Since its redefinition by Daams et al. (1987), the Lower Aragonian includes the Biozones B and C (∼ 17.00 Ma, ∼ 15.93 Ma). It is characterized by high relative abundance and species richness of glirids combined with the presence of representatives of the eomyid genus Ligerimys. Compared to the previous stage, the Ramblian, there is not a strong faunal rupture; most of the rodent genera recorded from Ramblian localities persist in the lower Aragonian (only Melissiodon and Palaeosciurus are absent in the Aragonian faunas from the Calatayud-Montalbán Basin). The main difference is the regular occurrence of the so-called “modern” cricetids in the Aragonian assemblages (Daams et al., 1998, Daams et al., 1999a and Van der Meulen et al., 2012).

Within the Lower Aragonian, three different migration events involving cricetids are clearly recognized and have been used to define its local biozonation (Van der Meulen et al., 2012). The first regular occurrence of Democricetodon is used to define the base of the Aragonian and its lowest biozone (Zone B). The Zone Ca is recognized by the presence of the genus Megacricetodon combined with Democricetodon decipiens. In this zone, the progressive depletion of glirid species started, which continues across the Lower Aragonian (Praearmantomys crusafonti, Armantomys jasperi, and Simplomys robustus). Most of these taxa had survived the Ramblian-Aragonian boundary, and their association is typical for Iberian Early Miocene faunas. Across Zone Cb, the decrease in glirid species continues. The cricetids increase in species richness with the entrance in the basin of a second lineage of Democricetodon of small size (D. franconicus) and the genus Eumyarion. The arrival of these cricetids, which present affinities with the central European representatives, co-occur with the first occurrence of a second Prolagus lineage (P. vargasensis). Across the zones Ca and Cb there is a momentary occurrence of the heterosoricid shrew Heterosorex. For the first time during the Early Aragonian, the assemblages are dominated by eomyids (Ligerimys ellipticus) and cricetids instead of eomyids and glirids.

The Middle Aragonian includes the Zones Da to E, from ∼ 15.93 Ma to ∼ 13.78 Ma, (Daams et al., 1999a and Van der Meulen et al., 2012). In central Spain, it represents a period of low richness in rodent species, with cricetids as the most abundant group, while the genus Ligerimys, dominant during the Ramblian and Lower Aragonian, went extinct. At the same time, the Talpidae, represented since the Ramblian by the genus Desmanodon, disappear from the area, as does the small gymnure Galerix symeonidisi. The glirids show low relative abundances; Microdyromys increases in relative abundances compared to the glirids of simpler dental pattern, which were dominant through the Early Miocene in the Iberian Peninsula. The beginning of the Middle Aragonian includes a period of high turnover with quick replacement of species, some of them with very short distributions. This rapid substitution of species is reflected in the short duration of the two first biozones, Da and Db, from ∼ 15.93 Ma to ∼ 15.60 Ma (Van der Meulen et al., 2012). Both, and especially Zone Db (∼ 15.85 Ma, ∼ 15.60 Ma), include rodent species with affinities with central European taxa, such as Megacricetodon vandermeuleni or Prodryomys (Oliver and Peláez-Campomanes, 2013 and García-Paredes, 2006, respectively). Zone Dc (∼ 15.60 Ma, ∼ 14.82 Ma) includes the last occurrence of Atlantoxerus blacki and Spermophilinus besanus. Microdyromys is for the first time represented by three species in the area (García-Paredes et al., 2010). Zone Dd (∼ 14.81 Ma, ∼ 14.75 Ma) contains impoverished faunas with a strong dominance of cricetids. Across all Zone Dd, there are three species of Microdyromys, although the species changed through time, leading to a more-complex dental pattern at the end of the biozone (García-Paredes et al., 2010). The upper part of Zone Dd is also characterized by an increase in the diversity of glirids with the first occurrence of Pseudodryomys rex, Tempestia ovilis and Muscardinus thaleri. For a short period of time, Keramidomys is recorded in the basin. This genus was a common representative in Middle Aragonian faunas from central Europe (Kälin and Kempf, 2009 and Van der Meulen et al., 2011). The end of the Middle Aragonian includes the extinction of a high proportion of the most characteristic taxa for the Early and Middle Aragonian. In this way, during the Zone E (∼ 14.75 Ma, ∼ 13.78 Ma), Pseudodryomys ibericus, Simplomys simplicidens, Microdyromys legidensis-koenigswaldi, and the lineage Democricetodon hispanicus-D. lacombai have their last occurrences in the basin. The biozone includes the first occurrence of Cricetodon (Cricetodon soriae) and Heteroxerus grivensis, which represents the third Heteroxerus species co-occurring in this biozone.

As pointed out by Van der Meulen et al. (2005), the rodent faunas from the Upper Aragonian represent a different community to that previously recorded in the area. There are new important taxa that characterize this time interval and have their first occurrences. The cricetids are still the dominant group with relative abundances usually higher than 75%. Across the Upper Aragonian, the species richness of glirids increases, although their relative abundance remained low. The biostratigraphy of this period is mainly based on the evolutionary degree of the Megacricetodon ibericus lineage combined with the presence of representatives of small-sized Megacricetodon. In addition to the Megacricetodon evolution, López-Guerrero et al., 2014a, López-Guerrero et al., 2013 and López-Guerrero et al., 2014b have recently showed the importance of the evolution of the representatives of the tribe Cricetodontini for the biostratigraphy. In this way, the Zone F (∼ 13.78 Ma, ∼ 13.62 Ma) can be recognized by the presence of Megacricetodon rafaeli and Cricetodon sansaniensis. The representatives of the M. ibericus lineage in Zone G1 show the same evolutionary degree than in the previous zone. However, Zone G1 (∼ 13.62 Ma, ∼ 13.27 Ma) can be easily recognized by the presence of Megacricetodon minor and Cricetodon jotae, a smaller and more advanced Cricetodontini (López-Guerrero et al., 2014b). The next local biozone, Zone G2 (∼ 13.27 Ma, ∼ 13.00 Ma), maintains the same micromammal structure as the previous one, continuing the increase of species of glirids present and the sporadic presence of the eomyid Eomyops. The first occurrence of the insectivore Turiasorex pierremeini is within this zone. The cricetids are very abundant, with Megacricetodon crusafonti as the most characteristic and abundant taxon.

Approximately the second half of the Upper Aragonian corresponds to the Zone G3 (∼ 13.00 Ma, ∼ 11.26 Ma). This long zone represents a transition towards the Vallesian faunas. In the basin, common taxa such as Armantomys, Paraglirulus, Lagopsis, and Galerix went extinct during this biozone. There are noticeable morphological changes in the Democricetodon and Megacricetodon lineages that have been used to recognize different species. Also notable is the achievement of the ‘Hispanomys evolutionary degree’ in the representatives of the tribe Cricetodontini. Although this tribe is not very abundant across this biozone, its representatives had an important turnover involving four species (López-Guerrero, 2014, López-Guerrero et al., 2014a and López-Guerrero et al., 2014b). Zone G3 also includes the re-entry of the castorids in the Calatayud-Montalbán Basin, after a time interval of absence spanning ∼ 7 Ma (Álvarez-Sierra et al., 2003 and López-Guerrero et al., 2007), and the first appearance of the gymnure Parasorex. Álvarez-Sierra et al. (2003) pointed out that representatives from other northern Spanish basins enter in the upper half of the Zone G3. These results, indicating a decrease in isolation among different basins, have been further supported by López-Guerrero et al. (2008) studying the Cricetodontini and, also, by Abella et al. (2012) with carnivores.

Zone H was defined by Daams and Freudenthal (1988) based on the presence of Megacricetodon ibericus. Álvarez-Sierra et al. (2003) informally used the co-occurrence of latter species and Hispanomys to define Zone H. Nevertheless, López-Guerrero et al., 2008 and López-Guerrero et al., 2014b, demonstrated the presence of Hispanomys representatives already in G3 localities. Therefore, Van Dam et al. (2014) implicitly modified that informal definition of the Zone H using, as lower boundary, the first occurrence of Hispanomys nombrevillae. According to Van Dam et al. (2014), the age of the lower boundary of Zone H could be older than 11.2 Ma, implying that it was about 500 ky before the first Hippotherium record in the basin at the Nombrevilla classic locality. This new way to define the Zone H therefore implies, as previously pointed out by Álvarez-Sierra et al. (2003), that the change towards ‘Vallesian’ micromammal faunas was already achieved in the upper part of the Aragonian.

The upper parts of the sections in the Daroca-Nombrevilla area correspond to the Zone I. The faunas of this period show strong differences with previous ones by the absence of members of the Megacricetodon ibericus lineage (the most abundant and characteristic lineage of the Upper Aragonian) and the presence of Cricetulodon. The glirid Ramys has also been considered as a typical genus for Zone I in the Calatayud-Montalbán Basin. However, after the study of the material available from the Cañada localities, a recently published stratigraphic section near the Carrilanga one, this taxon has been recorded associated with H. nombrevillae, indicating that its first occurrence in the basin predates the replacement of the Hispanomys forms (López-Guerrero et al., 2011). Around the same time, moles, now represented by Desmanella, also return to the basin (Van den Hoek Ostende et al., 2012). The uppermost part of this biozone contains the first occurrence of the murid Progonomys, the regular presence of which defines the lower boundary of the Upper Vallesian (10 Ma, Van Dam et al., 2014).

An extensive discussion on the tripartite division of the Aragonian and the history of the different definitions were summarized by Daams et al. (1999a). These authors justified the division used till now by the short duration of some of the biozones, which indicated a high replacement of species. In this way, they placed the boundary between Lower and Middle Aragonian at the base of Zone Da (∼ 16 Ma), and the boundary between Middle and Upper Aragonian at the base of Zone F (∼ 13.8 Ma). Therefore, in the Calatayud-Montalbán Basin, the three important faunal intervals recognized (Fig. 2) are: •

the Early Aragonian, characterized by a high diversity and relative abundance of endemic Spanish glirids, combined with the progressive presence of modern cricetids coming from the east;

Thus, the two limits on the Aragonian subdivisions correspond to two of the major changes. The first one corresponds to the start of the cricetid dominion over the glirids and eomyids (Fe 2 on Fig. 2), and the second one to the community reorganization (Fe 5 on Fig. 2) pointed out by Van der Meulen et al. (2005).

Three characteristic global climatic conditions, based on stable isotopes (Fig. 3), have been proposed for the period equivalents to Aragonian and Early Vallesian ages (Cramer et al., 2009, Miller et al., 1998, Miller et al., 1991 and Zachos et al., 2001). The Miocene Climatic Optimum (MCO), an interval between 17 and 15–14.5 Ma (Mudelsee et al., 2014 and Zachos et al., 2001) characterized by high temperatures and, in general, more humid environments (Zachos et al., 2001). The Middle Miocene Climate Transition (MMCT), between 15–14.5 Ma and ∼ 13.8 Ma (Holbourn et al., 2007, Mourik, 2010, Mourik et al., 2011 and Mudelsee et al., 2014), represents the beginning of the cooling trend of the Neogene (Mourik et al., 2011). The end of the MMCT marks the onset of the so-called “Icehouse”, a cold period during which an expansion of the Antarctic ice volume took place (Holbourn et al., 2007 and Zachos et al., 2001).

The beginning of the Aragonian (Fig. 2, Fe 1) coincides with the onset of the MCO around 17 Ma (Mudelsee et al., 2014 and Zachos et al., 2001). At that time, glirids were an important quantitative component of the rodent associations (García-Paredes, 2006 and Van der Meulen et al., 2005). The components of these communities are residents, which appear to be predominantly open country dwellers (Daams et al., 1999c and Van der Meulen et al., 2005). Hordijk (2010) pointed out that, in the Calatayud-Montalbán Basin, Zone B is dominated by herbivores, especially Lagopsis penai. Their dominance could indicate the presence of open landscapes at that time, as was also noted by Van der Meulen and Daams (1992).

The faunal change that occurred between the Lower and Middle Aragonian (Fig. 2, Fe 2), involving the beginning of the dominion of cricetids over the eomyids and glirids, is included within the MCO and the Monterrey Carbon Excursion (Fig. 3). It corresponds approximately with the Mi2 Oxygen isotope zone of Miller et al. (1991) that represents a relative cooling period within the warm conditions of the MCO (Fig. 3).

The strongest taxonomical turnover is marked by the collapse of the Early and Middle Aragonian community (Fig. 2, Fe 5) and their replacement by a new community (Daams et al., 1999a, Daams et al., 1999c, Hordijk, 2010, Van der Meulen and Daams, 1992 and Van der Meulen et al., 2005). In the general context, this corresponds approximately with the end of the MMCT (Fig. 3), marked by the Mi-3 of Miller et al. (1991), which is a major step in the Middle Miocene climatic cooling (Mi-3b in Abels et al., 2005). The new Upper Aragonian community is defined by the presence of a new set of sciurids and cricetids together with glirids associated to woodlands replacing the old resident open-land dwellers (Van der Meulen et al., 2005).

Van Dam et al. (2006) studied this relationship between climate and rodent turnover, detecting turnover cycles of 2.4–2.5 and 1.0 over a long record of more than 20 Myr based on the rodent record from the Spanish central basins. They calculated the statistically significant rodent turnover peaks and related them to the major marine benthic stable oxygen isotope events and with the predicted time of rodent turnover based obliquity and eccentricity cycles. Hordijk (2010), at a lower scale and including lagomorphs in the analyses, explained changes in the structure of the small mammal community based on their relationship with the local effects of global changes in climate.

The refined taxonomic and stratigraphic information available allows us to propose the correlation of smaller faunal turnover corresponding to biostratigraphic zonal boundaries with several global climatic changes. The discussion is not only based on the quantitative aspects, number of originations and extinctions, of the micromammal turnover (Van Dam et al., 2006), but also on the qualitative ecological preferences of the taxa (Van der Meulen et al., 2005), which help us to interpret the patterns of change.

The first of these smaller faunal turnovers that could be correlated with climatic changes is at the base of Zone Dd (Fig. 2, Fe 3), and approximately corresponds to the Mi-2a at the beginning of the MMCT and the carbon isotope decrease (Cramer et al., 2009) shown on Fig. 3. This was the onset of a presumably dryer period in the basin, leading the extinction of common taxa that where generally frequent in environments considered as more humid, such as Spermophilinus and Eumyarion (Casanovas-Vilar and Agustí, 2007).

Another important faunal turnover, but more gradual than the one discussed above, is that occurring across the end of Zone Dd and the beginning of Zone E (Fig. 2, Fe 4). The general climatic trend is towards colder temperatures and this could promote the migration of northern representatives that enter the central Spanish basins for the first time. The transition between Dd and E faunas can be related to the oxygen isotopic zone Mi-3a (Fig. 3) correlated to the base of the chron C5.Acr (Abels et al., 2005). This event could not be differentiated by Van Dam et al. (2006) from the turnover detected at the end of the Middle Aragonian due to the coarser taxonomical resolution of the data.

As discussed above, Zone G3 shows important community changes although there is not a high turnover rate restricted to a shorter period (Fig. 2, Fe 6). There are two important events within this zone such as the re-entry of the castorids into the basin after more than 7 Ma of absence (Álvarez-Sierra et al., 2003 and López-Guerrero et al., 2007) and the entrance of northern species of Hispanomys and carnivores into the basin (Abella et al., 2012, Álvarez-Sierra et al., 2003 and López-Guerrero, 2014). The latter turnover event could be related to Mi-5 at the base of chron C5r.2r.

Finally, there is an important turnover between Zones H and I, as pointed out by Álvarez-Sierra et al. (2003) and Van Dam et al. (2014), characterized mainly by the replacement of the large representatives of Democricetodon and Megacricetodon by the new immigrant Cricetulodon. This replacement may be correlated to Mi-6 (Fig. 3), situated at the lower part of chron C5N (Miller et al., 1998) with an approximate age of 10.45–10.35 Ma (Van Dam et al., 2006).