black et al, 2013;Gillespie et al, 2014). Further evidence for a middle Miocene age comes from recently obtained U-Pb radiometric ages of spatially related deposits on Riversleigh's Gag Plateau.…”

“…1) in a sequence of horizontally bedded sediments, topographically four meters below Ringtail Site (Creaser, 1997) which has been radiometrically dated at 13.56 ± 0.67 Ma (Woodhead et al, 2016). Jim's Jaw Site contains a large chronomorph (Black et al, 2013;fig. 7, table B) of the zygomaturine diprotodontid Neohelos tirarensis Stirton, 1967, a form antecedent to Neohelos stirtoni Murray et al, 2000.…”

“…7, table B) of the zygomaturine diprotodontid Neohelos tirarensis Stirton, 1967, a form antecedent to Neohelos stirtoni Murray et al, 2000. The latter species is known from AL90 Site (Black et al, 2013;Arena et al, 2014), a palaeocave deposit located on the southern section of the Gag Plateau (Arena et al, 2014), that has been radiometrically dated at between 14.17 and 15.11 Ma (Woodhead et al, 2016). Most recently, Arena et al (2016) have further temporally subdivided Riversleigh's middle Miocene Faunal Zones C assemblages on the basis of species successions in nine contemporaneous marsupial lineages.…”

“…Evidence for extensive rainforest habitats at Riversleigh has been well documented, at least until the end of the middle Miocene Climatic Optimum (Archer et al, 1994(Archer et al, , 1997bTravouillon et al, 2009Travouillon et al, , 2012Black et al, 2012bBlack et al, , 2012cArena et al, 2014;Woodhead et al, 2016). After this time (circa 15-13 Ma), palaeotemperatures dropped by as much as 7°C (Shevenell et al, 2004;Billups and Scheiderich, 2010) resulting in the regional collapse of rainforests and the expansion of sclerophyll communities, with Eucalyptusdominated forests becoming widespread by late Miocene times (Martin, 2006).…”

A quantitative comparative analysis of the size of the frontoparietal sinuses and brain in vombatiform marsupials

“…In K. torus, a diprotodontoid from the late Miocene (body size approximately 125-250 kg; Murray, 1997), pneumatisation becomes apparent when the M4 has begun to emerge. In N. lavarackorum (body size approximately 70 kg; Black et al, 2012), sinuses develop early, becoming larger and more expansive as the animal matures to accommodate the progressive separation of the outer and inner bony tables . found that in N. lavarackorum, a thick diploe layer was present in suckling pouch young (Stage II), and by the time the animal was fully weaned (Stage IV), sinuses had begun developing in the squamosal and were already well developed in the frontals.…”