A patterning system that has been shown to play a role in bird feather development is also apparent in the development of sharks' tooth-like skin, new theoretical and experimental evidence finds. This mechanism, say the authors, has likely controlled the progression of epithelial appendages - external structures like hair, feathers, scales, spines and teeth - for at least 450 million years, a timeframe that spans the evolution of vertebrates. From the scales of a snake to the feathers of a flamingo, modern vertebrates showcase an array of epithelial appendages. These structures all possess similar developmental positioning in relation to one another because they grow from a common foundation - the epithelial placode. Scientist Alan Turing famously developed what is now referred to as the reaction-diffusion (RD) model, which explains nature's diverse patterning as seen in modern animals, studies have proposed. To date, however, while research has found support for RD patterning in four-legged animals, a role for this system in earlier-diverging lineages is not clear. Rory L. Cooper and colleagues studied the small-spotted catshark - from the ancient shark family - at about 80 days post-fertilization using RD modeling and gene expression analysis. RD modeling showed that dorsal denticle rows acted as "initiator" rows, triggering the patterning of surrounding tooth-like skin. The authors then compared the patterning of shark denticles to chick feathers (in which RD modeling is at play) by examining the expression of the protein β-catenin (β-cat), an early regulator of chick epithelial placode signaling. The authors determined a similarity between shark and chick epithelial appendage patterning; the shark lateral line expressed β-cat soon before denticle patterning began, a comparable timeline to feather patterning. Cooper et al. additionally used the RD model to explain the diversity of denticle patterning in other ancient elasmobranchs (the thornback skate and the little skate). Within the elasmobranchs group, this system may have aided the evolution of various denticle functions, such as protective armor, hydrodynamic drag reduction, feeding and communication, say the authors.