Early Miocene rodent fossils reveal changing muroid assemblages in Spain’s Ribesalbes-Alcora Basin

A morphological and biometric study of fossil rodent teeth from the Campisano Ravine in Spain’s Ribesalbes-Alcora Basin identified five muroid taxa and documented a change in their relative abundance through an Early Miocene sequence. Megacricetodon primitivus predominated in the older L. florancei local biozone, while Democricetodon decipiens became more prevalent in the younger L. ellipticus biozone.

The researchers examined seven stratigraphic sections arranged broadly from oldest to youngest: Mas dels Coixos, Mas de Torner, Araia Cantera Sud, Barranc de Campisano, Foieta la Sarra, Mas d’Antolino B and Corral de Brisca. The approximately 100-m-thick Campisano Ravine sequence in Unit Three of the basin comprises grey and yellow mudstones, limestones and sandstones.

The deposits had previously been assigned broadly to regional biozone C in the early Aragonian MN4 interval, covering approximately 16.5 to 15.94 million years ago. An alternative interpretation places the boundary between the local Ligerimys florancei and L. ellipticus biozones at the MN4–MN5 transition, so the precise chronological placement of the younger deposits remains a subject of debate.

This was a descriptive palaeontological study of fossil material housed at the Natural History Museum of the University of Valencia. Dental measurements were recorded in millimetres with a binocular microscope, mechanical stage and measuring device, while scanning electron microscopy was used to image specimens. The authors compared the upper and lower molars of Megacricetodon primitivus, Democricetodon decipiens and Eumyarion weinfurteri with reference populations from European and Turkish localities.

Megacricetodon primitivus and Democricetodon decipiens were the most abundant taxa and occurred at nearly all sampled localities, but their representation differed through the sequence. Megacricetodon predominated in the older L. florancei biozone, whereas Democricetodon became increasingly common in the younger L. ellipticus biozone and was the dominant muroid at MAB11 and CBR0B. Eumyarion weinfurteri was confined to FS1, MAB3 and MAB5 in the younger biozone. Melissiodon was represented by five remains from BC1 and MAB3, some of them incomplete, and one M2 from MAB0A could not be identified below Muroidea.

The dental analysis supported the assignment of most material to Megacricetodon primitivus, Democricetodon decipiens and Eumyarion weinfurteri. The Megacricetodon specimens were generally smaller than several other species of the genus and differed from M. collongensis in features including the frequency of divided anteroconids, loph development and protoloph and metaloph patterns. The authors concluded that M. primitivus remained broadly stable in size and morphology through time, despite variation among localities and slight tendencies in individual dental dimensions and features such as mesolophid development.

The Democricetodon material was assigned to D. decipiens because its morphology and measurements fell within that species’ variability. It was generally larger than the reference D. hispanicus sample, smaller than D. moralesi and similar in size to comparative D. decipiens and D. franconicus material. The identification of Eumyarion was less straightforward because of extensive variation within the genus and differing treatments of E. weinfurteri and E. latior. Under the diagnostic framework used by de Bruijn, the basin material was assigned to E. weinfurteri, although it was slightly larger than that species’ type population.

The five Melissiodon remains were left under open nomenclature because the sample was too limited for a formal species assignment. The material was smaller than MN1–MN3 populations and within the reported size variation for MN4. The lower first molar from MAB3 had an isolated mesolophid and direct contact between the posterior metalophulid and entoconid, a combination not reported in another population. Although Melissiodon has traditionally been interpreted as frugivorous, its slender mandible, weak incisors and delicately ridged teeth resemble those of insectivorous rodents; the authors therefore regarded an insectivorous diet as plausible. Its association with karstic deposits also led them to suggest a preference for less-wet upland habitats.

The abundance patterns provide a biostratigraphic and ecological framework for the sequence, but the proposed links between rodent representation, climate and vegetation are associations rather than direct experimental results. The older L. florancei deposits were interpreted as comparatively arid and open, with Megacricetodon more abundant, while the younger L. ellipticus deposits were described as more humid and forested, with more Democricetodon. On this basis, the coastal Ribesalbes-Alcora Basin was interpreted as a transitional region between the forested areas of western and central Europe and the drier interior basins of Iberia. The analysis is bounded by fragmented remains, uncertain assignments for Melissiodon and the indeterminate tooth, unequal representation among localities, and continuing debate over the taxonomy and evolutionary history of several muroid groups.

Subject of Research: Morphological and biometric analysis of Early Miocene muroid rodent fossils from Spain’s Ribesalbes-Alcora Basin

Article Title: The Early Miocene muroids ( Muroidea , Rodentia ) of the Ribesalbes-Alcora Basin (Spain): A thriving haven during a time of migration

Article References: Crespo, V. D., Ríos, M., Marquina-Blasco, R., & Montoya, P. (2025). The Early Miocene muroids (Muroidea, Rodentia) of the Ribesalbes-Alcora Basin (Spain): A thriving haven during a time of migration. Fossil Record, 28(1), 187-218. https://doi.org/10.3897/fr.28.138478

Image Credits: AI Generated

DOI: 10.3897/fr.28.138478

Keywords: Early Miocene, muroid rodents, palaeontology, fossil teeth, biostratigraphy, Ribesalbes-Alcora Basin, Megacricetodon, Democricetodon, Eumyarion, Melissiodon, Early, Miocene