Bergmann´s Rule: a case study on Polydolopid marsupials.

CHORNOGUBSKY, L.; ZIMICZ, A.N.; PUJANA, R.R.

Congreso; 4th International Paleontological Congress; 2014

Polydolopidae is a family of Paleogene marsupials, recorded in Southern South America (Patagonia in Argentina, Central Chile) and the Antarctic Peninsula (in Marambio/Seymour Island). Two localities with polydolopids have been dated and are almost synchrounous: Laguna Fría (Ignimbrita Barda Colorada Formation), in the vicinities of Paso del Sapo (Chubut Province, Argentina) and Marambio/Seymour Island (Cucullaea I Allomember, La Meseta Formation; Antarctic Peninsula). The beds with fossil vertebrates at Laguna Fría are no older than 49.51 ± 0.32 my and not younger than 47.89 ± 1.21 my. On the other hand, the Cucullaea I Allomember beds at Marambio/Seymour Island have an age of ca. 49?51 my. The latter locality recorded fossil forests dominated by Podocarpaceae and with the presence of Nothofagaceae, and other families present in temperate forests such as Araucariaceae, Grossulariaceae, Proteaceae, Myrtaceae and Cupressaceae. The fossil floras near Laguna Fría represent more diverse rainforests with Australasian components (i.e. Eucalyptus, Casuarina) and also Podocarpaceae, but Nothofagaceae are absent. Comparatively, the Antarctic flora suggests cooler conditions than the floras from Patagonia. Bergmann´s Rule argue that homeothermous vertebrates from colder regions are larger than the ones presents in warmer lands. Here we analyze the body mass of polydolopids from the localities mentioned above in a phylogenetic framework. A phylogenetic comparative method (Phylogenetic Eigenvector Regression) was applied to detect the phylogenetic signal on the body mass data. A principal coordinate analysis was performed on the double-centered phylogenetic distance matrix in order to extract the significant eigenvectors. These eigenvectors that summarize the phylogenetic relationship amongst species were used as independent variables versus body mass in a multivariate regression. The residual portion of this regression represents the amount of variation independent from the phylogenetic structure. According to the analysis the 44 % of the body mass variation of polydolopids would be explained by phylogenetic inertia. The plot of detransformed residuals shows the same pattern that the single analysis of body mass. This means that the portion of body mass independent from phylogeny shows a pattern consistent with Bergmann`s rule: larger polydolopids were located at higher latitudes and cooler climates (Marambio/Seymour Island) while smaller ones were located at lower latitudes and warmest environments (Laguna Fría).