THE “CLOVER ANIMAL” – SPEARHEAD OF THE CAMBRIAN REVOLUTION

In many regions of the world, anabaritids dominate the earliest assemblages of skeletal fossils around the Precambrian–Cambrian boundary. Their triradially symmetrical, aragonitic tubes make them easy to recognize and set them apart from other shards of the Cambrian explosion. Discovered in 1919 in the Swedish Caledonides and nick-named klöverdjuret (”the clover animal”), they were largely neglected until the 1960s, when they started to turn up in the Precambrian–Cambrian boundary deposits of the Siberian Platform, where they comprise the first rich assemblages of skeletal fossils. The great morphological variability of the anabaritid tubes has led to some taxonomic oversplitting: 19 genera and more than 70 species have been proposed. Our revision of the type material suggests that the genera Anabarites, Cambrotubulus, Selindeochrea, Aculeochrea, and Mariochrea may be retained. Anabaritids have been compared to tubicolous polychaetes, but most evidence suggests a cnidarian affinity, corroborated by the presence of cnidarian-like embryos together with the oldest anabaritids in Siberia. Some characters are equivocal: Occasional replacement of the tube walls with celestite and barite preserves chevron-shaped growth laminations indicating that the tubes were secreted by a collar organ similar to that of serpulid polychaetes. Internal features of the anabaritid tubes speak against an annelid affinity, however.