Iberian Neogene–Quaternary biochronology: a refined framework based on maximum likelihood analysis of small mammal assemblages

The geological and fossil records in continental environments possess characteristics that make it challenging to establish regional-scale biozonations using traditional stratigraphic techniques. As a result, biochronological scales have become valuable frameworks. Nonetheless, proposed biochronological scales (e.g., Mammal Neogene Units, MN) are not exempt from criticism, particularly due to the use of fossil sites from different bioprovinces and the diachrony of paleobiological events across wide geographical areas. The fossil record of Neogene and Quaternary small mammals from the Iberian Peninsula is among the most complete and best -studied in the world. However, the distribution of outcrops and localities across multiple basins poses challenges for constructing a cohesive regional temporal framework. Here, we use the Maximum Likelihood Appearance Event Ordination (ML AEO) method to obtain a temporal ordination with numerical ages for 381 Neogene and Quaternary fossil sites from the Iberian Peninsula based on their small mammal fauna, including Rodentia, Lagomorpha, and Eulipotyphla. After comparison with established European and local scales for this temporal interval, as well as with previously calculated ages for the sites, our study dates fossil sites covering approximately from 25 Ma (Late Oligocene) to current days. The ordination results and calculated ages are mostly consistent with previously established chronological frameworks, both at European and regional scales. However, the latest Pleistocene and Holocene localities (<50 ka) show lower temporal resolution due to the limited number of appearance and extinction events occurring within a relatively short time interval. We also identified several localities whose inferred temporal positions differ from their traditional biochronological assignments, prompting a re-evaluation of their faunal composition and chronological significance. The detailed chronological framework established here constitutes an essential first step for future macroevolutionary and paleoecological studies on the extinct small mammal faunas of the Iberian Peninsula.