It is known that during the latest Pleistocene glaciations (2.59-0.01 Million years ago) the territory of Britain, as well as of Ireland and many other territories of the northern hemisphere, were covered by glacier and were uninhabitable for terrestrial fauna. During glaciations animals and birds either migrated southward or died out. Palaeontological and genetic evidence indicates that the majority of the contemporary fauna of Britain arrived from continental Europe dispersing across a land bridge that existed between Britain and mainland Europe during the short period after ice retreat and before it was submerged by rising sea level (ca. 0.45 Mya).
However, surprisingly, there are few British species that were able to survive the latest Ice Ages, for instance, the endemic Groundwater Shrimp (Niphargus glenniei; see on the photos below) currently known from cave ecosystems of Devon and Cornwall only. Another endemic species of the groundwater shrimps, restricted to Ireland, is Niphargus irlandicus. None of these species is known outside southern England and Ireland correspondingly.
As argued by McNerney et al. (2014), the most recent common ancestor of both species and all other Niphargus species (over 300 species distributed in cave ecosystems across Europe) was isolated approximately 87 Million years ago, i.e. during the late Cretaceous period (100–66 Mya). More importantly, that the two endemics (glenniei and irlandicus) should have been where they are now for at least 19.5 Mya and thus they have survived the entire Pleitocene period and many glaciations in the groundwater. This makes both groundwater shrimps the oldest known species of the British fauna.
This story is based on the paper by McNerney et al. (2014), The ancient Britons: groundwater fauna survived extreme climate change over tens of millions of years across NW Europe. Molecular Ecology, 23: 1153-1166; doi: 10.1111/mec.12664