Morfología del cráneo y complejo apendicular posterior de aves fororracoideas: implicancias en la dieta y modo de vida

• Autores: Federico J. Degrange
• Directores de la Tesis: Claudia Patricia Tambussi (dir. tes.)
• Lectura: En la Universidad Nacional de La Plata ( Argentina ) en 2012
• Idioma: español
• Enlaces
  • Tesis en acceso abierto en:  sedici.unlp.edu.ar  doi.org 
• Resumen
  • español

    Los Phorusrhacidae comprenden uno de los grupos de aves más característicos y diversos del Cenozoico sudamericano. Son considerados como depredadores dominantes en las cadenas tróficas durante el período de aislamiento acaecido en Sudamérica durante el Cenozoico, rol que habrían compartido con los marsupiales y cocodrilos terrestres. Su pico alto, angosto y largo terminado en gancho es un rasgo exclusivo. Sin embargo, no es su única característica peculiar. A diferencia del resto de las Neornithes, los fororracos perdieron la capacidad de flexionar el pico dorsoventralmente (una cualidad conocida como quinesis aviana). Esta inmovilidad craneana denominada aquinesis no es conocida en otras aves depredadoras y constituye sin dudas un rasgo derivado de este clado. Se estudiaron las 19 especies consideradas originalmente como fororracos y se analizaron todos los materiales de Phorusrhacidae depositados en museos nacionales e internacionales. Aplicando técnicas modernas de morfometría geométrica y clásica, biomecánicas y reconstrucción muscular en un contexto filogenético apropiado, se realizó la descripción cuantitativa y cualitativa del diseño del cráneo (como expresión de la manera en que se adquiere y manipula el alimento) y del complejo apendicular posterior (como relevante en la locomoción). Se analizó el funcionamiento del cráneo a partir de un análisis quinético y de un análisis de elementos finitos que sentó las bases para interpretar la estrategia de caza de estos animales.

  • English

    The Phorusrhacidae comprises one of the most characteristic and diverse groups of birds of the South American Cenozoic. They are considered as dominant predators in the trophic chains during the isolation period occurred in South America in the Cenozoic, a role that they have shared with marsupials and terrestrial crocodiles. Its high, narrow and long beak that ends in a hook is The 19 species originally considered as phorusrhacids were studied and all the Phorusrhacidae materials, housed in national and international museums, were analyzed.

    A quantitative and qualitative description of the skull design ( an exclusive feature; however it is not its only peculiar characteristic. Unlike the rest of the Neornithes, phorusrhacids lost their capacity to dorso-ventrally flex their beak (quality known as avian kinesis). This cranial immobility called akinesis is unknown in other predator birds and doubtless constitutes a derived feature of this clade.

    as an expression of how it acquires and manipulates food) and the posterior appendicular complex (as relevant in the locomotion) was made. This was accomplished using modern techniques of classic and geometric morphometry, biomechanics, and muscle reconstruction in a proper phylogenetic context. We analyzed the skull performance from a kinetic analysis and finite element analysis which set the basis for interpreting the hunting strategy of these animals.

    Considering all the lines of evidence together, it can be argued that the design of the skull, particularly the singular shape of the beak and the skull rigidity linked to akinesis, are, as a whole, well suited to kill. The skull is well designed to exert powerful bite forces and accurate vertical strikes against potential prey rather than to resist lateral and torsion loadings as they would be potentially harmful. The muscle reconstruction, the akinesis, and the biomechanical analysis of the skull and of the mandibular adductor muscles of the phorusrhacids show that the trophic apparatus is optimized for strength at the expense of speed. It is interpreted that the capture and manipulation of prey would consist in the quick opening of the mouth, submission from vertical strokes of the head, immobilization of the prey with a powerful bite force preventing any lateral or torsional effort that might be caused due to its movement. Finally, dismemberment and tear of the prey using the hook of the beak by pullback movements.

    As for the posterior appendicular complex, the phorusrhacids have morphological attributes that can be unequivocally associated with cursorial locomotion: a high and elongated postacetabularly pelvis, with a highly developed antitrochanter, elongated tarsometatarsus and tibiotarsus, the high development of the cristae cnemialis of the tibiotarsus, three relatively short digits directed forward and the elevated position of a digit that does not make contact with the ground. These features, together with the strong development of the extensor muscles, indicate the terrestrial locomotor habit and also highlight the cursorial ability of these birds. In terms of lifestyle, the posterior locomotor module characters indicate a technique for detecting prey by slow moving (walking) and a capture technique based on the active pursuit of prey (cursorial). The latter evidenced by high maneuverability that empowered to pursue their prey for long periods and even using elusive movements The morphological evidence analyzed in this thesis allows to discriminate two morphological patterns for both the skull and the pelvis: the “psilopterine” type and the “terror-bird” type. However, these two morphotypes may have had a very similar performance.

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    While the "terror bird" could be considered as an evolutionary specialization, form and function of both complexes (trophic and appendicular) show that the capacities of both morphotypes is to hit and to run, respectively, so it is inferred that the final biological role is the adaptation to a very particular type of hunting, chasing and killing using their beaks. In other words, all phorusrhacids are similar from the ecomorphological point of view and played the role of active cursorial predators.

    In predators, body mass is strongly linked to the prey selection: the larger the predator, the greater will be the size of potential prey. Available prey depends on the mass ratios and the behavior of both the predator and the prey, with bite force playing a secondary role. The morphology of the phorusrhacid beak and the high value of stress calculated for lateral forces suggest that large prey could be harmful. While birds cannot use the forelimbs to manipulate their prey and only depend on the actions of the head, neck and hindlimbs, it can be inferred that the phorusrhacids consumed prey smaller than themselves. It is possible to assume a distribution of niches on the size of prey between phorusrhacids of different sizes. The smaller prey such as rodents could have been eaten by Psilopterinae; mammals with masses up to 10 or 20 kg would be available to Patagornithinae while those of greater mass and size would be potential prey of Mesembriornithinae and Phorhusracinae, who probably could chase and kill animals that weight up to 100-130 kg. Forms of higher body masses were probably inaccessible to phorusrhacids.

    The execution of this thesis represents an important attempt to a better unification of the anatomy, functional morphology and ecology fields, and to complete the information about the Phorusrhacidae, trying to integrate functional, biomechanical, ecomorphological and paleontological studies in a phylogenetic framework suitable for testing hypotheses about the predatory role of these birds.