Pengana robertbolesi

Introduction

Pengana was a large, long-legged bird of prey from the Miocene of Riversleigh, with modified ankle joints that allowed it to bend its feet not just forwards, like most birds, but backwards and sideways. Two living hawks bend their feet in a similar way, using the flexible, mobile ankle to probe into hollows and recesses to reach prey. Penganamay have had a similar strategy, giving it access to prey unavailable to its competitors.

Identification

Pengana (from an Aboriginal word meaning 'hawk') was a very large raptor, or bird of prey. Its ankle joint (distal tibiotarsus) has features common only to hawks, eagles and their kin (Accipitridae). Pengana was larger than all other Australian accipitrids, fossil or recent, except the Wedge-tailed Eagle (Aquila audax) and White-breasted Sea Eagle (Haliaeetus leucogaster).

Most birds of prey have medially and laterally expanded condyles on the distal end of the tibiotarsus that articulate with the expanded proximal end of the tarsometatarsus, forming the intertarsal (ankle) joint. Pengana's intertarsal joint was modified to allow a much greater degree of flexion than in most other Accipitridae. In Pengana, the distal end of the tibiotarsus is not expanded but narrow (the distal condyles are laterally compressed). This narrowness would reduce the distance that ligaments must stretch, improving the flexibility of the joint. A second character, the orientation of the supratendinal bridge, is off the vertical in Pengana (in contrast to the vertical orientation of most raptors). These specializations, or modifications, are also seen (and to a greater degree) in harrier hawks (Polyboroides species) and the unrelated Crane Hawk (Geranospiza caerulescens), but not in other hawks or eagles. Polyboroides and Geranospiza both capture prey by reaching into tree hollows or crevices with their flexible clawed feet. Since none of these raptors - Polyboroides, Geranospiza or Pengana - is closely related, this is an example of convergent evolution, where unrelated animals faced with a similar problem independently develop (converge on) the same solution to that problem.

Habitat

The Riversleigh site ('Sticky Beak') where Pengana was found was probably a large, deep lake bordered by gallery rainforest, in a karst landscape.

Distribution

Pengana is known only from the Riversleigh World Heritage Fossil Site, northwestern Queensland.

Feeding and diet

Harrier hawks and the Crane Hawk take invertebrates, small vertebrates and eggs from tree hollows, other crevices and from under bark. The hanging nests of colonial weaverbirds are often raided and nestlings taken. The African Harrier-hawk, Polyboroides typus also includes the fruit of oil palms in its diet when palms are present. None of these hawks are swift, and adult mammals and birds are usually taken only when trapped.

Given its large size, Pengana may have hunted and eaten comparatively large prey such as possums or other mammals, lizards, frogs or perhaps baby birds nesting in tree hollows. Calculation of the length of its legs suggests Pengana could have reached 40 cm into crevices or hollows. Animals from the Riversleigh region that may have nested in tree hollows include parrots, kingfishers, owls, owlet-nightjars and possums. Passerines (many of which nest in bark recesses, and colonial nesters (starlings of the genus Aplonis) were also known to have been present, as were frogs, small lizards and even palms.

Life history cycle

Pengana may have lived in or frequented forest habitats, as harrier hawks and the Crane Hawk do, to take advantage of the possums, other mammals and birds living in hollow trees.

Fossils description

The only fossil of Pengana is the end of a leg bone (a distal tibiotarsus), held by the Queensland Museum, Brisbane.

Evolutionary relationships

The higher level relationships of Accipitridae are not well resolved, and it is especially difficult to place fragmentary fossil remains in a meaningful context. Most of the skeleton, as well as critical skull material, is lacking for Pengana, and little can be said other than that it is a specialised accipitrid. Although the harrier hawks (from Africa and Madagascar) and the Crane Hawk (from South America) are from southern continents, there is no evidence that they are either related to each other or to Pengana, or that they were part of a Gondwanan group of specialized raptors.

References

• Boles, W. E. 1993. Pengana robertbolesi, a peculiar bird of prey from the Tertiary of Riversleigh, northwestern Queensland, Australia. Alcheringa 17, 19-25.
• Boles, W. E. 2006. Chapter 21: The avian fossil record of Australia: an overview. Pp. 387-411 in Merrick, J. R., Archer, M., Hickey, G. M. and Lee, M. S. Y. (eds) Evolution and Biogeography of Australasian Vertebrates. Australian Scientific Publishing, Oatlands.

Further reading

• Debus, S. 1998. The Birds of Prey of Australia: A Field Guide to Australian Raptors.Oxford University Press, USA.
• Olson, P. 1995. Australian Birds of Prey: The Ecology and Biology of Raptors.University of New South Wales Press, Sydney/Johns Hopkins University Press, Baltimore.
• Olson, P. 2005. Wedge-tailed Eagle. CSIRO Press, Canberra.
• Thomson, C. and Dell, J. 1998. Australian Birds of Prey. Western Australia Department of Conservation and Land Management, Como.