There is no consensus about how terrestrial biodiversity was assembled through deep time, and in particular whether it has risen exponentially over the Phanerozoic. Using a
database of 60,859 fossil occurrences, we show that the spatial extent of the worldwide
terrestrial tetrapod fossil record itself expands exponentially through the Phanerozoic.
Changes in spatial sampling explain up to 67% of the change in known fossil species counts
and, because these changes are decoupled from variation in habitable land area that existed
through time, this therefore represents a real and profound sampling bias that cannot be
explained as redundancy. To address this bias, we estimate terrestrial tetrapod diversity for
palaeogeographic regions of approximately equal size. We find that regional-scale diversity
was constrained over timespans of tens to hundreds of millions of years, and similar patterns
are recovered for major subgroups, such as dinosaurs, mammals, and squamates. Although
Cretaceous/Paleogene mass extinction catalysed an abrupt two- to three-fold increase in
regional diversity 66 million years ago, no further increases occurred, and recent levels of
regional diversity do not exceed those of the Paleogene. These results parallel those
recovered in analyses of local community-level richness. Taken together, our findings
strongly contradict past studies that suggested unbounded diversity increases at local and
regional scales over the last 100 million years