Sunday, January 10, 2016

[Paleontology • 2015] A Vanished History of Skeletonization in Cambrian Comb Jellies (Ctenophora)

Fig. 3. Idealized three-dimensional models of Cambrian skeletonized ctenophores. (A to C) Side views of Gemmactena actinala gen. et sp. nov, Batofasciculus ramificans, and Thaumactena ensis gen. et sp. nov., respectively. (A′ to C′) Oblique aboral views corresponding to (A) to (C).


Ctenophores are traditionally regarded as “lower” metazoans, sharing with cnidarians a diploblastic grade of organization. Unlike cnidarians, where skeletonization (biomineralization and sclerotization) evolved repeatedly among ecologically important taxa (for example, scleractinians and octocorals), living ctenophores are characteristically soft-bodied animals. We report six sclerotized and armored ctenophores from the early Cambrian period. They have diagnostic ctenophore features (for example, an octamerous symmetry, oral-aboral axis, aboral sense organ, and octaradially arranged ctene rows). Unlike most modern counterparts, however, they lack tentacles, have a sclerotized framework, and have eight pairs of ctene rows. They are resolved as a monophyletic group (Scleroctenophora new class) within the ctenophores. This clade reveals a cryptic history and sheds new light on the early evolution of this basal animal phylum. Skeletonization also occurs in some other Cambrian animal groups whose extant members are exclusively soft-bodied, suggesting the ecological importance of skeletonization in the Cambrian explosion.

Keywords: Cambrian, Chengjiang biota, Ctenophora, skeleton

Fig. 4. Phylogenetic relationship of fossil and extant ctenophores based on a comprehensive cladistic analysis (tables S2 and S3).
The skeletonized ctenophores from the Chengjiang biota form a clade here described as the new class Scleroctenophora. The cladogram is a strict consensus of the three most parsimonious trees. Apomorphies (character number and state above and below nodes, respectively) are mapped on the cladogram. Tree length = 53; consistency index = 0.9231; retention index = 0.9394; rescaled consistency index = 0.8671. Illustrated taxa are marked in bold.

Qiang Ou, Shuhai Xiao, Jian Han, Ge Sun, Fang Zhang, Zhifei Zhang and Degan Shu. 2015. A Vanished History of Skeletonization in Cambrian Comb Jellies.  Science Advances. 1(6); e1500092. DOI: 10.1126/sciadv.1500092