Herradurichnus, Gyrochorte and Daedalus from the Balcarce Formation (?Silurian of the Tandilia System, Argentina) revisited.

PAZOS, PABLO J.; GUTIÉRREZ, CAROLINA; FERNÁNDEZ, DIANA E.

Congreso; ICHNIA 2016; 2016

The Balcarce Formation is a tide-dominated unit outcropping in the Tandilia System which contains abundant ichnofossils firstly studied by Borrello (1966). Original assignments in the unit like Cruziana and Arthrophycus remain indisputable and suggest a Late Ordovician lower Silurian age. Conversely, others were later reassigned to other ichnogenera. In particular the material reexamined in the field (Cabo Corrientes locality) received the attention of Poiré and del Valle (1996) and Seilacher et al. (2003) that introduced new assignments (e.g. Arenicolites, Didymaulichnus, Diplichnites and Diplocraterion). Interestingly, some trace fossils present there were analyzed and also discussed or reassigned in these or other works, sometimes informally (e.g. Heimdallia and Selenichnites). One unusual case is the material originally assigned to Palaeophycus that was later reassigned to Scolicia, Gyrochorte or Heimdallia. Some field specimens (fig. 1) partially resemble Arthrophycus brogniartii but differ in the variable course (winding, irregularor angularly meandering), subtriangular shape, variable width and asymmetrical ribs. These features do not fit in Arthrophycus and probably neither in the other previously attributed ichnogenera. Herradurichnus is recorded as the classical horse-shoe form, but also crescent and subtriangularin shape (fig. 2); variable incised forms suggest different producers than xiphosuran-like animals, probably a worm-like animal. The horizontal displacement of the Daedalus examples resembles the activity of siphons (fig. 3), like some sections of Hillichnus. Herradurichnus and Didymaulichnus arecross cut by Arthrophycus-like trace fossils. Finally, other trace fossils are meniscate endichnial forms without walls and showing basal marks.