Ornithorhynchus anatinus, the duckbill platypus, is a unique mammal native to Queensland, New South Wales, Victoria, South Australia, and Tasmania, where it frequents freshwater streams, rivers, lakes, and lagoons. It is not difficult to understand why early European explorers were initially puzzled with the classification of the platypus. This bizzare animal is about the size of a house cat and is covered by thick waterproof hair. It has a beak like a duck, webbed forelimbs for swimming, clawed hind feet for aid in burrowing, a common opening for the reproductive, excretory and digestive systems, and a broad, flat tail. In addition, the males have a single spur on each hind ankle that contains venom, and the females lay eggs (Grant, 1995).
It is now known that the platypus, along with the Australasian echidnas (Tachyglossus and Zaglossus),
comprise the living members of Monotremata, the egg-laying mammals, all of which have a single, common opening for the reproductive, excretory, and digestive systems. The fossil record for monotremes is poor, with few well preserved specimens. The oldest known specimen is a jaw fragment with teeth from the Cretaceous (110 million years old) of Australia (Archer et al., 1985). Other notable specimens include a 62 million year old tooth (Paleocene) from South America (Pascual et al., 1992), and a nearly complete skull of an extinct platypus (Obdurodon dicksoni) from the Miocene (15 million years old) of Australia (Archer et al., 1992; Musser and Archer, 1998).
There is some debate as to whether monotremes are more closely related to marsupials, or if marsupials and placentals are more closely related to each other. Morphological and some molecular data support the marsupial-placental clade, but some molecular studies support the monotreme-marsupial clade.
Although the osteology of Ornithorhynchus is well described (Pritchard, 1881; Wilson and Hill, 1908; Watson, 1916; de Beer and Fell, 1936; Simpson, 1938; Zeller, 1989a, b; Musser and Archer, 1998), the literature is scattered and access to specimens is difficult. Moreover, few studies are based on serial sectioning and concentrate on its internal cavities (Pritchard, 1881; Wilson and Hill, 1908; Watson, 1916; Zeller, 1989a, b) and these data have yet to be incorporated into systematic analysis. To remedy these problems, both adult and juvenile duckbill platypus skulls were scanned. Comparison of the CT images of the platypus specimens displayed on this site with other synapsids will undoubtedly reveal new morphological characters for phylogenetic analysis and help resolve the position of Monotremata within Mammalia.
Additional Information on the Skull
Click on the thumbnails below for labeled images of the juvenile and adult skulls in standard anatomical views.
The skull (cranium and mandible) of AMNH 200255 was scanned by Matthew Colbert at the UTCT on 6 September 2000 (see the UTCT webpage for more details on the scanner). The skull was scanned in its entirety (from the rostrum to the occiput) in the coronal plane at a 160% offset from horizontal, without a filter, using an air wedge. Slice thickness was 0.21 mm, with an interslice spacing of 0.21 mm (the space between consecutive slices). The source-object distance (the distance between the X-ray generation point and the specimen) was 90 mm.
Electrons from the filament (the source of electrons) set at 0.16 mA were accelerated through a potential of 150 kV. Two-dimensional slices were reconstructed from X-rays using the following parameters: field of reconstruction 43.5 mm, reconstruction offset 445, and reconstruction scale 26. Density is related to grayscale values with denser objects appearing as lighter grayscale values.
The scanner produced 438 12 bit, 512x512 pixel grayscale TIFF (tagged image file format) images that were saved as both 8 bit and 16 bit raw files in Adobe Photoshop 6.0®.
The terms "coronal", "sagittal" (= vertical or parasagittal), and "horizontal" are used here to name three orthogonal planes that transect the skull. The term "transverse" is not used because it has been used to represent two different anatomical planes in the literature (see Macrini, 2000 for further discussion).
Image processing for this project was conducted on both Windows and Macintosh platform machines. Conversion of 16 bit images to 8 bit and the initial grayscale level adjustments were done using Adobe Photoshop 6.0®. Digital reslicing of the coronal slice plane was done in NIH Image to produce images in the horizontal and sagittal planes using an interslice thickness (the pixel distance between slices) of 2.47 pixels. The horizontally resliced coronal stack comprised 356 horizontal slices (including one blank slice on each end) with an interslice spacing of 0.085 mm. The vertically resliced coronal stack comprised 476 sagittal slices (including one blank slice on each end) with an interslice spacing of 0.085 mm. The sagittal slices were renumbered in reverse order in RenameMan™ 2.1 because they are mirror images of the slices.
The images in all three slice planes were opened in Photoshop® and the grayscale levels were adjusted for a second time using a standard range of values to maximize the resolution of the images using the fewest number of grayscale values. Output values were adjusted to allow room in the palette for values for the color labels. The images were numbered in Scion Image 4.0.2 using a macro. Numbered TIFF image sequences for each plane were opened in QuickTime Player™ and then exported as self contained, uncompressed JPEG movies.
Canvas size was increased in Adobe Photoshop® to copies of all of the TIFF files to allow space for the anatomical labels (not shown here). The slices were renumbered in Scion Image and saved as TIFF files in IrfanView™. Every fifth slice in each plane was opened in Adobe Illustrator 9®, converted to RGB color, and anatomically labeled. Based on these anatomical labels, a description of the internal osteology will be written.
Three-dimensional volumetric rendering was done in VoxBlast®. The rendering was rotated 360 degrees (4 degrees at a time) in three planes to create rotation frames for the three 3-D movies. These images were opened as a sequence in Quicktime and then exported as a JPEG movie.
LiteraturePlatypus anatomy references:
Archer, M., F. A. Jenkins, Jr., S. J. Hand, P. Murray, and H. Godthelp. 1992. Description of the skull and non-vestigial dentition of a Miocene platypus (Obdurodon dicksoni n. sp.) from Riversleigh, Australia, and the problem of monotreme origins; pp. 15-27 in M. L. Augee (ed.), Platypus and echidnas. Royal Zoological Society of New South Wales, Mosman, NSW, Australia.
de Beer, G., and W. A. Fell. 1936. The development of the Monotremata.- Part III. The development of the skull of Ornithorhynchus. Transactions of the Zoological Society of London 23:1-42.
Fox, R. C., and J. Meng. 1997. An X-radiographic and SEM study of the osseous inner ear of multituberculates and monotremes (Mammalia): implications for mammalian phylogeny and evolution of hearing. Zoological Journal of the Linnean Society 121:249-291.
Green, H. L. H. H. 1937. The development and morphology of the teeth of Ornithorhynchus. Philosophical Transactions of the Royal Society of London B 228:367-420.
Hill, A. 1893. The cerebrum of Ornithorhynchus paradoxus. Philosophical Transactions of the Royal Society of London B 184:367-387.
Hines, M. 1929. The brain of Ornithorhynchus anatinus. Philosophical Transactions of the Royal Society of London B 217:155-287.
Musser, A. M., and M. Archer. 1998. New information about the skull and dentary of the Miocene platypus Obdurodon dicksoni, and a discussion of ornithorhynchid relationships. Philosophical Transactions of the Royal Society of London B 353:1063-1079.
Pritchard, U. 1881. The cochlea of the Ornithorhynchus platypus compared with that of ordinary mammals and birds. Philosophical Transactions of the Royal Society of London 172:267-282.
Shellshear, J. L. 1930. A study of the arteries of the brain of the spiny anteater (Echidna aculeata), to illustrate the principles of arterial distribution. Philosophical Transactions of the Royal Society of London B 218:1-36.
Simpson, G. G. 1929. The dentition of Ornithorhynchus as evidence of its affinities. American Museum Novitates 390:1-15.
Simpson, G. G. 1938. Osteography of the ear region in monotremes. American Museum Novitates 978:1-15.
Turner, W. 1885. The dumb-bell shaped bone in the palate of Ornithorhynchus compared with the prenasal of the pig. Journal of Anatomy and Physiology 19:214-217.
Watson, D. M. S. 1916. The monotreme skull: a contribution to mammalian morphogenesis. Philosophical Transactions of the Royal Society of London B 207:311-374.
Wilson, J. T. 1894. Observations upon the anatomy of the "dumbbell-shaped bone" in Ornithorhynchus, with a new view of its homology. Proceedings of the Linnean Society of New South Wales (2nd series) 9:44-45.
Wilson, J. T., and J. P. Hill. 1908. Observations on the development of Ornithorhynchus. Philosophical Transactions of the Royal Society of London B 199:31-168.
Zeller, U. 1988. The lamina cribrosa of Ornithorhynchus (Monotremata, Mammalia). Anatomy and Embryology 178:513-519.
Zeller, U. 1989a. Die Entwicklung und Morphologie des Schädels von Ornithorhynchus anatinus (Mammalia: Prototheria: Monotremata). Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 545:1-188.
Zeller, U. 1989b. The braincase of Ornithorhynchus. Fortschritte der Zoologie 35:386-391.
Zeller, U. 1991. Foramen perilymphaticum und Recessus scalae tympani von Ornithorhynchus anatinus (Monotremata) und anderen Säugern. Verhandlungen der Anatomischen Gesellschaft 84:441-443.
General mammal anatomy references:
de Beer, G. 1937. The development of the vertebrate skull. Clarendon Press, Oxford, 552 pp.
Evans, H. E. 1993. Miller's anatomy of the dog. 3rd edition. Philadelphia: W. B. Saunders Company, 1113 pp.
Gray, H. 1977. Anatomy, descriptive and surgical. 15th edition. Crown Publishers Inc., 1257 pp.
Macrini, T. E. 2000. High resolution X-ray computed tomography (CT) of the skull of an extant opossum (Monodelphis domestica) and a comparison of its ontogeny to synapsid phylogeny. Unpublished M.S. thesis, University of Texas, Austin, 158 pp. Includes CD-ROM.
General references on monotremes:
Archer, M., T. F. Flannery, A. Ritchie, and R. E. Molnar. 1985. First Mesozoic mammal from Australia- an early Cretaceous monotreme. Nature 318:363-366.
Augee, M. L. (ed.) 1978. Monotreme biology. The Australian Zoologist 20, part 1.
Augee, M. L. (ed.) 1992. Platypus and echidnas. Royal Zoological Society of New South Wales, Mosman, 296 pp.
Burrell, H. 1927. The platypus: its discovery, zoological position, form and characteristics, habitats, life history, etc. Angus and Robertson Limited, Sydney, 227 pp.
Grant, T. 1995. The platypus: a unique mammal. University of New South Wales Press Ltd., Sydney, 92 pp.
Griffiths, M. 1978. The biology of the monotremes. Academic Press, New York.
Lyne, G. 1967. Marsupials and monotremes of Australia. Taplinger Publishing Company, New York, 72 pp.
Musser, A. M. 1998. Evolution, biogeography and palaeoecology of the Ornithorhynchidae. Australian Mammalogy 20:147-162.
Pascual, R., M. Archer, E. O. Jaureguizar, J. L. Prado, H. Godthelp, and S. J. Hand. 1992. First discovery of monotremes in South America. Nature 356:704-706.
Pasitschniak-Arti, M., and L. Marinelli. 1998. Ornithorhynchus anatinus. Mammalian Species 585:1-9.
Mammalian Species account of Ornithorhynchus anatinus (American Society of Mammalogists)
Ornithorhynchus anatinus on The Animal Diversity Web (The University of Michigan Museum of Zoology)
See the brain of Ornithorhynchus at the Comparative Mammalian Brain Collection
For more information on Ornithorhynchus, including an extensive bibliography: What is a platypus?
Other Platypus web sites