The ancient Britons: groundwater fauna survived extreme climate change over tens of millions of years across NW Europe

Global climate changes during the Cenozoic (65.5–0 Ma) caused major biological range
shifts and extinctions. In northern Europe, for example, a pattern of few endemics and the
dominance of wide-ranging species is thought to have been determined by the Pleistocene
(2.59–0.01 Ma) glaciations. This study, in contrast, reveals an ancient subsurface
fauna endemic to Britain and Ireland. Using a Bayesian phylogenetic approach, we found
that two species of stygobitic invertebrates (genus Niphargus) have not only survived the
entire Pleistocene in refugia but have persisted for at least 19.5 million years. Other
Niphargus species form distinct cryptic taxa that diverged from their nearest continental
relative between 5.6 and 1.0 Ma. The study also reveals an unusual biogeographical pattern
in the Niphargus genus. It originated in north-west Europe approximately 87 Ma and
underwent a gradual range expansion. Phylogenetic diversity and species age are highest in
north-west Europe, suggesting resilience to extreme climate change and strongly contrasting
the patterns seen in surface fauna. However, species diversity is highest in south-east Europe,
indicating that once the genus spread to these areas (approximately 25 Ma), geomorphological
and climatic conditions enabled much higher diversification. Our study highlights
that groundwater ecosystems provide an important contribution to biodiversity and offers
insight into the interactions between biological and climatic processes.