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.
Highlights
Despite recent advances in molecular phylogenetics [1,2,3,4], the evolutionary history of Clitellata—earthworms, leeches and their relatives—is still poorly understood
Associated fossil bacteria consist mostly of rod-shaped, approximately 2-mmlong and approximately 0.8-mm-wide bacilli; many bear a characteristic dimple on the surface, and some occur in chains or clusters
Similarities in dimensions, structure and texture indicate that the isolated elements described above represent the various components of the specialized filiform spermatozoa typical of clitellate annelids
Summary
Despite recent advances in molecular phylogenetics [1,2,3,4], the evolutionary history of Clitellata—earthworms, leeches and their relatives—is still poorly understood This is in part because the delicate bodies of clitellates consist almost entirely of soft tissues, and can become fossilized only under exceptional circumstances [5]. Clitellate cocoons are common, though sporadically illustrated, components of plant micro- and mesofossil assemblages obtained via bulk dissolution of clastic sedimentary rocks [6,7,8,9] as old as Middle Triassic [10] This potential source of information about the origin and evolutionary history of clitellates has, received little scientific attention far, because it would appear impossible to determine the systematic affinities of the cocoon producers in any greater detail based on morphology alone. Some morphological features of the spermatozoa are reminiscent of those of extant Branchiobdellida, a peculiar group of leech-like worms whose extant representatives are ectosymbionts on freshwater crayfish
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