The following principles were employed for the assessment of paleotemperatures in our project. First, modern up-to-date data on several biotas (at the level of families or even orders, mainly for the Triassic and younger) make it possible to accurately estimate the ranges of habitats of certain taxa at various taxonomic levels (Bevilacqua et al. 2021).
Second, we considered the latitudinal biodiversity gradient, which indicates that biotic diversity is greater in warm-water environments than in cold water (Davydov 2014; Pianka 1966; Willig et al. 2003). An increase in biodiversity in similar facies and bathymetric settings within the different paleolatitudes indicates the warming of the basin waters and vice versa (Figure 5).
Third, we considered the facies, environments, and bathymetry of the biotas. Shallow-water environments realistically reflect the temperatures of surface waters, while transitional deep-water settings have permanent or intermediate temperatures. For the analysis of paleotemperatures in Siberia, we assessed only groups that occur in shallow water conditions at depths of no more than 250–300 m. Accordingly, the shallow water faunas indicate higher temperatures than those of deeper water. This criterion is valid for all benthic organisms, but
nektonic and planktonic faunas (ammonoids, conodonts, radiolarians, and some others). The temperature estimation of the latter required more skills and integrative sedimentological, geochemical, and other types of data.
The bathymetry of biotas is of critical importance in determining paleotemperatures, since at considerable depths (>300 m) the water temperature is very weakly or in no way related to the climate and, accordingly, to surface water temperature. Therefore, for the analysis of paleotemperatures in Siberia, only groups that occur in shallow water conditions at depths of no more than 250–300 m or less were assessed (Figure 3).
The evaluation of paleotemperatures from the taxonomic composition in Siberia was carried out according to several parameters. First, the position of taxa relative to the climatic zones: (1) equatorial, (2) subtropics (3) warm temperate, (4) cool temperate, and (5) polar (Figure 1). That is, if migrants from a warm-water climatic zone (for example, corals among cold-water faunas) are found in the studied assemblages’ characteristic of high latitudes, this gives grounds to associate this stratigraphic level in a given place with warmer, albeit short-term, conditions. Conversely, the appearance of cold-water conditions indices in warm-water taxa assemblages may indicate possible trends toward cooling.
In addition, we also used the following methodological approach. First, the taxa characteristics of warm-water and cold-water habitats were separated. All deep-sea taxa living at depths of more than 250-300 m can be classified as cold water (Hohenegger 2004; Murray 2006; Pinet 2019). Warm-water taxa live at depths of 250-300 m or shallower in tropical and subtropical zones. In the other climatic zones, they occur at much shallower depths (Figure 3). All other taxa of middle and high latitudes (>50-55°N-S) are categorized as cold water. We implement the search query "taxon name" + cold-water for cold-water taxa on the Internet web search and subsequent analysis of the obtained publications from the search results. A selected taxon that occurs in shallow water realistically shows surface water temperatures. Forms that are found only in deep-sea conditions are excluded from the analysis. These forms may appear at any latitude and therefore not directly document the SST (Sea Surface Temperature). The insertion of such data in the database will distort the results.
Since global and regional climate conditions change dynamically over time, multidirectional migrations of taxa occur because of these changes. For example, warm-water taxa can occur in middle and even higher latitudes together with cold-water forms, since the latter usually have a wide range of thermal tolerance and better adapt to warming environments (Dorey et al. 2019). In this case, there is a mutual overlap of taxa of different habitat conditions (warm and cool-cold water). Provisionally, we resolved this issue simply by excluding them from consideration. These rare exotic taxa had little effect on the average temperatures established for certain geographic areas and chronostratigraphic times. Such taxa must be evaluated separately in each specific case, which requires a lot of time and special knowledge.
In this paper, we explain how to determine paleotemperatures employing biota and focus only on the primary trends in surface water temperature changes. We apply this method to study the Late Paleozoic paleoclimate in Siberia. Shallow-water species living in the tropical zone (≤ 300 m depth) have average habitat temperatures ranging from approximately 20-35°C, while species in the subtropics range from 18-25°C. In middle latitudes, temperatures range from 12-20°C, and in high latitudes, they range from 4-10°C, while in polar regions, temperatures range from 0-4°C (see Figures 1 and 2b) (Pinet 2019; Segar and Segar 2018). Under deep-water conditions below 250-300 meters, but up to around 500 meters, habitat temperatures in the tropics fall below ~ 10-12°C, in mid-latitudes, they range from 5-7°C, and in high latitudes above 60° (N - S) temperature varies by about 1-4 degrees even in shallow water (Figures 2b-3) (Miller and Wheeler 2012). All these factors are also influenced by the climatic season, sea-ocean currents, and local and regional settings (such as the presence of large rivers, mountain ranges of glaciers, anomalous salinity of deep-water depressions, underwater volcanism, etc.) ) (Pinet 2019; Segar and Segar 2018). We have not considered these parameters in our project yet.
For more than a century, the concept of "biomes" has existed (Clements 1916), and was originally used to describe ecological communities in continental settings and construct climate maps (Archibold 1995). Biomes represent the second-to-last stage in the organization of life, ranging from a single cell to the biosphere (van der Maarel 2005), and are characterized by ecological and evolutionary traits under specific climatic and physiographic conditions. The biome is a crucial concept in ecology and biogeography (Goldstein and DellaSala 2020; Mucina 2019). Biomes are large-scale ecosystems of biotic assemblages of species associated with specific abiotic environments that interact within and between these assemblages and the physical space in which they function (Faber-Langendoen et al. 2020). The term "biome" in marine settings possessed controversial meaning since it was proposed (Clements and Shelpord 1939; Hedgpeth 1957). Nowadays, marine biomes are classified into hierarchical categories based on the level of organization, taxonomic composition, and scale of distribution, each with specific characteristics, including temperature: ecoregion, ecosystem, ecozone, ecoformation, and others (Costello and Chaudhary 2017; Goldstein and DellaSala 2020; Woodward 2008). Each of these categories is found in specific climatic and temperature conditions and physiography (Costello 2020; Fay and McKinley 2014). By establishing these categories for the sedimentary basins in Siberia, researchers can create reliable paleoclimate maps and identify areas with exotic biomes, the appearance of which can be explained by paleotectonic or other geological reasons.
Since temperature variations (temperature gradient) for the same taxon can be quite large (Dorey et al. 2019; Goldstein and DellaSala 2020; Nati et al. 2021; Williams et al. 1997-2007), the temperature distribution of different taxa can significantly overlap in one not only stratigraphic unit and region but also even in one locality (Figure 6). To solve this problem and establish a temperature close to the existing values, we divided all taxa into several classes according to the degree of reliability and narrowness of variations in their average habitat temperatures (Williams et al. 1997-2007):
1. Reliable - temperature variations do not go beyond ± 2-3 ° С
2. Slightly varying - temperature varies ± 3-5 ° С
3. Wide temperature variations within ± 5-10 °C
4. Extreme temperature variations within ±10-15 °С
The first two gradations can be confidently used in paleotemperature assessments. The third category is also placed on the graphs for the overall framework. The fourth category was excluded from the analysis of paleotemperatures.