A NEW NOTOUNGULATE(?) ICHNOTAXON FROM THE UPPER MIOCENE- LOWER PLIOCENE RÍO NEGRO FORMATION, PATAGONIA, ARGENTINA

MANUEL ROJAS MANRÍQUEZ; MELCHOR, RICARDO N.

Puerto Escondido

Simposio; V Simposio Latinoamericano de Icnología; 2023

Universidad Nacional Autónoma de México

The Río Negro Formation (Upper Miocene-Lower Pliocene) contains a remarkable diversity and abundance of trace fossils of continental and marine origin. The studied outcrops are located on coastal cliffs near El Cóndor town (41° 03' 05" S, 62° 51' 01" W), Rio Negro Province, Argentina. The deposits of the upper (continental) member correspond to aeolian dune, dry interdune, damp interdune, wet interdune, and ephemeral fluvial deposits. In wet interdune (lacustrine) and damp interdune deposits, a large variety of vertebrate and invertebrate trace fossils, as well as rhizoliths, was recorded. This contribution provides a detailed ichnological characterization of tridactyl footprints of an homopodial producer arranged in a quadrupedal trackway. The footprints are preserved in positive hyporelief in a fallen block and negative epirelief on a coastal platform. An isolated quadrupedal trackway is 3.07 m long and 0.29 m wide, comprising twelve tridactyl mesaxonic footprints arranged in six manus-pes sets. Manus prints are positioned in front of pes prints at an average distance of 0.284 m. On average, manus prints are wider than long (manus length = 156 mm, width = 173 mm, length/width ratio = 0.9), and larger than pes prints (pes length = 127 mm, width = 156 mm, length/width ratio = 0.8). In the pes print, digit imprints are united in a metatarsal-phalangeal pad, whereas in the manus prints digits imprints are disconnected from the metatarsal-phalangeal pad. In both prints, digit III is the longer (average 147 mm), and lateral digits are smaller and subequal in length (average II = 116 mm, IV = 129 mm). Average digit divarication is 89° (II-IV), and average angles formed by the lateral digits are similar (II-III = 46°, III-IV = 43°). Stride length is 1.036 m (manus) and 1.06 m (pes), pace length is 0.536 m (manus) and 0.542 m (pes), and pace angulation is 164° (manus) and 153° (pes). The ratio pace length/footprint length is 3.9. The manus is rotated inward (average = -5°, range= -11° to -3°, n= 6) and the pes is rotated outward (average = 12°, range= 5° to 18°, n= 6). On the coastal platform, the described footprints occur in a heavily trampled lacustrine flat surface with footprints arranged in various directions. This area contains fourteen distinct yet discontinuous trackways and isolated footprints. In addition to the described ichnotaxon, Macrauchenichnus cf. rector and Porcellusignum conculcator also occur. The newly described footprints are distinguished from the mentioned ichnospecies because are considerably larger and have elongated digits. Applying a mathematical equation to estimate the mass of the producer from the contact surface area of the footprint yields a weight of ~351 kg. The occurrence of tridactyl footprints in quadrupedal trackways is unusual. Considering the tridactyl footprint morphology, the quadrupedal trackway and homopodial character, the inferred body mass and the Late Miocene tetrapod fossil record, the most likely producer belongs to Toxodontidae and Leontiniidae (Mammalia: Notoungulata).