Changes in the Early Palaeozoic geography as a possible factor of echinoderm higher taxa formation: Delayed larval development to cross the Iapetus Ocean

Morphology, ontogeny and stratigraphic distribution convincingly show that Pisocrinacea originated from Homocrinacea in the Early Silurian by paedomorphosis. The Baltic basin was the center of origin and spreading of Pisocrinacea, as the most primitive and the earliest members discovered here were most diversified at the generic level. By the mid-Silurian to the beginning of Early Devonian they spread widely to the Urals, Tien Shan, Bohemia, Australia and, in part, to North America. Conversely ancestral Homocrinacea are known only in North America. That is why it may be suggested that the Pisocrinacea originated from the Homocrinacea during transit of the Iapetus Ocean from Laurentia to Baltica at the beginning of the Silurian. By this time Iapetus became considerably narrower, and the larvae of numerous benthic animals could cross it, though over a long period of time. Crossing of this ocean by larvae of one of the homocrinid members led to a delay in skeleton formation and to acceleration of maturity after attachment of larvae in the Baltic basin. As a result a new body plan which characterized the Pisocrinacea appeared, originally within the genus Pisocrinus. One of the Pisocrinus subgenera and its descendant genus Parapisocrinus may have immigrated by reverse currents to North American basins and there achieved high species diversity and abundance, particularly in shallow waters. This suggests that extremely long crossing of water barriers by benthic animal larvae after oceanic currents had changed their course may be an important and a rather common factor in the origin of new taxa.