THE PLIO-HOLOCENE LARGE MAMMALS OF THE WESTERN EURASIA: MACROECOLOGICAL AND EVOLUTIONARY ANALYSES OF THE FAUNAS
Carotenuto, Francesco (2009) THE PLIO-HOLOCENE LARGE MAMMALS OF THE WESTERN EURASIA: MACROECOLOGICAL AND EVOLUTIONARY ANALYSES OF THE FAUNAS. [Tesi di dottorato] (Inedito)
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The climate during the Plio-Pleistocene was deeply influenced by the so called Milankovitch cycles (Zachos et al.,2001). These are combinations of different astronomical phenomena involving the variation of the Earth’s orbital eccentricity, the axis obliquity and precession. The first phenomenon was estimated to occur every 400-100 Ka, whereas the others occur over a time period of 41 and 23-19 Ka, respectively. These astronomical variations influenced the Earth-Sun distance and the angular incidence of solar rays with a net effect on the global climate change (Zachos et al., 2001). The Early Pliocene was characterized by a decrease of the global temperature, a trend that began during the Late Miocene. The measure of the mean 16O/18O values indicates a trend of warmer climate until 3.2 Mya, the latter that represents the onset of a new temperature cycle. During these cycles the ice sheet expanded and contracted according to the variation of the mean global temperature. Indeed, in the Middle Pliocene, the ice sheets began to cover the Northern Hemisphere, an event indicated as NHG (North Hemisphere Glaciation) (Shackleton et al., 1998; Maslin et al., 1998; Zachos et al., 2001). At ~2.5 Mya the oscillations of the temperature became quite a regular pattern determining the alternation between warmer (Interglacials) and cooler (Glacials) time periods, that were to characterize the Pleistocene. Some 1 May (period coincident with the Jaramillo Event of Earth’s Magnetic Field inversion) the time interval of a complete climatic cycle changed its duration from the 41-23-19 Kya., to a new longer cycle of 100 Kya, probably determined Earth’s orbital eccentricity variation. With the increase of the duration of the cycles the mean global temperatures reached more extreme values and there were stronger climate ranges between warm and cold periods. Moreover, the measures of the oxygen isotopes ratios confirm a net decrement of mean temperature values recorded from the Late Pliocene to the Recent (Zachos et al., 2001). The strong oscillations of the temperatures during the Plio-Pleistocene deeply influenced the faunas of the whole world. These environmental changes determined local or mass extinctions in some cases or migration events in other cases (Lister, 2004). In particular, during the Glacials, the species adapted to warmer climates migrated southward, while, during the Interglacials, the species adapted to low temperatures migrated northward (Vrba, 1995b). There are many demonstrations of the influence of the climate changes on the evolution of mammals faunas, also at the community level (Vrba, 1995a,b; Alroy et al., 2000; Fortelius et al., 2002, 2006; Barnosky et al., 2003; Bobe and Behrensmeyer, 2004; Rodríguez et al., 2004; Raia et al., 2005; Barnosky and Kraatz, 2007; Meloro et al, 2008). Starting from these considerations on the evolution of mammal faunas, the aims of this doctoral thesis were focused on the detection of the paleocommunities of large mammals that lived since the Late Pliocene to the Early Holocene in the Western Eurasia and on their macroecological investigations in the light of climatic change. After collecting a great deal of data (811 Local Faunal Assemblages, LFA, and 220 large mammal species) the paleocommunities (here called EA PCOM) were detected using new statistical methods that avoid any subjective criteria in selecting the LFAs. Moreover, considering two recent articles (Alroy, 2000; Fortelius et al., 2006) a statistical-based time ordination of the collected fossil localities was obtained. As for their well defined temporal and geographical resolution, the EA PCOMs were discussed from the point of view of the distribution of the included LFAs. These analyses showed the presence of both spatial and temporal patterns, useful for inferring dispersal events of mammals in response to the environmental changes. From this point of view, EA PCOMs provide a paleontological framework for evolutionary and ecological investigations. All the collected LFAs were then used to perform statistical analyses to detect macroecological patterns that usually characterize the living mammal assemblages. At first, there were the reconstruction of the body masses and of the duration of both species and genera of the considered taxa. Then, the fossil mammal faunas were investigated to draw the occupancy and the range size trajectories over taxa life span, both at the species level and at the genera level. To this aim the occupancies and the range sizes were computed in different moment of taxon life time. Further, the data provided by occupancy and range size values were used to draw general models that decribe the most frequent coarses in both species and genera time life span. Some other statistics were computed over occupancy, range size and body size and, then, used to infer on their possible influences on the taxon duration. In addition, the data of the species occurrences were used to build the species-time, the species-area and the species-time-area relationship (STAR), that are all patterns typically recognized in living species. Moreover, the latter model (the STAR) was investigated for the first time in fossil species. The interdisciplinary approach of this doctoral thesis also involved the use of modern phylogenetic techniques to seek any relationships in all the measured traits between closely related taxa. To this aim, the phylogenetic trees of all the considered species and genera were reconstructed using data provided by genetic, morphologic and evolutionary studies. A second step was to combine the phylogenetic trees and all the other computed measures to infer about the models of evolution followed by the traits characterizing the large fossil mammals. All the computed results were used to draw possible evolutionary scenarios concerning the Plio-Pleistocene mammal faunas.
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