The cover highlights the mechanically self-organized pattern of scales on the head of a young Nile crocodile (Crocodylus niloticus). Typically, vertebrate skin appendages such as scales, hairs or feathers develop as genetically controlled units whose spatial patterning is dominated by a gene regulatory network of signalling molecules during embryo development. The patterning of scales on a crocodile’s head is an exception because it seems to emerge from a mechanical process whose precise nature and origin are unclear. In this week’s issue, Michel Milinkovitch and colleagues resolve this mystery. Working from Nile crocodile embryos, the researchers generated a 3D model of crocodile head patterning and found that the borders of the scales are skin folds that mechanically self-organize through compressive folding. This compressive stress results from the two layers of skin, which have different stiffnesses and grow faster than the underlying tissues.

The use of genetic ancestry to trace history and probe events of the past is challenging because ancestries in many locations are relatively similar, making it hard to distinguish groups and populations. In this week’s issue, Leo Speidel, Pontus Skoglund and colleagues present a new approach called Twigstats that allows subtle differences in ancestry to be reconstructed in high resolution. The researchers use their technique to examine the genomic history of early medieval Europe. This allowed them to track the expansion of two streams of Scandinavian-related ancestry across the continent, as well a later stream of ancestry expanding into Scandinavia before the Viking Age (around 750–1050). The cover is inspired by the serpentine carvings found on Viking Age runestones and features the Elder Futhark runes for the DNA nucleotides A, T, G and C (K).