Endocranial anatomy of Australasian flightless galloanseres, with a focus on evolutionary transformation and the phylogenetic utility of the avian brain

Author: Warren D. Handley

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Handley, Warren D., 2020 Endocranial anatomy of Australasian flightless galloanseres, with a focus on evolutionary transformation and the phylogenetic utility of the avian brain, Flinders University, College of Science and Engineering

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Endocast shape is a proxy for brain morphology in birds and has increasingly been used to infer aspects of both ecology and phylogeny. For taxa known only as fossils, model endocast shape is the only way to assess brain morphology. This project reconstructed model endocasts from Computed Tomography (CT) data of fossil and extant galloanseres (land- and waterfowl and kin), to quantify the evolution of brain shape within closely related avian taxa, and assess variation in previously unknown endocasts for fossil taxa over geological time scales. Geometric information described by anatomical landmark (Lm) coordinates, combined with univariate measurements derived from those data, were used to characterise shape distinctions between galloansere endocasts across diverse temporal scales. In a novel approach for birds, the use of discrete Lm modules to compare endocast shape and infer aspects of ecology and phylogenetic utility, allowed the quantification and assessment of endocranial morphology in a way not previously attained.

I assessed morphological changes over short time scales (~20 kys) using four Finsch’s duck (Chenonetta finschi) endocasts sampling a dated temporal sequence, documenting the transition to flightlessness in the taxon. Assessments identified a trend of hypertrophy of the rostrodorsal and dorsolateral forebrain areas, along with hypotrophy of the hindbrain across time. These endocranial changes are potentially related to increasing reliance on a visually accurate, terrestrial grazing mode of life. Novel descriptions of endocasts of several species of the giant Australian galloanseres in Dromornithidae facilitated an assessment of lineage evolution across deep time (~20–8 Ma). The oldest, an endocast reconstruction for the Oligo-Miocene (~20 Ma) Dromornis murrayi, was compared to the brain of the middle Miocene (~12 Ma) D. planei, digitally extracted from a limestone matrix, and to that of the middle Miocene (~12 Ma) Ilbandornis woodburnei. In addition, partial endocast reconstructions for the late Miocene (~8 Ma) D. stirtoni, aligned with that of D. planei, enabled assessment of changes associated with neurocranial foreshortening across the Dromornis lineage. The dromornithid two lineage hypothesis is supported by minimally five endocranial differences between Ilbandornis and Dromornis. Functional interpretations suggest dromornithids were specialised herbivores that likely possessed stereoscopic depth perception, visual proficiency, and targeted a soft browse trophic niche. The phylogenetic utility of brain morphology was assessed using a combined data set of 34 galloansere endocasts, including nine fossils of six species. The rhombencephalon and mesencephalon zones of the avian brain were shown to convey phylogenetic information. Endocranial morphological correlations suggest C. finschi may not belong in Chenonetta, and its taxonomic affinities require reinvestigation. Distinctive dromornithid eminentia sagittalis morphology supports hypotheses that dromornithids are more closely related to basal galliforms than anseriforms. Close associations noted between the European fossil Mionetta blanchardi, and the extant Australian Malacorhynchus membranaceus support the hypothesised Oligo-Miocene through Miocene basal erismaturine global radiation.

Keywords: Galloanseres, Finsch’s duck, Dromornithidae, brain morphology, geometric morphometrics

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Trevor H. Worthy