A new ichnospecies of Eatonichnus from the Middle Eocene of Patagonia Argentina: recording pupation chambers of dung beetles

KRAUSE, M. J.; SANCHEZ, M. V.; BELLOSI, E. S.; GONZÁLEZ, M. G.; SARZETTI, L. C.; PUERTA, PABLO; GENISE, J. F.

Colonia

Simposio; Tercer Simposio Latinoamericano de Icnologia; 2015

Universidad de la Republica Uruguay

Eatonichnus is an ovoid trace fossil characterized by an outer helical wall surrounding a central cylindrical to ellipsoidal cavity. It is known from Utah (USA) and eastern Chubut province (Argentina). The finding of a large number of specimens of Eatonichnus in paleosols from the lower Sarmiento Formation, at some localities of south-central Chubut, enabled the evaluation and improvement of the ichnotaxobases proposed in former contributions and the description of a new ichnospecies. Measures were taken from 159 specimens. The length is 17?61 mm (N=44; mean=44 mm) and the equatorial diameter 10?37 mm (N=144; mean=28 mm). The wall is composed of an outer thick, pelletal layer and an inner, smooth and thin one. The former shows externally a helical pattern composed of closely tight whorls, and ranges in thickness from 2 mm to 10 mm (N=129; mean=6 mm). The inner layer is mostly smooth, 1?2 mm thick, and surrounds completely the central cavity. The diameter of the cavity is 5?22 mm (N=99; mean=15 mm). In most specimens the central cavity displays a passive fill. Four ichnospecific taxobases were originally considered for distinguishing E. claronensis from E. utahensis: presence/absence of a bioglyph, size, degree of inclination of the whorls, and presence/absence of a meniscate backfill within the central cavity. The new ichnospecies shows that the former would be the key ichnotaxobase. The pelletal arrangement in the new ichnospecies is unique in showing transverse rows of small, juxtaposed to imbricated, rounded to elongate, rhomboidal pellets, 1.5?2 mm wide and 2?3 mm long, composing the outer helical wall. In contrast, E. utahensis displays coarse and poorly defined pellets, 6?8 mm wide and 8?10 mm long, and E. claronensis shows whorls with knobby surface, without defined pellets. Differences between extreme values in diameter and length of about 30%, and ranges of inclination from 0° to 30° within a same specimen, suggest that size and inclination of whorls are not a good ichnotaxobases. The meniscate filling inside the central cavity may indicate the emergence of the insect, and accordingly it may appear in specimens of different ichnospecies. The inner, smooth and thin layer of the wall, recognized in the three ichnospecies, is additionally proposed as a new ichnogeneric taxobase. Several pupation chambers of extant Scarabaeinae display the same helical, pelletal, two-layered wall as Eatonichnus. Both layers are interpreted as the result of the moulding of larval feces with a high concentration of soil material and probably larval secretions. In some specimens of the new ichnospecies the pelletal arrangement is strongly similar to that of Chubutolithes, an ichnotaxon originally assigned to a mud-dauber wasp nest. This new evidence suggests dung beetles would be also the producers of Chubutolithes.