The ecomorphology of the Monotreme cranium and the biogeography of Australia and New Guinea

Abstract

Extant Monotremata is an egg-laying mammal Order only found in Australia and New Guinea and is comprised of the genera, Ornithorhynchus, Tachyglossus and Zaglossus. Monotremes have been observed to have a large amount of variation as seen in the number of subspecies and species that were previously recognised for the taxa. This thesis studied the factors that may be responsible for this amount of variation. The thesis was broadly divided into five factors hypothesised to affect the variation of the crania; growth and development, sex differences, climate, phylogeny, and other macroecological and ecological factors.

Growth and development were analysed using both 3D geometric morphometrics and the timing of suture closure. In all three monotremes the suture closure shows the same patterning as the general pattern for mammals with the skull closing in order from vault, facial, and cranio-facial. The platypus demonstrated an increase in cranial size and shape related to the increase in cranial length and the two echidnas were found to have an increase in cranial vault height independent of cranial length.

The mating strategy can be used to indicate size differences between the sexes, with a polygynous mating system predicted to show sexual size dimorphism and a promiscuous mating system predicted to show reduced sexual size dimorphism. To find any differences between the sexes in the crania, a suite of supervised classification techniques were used and as Ornithrohynchus has a polygynous and Tachyglossus has a promiscuous mating system, the results found a large difference between the sexes in the platypus, with the echidna showing little sexual dimorphism in the cranium. When latitude is taken into account some difference between the sexes in Tachyglossus was found. Unfortunately there were not enough specimens to compare the difference between the sexes in Zaglossus.

All three monotremes were found to have a relationship between cranial size (platypus) and cranial vault height (echidnas) with latitude and elevation. The platypus was found to have a positive relationship with latitude and a negative relationship with elevation. Tachyglossus and Zaglossus were found to have a weaker positive relationship of cranial vault height with elevation and latitude.

In this thesis I used an unsupervised clustering method to find similarities in bioclimatic data to create bioregions of Australia and New Guinea. These bioregions, as well as the Interim Bioregions of Australia, and an Australian vegetation map, were used as classes for a suite of supervised learning analyses, to find whether these classes predict a change in the morphology of the monotreme cranium. The results show that latitude, elevation, basin, isolation and bioclimatic factors were important factors in predicting the size and shape change in Ornithorhynchus crania. In Tachyglossus and Zaglossus, bioclimatic factors, elevation, latitude, isolation, and vegetation were found to be important.

From the preceding work several different morphotypes of Ornithorhynchus, Tachyglossus, and Zaglossus are recognised and the thesis suggests populations that may require taxonomic revision.