A garden dormouse, one of the species covered by the study. / Animalia.

A research team led by the Complutense University of Madrid (UCM), with the participation of the Doñana Biological Station-CSIC, has reconstructed the last 25 million years of the history of the Iberian Peninsula by analysing tens of thousands of small-mammal fossils from 400 sites using an innovative quantitative model.

One of the study’s main contributions, published in Earth-Science Reviews, is the dating of sites that had previously lacked precise dates. In doing so, the researchers have refined a precise geobiological clock that sheds light on the key stages of evolutionary transitions during the Neogene–Quaternary, helping to understand how fauna has historically responded to environmental change.

“Establishing a precise timescale for terrestrial continental sites has historically been one of the greatest challenges in geology. Unlike marine sediments, which accumulate continuously, the terrestrial record is preserved in a fragmented form across isolated sedimentary basins,” says Manuel Hernández Fernández, professor in the UCM Department of Geodynamics, Stratigraphy and Palaeontology and one of the study’s authors.

Beyond mice and squirrels

One of the novel aspects of this study is the range of species included. Until now, analyses of this kind have focused mainly on rodents, a group that includes mice, squirrels, dormice, beavers and others.

"We also incorporated lagomorphs (rabbits, hares and pikas) and insectivores (shrews, moles and hedgehogs) into the analysis. Together, they account for 22% of all the species in the database," explains Rubén Juidíaz, a predoctoral researcher at UCM and first author of the paper. The results show, he adds, that including these groups can be crucial, as some of their species serve as excellent chronological "markers" for certain periods.

The study has calibrated the ages of the sites analysed with unprecedented precision for much of the record examined. However, the method loses resolution in the most recent millennia, with the last 50,000 years of the record representing the limit of the method’s precision.

“In this particularly recent period, its short timescale, the scarcity of small-mammal speciation or extinction events, the fragmentation of populations into Iberian climatic refugia during the last major glaciation, and the asynchronous arrival of human-associated commensal species such as rats and house mice distort the homogeneity required for an appropriate biostratigraphic ordering,” explains Fernando Blanco, a researcher at the Doñana Biological Station (CSIC).

The first step in the study was to compile information from the Iberian fossil record, which can be considered exceptional on a global scale: 322 species and more than 4,000 records of their presence across nearly 400 sites in 20 different Cenozoic basins.

To organise this vast volume of data, the researchers used the mathematical method known as Maximum Likelihood Appearance Event Ordination (ML AEO). This algorithm probabilistically analyses relationships of coexistence (conjunctions) or exclusion (disjunctions) between different species, while rigorously respecting the 55 stratigraphic sequences observed in the field, in order to determine the chronological order of appearance and extinction events and assign a temporal index to each site.

Finally, using a segmented regression statistical model calibrated against 156 dates previously obtained through geophysical dating techniques, such as radiometry, thermoluminescence and magnetostratigraphy, the model was able to assign a numerical age to all the sites studied.

Reviewing dates

In addition to dating the sites, the study has identified discrepancies with previous research. Some 6.8% of the fossil assemblages analysed were assigned numerical dates that differed significantly from those traditionally attributed to them.

In this way, the new model can serve as a biostratigraphic quality-control system, identifying sites and taxa that may require a reassessment of the composition of their faunas, their environmental contexts and their actual position in time.

"This framework is not intended to replace traditional timescales based on key species, but rather to serve as a complementary and independent clock," explains Soledad Domingo, associate professor at UCM and co-director of the UCM research group on Cenozoic Mammal Evolution and Continental Palaeoenvironments.

Looking ahead, by establishing a highly robust timeline, this work opens the door to a new generation of high-resolution studies in macroevolution, palaeoecology and palaeoclimatology that will shed light on how terrestrial biological communities responded to episodes of global climate change experienced by our planet over the past 25 million years.

“By transforming the remarkable catalogue of the Iberian fossil record into a tool for ecological and evolutionary discovery, we can move from ordering temporal events to interpreting the great mysteries of life on our planet,” concludes Manuel Hernández Fernández.