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

GIANECHINI, FEDERICO A.; ERCOLI, MARCOS DARÍO; DÍAZ MARTÍNEZ, IGNACIO

JOURNAL OF ANATOMY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2020 vol. 236 p. 772 - 797

0021-8782

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 determinemain morphofunctional appendicular properties, which ultimately reflect acompromise between different factors of the biological role. Dromaeosauridae is a dinosaurclade belonging to Theropoda, a group of bipedal predators. Dromaeosauridsrepresent an interesting study case, in which the hindlimbs have been proposed tobe involved in both locomotion and predation activity. A peculiar feature characterizingall dromaeosaurids is a modified second pedal digit, which is typically related topredation. This theropod group is closely related to birds and diversified during theCretaceous Period, mainly in the Northern Hemisphere (Laurasia). However, a subcladeof dromaeosaurids, the Unenlagiinae, was recently recognized for Gondwana.Nevertheless, there are morphological differences between derived Laurasiandromaeosaurids (eudromaeosaurs) and unenlagiines. Such differences are observedin the proportions between hindlimb bones and in the presence of a subarctometatarsaliancondition in unenlagiines, which is mainly characterized by a proximallyconstricted metatarsal III. To evaluate the function of these divergent morphologies,we conducted morphometric analyses and comparisons of qualitative morphologicalaspects, encompassing unenlagiines, other dromaeosaurids, as well as taxa fromother theropod groups, including extant birds. The former approach consisted of twophylogenetic principal component analyses, one based on the main measurementsof the hindlimb, and the other focused on the lengths of the pedal phalanges. Thefirst analysis drew the unenlagiines close to taxa with long tibiae, as well as long andslender metatarsi. Instead, eudromaeosaurs are closer to taxa with shorter tibiae, andshorter and wider metatarsi. The second analysis showed that eudromaeosaurs andunenlagiines have similar phalangeal proportions, including the elongation of distalphalanges. However, the shorter second phalanx of the pedal digit II of eudromaeosaurscould have increased the force generated by this digit, which was the mainpredatory tool of the autopodium. This, together with a shorter and wider metatarsus,and a marked hinge-like morphology of the articular surfaces of metatarsalsand phalanges, possibly allowed eudromaeosaurs to exert a great gripping strengthand hunt large prey. Conversely, the longer and slender subarctometatarsus, andless well-marked hinge joints of unenlagiines possibly gave them greater cursorialcapacities. Additionally, the longer second phalanx of digit II allowed unenlagiinesfast movements of this digit to hunt smaller and elusive prey. Thus, the distinctivemorphological evolutionary pathways of these two dromaeosaurid clades seem tohave been influenced by the particular locomotor and predatory specializations thatcharacterized each of these lineages.