An interdisciplinary approach to the ecology, morphology, and evolutionary relationships of Genyornis newtoni (Aves, Dromornithidae), with implications for the greater dromornithid radiation

Author: Phoebe McInerney

  • Thesis download: available for open access on 10 Apr 2027.

McInerney, Phoebe, 2024 An interdisciplinary approach to the ecology, morphology, and evolutionary relationships of Genyornis newtoni (Aves, Dromornithidae), with implications for the greater dromornithid radiation, Flinders University, College of Science and Engineering

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The Australasian continent has long been renowned for its diverse and unique avifauna, driven by its Gondwanan origin, long periods of isolation, and island nature. The avian family, Dromornithidae, a close relative of the living Galloanserae (land- and waterfowl), is endemic to Australia, with no fossils found off Australian shores despite their long fossil record spanning most of the Cenozoic era. The first dromornithid fossils were discovered in the mid-19th century and based on their gigantic sizes and geographic location, initially aligned with the ratites now in the infraclass Palaeognathae. This lineage comprises several well-known species of extant, giant birds, including two genera from Australia (Casuarius and Dromaius). Over the decades since the first description of a dromornithid, seven additional species have been described within four genera. Many of these species are now known from numerous well-preserved post-cranial and skull elements, derived from fossil localities across most of eastern Australia, and two in Western Australia. Questions were raised about the affinities of the dromornithids to the palaeognaths although it was not until skull material was assessed was it revealed that the dromornithids were instead affiliated with the galloanserans. Characteristics which aligned the dromornithids with the palaeognaths were thus identified as convergent and are related to the morphological changes driven by gigantism and the loss of flight capabilities.

Many people have contributed to the collection of, and research into, the Dromornithidae and resultantly, extensive knowledge on this radiation of birds is available. Nonetheless, several aspects of their morphology and ecology remained unknown due to their limited and patchy fossil record. Firstly, the greatest diversity of dromornithids occurred during the Miocene with minimally three coexisting species in two genera (Dromornis planei or D. stirtoni with two species of Ilbandornis), reducing to two species (Dromornis australis and Ilbandornis sp.) in the Pliocene, and culminated in only one known surviving species in the Pleistocene, Genyornis newtoni. This species has been the focus of debate regarding drivers of its extinction, predominantly due to tentatively assigned fossil eggshell, and resultantly little is conclusively known about population changes leading to the disappearance of G. newtoni from the fossil record. Secondly, dromornithids have an unusual skull morphology that differs between genera where known, but there is a lack of complete skulls for most taxa. Until now, no informative skull of G. newtoni was available. Thirdly, despite these limitations, the placement of the dromornithids within the galloanseran radiation is strongly supported and the dromornithids have been considered sister taxa to the giant, flightless Gastornithidae from the Northern Hemisphere. Their affinities with other galloanseran lineages are less clear though, resulting from a pull towards either of the modern orders Anseriformes and Galliformes, linked to the effects of extensive homoplasy and missing data. The discovery of additional fossil material to reduce the amount of missing data in phylogenetic studies and a reassessment of homoplasious characters were required to resolve these issues. Finally, dromornithids are typically sexually dimorphic in size, take a long time to asynchronously reach sexual and skeletal maturity, and have a brain morphology correlated with adaptations for picky eating habits, although it is unknown if other sensory organs have similar adaptations.

Since 2013, expeditions to Lake Callabonna have recovered numerous fossils of partial individuals of Genyornis newtoni, including elements not well represented by those excavated in previous years. As the associations of these fossils are retained, wide sampling of individuals for assessment of morphological and life-history intraspecific variation for Genyornis newtoni is possible, in addition to broader taxonomic comparisons and inferences relevant to all dromornithids. The research contained in this thesis utilises these fossils to form hypotheses on four key questions about Genyornis newtoni and the family, Dromornithidae, to address the limitations in current research identified above. First, I report the rare occurrence of palaeo-pathologies in G. newtoni; second, I describe new skull material for G. newtoni and use it to establish robust homologies of morphological features between dromornithids and extant galloanserans; third, from this, I establish a novel dataset to investigate the phylogenetic relationships of dromornithids; and fourth, I describe and analyse the vestibular structures of Genyornis and other dromornithids. From this, the thesis aims to contribute to a holistic understanding of Genyornis newtoni, and the dromornithids as a group in the context of the galloanseran radiation, building on the work of previous researchers to resolve the gaps in the literature, through novel approaches and modern methodologies.

The sample of Genyornis newtoni from Lake Callabonna revealed a surprising frequency of palaeo-pathologies. Assessment of the fossils attributed to Genyornis newtoni led to identification of minimally four individuals in a population of 34, which had suffered from osteomyelitis, a bone infection, during a time of severe drought around 48 thousand years ago. Similar pathologies were identified in several Dromornis stirtoni and Ilbandornis woodburnei phalanges from a fossil deposit in the Northern Territory which has also been associated with a drought event. These lesions indicate the possibility of immunosuppression in populations of dromornithids during times of environmental stress. Detailed morphological descriptions of skull material for Genyornis newtoni and broad taxonomic comparisons, paired with a revision of patterns of character state changes across early galloanserans, contributed to understanding previously elusive aspects of the mosaic dromornithid morphology. Characters were assessed phylogenetically, and conclusions considered within an ecological and palaeogeographic context, all revealing compelling support for closer affinities of dromornithids with the anseriforms (specifically the South American Screamers) than the galliforms; little evidence was identified for a sister-taxon relationship with the gastornithids. Functional assessment of the dromornithid morphology evidenced functional adaptations for fine, independent movement of upper and lower jaws, aquatic feeding, a wide gape, and a soft-browse diet. Visualisation of the vestibular apparatus (inner ear) via micro-computed tomographic scanning, 3D modelling methods, and morphometric landmark analyses revealed the cochlea of dromornithids to be unusually short, indicative of a restricted, low frequency hearing range, potentially simple, closed-mouth vocalisations, and evolutionary adaptations for dense environments or long-distance communication. Vestibular morphology suggests the birds were capable of fine head movement and significant morphological and size variation in the bony labyrinth morphology appears driven by evolutionary changes associated with large body and skull size and flightlessness. However, interactions between variables associated with these factors is indicative of complexities surrounding drivers of inner ear shape change, not yet well understood.

Keywords: Dromornithidae, flightless, skull morphology, evolution, galloansere, inner-ear, phylogenetics, Genyornis newtoni, pathology, palaeopathology

Subject: Science, Technology and Enterprise thesis

Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Trevor H Worthy