Remarkable Finding Extends the Age of Leeches by 200 Million Years

In the collective imagination, leeches evoke discreet parasites, associated with marshes or ancient medicine. However, a recent paleontological discovery reveals a much older and unexpected story. By uncovering a soft-bodied organism dating from the Silurian, researchers have pushed back the origin of these enigmatic annelids by more than 200 million years.

A primitive form frozen in rock for more than 430 million years

Researchers discovered the specimen in the Brandon Bridge Formation at the Waukesha Fossil Site in Wisconsin. This deposit is known for preserving organisms lacking a hard skeleton, a rarity in paleontology. The fossil, named Macromyzon siluricusmeasures a little more than five centimeters and preserves soft tissues in the form of a carbon film, an exceptional method of preservation.

The animal has an elongated, segmented body, and above all a large posterior sucker, characteristic of modern leeches. On the other hand, it does not have any anterior sucker, the one that current species use to pierce the skin of their hosts. This absence suggests a radically different lifestyle, far removed from blood sucking.

According to the study published in October 2025 in the scientific journal PeerJ by an international team led by Danielle de Carle, this fossil constitutes the very first leech body ever identified in the geological archives, with an age estimated between 437 and 436 million years.

© Andrew J Wendruff/Otterbein University and Takafumi Nakano/Kyoto University

The fossil leech compared to a modern leech. Double arrows indicate the large caudal sucker used for attachment, single arrows indicate the body rings.

This leech fossil reveals a way of life very different from its descendants

The anatomy of Macromyzon siluricus challenges the idea that leeches were always specialized parasites. The lack of a perforating apparatus, combined with a tropical marine environment, indicates that these early leeches probably did not feed on the blood of vertebrates.

Researchers favor the hypothesis of an opportunistic diet, based on predation or the absorption of internal fluids in small marine invertebrates. This behavior is consistent with the ecology of the Waukesha site, dominated at the time by arthropods such as trilobites, much more abundant than the rare primitive vertebrates present in these ancient seas.

SciTechDaily points out that the evolution of blood sucking requires complex biological machinery, including anticoagulants and specialized digestive enzymes. There is no indication that these adaptations already existed in this Silurian ancestor, which reinforces the idea of ​​a late transition to parasitism.

A lineage much older and more elusive than expected

Before this discovery, evolutionary models placed the appearance of leeches between 230 and 140 million years ago, based on indirect fossils such as cocoons and molecular analyses. The existence of such an ancient body fossil creates a major discrepancy between these estimates and the reality of the fossil record.

This leech fossil also sheds light on another enigma, that of the extreme rarity of these organisms in the geological archives. Their almost entirely soft bodies make them very vulnerable to decomposition, except in exceptional fossilization conditions like those at Waukesha. Many ancient lineages probably existed without ever leaving visible traces.

By revealing that leeches first evolved as marine hunters before becoming specialized parasites, this discovery profoundly changes the reading of their evolutionary history. Above all, it reminds us that even familiar biological groups can hide origins that are much more ancient and complex than what science thought it had established.