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New U-Th Dates from Vindija, Velika pećina (Kličevica) and Mujina pećina and Their Implications for Chronology of the Middle Paleolithic in Croatia

This paper reports new results obtained by Uranium-Thorium (U-Th) dating of animal bones, tooth and flowstone samples from three Croatian Middle Paleolithic sites. Dates were obtained on bones and teeth from Vindija (Hrvatsko zagorje) and flowstones from Velika pećina in Kličevica and Mujina pećina (both in Dalmatia). Obtained results support the previously established chronology of the Middle Paleolithic of Croatia by confirming that the oldest layers of Vindija belong to MIS 6 and that Velika pećina in Kličevica was visited by Neandertals after 40 ka BP.


Introduction
The stratigraphy of Vindija consists of over 9 m of sediment divided into 13 basic stratigraphic units which, according to Malez and Rukavina 11 , covered the period from the onset of the Riss glaciation (MIS6 or earlier) through the Holocene. Pleistocene layers contain finds from both the Middle and Upper Paleolithic 12 . Mousterian artifacts are made on local raw material, mainly quartz, chert and tuff 13,14 . Layer G3 contains a late Mousterian industry with notched and denticulated pieces, sidescrapers, retouched flakes and some Upper Paleolithic types, while the lithic assemblages from earlier layers have not been studied in detail 15,16 . The relatively meager lithic industry from layer G1 suggests continuation of the Mousterian technological tradition (with the lack of Levallois method, as in G3) although mixture of Middle and Upper Paleolithic typological characteristics is also present.
Fossil remains from the site were dated by radiocarbon method 17-25 and U-Th 10 ( Figure 2). In this paper we present new dates from Vindija obtained by U-Th on animal bones and teeth, and discuss the significance of these dates. Samples were selected by G. Rabeder from the material excavated by M. Malez (excavation seasons 1979, 1980 and 1983) and from the sampling conducted by G. Rabeder and M. Paunović (season 1994) ( Table 1).  (accessed 20.11.2020.). Designed by M. Banda. Wild et al. 10 .

Velika pećina in Kličevica
Velika pećina lies in the canyon of the Kličevica brook near Benkovac in northern Dalmatia ( Figure 1). The main channel of the cave is around 30 m long, 5 m wide on average and ca. 6 m high. At its end the channel turns left and forks after a dozen meters. M. Savić 26 collected several lithics from the cave and its surroundings. M. Malez also visited the site, collected several artifacts and did a smallscale excavation in the cave (M. Savić, personal communication). A test excavation was conducted by I. Karavanić and N. Čondić 27,28 in 2006, and systematic excavations were carried out from 2012 until 2015 29,30 . Five trenches were opened, one of which did not yield any finds. Mousterian artifacts with some animal bones and teeth were found in all other trenches. In the trench near the cave entrance several fragments of prehistoric ceramics (Bronze or Iron Age) were found in the upper layers.
Mousterian tools were made from local chert (Z. Perhoč, personal communication). They are small and strongly resemble the Micromousterian. Among the tools diverse sidescrapers are present; and among them microlithic transverse scrapers are noteworthy 31 , strongly suggesting the Charentian type of Adriatic Mousterian. Ani-mal bones and teeth from Middle Paleolithic layers are less common than artifacts. Although some finds (pottery) from later periods were found, there is no single find suggesting Upper Paleolithic affiliation.
Several radiocarbon dates were obtained on samples from the layers of Velika pećina 5,28,29 showing that the time range of the Middle Paleolithic occupations extended from >48 till about 38 ka cal BP. In this paper we present two dates on flowstone from this site, critical for confirmation of the validity of these radiocarbon dates.

Mujina pećina
Mujina pećina is situated in the hills north of Trogir and west of Split in southern Dalmatia ( Figure 1). The cave is about 10 m deep and 8 m wide. Finds were initially collected in 1977 from the surface inside and outside the cave 9 , and the first test excavation took place in 1978 32 . Systematic excavations conducted by I. Karavanić and I. Kamenjarin took place from 1995 until 2003 5,33,34 . All occupation layers contain Mousterian industries. The tools were largely made from locally available cherts. They are generally small and strongly resemble the Micromousterian. In some layers sidescrapers are dominant, suggest-

Materials and Methods
U-Th dating was conducted at the School of Earth Sciences, University of Melbourne (Australia). Isotopic measurements were performed using the multi-collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) on 20 animal bone and tooth (sub)samples from Vindija Cave and 4 flowstone samples from Mujina pećina and Velika pećina in Kličevica (2 for each). For the dating of bones and teeth from Vindija, in some cases two subsamples were analyzed from the same sample: one from the surface and one from the interior of the sample. Furthermore, three subsamples of a bone from layer K and four subsamples of a bone from layer M were dated. U-Th ages were calculated using 230 Th and 234 U decay constants by Cheng et al. 38 . All uncertainties are reported at 2σ level. Final ages are reported relative to 1950 AD.

Results
In Table 1 we present a listing of the bone and tooth samples collected from Vindija. The results of U-Th dating of these specimens are given in Table 2. We present all dates obtained on samples from this site. They show a time range approximately between 170 and 30 ka. Results on bones and teeth from Velika pećina in Kličevica and Mujina pećina are not listed since they gave unrealistic values, much younger in relation to the archaeological sequence and radiocarbon dating. More precisely, only 2 of the 26 obtained results on Mujina pećina samples show unequivocally Middle Paleolithic ages (one from layer E3A of 43.80±1.90 ka and another one from E3C of 51.9±3.0 ka) while others yielded Upper Paleolithic or later (Holocene) ages. This is inconsistent with the fact that all layers of Mujina pećina contain only the Mousterian industry and that the Middle Paleolithic age of these deposits has been confirmed by previous dating results 36,37 . From the same Middle Paleolithic stratigraphic unit (20) of Velika pećina in Kličevica, three Upper Paleolithic dates were obtained by U-Th, while one date is of Holocene age. On the other hand, dating of flowstones from these two sites gave valuable results that are presented in Table 3.  Figure 5). The first sample was taken at the top of the flowstone, while the second one derives from the bottom. Almost the entire flowstone was covered by sediment before this portion of the cave was excavated. During the sampling there was no sediment left in this part of the cave.

Discussion
Due to the disparities in ages from samples in the same layers and even two subsamples of the same specimen, it seems that at least some of the bones and teeth from Vindija exhibit open-system behavior of isotope uptake and loss. This means that some samples are unreliable for age determinations because the ratio of parent and daughter isotopes of the U-series is the result of both radioactive decay and the post-depositional exchange of uranium isotopes with the environment, producing either under-or overestimated ages for the deposition of bones and teeth [39][40][41][42] . Although a full comparison with the results reported in Wild et al. 10 should be performed after recalculating the ages according to up-to-date decay constants, we can make some general remarks. The new results indicate that layer M dates to MIS 6, probably around 160-170 ka BP, an age range that would have been previously reserved for layers J and/or K according to results reported in Wild et al. 10 . Going further, two subsamples from a tooth from layer H have yielded results of 102±6 and 88.7 +13.8/-12 ka BP, which is at least partially comparable to the average age of 88.2±2.3 ka for the sample from the border of layers H and I reported by Wild et al. 10 . However, the tooth used in the new dating was found ca. 40 cm above layer I, suggesting that even the lower part of layer H could broadly date to a period of ca. 90 ka. If ages for layers M and H are accepted, the dating of layers I to L is not reliable both in these results and those of Wild et al. 10 . The bone sample of the mandible from layer L failed, and the tooth from the same mandible produced incompatible ages (ca. 180 and 130 ka). For layer K one sample was dated only once, and one sample produced three incompatible ages (two of them infinite). Inconsistent dates were also obtained for two subsamples of bone from layer J. The reliability of the other date from a tooth from the same layer is also questionable because no other subsample from it was dated. Dates for these layers reported by Wild et al. 10 are much older on average and likewise have inconsistent results. New dates obtained for layer G3 are 33.5+9.0/-8.1 and 38.6+7.6/-6.7 ka. They are in agreement with previously obtained U-Th date from this layer 10 .    If the most recent dates by Deviése et al. 25 are correct, they do suggest that some mixing between layers occurred at Vindija. The presence of mixing between layers, particularly between G1 and Fd/d, has been considered possible by us for many years 16,46 . However, while archaeological evidence for mixing involving relatively small lithic pieces was demonstrated 47 , we considered it less likely that larger pieces were mixed. A specific example is the split-based osseous point (Vi 3437) found in association with a Neandertal mandible in layer G1. When excavated, this specimen exhibited the characteristic G1 sediment in crevices (J. Radovčić, personal communication) which strongly suggested its origin from G1. The red sediment is no longer in this specimen, but the situation is similar to the massive-based (so-called Mladeč) osseous point (Vi 3439) that was also found in layer G1 (Figure 6). But the new dates for G1 are too old for such a characteristic Aurignacian specimen (Vi 3437 and possible also for 3439) which indicates that, despite the other evidence, Vi 3437 was admixed into G1 from an upper level. The specimen likely derived from the definitively Upper Paleolithic level Fd/d. However, there is no evidence that admixture involved any of the significant Neandertal fossils from G1. The only possibility for mixing of Neandertal skeletal remains involves three isolated anterior teeth (Vi 286, 288, and 289) found in layer Fd but described as Neandertallike 48 . However, other Aurignacian-associated anterior teeth exhibit similar Neandertal-like features 1 .
Despite the new G1 radiocarbon dates, it is possible that the Neandertals from both layers (G3 and G1) of Vindija might overlap in time with earliest arrival of modern humans in Southeastern Europe. Specifically, the recent AMS radiocarbon dates from the Bacho Kiro cave in central Bulgaria indicate an age range for the earliest definitive evidence of early modern Europeans between 45.82-43.65 ka cal BP (95.4% probability), probably beginning from 46.94 ka cal BP (95.4% probability) 49 . It is important to note that these ranges are encompassed by the lower portion of the ranges for Vindija G1/G3. Thus, the documented pattern of "transitional" morphology in the Vindija Neandertals 50,51 could still be the result of biological admixture. If the appearance of the Bohunician in Central Europe during ca. 48-46 ka and certainly by 44 ka 52 , is associated with the arrival of modern humans, the Vindija Neandertals could have been contemporaries of those modern human populations also. Furthermore, the recent claim that modern humans may have reached Greece (southern Balkans) by ~210 ka 53 could mean that there are other early incursions of modern humans into this region that have yet to be detected. While this last possibility requires verification, it is certainly not impossible.
As mentioned earlier, U-Th dates from two flowstone samples were obtained from Velika pećina in Kličevica. Dated flowstone (sample ID: VPK-flowstone entrance trench) from the trench near the cave entrance (squares F-G 3-5) (Figure 4) yielded an age of 32.74±0.61 ka BP. This age is in stratigraphic agreement with a radiocarbon result of >47.7 ka uncal BP (OxA-33732) for a sample coming from layer 20 from the same trench 5 , since the flowstone was deposited between layers 17 and 18 (Figure 4). A bone sample from layer 17 resulted in an age of about 3.4 ka cal BP (OxA-32207), while charcoal sample from layer 19 that is below the flowstone produced the age of 4.3 ka cal BP (OxA-33808) 5 . The age of the dated charcoal sample OxA-33808 is not consistent with the U-Th date on the superimposed flowstone. The very recent age is probably the result of recent intrusion into layer 19. In this trench near the cave entrance, it seems that the flowstone represents a dividing line between Mousterian and Bronze Age layers. While stone artifacts are present in layer 18 which is deposited below the flowstone, they were not found in layer 17 that is deposited above the flowstone. Given the fact that between the dated flowstone and layer 20 there is a ca. one-meter-thick layer in which some Mousterian lithic finds were found, we can conclude that these remains of material culture were deposited before 32.74 ka BP while Mousterian artifacts from layer 20 and lower layers are older than 47.7 ka uncal BP.
The second dated flowstone sample from Velika pećina in Kličevica (sample ID: VPK-flowstone 2006 trench) comes from the interface of layers C and D (Figure 3). The age of the flowstone is 39.69±1.89 ka BP. Earlier radiocarbon dates from layers C and D correspond well with the U-Th age of the flowstone. From layer D, an AMS-dated bone of a mid-size ungulate gave a result of about 43 ka cal BP (Beta-228733) 28 . Another animal bone from layer D (cut in two pieces) yielded two results of ca. 40.9-39.6 ka cal BP (Beta-372935) and 37.4-36.3 ka cal BP (Beta-372934). The latter subsample was prepared by ultrafiltration 29 , and its age seems unrealistic if available dates are considered together with the stratigraphic position of layer D. AMS dating of a Cervus elaphus bone from layer C (square H 22) yielded an age of 36.2±0.75 ka uncal BP (OxA-33654), 42.2-39.9 ka cal BP (95.4%) 5 . A Rupicapra bone sample from layer D gave an unexpectedly recent AMS age of ca. 0.5 ka cal BP (OxA-34162). This could be a result of recent intrusion from the top layer or cave surface or result of bioturbations caused by badger burrows that are documented in the neighboring squares.
Obtained dates suggest that Mousterian humans visited the cave for the first time around or before 50 ka BP and were present there until slightly later than 40 ka BP. Such a late survival of Neandertals in the eastern Adriatic is also documented in Bioče rockshelter in Montenegro 54,55 . Therefore, this region joins other examples of post-40 ka Neandertal presence such as southern Iberia 56 and Spy 57 .
Two samples of flowstone from Mujina pećina were also dated. The upper part is dated to approximately 45 ka (the real age falls in between about 50 and 40 ka) which according to radiocarbon and ESR dates corresponds with human occupations of the cave 36,37 . Before this part of the cave (square H8) was excavated, a lower part of the flowstone was covered by layer D1. Unfortunately, the result of dating of this lower part of flowstone is not reliable due to very large uncertainties, which are caused by large detrital Th contamination ( 230 Th/ 232 Th ratio is 4.1).

Conclusion
New results of U-Th dating based on bones and teeth (Vindija) and flowstones (Velika pećina in Kličevica and Mujina pećina) presented in this paper update the chronology of the Middle Paleolithic of Croatia. The application of U-Th dating for these sites is important for two reasons. Firstly, it enabled dating of human occupational episodes whose age is beyond the limits of radiocarbon methods; and secondly, it was used to check the validity (consistency) of previously obtained radiocarbon dates. New U-Th dates confirm that the oldest layers of Vindija were deposited during MIS 6 and, compared to other dates, suggest no significant chronological difference between layers G3 (U-Th and AMS radiocarbon dates) and G1 (AMS radiocarbon dates based on amino acid hydroxyproline extraction). This indicates that Neandertals from both layers might overlap in time with arrival of early modern humans in Southeastern Europe 49 . Furthermore, results obtained on flowstone samples from Velika pećina in Kličevica confirm the possibility that Neandertals visited the site after 40 ka cal BP. This also strengthens the possibility that Neandertals lived in Dalmatia at the time when modern humans appeared in the eastern part of southeastern Europe 49 and on the Apennine Peninsula 37,58 . According to available radiometric dates, the Eastern Adriatic represents a region with very late survival of Neandertals in Europe.
Further chronometric dating of Croatian Paleolithic sites by various methods is needed to obtain a more precise chronology of the Middle/Upper Paleolithic interface, a period crucial for understanding temporal and behavioral relationships between Neandertals and early modern humans in this part of Europe.