Ninety-two samples have been collected in the three cropping out successions described above (Fig 1 ). Although sampling primarily focused on amber-bearing strata, all suitable levels of the sections were carefully sampled (Figs 3 –5 ) in order to obtain a more accurate and continuous biostratigraphic framework. The three logged sections were drawn using the software SedLog [66 ] in combination with Adobe Illustrator (www.adobe.com ).
A total of 15 rock samples from the San Just, 36 from the Cortes de Arenoso and 41 from the Arroyo de la Pascueta sections were prepared for palynological analysis. Palynological residues were processed using acid digestion with HCl and HF at high temperatures [67 , 68 ]. When required, a short oxidation with HNO3 (“nitric wash”) was performed in some residues. The residues were then concentrated by sieving through 500, 250 and 10 μm sieves, mounted in glycerin jelly on strew slides. In order to include rare taxa with potential biostratigraphic value, between 500 and 1000 palynomorphs were identified per sample (S1 Appendix ). Microscopic analysis of the palynological slides was performed with an Olympus BX51 microscope, incorporating a ColorView IIIu camera using a 100X oil immersion objective. Pollen diagrams were constructed by using Tilia/TGView 2.0.2 softwares [69 , 70 ]. All studied slides are provisionally stored in the museum of the Geological Survey of Spain (CN IGME-CSIC, Madrid). A list of the identified taxa along with their botanical affinity and occurrences in each section is given in Table 1 and S1 and S2 Appendices.
Principal Component Analysis (PCA) was performed to evaluate environmental gradients underlying the palynological dataset and to identify groups of taxa representing different types of vegetation. Only the most abundant or ecologically relevant taxa were analysed. PCA calculations were performed using the SPSS and software (version 4.0.2) with FactoMineE and Rioja packages for PCA and Cluster Analysis, respectively [100 , 101 ].
A total of 15 rock samples from the San Just, 36 from the Cortes de Arenoso and 41 from the Arroyo de la Pascueta sections were prepared for palynological analysis. Palynological residues were processed using acid digestion with HCl and HF at high temperatures [67 , 68 ]. When required, a short oxidation with HNO3 (“nitric wash”) was performed in some residues. The residues were then concentrated by sieving through 500, 250 and 10 μm sieves, mounted in glycerin jelly on strew slides. In order to include rare taxa with potential biostratigraphic value, between 500 and 1000 palynomorphs were identified per sample (
Principal Component Analysis (PCA) was performed to evaluate environmental gradients underlying the palynological dataset and to identify groups of taxa representing different types of vegetation. Only the most abundant or ecologically relevant taxa were analysed. PCA calculations were performed using the SPSS and software (version 4.0.2) with FactoMineE and Rioja packages for PCA and Cluster Analysis, respectively [100 , 101 ].
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