Oldest fossil evidence of a cloaca in the mammalian lineage

An international research team from the Museum für Naturkunde Berlin, together with colleagues from France and Canada, reports a spectacular discovery in Thuringia that provides new insights into the evolution of early terrestrial vertebrates. An exceptional fossil from the approximately 294-million-year-old Bromacker fossil locality in the UNESCO Global Geopark Thuringia represents the oldest known evidence of a cloaca in the mammalian lineage (Synapsida). The find fills a major gap in the fossil record and offers new insights into the evolution of the reproductive and excretory systems of early terrestrial vertebrates. The results have been published in the journal iScience.

Fossil tail impression provides rare insights into soft tissues

The cloaca is a single opening that serves the digestive, urinary, and reproductive systems. Today, it is found in amphibians, most reptiles and birds, as well as in a few mammals, such as monotremes. Because soft tissues are rarely preserved during fossilization, direct fossil evidence of this important anatomical structure has been virtually absent.

The newly described specimen, recovered from the renowned Bromacker locality, preserves an extraordinary tail impression associated with fossil footprints. Near the base of the tail, researchers identified two rows of elevated scales separated by a distinct vertical slit. These features are interpreted as the lips and opening of a cloaca.

Based on the morphology of the associated Dimetropus footprints and comparisons with skeletal material from the same locality, the trace is attributed to a caseid synapsid—an early, herbivorous relative of mammals that lived during the Permocarboniferous.

This remarkable specimen represents the first fossil evidence of a cloacal opening in Synapsida, the evolutionary lineage that ultimately gave rise to mammals.
 

Why did the cloaca change?

The reasons behind these repeated evolutionary shifts remain uncertain. The researchers discuss several possible explanations, including differences in reproductive anatomy, adaptations to increasingly arid environments, thermoregulation through cloacal evaporation, and changes in body shape such as the reduction of tails and limbs.

The Bromacker specimen was preserved in water-saturated sediment and is associated with swimming traces, suggesting that environmental conditions may have influenced cloacal evolution. The authors hypothesize that a horizontal vent may have offered advantages in drier habitats by facilitating evaporative cooling, although this idea requires further investigation.

A window into soft-tissue evolution

"The fossil demonstrates the unique nature of trace fossils, as they provide insights into soft tissues that are otherwise rarely preserved in the fossil record," says lead author Dr. Lorenzo Marchetti of the Museum für Naturkunde Berlin. Prof. Jörg Fröbisch, co-author of the study and project leader at the MfN, adds: "This discovery also shows that—alongside new finds from ongoing excavations—it is particularly important to re-examine historical specimens using new methods."

"The new finding highlights the exceptional scientific importance of the Bromacker locality, one of the world's most significant late Paleozoic fossil sites", says Dr. Tom Hübner, BROMACKER project partner from the Friedenstein Stiftung Gotha, where the remarkable Bromacker finds are housed.

By documenting the earliest known cloaca in the mammalian lineage, this study provides a unique glimpse into the biology of early synapsids and opens new avenues for investigating the evolution of reproductive and excretory systems in terrestrial vertebrates.

Publication

Marchetti, L., Logghe, A., Rebillard, A., MacDougall, M. J. and Fröbisch, J. 2025. The evolutionary significance of the earliest cloacal opening in Synapsida. iScience. doi: 10.1016/j.isci.2026.116752