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- Title
A new stem craniate from the Ordovician of Morocco and the search for the sister group of the craniata.
- Authors
CRIPPS, A. P.
- Abstract
A new mitrate from the Middle Ordovician (Llandeilo) or Morocco, Chauvelia discoidalis gen. et sp. nov., is reconstructed and described. Chauuelia discoidah is a stem craniate, as indicated by the presence of a slit-like lateral line and by dorsal sensory branches (n4 and n5) of the trigeminal nerve. Within the subphylum Craniata, C. discoidalis is more crownward than Chinianocarpos thorali and Aspidocarpus bohemicus, but less crownward than Mitrocystites mitra and the plesion of Mitrocystella incipiens. The soft anatomy of C. discoidalis is reconstructed based upon skeletal evidence and by comparison with other stem craniates, with tunicates and with hemichordates. Features of the nervous system (reconstructed by observation of internal moulds of the skeleton) include a large bipartite brain, a direct nervous connection from the deuterencephalon to the lateral-line ganglion on the right, a similar direct nervous connection from the deuterencephalon to the left auditory ganglion and a nerve passing from the left auditory ganglion to the left atrium. The observation that the left acoustic ganglion and the lateralis ganglion had direct connections with regions in the brain which, on recent analogies, would have carried the acoustico-lateralis nuclei, confirms that the ganglia have been correctly identified. Locomotion is deduced to have been similar to that of the stem craniate C. thorali, in which the hearing surfaces of the tail are confined to the dorsal surfaces of the terminal ossicles. Chauvelia discoidolis could have pulled itself rearwards through the substrate by pushing the hind tail into the mud and exerting a forward force with the rear end of the tail. The hydrodynamic shape of the head and the laterally flattened tail suggest that C. discoidalis could also swim. The fore tail is inserted further forwards into the head than in other mitrates so that, in life, the fore tail would not have been visible in dorsal aspect. The interrelations of the chordate subphyla are considered and, on the basis of features exhibited by living and fossil forms (mitrates), the Tunicata and Craniata ( sensu Janvier, 1981) are concluded to be sister taxa.
- Publication
Zoological Journal of the Linnean Society, 1990, Vol 100, Issue 1, p27
- ISSN
0024-4082
- Publication type
Article
- DOI
10.1111/j.1096-3642.1990.tb01860.x