Supernumerary scutes verify a segment-dependent model of the horny shell development in turtles

Turtle horny shell has a scute pattern, which is conservative through evolution and across species. The discovery of epidermal placodes as the scute primordia and their strict topographical association to the somites of the turtle embryo suggested a new interpretation of the developmental mechanism of the scute pattern. Here, we tested the hypothesis that horny scutes develop from a mosaic of placodes corresponding exactly to the paths of myoseptae, with vertebral and pleural scutes developing staggered in adjacent segments, and marginal scutes developing in every segment. This scheme predicts little variation in marginals and suggests intercalary supernumerary scutes as potential variations for the vertebral and pleural rows. We examined spatial and numerical variations of the horny shell in 655 newly hatched olive ridley sea turtle, Lepidochelys olivacea, which is known to have a highly variable horny shell. In total, 120 patterns of carapacial scutes and 10 patterns of scutes on plastron, differing in the number and position of scutes were found. The number of vertebral scutes varied from 4 to 10. Variations with five, six and seven vertebrals occurred with the greatest and nearly equally frequency (31.5% on average). Pleural scutes were from 5 to 10 at one or both sides, and the typical symmetric pattern for sea turtles with five pairs of pleurals was only seen in ca. 12% of specimens. In contrast, the majority of the specimens (92.7%) had just 13 pairs of marginals, showing a stable normal pattern. Similarly, on plastron the horny scutes were conservative, too; about 85% of specimens standardly had six pairs of plastral scutes and all specimens had four pairs of inframarginals. Despite a high level of variation of vertebral and pleural scutes in olive ridley turtle, all patterns fall into the theoretical spectrum of possible variants predicted by the segment-dependent model of development of the turtle horny shell. Therefore, the results of our analysis support the existence of direct morphogenetic correlation between the number and distribution of normal and supernumerary scutes and metamere organization of the turtle embryo.