Maotianshan-Shale nemathelminths — Morphology, biology, and the phylogeny of Nemathelminthes

We investigated individual-rich nemathelminth species from the Lower Cambrian Maotianshan-Shale (Chengjiang) biota comprising Chengjiang, Haikou and Shankou faunas near Kunming, southern China, with regard to morphology, life habits and their possible role in the early marine shallow-water ecosystems. Their body shapes can be grouped into four sets: long, worm-shaped forms, sac-like forms and tubiform forms with a conically tapering, stiff trunk, and forms with a body consisting of a grape-shaped anterior part and a similarly shaped trunk with cuticular plates along the long axis of the body. This latter shape matches the loricate trunk of extant larval priapulids and larval and adult loriciferans, both members of the scalidophoran Nemathelminthes. All Cambrian taxa share several features with the Scalidophora, comprising Loricifera, Priapulida and Kinorhyncha, such as the pharynx lined with internally curved spines and the frontal body region, introvert, which is similarly armed with hook-like spines. Living scalidophorans use their introvert for locomotion and the pharynx to swallow prey, which is assumed to be the case for the fossil forms too. Their inwardly curved pharyngeal spines prevent prey from escaping, while the introvert spines, the so-called scalids, may serve to anchor the body to the surface when pulling it forward. The living forms can withdraw their pharynx and introvert deeply into the body, which can be demonstrated only for the pharynx in the Lower Cambrian forms. Details of the arrangement and number of pharynx and introvert spines are considered useful features for a systematic positioning for the Lower Cambrian taxa. The structures themselves might have evolved, however, earlier than in the scalidophoran ground pattern and probably characterised already the stem species of cycloneuralian nemathelminths because of the presence of similar features in both taxa of the Nematoida, the nematomorphs (in the larvae) and the nematodes (circum-oral sensilla). Introverts with rings of six scalids in Nematoida opposite to rings of multiples of four or five in Scalidophora indicate a possible position of the fossil taxa close to the Scalidophora. We argue that the loricate Cambrian taxa (the fourth set) are derivatives of the stem lineage of the Loricifera. This implies that minute size, slight bilateralisation, a specific piercing apparatus developed along the evolutionary path of these animals toward life in the interstitial, characterise the