Temporal coverage of ammonoid measurement data in the Paleobiology Database

This preprint checks how well the ammonoid shell-measurement data in the Paleobiology Database (PBDB) represent the ammonoid fossil record through time. It links every measured ammonoid specimen in the PBDB to the database's own species-level occurrence record. It then asks, for each geological period from the Devonian to the Cretaceous, what share of the known species has ever been measured. Coverage turns out to be uneven. The late Paleozoic is well measured, while the Jurassic and Cretaceous are not. The pattern holds when every period is sampled equally and when source publications are removed one at a time. It also holds in a species-level model that separates the effect of geological age from the effect of shell form. The paper also shows that uncoiled (heteromorph) ammonoids are almost entirely missing from the measurement record, although they account for only part of the Cretaceous gap. The work is aimed at researchers who use PBDB measurements to study ammonoid body size, conch shape or morphological disparity through time. It closes with four practical recommendations for reporting and testing measurement coverage. The manuscript contains two figures and four tables. All analyses use an openly archived, curated dataset (Chakraborty 2026), so the results can be checked against public data.