A non-avian dinosaur with a streamlined body exhibits potential adaptations for swimming

Specialized diving traits in the generalist morphology of Fulica (Aves, Rallidae)

Foot-propelled diving comprises the primary locomotion-based feeding strategy for many birds, including families such as Phalacrocoracidae, Anhingidae, Podicipedidae, Gaviidae, and the diving ducks within Anatidae. While the morphology of specialized divers is well known, the corresponding morphology is less known for birds not as specialized but capable of diving, such as the coots (Rallidae, Fulica spp.). To compare the osteology of Fulica with other (non-diving) Rallidae, and with foot-propelled diving birds that are distantly related, we considered osteological characters, as well as the proportion of the hind limb bones and the femoral splay angle to construct a phylomorphospace, and to perform a comparative disparity analysis considering ecomorphologically relevant characters related to swimming and diving. Coots resulted to be significantly disparate from other Rallidae showing many traits of specialized foot-propelled divers, but only noticeable when compared with other rallids, as the degree of development of these traits is markedly less than in loons, grebes, or cormorants. This may correspond to a stabilizing selection of characteristics associated with a generalist morphology in Fulica. Studying adaptation in generalist taxa broadens our understanding of ecomorphologically significant features, thereby enabling us to generalize their evolutionary patterns.

Deinonychosaur trackways in southeastern China record a possible giant troodontid

The Longxiang tracksite (lower Upper Cretaceous, Shanghang Basin) includes twelve didactyl deinonychosaur tracks that fall into two morphologies, differentiated by both size and form. The smaller tracks (∼11 cm long) are referable to the ichnogenus Velociraptorichnus. The larger tracks (∼36 cm long) establish the ichnotaxon Fujianipus yingliangi. Based on the size of the tracks, F. yingliangi has an estimated hip height of over 1.8 m, a size comparable to that of the largest known deinonychosaurs, i.e., Austroraptor and Utahraptor. The reduced form of digit IV, relative to digit III, indicates that F. yingliangi is a probable troodontid. Gigantism evidently evolved independently at least four times within the Deinonychosauria and within at least three major lineages: the Eudromaeosauria, Unenlagiidae, and Troodontidae. In the mid-Cretaceous of Asia, the evolution of F. yingliangi overlapped with that of early large-bodied tyrannosauroids and with previously established large allosaurids (although the latter may have been in decline).

Morphological disparity and structural performance of the dromaeosaurid skull informs ecology and evolutionary history

Non-avialan theropod dinosaurs had diverse ecologies and varied skull morphologies. Previous studies of theropod cranial morphology mostly focused on higher-level taxa or characteristics associated with herbivory. To better understand morphological disparity and function within carnivorous theropod families, here we focus on the Dromaeosauridae, 'raptors' traditionally seen as agile carnivorous hunters.We applied 2D geometric morphometrics to quantify skull shape, performed mechanical advantage analysis to assess the efficiency of bite force transfer, and performed finite element analysis to examine strain distribution in the skull during biting. We find that dromaeosaurid skull morphology was less disparate than most non-avialan theropod groups. Their skulls show a continuum of form between those that are tall and short and those that are flat and long. We hypothesise that this narrower morphological disparity indicates developmental constraint on skull shape, as observed in some mammalian families. Mechanical advantage indicates that Dromaeosaurus albertensis and Deinonychus antirrhopus were adapted for relatively high bite forces, while Halszkaraptor escuilliei was adapted for high bite speed, and other dromaeosaurids for intermediate bite forces and speeds. Finite element analysis indicates regions of high strain are consistent within dromaeosaurid families but differ between them. Average strain levels do not follow any phylogenetic pattern, possibly due to ecological convergence between distantly-related taxa.Combining our new morphofunctional data with a re-evaluation of previous evidence, we find piscivorous reconstructions of Halszkaraptor escuilliei to be unlikely, and instead suggest an invertivorous diet and possible adaptations for feeding in murky water or other low-visibility conditions. We support Deinonychus antirrhopus as being adapted for taking large vertebrate prey, but we find that its skull is relatively less resistant to bite forces than other dromaeosaurids. Given the recovery of high bite force resistance for Velociraptor mongoliensis, which is believed to have regularly engaged in scavenging behaviour, we suggest that higher bite force resistance in a dromaeosaurid taxon may reflect a greater reliance on scavenging rather than fresh kills.Comparisons to the troodontid Gobivenator mongoliensis suggest that a gracile rostrum like that of Velociraptor mongoliensis is ancestral to their closest common ancestor (Deinonychosauria) and the robust rostra of Dromaeosaurus albertensis and Deinonychus antirrhopus are a derived condition. Gobivenator mongoliensis also displays a higher jaw mechanical advantage and lower resistance to bite force than the examined dromaeosaurids, but given the hypothesised ecological divergence of troodontids from dromaeosaurids it is unclear which group, if either, represents the ancestral condition. Future work extending sampling to troodontids would therefore be invaluable and provide much needed context to the origin of skull form and function in early birds. This study illustrates how skull shape and functional metrics can discern non-avialan theropod ecology at lower taxonomic levels and identify variants of carnivorous feeding.