Abstract
Fossil records generally inform paleobiologists about extinct taxa and rates of evolution measured at the scale of millions of years. Good records that are densely sampled through time can reveal species level details such as longevity in local sections. Yet fossil data normally do not address details of lineage microevolution because the density through time of lineage sampling is insufficient to perceive patterns at a precision finer than 106 years in most cases. This study concerns details of a splitting event in the evolution of murine rodents, an event for which multiple fossil samples dated to a precision of 105 years fortuitously document the tempo and mode of origin of sister species, the stems of two extant tribes of mice. Evolution of early Murinae in the northern part of the biogeographically restricted Indian subcontinent between 11.6 and 10.5 Ma involved cladogenesis of two crown taxa, the extant tribes Murini and Arvicanthini. Large samples of fossil rodent teeth document their divergence from a common morphological pool. Definitive basal Murini and Arvicanthini at 10.5 Ma are similar in size and differ by subtle features of the dentition. Those features occur sporadically in the common pool of older fossil teeth at 11.2, 11.4, and 11.6 Ma as inconsistent polymorphisms. Interpreted as a single lineage in the 11.6–11.2 Ma interval, variability of this abundant murine incorporated the roots of the two crown tribes. The pattern through time suggests morphological stasis for several hundred thousand years prior to splitting. This special case informs us on one example of evolution and shows that the tempo of splitting evolution in some cases may be measured in hundreds of thousands of years, followed by stasis once daughter species have differentiated morphologically.
Highlights
Gould (1985) and Reznick and Ricklefs (2009) discussed the importance of the fossil record for evolutionary biology in unveiling major events, especially those under the category of macroevolution
The Siwalik murine record has obtained more detail in recent years from review and analysis of carefully controlled data for over 3600 specimens and the evolution of murines can be traced in more detail
It is well worth emphasizing the unique set of conditions that allowed this evaluation of tempo and mode in evolution using the fossil record
Summary
Gould (1985) and Reznick and Ricklefs (2009) discussed the importance of the fossil record for evolutionary biology in unveiling major events, especially those under the category of macroevolution. Paleontology does not address details on how evolution proceeds at the population level because relevant data are not sampled densely enough in space and time. The potential for approaching microevolutionary patterns through the fossil record is great for muroid rodents in cases where stratigraphy and dating are well resolved, and multiple large samples of individuals are available. Steppan and Schenk (2017) resolved a massive amount of data to develop a well-supported hypothesis on relationships of over 900 species. This and previous works (e.g., Lecompte et al, 2008) distinguished major clades of muroids at the family, subfamily, and tribe levels, including tribes Arvicanthini and Murini, both of which have fossil members in South Asia
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