title: Fossilized spermatozoa preserved in a 50-Myr-old annelid cocoon from Antarctica.
abstract: The origin and evolution of clitellate annelids--earthworms, leeches and their relatives--is poorly understood, partly because body fossils of these delicate organisms are exceedingly rare. The distinctive egg cases (cocoons) of Clitellata, however, are relatively common in the fossil record, although their potential for phylogenetic studies has remained largely unexplored. Here, we report the remarkable discovery of fossilized spermatozoa preserved within the secreted wall layers of a 50-Myr-old clitellate cocoon from Antarctica, representing the oldest fossil animal sperm yet known. Sperm characters are highly informative for the classification of extant Annelida. The Antarctic fossil spermatozoa have several features that point to affinities with the peculiar, leech-like 'crayfish worms' (Branchiobdellida). We anticipate that systematic surveys of cocoon fossils coupled with advances in non-destructive analytical methods may open a new window into the evolution of minute, soft-bodied life forms that are otherwise only rarely observed in the fossil record.
authors.affiliation: Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden benjamin.bomfleur@nrm.se.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Bomfleur
authors.affiliation: Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden.
authors.forename: Thomas
authors.initials: T
authors.lastname: Mörs
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milano, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Ferraguti
authors.affiliation: División Paleontología de Vertebrados, Museo de La Plata, La Plata, Argentina CONICET: Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina Instituto Antártico Argentino, Balcarce 290, (C1064AAF), Buenos Aires, Argentina.
authors.forename: Marcelo A
authors.initials: MA
authors.lastname: Reguero
authors.affiliation: Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden.
authors.forename: Stephen
authors.initials: S
authors.lastname: McLoughlin
mesh_terms: D000818:Animals; D000845:Annelida / Q000648:ultrastructure*; D000864:Antarctic Regions; D005580:Fossils; D008297:Male; D010802:Phylogeny; D013094:Spermatozoa / Q000648:ultrastructure
keywords: Annelida; Antarctica; Clitellata; fossilization; spermatozoa; taphonomy