Vertebrate And Invertebrate Trace Fossils In Anastomosing Fluvial Deposits Of The Toro Negro Formation (Upper Miocene), La Rioja, Argentina

VERÓNICA KRAPOVICKAS; PATRICIA L. CICCIOLI; M. GABRIELA MÁNGANO; CARLOS O. LIMARINO

Albuquerque, New Mexico, USA

Simposio; Cenozoic Vertebrate Track Symposium; 2007

New Mexico Museum of Natural History and Science

The upper Miocene Lower Member of the Toro Negro Formation is a thick (approximately 1800 m) succession deposited in the Andean foreland basin in La Rioja province, western Argentina (Ciccioli et al., 2005). It is mainly composed of yellowish brown and greenish sandstone, red mudstone, gray conglomerate and several intercalated tuff layers. Trace fossils are recorded from the lower part of the Lower Member of the Toro Negro Formation at La TroyaCreek. This succession consists of sandstone, intraformational breccia, mudstone and scarce conglomerate deposited in a strongly incised sandy anastomosing fluvial system developed in a semi-arid climate (Ciccioli et al., 2004). Crevasse splay deposits mostly consist of massive medium- to fine-grained sandstone that is intensely bioturbated (BI 3: Droser and Bottjer, 1986). The invertebrate ichnofauna includes Taenidium barretti, Scoyenia gracilis, Palaeophycus striatus and Palaeophycus tubularis. This low-diversity assemblage is dominated by fodinichnionand domichnion structures most likely produced by insects.The vertebrate ichnofauna occurs at the top of channel deposits (emergent sandy bars) and is composed of avian and mammalian footprints. Three types of avian tracks were identified. Type I is most likely associated with the activities of shorebirds (possible Charadriiformes), and comprise tridactyl footprints of small size (average length 32 mm, average width 35 mm) with slender digits, terminal claw marks, average digit divarication angle of 103° and common metatarsal pad impressions. These tracks display random orientation and typically occur as clusters of high density. No trackways can be individualized. Type II is most likely related to Passeriformes. These tracks are functionally anisodactyl with slender digits (average length of 42 mm and average width of 32 mm). Digit divarication angle between digits II-IV is 103°. The halux (digit I) is directed backward. Tracks are organized in several trackways with consistent unidirectional movement. Type III is the largest avian track and possible producers are rheas or phorusrhacids. This type is only recorded by a single large tridactyl left footprint of 210 mm length and 150 mm width. It displays strong digits and conspicuous metatarsal pad impressions with an interdigital divergence angle of 60º. Four distinctive track and trackway morphologies have been assigned to mammal producers. Mammalian tracks include heteropod quadruped mammal footprints tentatively assigned to Notoungulata (Hegetotheriidae). Manus imprintsare tetradactyl with an average length of 34 mm and average width of 23 mm. Pes tracks are tridactyl with average length of 50 mm and average width of 41 mm. Manus-pes average distance is 60 mm. Average pace is 178 mm, and average stride is 329 mm. Other mammal footprints (cf. Venatoripes riojanus) are assigned to the superfamily Megatheroidea (Bonaparte, 1965). Pes tracks comprise oval impressions about 500 mm long and 260 mm wide.No manus impressions have been found. Average pace is 575 mm, stride is 876 mm and pace angulation is 100º.Tridactyl footprints, assigned tentatively to Macraucheniidae, have also been observed. These tracks are 120 mm long and 130 mm wide. They show short, broad and blunt digit imprints, with a divarication angle of 50º and posterior margin of slightly concave curvature. No clear trackway pattern has been identified. Finally, medium size kidney-like impressions (with concave medial lateral margin) about 220 mm in length and 110 mm in width, were also locally recorded. Average pace is 397 mm, stride is 777 mm and pace angulation is 146º. No clear producer was identified. Trackways are preserved as positive hypichnia on the surface of large loose blocks (average 4 per 6 m). Avian footprints are more abundant than mammalian ones, particularly those related to Passeriformes. Within mammal footprints, the smallest ones, most likely produced by Notoungulata (Hegetotheriidae), are the most abundant. This situation can be explained by the fragmentary surface exposed on blocks and the larger size of the other mammalianfootprints present. Some avian footprints cross-cut or are preserved within mammal tracks, indicating that they were 52 produced after formation of the mammal footprints.Interestingly, inter-channel areas (top of the emergent sandy bars), where vertebrate tracks and trackways are recorded, do not host invertebrate trace fossils. A well-developed invertebrate ichnofabric characterizes crevasse splay deposits associated with desiccation cracks and, locally, immature paleosols. The particular dynamics of the floodplain with high-energy crevasse splay events providing available sandy substrate to colonize followed by lowenergy fallout deposition resulted in optimal conditions for invertebrate trace fossil formation (i.e., via insect colonization)and subsequent preservation.The invertebrate ichnofauna is ascribed to the Scoyenia ichnofacies recording low-energy settings exposed to air and periodically inundated. The vertebrate ichnofauna can be included in the Grallator ichnofacies of Hunt and Lucas (2007), in particular the Avipeda ichnocoenoses (shore-bird ichnofacies of Lockley et al., 1994). This ichnofacies extends from the Triassic to the Recent, including different ichnocoenoses. Interestingly, its environmental distribution includes lacustrine margin environments (Hunt and Lucas, 2007), but exposed, non-vegetated areas of the floodplain and fluvial bars should also be included. Ichnodiversity is moderate to high, and in addition to birds, herbivorous mammals are clearly the dominant group in this Cenozoic example.