148 / 2025-04-09 21:54:37

Ultrastructural evidence for reptilian integument in Psittacosaurus during the evolutionary transition to feathers

Psittacosaurus,integument,ultrastructure

Theme 3: Earth-Life Co-evolution Through Time > Session 3k: The Fossil Record and Evolution of the Tetrapod Integument

As a basal ceratopsid dinosaur exhibiting both feather-like integumentary structures and scaled epidermal derivatives, Psittacosaurus serves as a pivotal transitional model for elucidating the evolutionary mechanisms of dinosaurian integument and its derivatives. This study employed an integrated analytical approach, combining stereomicroscopy (UV illumination), field emission scanning electron microscopy and micro-CT three-dimensional reconstruction, to systematically investigate the morphological and ultrastructural characteristics of integumentary tissues preserved in a Psittacosaurus specimen (NJUES-10). The research focused on regional differentiation of epidermal organization across body regions, revealing a partitioned skin development pattern: non-feathered regions retained a reptilian-type integument, while feathered areas exhibited avian-like epidermal specializations.  

Analysis demonstrated that scales with marked regional heterogeneity dominate the integumentary derivatives of NJUES-10. In non-feathered regions, three-dimensional silicified replicas preserved complete epidermal strata, including the stratum corneum, corneocytes, and melanosomes. The exceptionally thin stratum corneum, characterized by beta-keratin dominance rather than the alpha-keratin system observed in avian feathers, suggests reduced mechanical protection requirements associated with a bipedal stance. Melanosome distribution patterns align with those of extant crocodilians, supporting ancestral melanin-based pigmentation mechanisms. The forelimb and thoracic regions feature a hierarchical protective system comprising central hexagonal macro-scales interlocked with peripheral triangular micro-scales, while the pelvic area exhibits a complex multi-layered structure.  

Ultrastructural evidence indicates crocodilian-type positive ontogenetic scaling in scute growth, with stratum corneum renewal via fragmentary exfoliation and absence of molting-related Oberhäutchen layers. Although feather-like structures were not identified—potentially due to ontogenetic immaturity or taphonomic bias—the partitioned integumentary development implies functional dichotomy: reptilian-type epidermis in non-feathered regions maintained essential cutaneous functions during early feather evolution, while feathered domains initiated avian-like specialization. Remarkable nanoscale preservation of melanosomes and keratin filaments confirms rapid postmortem burial and silicification as critical taphonomic processes.  

This study pioneers the comprehensive reconstruction of multi-regional integumentary systems in dinosaurs. By integrating multi-scale analytical methodologies, it establishes a novel framework for investigating epidermal evolution, providing critical insights into the vertebrate transition from scaled to feathered integumentary architectures.