Cretaceous burrows of probable vertebrate origin from volcaniclastic interdune deposits of the Cerro Barcino Formation, Patagonia, Argentina

PEREZ, M.; UMAZANO, A.M.; MELCHOR, R.N.

Santa Rosa

Simposio; II Latin-American Symposium on Ichnology; 2013

The Albian alluvial-eolian Puesto La Paloma Member constitutes the basal unit of the Cretaceous Cerro Barcino Formation (Chubut Group in the Somuncurá-Cañadón Asfalto basin); which is mainly composed of sheet-like tuffaceous strata. For this contribution we measured one partial section (4.25 m thick) at Cerro Los Chivos (43°12?48,72?? S - 68°50?39,72?? W), near Pasode Indios, Chubut province. This section is approximately 7.5 m above the limit with the underlyingLos Adobes Formation. We recognized three types of tuffaceous deposits including sheet-flood, low relief eolian bedform and pyroclastic ash fall. All deposits represent interdune zones, which arecovered and laterally replaced by eolian dune facies. Sheet-flood deposits are composed of fine grained tuffaceous sandstones with plane parallel lamination and without ichnofossils. Low reliefeolian bedform deposits are constituted by fine-grained tuffaceous sandstones with low angle-crossbedding and lacking trace fossils. Pyroclastic ash fall deposits are composed of fine-grained tuffs with both rounded accretionary lapilli and ferruginous nodules. Trace fossil content is represented by root traces up to 10 cm long, Skolithos isp., Taenidium barretti and, in the top of the uppermost bed, we found silicified and mostly sub-horizontal large burrows, which are the subject of this contribution. The burrows are found in a well developed and intensely bioturbated paleosol. Studied material includes twelve collected specimens and several measured at the field. The best preserved specimens display an upper portion (ramp) with a maximum inclination of 20°, and then the burrows become subhorizontal. In plain view, several burrow casts display a curved path composed of two segments forming an obtuse angle (about 115°), with a rounded termination and no significant enlargement. The burrow casts, with a maximum length of 300 mm, have a flattened cross section, with a bilobed floor and a convex roof. A single specimen (probably a ramp) displays a cylindrical cross section. Average width is 61 mm (range: 53 - 75 mm) and average height is 26 mm (range: 19 - 35 mm). Burrow casts display a distinct surface texture in the form of groups of two or three parallel rounded ridges. Individual ridges are 1.9 to 2.5 mm wide and up to 31 mm long. The width of the pair of ridges is in the range 4.2 - 5.9 mm and that of three parallel ridges is 8.2 ? 9 mm. The ridges are clearer in the roof of the burrow where it forms a curved path, criss-crossing in the center and then changing outwardly to subparallel to burrow walls. In the floor of the burrow casts, the ridges have a chevron-like appearance or are sub-parallel to the burrow axis. Considering the size, these burrows could be constructed both by vertebrates and invertebrates. The known examples of moderately large burrows with a prominent surface texture in the form of well-defined ridges or grooves are commonly attributed to tetrapods or decapod crustaceans. One additional feature of these burrow casts is the presence of a bilobed floor, which has been only reported in fossil and modern vertebrate burrows. The burrow cross-section suggests a producer with a dorso-ventrally flattened body. A bilobed burrow floor and massive fill is compatible with an open burrow system that is used repeatedly. The small size of these burrows excludes most of the vertebrate remains recovered from the Cerro Barcino Formation, which include titanosauriform sauropods, theropod dinosaurs, crocodyliforms, turtles and lepidosauromorphs. The most likely producers are lepidosauromorphs, although anurans cannot be discarded. The latter have a flattened body and burrowing habits, but are not recorded in the formation and commonly lack strong claws that may produce the observed scratch marks.