The type species of Cyrtosymbole and the oldest (Famennian) cyrtosymboline trilobites

摘要

The hitherto poorly known type species of Cyrtosymbole, i.e., Dechenella escoti, is redescribed on the basis of an abundant new material recovered from the early Famennian Lower Griotte Limestone Formation at La Touriere, near Cabrieres, southern France. It includes sclerites (hypostome, librigenae, thoracic segments and external surface of the pygidium) that previously were either unknown, or represented by poorly preserved and incomplete specimens, together with a full suite of post-protaspid growth stages. The latter has revealed that certain characters, in particular the preglabellar region and postocular facial sutures, show marked changes between the early and late holaspid stages. In the past, some species have been based on immature specimens; for example the lectotype of C. escoti is an early holaspis, and the characters that it displays have been regarded as diagnostic of the genus. For confident specific assignments in cyrtosymboline trilobites it is important, therefore, to have to hand sufficient material, including late holaspids. A revised diagnosis of Cyrtosymbole is given, and only those species that share diagnostic adult characters with C. escoti are assigned to it. Insights into early growth development exemplified by C. escoti and allies corroborate the attribution of the Cyrtosymbolinae to the Proetidae.