The challenge of estimating the age of subterranean lineages: Examples from Brazil

Abstract

The applicability and effectiveness of different kinds of evidence used to estimate the age of lineages - morphological, molecular, phylogenetic, biogeographical, geological - are discussed. Examples from the Brazilian subterranean fauna are presented, using mainly fishes, one of the best studied groups, as a model. Only three taxa including troglobites are object of molecular studies, all in progress. Therefore, molecular clocks cannot be applied yet, and indirect evidence is used. Few phylogenies are available, e.g. for the catfish families Heptapteridae and Trichomycteridae. Theoretically, basal troglobitic clades are older than apical ones, but the possible existence of extinct epigean taxa belonging to such clades hampers the comparison. As well, the limitations of the use degrees of troglomorphism to estimate phylogenetic ages are analyzed with focus on the complexity of the mechanisms underlying morphological differentiation. Paleoclimatic reconstructions based on dating of speleothems from caves in northeastern and southeastern Brazil are available, but limited up to the last 200,000 years, thus useful for relatively recent lineages. Topographic isolation, probable for some fish groups from Central Brazil, is also within the time range of 105 years. Older dated events (in the order of 106 years or more) that may represent vicariant events affecting aquatic lineages with subterranean derivatives are related to the establishment of the modern South American main river basins. In view of the paucity of data useful for estimating the age of Brazilian troglobitic lineages, combined evidence, including morphology, systematics and biogeography, seems to be the best approach at the moment.