Paramonoclimacis sidjachenkoi (Obut & Sobolevskaya) and the evolution of the streptograptid thecal aperture in the Silurian (Graptolithina, Monograptidae)

Chemically isolated specimens of two species of the lower Silurian (Llandovery, mid–upper Aeronian) biform monograptid genus Paramonoclimacis , from Arctic Canada, show tubarium features evolutionarily linking it to the Streptograptus clade. The tubaria, coiled dorsally in the proximal end, show a number of streptograptid thecae with the distinct nozzle of a typical species of Streptograptus . Median thecae retain the nozzle, but are modified first into a wide, proximally directed flange with a shortened hook, while the ventral lobes are reduced. Distally, the thecal apertures form a strong ventral hood and two lateral lobes as an extension of the dorsal wall of the metatheca and not as a secondary construction or addition to a geniculum. The proximal streptograptid thecae of Paramonoclimacis may indicate the evolution of the Streptograptus clade through a Paramonoclimacis ancestor. The cupulae were previously thought to be restricted to the Streptograptus clade but are now also found in some species of Paramonoclimacis . There is a close connection between Paramonoclimacis and the genera Pribylograptus and Pernerograptus in the construction of complexly developed thecae and biform tubaria.