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

Abstract

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.