Structure and Secretion of the Graptolite Prosicula, and its Application for Biostratigraphical and Evolutionary Studies

Abstract

ABSTRACT. Studies on graptolite taxonomy and phylogeny in recent years have placed great emphasis on the proximal development of the rhabdosome, particularly the presence or absence of a virgella and early thecal growth patterns. As the prosicula was the earliest part of the graptolite skeleton to be secreted, it may also reveal fundamental information about evolutionary relationships within the Graptoloidea. The prosiculae from a variety of Ordovician taxa ranging in age from Tremadoc to Caradoc have been examined using a combination of light microscopy, SEM and TEM. Parameters investigated include the overall morphology, transition into the nema, pattern of longitudinal ridges and spiral line. Taxa show a change from early Tremadoc graptoloids which have a low diaphragm, prominent spiral line and lack longitudinal ridges, through late Tremadoc and early Arenig taxa which have longitudinal cortical bandages or spiralled, paired longitudinal ridges, into later Arenig and Llanvirn forms which have simple longitudinal ridges and indistinct spiral line and diaphragm. With additional work at higher stratigraphical levels, graptolite prosiculae may prove to be useful biostratigraphically when more complete material is absent, such as in palynomorph preparations from subsurface studies.

Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite

“…2A-D, G); 2 -tubular ( Fig. 2E); 3 -caudal (Figs 2F, 3D; see also Williams & Clarke 1999 2006) summarized the available information concerning the variety of forms of prosiculae among graptolites and Rhabdopleura. Epigraptus and Rhabdopleura possess a vesicular prosicula that lacks a helical line ( Fig.…”

“…F, Tetragraptus reclinatus? with conical nematophorous prosicula and longitudinal rods; zigzag suture between fusellar half rings along midline of metasicula (GSC 118638, Mid Ordovician I. victoriae victoriae Zone, Western Newfoundland; from Williams & Clarke 1999; Pl 2, fig. 11).…”

“…Emended definition. -The Graptoloida is the total clade descended from the first graptolite to possess a sicula with a caudal apex (nematophorous sicula; Figs 2F, 3D; see also Williams & Clarke 1999).…”

Middle Cambrian pterobranchs and the Question: What is a graptolite?

“…2B). The Graptoloidea possess a prosicula with a spiral thread (Kraft 1926: Schraubenlinie), a primary opening (prosicular aperture) and secondary longitudinal strengthening rods (Kraft 1926;Williams & Clarke 1999). In some dendroids the shape of the prosicula is slightly different due to the attachment site (Kozlowski 1949), but the spiral thread and the primary opening in the prosicula are present.…”

Dawsonia Nicholson: linguliform brachiopods, crustacean tail-pieces and a problematicum rather than graptolite ovarian vesicles

“…This would be consistent with graptolites producing a free-swimming lecithotrophic larva, that later secretes a prosicula (equivalent to the pterobranch prosiculum) in the plankton before maturing (cf. Williams & Clarke 1999) and budding to form a colony. tridens were originally described by Gurley (1896), but have long been considered to be tail-pieces of the crustacean Caryocaris (Ruedemann 1934;Rolfe in Theokritoff 1964).…”