Beta-keratin

Beta-keratin (β-keratin) is a member of a structural protein family found in the epidermis of reptiles and birds. Beta-keratins were named so because they are components of epidermal stratum corneum rich in stacked beta sheets, in contrast to alpha-keratins, intermediate-filament proteins also found in stratum corneum and rich in alpha helices. Because the accurate use of the term keratin is limited to the alpha-keratins, the term "beta-keratins" in recent works is replaced by "corneous beta-proteins" or "keratin-associated beta-proteins."

β-keratins add much more rigidity to reptilian skin than alpha-keratins alone do to mammalian skin. β-keratins are impregnated into the stratum corneum of the reptilian skin, providing waterproofing and the prevention of desiccation.

The scales, beaks, claws and feathers of birds contain β-keratin of the avian family. Phylogenetic studies of β-keratin sequences show that feather β-keratins evolved from scale β-keratins. The scale β-keratins form the basal group in avians. Duplication and divergence events then led to claw β-keratin genes, and further recombination resulted in new feather and feather-like avian β-keratin genes. Evidence for these duplication events comes from the correlation of feather β-keratin clade structure with their genomic loci.

Changes in β-keratins may have also influenced the development of powered flight. A recent study using molecular dating methods to link the evolution of avian β-keratin genes in general to that of feathers specifically reveals that the avian β-keratin family began diverging from the crocodile family about 216 million years ago. It also found that the feather β-keratin family did not begin diverging until 125 million years ago, a date consistent with the adaptive radiation of birds during the Cretaceous. β-keratins found in modern feathers have increased elasticity, a factor that may have contributed to their role in flight. Thus, feathered relatives of birds such as Anchiornis and Archaeopteryx, whose flight capabilities have been questioned, would have had avian, but not feather, β-keratins.

The small alvarezsaurid dinosaur Shuvuuia deserti shows evidence of a featherlike skin covering. Analysis by Schweitzer et al. (1999) showed that these featherlike structures consisted of beta-keratin. This has since been refuted by Saitta et al., finding that the fibers analyzed instead consisted of inorganic calcium phosphate as evidenced by fluorescence under cross polarised light. Signals from immunohistochemical analyses on fossil samples are prone to false positives and must be used with caution when dealing with geological samples.