Some aspects of evolution in the Lonsdaleia (Actinocyathus) crassiconus species-group

The Actinocyathus crassiconus species-group represents a separate trend of evolution within the rugose subgenus Lonsdaleia (Actinocyathus) (d'Orbigny, 1849). It is distinguished by consistently developed minor septa and by regular axial structures. In the Moscow Basin, this species-group comprises seven species, A. crassiconus (McCoy, 1849), A. lativesiculosus (Dobrolyubova, 1958), A. sarytschevae (Dobrolyubova, 1958), A. subtilis (Dobrolyubova, 1958), A. gorskyi (Dobrolyubova, 1958), Actinocyathus sp. A, and Actinocyathus sp. B. It ranges from the Mikhailov horizon (Brigantian) to the Protva horizon (lower upper Serpukhovian), and is especially characteristic of the Tarusa horizon (lowermost Serpukhovian) in the north-western part of the basin. Evolution in the group took place at the beginning of Mikhailov time and in Tarusa time and showed three trends: (i) increase in corallite size and number of septa; (ii) increase in number of septa and tabularia diameter; and (iii) a wide range of variability in septal number without important changes in tabularia diameter. Lonsdaleia ornata Dobrolyubova, 1958, L. heckeri Dobrolyubova, 1958, and L. longiseptata crassicolumellata Dobrolyubova, 1958 are synonymised with Actinocyathus subtilis, and Lonsdaleia subcrassiconus subcrassiconus Dobrolyubova, 1958 is with Actinocyathus gorskyi.