The false paradox of phylogenetic signal: repeated origins of fossoriality in Xenarthra

Phylogenetic signal is often read as evolutionary conservatism, but the same pattern can arise from retention of the ancestral state and repeated origins of the derived state. We tested this in Xenarthra using phylogenetic comparative analyses, ancestral state reconstructions, climatic associations, and tests of correlated evolution with diet, across 24 extant and extinct taxa and 2002 posterior trees. Fossoriality showed strong marginal phylogenetic structure (D = −0.85), yet ancestral state reconstructions indicated that this pattern reflects retention of the ancestral non-fossorial state rather than conservation of fossoriality. Fossoriality evolved repeatedly within Mylodontoidea, consistent with parallel evolution associated with shared morphological predispositions and similar ecological contexts. Precipitation regimes were associated with fossoriality, but the association reversed after excluding the lowest values, rejecting a simple climatic preference and pointing instead to precipitation as a viability filter rather than a driver. We also found support for correlated evolution between fossoriality and animal-based diets, although the direction of this relationship remains unresolved. Our results show that strong phylogenetic signal can mask repeated origins of a derived state, and that specialized lifestyles are better explained by interactions among phylogenetic history, environmental filtering, and foraging opportunity than by any single driver.