Kirkigraptus, Kozłowska & Bates, 2008

Kozłowska, Anna & Bates, Denis E. B., 2008, Kirkigraptus, a new retiolitid graptolite from Poland, Acta Palaeontologica Polonica 53 (1), pp. 105-112 : 108-110

Genus Kirkigraptus nov.

Type species: Kirkigraptus inexpectans sp. nov., monotypic.

Derivation of the name: After Nancy Kirk, the renowned palaeontologist, who discovered the actual structure of the retiolitids.

Species included.— Kirkigraptus inexpectans gen. et sp. nov.

Diagnosis.—Proximal end without ancora; two proxi−lateral lists join the two genicular lists of the first thecae, connecting the two sides of the rhabdosome. Thecal framework formed only of genicular lists, lateral apertural rods, thecal lips and mid−ventral lists. The ventral panels of lists (apertural and pleural lists) are straight. Ancora sleeve built of pleural lists and horizontal lists between the two rows of thecae. No transverse rods are present, their positions being marked by sockets. Large lateral proximal orifices are bordered by proxi−lateral lists and the first horizontal lists of the ancora sleeve.

Discussion.—The most striking feature of Kirkigraptus is that it is unique among the retiolitids in having no conventionally preserved ancora. The two lists, termed here the proxi−lateral lists, which unite the two ventral panels of lists, may in part represent some of the lists of a conventional ancora umbrella. Most probably the proxi−lateral lists are homologues of the distal edge of the ancora umbrella of typical retiolitids. There appear to be some seams on them, but with the limited material available, it is not possible to ascertain either their length or nature (insertion or enwrapping). The virgella and nema are also not preserved. Figure 2 shows the relationship of the two observed lists to a postulated ancora umbrella.

There are no young stages of the colony growth with the proximal end in the new material. Thus there is a possibility that the ancora umbrella had been developed in early stage of growth, but not thickened by bandages.

By analogy with the situation in Plectograptus , proximal to the th 11 and th 12 genicular lists, would be the two ventral orifices, each flanked by pleural list portions of the proxi−lateral lists. The central portions of these lists would be part of the ancora umbrella rim. But there are no broken lists or other traces found indicating the presence of an ancora umbrella connected with the proxi−lateral lists.

The size of rhabdosome of the new genus, and the arrangement of ventral panels of lists (thecal and ancora sleeve) resemble those of Plectograptus Moberg and Törnquist, 1909 . However, whereas Plectograptus has principal lateral ancora sleeve lists forming an alternating, zig−zag arrangement, the new genus has only simple sub−horizontal lists connecting its two sides. The new genus Kirkigraptus is also similar to Valentinagraptus simplex Piras, 2006 , a new form from the Neodiversograptus nilssoni and Lobograptus progenitor biozones of the Barrandian area of central Bohemia, Czech Republic. The similarities are in the size of rhabdosome, the horizontal arrangement of ancora sleeve lists, and the ventral rows of the thecal lists. However, without better study of Barrandian material (i.e., isolated in 3D) it is difficult to be sure of which portions of the longitudinal lists are the pleural and which the lateral apertural. It appears that Valentinagraptus Piras, 2006 has a normal ancora and well developed nema ( Fig. 1 ). The difference is in lack of mid−ventral lists in the ventral walls of Valentinagraptus , whereas the lists are well developed in Kirkigraptus . There is also a difference in shape of the longitudinal (pleural and lateral apertural) lists: they are straight in Kirkigraptus gen. nov., whereas in the Barrandian form they look distinctly concave, giving a “scalloped” appearance to the lateral profile of the graptolite. The Ludlow age as well as the strong similarities of the new form with Valentinagraptus Piras, 2006 suggests a close relationship between the two forms.

The new form shows some similarities to the form described as Sokolovograptus (?) ( Maletz et al. 1998). The form is associated with Plectograptus macilentus and Spinograptus clathrospinosus , suggesting the lower Ludlow age ( Maletz et al. 1998: table 1). There is only one flattened specimen to compare ( Maletz et al. 1998: pl. 1A). The form has similar horizontal ancora sleeve lists starting at about the level of theca 12. This is much lower than in the new form. Together with the well developed nema, Sokolovograptus (?) sp. is most similar to the Valentinagraptus . But it is difficult to recognize the details of the proximal end, as well as the presence of the mid−ventral lists. For the constructive comparison we need to study an isolated specimen under SEM.

There is some similarity of Kirkigraptus to Sokolovograptus Obut and Zaslavskaya, 1983 , especially to the most reduced form, Sokolovograptus polonicus ( Fig. 1B ). The ancora umbrella in S. polonicus is simple but well developed, built of strong lists. The distal edge of the ancora umbrella is rounded, similar to the proxi−lateral lists of the Kirkigraptus .

Stratigraphic and geographic range.—Bartoszyce borehole depths 1598.0 m and 1599.6 m, EEP, Poland; Neodiversograptus nilssoni Biozone, Ludlow.

Figures

Fig. 2. Drawings showing comparison of A: reconstruction of Kirkigraptus, with a postulated ancora umbrella and virgella, with B: Plectograptus mobergi Bates, Kozłowska, Maletz, Kirk, and Lenz, 2006.

Fig. 1. Drawings of representatives of retiolitids with strongly reduced ancora umbrella and ancora sleeve. A. Cometograptus kirki Lenz and Kozłowska− Dawidziuk, 2001. B. Reconstruction of Sokolovograptus polonicus Kozłowska−Dawidziuk, 1995. C. Reconstruction of Plectodinemagraptus gracilis Kozłowska−Dawidziuk, 1995. D. Plectograptus mobergi Bates, Kozłowska, Maletz, Kirk, and Lenz, 2005. E. Reconstruction of Valentinagraptus simplex Piras, 2006 after Piras 2006, modified. F. Reconstruction of Kirkigraptus inexpectans gen. et sp. nov.