The remarkable diversity in chromosome number within Planipapillus Reid 1996, the most speciose genus of peripatopsid onychophorans, centred in the alpine region of south-eastern Australia is documented. Robertsonian, whole-arm rearrangements account for the twofold range of diploid numbers in Planipapillus. Ooperipatellus Ruhberg 1985, another clade of oviparous onychophorans, shows a very different pattern, with no karyotypic diversity among species from Australia and New Zealand. Rate constancy in chromosomal evolution among peripatopsid genera would indicate an ancient radiation in Planipapillus, with extant species representing relictual survivors of more recent Pleistocene climatic upheavals. Conversely, if the rearrangements in Planipapillus are the result of recent and rapid karyotypic restructuring, the karyotypic and species diversity within the genus may be attributed to recent population fragmentation and isolation resulting from the repeated glaciation and warming cycles of the Pleistocene rather than stemming from a more ancient radiation. Data from other animal groups support a model of accelerated rates of Robertsonian centric fusions concordant with a recent radiation in Planipapillus. Karyotype change may be an important factor in the generation and maintenance of Planipapillus diversity.

Size and shape changes in the skull of South American spiny rats Proechimys spp. were investigated using geometric morphometrics. Six species were studied (P. guairae, P. poliopus, P. trinitatis, P. canicollis, P. mincae and Proechimys sp.) using 12 populations from Venezuela and Colombia. Proechimys poliopus, P. guairae and Proechimys sp. have undergone recent speciation coupled with chromosomal rearrangements. The separate analysis of size and shape showed that the only sexually dimorphic character is size; P. mincae is the smallest species and Proechimys sp. the largest. The populations are all recognizable by their shape, and populations of the same species share a common kind of shape modification. The prevalent non-uniform nature of morphological change in the skull shows how the various structures, such as those involved with audition, feeding and olfaction, are modified differently among species. Consequently, the modification of these different structures over a long time could reflect a progressive diversification of the microniche of each species.