MARTIN Michel (1), BUFFETAUT Eric (2),HAIYAN TONG (2) and SUTEETHORN VARADUDH (3)
1 – MUSEE d’HISTOIRE NATURELLE 115, Boulevard Eurvin 62200 – BOULOGNE SUR MER – FRANCE
2 – CENTRE NATIONAL de la RECHERCHE SCIENTIFIQUE (URA 1761) UNIVERSITE PARIS 6, Case 106, 4, Place Jussieu 75252 – PARIS CEDEX 05 – FRANCE
3 – GEOLOGICAL Survey Division Department of Mineral Resources Rama VI Road, BANGKOK – 10400 – THAILAND
Since 1981 The Mesozoic vertebrate assemblages from non-marine deposits of Thailand have provided new stratigraphical data. Moreover these discoveries provide interesting informations in terms of palaeobiogeo-graphical correlations (see a review in BUFFETAUT et al.1994).
GEOLOGICAL SETTING :
The fossil remains studied herein have been yielded by two different non marine formations. The first one in Southern peninsula of Thailand has yielded a small complete tooth plate in Mab Ching (Changwat Nakhon Si Thammarat Province). This section of alternating clays and limestone beds is supposed to be middle Jurassic in age on the basis of charophytes (BUFFETAUT et al 1994). The vertebrate fauna includes hybodont sharks, actinopterygian fishes, temnospondyl amphibians, turtles and crocodilians with in addition the tooth plate studied here. Recently a second more poorly preserved remain has been discovered in North Eastern Thailand in Ban Khok Sanam (Kalasin province). This locality in the Phu Kradung Formation has also yielded turtles, crocodilians and sauropod dinosaurs remains. This assemblage is suggestive of a Jurassic age. In addition the unnamed Formation visible in Mab ching could be correlated to the Phu Kradung Formation. But new biostratigraphical data will be necessary to provide a good basis for this correlation.
Subclass Dipnoi Muller 1844
One of us has recognized fives families in the order, Even on the basis of tooth plates (Martin 1982). However some recognized evolutive groups could be artificial and especially the group including Ptychoceratus phillipsi (Agassiz 1838) since on the basis of the holotype of this species and of new material from Provenchére Ptychoceratus philippsi would be closely related to P. rectangulus Link 1936 and consequently woulded be a true Ptychoceratus .
However the specific and generic determinations are always difficult for people who have not dealt with a good sample of tooth plates of the same species (more than 30 complete remains including different stages of growth) since an allometric growth is recorded in the tooth plates of some dipnoans (Kemp 1977, Martin 1984).
genus Ferganaceratodus Kaznyshkin and Nessov 1985
Material : one complete tooth plate from Mab Ching and an incomplete tooth plate from Ban Khok Sanam both remains being housed in the collection of the Departement of Mineral Resources, Bangkok.
The specimen discovered in Mab Ching is a rather small (BCP =11 mm) right upper tooth plate. It bears five sharp ridges which radiate from the tip of the inner angle. The first ridge is the shortest and the first notch is as broad as the second one. The inner angle reaches l00°. The first ridge is gently curved and the mesial edge reaches more than the 2/3 of the lingual edge (BC 1 / BCP = 75%). The other ridges are straight and serrated on their labial part. The pterygo palatine process arises above the second ridge and slightly behind it (fig.1)
Biometrical data : inner angle 100°, inner angle of the first ridge 74°, inner angle of the second ridge 47° inner angle of the third ridge 25°, BC2/BC1 100,3, BC1/BCP 75,3, CICP/BCP 103,0, C1C2/BC1 47,9 C1C2/C1Cp 34,3, C1CP/BCp 35,3, C2C3/C2CP 54,7.
The second specimen discovered in Ban Khok Sanam is longer (BCp =16 mm) but incomplete, the first ridge being broken off. This left lower toothplate possesses four clear ridges, there is a slight depression along the lingual margin which suggests the presence of a 5 th ridge but the absence of denticles on its labial part empedes to see in it a true ridge. It could be only the result of the wear against the left last upper ridge during the occlusion. The inner angle reaches 115° and this tooth plate was probably less stout than the former remain (fig. 2) Biometrical data : inner angle of the second ridge 52°, inner angle of the third ridge 25° , C2C3/C2CP / 61,5 The indices are given in percentage.
The old and erroneous referal to Ceratodus Agassiz 1838 will be used for different species herein but it does not mean that we think that it is true. In conclusion the probable generic determination will be provided.
One of us referred to C. cf. Szechuanensis a minute upper tooth plate (fig. 3) from the late Norian of the Korat plateau in North Eastern Thailand (Martin and Ingavat 1982 )
Morphologically this remain is very similar to the tooth plate from Mab Ching. From the biometrical point of view the tooth plates TF 1440-4 was different from the small triassic tooth plates described previously but morphologically similar to some of them.
When compared to the tooth plate from Mab Ching TF 1440-4 is very similar, only the index C2C3 / C2CP could be different but again the standard deviation of this index recorded in European Ptychoceratodontidae can explain this difference.
The incomplete tooth plate from Ban Khok Sanam is not different from the lower tooth plate of C. Szechuanensis for example V 230 (Liu and Yeh 1957) or from a junior synonyme of this species C. youngi Liu and Yeh 1957 (fig. 5 and 8). The curved ridges present in the chinese tooth plates are meaningless since this variation is recorded in a large sample of P. phillipsi(Martin et al. 1981). At the specific level it seems that the chinese Triassic and Jurassic tooth plates referred to C. szechuanensis and the Triassic and Jurassic tooth plates from Thailand are similar Especially the remain from Mab Ching fits V 2345 very well (fig. 6).
Recently Krupina ( l994) has described three new species from the upper Jurassic of Mongolia Especially C. sharategensis Krupina 1994 resembles both the thai specimen from Ban Khok Sanam and C. szechuanensis. But on the basis of biometrical data provided by Krupina these species are different.
Krupina has described C. Temporatus Krupina 1994 (fig. 13) and C. porrectus Krupina 1994 (fig.15) the latter species being probably a junior synonyme of the former one on the basis of range of variation recorded in the population of C. madagascariensis Priem 1924 well illustrated by Priem ( 1924) and Dechaseaux (1949). Krupina has well recognized the relationships between these Dipnoans and the Jurassic and Cretaceous dipnoans from North America recently revised and described by Kirkland. Unfortunately this author has described a chronocline leading to a ceratodus morphotype from a Ptychoceratodus morphotype meanwhile a single population from the same locality (Majunga) in Malagasy provides the same range of variation.
For this reason the validity of different new species erected by Kirkland must be discuss but it is not the topic of this paper. This group of Ptychoceratodontidae which does not exibit a clear tip of the inner angle on large tooth plates is probably closely related to C. concinnusPlieninger 1844 (Martin 1982). In this species it is always possible to distinguish C. kaupi like specimens from tooth plates of true C. kaupi Agassiz 1838 (Martin 1980). But some tooth plates of C. concinnus (fig.11) resemble very much C. youngi which is a junior synonyme ofC. szechuanensis.
THE GENERIC DETERMINATION :
FerganaCeratodus jurassicus Kaznyshkin and Nessov 1985 is described on the basis of tooth plates and isolated bones of the cranial roof (fig. 4, 7). It has been discovered in the Callovian of Kirghizistan and without the cranial element the remains would have been refered to Ceratodus (Nessov 1981 in litteris). The tooth plates are very interesting since they recall booth C. concinnus and C. Szechuanensis but the very deep first notch recorded in the callovian species empedes to see in it a synonyme of these species. The biometrical data are also different. C. sharategensis is also similar to F. jurassicus but the biometrical data are completely different. However these species are probably closely related. For these reason they are referred to Ferganaceratodus Kaznyshkin and Nessov 1985. The group of Ptychoceratodontidae without clear tips of inner angle on the large worn tooth plate is closely related to Ferganaceratodus.
PALAEOGEOGRAPHICAL and STRATIGRAPHICAL CORRELATIONS :
From the stratigraphical point of view the species similar to the Thai specimens are recorded during the triassic and the jurassic, for this reason the outcrops are probably Jurassic in age both in Mab Ching and in Ban Khok Sanam.
From the paleogeographical point of view the remains strengthen our previous conclusions (Buffetaut et al.1994). During Jurassic times the peninsular Thailand had faunal affinities with other East Asian Continental blocks and especially the chinese one. The Dipnoans from Thailand contradict palaeogeographical reconstructions wich imply a very late accretion of the south East Asian with the rest of Asia. (for exemple Tarling 1988) since they have close relationships when they are not similar to Triassic and Jurassic freshwater dipnoans from the rest of Asia.
These faunal affinities are in good agreement with reconstructions suggested by Besse and Courtillot ( 1988). According to these authors the South East Asian blocks were already in contact with South China during Jurassic Times.
Ferganaceratodus szechuanensis is present in Thailand from the late Triassic till the Middle to late Jurassic. This species is recorded in China by late Triassic to Jurassic.
Closely related species are recorded in Mongolia with F. sharategensis (late Jurassic) and in Kirghizistan with F. jurassicus (Callovian).
C. concinus from the European late Triassic is also refered to the genus Ferganaceratodus. These geographical distribution suggests accretion of the Indochina block to South China as soon as the late Triassic and contact of the South East Asian blocks with South China at least during Jurassic Times. The dipnoans are indicative of a Jurassic age in Mab Ching and Ban Khok Sanam localities.
The material has been collected during field trip supported by the Department of Mineral Resources Bangkok and the French Ministry of Foreign Affairs.
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