Platypus Ornithorhynchus anatomy
The Skull of the Platypus (Ornithorhynchus anatinus) Ornithorhynchidae
The skull of the platypus is adapted for the use of electro- and mechanosensory organs in the upper and lower bill to hunt prey in its freshwater environment. The upper bill has a core formed by the crura of the premaxilla. Most sensory innervation of the upper bill is provided by the infraorbital branch of the maxillary division of the trigeminal nerve [5max(inf)], which passes through the infraorbital foramen to the skull interior. The lower bill is formed around the forward projecting dentary bones (mandible). Each dentary has a large mandibular canal which carries the peripheral branch of the mandibular division of the trigeminal nerve (5mand). The mandibular division enters the skull through the foramen ovale. Adult platypuses do not have teeth, so prey are mechanically reduced by grinding between paired horny or keratinized plates on the maxilla and dentary.
Figure 1 shows superior (a), right lateral (b), and posterior (c) views of a CT imaged adult platypus skull. The skull is dominated by the bones forming the upper and lower bill anteriorly, and the braincase posteriorly. Teeth are absent, but bony plates in the maxilla and dentary are covered in thick keratin to provide grinding plates for breaking up chitinous invertebrate prey for digestion. Platypus skull from UNSW Zoology collection, CT scanned at Prince of Wales Radiology Department, Randwick.
The interior of the skull houses the pear-shaped brain with a smooth cerebrum. Unlike echidnas, the platypus does not have a cribriform plate and olfactory nerve fibres pass through narrow olfactory foramina from the nasal cavity to the small olfactory bulbs. The midline falx cerebri between the two cerebral hemispheres is ossified to form a falcial septum.
Figure 2 shows the interior of the skull of the platypus in superior (a), left (b), and right (c) views. Note the small nasal cavity in contrast to the echidna, the small olfactory foramina, and the presence of a bony falcial septum between the cerebral hemispheres. The brain connects with the spinal cord through the foramen magnum. Platypus skull from UNSW Zoology collection, CT scanned at Prince of Wales Radiology Department, Randwick.
Platypus skull modified from Cabrera, A (1919). Genera mammaliumMadrid,1919.biodiversitylibrary.org/page/32474795
Platypus anterior forelimb modified from Cabrera (1919). Genera mammalium Madrid,1919.biodiversitylibrary.org/page/32474795
Platypus posterior limb ventral view modified from Cabrera (1919), Genera mammaliumMadrid,1919.biodiversitylibrary.org/page/32474795