Abstract
Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Loricifera, Priapulida) and Nematoida (Nematoda, Nematomorpha), together constituting the monophyletic Cycloneuralia that is the sister group of the Panarthropoda. Kinorhynchs are unique among living cycloneuralians in having a segmented body with repeated cuticular plates, longitudinal muscles, dorsoventral muscles, and ganglia. Molecular clock estimates suggest that kinorhynchs may have diverged in the Ediacaran Period. Remarkably, no kinorhynch fossils have been discovered, in sharp contrast to priapulids and loriciferans that are represented by numerous Cambrian fossils. Here we describe several early Cambrian (~535 million years old) kinorhynch-like fossils, including the new species Eokinorhynchus rarus and two unnamed but related forms. E. rarus has characteristic scalidophoran features, including an introvert with pentaradially arranged hollow scalids. Its trunk bears at least 20 annuli each consisting of numerous small rectangular plates, and is armored with five pairs of large and bilaterally placed sclerites. Its trunk annuli are reminiscent of the epidermis segments of kinorhynchs. A phylogenetic analysis resolves E. rarus as a stem-group kinorhynch. Thus, the fossil record confirms that all three scalidophoran phyla diverged no later than the Cambrian Period.
Highlights
Phylogenetic analyses question the monophyly of cycloneuralians and propose that the nematoids are more closely related to panarthropods than to scalidophorans[10,11,12], others do support a monophyletic cycloneuralian clade[13]
The lack of kinorhynch fossils is a prominent knowledge gap, given that molecular clocks suggest an Ediacaran divergence of the scalidophorans[10] and that Cambrian priapulid-like and loriciferan-like fossils imply the presence of total-group kinorhynchs in Cambrian or older rocks if kinorhynchs are a sister group to loriciferans[3] or priapulids[11]
Several key morphological features of Eokinorhynchus rarus, as reconstructed based on available material (Fig. 5), suggest that this taxon is placed within total-group scalidophorans
Summary
Phylogenetic analyses question the monophyly of cycloneuralians and propose that the nematoids are more closely related to panarthropods than to scalidophorans[10,11,12], others do support a monophyletic cycloneuralian clade[13]. Trunk has at least 20 annuli and each annulus is covered with a circlet of tightly sutured small plates and armored with spinose sclerites. The neck region consists of five circlets of neck scalids, which do not form longitudinal rows (Supplementary Fig. S2).
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