The generative phase and the first radiation event in the Early Silurian monograptid history

The generative phase in the monograptid history, that is a period between the appearance of the uniserial colony (the Persculptograptus persculptus Biozone) and the first radiation event (the Cystograptus vesiculosus Biozone), shows neither morphological, nor taxonomical variety in the rhabdosomal and thecal structure. It is characterized by the speciation events in the single Atavograptus linage. These events coincide with the recovery phase of the Early Silurian diplograptids and are associated mostly with deep-water environments within the marginal basins in the low-latitude equatorial belt. The monograptid radiation in the C. vesiculosus Biozone is well represented in the Bornholm and South Urals sections (Korenʹ and Bjerreskov, 1997). It resulted in a significant increase in diversity of derived thecal morphologies based on a simple Atavograptus primitivus morphotype. Some important morphological novelties that were typical for almost all early monograptids (such as the position of the theca 1 bud at the metasicula) were also introduced in several akidograptid and dimorphograptid lineages within the Parakidograptus acuminatus and C. vesiculosus Biozones. The vesiculosus radiation led to geographical and ecological expansion of monograptids which became globally recognizable within the epipelagic and marginal basins in the low to moderate palaeolatitudes.