Ancient Parasites Revealed: The Riddler Mollusk Fossil and the First Leech in Wisconsin

Overview

Parasites have a long, little-seen history in the fossil record, from ancient marine worms to rare leeches. In this NPR Short Wave episode, scientists Karma Nanglu and Danielle DeCl discuss how fossils reveal host–parasite relationships across hundreds of millions of years. A mysterious mollusk fossil marked with a stylized question mark, nicknamed the Riddler, prompts detective-like analysis that links ancient trace fossils to the behavior of modern parasitic worms. The conversation also covers a rare leech fossil discovery from Wisconsin that pushes back the age of leeches and clarifies early parasite lifestyles and habitats. The episode underscores how existing collections and new imaging tools are expanding our understanding of parasite evolution in deep time.

Introduction: Parasites in Deep Time

In this episode, NPR science hosts and guests explore what parasitism means in biology and why fossil parasites are surprisingly scarce yet informative. The discussion clarifies that parasitism is a specialized form of symbiosis in which a parasite derives nutrients from a host, often leaving distinctive traces in the fossil record. The guests note that soft tissues rarely fossilize, but examples such as insects in amber with nematodes emerging provide crucial clues about ancient parasite–host interactions.

"There is huge potential for discovering new fossils in collections that already exist." - Danielle DeCl

The Riddler Mystery: Markings on Ancient Mollusks

The scientists describe a peculiar fossil with markings that resemble a stylized question mark, earning it the nickname the Riddler. Through iterative examination and elimination of non-parasitic explanations, they align the marks with trace patterns produced by modern spionid worms that bore into shells and extend feeding tubes. The detective analogy captures the method: ruling out suspects and converging on a plausible parasitic behavior that fits the fossil evidence.

"We had this weird marking on them, looked pretty much like a perfect sort of stylized question mark." - Karma Nanglu

Leeches in the Fossil Record: The First Leech

The episode then covers a rare fossil candidate for the earliest leech. Although initial opinions disagreed, new materials from Wisconsin's Wisconsin-Waukesha site provided stronger evidence, including a posterior sucker that marks a defining feature of modern leeches. This discovery pushes the origin of leeches back by roughly 200 million years and informs debates about whether early leeches fed on vertebrates or invertebrates and where they lived.

"The big sucker at the posterior end is a huge hallmark of leeches today." - Danielle DeCl

Implications for Evolution and Habitat

Experts explain that early leeches and other parasites inhabited a range of habitats, with implications for how parasitic lifestyles endure through mass extinctions and diversify to the present. The findings suggest a longer, more complex history for marine and freshwater parasites and invite reevaluation of assumptions about parasite–host interactions in ancient ecosystems.

"Over the course of basically every major mass extinction, this group has continued to be successful to the point that we still are studying this modern group of organisms." - Karma Nanglu

Future Prospects: Reexamining the Fossil Record

Attention turns to the potential of revisiting existing fossil collections with new technologies like CT scanning to uncover hidden organisms or traces. Danielle notes that many fossils sit unused in drawers, awaiting fresh analyses, while Karma emphasizes that the fossil record is incomplete but becoming richer as methods and data advance. The conversation highlights optimism for paleontology and the study of parasites as data and techniques evolve.

"CT scanning was one of the technologies used to get a lot more information." - Danielle DeCl

Conclusion: A Culture of Curiosity and Collaboration

The episode closes with reflections on collaboration, humility, and the ongoing quest to understand life’s ancient parasites and their modern descendants in the oceans today.