First Cretaceous Fish Fauna from Malaysia

Journal of Vertebrate Paleontology

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First Cretaceous fish fauna from Malaysia

Yu He Teng, Masatoshi Sone, Ren Hirayama, Masataka Yoshida, Toshifumi Komatsu, Suchada Khamha & Gilles Cuny

To cite this article: Yu He Teng, Masatoshi Sone, Ren Hirayama, Masataka Yoshida, Toshifumi Komatsu, Suchada Khamha & Gilles Cuny (2019): First Cretaceous fish fauna from Malaysia, Journal of Vertebrate Paleontology, DOI: 10.1080/02724634.2019.1573735 To link to this article: https://doi.org/10.1080/02724634.2019.1573735

Published online: 01 May 2019.

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ARTICLE

FIRST CRETACEOUS FISH FAUNA FROM MALAYSIA

YU HE TENG,1 MASATOSHI SONE, *,1 REN HIRAYAMA, 2 MASATAKA YOSHIDA,2,3 TOSHIFUMI KOMATSU,4 SUCHADA KHAMHA,5 and GILLES CUNY 6 1Department of Geology, University of Malaya, Kuala Lumpur 50603, Malaysia, [email protected]; 2School of International Liberal Studies, Waseda University, Nishiwaseda 1-6-1, Shinjuku, Tokyo 169-8050, Japan; 3Graduate School of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8654, Japan; 4Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan; 5Palaeontological Research and Education Centre, Mahasarakham University, Khamrieng, Kantharawichai, Maha Sarakham 44150, Thailand; 6Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France

ABSTRACT—Cretaceous fish fossils are reported from Malaysia for the first time. They were found with dinosaur and turtle remains in nonmarine sediments in the interior of Pahang State, Peninsular Malaysia. This fish assemblage consists mostly of isolated teeth, with minor amounts of noncranial remains, including vertebral centra, scales, coprolites, and hybodont fin spines. Over 100 fish teeth were examined. Nine taxa were confirmed from this fish assemblage: six (identified and unidentified) species of hybodont sharks, Heteroptychodus kokutensis, Isanodus paladeji, Lonchidion aff. khoratensis, Mukdahanodus aff. trisivakulii, Egertonodus sp., and Hybodontidae indet., and three species of actinopterygians (ray-finned fishes): ‘Lepidotes’ sp., Halecomorphi indet., and Ginglymodi indet. This fish assemblage has strong affinities with Early Cretaceous (Barremian–early Aptian) faunas of Thailand, because the four hybodont species, H. kokutensis, I. paladeji, L. khoratensis, and M. trisivakulii, were previously known only from the Sao Khua Formation (Khorat Group) and equivalent strata of Ko Kut (Kut Island). Egertonodus has been confirmed in Asia for the first time. Overall, this fish assemblage shows a close linkage to Early Cretaceous (Barremian–early Aptian) faunas of Thailand. Based on faunal composition and biostratigraphic correlation, we suggest a Barremian–early Aptian age for this new fauna from Malaysia.

Citation for this article: Teng, Y. H., M. Sone, R. Hirayama, M. Yoshida, T. Komatsu, S. Khamha, and G. Cuny. 2019. First Cretaceous ﬁsh fauna from Malaysia. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2019.1573735.

INTRODUCTION GEOLOGICAL SETTING For the past 20 years, our knowledge of Cretaceous freshwater The fossil-bearing unit is informally referred to as ‘the Pahang fishes in Southeast Asia has been based mainly on finds from the vertebrate bed’ in this report (Fig. 1). The unit consists of thick Khorat Basin in northeast Thailand. Abundant isolated fish teeth fine-grained sandstones at the base, fining upward into medium- and scales have been reported from the Khorat Basin (e.g., grained sandstone-siltstone laminations. Small-scale tabular Cappetta et al., 1990, 2006; Cuny et al., 2003, 2006, 2008, 2009, cross-bedding is common in some sandstone beds. The presence 2014; Khamha et al., 2016). On the other hand, no information of dinosaurs, terrestrial turtles, and some plant fossils in the unit has been available from the other Cretaceous nonmarine basins suggests that the depositional site might have been near to a ter- in the region. restrial environment where these organisms lived. The absence of In 2014, the University of Malaya’s paleontological team marine fossils suggests that the depositional environment had reported the first discovery of Cretaceous nonmarine vertebrate little to no marine influence. Based on the Early Cretaceous fossils from the state of Pahang, Peninsular Malaysia. This age of the Pahang vertebrate bed, we consider that it is correlated included the first dinosaurs (spinosaurid and ornithischian) to the Early Cretaceous part (Temus Shale) of the Tembeling from Malaysia (Sone et al., 2015). Together with the dinosaurs, Group. The depositional environment of the Tembeling Group abundant fossils of freshwater fish and terrestrial turtle remains is generally accepted to be fluvial-lacustrine (Koopmans, 1968; were recovered, including hybodont sharks. This paper aims to Khoo, 1977; Ahmad et al., 1991; Shamsudin and Morley, 1994; systematically study this fish assemblage and to interpret its Ainul et al., 2005). likely age based on biostratigraphic correlation. Traditionally, most Mesozoic nonmarine sediments in Malaysia Institutional Abbreviations—PRC, Palaeontological Research were dated using plant macrofossils, palynomorphs, and/or fresh- and Education Centre, Mahasarakham University, Maha Sarakham, water bivalves, for example, the plant Gleichenoides gagauensis, Thailand; TF, Thai Fossil specimens, Sirindhorn Museum, Kalasin, the bivalve Trigonioides sp. (Koopmans, 1968), and spores and Thailand; UM, Department of Geology, University of Malaya, pollen of Clavatipollenites sp., Classopollis sp., and Circulina sp. Kuala Lumpur, Malaysia. (Khoo, 1977; Shamsudin and Morley, 1994; Ainul et al., 2005; Uyop et al., 2007). These earlier studies suggested an Early Cre- taceous age for those nonmarine Mesozoic sediments in Malaysia. Due to our agreement with Pahang State Government to *Corresponding author. protect the fossiliferous site, the exact location of the sampling Color versions of one or more of the figures in the article can be found outcrop cannot be provided in this report. The Global Positioning online at www.tandfonline.com/ujvp. System (GPS) information of the location is kept at the

Published online 01 May 2019 Teng et al.—First Cretaceous ﬁsh fauna from Malaysia (e1573735-2)

FIGURE 1. A, map showing the distribution of Jurassic–Cretaceous nonmarine sediments in Southeast Asia. Information combined from Lovatt Smith et al. (1996), Yunus (2014), and Hinthong (1999). B, sedimentary log of the fossil locality with the position of the reported materials.

University of Malaya. Any scientific inquiry concerning this fossil magnification of ×1–2 aided by a teleconverter. The lighting deposit can be referred to the Department of Geology, University sources used were a Nikon fiber optic illuminator and a Nikon of Malaya, Kuala Lumpur, or to the Pahang State Government, ring light. To take images at different depths of field, step size Kuantan, or directly to any of the authors. was determined by focusing the top and bottom of the specimens. The gaps were then divided into an average of eight steps. The raw images were aligned and stacked using graphic design soft- MATERIALS AND METHODS ware program CombineZP. The fossil fish assemblage from the Pahang vertebrate bed con- Another method was used for imaging smaller teeth with sists mostly of isolated teeth, many of which show signs of lengths less than 5 mm. A field emission scanning electron micro- abrasion. This implies that they were likely transported for scope (FESEM) was used for taking photomicrographs of the some distance prior to deposition. Rare fish scales, coprolites, specimens. The acceleration voltage used was 15.0 kV, and the hybodont fin spines, and osteichthyan (bony fish) vertebral working distance used ranged from 12 to 27.7 mm. Specimens centra also occur in this deposit. were pre-coated with gold by using a Leica EM SCD005 sputter All fossils were obtained from the field by surface collecting coater. and bulk sampling. Fossil preparation was performed chiefly using a mechanical engraver, knives, and sharp-pointed tools under a stereomicroscope. Wherever necessary, both liquid and SYSTEMATIC PALEONTOLOGY gel Paraloid B-72 thermoplastic resin and cyanoacrylate glue were applied to prevent fragmentation. Screen-sieving was Phylum CHORDATA Haeckel, 1874 attempted, but it was not an effective method to free the fossils Class CHONDRICHTHYES Huxley, 1880 from the strongly cemented sandstone. More than 100 isolated Subclass ELASMOBRANCHII Bonaparte, 1838 fish teeth from both hybodont sharks and actinopterygians (ray- Order HYBODONTIFORMES Patterson, 1966 finned fishes) were examined for this study. Family HYBODONTIDAE Owen, 1846 For imaging fish teeth, we used two different methods depend- HYBODONTIDAE indet. ing on the specimen size. For specimens larger than 0.5 mm in (Fig. 3A, B) length, a focus stacking technique was applied. Images at different depths of field were taken and then were combined to produce a focus-stacked image in which the whole specimen is Hybodus sp. Cuny, Laojumpon, Cheychiw, and Lauprasert, in focus (Brecko et al., 2014). We used a Nikon D300 digital 2010:416, fig. 2a–c. single-lens reflex camera with a resolution of 12 megapixels. It was equipped with a 105-mm focal length macro lens, with a Material—UM10644, an isolated tooth, without root. Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-3)

FIGURE 2. A–C, Egertonodus sp. from Pahang, Peninsular Malaysia. A, UM10642, posterior tooth in labial view; B, C, UM10643, anterior tooth in B, lingual and C, mesial/distal views. D, E, tooth TF7662 reported as ‘Hybodus’ sp. A from the Sao Khua Formation in D, lingual and E, mesial (or distal) views. Images in A–C produced using focus stack technique; D, E reproduced from Cuny et al. (2006:ﬁg. 2c–d).

Description—UM10644 has a conical cusp that is proportionally from Malaysia and Ko Kut are probably conspecific. Overall, short with a blunt apex. The crown is compressed labiolingually. the limited characteristics present in the incomplete Malaysian The tooth measures 2 mm labiolingually, 4 mm mesiodistally, and specimen hinder identification at a more refined level. is 3 mm high. Strong longitudinal ridges are equally well developed over all facets of the tooth, and some of them reach the apex. The Genus EGERTONODUS Maisey, 1987 lower part of the tooth is not preserved, and it is unclear whether the specimen is part of a multicuspid tooth. Remarks—UM10644 is well differentiated from Acrodus by Type Species—Hybodus basanus Egerton, 1845. not having a transverse crest across the occlusal face of the crown. The longitudinal ridges in UM10644 are not ramified EGERTONODUS sp. like those in Acrodus described by Cappetta (1987, 2012). The (Fig. 2A–C) blunt conical cusp is similar to ‘Hybodus’ aequitridentatus Cuny, Suteethorn, Khamha, and Buffetaut, 2008, and both of them have longitudinal ridges that reach the apex. Nevertheless, ‘Hybodus’ sp. A Cuny, Suteethorn, Khamha, Buffetaut, and Phi- UM10644 is well distinguishable from ‘H.’ aequitridentatus by lippe, 2006:23, fig. 2c–e. having nonanastomosing ridges and a compressed labiolingual outline. Material—UM10642, UM10643, UM10689 –10694, eight iso- On the other hand, UM10644 strongly resembles the large lated teeth. crown PRC19 from Ko Kut reported as Hybodus sp. by Cuny Description—The crown is high and conical in shape, with a fi – et al. (2010b: g. 2a c). They share a low, blunt cusp with ridges pointed apex. The height of the crown ranges from 2 to 6 mm, reaching the crown apex. They have similar strong ridges, and although in most available specimens the apex is either worn or the ridges are quite straight on the lingual side. Although the damaged. The lower two-thirds of the lingual face is ornamented mesiodistal length of the Malaysian specimen is half of PRC19, by numerous longitudinal ridges. These ridges are strong and their labiolingual length and crown height are comparable (Fig. more or less regularly spaced but do not attain the apex. 3). A minor difference is that the Ko Kut specimen possesses Cutting edges are present on both the mesial and distal sides of small ridges at the base of the labial face. This feature was not the cusp. The posterior tooth, UM10642, develops three small observable in the Malaysian specimen due to its incomplete pres- cusplets flanking the main cusp, two on the mesial side and one ervation. Based on their shared characteristics, the specimens on the distal side (Fig. 2A). The anterior tooth, UM10643, has a narrow, high cusp with a sigmoidal outline in mesial or distal view (Fig. 2B, C), but the presence of cusplets is unclear. The longitudinal ridges are weaker in the anterior tooth compared with those in the posterior tooth. Remarks—The Malaysian teeth are superficially similar to those of Hybodus. However, the presence of a sigmoidally curved main cusp, with high and slender lateral cusplets and strong cutting edges on both the mesial and distal sides of the cusp and cusplets, reflects characteristics of the genus Egertono- dus defined by Rees and Underwood (2008). These characteristics are shared by Egertonodus basanus (Egerton, 1845) and fi FIGURE 3. A, B, Hybodontidae indet. from Pahang, Peninsular Malay- E. duf ni Rees and Underwood, 2008. The difference between sia. UM10644, tooth of indeterminate position in A, lingual and B, these two species and the Malaysia specimen is that all teeth mesial/distal views. C, tooth PRC19 from Ko Kut reported as Hybodus of both E. basanus and E. duffini are multicuspid, but the sp. in lingual view. Images in A, B produced using focus stack technique; anterior teeth in the Malaysian specimens show only one high C reproduced from Cuny et al. (2010b:fig. 2a2). cusp (Fig. 2B). This may be a result of incomplete preservation, Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-4) and it is unknown whether the anterior teeth have a cusplet. without ornamentation. A prominent transverse crest extends Nevertheless, the present teeth can be confidently assigned from the mesial extremity to the distal one. The crest is sharp to Egertonodus, and this is the first confirmation of the genus and smooth. A well-developed narrow peg is present on the in Asia. labial side of the crown, although a crack at this level prevents The ‘Hybodus’ sp. A reported from the Sao Khua Formation seeing its exact shape and notably whether its mesial and distal (Cuny et al., 2006:fig. 2c–e) also has a sigmoidal outline, short margins are parallel. It is situated on the lower part of the ridges on the lingual face and strong cutting edges on both crown, well below the transverse crest. At the same level as the mesial and distal sides, suggesting that it is probably a species labial peg, there is a small protuberance at the distal part of of Egertonodus, not ‘Hybodus.’ We compared the Malaysian the crown. There is no ascending ridge. The lingual face of the specimens and the Thai ‘Hybodus’ sp. A (see Fig. 2). Besides crown is steep, whereas the labial face is wider and gently des- sharing a similar outline, they are comparable in size: the avail- cending to the base of the crown. able specimens from both faunas have similar heights of up to Remarks—Based on the diagnosis by Rees and Underwood 6 mm. The ridges on the lingual face of both specimens share (2002), the Malaysian specimen is assigned to the genus Lonchi- similar patterns, namely, they cover more than half of the dion by having a well-developed labial peg, a mesiodistally wider length of the crown and do not anastomose. Due to incomplete than high crown and a prominent transverse crest running mesio- preservation, we are unable to tell whether the ridges on the distally. Its morphology is comparable to that of the type species labial face of the Malaysian specimens are shorter than those Lonchidion selachos Estes, 1964. Both of them are compressed on their lingual face, a characteristic shown by the Thai labiolingually, with a strong transverse crest and have no orna- ‘Hybodus’ sp. A. However, we can be certain that the ridges on mentation on their crown. The Malaysian specimen can be separ- the labial face of the Malaysian teeth are strong and equally ated from L. selachos by not having a main cusp at the center of spaced, and this pattern is also demonstrated by ‘Hybodus’ the crest, and it has no lateral cusplets on the mesial and distal sp. A. Based on their shared characteristics, this Thai form is con- ends of the crown. The crown of the Malaysian specimen is also sidered closely related to the Malaysian Egertonodus sp. More not as gracile as that of L. selachos figured in Rees and Under- complete specimens are needed to investigate their relationship. wood (2002:fig. 2a–c). Compared with Lonchidion breve Patter- son, 1966, the Malaysian specimen shares with the latter a Family LONCHIDIIDAE Herman, 1977 smooth crown and no cusplet. However, L. breve has a low Genus LONCHIDION Estes, 1964 cusp with an ascending ridge connecting to the labial peg. This characteristic is common in other species of Lonchidion, such as Lonchidion humblei Murry, 1981, and Lonchidion striatum Pat- Type Species—Lonchidion selachos Estes, 1964. terson, 1966, but this is absent from the Malaysian specimen. The characteristic of a sharp crest without a cusp recalls Lonch- LONCHIDION aff. KHORATENSIS Cuny, Suteethorn, idion khoratensis Cuny, Suteethorn, Kamha, Buffetaut, and Phi- Khamha, Buffetaut, and Philippe, 2006 lippe, 2006. Lonchidion khoratensis also displays a smooth (Fig. 4A–C) ornamentation and no ascending ridge on the labial peg. These shared characteristics suggest that the Malaysian specimen is Material—UM10641, an isolated tooth. closely related to L. khoratensis. However, unlike the Malaysian Description—UM10641 measures 2 mm mesiodistally and specimen, L. khoratensis does not possess any accessory node on 0.9 mm labiolingually. The crown is asymmetrical where the the distal side of the labial peg. The Malaysian specimen hence distal end is tapering and the mesial end is blunter. It is smooth possibly represents a different species. More material is necessary to explore this possibility.

Family incertae sedis Genus HETEROPTYCHODUS Yabe and Obata, 1930

Type Species—Heteroptychodus steinmanni Yabe and Obata, 1930. Remarks—The genus Heteroptychodus is endemic to Asia. It is thus far known from the Early Cretaceous of Japan (e.g., Yabe and Obata, 1930; Tanimoto and Tanaka, 1998; Ohara and Yamada, 2005; Okazaki, 2016), Mongolia, and possibly Kyrgyz- stan (Mertiniene and Nessov, 1991; Nessov, 1997 ), south China (Mo et al., 2015), Thailand (e.g., Cappetta et al., 2006; Cuny et al., 2006, 2008, 2010b), and Malaysia (this study). Three species are known for the genus, viz., Heteroptychodus steinmanni Yabe and Obata, 1930, from Japan, Heteroptychodus chuvalovi (Nessov, Glückman, and Mertiniene in Mertineine and Nessov, 1991) from Mongolia (originally ascribed to the genus Asiadon- tus, transferred to Heteropthychodus by Cuny et al., 2008), and Heteroptychodus kokutensis Cuny, Laojumpon, Cheychiw, and Lauprasert, 2010, from Thailand. In addition to Heteroptychodus, other Cretaceous genera also develop numerous surface ridges that are mesiodistally oriented, for example, Ptychodus Agassiz, 1837, and Acrorhizodus Cap- FIGURE 4. A–C, Lonchidion aff. khoratensis from Pahang, Peninsular Malaysia. A–C, UM10641, tooth of indeterminate position in A, occlusal, petta, Buffetaut, Cuny, and Suteethorn, 2006. Ptychodus can be B, lingual, and C, labial views. D–F, TF7668, holotype of Lonchidion khor- readily differentiated from Heteroptychodus by having no second- atensis from Thailand in D, lingual, E, occlusal, and F, labial views. Images ary ridges and a less elongate parallelogram outline. In addition, a in A–C produced using focus stacking technique; D–F reproduced from recent study of its enameloid microstructure shows that Ptycho- Cuny et al. (2006:ﬁg. 2j–l). dus is unlikely to be a hybodont and is instead a neoselachian Teng et al.—First Cretaceous ﬁsh fauna from Malaysia (e1573735-5)

(Hoffman et al., 2016). Acrorhizodus has teeth with labiolingually related to each other than to the type species H. steinmanni.It oriented ridges displaying a radiating pattern, unlike also implies that the current diagnosis of H. chuvalovi originally Heteroptychodus. provided by Nessov (1997) can not clearly distinguish the species from H. kokutensis. Nevertheless, based on the currently HETEROPTYCHODUS KOKUTENSIS Cuny, Laojumpon, limited information on H. chuvalovi, it is not possible to deter- Cheychiw, and Lauprasert, 2010 mine the exact relationship between the Thai-Malaysian and (Fig. 5) the Mongolian species. A revision of the latter species is strongly recommended. Among all the Malaysian specimens, UM10649 (Fig. 5K–L) is Heteroptychodus steinmanni Yabe and Obata, 1930: Cuny et al., distinctively narrower labiolingually compared with the other fi 2007:351, g. 1 only. specimens. A bulge is situated near its mesial third, indicating Heteroptychodus kokutensis Cuny, Laojumpon, Cheychiw, and that the tooth is an anterior tooth. It possesses a surface ornamen- fi – Lauprasert, 2010b:419, g. 3a l (original description). tation that is characteristic of H. kokutensis, although the secondary ridges are weakly developed. Therefore, UM10649 may represent a juvenile or immature tooth of H. kokutensis. Material—UM10645–UM10650, UM10727–UM10755, 35 isolated teeth, without roots. Genus ISANODUS Cuny, Suteethorn, Khamha, Buffetaut, Description—These teeth are characterized by a parallelo- and Philippe, 2006 gram-shaped outline that is narrow labiolingually. Their size ranges from 3 to 8 mm labiolingually and from 6 to 16 mm mesio- Type Species—Isanodus paladeji Cuny, Suteethorn, Khamha, distally. The tooth crown is generally low, and its surface is orna- Buffetaut, and Philippe, 2006. mented with 4 to 20 transverse ridges that extend from the mesial Remarks—Isanodus was previously known only from the Sao to the distal extremity. Numerous short secondary ridges orig- Khua Formation and its equivalent strata of Ko Kut in Thailand inate from the lingual side of each transverse ridge. Some trans- (Cuny et al., 2006, 2010b; Khamha et al., 2016). Only two verse ridges tend to anastomose. Both parallel and species are known for the genus, viz., Isanodus paladeji Cuny, anastomosing ridges can be recognized within UM10650 (Fig. Suteethorn, Khamha, Buffetaut, and Philippe, 2006, and Isanodus 5M–O). The labial surface is slightly concave in mesial and nongbualamphuensis Khamha, Cuny, and Lauprasert, 2015(‘2016’). distal views, and it is perpendicular to the occlusal surface, Isanodus was originally placed in the family Lonchidiidae whereas the occlusal surface overhangs its base at the lingual Herman, 1977, because its teeth exhibit a low cusp, a shallow side of the crown. The anterior teeth UM10645, UM10646, and root, a well-developed labial protuberance, and a moderately to UM10647 possess a labiolingually oriented bulge near their strongly developed occlusal crest (Cuny et al., 2006). This South- mesial third. They have kinked transverse ridges, forming chev- east Asian genus was considered close to Lissodus nodosus (Sei- rons across the bulge (Fig. 5A–I). The tips of the chevrons are lacher, 1943) as described by Duffin(1985), in terms of their connected by secondary ridges. The lateral tooth UM10650 has similar heterodonty and the presence of labial nodes (Cuny a nonbulged convex surface, but some weakly developed chev- et al., 2006). Lissodus was excluded from the Lonchidiidae by rons are still present on the mesial third of its crown (Fig. 5M–O). Rees (2008) because the genus has a crown morphology and a Specimen UM10649 displays an elongate outline with a taper- heterodonty pattern that are notably different from other lonch- ing mesial end (Fig. 5K–L). It is smaller than the other specimens idiid genera. In like manner, Cuny et al. (2010b) preferred to at 2 mm labiolingually and 6 mm mesiodistally. Up to seven trans- remove Isanodus from the Lonchidiidae. The familial status of verse ridges are present on the occlusal surface of UM10649, and Isanodus remains indeterminate (incertae sedis). the secondary ridges are weakly developed or absent. Isanodus was interpreted as showing a heterodonty pattern Discussion—UM10645–UM10650 and UM10727–UM10755 made of four types of teeth, namely, anterior, anterolateral, pos- are assigned to Heteroptychodus because they display the diag- terolateral, and posterior teeth (Cuny et al., 2006; Khamha et al., nostic surface ornamentation of the genus. Numerous mesiodis- 2016). However, such a dental arrangement for Isanodus is still tally oriented transverse ridges are coupled with secondary hypothetical. ridges. Among all the known species of Heteroptychodus, UM10645– ISANODUS PALADEJI Cuny, Suteethorn, Khamha, Buffe- UM10650 and UM10727–UM10755 appear most similar to taut, and Philippe, 2006 those of Heteroptychodus kokutensis from Ko Kut, as illustrated (Fig. 6) by Cuny et al. (2010b :fig. 3a–l), with a low crown and mesiodistally elongate outlines with rounded corners. A direct comparison between the Ko Kut form and Malaysian form (Fig. 5R) also Isanodus paladeji Cuny, Suteethorn, Khamha, Buffetaut, and Phi- fi – reveals that they both have similar transverse ridges that tend lippe, 2006:26. g. 3a o (original description). to anastomose. We therefore refer UM10645–UM10650 and Isanodus paladeji Cuny, Suteethorn, Khamha, Buffetaut, and Phi- fi – UM10727–UM10755 to H. kokutensis. lippe, 2006: Khamha et al., 2016:5, g. 2a l. One distinctive feature of the anterior teeth UM10646 and UM10647 is the kinked transverse ridges, which form chevrons on top of the bulge. This feature is shared by the teeth of Hetero- Material—UM10651–UM10654, UM10714–UM10726, 17 iso- pthychodus chuvalovi from the Aptian of Mongolia (= Asiadontus lated teeth, none with the root preserved. chuvalovi in Nessov, 1997, reproduced in Cuny et al., 2008:fig. 4f Description—The crown has a highly arched crown base both −i) and is clearly absent in the teeth of H. steinmanni from the labially and lingually. The crown base is elongate mesiodistally younger Khok Kruat Formation (illustrated by Cappetta et al., (Fig. 6A–I), with a length of between 2 and 8 mm. The distal 2006; see also Cuny et al., 2008). Upon close examination of the end is tapered, and it tends to arch labially. A well-developed Thai material by the authors, such chevrons also have been cusp is located at the center of the crown. One longitudinal found on the anterior teeth of H. kokutensis stored in the PRC, crest extends from the mesial to the distal end across the crown. although this feature was not mentioned in Cuny et al. (2010b). Up to four ridges are developed parallel to the longitudinal The presence of chevrons in both H. kokutensis and crest; these ridges are developed on the lingual side of the cusp, H. chuvalovi suggests that these species are more closely becoming shorter toward the base of the crown. The ridges are Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-6)

FIGURE 5. A–O, Heteroptychodus kokutensis Cuny, Laojumpon, Cheychiw, and Lauprasert, 2010, from Pahang, Peninsular Malaysia. A–C, UM10645, anterior tooth in A, occlusal, B, labial, and C, lingual views. D–F, UM10646, anterior tooth in D, occlusal, E, labial, and F, lingual views. G–I, anterior tooth UM10647 in G, occlusal, H, lingual, and I, labial views; J, UM10648, anterior tooth in occlusal view. K, L, UM10649, possible juvenile/immature tooth in K, occlusal and L, lingual views. M–O, UM10650, lateral tooth in M, occlusal, N, lingual, and O, labial views. P–R, non-Malaysian materials for comparison. SDRC14, tooth reported as Heteroptychodus steinmanni Yabe and Obata, 1930, from the Khok Kruat Formation, Thailand, in P, occlusal and Q, lingual views; images reproduced from Cappetta et al. (2006:ﬁg. 6i–j) with permission from Wiley. R, PRC8, holotype of H. kokutensis from Ko Kut, Thailand, in occlusal view; image reproduced from Cuny et al. (2010b:ﬁg. 2i1) with permission from Elsevier. Images in A–O produced using focus stacking technique.

kinked and are ‘V’-shaped at the level of the cusp. Some short Besides having the mesiodistal transverse ridges at the lingual secondary ridges perpendicular to the transverse ridges are side of the longitudinal crest, its surface is ornamented by honey- present in certain teeth (e.g., UM10651 and UM10653). Labial comb-like ornamentation (Fig. 6A–C). Morphotype B pegs are generally poorly developed. (UM10652) has an outline similar to Morphotype A, except Three morphotypes are deﬁned. Morphotype A (UM10651) that it is more elongated mesiodistally than Morphotype has a high and pyramidal cusp, whereas labial nodes are absent. A. Additionally, its surface is smooth apart from having Teng et al.—First Cretaceous ﬁsh fauna from Malaysia (e1573735-7)

FIGURE 6. Isanodus paladeji Cuny, Suteethorn, Khamha, Buffetaut, and Philippe, 2006, from Pahang, Peninsular Malaysia. A–C, UM10651, tooth of Morphotype A in A, lingual, B, occlusal, and C, labial views. D–F, UM10652, tooth of Morphotype B in D, lingual, E, occlusal, F, labial views. G–I, UM10653, tooth of Morphotype C in G, lingual, H, occlusal, and I, labial views. J–L, UM10654, tooth of Morphotype C in J, lingual, K, occlusal, and L, labial views. Images produced by FESEM.

mesiodistal transverse ridges (Fig. 6D–F). Morphotype C ‘anterolateral’ and ‘posterolateral’ tooth types, and Morphotype (UM10653 and UM10654) has a low cusp and up to 10 well-devel- B is referable to the ‘posterior’ tooth type. The ‘anterolateral’ oped labial nodes. These labial nodes point downward at the base tooth was interpreted as having a more well-developed cusp as of the crown (Fig. 6G–L). compared with the ‘posterolateral’ tooth (Cuny et al., 2006). Remarks—As outlined above, three different morphotypes of However, this difference is not clearly reflected by the Malaysian Isanodus teeth are present in the Malaysian assemblage, specimens, because the specimens (e.g., UM10654) that are seem- suggesting a strongly heterodont dentition pattern for the ingly similar to the ‘posterolateral’ tooth often have an abraded genus. This observation is in agreement with Cuny et al. (2006), occlusal surface (Fig. 6J–L). On the other hand, specimen who suggested the presence of considerable heterodonty in Isa- UM10652 of Morphotype A is similar to the holotype, TF7674, nodus. Cuny et al. (2006)defined four types of teeth in the type of Isanodus paladeji in Cuny et al. (2006). These specimens series. Three of them, namely, the ‘anterolateral’ tooth, the ‘pos- share a high crown, an asymmetrical outline with arching of the terolateral’ tooth, and the ‘posterior’ tooth, are also recognized in distal end, and transverse ridges decreasing in length toward the Malaysian assemblage. Morphotype C is referable to both the lingual side of the crown. The Malaysian Isanodus is therefore Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-8) confidently assigned to I. paladeji. Nevertheless, the absence of an ‘anterior’ tooth for I. paladeji in the Malaysian assemblage invites revision of the diagnosis and heterodonty pattern of I. paladeji. Recently, another species of Isanodus, I. nongbualamphuensis, was proposed by Khamha et al. (2016) based on teeth from the Sao Khua Formation of Phu Phan Thong, northeast Thailand. Isa- nodus nongbualamphuensis is defined by having stronger labiolingually oriented ridges and better-developed labial nodes than I. paladeji. It also has a bulky outline, whereas I. paladeji has a narrow and elongated outline (Khamha et al., 2016). However, this diagnosis of I. nongbualamphuensis was derived mostly from incomplete specimens that were available from Phu Phan Thong. The more complete teeth of Isanodus from Malaysia now reveal that the diagnosis of I. nongbualamphuensis is inac- curate. First, the claim that I. nongbualamphuensis has a bulkier outline is based on specimens that have broken mesial or distal parts. The most complete specimens from Malaysia show that the outline of Isanodus teeth is generally elongate. Second, the Malaysian specimens show a range of weak to strong labiolingually oriented ridges. This indicates that the strength of the ornamentation might be part of the pattern of heterodonty in I. paladeji,orit could be a result of surface abrasion. In either case, it should not be used as a diagnostic character. Hence, I. nongbualamphuensis is not readily distinguishable from I. paladeji, and the proposal of the new species I. nongbualamphuensis is not justified.

Genus MUKDAHANODUS Cuny, Cavin, and Suteethorn, 2009 FIGURE 7. Mukdahanodus aff. trisivakulii from Pahang, Malaysia, and Mukdahanodus trisivakulii Cuny, Cavin, and Suteethorn, 2009, from Muk- Type Species—Mukdahanodus trisivakulii Cuny, Cavin, and dahan, Thailand, for comparison. A–C, Mukdahanodus aff. trisivakulii. Suteethorn, 2009, by monotypy. UM10655, tooth of indeterminate position, with A, occlusal cutting edge in close-up view and B, entire tooth in labial view. C, UM10656, tooth of indeterminate position in labial view. D, M. trisivakulii, MUKDAHANODUS aff. TRISIVAKULII Cuny, Cavin, and A–C Suteethorn, 2009 TF9041, holotype, in labial view. Images in produced using focus stacking technique; D reproduced from Cuny et al. (2009:ﬁg. 2a). (Fig. 7)

Material—Two nearly complete isolated teeth without their Malaysian teeth is much coarser than the one in the Thai speci- root, UM10655 and UM10656. mens. In the Thai specimens, a large notch is commonly devel- Description—The teeth are mesiodistally elongate and labio- oped in the middle of the occlusal cutting edge, interrupting the lingually compressed, giving a blade-like shape. Specimens serration, as seen in the holotype, TF9041 (Fig. 7D; reproduced UM10655 and UM10656 measure 10.5 and 11 mm mesiodistally, from Cuny et al., 2009:fig. 2a). By comparison, the Malaysian respectively, and both are 2 mm in height. Both the mesial and teeth have weakly developed notches (Fig. 7A). These differences distal extremities of the crown have one well-developed lateral suggest that the Malaysian teeth probably belong to a new species cusp that is directed almost horizontally. Up to five accessory cus- of the genus Mukdahanodus. plets are located beneath one of the lateral cusps, becoming smaller toward the base of the crown (Fig. 7B). The mesiodistally Class OSTEICHTHYES Huxley, 1880 oriented cutting edge is ornamented with strong, irregularly Subclass ACTINOPTERYGII Klein, 1885 spaced serration (Fig. 7A). A weakly developed notch is located Superdivision HOLOSTEI Müller, 1844, sensu Grande, 2010 near the center of the cutting edge. The presumed lateral face is Division GINGLYMODI Cope, 1872, sensu Grande, 2010 smooth, without ornamentation. Remarks—Mukdahanodus trisivakulii was previously known GINGLYMODI indet. only by isolated teeth from the Sao Khua Formation in Mukda- (Fig. 8) han Province, northeast Thailand (Cuny et al., 2009). It is the only species of the genus so far known. The Malaysian teeth share similarities with the Thai type specimens illustrated in Indeterminate actinopterygian tooth in Cuny et al. 2006:fig. 2a, b. Cuny et al. (2009:fig. 4) in having a serrated cutting edge on the Semionotiformes indet. Cuny et al. 2010b:417, fig. 2k1, l1. occlusal face, a low crown, an almost horizontally directed lateral cusp at the mesial and distal extremities, and small acces- Material—UM10688, a complete tooth. sory cusplets below the lateral cusp. Description—This tooth has an elliptical outline in occlusal Nevertheless, some differences are noted between the Thai and view. It measures 2.5 mm mesiodistally, 4 mm labiolingually, and Malaysian teeth. The Thai teeth were described by Cuny et al. is 1 mm high. The margin of the apical face is crenulated. A (2009) as possessing up to four accessory cusplets, whereas the blunt-tipped papilla is located near the center of the crown. The Malaysian teeth possess up to five accessory cusplets. The Thai marginal area surrounding the papilla is flat and wide and orna- specimens display a double serration pattern—small-scale serra- mented by some irregular granules. tions occur within the larger-scale serration, along the occlusal Remarks—This actinopterygian tooth is peculiar, because cutting edge (T.Y.H., pers. observ.), whereas the Malaysian speci- only a few similar forms were known previously from the Barre- mens do not have this double serration. Also, the serration in the mian–Aptian of Thailand and Tunisia. Specimen UM10688 is Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-9)

FIGURE 8. A, B, Ginglymodi indet. from Pahang, Peninsular Malaysia. UM10688, tooth in A, occlusal and B, labial/lingual views. C, D, Semionoti- formes indet. from Ko Kut, Thailand. Tooth in C, occlusal and D, labial/lingual views; images reproduced from Cuny et al. (2010b:fig. 2k1, l1) with permission from Elsevier. E, F, indeterminate actinopterygian from the Sao Khua Formation. Tooth in E, occlusal and F, labial/lingual views; images reproduced from Cuny et al. (2006:fig. 2a, b) with permission from Taylor & Francis. G, Semionotidae indet. from Tunisia, tooth in occlusal view. H, Semionotidae indet. from Tunisia, tooth in labial/lingual views; images reproduced from Cuny et al. (2010a:fig. 2.11–2.12) with permission from Elsevier. Images in A and B produced using focus stacking technique.

comparable to the Thai teeth that were reported as an indeterminate actinopterygian from the Sao Khua Formation (Cuny et al., 2006:fig. 2a, b) and Semionotiformes indet. from Ko Kut (Cuny et al., 2010b:fig. 2k–o). Fifteen teeth were reported from Ko Kut, all with irregular ridges radiating from the center of the crown toward the edge (Cuny et al., 2010b). All of these teeth share a similar morphology, a coarse edge and a papilla at the center of the crown. The Thai teeth are considered to be conspecific with UM10688. Although possessing a similar radiating pattern, the ridges of the Sao Khua teeth (Cuny et al., 2006) and UM10688 are coarser than those of the Ko Kut teeth, forming granules instead of fine ridges (Fig. 8). Other teeth reported as Semionotidae indet. by Cuny et al. (2010a:fig. 2.11, 2.12) from the Douiret Formation (Aptian) of Tunisia are also comparable, although they tend to have a thicker disc than the Southeast Asian teeth. FIGURE 9. ‘Lepidotes’ sp. from Pahang, Peninsular Malaysia. A–C, Genus LEPIDOTES Agassiz, 1832 UM10657, tooth in A, labial, B, mesial, and C, lingual views. D, E, UM10658, tooth in D, occlusal and E, mesial views. Images produced Type Species—Lepidotes gigas Agassiz, 1832. using focus stacking technique.

‘LEPIDOTES’ sp. Lepidotes, isolated teeth alone are never sufﬁcient to characterize (Fig. 9) Lepidotes due to their generalized characteristics. Accordingly, the assignment to Lepidotes is tentative.

Material—UM10657, UM10658, UM10695–UM10701, nine Division HALECOMORPHI Cope, 1872, sensu Patterson, isolated teeth. 1973 Remarks—By convention, these button-shaped crushing teeth from Cretaceous freshwater deposits are often referred to as HALECOMORPHI indet. either Lepidotes or ‘semionotids’ in Thailand (Cavin et al., (Fig. 10) 2009; Cuny et al., 2010b) and North America (Garrison et al., 2007; Brinkman et al., 2013). Specimens UM10657, UM10658, and UM10695–UM10701 are similar to the Thai specimens Material—UM10659, UM10660, UM10702–UM10713, 14 iso- reported as Lepidotes sp. from Ko Kut (Cuny et al., 2010b) and lated teeth. as ‘semionotids’ from the Sao Khua Formation and the Khok Description—The teeth are composed of two fundamental Kruat Formation (Cavin et al., 2009). All of them have a parts: an arrow-like acrodin cap with a pointed apex and a smooth surface and a button-shaped morphology that are gener- ganoine shaft (Fig. 10). The teeth measure 0.5 to 2 mm mesiodis- ally comparable to the teeth of Lepidotes, which are short and tally and are up to 6.5 mm in height. The acrodin cap is slightly stout, as described by Woodward (1916). Although UM10657, compressed labiolingually. Two distinct cutting edges (carinae) UM10658, and UM10695–UM10701 here are assigned to are present on two sides of the acrodin cap, extending from the Teng et al.—First Cretaceous ﬁsh fauna from Malaysia (e1573735-10)

ganoine shaft, such as those from the Middle Jurassic of southern France (Knoll and Lopez-Antonanzas, 2014) and the Aptian of south China (Mo et al., 2015). It is hitherto uncertain as to whether both variations of the ganoine shaft belong to Caturus. Therefore, we hesitate to assign the Malaysian form to Caturus. On the other hand, the sinamiid ﬁsh Siamamia was proposed by Cavin et al. (2007) from the Sao Khua Formation in Phu Phok, northeast Thailand. Unfortunately, teeth of its monotypic species Siamamia naga were not clearly presented in Cavin et al. (2007); our recent observation of the Siamamia specimens in the collection of the Sirindhorn Museum (Thailand) reveals that the teeth of S. naga possess an arrow-shaped acrodin cap (T.Y.H., pers. observ., 2017). Thus, the Malaysian halecomorph teeth may be related to Siamamia, but conﬁrmation is needed. At this stage, due to the lack of material for comparison, we report this form as Halecomorphi indet.

AGE OF THE MALAYSIAN FAUNA Species comparison of the Malaysian fish fauna with the Thai faunas (from the Khok Kruat, Sao Khua, and Phu Kradung formations, and the Ko Kut beds) is shown in Table 1. In addition to the Khorat Group, we also compare the Malaysian fauna with the Ko Kut fauna. The Ko Kut fossil beds, located offshore of Trat, were correlated to the Sao Khua Formation based on the discovery of a new hybodont fauna, including Isanodus paladeji and Heteroptychodus kokutensis (Cuny et al., 2010b). We also consider that the Ko Kut fish assemblage is equivalent to part of the Sao Khua fauna. The results of these comparisons show a striking similarity between the Malaysian and Sao Khua faunas, i.e., most species found in the Malaysian fauna are also present in the Sao Khua and Ko Kut faunas. On the other hand, the Malaysian fauna does not have any species in common with the older Phu Kradung fauna, and it shares only the ‘Lepidotes’ button- FIGURE 10. Halecormophi indet. from Pahang, Peninsular Malaysia. shaped tooth with the younger Khok Kruat fauna. UM10659, tooth in A, labial and B, mesial views; UM10660, tooth in C, The three hybodont species that are shared between the Sao labial and D, mesial views. Images produced using focus stacking Khua and Malaysian faunas are Heteroptychodus kokutensis, Isano- technique. dus paladeji, and Hybodontidae indet. The two faunas also share the genus Egertonodus. Although the species identification is kept open, the Malaysian forms of Mukdahanodus and Lonchidion are very comparable to Mukdahanodus trisivakulii and Lonchidion khora- tip toward the base of the cap. The surface of the acrodin cap is tensis, respectively, from the Sao Khua Formation (Table 1). smooth without ornamentation, whereas the ganoine shaft is reg- Heteroptychodus kokutensis, known previously only from the ularly ornamented by coarse striations from the tooth neck to the Sao Khua Formation and Ko Kut fossil beds, is now confirmed bottom of the shaft. in the Pahang vertebrate bed. In terms of age, the genus Hetero- Remarks—The presence of cutting edges on both sides of the ptychodus occurs in the Aptian (Napai Formation) of south China arrow-shaped acrodin cap is a common characteristic shared by (Mo et al., 2015) and upper Lower Cretaceous in Thailand (Cap- several different halecomorph genera, such as Caturus Agassiz, petta et al., 2006; Cuny et al., 2007). In Japan, most reports of Het- 1834, Ionoscopus Costa, 1853, Melvius Bryant, 1987, Calamo- eroptychodus are from the Lower Cretaceous formations, such as pleurus Agassiz, 1841, Pachyamia Chalifa and Tchernov, 1982, the Sebayashi Formation (Barremian–Aptian) of Gunma (Yabe Tomognathus Dixon, 1850, and Siamamia Cavin, Suteethorn, and Obata, 1930; Goto et al., 1996; Tanimoto and Tanaka, 1998; Buffetaut, Claude, Cuny, Loeuff, and Tong, 2007. The sharpness Okazaki, 2016). Thus, the occurrence of Heteroptychodus is of the cutting edges may differ from genus to genus. indicative of an Early Cretaceous age in general. Compared with the genera listed above, UM10659, UM10660, To further constrain the age of the fauna, Mukdahanodus,a and UM10702–UM10713 appear to be most similar to the teeth of rare hybodont with a specialized cutting dentition, may provide Caturus, which have strong and sharp cutting edges. Caturus teeth some important clues. Our new find from Malaysia marks the are also characterized by an arrow-like acrodin cap, strong labio- first confirmation of this Sao Khua genus outside Thailand. It is lingual compression, and a circular base (Thies and Mudroch, interesting to note that this genus is believed to have been 1996; Grande and Bemis, 1998; Thies et al., 2007). These features replaced by Thaiodus, another hybodont with a similar cutting are also displayed by UM10659, UM10660, and UM10702– dentition from the Khok Kruat Formation, during the early– UM10713. Similar Caturus-like teeth are present in the Sao middle Aptian (Cuny et al., 2009). If this is the case, Mukdahano- Khua Formation as mentioned by Cuny et al. (2006) and Cavin dus may be indicative of a pre-middle Aptian age (prior to the et al. (2009): these are likely to be conspecific with the Malaysian deposition of Khok Kruat sediments in Thailand). form (T.Y.H., pers. observ., 2017). On the other hand, although Lonchidion is known to have a It is noteworthy that some teeth assigned to Caturus have a long age range, from the Triassic to the Late Cretaceous (Rees smooth ganoine shaft (Thies and Mudroch, 1996; Cumbaa and Underwood, 2002), it only appears in pre-middle Aptian et al., 2006; Müller, 2011), whereas others have a striated deposits in Thailand, i.e., the Phu Kradung and the Sao Khua Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-11)

TABLE 1. Taxa of hybodonts and actinopterygians present in faunas of Southeast Asia.

Thailand Malaysia Pahang beds Fish Species Khok Kruat Fm. Sao Khua Fm. Ko Kut beds Phu Kradung Fm. (this study) Hybodont Thaiodus ruchae1 ✓ Acrorhizodus khoratensis2 ✓ Khoratodus foreyi3 ✓ Heteroptychodus steinmanni2,11 ✓✓ ✓ Heteroptychodus kokutensis4 ✓✓ ✓ Mukdahanodus trisivakulii5 ✓ Mukdahanodus aff. trisivakulii ✓ Lonchidion khoratensis6 ✓ Lonchidion aff. khoratensis ✓ Lonchidion sp.7 ✓ Parvodus sp.6 ✓ Isanodus paladeji6 ✓✓ ✓ Isanodus nongbualamphuensis8 ✓ Jaiodontus sp.7 ✓ Acrodus kalasinensis7 ✓ Acrodus sp.9 ✓ Egertonodus spp.6 A ✓ Hybodontidae indet.4 B ✓ Actinopterygian ‘Lepidotes’ spp.10 CDE ✓ Halecormophi indet.11 F ✓ Ginglymodi indet.4,6 GH ✓

Key: A, “Hybodus” sp. A; B, Hybodus sp.; C, semionotid; D, semionotid; E, Lepidotes sp.; F,cf.Caturus; G, actinopterygian indet.; H, Semionotiformes indet. References: 1Cappetta et al., 1990; 2Cappetta et al., 2006; 3Cuny et al., 2008; 4Cuny et al., 2010b; 5Cuny et al., 2009; 6Cuny et al., 2006; 7Cuny et al., 2014; 8Khamha et al., 2016; 9Cuny et al., 2003; 10Cavin et al., 2009; 11T.Y.H., pers. observ. in PRC.

formations (Cuny et al., 2006, 2007, 2014). The genus is notably long-ranging age from Bathonian to Aptian, the Halecomorphi absent from the younger Khok Kruat Formation (Cuny et al., indet. is less significant in refining the age of the Malaysian fauna. 2008). Lonchidion in Thailand is suggested to have been replaced The faunal comparison shows strong similarities between the by other endemic hybodonts during the early–middle Aptian Malaysian and Sao Khua faunas and they are therefore likely (Cuny et al., 2008). In that case, the occurrence of Lonchidion to be coeval. The Malaysian fauna is thus interpreted to be Bar- aff. khoratensis in Malaysia is likely to indicate a pre-middle remian to early Aptian in age (Fig. 11). This age estimation Aptian age. gains additional support from the freshwater turtle remains This report marks the first occurrence of I. paladeji outside from the same locality, which include members of the Trionychia Thailand, the species previously known only from the Sao Khua such as the soft-shelled turtles (family Trionychidae). This family Formation and the Ko Kut beds. In addition, Egertonodus spp. first appeared during the Barremian to the Aptian of Asia (Hir- from the Malaysian and Sao Khua faunas are likely to be ayama et al., 2000; Hirayama, 2002). Dinosaurs and turtles from closely related, although more specimens are needed to check this locality will be described in separate reports. whether they are conspecific. These common occurrences are substantial evidence for a faunal connection between Malaysia CONCLUSIONS and Thailand during Barremian–early Aptian. Apart from the hybodonts, the Malaysian and Sao Khua faunas The Malaysian fish fauna described above indicates for the also share three forms of actinopterygians, viz., ‘Lepidotes’ sp., first time a faunal relationship between the Cretaceous fresh- Ginglymodi indet., and Halecomorphi indet. ‘Lepidotes’ sp. water faunas of Malaysia and Thailand. Of the nine taxa (those button-like teeth) are present in both the Sao Khua and found in Malaysia, at least six (both identified and unidentified) Malaysian faunas. Similar teeth are also reported from the species and three genera are shared with the Sao Khua fauna. Khok Kruat Formation (Cavin et al., 2014). This tooth mor- Of these six are hybodonts and three are actinopterypgians phology is rather generalized and could not be firmly assigned (Table 1). Four of these nine taxa, viz., Heteroptychodus koku- to a genus. It is also uncertain whether the Thai and Malaysian tensis, Isanodus paladeji, Mukdahanodus aff. trisivakulii,and forms belong to the same species based on the teeth alone. Lonchidion aff. khoratensis, were hitherto considered to be Hence, the Malaysian ‘Lepidotes’ sp. is of little stratigraphic endemic to Southeast Asia. Based on this faunal linkage, the value. On the other hand, Ginglymodi is a broad monophyletic age of the Malaysian fauna is interpreted as being coeval with group, ranging in age from the Triassic up to the Recent that of the Sao Khua Formation, which is Barremian to early (Lopez-Arbarello, 2012; Cavin et al., 2013). The Malaysian Gin- Aptian in age. This is the first time that vertebrate fossils glymodi indet. (a disk-shaped ginglymodian tooth) is an enig- have been used to correlate the Khorat Group and Malaysian matic fish. Although it is rare, similar teeth have also been Mesozoic sediments, implying that biostratigraphic correlation found in the Sao Khua Formation (Cuny et al., 2006) and Ko based on vertebrate fossils is possible for terrestrial deposits Kut (Cuny et al., 2010b), further supporting a connection in Southeast Asia. between the Malaysian and Sao Khua faunas. In terms of systematic paleontology, the Malaysian fauna pro- The Malaysian form of the Halecomorphii is comparable to the vides new information on several important hybodonts that are specimens identified as Caturus sp. from the Bathonian of endemic to Southeast Asia. Based on the morphologies demon- southern France (Knoll and Lopez-Antonanzas, 2014) and the strated by the Malaysian specimens of I. paladeji, the diagnosis possible Aptian of south China (Mo et al., 2015). With a rather of I. nongbualamphuensis defined by Khamha et al. (2016)is Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-12)

FIGURE 11. Chronostratigraphic correlation of the Pahang ﬁsh bed to other Cretaceous faunal/ﬂoral deposits of Malaysia and Thailand. Ages from Smiley (1970), Khoo (1977), Shamsudin and Morley (1994), Uyop (2002), Uyop et al. (2003, 2007), Ainul et al. (2005), Zainey et al. (2007), and Racey and Goodall (2009).

unable to differentiate the species from I. paladeji. Also, the geographic and/or stratigraphic differentiation. More specimens absence of an ‘anterior’ tooth of I. paladeji in the Malaysian from Malaysia are needed to further study the relationship assemblage calls into question the heterodonty pattern of the between the two species. Thai type series. As pointed out by Cappetta (2012), the type Overall, this study enhances our understanding on the taxon- series might include more than one species or even genus. The omy and spatial distribution of the Mesozoic fishes in Southeast heterodonty reflected by the Malaysian specimens favor this sug- Asia, which is crucial for future studies into the evolution of fresh- gestion, pointing to the need for a revision of the genus Isanodus. water fishes and the Mesozoic ecosystem in this region. As discussed in the Systematic Paleontology section, the Malaysian specimens of H. kokutensis show a distinctive feature ACKNOWLEDGMENTS of this species, i.e., the presence of chevrons over the bulge in the anterior teeth. This feature highlights a possible close S. Suteethorn and other members of the Palaeontological relationship between H. kokutensis and the Mongolian Research and Education Centre (PRC) and U. Deesri and H. chuvalovi, showing a need to revise these species. K. Lauprasert of the Department of Biology, all in Mahasarak- The Malaysian Mukdahanodus shares very similar tooth mor- ham University, are thanked for facilitating our study of the phology with M. trisivakulii, which makes them undoubtedly Thai specimens in the PRC. L. Cavin from the Natural History related, but their differences in several features (see Systematic Museum of Geneva (Switzerland) is thanked for providing Paleontology) suggest that there is a possible new species from helpful information on the fish taxonomy. T. F. Ng (University Malaysia. This variation might be a result of geographic and/or of Malaya) is also thanked for assisting in the digital single-lens stratigraphic differentiation, but further study is necessary to reflex photography of the Malaysian specimens. C. Duffin and investigate potential causes. G. Guinot are thanked for reviewing the manuscript and giving Likewise, the slight difference of the Malaysian Lonchidion valuable comments. All SEM images in this paper were produced from the Thai Lonchidion khoratensis may also be a result of with the FESEM in the Institute of Biological Science (ISB), Teng et al.—First Cretaceous fish fauna from Malaysia (e1573735-13)

University of Malaya. Permissions to reproduce some published Cope, E. D. 1872. On the families of fishes of the Cretaceous formations in photographs were kindly granted by Elsevier, Taylor & Francis, Kansas. Proceeding of the American Philosophical Society, and Wiley. This study was part of T.Y.H.’s M.Sc. project at Philadelphia 12:327–357. – the University of Malaya, which was funded partly by a PPP Costa, O. G. 1853 1860. Ittiologia fossile italiana. Opera da servire di sup- plemento alle ricerche su i pesci fossili di L. Agassiz. xxx, Naples, I– Postgraduate Research Grant (PG190-2014). IGCP608 (Asia- – fi VIII:1 67. Paci c Cretaceous Ecosystems) also provided a Grant-in-Aid Cumbaa, S. L., C. Schröder-Adams, R. G. Day, and A. J. Phillips. 2006. to T.Y.H. Cenomanian bonebed faunas from the northeastern margin, Western Interior Seaway, Canada. New Mexico Museum of Natutal History and Science Bulletin 35:139–156. ORCID Cuny, G., L. Cavin, and V. Suteethorn. 2009. 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