Author Identifier

Shelby Middleton: http://orcid.org/0000-0001-5570-3510

Date of Award

2025

Document Type

Thesis - ECU Access Only

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Science

First Supervisor

Robert Davis

Second Supervisor

Kenny Travouillon

Third Supervisor

Linette Umbrello

Fourth Supervisor

Anna Hopkins

Fifth Supervisor

Harriet Mills

Abstract

Taxonomy is a fundamental science, yet with the current rate of biodiversity loss and insufficient resources being contributed, many species may be at risk of extinction prior to their discovery and description. Accurate species delimitation is essential and ideally should precede the planning of conservation efforts to ensure management and monitoring plans are appropriate and effective. The phenotypic variation observed in the common brushtail possum (Trichosurus vulpecula) has led to a complex and contested taxonomic history. Ongoing uncertainty regarding species boundaries, combined with population declines across much of their range, highlights the need for a modern and systematic reassessment of T. vulpecula in Australia. This study represents the first total evidence approach applied to this group, integrating comprehensive morphological and molecular data from across the continent to clarify the taxonomy and historical biogeography of Trichosurus populations.

In Chapter 2 of this thesis, I conducted a literature review summarising the current and historical taxonomic understanding of the genus Trichosurus. Early taxonomic descriptions were often based solely on morphology, lacked sufficient locality data, and frequently had no holotype designated, resulting in the naming of numerous species that are no longer considered valid. This review established a foundation for identifying key gaps addressed in subsequent chapters, as well as presenting photographic documentation of type specimens obtained from museums worldwide.

In Chapter 3, I used both single nucleotide polymorphism (SNP) markers and mitochondrial cytochrome b sequences to assess phylogenetic relationships, ancestry, and genetic differentiation across Trichosurus populations. The results revealed that current subspecies classifications of T. vulpecula do not reflect underlying genetic structure. Populations from the Pilbara and Midwest in Western Australia, previously assigned to T. v. hypoleucus, clustered genetically with southeastern and central T. v. vulpecula. Samples from Broome in Western Australia grouped with southwest and Barrow Island populations, indicating that they are T. v. hypoleucus, not T. v. arnhemensis as previously assumed. The genetic divergence between T. caninus and T. cunninghami was also comparable to the level of differentiation observed among genetic clusters within T. vulpecula, raising further questions about current taxonomic boundaries.

In Chapter 4, I examined museum specimens in a comprehensive morphological analysis, using both linear and 3D geometric morphometrics of cranial and dental features, along with external body measurements and pelage coloration. Morphological variation was assessed across genetically defined clades and geographic populations, and environmental drivers of cranial shape were evaluated. Significant differences in skull size and shape were identified across species, subspecies, and populations. While size explained the majority of the variation, shape differences remained significant even after accounting for size. Morphological variation was strongly associated with habitat type and vegetation structure, indicating that adaptive divergence has occurred in response to diet and environmental gradients. Our findings revealed that morphological divergence within Trichosurus vulpecula exceeded that between the currently recognised species in the genus.

Finally, in Chapter 5, I conducted a total evidence analysis to assess the alignment between genetic clades and morphological characters, estimate divergence times, and infer an evolutionary hypothesis for speciation within Trichosurus. A character-based phylogeny was constructed from cranial and dental characters, and was used to test whether morphological differentiation correlated with genetic divergence. Divergence time estimates suggested that many populations of T. vulpecula have experienced limited gene flow since the Pleistocene, with T. v. hypoleucus diverging as early as the Pliocene. Ancestral range reconstructions indicated that dispersal played a significant role in the historical range expansion of brushtail possums, while founder events were important in the establishment of island populations. Based on these integrated data, a taxonomic revision of the T. vulpecula species complex is proposed, including the formal description of new taxa. This research demonstrates the value of using multiple lines of evidence to resolve species complexes, and the results from this study will have important implications on the conservation of brushtail possums, especially in the context of management decisions such as selecting source populations for translocations.

Access Note

Access to this thesis is embargoed until 13th February 2031

DOI

10.25958/9v40-9473

Download

Available for download on Thursday, February 13, 2031

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