Evolution of dinosaur epidermal structures

Abstract

Spectacularly preserved non-avian dinosaurs with integumentary filaments/feathers have revolutionized dinosaur studies and fostered the suggestion that the dinosaur common ancestor possessed complex integumentary structures homologous to feathers. This hypothesis has major implications for interpreting dinosaur biology, but has not been tested rigorously. Using a comprehensive database of dinosaur skin traces, we apply maximum-likelihood methods to reconstruct the phylogenetic distribution of epidermal structures and interpret their evolutionary history. Most of these analyses find no compelling evidence for the appearance of protofeathers in the dinosaur common ancestor and scales are usually recovered as the plesiomorphic state, but results are sensitive to the outgroup condition in pterosaurs. Rare occurrences of ornithischian filamentous integument might represent independent acquisitions of novel epidermal structures that are not homologous with theropod feathers.

Keywords: Dinosauria; feathers; integument; phylogeny; scales.

Figure 1.

Ancestral state reconstructions for ornithodiran integument. Ancestral states based on unordered character states across the ‘equal’ time-scaled tree (a–b): pterosaurs coded as (a) plesiomorphically scaled and (b) filament-covered. (c) Range of probabilities for each state at particular nodes based on all permutations described in the text. Percentages reflect the relative support for a given state with respect to the others. (Online version in colour.)