A high degree of anatomical variability in many structures of Otaria byronia (Blainville, 1820) and Arctocephalus australis (Zimmerman, 1783) was observed, which should yield serious implications to the systematics of Otariidae. The main differences observed between the two species were: the breadth of the rostrum, the exposure (or not) of the ethmoid in the orbit, the palatine/pterygoid vacuity, the extension of the maxilla, the concaveness and shape of the palate, the size/shape of the orbit/supra-orbital process, the height/shape of the zygomatic arch, the shape of the hamulus pterygoideus, the size of processes and crests, the proportions of the sphenoid and of neurocranium, the occurrence or not of the vidian and transverse canals, the relieves of the cranial fossae, and the size of secondary angular process. While some differences (ethmoid) are rather unusual between contemporary species belonging to the same family, many others are of allometric nature. Some differences are peramorfic: skull size, palate size and crests size (hypermorphic in O. byronia). Others caracters are pedomorphic in this species: incisive foramen, maxille process of the frontal and vidian channels. The main differences between males and females of O. byronia, and similitarities between de males of this species and A. australis are related to developmental modification in rate or in the timing too. Some characters usually used in the group's systematics were not corroborated, mostly those related to the naso-labialis fossa; zygomatic-temporal suture; internal acoustic meatus and entotimpanic.
Syncranium; anatomy; osteology; south american sea lion; south american fur seal
