Dactyloidites peniculus D’Alessandro and Bromley, 1986

Uchman, Alfred & Pervesler, Peter, 2007, Palaeobiological and palaeonvironmental significance of the Pliocene trace fossil Dactyloidites peniculus, Acta Palaeontologica Polonica 52 (4), pp. 799-808 : 800-804

Dactyloidites peniculus D’Alessandro and Bromley, 1986

Figs. 4–6.

*1986 Dactyloidites peniculus isp. nov.; D’Alessandro and Bromley 1986: 79, pl. 12: 3; pl. 13: 3.

Amended diagnosis.—Simple, numerous and crowded cylindrical elements radiating laterally from a central shaft. They form a radial pile arranged in a part or full circle brush−like structure. The radial elements consist of tubes that can overlap and form spreite−like structures. The tubes can be lined and irregularly coated with pellets.

Material.— 7 specimens, 5 thin sections, and numerous field observations documented by 88 photographs.

Description.—A radial structure, which appears as an oblate ellipsoid of revolution in outline. It is 82 to 270 mm wide (mean value 115.6 mm) and 42 to 120 mm high (mean value 77.3 mm). The radiating elements are thin, 4–5 mm thick, straight to curved, unbranched tubes. The tubes can coalesce, especially in their proximal parts ( Figs. 4B, 5A). In vertical section, the tubes may overlap partially and appear as indistinct spreiten. The tubes run from the common centre. Some of them interpenetrate ( Fig. 6A ). Commonly more than 100 radial elements are present in one specimen.

The radial tubes are built of and lined with light−coloured siltstone material contrasting in grain size with the surrounding very fine−grained sand. They are coated and partly filled with the same material. The coating and the fill material displays a pelleted structure. Only some tubes are filled with fine−grained sand. Locally, the pelletal structure is not seen and a gradual transition from pelleted to massive fill structure or coating can be observed. Some pellets can be seen in thin sections ( Fig. 5B ), on polished surfaces ( Fig. 6A, C ) and on the surface after sand jet preparation ( Fig. 6B ). The pellets are 1.5–1.7 mm long and 0.5–0.7 mm wide, cylindrical in shape, with hemispherical terminations. The coatings can coalesce and form irregular clumps, which are up to 35 mm thick and 45 mm wide. The pellets are concentrically arranged around some tubes ( Fig. 6A ). Locally, the contact between pellets in the tubes is angular ( Fig. 6C ). The filling of tubes is locally meniscate ( Fig. 6A ).

Margins of the radial structure are commonly uneven and seem to be disturbed by burrowing animals other than the trace maker ( Fig. 4A, B). The degree of destruction is variable. In the extreme situation only “ghosts” of the structure are visible. Some of the radial elements protrude beyond the structure. The marginal parts of the radiating elements have a tendency to be curved up. Some specimens are slightly concave in the central lower part and convex in the central top part. This part of the trace fossil is commonly preferentially cemented and it sticks out from a weathered rock as a ball ( Fig. 4C).

A vertical or subvertical, straight to slightly curved shaft runs up from the central top part ( Fig. 4D, E). The shaft was traced up for a distance of 410 mm but it is probably much longer. The shaft is 2 to 4 mm wide (outer dimension). It is thinly lined with silty material, similar to that found along the radial elements. The thickness of the lining ranges from 0.5 to 1.2 mm. Locally, the outer margin of the shaft is finely lobate in longitudinal section, what suggests its external surface could be pelleted.

Remarks.—The original diagnosis is as follows “A Dactyloidites having numerous, regular, symmetrically, radiating branches, themselves apparently unbranched and containing a limited retrusive spreite” (after D’Alessandro and Bromley 1986). In the amended diagnosis, the reference to Dactyloidites , taking in account problems of its type ichnospecies, is avoided. Moreover, a relation to the shaft and the presence of pellets is underlined.

Figures

Fig. 6. Morphological details of Dactyloidites peniculus. A. Specimen INGUJ 200P2, vertical cross section of the radial structure showing light pelletal masses, tubes and dark sandy sediment; distinct coatings containing the pellets (c) are seen around some tubes. Some tubes display a meniscate fill structure (m). B. Specimen INGUJ 200P1, a clump of pellets on the surface of the radiating structure.C. Specimen INGUJ 200P3, compressed pellets in a tube showing angular contacts.

Fig. 5. The radial part of Dactyloidites peniculus after preparation. A. Specimen INGUJ 200P1 after sand jet preparation, in top (A1) and bottom (A2) views. B. Specimen INGUJ 200P4, extra large vertical thin section.