Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features

Abstract

Troodontids are important towards our understanding of avian origins because they are considered as the closest relatives of birds, either on their own or together with dromaeosaurids. Here we report a new troodontid Jianianhualong tengi gen. et sp. nov. from the Lower Cretaceous Jehol Group of China, that has anatomical features that are transitional between long-armed basal troodontids and derived short-armed ones, shedding new light on troodontid character evolution. This taxon displays a mosaic of plesiomorphic and apomorphic features with a distinct spatial organization, like Sinusonasus, another troodontid with transitional anatomical features. Jianianhualong has forelimbs and a pelvis closely resembling those of basal troodontids, but a cranium and hindlimbs that are more similar to those of derived troodontids. Sinusonasus has a cranium that closely resembles those of basal rather than derived troodontids and a pelvis and hindlimbs that are more similar to those of derived troodontids than to basal ones. This new information helps to develop a foundation for investigating modular evolution in this clade, a phenomenon known in birds. Several recent phylogenetic studies have questioned the troodontid affinities of Anchiornis, Xiaotingia and Eosinopteryx making Jianianhualong the only unequivocal troodontid with preserved feathering. This indicates that troodontid feathering is similar to Archaeopteryx in having large arm and leg feathers as well as frond-like tail feathering, confirming that these feathering characteristics were widely present among basal paravians. Most significantly, this troodontid has asymmetrical feathers in the form of long, narrow and square-tipped tail feathers. Parsimony-based ancestral state reconstruction of paravian feather symmetry suggests that asymmetrical arm feathers were ancestral to Paraves, whilst asymmetrical tail feathers were ancestral to a more inclusive paravian clade that excludes Scansoriopterygidae and Avialae. Asymmetrical feathers have been associated with flight capability, but are also found in species that do not fly. Among non-avialan theropods, they are also known in microraptorine dromaeosaurids and have been linked to gliding behaviours. The asymmetrical tail feathers of Jianianhualong presumably kept the frond stable in an airflow, whilst its ‘slotted’ and distally-expanded shape presumably aided drag reduction. Biomechanical modeling and more fossil discoveries will be important towards uncovering the function of early asymmetrical feathers.