Age of the Early Triassic Ichthyopterygian Thaisaurus Inferred from Ammonoid Biostratigraphy

Early Triassic ichthyopterygian fossils from the Russian Far East

“…1 ). Some previous studies have noted that Utatsusaurus ranges only within the Subcolumbites zone and not in the Columbites zone 2 , 5 , partly because of the misleading definition of Columbites and Subcolumbites subzones within the Subcolumbites zone sensu Bando and Shimoyama 58 , which is clearly not equivalent to the currently accepted middle Spathian Subcolumbites zone. To avoid confusion, we refer to the Subcolumbites zone sensu Bando and Shimoyama (1974) as the “ Columbites – Subcolumbites zone” in this study.…”

“…As noted above, the vertebral size of the oldest known ichthyopterygian from Tirolites beds reaches ~ 3.5 cm in diameter 3 , which presumably corresponds to the largest vertebra of a ~ 3-m-long ichthyopterygian. Other lower Spathian ichthyopterygians are generally small; Chaohusaurus is approximately 1 m long and smaller than the largest known contemporary predatory fish 2 , 5 . Thaisaurus chonglakmanii is known from a single specimen TF 2454, consisting of an almost complete skull and incomplete postcranial skeleton, and is close to C. geishanensis in size and shape 4 , 37 .…”

“…Thaisaurus chonglakmanii is known from a single specimen TF 2454, consisting of an almost complete skull and incomplete postcranial skeleton, and is close to C. geishanensis in size and shape 4 , 37 . The cranial length of T. chonglakmanii is ~ 10 cm, and the estimated body length is 53–84 cm based on the skull/body proportion in embryonic and adult C. geishanensis 4 , 5 . Although the growth stage of the T. chonglakmanii holotype has not been estimated precisely, the ossification stage of carpal bones in the only specimen is close to that of adult C. geishanensis 4 , 63 .…”

“…The oldest known ichthyopterygian material is a vertebral centrum from Tirolites ammonoid biozone (lower lower Spathian, ~ 3 myr after the P–T boundary) in NoName locality of northeastern Nevada 3 . Thaisaurus chonglakmanii , a small ichthyopterygian from the Phukhaothong Dolomite Member of the Chaiburi Formation in the Phatthalung area, southern Thailand 4 , is one of the oldest named taxon of Mesozoic marine reptiles; the Thaisaurus horizon belongs to the middle lower Spathian 5 (Fig. 1 ), which is slightly younger than NoName locality centrum.…”

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Figure 1 Ammonoid biostratigraphy of Spathian (Olenekian, Lower Triassic) ichthyopterygian and possible ichthyopterygian fossil localities. Ammonoid biostratigraphic data were compiled after Ji et al 7 for Chaohu (South China); Tongtherm et al 5 for Phatthalung (Thailand); Zakharov et al 20 , 22 and Shigeta and Kumagae 73 for South Primorye (Russia); Bando and Shimoyama 59 , Ehiro et al 61 , Shigeta and Nakajima 29 , and Shigeta 60 for Kitakami (Northeast Japan); Hurum et al 11 and Hansen et al . 12 for Svalbard (Norway); Guex et al 56 and Brayard et al 55 for the Western United States; Tozer 57 for British Columbia (Canada).

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