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| Adocus kohaku Hirayama, Sonoda, Uno, Horie, Tsutsumi, Sasaki, Mitsuzuka & Takisawa, 2021 Illustration: Oda Takashi facebook.com/studiocorvo |
Abstract
A nearly complete shell of the genus Adocus (Adocidae; Pan-Trionychia; Cryptodira; Testudines) was collected from the late Cretaceous (Turonian) Tamagawa Formation of Kuji Group at Kuji City, Iwate Prefecture, northeast Japan. This turtle shows unique features such as the loss of cervical scute, extreme expansion of marginal scutes overlying costal plates, and exclusion of the humeral- pectoral sulcus from entoplastron. Thus, Adocus kohaku is erected as a new species. As A. kohaku shows most derived position of A. kohaku within this genus, morphological diversity of the genus Adocus seems to have occurred rather early in its evolution in Eastern Asia.
Keywords: Adocus kohaku sp. nov.; Cretaceous; Kuji Group; Reptilia; Testudines; U-Pb Dating
Systematic Paleontology
Testudines Batsch, 1788
Cryptodira Cope, 1868
Pan-Trionychia Hummel, 1929
Adocidae Cope, 1870
Type Genus. Adocus Cope, 1868
Known Distribution. Early Cretaceous to Eocene of Asia and Late Cretaceous to Paleocene of North America.
Genus Adocus Cope, 1868
Type Species. Adocus beatus (Leidy, 1865)
Known Distribution. Early Cretaceous to Eocene of Asia and
Late Cretaceous to Paleocene of North America.
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| : Turtle shell of Adocus kohaku sp. nov., holotype, KAM 01. A, carapace in dorsal view; B, plastron in ventral view; C, plastron in dorsal view. Red lines show scute sulci on shell surface. |
Adocus kohaku, New Species
Diagnosis. Adocus with cervical scute completely lost; fifth to tenth marginal scutes medially enlarged, overlying at least distal half of second to seventh costal plates; three pairs of inframarginals.
Etymology. Named after Japanese name of amber as this fossil was associated with numerous ambers.
Conclusion:
A nearly complete shell of the genus Adocus (Adocidae;
Trionychia; Cryptodira; Testudines) was collected from the
late Cretaceous (Turonian) Tamagawa Formation of Kuji
Group at Kuji city, Iwate Prefecture, northeast Japan. This
turtle shows unique features such as the loss of cervical scute,
extreme expansion of marginal scutes overlying costal plates,
exclusion of the humero-pectoral sulcus from entoplastron,
and three pairs of inframarginals. Thus, A. kohaku is proposed as a new species. Additional remains of A. kohaku suggest
this species had carapace length of at least a 60 cm long,
the largest among Adocus in Asia. As A. kohaku is a derived
taxon within this genus, its occurrence in the early Late
Cretaceous suggests early development of morphological
diversifications within this genus, as previously proposed for
the Nanhsiungchelyidae. Occurrences of both most basal (A.
sengokuensis) and most derived (A. kohaku) taxa from Japan
implies eastern Asia was the center of diversification of this
clade.
Ren Hirayama, Teppei Sonoda, Hikaru Uno, Kenji Horie, Yukiyasu Tsutsumi, Kazuhisa Sasaki, Shunsuke Mitsuzuka and Toshio Takisawa. 2021. Adocus kohaku, A New Species of Aquatic Turtle (Testudines: Cryptodira: Adocidae) from the Late Cretaceous of Kuji, Iwate Prefecture, Northeast Japan, with Special References to the Geological Age of the Tamagawa Formation (Kuji Group). International Journal of Paleobiology & Paleontology. medwinpublishers.com/IJPBP
