Silvabestius johnnilandi

Introduction

Silvabestius johnnilandi was a rare, sheep-sized diprotodontoid marsupial, one of the smallest and most primitive discovered to date. It is late Oligocene in age, known from two skulls found in close proximity: almost certainly that of a female adult and its pouch young. Silvabestius would have been a browser, feeding on leaves, stems and other soft parts of plants.

Identification

Members of the family Diprotodontidae were quadrupedal, herbivorous marsupials with proportionately large heads (but small brains) and stout limbs. There are two subfamilies in Diprotodontidae: Diprotodontinae and Zygomaturinae. Zygomaturines are distinguished from diprotodontines by a complex upper third premolar (P3/) with from three to five cusps.

Silvabestius (known from two complete skulls, an adult and juvenile) is placed in Zygomaturinae because it has a typically 4-cusped zygomaturine premolar (a parastyle on P3/ is developed) and it has a ventral alisphenoid tympanic process, as in other zygomaturines. It is distinguished from other zygomaturines by the following combination of features: small size; possession of upper canine teeth; features of the upper third premolar (absent to small hypocone; a small, poorly developed parastyle dominated by an anterolingual blade; absence of a deep trench separating parastyle from parametacone, and prominent anterobuccal blade extending forward from metacone); small parastyle on M1/; absence of a distinct gradient in molar size from M1-4/; and other features of the dentition (for further characters, see Black and Archer 1997b). Unlike later species of Zygomaturus, Maokopia and Hulitherium, the skull of Silvabestius lacks a constricted diastema on the upper jaw, an inferior premaxillary suture close to I3/; an anteriorly concave lateral premaxilla/maxilla suture; high, steep frontals; and frontated (broadened) orbits (from Black 2007). Silvabestius shares several skull and dental features with the small zygomaturine Nimbadon lavarackorum, also from Riversleigh.

Silvabestius johnnilandi differs from the older and more archaic Silvabestius michaelbirti (known from a relatively complete cranium) in that its teeth are more high-crowned, the P3/ is more complex in form (S. michaelbirti lacks a parastyle, the protocone is larger and it lacks a distinct hypocone); and the protoloph on M1/ is longer and more anteriorly convex. The S. michaelbirti skull has a well developed sagittal crest, which may indicate that it was either an older animal or a male of the species.

Both an adult skull and the skull and lower jaw of a juvenile (a pouch young, designated as the holotype) are known. The skull of the adult (probably the mother of the juvenile) is about 25cm long, indicating that adult Silvabestius were sheep-sized. The juvenile was most likely a pouch young: its dentition, still in the process of erupting, is completely unworn (providing crucial information on crown morphology); the cranial bones are still unfused; and upper and lower incisors do not meet when the jaws are brought into occlusion, (a small gap for the mother's nipple). Furthermore, the skulls were discovered nose-to-nose, suggesting a close relationship between the adult and young.

Silvabestius was most closely related to a second species of Silvabestius, S. michaelbirti from the late Oligocene of Riversleigh. The genus Silvabestius was most closely related to Nimbadon lavarackorum, another small diprotodontoid from Riversleigh, and more distantly related to the Pleistocene Zygomaturus. The closest living relatives of diprotodontoids are wombats (Vombatiformes).

Habitat

The Riversleigh region during the late Oligocene was a mix of vegetation types, suggesting a drier and more open environment than later Miocene Riversleigh habitats. There may have been a large lake in the area, a karst (limestone) landscape.

Distribution

Silvabestius johnnilandi is known only from the Riversleigh World Heritage Fossil Area in northwestern Queensland.

Feeding and diet

All zygomaturines were browsers rather than grazers. Silvabestius would have fed on leaves, stems and shoots of forest plants. Its relatively low-crowned teeth and lack of extensive wear suggest that the diet of Silvabestius was fairly soft (non-abrasive) plant matter.

Life history cycle

Zygomaturines, like all marsupials, began life as tiny neonates that completed their growth after birth in the mother's pouch. Living vombatiforms (koalas and wombats) have a backwardly-facing pouch, and it is probable that diprotodontoids like Silvabestius had the same. The discovery of a pouch young of Silvabestius gives palaeontologists a good look at the stage of development of early diprotodontoids during this critical life stage.

Silvabestius is known from just five specimens in two species, suggesting that these were rare and possibly solitary animals. There were many potential predators known from the late Oligocene of Riversleigh that may have been a threat to the sheep-sized Silvabestius, including thylacines, marsupial 'lions' (thylacoleonids) and large crocodiles.

Fossils description

Silvabestius johnnilandi is known from two well-preserved fossil skulls, those of an adult and juvenile found close together at the Riversleigh World Heritage fossil site (VIP Local Fauna: 'System A' or late Oligocene) in northwestern Queensland. The juvenile skull and lower jaw have been designated as the holotype, and all material is held by the Queensland Museum, Brisbane.

A third species of Silvabestius, originally described as a species of Palorchestinae, has been proposed. This specimen is poorly preserved and some features said to distinguish it from other species are variable, so it most likely does not represent a valid third Silvabestius species (Black 2007).

Evolutionary relationships

Diprotodontids are included in the suborder Vombatiformes, along with wombats and koalas. They first appeared in the late Oligocene, about 25 million years ago. These small, early diprotodontoids were probably descended from wynyardiids (small marsupials with a dentition intermediate between that of possums and diprotodontoids). Diprotodontidae is divided into the subfamilies Diprotodontinae and Zygomaturinae, distinguished from each other primarily on dental grounds.

Zygomaturines may have evolved from a primitive diprotodontine-like ancestor with a small parastyle on the P3/. The subfamily includes several species of small diprotodontids from New Guinea, Alcoota and Riversleigh as well as the larger Zygomaturus from the Pleistocene of Australia. Silvabestius may be at the base of the zygomaturine tree as its most primitive (plesiomorphic) member, making the juvenile an especially important discovery. S. michaelbirti, from slightly older late Oligocene deposits, is more primitive (plesiomorphic) than S. johnnilandi. However, species of Silvabestius may not be ancestral to later zygomaturines.

References

• Black, K. 1997. Diversity and biostratigraphy of the Diprotodontia of Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 42, 187-192.
• Black, K. (thesis)
• Black, K., and Archer, M. 1997b. Silvabestius gen. et sp. nov., a primitive zygomaturine (Marsupialia, Diprotodontidae) from Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 41, 193-208.
• Black, K. and Mackness, B. 1999. Diversity and relationships of diprotodontoid marsupials. Australian Mammalogy 21, 20-21; 34-45.

Further reading

• Archer, M., Hand, S. J. and Godthelp, H. 1994. Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Chatswood.
• Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore, 240 pp.