Acanthochitona avicula

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 epochs (Pliocene, Pleistocene…) every museum uses, shown so you can see which chapter of Earth's history this lineage lived in. This lineage is a young one, so the strip is zoomed in to epochs — the finer subdivisions inside a period. 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 Acanthochitona avicula. Above itBeside it, each dot is one dated fossil find — few enough to count, so they are drawn individually rather than as a graph. 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.

Extinct — but the bar still reaches today. PBDB flags this lineage as extinct, yet its fossil range ends at 0 Ma — the present day. Both cannot be literally true. This is what it looks like when the youngest fossils fall inside the most recent slice of the time scale: the endpoint rounds to “today” rather than to the actual disappearance, which may be far too recent for an axis measured in millions of years to resolve. Read the young end of the bar (and the †) as the limit of the dated record, not as the date it died out.

How it livedPBDB