GINGLYMODI

Uthumporn Deesri, Pratueng Jintasakul & Lionel Cavin, 2016, A new Ginglymodi (Actinopterygii, Holostei) from the Late Jurassic-Early Cretaceous of Thailand, with comments on the early diversification of Lepisosteiformes in Southeast Asia, Journal of Vertebrate Paleontology 36 (6), pp. 1-11 : 9-10

GINGLYMODI FROM THE PHU KRADUNG FORMATION

The Phu Kradung Formation can be divided into a lower and an upper part. The latter is sandier and shows a gradational conformable contact with the overlying Phra Wihan Formation. It corresponds mostly to meandering river environments under a probable two-season semiarid/humid climate, whereas the lower part corresponds to a lacustrine-dominated alluvial floodplain ( Mouret, 1994; Racey et al., 1996; Racey, 2009). However, a lower and an upper member have not yet been officially established, although the uppermost part was sometimes considered as a separate formation, the Waritchaphum Formation ( Mouret, 1994; Philippe et al., 2004). It consists of sandstone beds alternating with silty to sandy claystones, with pedogenic horizons sometimes reminiscent of the massive quartzitic sandstone body of the Phra Wihan Formation. Liard and Martin (2011) have examined the stratigraphy of seven vertebrate-bearing localities (i.e., Phu Nam Jun, Chong Chat, Phu Noi, Khok Sanam, Dan Luang, Dan Kerng, and Kham Phok) in the Phu Kradung Formation and located them in a general framework including the overlying Phra Wihan Formation. Within this chart, the Phu Nam Jun locality, which has yielded Thaiichthys buddhabutrensis and Isanichthys palustris , is located high in the formation, just below a sandstone bed from the Phra Wihan Formation. The Chong Chat locality, which has yielded numerous isolated remains of ginglymodians and a subcomplete specimen closely related to Thaiichthys ( Cavin et al., 2009), is located 10 m above to the last greenish-gray sandstone bed of the upper part of the Phu Kradung Formation and 10 to 20 m below the contact with the Phra Wihan Formation. The contact itself is not visible in that place due to rock falls and vegetation cover. The precise location of the Kham Phok site in this synthetic stratigraphy is difficult to establish due to the complex topographic structure of the area, with rock falls and heavy weathering that prevent any clear positioning. It is clearly located within the upper part of the Phu Kradung Formation, not more than 50 m below its upper limit. The Dan Luang and Dan Kerng sites are also difficult to locate precisely stratigraphically. Two vertebrate-bearing sites at least, Phu Noi (the type locality of Isanichthys lertboosi ) and Khok Sanam, are located far below the identified transition between the lower and upper parts and are likely candidates for a Late Jurassic age ( Liard and Martin, 2011). The stratigraphic location of the specimen from Ban Non Sao-Ae, described above, is uncertain, but the presence of outcrops of the Phu Kradung Formation and the thick sandy matrix containing the imprint lead us to assume that it comes from the upper part of Phu Kradung Formation.

So far, three different taxa of ginglymodians have been recorded in the Phu Kradung Formation ( Cavin et al., 2014), to which should be added Khoratichthys .

The abundance of semionotiform remains in the Phu Kradung Formation, as well as in the overlying formations of the Khorat Group constituted by freshwater deposits, is possibly due in part to taphonomic biases. Indeed, ganoid scales and thick dermal ossifications of the skull are more prone to fossilize than much more fragile bones of most other ray-finned fishes, in particular teleosts. However, the thorough researches on fossil fishes recently conducted in these formations, including systematic excavations and sieving of sediment, have thus far revealed no teleost remains, and only a handful of taxa belonging to other groups than to the ginglymodians (a Ptycholepis -like form in Khok Sanam; Cavin et al., 2009). These observations indicate that the high taxic diversity of ginglymodians and their dominance in the assemblages are not only artifactual, but real. This abundance is observed not in the sedimentary basin of the Indochina block, but also in the northwardly located Jurassic basin corresponding to modern Sichuan ( Murray et al., 2015). Although the high diversity cannot be regarded as the result of a single biological radiation, because the taxa that compose it belong to different lineages according to the available phylogenies (but most are basal lepisosteiforms), it is evidence that ginglymodians occupied a dominant position in freshwater ecosystems in that part of the world. This observation does not fit recent assumptions about the origination of cypriniforms ( Saitoh et al., 2011). These authors suggested, based on a mitochondrial genomic study, that the order Cypriniformes originated during in the Late Triassic, and that basal cypriniform divergences should have taken place during the Late Jurassic in southern Asia, excluding the Indian subcontinent. The deduced place and time found by Saitoh et al. (2011) correspond precisely to the age of deposition and geographical location of the Phu Kradung Formation. So far, fossil cypriniforms are unknown in Mesozoic sediments of Southeast Asia (and in the Mesozoic worldwide). The discrepancy between paleontological and molecular evidence might be caused by the incompleteness of the fossil record, by uncertainty in molecular dating, or both ( Saitoh et al., 2011). It has been suggested ( Cavin, 2010) that high taxic diversity of semionotiforms in the Early Cretaceous, in particular taxa from the Lepisosteiformes lineage such as Araripelepidotes ( Thies, 1996) and Thaiichthys ( Cavin et al., 2013), is comparable to the present-day diversification of cypriniforms. The fact that paleontological evidence shows a high taxic diversity of semionotiforms in the Late Jurassic–Early Cretaceous of Southeast Asia, whereas molecular evidence indicates at the same time and place a diversification of the cypriniforms, which share a similar ecological niche, is an intriguing issue to be addressed in future studies.

CONCLUSION

The occurrence of Khoratichthys gibbus in the Phu Kradung Formation is further evidence of the high diversity of freshwater lepisosteiforms in the Upper Jurassic and Lower Cretaceous of Thailand and, beyond, of southeastern mainland Asia, including South China. This new taxon is phylogenetically situated in a more basal position among lepisosteiforms than the other genera found in the same formation, i.e., Isanichthys and Thaiichthys. Our phylogenetic analysis indicates that Khoratichthys belongs to a freshwater lineage that is distinct from the main shift towards freshwater in the lepisosteid lineage. This main shift, which eventually led to the modern gar lineage, contains Isanichthys, Thaiichthys , and other extinct Lepisosteoidei (sensu López- Arbarello, 2012). However, the branches of this part of the cladogram are still poorly supported, and further analyses based on more complete material are necessary before we can provide a strong palaeoenvironmental scenario for the evolutionary origin of the lepisosteiforms.