Temporal coverage of ammonoid measurement data in the Paleobiology Database

To understand how life on Earth has changed over hundreds of millions of years, scientists often look at the shells of ancient sea creatures called ammonites. These spiral-shelled animals, which swam in oceans from the Devonian period through the Cretaceous, left behind a vast record of fossils. Because their shells are abundant and their shapes are relatively simple to describe, researchers use them to track how body sizes and shell forms evolved over deep time. They measure the diameter of the shell, the height of its whorls, and the width of the opening to build a picture of how these creatures adapted, grew, and eventually went extinct. However, the fossil record is never a perfect mirror of the past; it is a patchwork of what happened to be preserved and what scientists have chosen to study. While scientists have long known that some time periods are better represented in the fossil record than others, a newer question has emerged: even when a species is known to exist, has anyone actually measured its shell? If the effort to measure these shells is uneven across time, then any story about how ammonite sizes changed might be telling us more about modern research habits than about ancient biology.

A researcher at the Blue Blocks Micro Research Institute set out to test this idea by examining the Paleobiology Database, a massive online collection that catalogs fossil occurrences and measurements. The goal was to see if the measurements of ammonite shells were spread out evenly across the geological timeline or if they were clustered in specific eras. The researcher compared two sets of data: a list of 64,733 known occurrences of ammonite species from the Devonian to the Cretaceous periods, and a separate list of 20,272 individual specimens that had actually been measured. By lining these up against each other, the study revealed a striking imbalance. The data showed that measurements were heavily concentrated in the Paleozoic era, specifically the Carboniferous and Permian periods, where measured specimens appeared about twice as often as the number of known fossil occurrences would predict. In contrast, the Mesozoic era, particularly the Jurassic and Cretaceous periods, was severely under-measured. The Cretaceous period, which holds nearly 31 percent of all known ammonite occurrences, accounted for only about 16.5 percent of the measured specimens.

This gap was not just a matter of having fewer fossils from later periods; it was a matter of how thoroughly scientists had studied the species that were known. The study found that in the Permian period, roughly 76 percent of known species had at least one measured specimen. By the time of the Cretaceous, that number had plummeted to just 27.5 percent. To ensure this pattern was real and not just an artifact of having more fossils in one era than another, the researcher used a statistical method to draw an equal number of fossil records from each time period. Even when the sample sizes were forced to be identical, the trend remained: the Permian still had high coverage, while the Cretaceous remained low. The analysis also checked whether a single famous book or a few large research papers were skewing the results. It turned out that the high coverage in the Triassic and Carboniferous periods relied heavily on a handful of massive, detailed monographs that listed hundreds of measurements at once. If those specific books were removed from the database, the coverage in those eras would drop significantly. However, the lack of measurements in the Cretaceous was not caused by missing a single big book; the deficit persisted even when every single source publication was removed one by one, proving that the gap is a robust feature of the data, not a fluke of a few influential authors.

One potential explanation for the low numbers in the Cretaceous is the presence of heteromorph ammonites, a group of creatures that did not have the classic spiral shell but instead had strange, uncoiled, or hooked shapes. Because standard measurement techniques are designed for spiral shells, these odd-shaped creatures are notoriously difficult to measure using traditional methods. The study confirmed that heteromorphs are indeed almost entirely missing from the measurement record, with only 2.7 percent of Cretaceous heteromorph species having any measurements at all. However, even when these difficult-to-measure creatures were excluded from the analysis, the coverage for the remaining, standard spiral-shelled Cretaceous species only rose to 34.7 percent. This means that the uncoiled shape of the heteromorphs explains only a small part of the problem. The real issue is that Cretaceous ammonites, regardless of their shape, are simply much less likely to be measured than their Permian ancestors. A statistical model confirmed that, even when accounting for how common a species is and whether it has a strange shell, a Cretaceous species is still nearly eight times less likely to have been measured than a Permian species.

The implications of this finding are significant for anyone trying to understand the evolution of ammonite size or shape. When scientists compare the average size of ammonites in the Permian to those in the Cretaceous using this database, they are not comparing two equal groups. They are comparing a group where three-quarters of the known species have been measured against a group where only about a quarter have been measured. If the unmeasured species in the Cretaceous were systematically different in size or shape from the ones that were measured, the entire story of their evolution could be distorted. The study does not claim that previous scientific conclusions are wrong, but it does insist that researchers must now be aware of this unevenness. Future studies using these measurements need to report exactly how many species were covered in each time period and test whether their results hold up when the data is adjusted for these gaps. The path forward involves filling in the missing pieces, particularly by entering more detailed measurement tables from Cretaceous and Jurassic research into the database, ensuring that the story of ammonite evolution is built on a foundation that is as complete as the fossils themselves.