For the first time, footprints made by an extinct terror bird (family Phorusrhacidae) have been identified by paleontologists.
Holotype trackway of Rionegrina pozosaladensis: (a-c) orthomosaic, digital elevation model and interpretative drawing of the holotype; (d, e) detail of orthomosaic and digital elevation model of fifth right footprint (R5); (f, g) detail of orthomosaic and digital elevation model of the fourth left footprint (L4). White arrow points to wrinkle traces and black arrows to tip of digit II claw trace. R1-R6: consecutive right footprints; L1-L5: consecutive left footprints; II: digit II. Image credit: Melchor et al., doi: 10.1038/s41598-023-43771-x.
Terror birds belong to Phorusrhacidae, an extinct family of large, carnivorous, flightless birds.
These birds had a very large body mass, up to 70 kg, and were between 0.9-2 m (3-6.6 feet) in height.
They were the predominant predators during the Cenozoic in South America, but they are also from the Plio-Pleistocene of North America and the Eocene of Africa.
Phorusrhacidae contains almost 20 species in 14 genera and 5 subfamilies (Brontornithinae, Mesembriornithinae, Patagornithinae, Phorusrhacinae, and Psilopterinae).
Their closest living relatives are believed to be seriemas, the sole living members of the family Cariamidae.
“Terror birds comprise the most outstanding group of South American Cenozoic avifauna, and have been considered as dominant predators in Cenozoic ecosystems,” said Universidad Nacional de La Pampa paleontologist Ricardo Melchor and his colleagues.
“Their terrestrial habits have been well established based on the reduction of their forelimbs and high body mass.”
“One of the pillars for the hypothesis of a predatory mode of life for phorusrhacids is based on the morphology of their hind limbs, which seem to be suitable for pursuing prey.”
“Comparison of the hindlimb of phorusrhacids with those of extant groups suggests that Mesembriornithinae and Patagornithinae were likely cursorial birds, but members of the Psilopterinae were likely walkers and waders.”
Llallawavis scagliai, a species of terror bird that lived in what is now Argentina during the Pliocene epoch, around 3.5 million years ago. Image credit: H. Santiago Druetta.
The excellently-preserved footprints of a medium- to large-sized terror bird, probably belonging to the Mesembriornithinae subfamily, were recently found in coastal outcrops of the Río Negro Formation at the San Matías Gulf (Atlantic coast of Argentina).
“These are the first and only known fossil footprints assigned to this group of birds,” the paleontologists said.
The footprints date back nearly 8 million years (Late Miocene epoch) and were assigned to a new ichnogenus and ichnospecies called Rionegrina pozosaladensis.
“The footprints contain wrinkle traces (related to integumentary imprints of the producer), display cross-cutting relationships with wave ripples, and mud-cracks follow their boundary,” the researchers said.
“It is possible to infer that footprints were registered in a lake mudflat after wave ripple formation and were later exposed and mud-cracked.”
“The presence of wider mud-cracks toward the south, suggest that the producer was moving away from the lake coast.”
“Considering a height at hip of 0.81 m (2.7 feet), the estimated speed of the footprint producer is 2.74 m/s (9 feet per second).”
The average body mass of the producer of Rionegrina pozosaladensis is estimated at about 55 kg.
“These footprints yield unprecedented information on the locomotor habits of Phorusrhacidae,” the authors said.
“The finding implies that medium-sized, Late Miocene terror birds developed noticeable adaptations for running and predation.”
“The latter features include functionally didactyl posture, with stabilization during running facilitated by digit IV, and a combination of a large curved claw in digit II with an adjoining metatarso-phalangeal pad that assisted in the pinning and grasping of prey.”
A paper on the findings was published in the journal Scientific Reports.
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R.N. Melchor et al. 2023. First terror bird footprints reveal functionally didactyl posture. Sci Rep 13, 16474; doi: 10.1038/s41598-023-43771-x
