Comments on the generic position of Typton australis Bruce, 1973, and some related taxa (Crustacea: Decapoda: Pontoniinae)

Abstract

A recent communication by Marin (2008) discussed the generic position of Typton australis Bruce, 1973, with the conclusion that it should be transferred to the genus Onycocaridella Bruce, 1981, with further changes to the generic attributions of Onycocaridella antokha Marin, 2007 and Onycocaridella monodoa (Fujino & Miyake, 1969) and Onycocaridella stenolepis (Holthuis, 1952), both returned to the genus Onycocaris Nobili, 1904. The assessment of the generic situation of T. australis is not helped by the lack of a detailed illustrated description of T. spongicola Costa, 1844, the type species of the genus, which is in the process of rectification. The range of morphological variation found between the 16 species (including T. australis) referred to Typton strongly suggests that that assemblage is artificial and that the genus is in need of revision. Based on the key of Holthuis (1993), the synoptic diagnosis of the genus Typton is: rostrum present, scaphocerite rudimentary, mandibular palp absent, exopods present on all maxillipeds, ambulatory dactyls biunguiculate. The rudimentary scaphocerite distinguishes Typton from Onycocaris and Onycocaridella in which it is well developed. This character is shared with Typtonychus Bruce, 1996, but both genera can be readily distinguished by the presence of filtratory mouthparts in the latter genus. However, an autapomorphic generic character not utilized in the key by Bruce (1995), but included in the diagnosis given for the genus Typton (see Bruce 1995: 144), is the presence of shearing cutting edges on the fingers of the minor second pereiopod. This configuration of the minor cheliped fingers is present in all species currently referred to Typton, including T. australis, and is a character of major significance in the diagnosis of this genus, being absent from species of Onycocaris and Onycocaridella.