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| Macromyzon siluricus de Carle, Iwama, Wendruff, Babcock & Nanglu, 2025 |
Abstract
Clitellata is a major annelid clade comprising oligochaetes (e.g., earthworms) and hirudineans (e.g., leeches). Due to their scant fossil record, the origins of clitellates, particularly Hirudinea, are poorly known. Here, we describe the first leech body fossil, Macromyzon siluricus, gen. et sp. nov., from the Brandon Bridge Formation (Waukesha Lagerstätte). This fossil, which is preserved in exceptional detail, possesses several hirudinean soft-tissue synapomorphies–including a large sucker at the posterior end and sub-divided segments–and phylogenetic analyses resolve Macromyzon siluricus as a stem leech. Its age, 437.5–436.5 Ma, is consistent with early age estimates for the origin of clitellates, and predates molecular-clock-based estimates of hirudinidan origins by at least 200 million years. These findings suggest that the earliest true leeches were marine and that, contrary to prevailing hypotheses, were unlikely to have fed on vertebrate blood.
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| Macromyzon siluricus gen. et sp. nov. from the Lower Brandon Bridge Formation (Waukesha Lagerstätte), Silurian (Llandovery: Telychian), Waukesha, Wisconsin, USA. (A) Holotype specimen UWGM 7056. (B) Schematic of the external morphology of Macromyzon siluricus based on the holotype. (C) Detail of the anterior region, dorsal view, showing sexannulate segments with annuli numbered; black arrows indicate putative tubercles. (D) Schematic of the anterior region showing tubercles in light grey and sexannulate segments with annuli numbered. (E) Schematic of segmentation pattern for M. siluricus. Green borders indicate extant Hirudinida introduced for comparison: (F) Ventral view of Myxobdella sinanensis (Zoological Collection of Kyoto University, specimen KUZ Z1794); photo by T. Nakano. (G) Schematic of Haementeria lutzi (dorsal view) with inset showing the species’ segmentation pattern. This specimen is deposited in the collections of the Museum of Zoology of the University of São Paulo (MZUSP 0026). Abbreviations: Ca, caudal sucker; S, segment; Tu, tubercles; Tu?, putative tubercles (metameric circular organs). White arrow indicates mid-body torsion, the point of torsion is shown in blue on schematics; breakage in the specimen is indicated in dark grey. |
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| Phylogenetic analyses recover Macromyzon siluricus gen. et sp. nov. as a stem leech. (C) Life reconstruction of Macromyzon siluricus. Illustration by E. K. Chan. |
Phylum: Annelida, Lamarck 1809
Class: Clitellata, Michaelson 1919
Subclass: Hirudinea, Lamarck 1818
Genus: Macromyzon gen. nov.
Macromyzon siluricus sp. nov.
Etymology: Macromyzon (gender: masculine), from Greek, makros, “large” + myzon, “sucker” in reference to the large caudal appendage. The specific epithet refers to the Silurian age of the fossil.
Locality: Waukesha Lime and Stone Company, west quarry, north of State Highway 164, Waukesha, Wisconsin, USA.
Stratigraphic occurrence: Lower part of the Brandon Bridge Formation (Silurian: Llandovery, Telychian; Mikulic & Kluessendorf, 1999).
Diagnosis: Hirudinean having a vermiform-sublanceolate shape, truncated anteriorly, width expanding toward posterior, reaching maximum width slightly anterior of margin. Posterior margin approximately twice the width of anterior margin. Body segmented, with regularly spaced, sexannulate divisions. Posterior terminates in a large caudal sucker. The genus is monospecific; the diagnosis applies to both genus and species.
Danielle de Carle, Rafael Eiji Iwama, Andrew J. Wendruff, Loren E. Babcock and Karma Nanglu. 2025. The First Leech Body Fossil predates estimated hirudinidan origins by 200 million years. PeerJ. 13:e19962. DOI: doi.org/10.7717/peerj.19962 [October 1, 2025]
