Turtle shell bone and osteoderm histology of Mesozoic and Cenozoic stem-trionychian Adocidae and Nanhsiungchelyidae (Cryptodira: Adocusia) from Central Asia, Mongolia, and North America
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 ha...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Pensoft Publishers
2017-02-01
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| Series: | Fossil Record |
| Online Access: | http://www.foss-rec.net/20/69/2017/fr-20-69-2017.pdf |
| Summary: | 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 <i>Basilemys</i> 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 <i>Adocus</i> 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. |
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| ISSN: | 2193-0066 2193-0074 |