Morphological and histological evidence for the oldest known softshell turtles from Japan

Abstract

Herein we report morphologically and histologically diagnostic trionychid specimens from the Barremian-Aptian (129.4-113.0 Ma) of Japan. One specimen (FPDM-V9487) is an associated skeleton that consists of a scapula, a humerus, an ischium, and hypoplastra. The limb and girdle bones are similar in morphology to those of modern trionychids.The hypoplastron also resembles that of modern trionychids; however, it is distinctive in that it completely lacks callosities.The other three new specimens are fragmentary costals that show a reduction in the shell bones and an absence of scute sulci.Two of the newly reported costals and FPDM-V0127 were examined histologically. Bone fiber bundles organized in a plywood-like structure, which is unique to trionychids, were identified. Fossil occurrence data indicate that morphologically and histologically typical trionychids already inhabited the coastal region of Asia (e.g., western Japan) as early as the Aptian. In contrast, the Hauterivian-Aptian stem trionychid Kappachelys okurai from Japan does not show a plywood-like shell microstructure, suggesting that K. okurai could be the earliest-branching taxon of known stem trionychids or a carettochelyid. Our paleobiogeographic compilation suggests that the spread of wetlands in the Northern Hemisphere and the high global temperature during the middle Cretaceous (Aptian-Turonian, 125.0-89.8 Ma) might have contributed to the dispersal of trionychids from Asia, allowing for the establishment of this modern freshwater reptilian fauna.

Turtle shell bone and osteoderm histology of Mesozoic and Cenozoic stem-trionychian Adocidae and Nanhsiungchelyidae (Cryptodira: Adocusia) from Central Asia, Mongolia, and North America

“…Nanhsiungchelyid and adocid turtle shell bones share with carettochelyids and trionychids the external bone sculpturing, the diploe structure of shell bones, the presence of interwoven structural fibres in the external cortex, and the presence of parallel-fibred bone in the internal cortex (see Sect. 3.3 and 3.4;Scheyer et al, , 2012Delfino et al, 2010;Nakajima et al, 2017).…”

“…Instead, the fibres extending towards the external bone surface appear to be only loosely organised. Developmentally, ZIN PH 11/101 could thus fall between Kappachelys okurai Hirayama et al, 2013 from the Kitadani Formation (Hauterivian-Aptian) of Japan, lacking keratinous scute cover but retaining a ring of peripherals similar to carettochelyids (but showing some histological affinities to crown-trionychids), and indeterminate trionychid material from the Akaiwa Formation (Barremian-Aptian) of Japan and "Trionyx" kyrgyzensis from the Almyshik Formation (lower-middle Albian) of Kyrgyzstan, both of which unambiguously show the complex fibre bundle quadrangle arrangement in their plywood-like structure (Hirayama et al, 2013;Nakajima et al, 2017).…”

The evolution of dermal shield vascularization in Testudinata and Pseudosuchia: phylogenetic constraints versus ecophysiological adaptations

“…Instead, they exchange blood gases with those dissolved in the surrounding water using pharyngeal, cloacal and cutaneous respiration [88]. Although gas exchanges are not possible through scales of keratin in sauropsids (including crocodylians and testudinatans), this mechanism is rendered possible in trionychids by the secondary loss of their superficial keratin layer [63]. It is worth mentioning that the use of cutaneous respiration in Trionychia correlates with a rare expression of shell apical ornamentation in testudines.…”

“…For both Pseudosuchia and Testudinata, time-scaled phylogenetic relationships of the sampled specimens were reconstructed in Mesquite [50] by relying on published references [5,41,42,[51][52][53][54][55][56][57][58][59][60][61][62][63] (figure 1). We then traced the evolution of the post-cranial dermal bone vascular area using the least-squared parsimony to calculate the ancestral states for each clade.…”

Kalasinemys, a new xinjiangchelyid turtle from the Late Jurassic of NE Thailand

“…2017). The youngest formation of the Tetori Group, the Kitadani Formation (Barremian-Aptian), has yielded four taxa of trionychoids (a trionychid (Nakajima et al 2017), two taxa of adocids and a nanhsiungchelyid) and two other eucryptodires (xinjiangchelyids and sinemydids, represented by large peripherals; Hirayama, pers. comm.…”