Stratigraphic and Paleoecological Signiﬁcance of the Early/Middle Pleistocene Vertebrate Fauna of the Sütt ˝o 21 Site

: The Sütt˝o 21 site is a ﬁssure ﬁll of the freshwater limestone of the Gazda quarry in Sütt˝o. The material was collected between 2017 and 2019, and the results are summarised in this article, with a special focus on the small vertebrate fauna of the site and its stratigraphic and paleoecological signiﬁcance. The ﬁssure ﬁll can be placed around the Early/Middle Pleistocene boundary (ca. 1.1 and 0.77 Ma). The paleoecological analysis of the herpeto- and mammal fauna of the sequence indicates the proximity of a permanent water body. The lower part of the sequence is dominated by open habitat indicator taxa indicating a cool, dry climate. Towards the upper part of the sequence, the climate remained cool, but became wetter, and the vegetation gradually changed to forest-steppe/open forest. The fauna of the Sütt˝o 21 site can be compared with the material of sites that are of a similar age, thus revealing taxonomic and paleoecological differences between different areas of the country. While a warm, dry climate and open vegetation can be reconstructed in the Vill á ny Hills around the Early/Middle Pleistocene boundary, the Northern Hungarian areas had a cooler, wetter climate and a slightly more closed (sparse forest, forest-steppe) vegetation during this period.


Introduction
One of the most-important travertine quarries in Hungary, exploited until the present day, can be found south of Süttő village (Northern Hungary). This mine is one of the most-important Pleistocene vertebrate sites in Hungary and, through the site, Süttő 6, is the type locality of the Süttő Biochronological Phase (MIS 5) [1,2]. Since the middle of the 19th Century, vertebrate remains have been recovered from 20 sites in different parts of the mine (Cukor quarry, Hegyháti quarry, Diósvölgyi quarry, and Haraszti quarry), both from travertine and fissure fills in the rock. The sites described from the Süttő Travertine Complex were last summarized by Pazonyi et al. [3], and the age of the travertine and fissure faunas was correlated with the Danube terrace chronology by Ruszkiczay-Rüdiger et al. [4]. The age of the travertine is 2.0-1.8 Ma based on the fossil material recovered from it, while the age of most of the fissure fills correlate to the different phases of MIS 5 (Süttő 3,7,9,and 12). However, there are also younger (Süttő 16; MIS 2) and older (Süttő 17 and 19; 1.0-0.9 Ma) fissure faunas than the abovementioned age [3,4].
The site presented in this paper was discovered on the field day of the 20th Hungarian Paleontological Meeting in 2017. It is the first known fossiliferous fissure fill from the Gazda quarry ( Figure 1). The site, called Süttő 21, is an unstratified loess fill deposited in Although the analysis of the samples collected in 2019 is still ongoing, the taxonomic processing of the upper nine samples has already been performed (between 6 and 2.4 m). Fortunately, the previous collections were mainly concentrated in the lower part of the stratigraphy, so sufficient information is available for this part (between 2.4 and 0 m) also.
The main aim of this paper was to present the stratigraphic and paleoecological significance of the small vertebrate material, so the taxonomic results are only discussed for those taxa where relevant for this paper. We also aimed to compare the material of the Süttő 21 site with other sites of a similar age in Hungary, which may help to reveal climatic and vegetation differences between different areas of the country.

Süttő 21 Site
The deposit discovered in 2017 was a 6 m-high, unstratified loess-filled fissure at the second mining level. Figure 2 clearly shows that the infilling continued below and above Although the analysis of the samples collected in 2019 is still ongoing, the taxonomic processing of the upper nine samples has already been performed (between 6 and 2.4 m). Fortunately, the previous collections were mainly concentrated in the lower part of the stratigraphy, so sufficient information is available for this part (between 2.4 and 0 m) also.
The main aim of this paper was to present the stratigraphic and paleoecological significance of the small vertebrate material, so the taxonomic results are only discussed for those taxa where relevant for this paper. We also aimed to compare the material of the Süttő 21 site with other sites of a similar age in Hungary, which may help to reveal climatic and vegetation differences between different areas of the country.

Süttő 21 Site
The deposit discovered in 2017 was a 6 m-high, unstratified loess-filled fissure at the second mining level. Figure 2 clearly shows that the infilling continued below and above this level, but we were unable to collect material from these infillings because, by the year 2022, both the original and the fissures above and below the original site had been mined.
Diversity 2023, 15,736 this level, but we were unable to collect material from these infillings because, 2022, both the original and the fissures above and below the original site had b Between 2017 and 2019, a total of 33 samples were collected from the site different collections. Some of these were only test collections, where we exami bris collected at the bottom of the fissure, but most of the samples were taken f defined part of the fill. This article is based mainly on material from the 2019 when we systematically collected the entire fissure fill every 40 cm, starting f of the stratigraphic sequence. Since the fissure was narrower in some places a others, the amount of material in each sample was different, but the smallest s resented several bags (at least ca. 200 kg) of material, which did not cause an with the interpretation. Unfortunately, mainly due to the large amount of m material from this excavation is not yet fully processed, so the lower 2.4 m o graphic sequence is presented using material from the other excavations. Alth were smaller samples, as the material was collected from the same height in s vations, we were able to combine them and, thus, have sufficient material to c analyses (Tables 1 and 2).  Between 2017 and 2019, a total of 33 samples were collected from the site during six different collections. Some of these were only test collections, where we examined the debris collected at the bottom of the fissure, but most of the samples were taken from a welldefined part of the fill. This article is based mainly on material from the 2019 excavation, when we systematically collected the entire fissure fill every 40 cm, starting from the top of the stratigraphic sequence. Since the fissure was narrower in some places and wider in others, the amount of material in each sample was different, but the smallest sample represented several bags (at least ca. 200 kg) of material, which did not cause any problems with the interpretation. Unfortunately, mainly due to the large amount of material, the material from this excavation is not yet fully processed, so the lower 2.4 m of the stratigraphic sequence is presented using material from the other excavations. Although these were smaller samples, as the material was collected from the same height in several excavations, we were able to combine them and, thus, have sufficient material to carry out the analyses (Tables 1 and 2). Triturus cristatus The collected material was washed through a 0.5 mm sieve in the Department of Palaeontology and Geology laboratory at the Hungarian Natural History Museum. The small fossils were sorted out under a stereo light microscope (Nikon SMZ 445).
The material contained seeds and terrestrial snail shells, as well as bones of fishes, amphibians, reptiles (Table 1), and mammals ( Table 2). When calculating the minimum number of individuals (MNI) values, all teeth and bones found in one sample and determined as the same species were taken into account. We examined at least how many individuals belonging to this species had to occur here for these remains to be found in the sample.

Sites of a Similar Age Included in the Study
Altogether, five sites were studied here from the Pannonian Basin that contained vertebrate fauna of a similar age: two were situated in the Villány Hills (Southern Hungary), while three in Northern Hungary ( Figure 3).
Diversity 2023, 15, 736 5 The material contained seeds and terrestrial snail shells, as well as bones of fi amphibians, reptiles (Table 1), and mammals ( Table 2). When calculating the minim number of individuals (MNI) values, all teeth and bones found in one sample and d mined as the same species were taken into account. We examined at least how many viduals belonging to this species had to occur here for these remains to be found i sample.

Sites f a Similar Age Incl ded in the St dy
Altogether, five sites were studied here from the Pannonian Basin that contained tebrate fauna of a similar age: two were situated in the Villány Hills (Southern Hung while three in Northern Hungary ( Figure 3).

Villány Hills (Southern Hungary)
Somssich Hill 2 is an 8 m-deep karst cavity with a diameter of 5 m on the pre surface. It is situated within Upper Jurassic (Oxfordian) limestone at the top of Som Hill, to the west of Villány. The infilling sediment is a breccia with a brownish-red clay matrix below 4 m, whereas it becomes light yellowish-brown carbonate-cemente in the upper part [5,6]. The age of the locality can be correlated with the Latest Early P tocene (c. 1.0 Ma), the equivalent of the Mim mys savini-Mim mys p sill s Assemb Zone of Kordos [7] based on the co-occurrence of the "advanced" All phai mys pli cae and Micr t s (M.) nival ides together with Mim mys savini and Mim mys p sill s (see [8]). Villány 8 is a karst cavity connected to a fissure system. It is located on the sout wall of the same quarry at Templom Hill as Villány 5. The lower layers of the infi sediment are reddish-brown clays with calcareous interbeds, whereas the upper par comes yellowish-brown silt, similar to the Somssich Hill 2 site [2,9]. This site is the st type of the Templomhegyian Phase within the Biharian Stage of the local biochronolo system [2,10]

Villány Hills (Southern Hungary)
Somssich Hill 2 is an 8 m-deep karst cavity with a diameter of 5 m on the present surface. It is situated within Upper Jurassic (Oxfordian) limestone at the top of Somssich Hill, to the west of Villány. The infilling sediment is a breccia with a brownish-red silty clay matrix below 4 m, whereas it becomes light yellowish-brown carbonate-cemented silt in the upper part [5,6]. The age of the locality can be correlated with the Latest Early Pleistocene (c. 1.0 Ma), the equivalent of the Mimomys savini-Mimomys pusillus Assemblage Zone of Kordos [7] based on the co-occurrence of the "advanced" Allophaiomys pliocaenicus and Microtus (M.) nivaloides together with Mimomys savini and Mimomys pusillus (see also [8]). Villány 8 is a karst cavity connected to a fissure system. It is located on the southern wall of the same quarry at Templom Hill as Villány 5. The lower layers of the infilling sediment are reddish-brown clays with calcareous interbeds, whereas the upper part becomes yellowish-brown silt, similar to the Somssich Hill 2 site [2,9]. This site is the stratotype of the Templomhegyian Phase within the Biharian Stage of the local biochronological system [2,10]. The beginning of the Templomhegyian Phase can be dated to around the Early/Middle Pleistocene boundary (780 KA). Based on the absence of Allophaiomys and Mimomys pusillus, the vole fauna can be assigned to the Mimomys savini Biozone [7,11].

Northern Hungary
Süttő 17 is a fissure infill with two layers of the freshwater limestone of the Hegyhát quarry in Süttő, approximately 400 m from the Süttő 21 site. The vole fauna is characterized by Mimomys savini, Allophaiomys praehintoni, Lasiopodomys hintoni, and Microtus (M.) nivaloides, while Mimomys pusillus is missing. Based on these dates, the age of this site is equal to the Mimomys savini Biozone [3].
Újlaki Hill is located to the north of Budapest as a part of the Buda Mountains. The fauna was recovered from a small karst cave filled with terra rossa, a few meters from the top of the hill, surrounded by Upper Eocene limestone [12]. Mimomys pusillus, Allophaiomys praehintoni, Terricola arvalidens, and "true" Microtus species were found together at the site, which means that the material belongs to the Mimomys savini-Mimomys pusillus Assemblage Zone of Kordos [7], and its age can be correlated with the Latest Early Pleistocene (c. 1.2-1.0 Ma) [8].
Kövesvárad locality is a 5 m-high and 3 m-wide karst cavity situated to the east of Répáshuta in the Bükk Mountains. The vole material was recovered from its orange and reddish brown clay infilling [2,13]. Based on the co-occurrence of Mimomys savini and Allophaiomys praehintoni together with Terricola arvalidens and Microtus (M.) nivaloides, this site is probably contemporaneous with Újlaki Hill and Somssich Hill 2.

Biostratigraphic Results
The vole fauna of the Süttő 21 site is quite rich, with a total of 10 taxa, of which Mimomys pusillus and Mimomys savini are the zonal index species. In addition to these, two steppe lemming species, Lagurodon arankae and Prolagurus pannonicus, are very common, as are species belonging to the Allophaiomys-Microtus evolutionary lineage (Allophaiomys praehintoni, Lasiopodomys hintoni, Microtus (M.) nivaloides). Less common taxa are Pliomys sp., Clethrionomys sp., and the single example of Microtus (M.) nivalinus.
Based on its vole fauna, the Süttő 21 site can be dated to the so-called Mid-Pleistocene climatic revolution. During this period (1.2-0.8 Ma), the Earth's orbital ice-age cycles intensified, lengthening from~40 ky to~100 ky, and became markedly asymmetric. After this transition, the climate became drier and cooler, increasing the areas covered with open vegetation. The fauna of Süttő 21 represents the last phase of the transitional period, characterized mainly by the appearance of Microtus (M.) nivalinus, but also Lasiopodomys hintoni and Prolagurus pannonicus. Such faunas have been associated with the end of the Matuyama paleomagnetic chron (e.g., Karai-Dubina, Petropavlovka 2) [14]. Overall, the vole fauna is very similar to the late Early Pleistocene (0.9 Ma) Somssich Hill 2 material [6], but the Süttő 21 fauna lacks Terricola arvalidens, which dominates the Somssich Hill 2 material, and Allophaiomys pliocaenicus.
The situation is similar for shrews. In most parts of the sequence (except the uppermost strata), the faunal composition is very similar to that of the Soricidae assemblage of the Somssich Hill 2 site. The dominant species are Beremendia fissidens and Sorex runtonensis, with abundant Sorex (Drepanosorex) savini and Sorex minutus, while Asoriculus gibberodon is also present with a low number. Neomys newtoni is absent, but this is not surprising, as the species was not widespread yet in Central Europe at the end of the Early Pleistocene. Its sparse occurrence at Somsich Hill is the first report of this species from Hungary.
However, there is a conspicuous absence of Crocidura species, one of the most-abundant genera at the Somssich Hill 2 site. This would suggest that the age of the assemblage may be older than when the Crocidura shrews migrated from Africa to Europe, but the other Pleistocene faunal elements preclude the site pre-dating the "Crocidura date" (MN16/17 zonal boundary). Thus, the absence of Crocidura, like the variation in the vole fauna, is probably due to ecological reasons.
The mammalian fauna of the Süttő 21 site is very similar to that of the late Early Pleistocene Untermassfeld fauna [15,16] also, although the latter, probably due to paleobiogeographical reasons, lacks the lagurins (Lagurodon arankae, Prolagurus pannonicus) typical of Süttő 21. Since, in contrast to Süttő 21, magnetostratigraphic analysis of the Untermassfeld fauna was possible, we know that a paleomagnetic reversal occurred at this site, which is associated with the Jaramillo subchron (1.05 Ma), mainly based on the evolutionary level of Microtus (Allophaiomys) thenii [16]. The Süttő 21 site is certainly younger than Untermassfeld, since in addition to Lasiopodomys hintoni and Allophaiomys praehintoni, which have very similar morphologies to Microtus (Allophaiomys) thenii, two "true" Microtus species (Microtus (M.) nivaloides, M. (M.) nivalinus) also occur at this site ( Figure 4). Further studies will be needed to clarify whether the morphology of Lasiopodomys hintoni and Allophaiomys praehintoni is a continuous transition and how these species relate to each other and Microtus (Allophaiomys) thenii. studies will be needed to clarify whether the morphology of Lasi p d mys hint ni and All phai mys praehint ni is a continuous transition and how these species relate to each other and Micr t s (All phai mys) thenii.
It should be noted here that the following genus or subgenus of "Micr t s" voles were found at the Süttő 21 site: All phai mys, Lasi p d mys, and Micr t s (Micr t s). We used the taxonomic nomenclature within the Arvicolini sensu stricto tribe based on the proposed, phylogenetic-based system of Abramson et al. [17].  It should be noted here that the following genus or subgenus of "Microtus" voles were found at the Süttő 21 site: Allophaiomys, Lasiopodomys, and Microtus (Microtus). We used the taxonomic nomenclature within the Arvicolini sensu stricto tribe based on the proposed, phylogenetic-based system of Abramson et al. [17].
The Micr t s (M.) nivalin s is a junior synonym of M. ratticep ides [18] and is one of the species typical of the last phase of the Mid-Pleistocene transition [14]. The species has been described previously from Hungary, from the Early/Middle Pleistocene Villány 8 site [9]. As this species is absent from the material of late Early Pleistocene sites (e.g., Somssich  Microtus (M.) nivalinus is a junior synonym of M. ratticepoides [18] and is one of the species typical of the last phase of the Mid-Pleistocene transition [14]. The species has been described previously from Hungary, from the Early/Middle Pleistocene Villány 8 site [9]. As this species is absent from the material of late Early Pleistocene sites (e.g., Somssich Hill 2), it can be assumed that M. (M.) nivalinus appeared in the area somewhat later than M. nivaloides.
The cap morphology of M. (M.) nivalinus is somewhat similar to that of the recent Alexandromys oeconomus. The neck is open; BRA4 is very poorly developed, but BSA4 is clearly distinguishable. The cap is slightly flattened (similar to recent Chionomys nivalis), but not rounded as in M. (M.) nivaloides and recent A. oeconomus (Figure 4).
In Mimomys savini, in the lower layers, only relatively small (L mean = 2.98 mm, n = 3) forms with roots appear, with a high SDQ (SDQ mean = 165) and a low A/L ratio (A/L mean = 40.56, A mean = 1.21 mm). In contrast, the top of the sequence is characterized by rootless, large forms (L mean = 3.52, n = 3), with a lower SDQ (SDQ mean = 144.45), but a higher A/L ratio (A/L mean = 44, A mean = 1.54). Rootless forms are young Mimomys savini, not early Arvicola species (Figure 4).
Prolagurus pannonicus is dominated by rounded anterioconid cap forms with a very simple morphology at the bottom of the sequence. At the top of the sequence (from 4 m upwards), however, the "posterius" morphotype becomes dominant, although the "pannonicus" morphotype is also present. A single specimen of the "transiens" morphotype was recovered from the material (between 3.6 and 3.2 m). Based on these observations, the age of the sequence is estimated to be between 1.1 and 0.8 Ma (Figure 6).
Diversity 2023, 15,736 Pr lag r s pann nic s is dominated by rounded anterioconid cap forms simple morphology at the bottom of the sequence. At the top of the sequenc upwards), however, the "p steri s" morphotype becomes dominant, although nic s" morphotype is also present. A single specimen of the "transiens" morp recovered from the material (between 3.6 and 3.2 m). Based on these observati of the sequence is estimated to be between 1.1 and 0.8 Ma (Figure 6). There is also a difference between the upper and lower part of the site i fauna. At the top of the sequence, Beremendia, which dominate the lower leve rare and a large S rex species (present only in these layers) appears.
In contrast, hamsters appear throughout the sequence of the Süttő 21 s same two species (Cricet s cricet s ssp., All cricet s b rsae). Cricet s cricet s ssp in Figure 7, is significantly smaller than Cricet s praeglacialis in the Middle Plei terial of Villány 8 and the recent Cricet s cricet s; however, the dimensions a larger than Cricet s nan s. The characteristic data of this latter species are giv 3. A similarly small jaw with teeth m1 and m2 was also recovered from Köve L = 2.9 mm W = 1.75 mm; m2: L = 2.35 mm W = 1.95 mm), together with some probably Cricet s r nt nensis or C. maj r. Unfortunately, such a small numbe are known from both Süttő 21 and Kövesvárad that we cannot give a more p nomic definition for this small form.
Based on the observed faunistic and taxonomic changes, the sequence of can be placed around the Mim mys p sill s-Mim mys savini and Mim mys s i.e., the Early/Middle Pleistocene boundary (between ca. 1.1 and 0.77 Ma) (Fi lower part of the sequence is Early Pleistocene, while the upper part probably There is also a difference between the upper and lower part of the site in the shrew fauna. At the top of the sequence, Beremendia, which dominate the lower levels, becomes rare and a large Sorex species (present only in these layers) appears.
In contrast, hamsters appear throughout the sequence of the Süttő 21 site with the same two species (Cricetus cricetus ssp., Allocricetus bursae). Cricetus cricetus ssp., as shown in Figure 7, is significantly smaller than Cricetus praeglacialis in the Middle Pleistocene material of Villány 8 and the recent Cricetus cricetus; however, the dimensions are definitely larger than Cricetus nanus. The characteristic data of this latter species are given in Table 3. A similarly small jaw with teeth m1 and m2 was also recovered from Kövesvárad (m1: L = 2.9 mm W = 1.75 mm; m2: L = 2.35 mm W = 1.95 mm), together with some large teeth, probably Cricetus runtonensis or C. major. Unfortunately, such a small number of remains are known from both Süttő 21 and Kövesvárad that we cannot give a more precise taxonomic definition for this small form.   Based on the observed faunistic and taxonomic changes, the sequence of the Süttő 21 can be placed around the Mimomys pusillus-Mimomys savini and Mimomys savini zones, i.e., the Early/Middle Pleistocene boundary (between ca. 1.1 and 0.77 Ma) ( Figure 5). The lower part of the sequence is Early Pleistocene, while the upper part probably dates to the very beginning of the Middle Pleistocene. This may be indicated by the occurrence of Microtus (M.) nivalinus, as this species has so far only been described from the beginning of the Middle Pleistocene in Hungary (Villány 6 and Villány 8 sites) [9]. The stratigraphic significance of the site is that we can study the changes at the Early/Middle Pleistocene boundary in a continuous stratigraphic sequence.
Large quantities of large mammal remains were found, but unfortunately, most of them are fragmentary. The intact specimens are dominated by isolated teeth, phalanges, carpal, tarsal bones, etc., which in most cases allow more or less questionable species identification only. Thus far, the following large mammal faunal elements have been identified: Mammuthus meridionalis, Equus sp. (very probably E. altidens or E. suessenbornensis), Capreolus sp., Eucladoceros cf. giulii, Bison cf. schoetensacki, and Homotherium ex gr. H. latidens. The identification of Mammuthus meridionalis and Homotherium is quite certain, based on the dimensions and morphological characters of some of their remains. In the case of M. meridionalis, the enamel thickness measured on a tooth plate fragment is 3.8-4.2 mm, and the crown height is 75 mm. The identification of the Homotherium is based on an upper P4. It has no trace of preparastyle, and its protocone is extremely reduced. The maximum length of the crown is 40.25 mm; the height of the paracone is 24.75 mm; the width behind the paracone is 11.67 mm. The taxonomic classification of the other species was partly based on the dimensions and overall shape of some isolated teeth and limb bone fragments, but the most-important factor was the comparison with other European sites of a similar age (e.g., La Vallonet, Redicicoli, Atapuerca-Gran Dolina, and first of all, Untermassfeld) and the species that were identified from them [24][25][26].

Paleoecological Evaluation
For the paleoecological evaluation of the small vertebrate fauna of the stratigraphic sequence, in addition to taxonomic processing, it was also important to determine the MNI of the species in the case of small mammals ( Table 2). For the herpetofauna, no such minimum number of individuals was determined; in Table 1, only the occurrence of each taxon in the different samples is indicated.
The remains of the spadefoot toads (Pelobates fuscus) are present in all samples (Table 1). This is probably due to the fact that the soil was favourable, soft, and plastic [27], throughout for these burrowing frogs. The burrowing toads such as the Bufotes viridis are also relatively frequent, and some remains of Bufo bufo also occur in two samples. Other open land-loving animals [28,29] such as green lizards and whip snakes are significantly rarer, while the uncertain remains of the cat snake (cf. Telescopus fallax) appear only in the 1.6-2.0 m, while the Vipera ursinii is known from two samples in the profile. Except for a sample from 0.8-1.2 m, the moisture-loving herpeto elements (newts, the remains of the Pelophylax esculentus group, and the grass and dice snakes) are everywhere. On the other hand, fish remains (otoliths) were present from this screen-washed material, that is the permanent water existed, suggesting some fragmentary Natrix sp. vertebrae, as well. Hyla arborea, Anguis fragilis, Elaphe quatuorlineata, Zamenis longissimus, and Vipera ammodytes suggest that the paleoenvironment was woody or at least woody and bushy [28,30,31]. The presence of opportunist Rana temporaria and the more frequent Coronella austriaca suggest the presence of a mosaic, forest edge paleoenvironment. Highly adapted to various habitats, Latonia gigantea cannot be used as a paleoenvironmental indicator [32][33][34][35][36].
In summary, the composition of this paleo-herpetofauna suggests the paleoenvironment of the site Süttő 21 was an ephemeral water body (lake) and around this loose soil and woody land, which could come into contact with a steppe.
In the case of small mammals, the ecological requirements of the species were based on recent analogies [37,38], our previous research [6,39], and the results of similar paleoecological studies [40]. We classified taxa into four categories: open habitat indicator taxa, forest-shrub indicator taxa, mesophilous taxa, and taxa with unknown ecological preferences or opportunistic species (Table 4). In addition to analysing the total small mammal fauna, we also analysed the so-called steppe species, which represent about half of the small mammal fauna and are indicative of open environments and within which significant changes in the sequence can be observed due to environmental differences. Table 4. Grouping of small mammal taxa according to their habitat preferences [6,[37][38][39][40].

Open Habitat Indicator Taxa
Forest-Shrub Indicator Taxa  As shown in Figure 8, the ecological requirements are unknown of only 16.4% of the species that make up the fauna on average, so the analysis is quite informative. In the small mammal fauna of the stratigraphy, the taxa (Sorex (Drepanosorex) savini, Mimomys savini, Desmaninae gen. et sp. indet.) that are mesophilous are present throughout in subordinate abundance, which may indicate that there was a permanent watercourse or lake in the wider environment of the site. Although the open habitat preferences taxa (Sorex runtonensis, Spalax sp., Cricetus cricetus ssp., Allocricetus bursae, Spermophilus cf. primigenius, Sicista cf. praeloriger, Ochotona sp., Lagurodon arankae, and Prolagurus pannonicus) dominate the small mammal fauna, the forest-shrub indicator taxa (Talpa sp. indet., Sorex sp. indet., Asoriculus gibberodon, Sorex minutus, Apodemus cf. sylvaticus, Sciurus sp., Eliomys aff. quercinus, Muscardinus cf. dacicus, Clethrionomys sp., Allophaiomys praehintoni, Lasiopodomys hintoni, and Microtus (M.) nivaloides) are also present throughout the sequence. The relative proportions of the two categories allowed us to distinguish between periods of more forested and more open vegetation.
To determine the extent to which the vegetation in the area was forested, we looked at the ratio       Given the total absence of Crocidura species typical of the warm steppe, this environment could have been a cold-climate steppe or forest-steppe throughout.
The taxonomic shifts discussed in the biostratigraphic section are also reflected in ecological features in the upper part of the sequence. The dominance of Sorex runtonensis, Lagurodon arankae, and Prolagurus pannonicus, which are specifically cold steppe species, is reduced, and the already present mesophilous shrews and voles (Sorex minutus, Asoriculus gibberodon, Allophaiomys praehintoni, Lasiopodomys hintoni, Microtus (M.) nivaloides) are joined by a new Sorex species, probably also in forested vegetation. The presence of a Talpa species confirms the increased proportion of forested or scrubby areas at the top of the sequence, as moles prefer the looser soils of forest or scrub.
Three more closed and two more open periods of vegetation can be distinguished in the sequence. Figure 8 shows that the vegetation became more closed as it moves toward the top of the sequence. While at the bottom of the infilling (0.5-2.8 m), the proportion of those taxa preferring forest-shrub vegetation was 80-100% of those taxa preferring open habitat, in the next wave (3.6-4 m), this proportion rose to 113%, while at the top of the sequence (5.2-6 m), it reached 141%. In the other phases, we can imagine a more open steppe environment with shrubby vegetation limited to the area around the water body. Given the total absence of Cr cid ra species typical of the warm steppe, this environment could have been a cold-climate steppe or forest-steppe throughout.
The taxonomic shifts discussed in the biostratigraphic section are also reflected in ecological features in the upper part of the sequence. The dominance of S rex r nt nensis, Lag r d n arankae, and Pr lag r s pann nic s, which are specifically cold steppe species, is reduced, and the already present mesophilous shrews and voles (S rex min t s, As ric l s gibber d n, All phai mys praehint ni, Lasi p d mys hint ni, Micr t s (M.) nival ides) are joined by a new S rex species, probably also in forested vegetation. The presence of a Talpa species confirms the increased proportion of forested or scrubby areas at the top of the sequence, as moles prefer the looser soils of forest or scrub.
As the proportion of different steppe species varies widely across the series, we attempted to provide a more accurate reconstruction of how open area characteristics have changed, based on recent analogies. To do this, we classified the taxa according to their habitat into four categories: arid or semi-arid grassland (Lag r d n arankae, Pr lag r s pann nic s, S rex r nt nensis), fertile lowland steppe (All cricet s b rsae, Cricet s cricet s ssp.), shortgrass-steppe (Spalax sp., Sperm phil s cf. primigeni s), and forest-steppe (Sicista cf. prael riger, Och t na sp.) ( Figure 10).  Figure 10 shows that, during the deposition of the lower part of the sequence (0-4 m), the area around the site was mainly cold, dry steppe, and then from about 3.6-4 m upwards, a higher proportion of more forested steppe types with more precipitation Figure 10. Percentage distribution of the so-called steppe taxa by habitat in the stratigraphic sequence of the Süttő 21 site. Arid or semi-arid grassland-Lagurodon arankae, Prolagurus pannonicus, Sorex runtonensis. Fertile lowland steppe-Allocricetus bursae, Cricetus cricetus ssp. Shortgrass-steppe-Spalax sp., Spermophilus cf. primigenius. Forest-steppe-Sicista cf. praeloriger, Ochotona sp. Figure 10 shows that, during the deposition of the lower part of the sequence (0-4 m), the area around the site was mainly cold, dry steppe, and then from about 3.6-4 m upwards, a higher proportion of more forested steppe types with more precipitation appeared. This supports the results of the paleoecological analysis of the entire small mammal fauna and may explain the disappearance of some species of voles and shrews in the upper part of the series.
The large mammal remains indicate a mosaic environment with mainly open, grassysteppe vegetation, which may have been interspersed with patches of forest. Compared to other localities, the strikingly few finds of small carnivores (mustelids) and the complete absence of small-sized large carnivores (canids, i.e., wolf and fox) in the large mammal material is a very strange feature. The absence of the canids, combined with the relatively high number of remains from juvenile herbivorous large mammals (especially cervids), suggests that these phenomena are probably due to the presence of a large predator (i.e., the sabre-toothed Homotherium).

Comparison of the Süttő 21 Site with Sites of a Similar Age in Hungary
It has been known for a long time that significant ecological differences can be detected between the northern and southern parts of the Carpathian Basin, just as it is now, starting from the Pliocene [2]. Exploring these differences is difficult because the periods in which vertebrate faunas are known from both areas are rare. In addition to the relatively rich small and large vertebrate material, the value of the site is also enhanced by the fact that it provides well-documented information on a Pleistocene interval known from only a few sites in Hungary. The fauna of the Süttő 21 stratigraphic sequence in Gerecse Mountains (Northern Hungary) can be compared well with the material from another site of Süttő (Süttő 17) [3], the sites of Somssich Hill 2 and Villány 8 in the Villány Hills (Southern Hungary), Kövesvárad in the Bükk Mountains, and Újlaki-hegy in the Buda Hills (Northern Hungary), which are of a similar age, thus revealing taxonomic and paleoecological differences between different areas of the country [9]. The stratigraphic position of these sites used in the comparison is shown in Figure 5.
When comparing the sites in Northern and Southern Hungary, the first thing that stands out is the taxonomic differences. The first two of the species that play an important role in the vole fauna of the Villány Hills (Allophaiomys pliocaenicus, Terricola arvalidens, Lasiopodomys hintoni, Microtus (M.) nivaloides) are completely absent from the Northern Hungarian sites. Instead of Allophaiomys pliocaenicus, Allohaiomys praehintoni is present everywhere in the northern sites, along with Lasiopodomys hintoni and Microtus (M.) nivaloides.
The taxonomic differences are clearly due to ecological differences between the two areas. While in the Villány Hills, a generally warm, dry climate and open vegetation (probably the most-closed vegetation is karst shrubland) [6] can be reconstructed in the late Early Pleistocene and Early/Middle Pleistocene; the Northern Hungarian areas had a cooler, wetter climate and slightly more forested vegetation (sparse forest, forest-steppe) [3]. The different ranges of the two Allophaiomys species may be explained by the different climates and environments. Allophaiomys pliocaenicus may have been a warm, dry steppe species, while Allophaiomys praehintoni may have been a species of vole living in a cooler, wetter climate in forest-steppe vegetation.

Discussion and Conclusions
The rich vertebrate material of the Süttő 21 site provides a better understanding of the taxonomic and environmental changes around the Early/Middle Pleistocene boundary. A relatively dense sampling every 40 cm in the 6 m-high continuous sequence of the site has allowed both the discovery of taxonomic changes within the sequence and the paleoecological studies that have allowed the reconstruction of environmental changes during the deposition of the sediment.
The taxonomic changes at the presumed Early/Middle Pleistocene boundary may have been caused by environmental changes that can be well reconstructed by paleoecological studies of vertebrate fauna. In general, the vegetation became more forested as it moved up the stratigraphic sequence, as indicated by changes in the proportions of steppe taxa and forest-shrub environment taxa, and by an increase in the proportion of grasslandshrubland and open forest species within the steppe species and the disappearance of dry steppe species. The same changes suggest that the climate became more humid, with higher levels of precipitation towards the top of the stratigraphy, but the complete absence of Crocidura shrew species suggests that this change was not accompanied by warming. This could be interpreted as a change from the dry, cold steppe vegetation at the bottom of the sequence to forest-steppe vegetation developed in also cold climates at the top of the series.
A comparison of the Süttő 21 site and other Hungarian vertebrate sites of a similar age (between 1.1-0.7 Ma) revealed taxonomic and paleoecological differences between the northern and southern parts of the country. In contrast to the warm, dry climate and steppe vegetation typical of the Villány Hills, the northern part of the country (Gerecse, Buda Hills, Bükk) was characterized by a cool, wet climate and more closed (sparse forest) vegetation in the late Early Pleistocene and Early/Middle Pleistocene. For species occurring exclusively in one area, this allowed autecological conclusions to be drawn.
Of the vole species belonging to the Microtus evolutionary lineage, only Lasiopodomys hintoni and Microtus (M.) nivaloides were previously assumed to prefer cooler, wetter climates and semi-enclosed vegetation and Terricola arvalidens to have a clear preference for warm, dry climates and open vegetation [6], while the ecological requirements of the two Allophaiomys species were not clearly understood. However, the current work helps to clarify the ecological preferences of the two Allophaiomys species. The co-occurrence of Allophaiomys pliocaenicus with Terricola arvalidens at the Somssich Hill 2 site and its absence from the Süttő 21 site suggests that this species lived in warm, dry climates and open environments. In contrast, the present work suggests that Allophaiomys praehintoni preferred cool, moist climates and semi-enclosed (forest-steppe) environments. Their different ranges are, therefore, due to ecological differences.
Author Contributions: P.P. and M.G. conceptualised the idea and designed the methodology along with Z.S. All authors collected and analysed the data. The original draft was written by P.P., while the manuscript was reviewed and edited by Z.S., L.M., J.H. and M.G. All authors contributed to the drafts and gave final approval for publication. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available in the article.