Abstract
Astogeny and Phylogeny of the Last Retiolitids (Graptolithina; Silurian)Some twenty retiolitid species of the latest Wenlockian (Zone of Cyrtograptus lundgreni) and much of the Early Ludlovian (up to the Zone of M. chimaera), i.e., through about five million years have been etched out of the rock. The astogeny of many species is now known in great detail, and the phylogeny can be traced fairly well. The major evolutionary trend consists of the progressive simplification and diminution of the rhabdosome (colony) as follows: from six main supporting rods to four, and from complex structures to such without any fine mesh‐work (reticula) and without thecal apertural processes; and from 10‐25 thecal (zooidal) pairs to only two pairs in the very last species. It would appear that the retiolitids evolved themselves out of history. The observed very profound structural transformations are operative throughout the considered time span as rapid, but gradual and stepwise processes.
Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
