Planolites Nicholson, 1873

Anistoroae, Anca & Miclăuș, Crina, 2015, The Cruziana Ichnofacies In The Lower Member Of The Bisericani Formation (Eastern Carpathians, Romania), Acta Palaeontologica Romaniae 11 (2), pp. 9-23 : 15

Ichnogenus Planolites Nicholson, 1873

Planolites isp.

Material: Many specimens observed in the background mudstone beds as full reliefs ( Fig. 4g ); few specimens on the lower surface of plan-parallel laminated or ripple cross laminated sandstone beds as positive hyporeliefs ( R 6, Fig. 4h ).

Description: subcylindrical unbranched burrow, horizontal to subhorizontal, straight to slightly curved, occasionally overlapping one another ( Fig. 4h ). The infilling contrasts with the surrounding matrix by color, texture, and composition. In some cases it reveals rusty ( Fig. 4i ) or black ( Fig. 4j ) infill color. The unbranched cylinder is less than 1.5 cm, rarely 2 cm in diameter ( Fig. 4j ), while the length does not exceed 15 cm. In several whitish-gray mudstone beds Chondrites co-occurs with and horizontal flattened galleries of Planolites isp., which are largely curved, 0.7 cm wide and 8 cm long, darker than the host deposit ( Fig. 4f ).

Remarks: Planolites is most likely a tunnel produced by deposit feeding worms, which actively back-filled with biologically processed sediment Häntzschel (1975). The author noticed that Planolites is quite easily confused with Palaeophycus due to striking external morphological resemblance, the difference being based on the type of filling. The rusty infilling of a Planolites is a good example of color contrast which might result either from the limonitization of sediment bypassed trough the tracemaker gut or from sediment sorting resulted from selective feeding ( Pemberton & Frey, 1982). The transversal fine ornamentation noted on some specimens ( Fig. 4k ) would demonstrate the back-filling activity ( Häntzschel, 1975). The flattened endichnial Planolites is the result of compaction. Calculating a:b ratio (a = 0.7 cm; b = 0.2 cm), the obtained 3.5 value suggests a fluid to soft substrate consistency at colonization time ( Wetzel & Aigner, 1986, Schieber, 2003).

Figures

Fig. 4 Trace fossils: a plantlike Chondrites targionii; b featherlike Chondrites targionii; c, d large specimens of Chondrites intricatus; e, f small specimens of Chondrites intricatus; g Planolites in the background mudstone - full relief; h Planolites – positive hyporeliefs; i Planolites with rusty material infill; j Planolites with dark material infill; k Planolites with transversal fine ornamentation.