The fibrovascualar ring: A synapomorphy of hystricognath Rodentia newly described in Petromus typicus and Octodon degus

Hystricognathi is a higher monophyletic taxon within Rodentia that is supported particularly by characters of early ontogeny and placentation. Since the original findings are based on a small sample of species, further information on character distribution is desirable. The present paper provides new insight, into the morphology of one of the most significant characters in two additional species of Hystricognathi, i.e. Petromus typicus A. SMITH, 1831 and Octodon degus MOLINA, 1782. In both species an arterial ring within the inverted yolk sac splanchnopleura that is associated with a network of capillaries is present throughout pregnancy. Consequently, the existence of the so called ``fibrovascular ring'' is confirmed for these species and distributional data Suggest that it can be regarded as a synapomorphic feature of Hystricogrnathi. Thus, two of the three original diagnostic characters of Hystricognathi are now confirmed for a larger set of taxa.