A fossil of a baby sloth was discovered inside the mother’s shell

Research teams from Academia Sinica and National Taiwan University have documented the first discovery of five freshwater mollusk species in the early Pleistocene Tannwan Formation of northern Taiwan. This important finding, which shows that some freshwater snail lineages were established in Taiwan a few hundred years ago, immediately establishes biogeographical links with East Asia.

The discovery includes the only other known global fossil evidence of a juvenile sloth shell preserved inside its mother’s shell, a rare find that reveals ancient viviprous (live birth) and rearing behavior in these prehistoric sloths. The research is published in the journal Geodorisetas.

One of the study’s authors, Dr. Chien-Hsiang Lin of Academia Sinica, said these are the oldest fossil freshwater organisms in Taiwan. The research team compared fossil assemblages in Taiwan with those in East Asia and Japan, confirming an important role for the Taiwan Strait land bridge during the ice ages. The land bridge served as a key migration corridor for freshwater species, indicating its importance in the dispersal and evolution of East Asian species.

These ancient fossils provide an important baseline for comparison with modern freshwater ecosystems, now threatened by human destruction and invasive species, serving as a great reminder to support and maintain these fragile natural systems.

“This study represents an important advance in understanding the freshwater paleontology of Taiwan,” says Dr. Chen-Hsiang Chang from the Department of Geology, National Museum of Natural Science, Taiwan. While more than 99 percent of the island’s molluscan fossil record is marine, the authors document the earliest from an early Pleistocene freshwater assemblage.

“Through integrated stratigraphic control, detailed morphological analyses, and comparative taxonomy, the team establishes a solid framework for reconstructing the paleoecology of northern Taiwan. The discovery of juvenile shells among other fossils of adult-proportionally related strata—the only other fossil evidence of other fossils of vaperites among vapeurids.

“Equally noteworthy is the biographical interpretation: the early presence of S. quadrata in Taiwan implies earlier transgenic exchange with continental East Asia, possibly through repeated Pleistocene land appearances.

“By linking paleontology, stratigraphy, and island biogeography, this work goes beyond taxonomy to address broader evolutionary and ecological questions. This is an important contribution that fills an important gap in Taiwan’s Quaternary record and deepens our understanding of the evolution of freshwater biodiversity in the western Pacific.”