FIRST RECORD OF TERROR BIRD FOOTPRINTS FROM THE UPPER MIOCENE OF PATAGONIA, ARGENTINA

MELCHOR, RICARDO N.; FEOLA, SILVERIO F.; CARDONATTO, M. CRISTINA; ESPINOZA, NAHUEL; ROJAS-MANRIQUEZ, MANUEL A.

Puerto Escondido

Simposio; V Simposio Latinoamericano de Icnología; 2023

Terror birds (Aves, Phorusrhacidae) comprise the most outstanding group of South American Cenozoic avifauna, and have been considered dominant terrestrial predators. Phorusrhacids were considered functionally tridactyl with three relatively short digits II–IV and a small, elevated digit I. Incomplete or lack of preservation of foot bones have hampered understanding of the evolution and diversification of Phorusrhacidae. Here we report the first known and well-preserved footprints of Phorusrhacidae with a didactyl posture that yield unprecedented information on the locomotor habits of the group. The described trackway occurs in the Río Negro Formation (Upper Miocene-Lower Pliocene) near Pozo Salado locality (41° 00´ 49.45"S; 64° 09´ 54.17"W), Río Negro Province, Argentina. The sedimentary sequence is interpreted as eolian dunes alternating with interdune lakes, and the trackway occurs in a mudflat of the interdune lake. A vitric tuff bed occurring 9.7 m above the footprint-bearing level, yielded an age of 8.0 ± 0.1 Ma (U/Pb method on zircon). The Phorusrhacidae footprints are dated as Tortorian (Late Miocene). The described footprints compose a nearly straight trackway including eleven consecutive footprints with a very good preservation. Large (average length = 371.4 mm, width = 253 mm) functionally didactyl footprints with moderate elongation (average FL/FW = 1.47), marked impressions of digit III (up to 57 mm deep), then of digit IV, and shallower metatarso-phalangeal pad and digit II impressions. Digit impressions are not connected to the metatarso-phalangeal impression. Impression of digit III is fusiform, the longest (average = 250 mm) and thickest (average width = 88 mm) with marginal ridges on the medial and lateral sides, and large (average length = 71.4 mm, width 33.1 mm) and deeply set subtriangular claw traces. Digit IV impression is the second longest (average length 163.4 mm, average width 60.3 mm) and curved inward with a smaller claw trace (average length = 30.5 mm, width = 27.9 mm). The elliptical digit II impression is the shortest (average length = 85.1 mm, width = 45.5 mm), shallower and exhibits an elliptical to subtriangular imprint (about 12 mm in diameter) located ~72 mm in front of the digit impression that is a probable claw imprint. Subcircular to pear-shaped metatarsal-phalangeal pad impression (73.3 mm by 75.6 mm) is shallow. Transverse to oblique wrinkles occur inside digit impressions III, IV and the metatarsal-phalangeal pad impression. The trackway is straight and have an average pace angulation of 168.8°, pace length of 918 mm, and stride length of 1830 mm. Individual footprints display an inward rotation averaging 6.8°, the external trackway width is 407.8 mm and the breadth between tracks is negative (average 75.8 mm). This finding implies that medium-sized, Late Miocene phorusrhacids developed strong cursorial adaptations; achieved through reduction of digit II, raised metatarsal-phalangeal pad, main body support in a large and thick digit III, and digit IV as outrigger. Raised and long claw of digit II was probably used in pining of prey. Phorusrhacidae footprints differs from the Early Cretaceous didactyl footprints of deinonychosaurian dinosaur affinity by its larger size and strong mesaxony.