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

Abstract

Tetrapod limbs morphology is a reliable proxy of locomotor capacities. Beyond this, other aspects of life habits, such as predation abilities, can also be relevant to determine main morphofunctional appendicular properties, which ultimately reflect a compromise between different factors of the biological role. Dromaeosauridae is a dinosaur clade belonging to Theropoda, a group of bipedal predators. Dromaeosaurids represent an interesting study case, in which the hindlimbs have been proposed to be involved in both locomotion and predation activity. A peculiar feature characterizing all dromaeosaurids is a modified second pedal digit, which is typically related to predation. This theropod group is closely related to birds and diversified during the Cretaceous Period, mainly in the Northern Hemisphere (Laurasia). However, a subclade of dromaeosaurids, the Unenlagiinae, was recently recognized for Gondwana.Nevertheless, there are morphological differences between derived Laurasian dromaeosaurids (eudromaeosaurs) and unenlagiines. Such differences are observed in the proportions between hindlimb bones and in the presence of a subarctometatarsalian condition in unenlagiines, which is mainly characterized by a proximally constricted metatarsal III. To evaluate the function of these divergent morphologies, we conducted morphometric analyses and comparisons of qualitative morphological aspects, encompassing unenlagiines, other dromaeosaurids, as well as taxa from other theropod groups, including extant birds. The former approach consisted of two phylogenetic principal component analyses, one based on the main measurements of the hindlimb, and the other focused on the lengths of the pedal phalanges. The first analysis drew the unenlagiines close to taxa with long tibiae, as well as long and slender metatarsi. Instead, eudromaeosaurs are closer to taxa with shorter tibiae, and shorter and wider metatarsi. The second analysis showed that eudromaeosaurs and unenlagiines have similar phalangeal proportions, including the elongation of distal phalanges. However, the shorter second phalanx of the pedal digit II of eudromaeosaurs could have increased the force generated by this digit, which was the main predatory tool of the autopodium. This, together with a shorter and wider metatarsus, and a marked hinge-like morphology of the articular surfaces of metatarsals | 773 GIANECHINI Et Al.

A new unenlagiine (Theropoda, Dromaeosauridae) from the Upper Cretaceous of Brazil

“…According to Gianechini et al . (2020), tooth morphology is not unequivocal evidence for piscivory; and other evidence must be considered (to which we fully agree; Souza et al . in press).…”

“…Furthermore, through morphometric analyses and morphofunctional comparisons, Gianechini et al . (2020) showed that the hindlimbs of unenlagiines could have performed cursorially even better than other dromaeosaurids, being similar to ground birds that feed on terrestrial prey, such as Cariama , implying that unenlagiines too could have foraged on the ground for terrestrial prey. Based on the faunal record of La Buitrera, Gianechini et al .…”

“…Based on the faunal record of La Buitrera, Gianechini et al . (2020) inferred that small tetrapods, such as sphenodonts, crocodyliforms and mammals, were the potential prey of Buitreraptor . However, Gianechini et al .…”

“…However, Gianechini et al . (2020) did not include in their analyses ground birds that forage for fish (e.g. ardeids, ciconiids), which could be appropriate analogues for possible piscivorous unenlagiines such as we describe here.…”

“…Still, Gianechini et al . (2020) recently proposed that unenlagiines were not mainly piscivorous ( contra Gianechini et al . 2011).…”