Abstract
This thesis began as an investigation into evolution of the platypus family (Ornithorhynchidae,
Monotremata), now known from both Australia and South America. The thesis broadened its scope
with inclusion of non-ornithorhynchid Mesozoic monotremes from Lightning Ridge, NSW. This
change in direction brought an unexpected result: a fossil mammal from Lightning Ridge
investigated for this thesis (presumed to be monotreme: Flannery et al., 1995) appears to be a new
and unique type of mammal. Specimens were procured through Queensland Museum (Riversleigh
material); Australian Museum (Lightning Ridge material); and Museum of Victoria and the South
Australian Museum (fossil ornithorhynchids). Specimens were examined under a light microscope
and scanning electron microscope; specimens were photographed using light photography and a
scanning electron microscope; and illustrations and reconstructions were done with a camera lucida
microscope attachment and photographic references. Parsimony analysis utilised the computer
programs PAUP and MacClade. Major conclusions: 1) analysis and reconstruction of the skull of
the Miocene platypus Obdurodon dicksoni suggest this robust, large-billed platypus was a derived
northern offshoot off the main line of ornithorhynchid evolution; 2) the well-preserved skull of
Obdurodon dicksoni shows aspects of soft anatomy previously unknown for fossil
ornithorhynchids; 3) two upper molars from Mammalon Hill (Etadunna Formation, late Oligocene,
central Australia) represent a third species of Obdurodon; 4) the South American ornithorhynchid
Monotrematum sudamericanum from the Paleocene of Argentina is very close in form to the
Oligocene-Miocene Obdurodon species from Australia and should be considered congeneric; 5) a
revised diagnosis of the lower jaw of the Early Cretaceous monotreme Steropodon galmani includes
the presence of two previously undescribed archaic features: the probable presence of postdentary
bones and a meckelian groove; 6) morphological evidence is presented supporting a separate family
Steropodontidae; and 7) analysis of new fossil material for Kollikodon ritchiei suggests that this
taxon is not a monotreme mammal as originally identified but is a basal mammal with close
relationships to allotherian mammals (Morganucodonta; Haramiyida). Kollikodon is provisionally
placed as basal allotherian mammal (Allotheria sensu Butler 2000) and is unique at the ordinal
level, being neither haramiyid nor multituberculate. A new allotherian order – Kollikodonta – is
proposed.