Shell bone histology of the pan-carettochelyid turtle Kizylkumemys schultzi from the Upper Cretaceous of Uzbekistan and shell bone morphology transformations in the evolution of pan-trionychian turtles

This paper presents histological data on the shell bones of different size (age) individuals of the basal pan-carettochelyid Kizylkumemys schultzi from the Upper Cretaceous (Cenomanian) of Usbekistan. Ontogenetic changes in the shell bone histology of Kizylkumemys schultzi correspond to those characteristic of other turtles. Comparison of the shell bone histology of Kizylkumemys schultzi with those of other pan-carettochelyids show similar histological structures, suggesting that this group is very conservative in the shell bone histology, which did not change significantly since the Cretaceous. Kizylkumemys schultzi and other pan-carettochelyids share prominent shell sculpture with all other pan-trionychians; shell sculpture of large pits and grooves and network of vertical and subvertical canals in the external cortex, with nanhsiungchelyids and pan-trionychids; reduced or absent scute sulci and lost or loose contact between peripherals and costals, with pan-trionychids; and loss of rib thickenings of costals, with adocids. According to new data, Kappachelys okurai from the Hauterivian-Barremian of Japan, previously considered as a pan-trionychid or pan-carettochelyid, could be either a pan-trionychian (sister taxon to Trionychia) or a pan-carettochelyid.