Megacricetodon primitivus ( Freudenthal, 1963 )

Crespo, Vicente D., Ríos, María, Marquina-Blasco, Rafael & Montoya, Plini, 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), pp. 187-218 : 187-218

Megacricetodon primitivus ( Freudenthal, 1963)

Figs 2 , 3 , 4

Localities.

MCX 2, MCX 3, MCX 4, MTR 1, MTR 2, MTR 3, BC 1, BC 2, FS 1, MAB 0 A, MAB 0 B, MAB 2, MAB 3, MAB 4, MAB 5, MAB 11, MAB 11 B, MAB 12, CBR 0 B, CBR 0 E, CBR 0 F, CBR 1, CBR 2, and CBR 4.

Material (number of remains).

Suppl. material 1; MCX 2 (1): 1 mandible, 1 m 1; MCX 3 (62): 1 mandible, 12 m 1, 9 m 2, 9 m 3, 10 M 1, 12 M 2, 9 M 3; MCX 4: 1 M 1; MTR 1 (3): 1 m 1, 2 M 2; MTR 2 (47): 13 m 1, 9 m 2, 3 m 3, 12 M 1, 5 M 2, 5 M 3; MTR 3 (2): 1 M 1, 1 M 2; BC 1 (51): 10 m 1, 8 m 2, 7 m 3, 15 M 1, 7 M 2, 4 M 3; BC 2 (1): 1 M 1; MAB 0 A (22): 3 m 1, 4 m 2, 1 m 3, 7 M 1, 4 M 2, 3 M 3; MAB 0 B (9): 2 m 1, 3 m 2, 2 m 3, 2 M 1; FS 1 (15): 3 m 1, 1 m 3, 3 M 1, 7 M 2, 1 M 3; MAB 2 (2): 1 m 1, 1 m 3; MAB 3 (92): 14 m 1, 20 m 2, 19 m 3, 17 M 1, 12 M 2, 10 M 3; MAB 3 A (1): 1 m 2; MAB 4 (1): 1 M 2; MAB 5 (85): 18 m 1, 13 m 2, 14 m 3, 14 M 1, 15 M 2, 11 M 3; MAB 11 (10): 1 m 1, 3 m 2, 1 m 3, 3 M 1, 2 M 2; MAB 11 B (1): 1 M 3; MAB 12 (1): 1 m 3; CBR 0 B (1): 1 m 2; CBR 0 E (1): 1 M 1; CBR 0 F (1): 1 m 2; CBR 1 (19): 1 m 1, 4 m 2, 2 m 3, 5 M 1, 4 M 2, 2 M 3; CBR 2 (1): 1 m 3; CBR 4 (1): 1 M 1.

Measurements.

Suppl. material 2.

Description.

(See Suppl. material 3).

Mandible ( MCX 3; Fig. 2 A ): In occlusal view, the mandible exhibits a slight inclination, which does not obscure the mental foramen. This foramen is located between the incisor and the first molar on the labial side. The masseteric process is deep and originates at the level of the anterior part of the m 1 with a ridge.

Concerning other sites, no significant disparities were observed in the material of MCX 2.

m 1 ( MAB 5; Fig. 2 F – I ): the anteroconid is simple and rounded, and slightly lower than the rest of the cusps (between morphology B and C as described by Oliver and Peláez-Campomanes (2016) (Fig. 2 H )). The labial spur of the anterolophulid may be developed (1 out of 16), incipient (1 out of 16), or absent (14 out of 16). The metalophulid is positioned anteriorly and is directed forward, not connected to the protoconid. The mesolophid may be categorised as follows: long and developed to the lingual side (1 out of 16), long without reaching the lingual side (4 out of 16), medium (8 out of 16), or short (3 out of 16). The ectomesolophid is absent. The hypolophulid is directed forward. The posterolophid descends towards the base of the entoconid but does not connect to it.

Variability in other sites: In the remains of MCX 2 and MCX 3 (Fig. 2 B ), the anterolophulid is consistently absent, and the mesolophid is either short or medium. In MTR 1, the anterolophulid is absent. In the remains from MTR 2 (Fig. 2 C ), two specimens have a double metalophulid, one has no mesolophid, one has an ectomesolophid and one has the posterolophid attached to the entoconid. In BC 1, the mesolophid is shorter (Fig. 2 D ). In one individual, the posterolophid is joined to the entoconid, and there is greater development of the labial ridge of the anterolophulid. In MAB 0 B (Fig. 2 E ), one individual is observed to possess two ridges at the lingual-anterior and labial side of the anteroconid; the anterolophulid is more developed, and in another, the metalophulid is isolated. In FS 1, an individual is noted with the metalophulid isolated from the protoconid. In MAB 3, a tooth with an incipient ectomesolophid is observed. The material from the other sites ( MAB 2, MAB 0 A, MAB 11, and CBR 1) shows no significant differences. Generally, there seems to be a slight tendency for the mesolophid to elongate over time. As illustrated in Fig. 3 A , Suppl. material 4, the biometric values of M. primitivus from localities MCX 2 and MCX 3 are the smallest observed in this basin. In addition, there is a tendency for specimens to lengthen in time.

m 2 ( MAB 3; Fig. 2 M – P ): the lingual anterolophid may terminate at the antero-lingual border of the metaconid (2 out of 16), fail to reach the corner (10 out of 16), or be absent (4 out of 16). The anterosinusid may be small and narrow (7 out of 16) or absent (9 out of 16). The labial anterolophid is long and descends towards the base of the protoconid. The mesolophid can be categorised into several distinct forms, including long and reaching the lingual margin (2 out of 18), long without reaching the lingual margin (3 out of 18), medium (6 out of 18), short (5 out of 18), or absent (3 out of 18). The ectolophid is continuous. The ectomesolophid is absent, and the posterolophid is not connected to the entoconid.

Variability in other sites: In MCX 3 (Fig. 2 K ), the lingual anterolophid is always present and longer, and the mesolophid is short or medium in length. In BC 1 (Fig. 2 L ), the lingual anterolophid is always present; in one specimen, the labial anterolophid joins the labial mesocingulum, and the mesolophid is usually longer. In MAB 0 A, the mesolophid is usually longer. In MAB 0 B, the posterolophid may be connected to the entoconid. In MAB 5 (Fig. 2 Q ), the anterolophid is always present. In CBR 1, the posterolophid is connected to the entoconid; in another specimen, the labial anterolophid is connected to the labial mesocingulum. The m 2 morphology of MTR 2, FS 1, MAB 3 A, MAB 11, CBR 0 B (Fig. 2 R ), and CBR 0 F does not differ significantly from that of MAB 3. About the biometry (Fig. 3 B , Suppl. material 4) between the material from the different sites of this species in the Ribesalbes-Alcora Basin, the material from MCX 3 is slightly smaller, while that from MAB 11 is slightly longer, and that from BC 1 is slightly narrower. In addition, there is a slight tendency for specimens to become larger over time.

m 3 ( MAB 3; Fig. 2 V – Z ): The lingual anterolophid may be of medium size (7 out of 19), short (6 out of 19), or absent (6 out of 19). A small anterosinusid is present in six specimens. The labial anterolophid may be long (9 out of 18) or short (9 out of 18), descending towards the base of the protoconid (9 out of 18). The metalophulid is short and complete, while the mesolophid is absent. The mesosinusid is narrow and transverse. The posterosinusid is medium to large in size. The protoconid and the hypoconid may be separate (6 out of 18) or not (12 out of 18). The metaconid, entoconid, and posterolophid form a continuous wall along the lingual margin (10 out of 17), or the metaconid is independent (7 out of 17).

Variability in other sites: In MCX 3 (Fig. 2 S ), the lingual anterolophid is shorter and the labial is longer, with the anterosinusid occurring more frequently. In BC 1 (Fig. 2 T ), there is a tooth with an anterior cingulum in front of the anterolophid. In MAB 2 (Fig. 2 U ), an individual is observed with a developed mesolophid. In MAB 5 (Fig. 2 AA – AB), the lingual anterolophid is long in two individuals, while the metalophulid is absent in another and the mesolophid is present in a third specimen. In CBR 1, in addition to the mesolophid, the metaconid has a ridge that contacts the lingual part of the ectolophid in one specimen. In the more modern sites of the basin, such as MAB 5 and CBR 1, the presence of mesolophids is more prevalent. The morphology of m 3 from localities MTR 2, MAB 0 A, MAB 0 B, FS 1, MAB 11, MAB 12, and CBR 2 does not differ significantly from that described in MAB 3. The comparison of the biometry (Fig. 3 C , Suppl. material 4) of the m 3 of M. primitivus from the various localities of the Ribesalbes-Alcora Basin shows that the specimen from MAB 0 B is the largest, while that from CBR 2 is the smallest ( MAB 0 B). In addition, there is a slight tendency for specimens to become shorter and wider over time.

M 1 ( MAB 3; Fig. 4 E – I ): the anterocone may be not subdivided (1 out of 15), be slightly subdivided and the sulcus shallow (1 out of 15), or deeply subdivided and have a deeply subdivided with a platform in front of the anterocone (10 out of 15) and with even a cingulum (3 out of 15). The labial part of the anterocone may be larger (13 out of 15) or equal (2 out of 15) to the lingual one. The protolophule may be posterior to the protocone (5 out of 15), posterior almost double, formed by the labial ridge of the anteroloph or by the front ridge of the paracone (10 out of 15). The anterolophule may connect over the lingual cusp (8 out of 15) or between the two cusps of the anterocone (7 out of 15). The labial spur of the anterolophule may be short (4 of 15), incipient (7 of 15), or absent (4 of 15). The lingual mesocingulum is present but poorly developed (8 of 16) or absent (8 out of 16). The ectoloph (or paracone ridge) may be long (8 out of 16) or short (8 out of 16). The length of the mesoloph is recorded as either long and reaching the labial margin (2 out of 17), long (11 out of 17), medium (2 out of 17), or short (2 out of 17). The connection between the mesoloph and ectoloph of the paracone is categorised as either present (4 out of 18) or absent (14 out of 18). The metalophule is directed backwards, thereby reducing the posterosinus. The posterosinus may be narrow (10 out of 17) or of medium width (7 out of 17), being either deep (8 out of 15) or deep (8 out of 15) or shallow (7 out of 15).

Variability in other sites: In the M 1 of MCX 3, there is a higher prevalence of uniform size anterocone cusps, the absence of a double protolophule, the anterolophule connection with the lingual cusp, the ectoloph is absent in one specimen, and the mesoloph is long and lack of contact with the posterior crest of the paracone. In M 1 from MTR 2 (Fig. 4 A ), the protolophule is posterior, the anterolophule connects to the lingual cusp, the labial crest of the anterolophule is absent, the lingual mesocingulum is rare, the mesoloph and ectoloph of the paracone never seem to be connected, and finally the metalophule usually connects to the most anterior part of the posteroloph. It may even be directed more posteriorly, even transversely. In BC 1 (Fig. 4 B, C ), there are two specimens with a deeply divided anterocone without platform or cingulum, the protolophule is simple and posterior, the anterolophule connects with the lingual cusp, its labial crest is always absent, the lingual mesocingulum is less frequent, the mesoloph never connects with the ectoloph of the paracone and finally the metalophule connects with the posteroloph posterior to the hypocone in six out of fifteen cases. In MAB 0 A (Fig. 4 D ), most individuals have an anterior cingulum of the anterocone, the protolophule is usually posterior, the ectoloph is long in three out of six individuals, and in another, the metalophule does not reduce the posterosinus. In FS 1, the anterocone is deeply divided and has no anterior platform or cingulum; in one individual, the protolophule is double; in another, the protolophule is double; in another, the ectoloph is absent. In MAB 5 (Fig. 4 J ), the anterocone is less divided and there are no specimens with a cingulum, the proportion of material with anterocone cusps of equal size is higher, the lingual mesocingulum is less common, the protolophule is mostly posterior, the ectoloph is absent in one tooth, and there are four specimens with a metalophule that connects posteriorly but without reducing the posterosinus. In MAB 11 (Fig. 4 K ), the anterocone is less divided; in another, the metalophule does not reduce the posterosinus. In CBR 1 (Fig. 4 L ), the protolophule is posterior, the anterolophule touches the lingual cusp, and the metalophule does not reduce the posterosinus in three individuals and is transverse in another. In the remaining sites ( MCX 4, MTR 3, BC 2, MAB 0 B, CBR 0 E, and CBR 4), no significant morphological differences were observed. About biometrics (Fig. 3 D , Suppl. material 4), the material from the study sites belongs to the L. florancei local biozone, which is generally slightly smaller in size. In addition, there is a slight tendency for specimens to become larger over time.

M 2 ( MAB 5; Fig. 4 Q – T ): the protolophule may be single and transverse, connected to the protocone (11 out of 14), or double with an incomplete posterior protolophule (3 out of 14). The ectoloph may be long and contact the labial side (4 out of 13), long (2 out of 13), short (4 out of 13), or absent (3 out of 13). The mesoloph is long and reaches the labial edge (4 out of 13), long without reaching the labial edge (6 out of 13), or medium length (3 out of 13). The ectoloph and the mesoloph may be connected (5 out of 13) or not (6 out of 13), with two specimens in which the ectoloph is connected to the labial border. The sinus may be directed slightly anteriorly (2 out of 16) or perpendicular (14 out of 16). The metalophule may be directed posteriorly, connecting with the posteroloph posterior to the hypocone (3 out of 13), or directed forward and connecting with the entoloph (10 out of 13). The posteroloph may be connected to the posterior part of the metaconid (8 out of 13) or not (5 out of 13). The posterosinus may be either long (8 out of 13) or short (5 out of 13).

Variability in other sites: In MCX 3 (Fig. 4 M ), there is one individual with a double protolophule, another with a lingual mesocingulum, and the ectoloph is usually longer. In BC 1 (Fig. 4 N ), the morphology of the protolophule is more variable, even with a double protolophule, a double protolophule with both protolophules complete in one individual, the mesoloph is short in another. It does not appear to be connected to the ectoloph. In MAB 0 A, the protolophule may be directed either anteriorly or posteriorly. In FS 1 (Fig. 4 O ), there is one individual with a double protolophule and another with a metalophule attached to the posteroloph. In MAB 3 (Fig. 4 P ) there are three individuals with the protolophule attached to the anteroloph anterior to the protocone another with a double protolophule, the ectoloph always present, another with a short mesoloph, and three others with a transverse metalophule. In MAB 11 (Fig. 4 U ) has one specimen with an anterior protolophule and another with a transverse metalophule. In CBR 1 (Fig. 4 V ), there is one individual with an incomplete anterior protolophule, an oblique sinus in three out of four specimens, another with a transverse metalophule, and another with a reduced posterosinus. No significant differences exist in MTR 1, MTR 2, MTR 3, and MAB 4. Regarding biometry (Fig. 3 E , Suppl. material 4), the examined material belonging to local biozone L. florancei tends to be smaller than that belonging to local biozone L. ellipticus . In addition, there is a slight tendency for specimens to become larger over time.

M 3 ( MAB 5; Fig. 4 AC – AF): the lingual anteroloph and the protosinus may be poorly developed (2 out of 11) or absent (9 out of 11). The labial anteroloph may be short (5 out of 11) or long (6 out of 11), connecting with the paracone (7 out of 11) or not (4 out of 11). This morphology of the labial anteroloph influences the presence of a short but relatively wide anterosinus (2 out of 11), a long anterosinus (6 out of 11), or the absence of an anterosinus (3 out of 11). The metalophule (centroloph) may be connected to the neo-entoloph (3 out of 11), to the anterior ridge of the hypocone (4 out of 11), to the anterior ridge of the hypocone and protolophule (2 out of 11), to the posterior ridge of the protocone (1 out of 11) or the axioloph (1 out of 11). The sinus may be shallow (4 out of 11) or deep (7 out of 11). The mesoloph may be long (2 out of 11), short (4 out of 11), incipient (2 out of 11), or absent (3 out of 11). The neo-entoloph may be short and connected to the protocone and hypocone (8 out of 11) or absent (3 out of 11). The axioloph may be incipient (3 out of 11) or absent (8 out of 11). The posteroloph may be long and curved (10 out of 11) or straight (1 out of 11), forming a labial wall together with the metacone and the posterior wall of the paracone (7 out of 11), and may not connect with the metacone (4 out of 11).

Variability in other sites: In MCX 3 (Fig. 4 W ), the labial anteroloph is shorter; in two out of eight individuals, the neo-entoloph is absent, and the protocone and hypocone are directly connected. In MTR 2 (Fig. 4 X ), there is one individual without a labial anteroloph, and the metaloph seems more often connected to the hypocone. In BC 1 (Fig. 4 Y ), there is one individual without a metalophule; the sinus is always deep, and the mesoloph is absent. In MAB 3 (Fig. 4 Z – AA), the metalophule disappears in some individuals, while the mesoloph is less common. There are no significant differences in FS 1, MAB 0 A, MAB 11 B, and CBR 1. Regarding biometry (Fig. 3 F , Suppl. material 4), no significant differences can be observed between the different sites in the Ribesalbes-Alcora basin. In addition, there is a slight tendency for specimens to become larger over time.

Remarks.

The genus Megacricetodon has been the focus of significant research interest due to its abundance and wide distribution during the Early and Middle Miocene ( Daams and Freudenthal 1988; Lazzari and Aguilar 2007; Wessels and Reumer 2009; Maridet et al. 2011; Oliver and Peláez-Campomanes 2013, 2014, 2016; Bonilla-Salomón et al. 2021; Jovells-Vaqué and Casanovas-Vilar 2021; Čermák et al. 2023, among others). The probable origin of this genus is in Asia, with the possibility of Megacricetodon beijiangensis Maridet et al., 2011 in China ( Maridet et al. 2011) or Megacricetodon dzhungaricus ( Kordikova and de Bruijn, 2001) in Kazakhstan ( Kordikova and de Bruijn 2001), being possibly the oldest species described so far. An alternative hypothesis proposes an origin in Turkey, with an indeterminate form found in Keseköy ( Wessels et al. 2001). These localities correlate with the European MN 3 ( Wessels et al. 2001; Wessels 2009; Maridet et al. 2011; Flynn and Wessels 2013). The temporal range of this genus extends to the island site of Gargano ( MN 13). However, the karstic origin of the latter site means that the record is a mixture of materials of different ages, according to Freudenthal and Martín Suárez (2010). Therefore, it can be concluded that the most recent record is that cited from the MN 9 by Kälin (1999).

The species M. primitivus , in conjunction with Megacricetodon bezianensis Bulot, 1980 , represents the most primitive species identified in south-western Europe ( Oliver and Peláez-Campomanes 2014; Čermák et al. 2023). Furthermore, the existence of multiple species of this genus in the Iberian Peninsula dates back to the Middle Miocene, as evidenced by the coexistence of M. primitivus with other species of the same genus during this period (Sesé 2006; van der Meulen et al. 2012; Casanovas-Vilar et al. 2016). In the Early Miocene of France, M. primitivus undoubtedly coexists with other species of the same genus, including M. bezianensis and Megacricetodon aunayi Lazzari & Aguilar, 2007 ( Antoine et al. 2000; Ginsburg and Bulot 2000; Bulot et al. 2009).

The populations of the Calatayud-Montalbán Basin, as meticulously documented by Oliver and Peláez-Campomanes (2014), provide a detailed understanding of the morphology of M. primitivus . This understanding is defined by specific characteristics: the m 1 with a rounded and generally simple anteroconid, a short anterolophulid, and an ectomesolophid that is typically absent; the m 3 mesolophids are usually present at low frequency; the m 1 has double anteroconid, which are typically accompanied by a small anterior platform, a strong lingual mesocingulum that is present in 20 % of specimens, and a mesoloph that is usually short to medium length. This detailed morphology is comparable to that observed in the material studied here, although with some minor variations, which will be discussed in further detail below.

The material under study is distinguished from other small and medium-sized species of this genus from Europe and Turkey by its relatively smaller size, Megacricetodon andrewsi Peláez-Campomanes & Daams, 2002 , Megacricetodon similis ( Fahlbusch, 1964) , and Megacricetodon grueneri Čermák et al., 2022 ( Peláez-Campomanes and Daams 2002; Čermák et al. 2023). The material under study exhibits significant distinguishing characteristics in comparison with Megacricetodon collongensis ( Mein, 1958) , including a higher frequency of double anteroconids, longer anterolophs / lophids, more symmetrical anterocones of M 1, and a greater prevalence of protolophs and double metalophs in M 1 and M 2. It differs from Megacricetodon debruijni Freudenthal, 1968 , in that it exhibits split anteroconids on M 1 and short mesolophids. It is also distinguished from Megacricetodon freudenthali García Moreno (in Álvarez-Sierra and García Moreno, 1986), Megacricetodon lopezae García Moreno (in Álvarez-Sierra and García Moreno, 1986), Megacricetodon minor ( Lartet, 1851) , Megacricetodon minutus Daxner, 1967 , and Megacricetodon rafaeli Daams & Freudenthal, 1988 . The material under study differs from these in possessing an undivided anterocone. Furthermore, it differs from Megacricetodon tautavelensis Lazzari & Aguilar, 2007 in terms of its smaller size, a higher percentage of divided anterocones, shorter mesolophs / ids, and a higher percentage of representation of crescent-shaped anteroconids. Finally, Megacricetodon hellenicus Oliver and Peláez-Campomanes, 2016 differs from the aforementioned material by having the M 1 in that it has a less deeply divided anterocone, longer mesolophs and entolophs; M 2 has a higher percentage of ectolophs; M 3 has longer entolophs; the anteroconid of m 1 is lower, and the mesolophids and lingual anterolophid of m 2 are the longest ( Oliver and Peláez-Campomanes 2014, 2016).

Given the substantial number of localities in which this species is present, a comparison was made between the present material and that from the localities of the Calatayud-Montalbán Basin. The most numerous population from each of the localities, Ca (Artesilla), Cb (Vargas 1 a), and the type locality (Valtorres, MN 5, local area Db), was selected for analysis ( Oliver and Peláez-Campomanes 2014).

In the M 1 s of the Calatayud-Montalbán Basin, the anterocone generally exhibits a small anterior platform; however, in a smaller proportion of specimens, this feature is absent, and in a lesser degree, the anterocones display a small cingulum or the anterocone is barely divided. In the material studied, this form is dominant only in MTR 2. In the remaining localities, the presence of this anterior platform or the cingulum is exceedingly rare. It is noteworthy that MAB 5 contains multiple specimens exhibiting a slight division of the anterocone. About the symmetry of the anterocone of M 1, the prevailing morphology in the populations used for comparisons is that in which the labial cusp is larger than the lingual cusp, except Artesilla, where the two cusps are almost of the same size. A similar phenomenon is observed in the localities of the Ribesalbes-Alcora Basin, where something similar occurs, where three sites ( MCX 3, MAB 5, and CBR 1) exhibit comparable sizes between the anterocone cusps. In the type locality, the connection of the anteroloph of M 1 to the anterocone occurs at similar frequencies along the middle part of the cusps of the anterocone or on the lingual cusp, with a slightly higher frequency in the latter case.

In the Ribesalbes-Alcora Basin, except MAB 3 and MAB 5, where both morphologies are represented in equal proportions, there is a striking prevalence of the anteroloph attached to the lingual cusp in the remaining materials. The presence of the partially developed or incipient labial ridge of the anteroloph of M 1 is an infrequent occurrence in the localities with which it is compared. In contrast, it is similar in our localities, with the incipient stage slightly more common. The protoloph of M 1 in the localities used for comparisons of the Calatayud-Montalbán Basin is predominantly single and posterior, with a lesser prevalence of complete or interrupted double forms. These also occur in our material, although in MAB 3 the dominant form is that with the small unconnected ridge.

Concerning the lingual mesocingulum of M 1, the number of individuals exhibiting a robust lingual mesocingulum was documented in the populations utilised for comparisons. This character is not particularly prevalent in the material under consideration. However, when only the presence or absence of the mesocingulum-anteroloph connection is considered, the proportions are similar or even higher. The ectoloph is typically longer in the localities of the Ribesalbes-Alcora Basin than in the localities used for comparisons of the Calatayud-Montalbán Basin, and a comparable development is observed solely at the Artesilla site. In contrast, the number of connections with the mesoloph is very similar in both groups of localities. The mesoloph of M 1 demonstrates a comparable development, except for MCX 3, where they are longer. At the same time, the metalophule of M 1 is predominantly connected to the base of the posteroloph in the localities used for comparisons. In the localities of the Ribesalbes-Alcora Basin, there is material where a posteriorly directed metalophule dominates, thereby reducing the size of the posterosinus.

Regarding the development of the M 2 protolophule, in most of the populations utilised for comparisons, it is anterior, or an incomplete posterior protolophule manifests with significantly reduced frequency, and the transverse, double, or posterior forms are residual. Conversely, within the material studied, the proportions observed in the sites belonging to the L. florancei biozone are consistent with those seen in other populations, while in the sites belonging to the L. ellipticus biozone, the transverse forms predominate. The ectoloph’s length and percentage of connections with the mesoloph are comparable. Notably, the ectoloph-mesoloph junction is generally less frequent in the oldest deposits belonging to the local L. florancei biozone of the Ribesalbes-Alcora Basin. The development of the M 2 metalophule is very similar in both populations, with the anterior form dominating, although the transverse form predominates in MCX 3 and the posterior metalophule in MAB 0 A.

The morphology of the metalophule of M 3 is similar in both basins, predominantly connected to the anterior ridge of the hypocone, except for the exception of BC 1, where the majority of specimens are devoid of a metalophule.

In m 1, the anteroconid is consistently simple, although at very low frequencies, the populations employed for comparisons from the Calatayud-Montalbán Basin may be slightly divided. The labial ridge of the m 1 anterolophulid is predominantly absent in both basins, although incipient ridges are well represented in BC 1 and well-developed ridges are present in Vargas 1 A. The anterior metalophulid of m 1 is present in the majority of specimens from both basins, more frequently separated from the protoconid in the Ribesalbes-Alcora Basin than in the populations used for comparisons of the Calatayud-Montalbán Basin. The mesolophid lengths of m 1, m 2, and m 3 are similar in the localities of both basins, although in m 2 of the localities studied, there are proportionally more specimens that reach the labial edge. On the other hand, the mesolophids of m 3 occur only in the more modern deposits belonging to local biozone L. ellipticus in the Ribesalbes-Alcora Basin. The absence of ectomesolophids in m 1 and the length of the anterolophid of m 2 (short) are similar in both basins. Although in the latter case, the length of the m 2 anterolophid is shorter, the well-developed form is more abundant in the MCX 3, BC 1, and MAB 5 sites.

The specimens from the Calatayud-Montalbán and the Ribesalbes-Alcora basins studied here are similar in size (see Fig. 3 ). However, minor discrepancies do emerge: the m 1 is similar to those of the populations utilised for comparisons of local biozone C (Ca and Cb), though with some shorter specimens in MCX 2, MCX 3, and MTR 2; the m 2 are also more similar to the local biozone C populations, with maximum length values slightly higher than in the material under study; the m 3 the sizes are similar to those of the deposits of the same local biozone, except for one specimen found in MAB 0 B; the M 1, although similar, is generally closer to the range of the locality used for comparisons in local biozone Cb; the M 2 are identical to those of this locality, only the material from MAB 0 A is longer; and finally, regarding the M 3, there is one specimen from BC 1 and one from MAB 3 that are narrower than those from this locality, while the rest are of similar size.

In relation to the Buñol zone, the Barranco de Candel site is distinguished by the following features in comparison with our material: one specimen with a slightly bilobed anteroconid of the M 1; the anterolophid of the M 2 is typically more developed; the M 2 mesolophid is longer; the M 1 ectolophs are more prevalent; and the M 1 mesolophs are longer. Regarding biometrics, the specimens from the Barranco del Candel site are broadly similar to those studied from the Ribesalbes-Alcora Basin, although the M 1 s are slightly smaller ( Adrover et al. 1987). Furthermore, the morphology of the material from the Buñol site is comparable to that of the Ribesalbes-Alcora Basin sites, with the exception of the mesolophid of m 1, which is slightly longer, and the anterolophid of m 1, which is somewhat shorter. Also, the lingual anterolophid of m 2 is more developed, the protolophule is almost double in half of the specimens, which is the same as in MAB 3, and the mesoloph of M 1 is shorter. The measurements are similar, except M 2 is slightly smaller and M 3 is slightly broader ( Daams and Freudenthal 1974).

In the Morteral section (1–20 A), within the Magro Basin (including M. primitivus and some taxa in open nomenclature ( Megacricetodon sp. 1 and 2), the morphology of these taxa is similar. However, there are some differences when compared to our sites. In the more modern sites, there are representatives with a slightly doubled anteroconid of m 1 and shorter mesolophids. The m 2 exhibits a slightly shorter mesolophid, and the labial ridge of the anterolophid of M 1 is less frequently observed. The ectoloph of the M 1 is longer. In M 2, there is a reduction in the number of partially double and transverse protolophids. Furthermore, in the more modern deposits, the ectoloph is more frequently attached to the mesoloph. The metalophule of M 2 exhibits a comparable morphology, with the exception of MCX 3, where it is transverse. Biometrically, they are similar, except for Megacricetodon sp. 2 , which has larger lower molars, and the upper molars are of a similar size ( Ruiz-Sánchez 1999; Ruiz-Sánchez et al. 2003).

In the Vallès-Penedès Basin, the remains analysed in this study demonstrate notable similarities to those exhibited by the material found in the following sites: Can Julià 6 and Can Martí Vell I and II, Els Casots 73 and 76. However, a distinguishing feature emerges in the analysis of Can Martí Vell I, where the protolophid is consistently located posteriorly, and the mesolophid of m 1 is typically shorter. Biometrically, the material from Catalonia corresponds with the material studied here, including the Megacricetodon sp. described by Agustí (1983), for which he emphasized its larger size. While the most recent publications from this basin indicate an increase in size in the more recent deposits, similar to what occurred in the present basin, the measurements are, on average, slightly larger than those of the material studied ( Jovells-Vaqué and Casanovas-Vilar 2021).

In the Loranca Basin, particularly at the La Retama site, the mesoloph of M 1 is characterised by a reduction in length, the lingual mesocingulum exhibits increased prevalence, the protolophule is posteriorly positioned, and the metalophule is anteriorly situated. In the M 2, the mesoloph is shorter; in the M 1, there is a higher prevalence of divided anteroconids. In the m 2, the lingual anterolophid is less prevalent, and the mesolophids are shorter in length. Biometrically, M 2, m 2, and m 3 are typically smaller in size ( Álvarez-Sierra et al. 2006).

Therefore, the morphological and biometric study of the material of M. primitivus is in accordance with the findings of Oliver and Peláez-Campomanes (2014), which indicate that the species is highly stable over time, exhibiting intraspecific variability. Additionally, there is no evident evolutionary trend in either size or morphology. Furthermore, the distribution of dental characters varies considerably across different sites, yet they always retain a resemblance to the type of material.

Figures

Figure 2. Lower teeth of Megacricetodon primitivus from the Ribesalbes-Alcora Basin. A. Left mandible + m 2 + m 3 (MCX 3-33); B. Right m 1 (MCX 3-73); C. Right m 1 (MTR 2-72); D. Left m 1 (BC 1-55); E. Right m 1 (MAB 0 B- 10); F. Right m 1 (MAB 5-9); G. Right m 1 (MAB 5-61); H. Right m 1 (MAB 5-499); I. Left m 1 (MAB 5-798); J. Right m 1, labial view (MCX 3-65); K. Right m 2 (MCX 3-78); L. Left m 2 (BC 1-70); M. Left m 2 (MAB 3-170); N. Right m 2 (MAB 3-171); O. Right m 2 (MAB 3-172); P. Left m 2 (MAB 3-173); Q. Right m 2 (MAB 5-540); R. Right m 2 (CBR 0 B- 13); S. Left m 3 (MCX 3-87); T. Right m 3 (BC 1-71); U. Left m 3 (MAB 2-2); V. Right m 3 (MAB 3-6); W. Left m 3 (MAB 3-209); X. Left m 3 (MAB 3-213); Y. Right m 3 (MAB 3-214); Z. Right m 3 (MAB 3-219); AA. Right m 3 (MAB 5-512); AB. Right m 3 (MAB 5-803). Scale bar: 1 mm.

Figure 3. Comparison of length and width measurements (in mm) of Megacricetodon primitivus from Ribesalbes-Alcora Basin. M. primitivus from Valtorres (type locality (t. l. )); M. vandermeuleni from Fuente Sierra 4 (t. l.); M. hellenicus from Aliveri NQ (t. l.); M. collongensis from Vieux Collonges (l. t.); M. bezianensis from Bézian; M. bavaricus from Langenmoosen (t. l.); and the material from Ribesalbes-Alcora Basin (Fahlbusch 1964, Bulot 1972, Maridet 2003, Oliver and Peláez-Campomanes 2013, 2014, 2016; this publication).

Figure 4. Upper teeth of Megacricetodon primitivus from the Ribesalbes-Alcora Basin. A. Right M 1 (MTR 2-47); B. Right M 1 (BC 1-33); C. Right M 1 (BC 1-36); D. Right M 1 (MAB 0 A- 22); E. Left M 1 (MAB 3-53); F. Left M 1 (MAB 3-57); G. Left M 1 (MAB 3-59); H. Right M 1 (MAB 3-62); I. Left M 1 (MAB 3-66); J. Left M 1 (MAB 5-78); K. Left M 1 (MAB 11-1); L. Right M 1 (CBR 1-12); M. Left M 2 (MCX 3-51); N. Left M 2 (BC 1-46); O. Right M 2 (FS 1-20); P. Left M 2 (MAB 3-87); Q. Left M 2 (MAB 5-65); R. Right M 2 (MAB 5-74); S. Right M 2 (MAB 5-489); T. Right M 2 (MAB 5-509); U. Left M 2 (MAB 11-31); V. Left M 2 (CBR 1-1); W. Left M 3 (MCX 3-59); X. Left M 3 (MTR 2-204); Y. Right M 3 (BC 1-53); Z. Left M 3 (MAB 3-112); AA. Left M 3 (MAB 3-114); AB. Left M 3 (MAB 3-118); AC. Left M 3 (MAB 5-13); AD. Left M 3 (MAB 5-62); AE. Left M 3 (MAB 5-494); AF. Left M 3 (MAB 5-809). Scale bar: 1 mm.