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

Abstract

Abstract. The record of fossil turtles from the Cretaceous and Cenozoic of Asia and North America is very rich, including several lineages of cryptodiran turtles. Here we survey the shell bone histology of two important closely related groups of stem trionychians, the Adocidae and Nanhsiungchelyidae, which have representatives in both Asia and North America. All studied taxa show shell bones in which the diploe is framed by well-developed cortical compact bone layers. Taxa of both groups also express external regular surface sculpturing of their shell bones, and in the case of the nanhsiungchelyid genus Basilemys also on the osteoderms, which is also reflected in the internal histological bone structures. Besides similarities of the regular ornamentation patterns, both groups share a number of microanatomical and histological characters such as the zonation of external cortex with rather homogeneous fine-fibred interwoven structural fibres (ISF) in the more internal zone and a dominance of vertically oriented fibres in the ISF and the presence of growth marks in the more external zone. On the other hand, growth marks, i.e. lines of arrested growth, which are visible as wavy lines in thin sections, extend subparallel to the external bone surface in adocids, but they are not parallel/subparallel in nanhsiungchelyids. Thickness and structure of bone trabeculae in the cancellous interior regions depends on the shell bone thickness of the individual samples. The internal cortices of all taxa except the North American samples of Adocus usually consist of parallel-fibred bone that locally grades into lamellar bone. Secondary bone remodelling is more frequent in nanhsiungchelyids compared to adocids, and Sharpey's fibres that extend perpendicular to the bone margins extending across subparallel growth marks are more commonly found in adocids. In addition, bone histology served to identify trionychid specimens in the adocid and nanhsiungchelyid samples, especially as bone surface sculpturing patterns were weathered or eroded in those cases. The histological data present thus supplement the numerous previously reported differences in external shell morphology between adocids and nanhsiungchelyids on the one hand and trionychids and carettochelyids on the other.

A new microvertebrate assemblage from the Mussentuchit Member, Cedar Mountain Formation: insights into the paleobiodiversity and paleobiogeography of early Late Cretaceous ecosystems in western North America

“…5H – 5J ). The identification of these specimens as adocid is supported by similarities of internal microstructure ( Scheyer, Syromyatnikova & Danilov, 2017 ). All samples possess thick, densely packed external and internal cortical bone with well-developed interior cancellous bone ( Fig.…”

Osteohistology and microanatomy of a new specimen of Cearachelys placidoi (Testudines: Pleurodira) a side‐necked turtle from the lower Cretaceous of Brazil

“…Our findings corroborate the exoskeletal origin of the plastron, based on the metaplastic incorporation of dermal interwoven structural fiber bundles in xiphiplastron (Scheyer & Sander, 2007). Plastral plates of fossil and extant cryptodires and the Triassic Testudinata Waluchelys cavitesta Sterli, Martínez, Cerda and Apaldetti 2020 demonstrated similar process of metaplastic woven bone formation (Scheyer & Anquetin, 2008; Scheyer & Sander, 2007; Scheyer, Syromyatnikova, & Danilov, 2017; Sterli, Martínez, Cerda, & Apaldetti, 2020).…”

“…However, plastral plates of Phrynops are more vascularised than xiphiplastron of Cearachelys (Figure 7 and Table S1). On the other hand, plastral plates of Cretaceous chelid Linderochelys rinconensis and cryptodires (both extinct and extant) exhibit a slower growth rate, characterized by parallel‐fibered bone which suggests a more conservative pattern in shell osteohistology (Danilov, Obraztsova, Boitsova, & Skutschas, 2018; Jannello, Maniel, Previtera, & dela Fuente, 2018; Scheyer, Syromyatnikova, & Danilov, 2017; Scheyer & Sander, 2007; Skutschas et al, 2020; Skutschas, Boitsova, Cherepanov, & Danilov, 2017). An additional and unique aspect reported in C .…”

An early bothremydid from the Arlington Archosaur Site of Texas

“…The predominance of woven bone suggests fast skeletal growth, a derived characteristic www.nature.com/scientificreports/ seen in C. placidoi and the extant pleurodiran genus Phrynops 46,47 . In contrast, extinct and extant cryptodires and the Cretaceous chelid Linderochelys rinconensis exhibit greater parallel-fibered bone, characteristic of a slower growth rate and a more conservative osteohistological pattern 46,[48][49][50][51][52][53] .…”