Hind limb morphometry of terror birds (Aves, Cariamiformes, Phorusrhacidae): functional implications for substrate preferences and locomotor lifestyle

Abstract

The hind limbs of birds have long been considered a key feature in the conquest of different environments. However, the high level of morphological diversity encountered complicates the foundation of a good theoretical correlation between morphology, locomotor habits and substrate preference and this, in turn, complicates palaeobiological interpretations. Phorusrhacids (Aves, Cariamiformes) are a good example, since they have been unequivocally categorised as terrestrial birds due to their reduced forelimbs; and as apex predators with the ability to pursue prey based only on their hind limb morphology. Multivariate techniques (PCA and discriminant analysis), based on traditional metrics and geomorphometrics of the hind limb and pelvis, were applied in order to explore terrestriality and cursoriality in phorusrhacids. Although several groups of birds could be identified, when looking solely at hind limb metrics, some phorusrhacids appear to be associated with walking birds, while others are associated with cursorial birds. However, the pelvis separates cursorial birds and phorusrhacids from walking and wading birds. This scenario is complicated further by a lack of clear definition of the different locomotor modes and substrate preferences in extant birds, and this makes it difficult to confirm phorusrhacid cursoriality based solely on morphometrics. However, some qualitative features of the pelvis and foot make the picture a little clearer. To study limb adaptations in fossil birds, a more holistic study, with an emphasis on qualitative features of the whole posterior locomotor module, is necessary, since morphometrics leaves some issues unresolved. A comparison with the wings is also needed, in order to make a more complete analysis of locomotor behaviour.

Differential locomotor and predatory strategies of Gondwanan and derived Laurasian dromaeosaurids (Dinosauria, Theropoda, Paraves): Inferences from morphometric and comparative anatomical studies

“…Based on the results of the second phylogenetic PCA made using lengths of the phalanges, taxa such as Avimimus , Cariama and Rhea are considered to have greater cursorial abilities (Gonzaga, ; Picasso, ; Degrange, ). These taxa have elongated proximal phalanges and a long digit III (Figs , and ).…”

“…Rhea are considered to have greater cursorial abilities (Gonzaga, 1996;Picasso, 2010;Degrange, 2017 I II III IV I II III IV I II III IV I II III IV I II III IV II III IV II III IV I II III IV I II III…”

Morphology of the forelimb of Psilopterus bachmanni (Aves, Cariamiformes) (Early Miocene of Patagonia)

“…Even considering the weight limit exemplified by living birds, body mass would not have impeded the ability of flight in Psilopterus, whose mass is estimated to have ranged between 5 and 10 kg among its various species. It is possible that P. bachmanni (and the rest of the Psilopterinae) had both cursorial and flying capabilities, although the fore-and hindlimb proportions seem to indicate a more accentuated use of the legs (Degrange 2012(Degrange , 2014. This locomotor style is present in Otididae and Cariamidae, which exhibit a similar proportion of wing bones to psilopterines.…”

The locomotor and predatory habits of unenlagiines (Theropoda, Paraves): inferences based on morphometric studies and comparisons with Laurasian dromaeosaurids

“…Based on the results of the PPCA made from phalanges length, taxa such as Avimimus, Cariama, and Rhea are considered with greater cursorial capacities [86][87][88], which are 25 associated with more elongated proximal phalanges and a long digit III (Figs 3, 4 and 7).…”