Morphological trends and new species of Cyphaspis (Trilobita, Otarioninae) in the Devonian of Morocco, Turkey, Germany and Belgium

1. IntroductionThe systematics and ontogeny of the Otarioninae were subjected to the comprehensive studies of Adrain & Chatterton (1994, 1995, 1996) who underlined the importance of heterochrony to the evolution of the nominal Otarionini. The tribe’s chief genera in sheer species numbers are the Siluro-Devonian Cyphaspis Burmeister, 1843 and Otarion Zenker, 1833 which were regarded as sister taxa by Adrain & Chatterton (1994). These workers provided a differential diagnosis chiefly intended to discriminate the basal Silurian members of these genera for it was stated, named differences may not reflect possible convergences during the Devonian. Indeed when considering Devonian members of Cyphaspis –focal point of the present study– the state of knowledge in the early 1990s was far from satisfactory. The majority of species known at that time came from continental Europe and Morocco, many of which had been established on poorly preserved or otherwise inadequately documented material. Subsequent records including some well-preserved complete specimens were provenant from the Ardenno-Rhenish Mountains (e.g. Basse, 1997, 2002, 2006; Basse & Lemke, 1996; van Viersen, 2006; van Viersen & Prescher, 2007), Morocco (Chatterton et al., 2006; van Viersen & Prescher, 2014), Algeria (Khaldi et al., 2016), North America (Adrain & Kloc, 1997) and Australia (Ebach, 2002), among others. The morphological variety is compelling and along with the descriptions of new species arose the challenges o