A palaeontological case study for species delimitation in diverging fossil lineages

Abstract

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.

Keywords::

• Murinae
• Siwaliks
• general lineage concept for fossils
• sympatric speciation
• anagenesis
• cladogenesis
• lineage separation event

Acknowledgements

Y. Kimura thanks Yukimitsu Tomida, who retires from his position at the National Museum of Nature and Science in 2015, for his dedicated academic support since before her undergraduate career. This study is based on 38 years (1977–2015) of fieldwork and investigations that built a classic record of fossil rodents from Pakistan, in which Yukimitsu Tomida played a part in the field in early 1979. Y. Kimura undertook much of her contribution to this work at the Peabody Museum of Archaeology, Harvard University, thanks to David Pilbeam and at the University of Utah, thanks to Thure Cerling. L.J. Flynn and L.L. Jacobs acknowledge their long friendship with Yukimitsu Tomida, beginning with their rich learning experiences shared in graduate school and continuing unabated ever since. We thank Gareth Dyke, Kevin de Queiroz and anonymous reviewers for providing constructive critiques on this article.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Research grants supporting this research were provided by the Institute for the Study of Earth and Man at Southern Methodist University, the Society of Vertebrate Paleontology (Mary R. Dawson Grant) and the Geological Society of America (Graduate Student Research Grant), and numerous National Science Foundation grants supporting field work in Pakistan and subsequent laboratory studies.