The ontogeny of Taihungshania miqueli (Bergeron, 1893) from the Arenig of the Montagne Noire, southern France

The very large trilobite Taihungshania miqueli occurs abundantly in siliceo us nodules in the Ordovician of the Montagne Noire, southern France, but in these, larval individuals are not preserved. At a new locality at Peret, n ear Cabrieres, protaspid and early meraspid growth stages occur in great ab undance, together with a few complete specimens belonging to meraspid degre es 1, 2, 3, 5 and 7, and a single young holaspis. Although this material is largely flattened, and the originally globular protaspides crushed, it is still possible to establish the development of several axial rings before t he transitory pygidium becomes liberated. In the degree 0 meraspis, the cep halon and transitory pygidium are about the same size, and the presence of a partially enrolled degree 3 meraspis indicates that protective coiling wa s possible by this early stage. Palpebral lobes are incipient in degree 2 m eraspides, and fully formed by degree 7, otherwise the morphology of the ce phalon becomes quite adult-like by degree 4. The pygidial spines begin to a ppear in degree 5, at which stage there are about 10 axial rings; there are about 13 in a degree 7 meraspis, and in the adult, when the posterior cont our is semi-elliptical, there are some 22. The smallest hypostomes present belong to a degree 6 meraspis, and these are morphologically similar to tho se of adults. Observations on the later growth stages were made on material from another locality (Caramaou), as well as that from Peret. In the eye t he upper lenses an large and slightly separated, but those towards the base are much diminished in size, this is a function of an unusual growth geome try. At Peret, all stages of development, at least as far as young adults, are r epresented, albeit often in a fragmentary state. As with other asaphines, t he protaspides are assumed to be planktic, and T. miqueli presumably left t he plankton and became benthic between the protaspid and meraspid periods.