Arctica islandica

How far back this lineage goes — and how we know. Everything here is measured in Ma, short for “mega-annum”: millions of years ago. The chart reads left to right like a calendar of the Earth, from the deep past on the left to today at the right edgetop to bottom like a core drilled through the Earth, from the deep past at the top down to today at the bottom.

When this lineage existed

How to read this: the coloured strip along the bottomdown the left is the geological calendar — the standard periods (Jurassic, Cretaceous…) every museum uses, shown so you can see which chapter of Earth's history this lineage lived in. The dashed rules marked ✦ are the five great mass extinctions. The solid bar is the fossil range: the span between the oldest and the youngest fossil that palaeontologists have assigned to Arctica islandica. Above itBeside it, the bars count how many dated finds fall in each slice of time; the tallest bar is labelled, and heights use a square-root scale so that thin slices stay visible next to rich ones. Read this as how well each stretch of time is preserved and studied — thick bars mean plenty of the right kind of rock and plenty of collectors, which is related to, but not the same as, how common it actually was. The orange marker is the DNA clock: DNA accumulates mutations at a roughly steady rate, so comparing this species' DNA with its relatives estimates when the lineage split off — independently of any fossil. Where the DNA reaches further back than the oldest fossil, the gap is hatched: the ghost lineage. It means the lineage was already out there, but has left us nothing we have dug up yet.

How it livedPBDB