Phylogenetic relationships of Malarossia Berezovsky (Eocene, Ukraine) and trends in the size evolution of the carditids (Bivalvia, Carditidae)

Phylogenetic studies of fossil bivalves are crucial for understanding evolution in both extinct and extant species, and the family Carditidae has received limited research attention in this regard. The endemic genus Malarossia comes from the Eocene deposits of Ukraine and the diversity of carditids during this time have been broadly described from North America and Western Europe, although little is known beyond these regions. The aim of this study is to explore the phylogenetic position of the genus Malarossia and discuss trends in shell size among carditids. To accomplish this, a phylogenetic analysis was conducted using 51 species and 153 morphological characters. Our findings indicate that Malarossia represents an early diverging genus within the subfamily Scalaricarditinae, supported by morphological characters related to development and number of radial ribs, placement of pallial line, and umbones, among others. A wide range in shell size can be seen among carditids, with average values ranging from 3 to 100 mm. Notably, the larger sizes were predominantly observed during the Paleocene–Eocene, in the Venericorini. The Miodomeridinae and Scalaricarditinae had the smallest shell sizes, spanning from the Eocene to the present. Paedomorphic heterochronic processes are suggested for the origin of the small size observed in the Scalaricarditinae subfamily.