The postembryonic development of the turtle carapace was studied in the aquatic Emys
orbicularis and the terrestrial Testudo graeca. Differences in the structure of the
bony shell in aquatic and terrestrial turtles were shown to be associated with varying
degrees of development of epidermal derivatives, namely, the thickness of the scutes
and the depth of horny furrows. Sinking of the horny furrows into the dermis causes
local changes in the structure of the collagen matrix, which might precondition the
acceleration of the ossification. Aquatic turtles possess a relatively thin horny
cover, whose derivatives are either weakly developed or altogether absent and thus
make no noticeable impact on the growth dynamics of bony plates. Carapace plates of
these turtles outgrow more or less evenly around the periphery, which results in uniform
costals, relatively narrow and partly reduced neurals, and broad peripherals extending
beyond the marginal scutes. In terrestrial turtles (Testudinidae), horny structures
are much more developed and exert a considerable impact on the growth of bony elements.
As a result, bony plates outgrow unevenly in the dermis, expanding fast in the zones
under the horny furrows and slowly outside these zones. This determines the basic
features of the testudinid carapace: alternately cuneate shape of costals, an alternation
of broad octagonal and narrow tetragonal neurals, and the limitation of the growth
of peripherals by pleuro-marginal furrows. The evolutionary significance of morphogenetic
and constructional differences in the turtle carapace, and the association of these
differences with the turtle habitats are discussed.
