Abstract
Phylogenomic studies have greatly improved our understanding of the animal tree of life but the relationships of many clades remain ambiguous. Here we show that the rare soft-bodied animal Amiskwia from the Cambrian of Canada and China, which has variously been considered a chaetognath, a nemertine, allied to molluscs, or a problematica, is related to gnathiferans. New specimens from the Burgess Shale (British Columbia, Canada) preserve a complex pharyngeal jaw apparatus composed of a pair of elements with teeth most similar to gnathostomulids. Amiskwia demonstrates that primitive spiralians were large and unsegmented, had a coelom, and were probably active nekto-benthic scavengers or predators. Secondary simplification and miniaturisation events likely occurred in response to shifting ecologies and adaptations to specialised planktonic habitats.
Highlights
Phylogenomic studies have greatly improved our understanding of the animal tree of life but the relationships of many clades remain ambiguous
Thanks to continuous progress in our understanding of animal relationships, new fossil discoveries, and the application of modern imaging and analytical techniques for studying fossils, many problematic taxa have become vital in reconstructing the early evolution of bodyplans and the sequence of acquisition of morphological characters that led to modern phyla[2]
Our results suggest affinities with gnathiferans, a clade containing some of the smallest animals on Earth, and challenge previous views that the earliest spiralians were meiofaunal acoelomates or pseudocoelomates[16,17,18]
Summary
Phylogenomic studies have greatly improved our understanding of the animal tree of life but the relationships of many clades remain ambiguous. We show that the rare soft-bodied animal Amiskwia from the Cambrian of Canada and China, which has variously been considered a chaetognath, a nemertine, allied to molluscs, or a problematica, is related to gnathiferans. Thanks to continuous progress in our understanding of animal relationships, new fossil discoveries, and the application of modern imaging and analytical techniques for studying fossils, many problematic taxa have become vital in reconstructing the early evolution of bodyplans and the sequence of acquisition of morphological characters that led to modern phyla[2]. Amiskwia sagittiformis remains one of the most enigmatic fossils of the Burgess Shale. It was originally described by Walcott in 1911 as a chaetognath[3]. A second species, Amiskwia sinica, has been described succinctly based on a single poorly preserved specimen recovered from the lower Cambrian of China[13]. A possible second specimen of Amiskwia sinica (contra[15], see Fig. 3 of that paper) does not show convincing grasping spines or fin rays
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