A palaeontological case study for species delimitation in diverging fossil lineages

A general lineage concept is widely adopted in studies of species delimitation for extant taxa with DNA sequence data. In the general lineage concept, species are separately evolving metapopulations that acquire sets of properties during the process of speciation. Murine rodents from the Miocene Siwalik Group of Pakistan have an excellent fossil record showing temporal trends of morphological change, and are therefore ideal to observe the process of lineage divergence as fossil evidence. Here, we review the evolution of Siwalik murine rodents in the sense of the general lineage concept and draw lines of evidence for lineage separation. For currently available datasets, we applied the initial split criterion as well as three operational criteria (phenetic, diagnosable and ecological properties). In early Siwalik murine rodents, the acquisition of distinct phenetic properties emerges subsequent to the event of the evolutionary split. The phenetic evidence occurs closer in time to ‘true’ divergence, while distinct diagnosable and ecological properties fully arise later. Within a sequence of time-controlled samples, these observations explicitly show that phenetic properties (here, shape data and a combination of linear measurements) more precisely delimit the divergence of fossil species than does the fixation of diagnostic properties.