Systematic relationships of the blind phacopine trilobite Trimerocephalus, with a new species from Causses-et-Veyran, Montagne Noire

The paper describes a new species of blind trilobite from the lower Fammenian of Concours−le−Haut at Causses−et−Veyran, Montagne Noire (France). Trimerocephalus (Trifoliops) nigritus subgen. et sp. nov. is assigned to a new subgenus together with Tr. (Trif.) trifolius (Osmólska, 1958). This grouping is supported by the results of phylogenetic analysis of thirteen species attributed to the Fammenian genus Trimerocephalus McCoy, 1849; of 16 previously known species attributed to this genus, only 12 were represented by data of quality sufficient to be included in the analysis, using 23 morphological characters. The Frasnian phacopine Acuticryphops acuticeps (Kayser, 1889) is used as the outgroup. The three most parsimonious trees have a length of 51 steps and a consistency index of 0.82. The new subgenus Trifoliops forms a clade together with Trimerocephalus? steinachensis (Richter and Richter, 1926), supported by an exclusive synapomorphy: widening of the cephalic antero−lateral border. Tr.? steinachensis seems to be more closely related to Tr. (Trif.) trifolius (sharing two synapomorphies) and may represent a more derived taxon (possibly deserving a separate subgeneric status). The remanig Trimerocephalus species are not formally assigned to subgeneric taxa, pending further studies (their relationships are shown in cladograms). The results do not confirm the classification suggested by Chlupač (1966) for Trimerocephalus.