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New fossil footprints reveal terror birds (Phorusrhacidae) were fast runners with cursorial adaptations. These South American predators used their didactyl feet for efficient locomotion and prey capture.

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Area of Science:

  • Paleontology
  • Vertebrate Zoology
  • Ichnology

Background:

  • Terror birds (Phorusrhacidae) were dominant South American predators during the Cenozoic.
  • Previous inferences of their terrestrial habits were based on reduced forelimbs and high body mass.
  • The exact function of terror bird foot morphology, particularly digit II, remained debated due to incomplete fossil records.

Purpose of the Study:

  • To analyze the first well-preserved fossil footprints of Phorusrhacidae, named Rionegrina pozosaladensis.
  • To reconstruct the locomotor habits and functional morphology of these extinct predatory birds.
  • To understand the evolutionary adaptations of terror birds based on ichnological evidence.

Main Methods:

  • Analysis of newly discovered, well-preserved fossil footprints attributed to Phorusrhacidae.
  • Comparative ichnological analysis with other fossil footprints, including deinonychosaurian dinosaurs.
  • Functional morphological interpretation of foot structure and inferred locomotion.

Main Results:

  • Discovery of the first didactyl footprints of Phorusrhacidae, Rionegrina pozosaladensis.
  • Evidence of strong cursorial adaptations in medium-sized, Late Miocene terror birds (~8 Ma).
  • Footprints indicate a reduced digit II, a raised metatarso-phalangeal pad, primary support on digit III, and digit IV acting as an outrigger.

Conclusions:

  • Fossil footprints provide unprecedented insights into terror bird locomotion, revealing cursorial adaptations.
  • The raised claw of digit II was likely used for pinning prey.
  • Phorusrhacid footprints are distinct from deinonychosaurian footprints due to size and mesaxony, highlighting unique evolutionary paths.