Evolutionary developments in the silurian retiolitidae (Graptolites)
Silurian retiolitid graptolites first appeared during the early part of the Aeronian Stage, and underwent considerable diversification in late Aeronian - earliest Telychian, during which time interval more than thirty species were present globally. A lesser, but significant species peak was again reached in the early Homerian (the lundgreni Biozone) prior to the major lundgreni extinction event, as a result of which only one species survived. The last retiolitid became extinct during the leintwardinensis Biozone (mid Ludlow). The two most striking evolutionary changes, important at the subfamily level, involved the change from outward-facing list seams on the ancora sleeve and smooth or linear striations on the list surfaces (seen in most older forms), to inward-facing seams and lists with a pustulose surface (seen in lundgreni Biozone and all younger forms as well as in the older Sokolovograptus). Other generalized trends up the stratigraphic column include: loss of preservation of the sicula; more than a twofold lengthening of the sicula; shallowing and decrease in complexity of the ancora umbrella; reduction of the ancora sleeve; change in thecal profile from early orthograptid to later climacograptid; change from a nema attached to the lists throughout, to a nema either attached only distally, often to an appendix, or being a completely "free" nema; development of an appendix in younger forms, and an internal or external nematularium only in some "open-ended" forms; development of the upper apertural list in the early Homerian with complex apertural processes in some forms; marked decrease in size from rhabdosome lengths greater than 100 mm in early forms to two to three millimeters in youngest forms, as well as a tendency towards the simplification of the rhabodosome as expressed in the reduction of thecal and ancora sleeve lists, and accompanied by a marked increase in the length of the sicula.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
Policy: Open Access