The formation is considered as forming the base the Khorat Group which outcrops mostly on the Khorat Plateau in north-eastern Thailand (Racey 2009). The age of the Phu Kradung Formation is still uncertain, with contradictory signals coming from vertebrate palaeontology and palynology and it is conventionally considered as either Late Jurassic or Early Cretaceous in age (Racey et al. 1996). The presence of Dicheiropollis etruscus in the Phu Kradung Formation seems to confirm an Early Cretaceous age, although some of the lower part of the Phu Kradung Formation could still be Late Jurassic (?Tithonian) (Racey and Goodall 2009). The rich vertebrate site Phu Noi and the nearby Ban Khok Sanam localities, which are in the lower part of the Phu Kradung Formation have yielded fossil assemblages similar to those found in the Middle-Late Jurassic of China (Buffetaut et al. 2003; Cuny et al. 2014; Chanthasit et al. 2019). Currently, no named ornithischian taxa have not been reported from the Phu Kradung Formation.

The Phra Wihan Formation underlies the Sao Khua Formation, is underlain by the Phu Kradung Formation and is dated as Early Cretaceous (Berriasian to early Barremian) by a rich palynological assemblage (Racey 2009; Racey and Goodall 2009). Lithology and stratigraphy indicate a depositional environment of braided streams and occasional meandering rivers in a humid climate. The formation has not yielded skeletal material, but the biodiversity of dinosaurs can be assessed by study of its footprint faunas. The site of Hin Lat Pa Chad is located at Phu Wiang, Khon Kaen Province. Dinosaur footprints are present on the upper surface of the Phra Wihan Formation’s sandstone (Buffetaut and Suteethorn 1993) and the palaeoenvironment is thought to be brackish water or fluvial shallow (Kozu 2017). At least one trackway was made by a small-sized theropod. Others belonged to a small quadrupedal or bipedal ornithischian (Fig. 6), as the pes tracks are tetradactyl, and that at least one manus track is also tetradactyl, and may be interpreted as pentadactyl (Lockley et al. 2009; Kozu 2017). The pes axis is inwardly rotated and the pes digit I is short and anteromedially directed, while manus is outwardly rotated and situated lateral to pes digit III or IV (Lockley et al. 2009). It was classified in the ichnogenus Neoanomoepus isp. on the basis of the type material from Canada, and heteropody was noted (pes much larger than manus). Neoanomoepus is attributed to basal ornithischians and ornithopods, suggesting that these hitherto unknown earliest Cretaceous ichnofaunas may represent a radiation of ornithopods (Le Loeuff et al. 2002; Lockley et al. 2009; Kozu 2017).

The formation is composed of reddish-brown, reddish-purple sandstone, siltstone and mudstone, with some conglomerate beds (Department of Mineral Resources 2014). The Khok Kruat Formation conformably overlies the Phu Phan Formation and is widespread in the outer parts of the Phu Phan Range. The Khok Kruat Formation of Thailand is equivalent to the upper part of the Grès Supérieurs Formation of southern Laos (Weishampel et al. 2004) and is considered as Aptian-Albian in age on the basis of its vertebrate fauna and palynology (Cappetta et al. 1990; Buffetaut et al. 2005b; Racey and Goodall 2009). By contrast with other formations in the Khorat Group, the ornithischian dinosaur remains are fairly abundant at various localities in north-eastern Thailand. An ornithopod track was previously mentioned in the Khok Kruat Formation at Huai Dan Chum (Tha Uthen) track-site, Nakhon Phanom Province (Buffetaut et al. 2005a). From hundreds of footprints and dozens of trails on reddish-brown sandstone, it is assumed that there were dinosaurs similar to ornithomimosaurs and small-sized crocodylomorphs (Buffetaut et al. 2005a; Le Loeuff et al. 2009). Le Loeuff et al. (2009) remarked that the Tha Uthen specimen (T23) is remarkably similar to ornithopod tracks. However, Kozu and his team suggested that it belongs to a small-sized theropod (Kozu et al. 2017).

All of the dinosaur-bearing beds in the Savannakhet Basin belong to the top of the Grès Supérieurs Formation (Racey 2009; Cavin et al. 2019). The age of dinosaurs in Savannakhet Province is Aptian–Albian, constrained by the non-marine Cretaceous bivalve Trigonioides kobayashi-Plicatounio Suzuki (Allain et al. 1999; Cavin et al. 2019). The Grès Supérieurs Formation is considered the lateral equivalent of the Khok Kruat Formation in neighbouring Thailand. Both are considered as Aptian-Albian in age, based on their vertebrate assemblages, bivalves, and palynomorphs (Cappetta et al. 1990; Buffetaut et al. 2005b; Racey 2009; Allain et al. 2012) . Laos has yielded not only skeletal materials, but possibly an ornithopod trackway from Muong Phalane (Allain et al. 1997; Le Loeuff et al. 2009).

This non-marine fossil-bearing unit was informally referred to as ‘the Pahang vertebrate bed’ and is located in the interior of Pahang State, but the exact location of the site has been kept confidential. Hybodont sharks and ray-finned fish fishes were reported from this assemblage, which have strong affinities with fauna in the Early Cretaceous of Thailand (Teng et al. 2019). The four hybodont taxa, including Heteroptychodus kokutensis, Isanodus paladeji, Lonchidion aff. khoratensis, and Mukdahanodus aff. trisivakulii, were previously known only from the Sao Khua Formation (Khorat Group) and equivalent strata of Ko Kut (Kut Island) (Teng et al. 2019). Plants, bivalves, turtles and dinosaurs (teeth of a spinosaurid and an ornithischian) also reported from the same bonebed (Sone et al. 2015; Teng et al. 2019). They considered this site is correlated to the Early Cretaceous part (Temus Shale) of the Tembeling Group, and is equal to the Sao Khua Formation of Thailand in age based on faunal composition and biostratigraphic correlation (Teng et al. 2019).

So far, the ornithischians from the Tembling Group have not been published, but some information was released in the Malaysian media (University of Malaya 2014). We are unable to provide further details currently.

Far northeast from the Pahang vertebrate bed, another dinosaur site was found in the Chichir River of Hulu Terengganu in the north-eastern part of the Mount Gagau Area (Rahman 2019). The fossil area is underlain by the Jurassic-Cretaceous Gagau Group, comprising footprints and teeth within the Lotong Sandstone (Akhir et al. 2015). The teeth were found in ex-situ conglomerate boulders, and identified as belonging to iguanodontian dinosaurs.

Material of ornithischians was reported from the Napai Basin in south-western Guangxi Zhuang Autonomous Region (Dong 1979). The assemblage belongs to the Early Cretaceous (Aptian) Xinlong Formation (sometimes called the Napai Formation), and shows resemblance to the Khok Kruat Formation Southeast Asia (Mo et al. 2016). The four species of hybodonts from the assemblage (Acrorhizodus khoratensis, “Hybodus” aequitridentatus, Thaiodus ruchae and Khoratodus foreyi) are endemic to Southeast Asia and South China, and restricted to the Aptian-Albian interval (Cuny 2012; Cuny et al. 2017). These vertebrate fossils support the Xinlong Formation as coeval with the Khok Kruat Formation of Thailand and the ‘Grès Supérieurs’ Formation of Laos (Mo et al. 2016).

The origin of ornithischians remains controversial (Baron and Barrett 2018). Chilesaurus diegosuarezi from the Late Jurassic (Tithonian) of Chile and Pisanosaurus mertii from the Early Late Triassic (Carnian) of Argentina, were considered to be the most basal ornithischians (Butler et al. 2008; Baron and Barrett 2017; Baron et al. 2017). However, some palaeontologists consider them as primitive dinosauriforms (Müller et al. 2018; Madzia et al. 2021). Other primitive true ornithischians were also discovered from the Early Jurassic in southern Gondwana, for example, Eocursor parvus (potentially Sinemurian) in South Africa (Butler et al. 2007), Heterodontosaurus tucki (potentially Sinemurian) in South Africa (Sereno 2012), Lesothosaurus diagnosticus (= Stormbergia dangershoeki) in Lesotho and South Africa (Baron et al. 2017), and Laquintasaura venezuelae (Hettangian) in Venezuela (Barrett et al. 2014). Ornithischians evolved later to become remarkably diverse in the Jurassic and one of the most successful groups of Cretaceous herbivores.

So far, Asian ornithischians have been found from five epochs as follows:

1) Early Jurassic

A few basal thyreophorans have been reported from Yunnan Province, China (Norman et al. 2007; Yao et al. 2022). During this epoch, basal thyreophorans are known from Africa, Europe and North America (Barrett et al. 2014). However, evidence of their existence is yet to be found in Southeast Asia. Although dinosaur remains have come from the Late Triassic-Early Jurassic Nam Phong Formation of north-eastern Thailand, the fossils discovered include only postcranial material of sauropodomorphs (Buffetaut et al. 2000; Laojumpon et al. 2017) and large theropod footprints (Liard et al. 2015). In the Mesozoic red beds of northern Thailand, an indeterminate sauropod from the Chiang Muan District, Phayao Province was found and dated as post-Toarcian in age, possibly at the boundary between the Early and Middle Jurassic (Chanthasit et al. 2018). However, ornithischian material has yet to be discovered.

2) Middle Jurassic

A few basal neornithischians (such as Agilisaurus and Kulindadromeus) were reported from China and Russia (Barrett et al. 2005; Godefroit et al. 2014). Heterodontosaurids, possibly ankylosaurians and stegosaurians appeared in China (Dong et al. 1983; Dong 1993; Zheng et al. 2009). Interestingly, filamentous integumentary structures in ornithischians have only been from Asian taxa so far (Tianyulong and Kulindadromeus and appeared again in Psittacosaurus in the Early Cretaceous). These provide evidence for protofeathers being basal to ornithischians, rather than just to theropods, as previously suspected (Godefroit et al. 2014). The Middle Jurassic dinosaurs in Southeast Asia are still obscure.

3) Late Jurassic

As in Africa, Europe, and the USA, Asian ornithischian faunas were dominated by stegosaurs during this time. Some basal neornithischians are reported from the USA, China, and Thailand. Basal iguanodontians (such as Dryosaurus, Dysalotosaurus and Camptosaurus) evolved in North America, Africa, and Europe, but there is no evidence for these taxa in Asia (Norman 2004; Xu et al. 2018). However, there were the earliest known marginocephalians, the Chaoyangsauridae, in China. The presence of Jurassic ceratopsians restricted to Asia indicates an Asian origin for the group (Zhao et al. 1999). In Thailand, the lower part of the Phu Kradung Formation yields stegosaurians and basal neornithischians. While the upper part, which is likely considered as Lower Cretaceous in age (Tong et al. 2015, 2019b), still includes basal neornithischians and other groups of dinosaurs (such as metricanthosaurids and mamenchisaurids) similar to those from the lower Phu Kradung Formation.

4) Early Cretaceous

By this time, the number of stegosaurids decreased and these were eventually lost (Tumanova and Alifanov 2018). In contrast, we observe an apparent increase of ankylosaurians and the appearance of jeholosaurids, considered to have been an endemic group in East Asia (Han et al. 2012). During the late Early Cretaceous, iguanodontians evolved larger body sizes and became the dominant herbivores in the ecosystem. Several non-hadrosaurid iguanodontians have been reported from China, Japan, Mongolia, Thailand, Laos, and possibly Malaysia (Norman 2004). Several species of Psittacosaurus and basal neoceratopsians have been found in China, Japan, Mongolia, Russia, South Korea, Thailand, Laos, and possibly Uzbekistan (You and Dodson 2004; Averianov et al. 2006).

5) Late Cretaceous

The Beringian land bridge between present-day Siberia and Alaska, which opened during the Aptian-Albian, served as a migration route for terrestrial vertebrates between Asia and North America during the Late Cretaceous (Russell 1993). Hence, we can see the similarities between the dinosaur fauna from these two continents. Many dinosaur groups (including ankylosaurids, hadrosaurids, neoceratopsians, pachycephalosaurians, tyrannosaurids and troodontids) supposedly originated in Asia (Bell 2011). Asian ornithischian dinosaurs diverged considerably in the Late Cretaceous. Non-hadrosaurid hadrosauroids were replaced with hadrosaurids, the medium to large-sized duck-billed dinosaurs (Tsogtbaatar et al. 2019; Kobayashi et al. 2021). Hadrosaurids became dominant, and extended across China, Japan, Kazakhstan, Mongolia, and Russia. There are two thescelosaurids known from Mongolia and South Korea (Huh et al. 2011; Makovicky et al. 2011). Several pachycephalosaurians, exclusively known from Laurasia, were found in China and Mongolia (Sullivan 2006). Several ankylosaurians are reported from China, Japan, Mongolia, and Uzbekistan (Park et al. 2021). Several neoceratopsians (such as leptoceratopsids and protoceratopsids) were reported from China, Mongolia, and Uzbekistan, but there is only one taxon of ceratopsid from China, contrary to their prevalence in North America (Xu et al. 2010). However, although the diversity of ornithischian dinosaurs during this epoch was highest, no dinosaur bones of this age have been reported in Southeast Asia.

Southeast Asia consists of a mosaic of microcontinents. In the late Palaeozoic and Mesozoic, the northern margin of eastern Gondwana, after drifting northwards, collided with South China and other microcontinents (Metcalfe 1998). These terranes were united by the Late Triassic. Most of the dinosaur fossils in north-eastern Thailand, Laos, and Cambodia have been found on the Indochina Terrane.

Stage 1: Late Jurassic to Early Cretaceous

The oldest record of ornithischian dinosaurs in southeast Asia so far is from the Phu Kradung Formation of north-eastern Thailand, which is the basal unit of the Khorat Group (Racey et al. 1996; Racey 2009). Stegosaurids and small-bodied basal neornithischians have been unearthed, together with mamenchisaurid sauropods, and metricanthosaurid theropods, from the rich vertebrate assemblages of the Phu Noi locality and the nearby Khok Sanam locality, which is considered as the lowermost part of Phu Kradung Formation (Buffetaut et al. 2001; Cuny et al. 2014; Chanthasit et al. 2019; Manitkoon and Deesri 2019). Racey and Goodall (2009) supposed that the lower Phu Kradung Formation could be Late Jurassic (?Tithonian) in age, which was also supported by the evidence from vertebrate remains (Buffetaut et al. 2001; Buffetaut et al. 2006).

Interestingly, the vertebrate faunas from the lower Phu Kradung Formation share similarities with the those from the Middle-Late Jurassic (Bathonian-Callovian) Khlong Min Formation of the Thai southern peninsula from the Sibumasu Terrane (Buffetaut et al. 2005a; Cuny et al. 2014). Hybodont sharks (Cuny et al. 2014), brachyopoid temnospondyls (Buffetaut et al. 1994; Nonsrirach et al. 2021), teleosaurid crocodylomorphs (Buffetaut et al. 1994; Cuny et al. 2009; Martin et al. 2019), and mamenchisaurid sauropods (Buffetaut et al. 2005a; Suteethorn et al. 2013) from both the lower Phu Kradung and Khlong Min Formations indicate a wide distribution.

The upper part of Phu Kradung Formation is Early Cretaceous in age, based on the presence of Dicheiropollis etruscus (Racey and Goodall 2009) and a turnover of hybodont shark, turtle and crocodylomorph faunas (Tong et al. 2009, 2019b; Cuny et al. 2014; Martin et al. 2019). Microremains of the hybodont Acrodus kalasinensis, Jaiodontus sp. and related denticles from the Khlong Min Formation are found in the lower Phu Kradung (Cuny et al. 2014). The genus Acrodus is known from the Triassic and Jurassic (Rees and Underwood 2006), and Jaiodontus is restricted to the Oxfordian of China (Klug et al. 2010). In addition, the absence of the more common Heteroptychodus from the Early Cretaceous assemblages and its presence in the upper Phu Kradung Formation supports the age difference between the upper and lower parts of the Phu Kradung Formation (Cuny et al. 2014).

The xinjiangchelyid turtles (such as Phunoichelys kalasinensis and Kalasinemys prasarttongosothi) from Phu Noi can be correlated with those from the Late Jurassic of China as follows: the Late Jurassic Shangshaximiao (= upper Shaximiao) Formation of Sichuan Basin, the Middle-Late Jurassic Shishugou, Toutunhe and Qigu formations of the Junggar and Turpan Basins, and the Middle Jurassic Chuanjie Formation in Yunnan Basin. In contrast, records of this group in Early Cretaceous deposits are scarce in Asia (Tong et al. 2015, 2019b). The turtle fauna from the lower Phu Kradung are distinct from assemblages in the upper part, which include abundant remains of more advanced turtles, such as the trionychoid Basilochelys macrobios (Tong et al. 2009, 2015, 2019b).

The presence of teleosaurid crocodylomorphs (such as Indosinosuchus potamsiamensis) from Phu Noi suggest a Middle-Late Jurassic age contrary to crocodylomorphs from the upper part, which are characterised by pholidosaurids (such as Chalawan thailandicus) and atoposaurids (Lauprasert et al. 2011; Martin et al. 2014), indicating a faunal turnover in Southeast Asia through the Jurassic-Cretaceous (Martin et al. 2019).

The saurischian dinosaur faunas from both the lower and the upper Phu Kradung Formation consist of mamenchisaurids and metriacanthosaurids, which are well-known from the Middle-Late Jurassic/Early Cretaceous Formations in the Sichuan-Yunnan-Northern Junggar Basin of China. Mamenchisaurids (such as Mamenchisaurus and Omeisaurus) are eusauropods, and are also present in the Chuanjie Formation, Shishugou Formation, lower and upper Shaximiao Formation, Suining Formation, and Penglaizhen Formation (Buffetaut et al. 2006; Xing et al. 2015; Wang et al. 2019; Ren et al. 2021). Metriacanthosaurids (= sinraptorids) from Phu Noi show similar characteristics to Sinraptor dongi from the Upper Jurassic Shishugou Formation of the Junggar Basin in north-western China, and Yangchuanosaurus from the Middle-Late Jurassic Shaximiao Formation and possibly Late Jurassic-Early Cretaceous Suining Formation (Chanthasit et al. 2019). Both mamenchisaurids and metrianthosaurids were once thought to be endemic to east Asia. However, the report of isolated fossils from the Itat Formation of Russia and the taxon Wamweracaudia from Tendaguru extends the geographic distribution of mamenchisaurids into Siberia and Africa (Averianov et al. 2019; Mannion et al. 2019). Furthermore, Metriacanthosaurus, a close relative of Sinraptor, has been found in the upper Oxford Clay Formation of England.

The single stegosaur bone from the lower Phu Kradung Formation cannot provide definitive proof, except that it is more advanced than the Middle Jurassic Huayangosaurus and likely to be closer to those from the Late Jurassic taxa, such as Tuojiangosaurus (Buffetaut et al. 2001). Basal neornithischians from Phu Noi show characters that resemble taxa from the lower and upper Shaximiao Formation of China and the Late Jurassic Morrison Formation of US (Barrett et al. 2005; Carpenter and Galton 2018; Manitkoon et al. 2019).

As mentioned above, the dinosaur faunas including metriacanthosaurids and mamenchisaurids, and basal neornithischians have been found in both the lower and upper parts of the Phu Kradung Formation indicating that, despite the change in other groups of vertebrate faunas, dinosaurs remained the same and had long stratigraphic ranges. Another noteworthy point is the Phu Kradung Formation, and the Klong Min Formation show a remarkable biodiversity and reveal a close relationship with Chinese Jurassic vertebrate assemblages suggesting that the vertebrate faunas are more widespread than previously thought. This is probably not surprising as the vertebrate-bearing terranes of Southeast Asia and China were fused by the Late Triassic and Mesozoic terrestrial sandstones are widespread from China south to Malaysia (Sone and Metcalfe 2008; Cai and Zhang 2009; Choong et al. 2022).

Stage 2: Early Cretaceous (?Berriasian to pre-Barremian)

The trackway of a small quadrupedal ornithopod is found in the Phra Wihan Formation of Thailand. This Formation was dated as Lower Cretaceous (Berriasian-Early Barremian) from a rich palynological assemblage (Racey 2009; Racey and Goodall 2009). However, a subsequent study using radiometric dating on detrital zircon grains suggested that the dates for the underlying Sao Khua Formation are much older than previously suspected (Tucker et al. 2022). The study restricted the upper part of the Sao Khua Formation to no earlier than early Hauterivian, which means that the Phra Wihan Formation is likely not younger than Valanginian in age (Tucker et al. 2022).

The Sao Khua Formation of Thailand was assigned to the Early Cretaceous on palynological evidence (Racey et al. 1996), and probably Valanginian-Barremian on the basis of dinosaurs and bivalves (Meesook 2000). In the most recent publications, Sao Khua vertebrate fauna, including turtles and theropod dinosaurs, are considered to be Barremian (Samathi et al. 2019b; Tong et al. 2019a), and following a more refined regional biostratigraphic correlation of the freshwater bivalves Pseudohyria (Matsumotoina) matsumotoi suggesting a late Barremian age (Tumpeesuwan et al. 2010). Lastly, the radiometric data on detrital zircon grains indicated a tightly restricted late Valanginian-early Hauterivian age (133.6–132.1 Ma) for the Sao Khua Formation, and establishes that the dinosaur fauna is ~ 5–9 million years older than currently known (Tucker et al. 2022).

The Sao Khua Formation is dominated by sauropods (somphospondylian titanosauriforms, other titanosauriforms and diplodocoids) and theropods (spinosaurids, megaraptorans and coelurosaurs) in terms of species-richness and overall abundance (Buffetaut and Suteethorn 1998a; Buffetaut et al. 2002; Samathi et al. 2019a) (Fig. 9). Amongst the thousands of saurischian bones that have been collected from north-eastern Thailand, no evidence of ornithischians has yet been found in this formation (Buffetaut et al. 2015). The lack of ornithischians in the Sao Khua Formation is possibly consistent with the ecological composition of Valanginian/Hauterivian aged assemblages from Gondwana, for example, the “wood beds” of the upper Kirkwood Formation of South Africa and the Bajada Colorado Formation of South America, which are dominated by various-sized theropods and multiple cohabiting species of sauropods (Tucker et al. 2022). This is in contrast to those pre-Barremian dinosaur assemblages of Laurasian landmasses from Europe and North America, which have a diverse ornithischian record including ankylosaurians and iguanodontians and which shared habitats with other sauropod and theropod dinosaurs (Norman 2010; Kirkland et al. 2016; Tucker et al. 2022). By comparison with contemporaneous formations in Asia, ornithischian remains are still somewhat limited (Tucker et al. 2022). Material of an indeterminate stegosaurian has been reported from the Mengyin Formation (Berriasian-Valanginian), Shandong Province of China (Tucker et al. 2022). A stegosaurine Wuerhosaurus homheni and probable basal ceratopsians have been reported from the Lianmuqin Formation (?Valanginian) of Tugulu Group, Xinjiang Region of China (Sereno and Shichin 1988; Maidment et al. 2008).

Although the late Valanginian-early Hauterivian seems to be a crucial period for the ornithischian dinosaurs in southeast Asia, it does not mean that they were completely absent, but possibly reflected niche overlap, competition between herbivores considerably, and/or different timing of biogeographic dispersal. However, the vertebrate assemblage from ‘the Pahang vertebrate bed’ of Malaysia shows strong affinities with faunas in the Sao Khua of Thailand (Teng et al. 2019). An ornithischian tooth from this site may help fill the gap in the disappearance of ornithischian dinosaurs from Southeast Asia during this time.

The Early Cretaceous Phu Phan Formation is unconformably underlain by the red siltstones of the Sao Khua Formation, whereas the contact with the overlying Khok Kruat Formation is conformable suggesting that the Phu Phan Formation must fall within the interval early Hauterivian to Aptian, based on the ages of the over- and underlying formations (Racey 2009; Tucker et al. 2022). Theropod tracks have been reported, while vertebrate bones are extremely rare. Only a large sauropod limb bone has been found in a cliff face at Phu Kum Kao locality, Kalasin Province (Buffetaut et al. 2002; Buffetaut et al. 2003). No evidence of ornithischians has hitherto been reported.

Stage 3: Middle Cretaceous (Aptian to Albian)

In contrast with the Sao Khua Formation, the younger Khok Kruat Formation contains abundant neornithischian dinosaurs remains including iguanodontians and basal ceratopsians, fewer theropods (spinosaurids and carcharodontosaurians) and titanosauriforms sauropods have been found (Buffetaut et al. 2005b; Chokchaloemwong et al. 2019; Manitkoon et al. 2022).

The Khok Kruat Formation of Thailand, together with the Grès Supérieurs Formation of Laos and the Xinlong Formation of southern China share the same palaeobiogeography, supported by vertebrate fossils (Buffetaut and Suteethorn 1998b; Mo et al. 2016; Cuny et al. 2017), and have yielded five iguanodontian taxa, and at least one taxon of Psittacosaurus (Buffetaut and Suteethorn 1992; Buffetaut and Suteethorn 2011; Shibata et al. 2015; Ji and Zhang 2022). This stage represents the highest diversity of ornithischians, and also refines the temporal shift from sauropod-dominated to iguanodontian-dominated ecosystems during the Cretaceous in southeast Asia. Both iguanodontians and ceratopsians possibly spread in this region during the Aptian. One hypothesis suggests Cerapoda (ornithopods+ceratopsians) dentitions are suited for efficiently grinding vegetation as a reason for their successful competition with other herbivores in later Cretaceous time (Strickson et al. 2016).

Basal iguanodontians first appeared in North America, Africa, and Europe during the late Jurassic (possibly Kimmeridgian) (Norman 2004). Ankylopollexia, a derived clade of iguanodontian, is inferred to have immigrated to West Europe from North America prior to the Valanginian, and dispersed into East Asia from West Europe via the elongated archipelago around the Jurassic-Cretaceous boundary, probably associated with the coeval global marine regression (Xu et al. 2018). Asian basal ankylopollexians were diverse during the Early Cretaceous including taxa like Fukuisaurus and Koshisaurus (Barremian) from Japan (Shibata and Azuma 2015), Lanzhousaurus (Barremian) from China (You et al. 2005), and Bayannurosaurus (early Aptian) from Inner Mongolia, China (Xu et al. 2018). Later, the non-hadrosaurid hadrosauroids are mainly known from the middle Cretaceous (Aptian-Albian). For instance, Altirhinus and Choyrodon from Mongolia (Norman 1998; Gates et al. 2018); Equijubus, Gongpoquansaurus, Jintasaurus, and Xuwulong from Gansu Province, China (You et al. 2003, 2014; You and Li 2009); Probactrosaurus and Penelopognathus from Inner Mongolia, China (Rozhdestvenskiy 1967; Godefroit et al. 2005); Ratchasimasaurus, Sirindhorna, and possibly Siamodon from Thailand (Shibata et al. 2015).

The earliest known ceratopsians (or even marginocephalian dinosaurs) are in the family Chaoyangsauridae (Chaoyangosaurus and Yinlong), from the Late Jurassic of China (You and Dodson 2004). Until the Early Cretaceous (late Barremian-Aptian), the genus Psittacosaurus roamed across China, Mongolia, Russia, Thailand and possibly Laos (Buffetaut et al. 2007). Although as many as 19 species have been referred to this genus, about 9–12 are currently considered valid (Napoli et al. 2019). Although psittacosaurids were abundant in the Early Cretaceous of Eastern Asia, they appear to be scarce in Southeast Asia (Buffetaut and Suteethorn 1992; Buffetaut et al. 2007). Specimens of Psittacosaurus are often found in lacustrine deposits (Averianov et al. 2006; Buffetaut et al. 2007), which differs from the fluvial deposits with an arid or semi-arid subtropical climate of the Khok Kruat Formation and the Grès Supérieurs Formation (Racey et al. 1996; Wongko 2018). Both factors, depositional environment and palaeoclimate, may provide an explanation for the scarcity of psittacosaurid materials in Thailand and Laos (Manitkoon et al. 2022).

After the Aptian-Albian stages, no further Mesozoic vertebrate fossils have been reported from Southeast Asia. The Khok Kruat Formation is unconformably overlain by the Maha Sarakham Formation (Albian-Cenomanian), which was deposited in a hypersaline, land-locked salt lake within an arid, continental desert, coinciding with worldwide high sea level in the Late Cretaceous and the flooding of marine-sourced water over what is now the Khorat Plateau (Racey et al. 1996).