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Tomopterna cryptotis | Cryptic Sand Frog | Photo by Alberto Sanchez-Vialas

Those trying to understand global amphibian declines in the context of the fossil record are probably out of luck. It’s not that the fossils aren’t there; rather, it’s that the fossils couldn’t be there. That’s the conclusion Krone et al. (2023) draw from a novel analysis of fossilization potential that includes nearly all modern tetrapods. To reach this rather jarring conclusion, the authors measured the area of overlap between the geographic ranges of modern tetrapods and areas of current sediment deposition. If no sediments are being deposited somewhere, no sedimentary rocks from our time period will exist there, which means that the animals living there won't have the opportunity to leave behind fossils. Among major tetrapod groups, amphibians have, by far, the poorest preservation potential, with only 40% of amphibian species having more than 1 km^2 of their geographic range in a sedimentary basin. Amphibians with a large amount of sedimentary basin overlap are even rarer. Additionally, more phylogenetically unique lineages of amphibians tend to be excluded from the fossil record. The authors link all of this to the particular affinity of amphibians for mountainous environments, which are areas of net erosion, rather than sedimentation. If this regime of under-preservation has affected amphibians throughout deep time, fluctuations in their diversity may be in principle impossible to recover. For instance, they show that, were the 40% of currently endangered amphibian species to go extinct, the fossil record would record, at best, a 13.6% drop in amphibian diversity, and more realistically, a drop of less than 5%. While these results suggest that the vast majority of amphibian history has gone unfossilized, their model can help point amphibian paleontologists away from hopeless dead-ends and remind us how precious and ephemeral modern amphibian species are.

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Current number of amphibian species in our database

As of (Feb 29, 2024)

8,729

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Total Amphibian Species by Order

222 Caecilians 816 Salamanders 7,691 Frogs