Holoretiolites helenaewitoldi, Kozłowska-Dawidziuk, 2004

Kozłowska-Dawidziuk, Anna, 2004, Evolution of retiolitid graptolites-a synopsis, Acta Palaeontologica Polonica 49 (4), pp. 505-518 : 515-517

Holoretiolites helenaewitoldi sp. nov.

Figs. 4F , 5L, 7B, 8A, B, E–I.

Holotype: ZPAL G.34/2, Bartoszyce borehole, 1579.9 m, EEP, Poland, upper part the Lobograptus progenitor Biozone, Silurian.

Derivation of the name: In honour of my parents Helena and Witold Kozłowski.

Diagnosis.— Holoretiolites with almost completely reduced ancora sleeve lists, a partly reduced ancora umbrella, and mid−ventral lists. Differs from H. atrabecularis in the greater reduction of ancora sleeve lists and of the ancora umbrella, and in possessing no ancora sleeve lists on lateral sides of rhabdosome.

Studied material.—Ten rhabdosomes and 20 fragments. Bartoszyce borehole, depth 1579.9 m.

Description.—Rhabdosomes with two to three pairs of thecae. Ancora umbrella in most specimens reduced to small lists, in a few specimens lists are more or less complete ( Fig. 8), very thin. Maximum length of rhabdosome containing three pairs of thecae is 3.5 mm. Width of proximal part (corona) is 0.6 mm. Length of lips about 0.25 mm. Lips and primary ancora umbrella lists much stronger than the secondary ancora umbrella lists, genicular lists, and reduced mid−ventral lists ( Fig. 8H). In some specimens the genicular lists are partly reduced. Appendix not observed, although some distal list suggests its presence, possibly simplified ( Fig. 8E). Long virgella with preserved prosicular rim, observed in one specimen ( Fig. 8F). The prosicular rim is 1.6 mm from the ancora, suggesting a sicula length of about 2 mm. There are no ancora sleeve lists on the lateral sides of the rhabdosome.

Discussion.— Holoretiolites helenaewitoldi sp. nov. represents the most advanced stage of rhabdosomal reduction in the Gothograptus lineage. The new species is most similar to H. atrabecularis in the reduction of mid−ventral lists. H. atrabecularis has a well developed ancora umbrella and ancora sleeve lists, and a reticulum in the proximal part, whereas H. helenaewitoldi sp. nov. has a strongly reduced ancora umbrella and ancora sleeve lists. The zigzag lists, which may in part, be thecal lists and ancora sleeve lists, are similarly developed in these two species. In some specimens of H. helenaewitoldi sp. nov., the reduction of the ancora umbrella is more advanced ( Fig. 8A, F) and similar to that in Plectograptus macilentus , Semiplectograptus urbaneki , and Plectodinemagraptus gracilis ( Fig. 2 ) from the Plectograptus lineage.

Holoretiolites mancki is regarded as a possible ancestor of H. atrabecularis . The next evolutionary stage is Holoretiolites helenaewitoldi sp. nov., representing the most extreme skeletal reduction. The measurable length of sicula in H. helenaewitoldi convincingly indicates, for the first time, the sicula length in the genus Holoretiolites in general.

Plectodinemagraptus gracilis in the Plectograptus lineage, and Holoretiolites helenaewitoldi sp. nov. of the Gothograptus lineage ( Figs. 2 , 4 , 5) represent the final stages of two kinds of rhabdosomal reduction. In both species the ancora umbrella is similarly simple whereas there are differences in the construction of the rhabdosome lists. In H. helenaewitoldi sp. nov., zigzag lists and lips are well developed in contrast to P. gracilis , which retains only mid−ventral lists, lips and well developed genicular lists ( Figs. 2 , 4 ).

Figures

Fig. 4. Species of Holoretiolites showing successive steps of rhabdosomal changes. A. Largest rhabdosome of H. manckoides Kozłowska−Dawidziuk, 1995. B. H. romani Kozłowska−Dawidziuk, 1995 with reticulum.C. H. mancki with long appendix. D. H. simplex Eisenack, 1951 having short rhabdosome with two pairs of thecae and reduced ancora sleeve. E. H. atrabecularis, Kozłowska−Dawidziuk, 1995 with ancora sleeve preserved only in the proximal part and reduced mid−ventral lists. F, G. H. helenaewitoldi sp. nov. with strongly reduced ancora umbrella, ancora sleeve, and mid−ventral lists; ZPAL G.34/2 (F) and ZPAL G.34/4 (G). Figures modified from: A, B, E, Kozłowska−Dawidziuk (1995); C, Kozłowska−Dawidziuk and Lenz (2001); D, Eisenack (1951); E–G, originals. Not to scale.

Fig. 2. Examples of the most reduced retiolitid rhabdosomes. A. Sokolovograptus polonicus Kozłowska−Dawidziuk, 1995. B. Paraplectograptus eiseli Manck, 1917, ZPAL G.34/12, Bartoszyce borehole, 1690.6 m, C. lundgreni/T. testis Biozone. C. Cometograptus kirki Lenz and Kozłowska−Dawidziuk, 2001. D. Semiplectograptus urbaneki Kozłowska−Dawidziuk, 1995. E. Holoretiolites helenaewitoldi sp. nov., holotype ZPAL G.34/2. F. Plectodinemagraptus gracilis Kozłowska−Dawidziuk, 1995. Figures modified from: A, D, F, Kozłowska−Dawidziuk (1995); C, Lenz and Kozłowska−Dawidziuk (2001); B, E, originals. Not to scale.