Alliansidid

Alliansidid is a monotypic genus of extinct trilobites that belongs to the order Phacopida within the class Trilobita. The fossil record places its single species, Alliansidid silvestrii, in the Silurian period, approximately 430 million years ago. The genus was first described by the Italian paleontologist Luigi Maria d’Artri in 1934 based on specimens collected from the Valdai Hills of Russia. The generic name derives from the Latin "allian" meaning "joined" and the suffix "-sidid" common to trilobite nomenclature, reflecting the distinctive thoracic segments that appear fused in lateral view.

Morphologically, Alliansidid silvestrii displays a relatively small cephalon with a smooth glabella and fine spines along the border. The thorax consists of eight articulated segments, each bearing a subtle caudal spine that suggests a benthic lifestyle in shallow marine environments. The pygidium is moderately enlarged, with a well-defined axis and spiny pleural furrows. Comparative analysis with contemporaneous phacopids indicates that Alliansidid likely fed on detritus and microalgae, using its sediment-digging legs to search the seafloor.

Geographic distribution of Alliansidid fossils is limited to the Baltica region, with additional finds reported from the Appalachian Basin in North America after reevaluation of late Silurian strata in the 1990s. The genus has been used as a biostratigraphic marker for correlating Silurian deposits across Eurasia and the Americas. Because of its restricted range and distinctive morphology, Alliansidid remains an important datum for reconstructing paleoenvironmental conditions during the Silurian. The primary literature on this genus includes works by d’Artri (1934), Smith (1970), and the revised monograph by Johnson and Lee (2005) which incorporated new field data and reassessment of the original specimens.