Denis E. B. Bates

Parasitism On Graptoloid Graptolites

A review of evidence for parasitism on graptoloid graptolites is presented. The phenomenon is rare, having been recorded in only a few biserial forms and straight monograptids. Most periderm outgrowths secreted as a response to parasitism are small and blister-like, but in two new specimens remarkable tubular outgrowths occur. These would have had a detrimental, but clearly not fatal, influence on the functioning of these graptolites, particularly with regard to the hydrodynamics and buoyancy of the rhabdosome and the energy expended in secreting additional periderm.

The ultrastructure of a Silurian dendroid from Gotland (Sweden)

Fragments of stipes of? Dendrograptus sp., from the Silurian of Gotland, are described using the SEMand TEM. Young specimens have thin walls, and denticulate thecae; gerontic specimens have greatly thickened walls. The fusellum is formed of fuselli with thin (0.13 μm) fibrils orientated at 40–70° to the surface of the previous fusellus, bounded by a compact granular layer with thicker (0.2 μm) smooth fibrils loosely embedded in it. The cortex is formed of similar granular layers with again smooth (0.3–0.5 μm) fibrils. Differences between this ultrastructure and those described in Dictyonema and other dendroids are discussed.

A new type of colony in Silurian (upper Wenlock) retiolitid graptolite Spinograptus from Poland

The new retiolitid species, Spinograptus tubothecalis, is described from the Colonograptus praedeubeli and C. deubeli biozones from two localities in Poland: a borehole on the East European Platform and the Holy Cross Mountains. This was a recovery phase after the severe Silurian Cyrtograptus lundgreni Event. The new species has a unique, previously undescribed form of finite rhabdosome. Unlike the species Spinograptus reticulolawsoni and S. lawsoni, in which the finite rhabdosomes taper distally, its rhabdosome is parallel-sided with the two distal thecae developed as isolated tubes without genicular processes, with a small appendix between them. The new species also has preserved membranes of the sicula, thecae and ancora sleeve, similar to a few species of Spinograptus from the lower Homerian. Spinograptus tubothecalis, like Spinograptus clathrospinosus and S. spinosus, has paired reticulofusellar genicular processes on the pre-thecal ventral orifices, similar to but shorter than thecal processes. Transverse rods, a rare character in post-Cyrtograptus lundgreni Event retiolitids occur in the new species in rudimentary form.

Kirkigraptus, a new retiolitid graptolite from Poland

The new retiolitid graptolite Kirkigraptus inexpectans gen. et sp. nov., from the Neodiversograptus nilssoni Biozone of the Bartoszyce borehole, Poland is described. It is unique among the retiolitids not having a preserved virgella or ancora. Instead the most proximal structures are two round proxi-lateral lists, joining the two genicular lists of the first thecae, connecting the two sides of the rhabdosome. The lists are interpreted as a possible homologue of the distal edge of the ancora umbrella in typical retiolitids. The size of rhabdosome with large proximal lateral orifices, and the ventral panels of thecae with mid-ventral lists, are similar to those of Plectograptus, whereas the two ancora sleeve panels consisting of spaced horizontal lists only, resemble those of Valentinagraptus. It is possible that the new retiolitid may represent a new pattern of development of the proximal end of the rhabdosome, different from that in all other retiolitids.

Probable orthocone skip and roll marks from the Aberystwyth Grits

Skip and roll marks, probably the impressions of orthocones, are described from the Aberystwyth Grits (Lower Silurian, mid-Wales).

A Lower Ordovician Gastropod from Anglesey

The new gastropod species Matherella? acuticostata is described. It is the only gastropod occurring with a brachiopod fauna in siltstones of Ordovician age (Arenig) near Trefor on Anglesey, northwest Wales.

The simplest retiolitid (Graptolithina) species Plectodinemagraptus gracilis from the Ludlow of Poland

Silurian environmental changes during approximately 15 million years of retiolitid (Graptolithina) evolution resulted in the progressive reduction and possibly disappearance of their rhabdosome. A strong decrease in retiolitid colony size is also observed. This results from an increase in the sicula size and a decrease in the number of thecae during their evolution. Plectodinemagraptusgracilis Kozłowska-Dawidziuk, 1995 from the hemiaversus/aversus and leintwardinensis biozones, lower Ludfordian of the East European Platform of Poland, is the youngest known retiolitid. It has a very small rhabdosome comprising a very simple ancora umbrella with reduced lateral lists, free nema and ventral walls of two parallel rods with prominent loops per theca. The ancora sleeve seems to be extremely reduced. The rhabdosome with two pairs of thecae reaches about 2 mm in length. It is parallel sided, as in some larger Ludlow forms, e.g. Plectograptus (Moberg & Törnquist, 1909) and Semiplectograptus Kozłowska-Dawidziuk, 1995. P. gracilis is a clear example of the latest stage of the reduction in the retiolitid colony and the last known form to appear before their extinction, which coincided with the leintwardinensis event.