Atollites italicum

Serpagli, Enrico, 2005, First record of the ichnofossil Atollites from the Late Cretaceous of the Northern Apennines, Italy, Acta Palaeontologica Polonica 50 (2), pp. 403-408 : 404-406

Atollites italicum ichnosp. nov.

Fig. 2A, B.

Holotype: Fig. 2. CMV 240787.

Type locality: Castelletta Quarry located in the territory of Prignano (Modena District).

Derivation of the name: The species name refers to the occurrence of the new taxon in Italy.

Diagnosis.— Rosetted (star−shaped) circular body having a central field with a circular area (bearing a small hole at the centre) encircled by a sharp, narrow, raised ridge−like collar, surrounded by an intermediate depressed zone radially crossed by numerous, narrow ray−like cylinders each connecting globular−subglobular knob−like clubs of the external zone. The boundary between the intermediate and external zones is not very well defined owing to the partial overlapping of some “clubs”. On the other hand, the boundary between the trace and the “housing” rock is well defined.

Description.— The diameter of the whole rosetted specimen varies from 60 to 65 mm and its thickness from 4 to 7 mm. It occurs inside a sediment rich in iron, black in colour, made by fine spherulites of dolomite which have substituted the original pelitic sediment.

The central circular area, including the encircling ridge−like collar, ranges from 10 to 11 mm in diameter. Inside the 2 mm thick ridge, there is a flat area (6 mm in diameter) with a narrow central hole (1–1.5 mm in diameter) producing a neck−like constriction. The intermediate area is mostly occupied by the densely packed ray−like cylinders whose length ranges from 7 to 12 mm whereas the diameter seems to be more constant being about 1–1.5 mm. Locally, near the axis of the system, the cylinders overlap leaving only a small area of substrate undisturbed in the vicinity of the ridge. This also prevents the exact number of cylinders from being established, but, owing to obliteration, they are apparently less in number than the clubs.

The clubs, 30–35 in number, are very close together and variable in shape (globular or subglobular) and size, ranging from 4× 5 mm (for the smallest, mostly globular) to 8× 11 mm (for the largest, mostly subglobular). The two groups of clubs are randomly distributed even if wider clubs are apparently more common at the periphery of the body. Whereas the clubs are in groups of two sizes, the ray−like cylinders have more or less the same diameter which corresponds to the diameter of the hole in the central area.

All the structures (cylinders and clubs) are in the form of internal moulds.

Reconstruction.—No attempt to identify the trace−maker can be made without restoration of the structure before it was affected by diagenesis and/or tectonics.

The specimen appears to be approximately planar owing to compaction, but it is nevertheless three−dimensional having a residual thickness between 4 and 7 mm. I have not been able to determine the precise degree of compaction affecting the specimen but reduction by 90% is probably consistent. Because of that, I interpret the pre−diagenetic situation as being very close to the idealised reconstruction showed in Fig. 3A , whereas in the Fig. 3B–D a computer visualization of the gradual compaction affecting the structure is illustrated. In all the figures the number of lateral clubs and cylinders has been deliberately and markedly reduced in order to produce clear, light and intelligible drawings. A conical reconstruction of the whole building is favoured because wider clubs are apparently more common at the periphery of the body.

In the structure three main elements can be recognised: a vertical, axial shaft, about 30–35 lateral clubs and as many connecting cylinders.

From the axial shaft, whose entrance is clearly visible in the centre of the specimen, about 30–35 clubs radiate subhorizontally via narrow cylinders. In fact, the pre−compaction angle formed by the cylinders and the central shaft was probably close to 90 ° or so. The club−cylinder systems may follow a slight helical arrangement (faint spiral pattern) and in most cases they are arranged in closely packed whorls.

Discussion.—The trace fossil described has no equivalent, at least in literature known to the author, and therefore the establishing of a new species is proposed. Atollites zitteli Maas, 1902 and A. minor Maas, 1902 are apparently the only valid species of the ichnogenus. Other ichnotaxa originally referred to Atollites like A. carpathicus Zuber, 1910 and A. caucasicus Sobolev, 1957 have been subsequentely placed in Lorenzinia Gabelli, 1900 and Kirklandia Caster, 1945 respectively ( Nowak 1957; Ślączka 1964), whereas validity of A. kulczinskii Kuźniar, 1911 needs verification.

This ichnospecies shows important diagnostic characters that distinguish it from other species of the genus: (a) the clubs are of different sizes with the largest mostly at the periphery of the crown whereas either in Atollites minor or in A. zitteli they are more or less the same size; (b) furthermore, A. italicum has at least a three times as many chambers as the former two species.

The possible helical arrangement of the clubs evidenced by our specimen confirms also a hypothetical link between Atollites and Gyrophyllites Glocker, 1841 ( Seilacher 1977) and not between Atollites and Lorenzinia as previously supposed ( Kieslinger 1939).

There are similarities between the central area of the structure encircling the entrance of the shaft, and the same area of Palaeosemaeostoma geryonides ( Huene, 1901) where a raised narrow collar is also present. Faint architectural similarities exist with elements of the ichnogenus Cellicalichnus Genise, 2000 erected for fossil bee cells which is one of the most common traces in palaeosols.

Figures

Fig. 3. A. Schematic and simplified pre−diagenetic reconstruction of the whole architecture of Atollites italicum ichnosp. nov., For drawing purposes, the number of lateral cylinders and clubs has been reduced to less than half of those actually present. B–D. Computer visualization of gradual compaction of the structure.