a new iguanodontian dinosaur from the khok kruat formation (early cretaceous, aptian) of...

Annales de Paléontologie 97 (2011) 51–62

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Original article

A new iguanodontian dinosaur from the Khok KruatFormation (Early Cretaceous, Aptian) of northeastern

Thailand

Un nouveau dinosaure iguanodontien de la Formation Khok Kruat(Crétacé inférieur, Aptien), NE de la Thaïlande

Eric Buffetaut a,∗, Varavudh Suteethorn b

a CNRS, UMR 8538, Laboratoire de géologie de l’École normale supérieure, 23, rue Lhomond, 75231 Paris cedex 05,France

b Palaeontological Research and Education Centre, Mahasarakham University, Khamrieng Sub-district,Kantharawichai District, Maha Sarakham Province, 44150 Thailand

Available online 8 September 2011

Abstract

A new taxon of ornithopod dinosaur is described as Siamodon nimngami nov. gen, nov. sep., on thebasis of a well-preserved maxilla from the Khok Kruat Formation (Aptian) of northeastern Thailand. Anisolated tooth and a braincase are referred to this taxon, and the status of other ornithopod specimens fromThailand and Laos is discussed. S. nimngami nov. gen, nov. sep. is considered as an advanced iguanodontian,apparently close to Probactrosaurus, from which it differs by various characters of the maxilla. Siamodon isan addition to the already long list of advanced iguanodontian taxa from the late Early Cretaceous of Asia.The diversity and abundance of these forms may suggest that advanced iguanodontians first appeared inAsia, before spreading to other parts of the world.© 2011 Elsevier Masson SAS. All rights reserved.

Keywords: Ornithopoda; Iguanodontia; Early Cretaceous; Aptian; Thailand; New taxon

Résumé

Un nouveau taxon de dinosaure ornithopode est décrit sous le nom de Siamodon nimngami nov. gen, nov.sep. à partir d’un maxillaire bien conservé provenant de la Formation Khok Kruat (Aptien) du NE de laThaïlande. Une dent isolée et une boîte crânienne sont rapportées à ce taxon et le statut d’autres spécimensd’ornithopodes de Thaïlande et du Laos est discuté. S. nimngami nov. gen, nov. sep. est considéré comme uniguanodontien évolué, apparemment proche de Probactrosaurus, dont il se distingue par divers caractères

∗ Corresponding author.E-mail address: [email protected] (E. Buffetaut).

0753-3969/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.annpal.2011.08.001

dx.doi.org/10.1016/j.annpal.2011.08.001

mailto:[email protected]

dx.doi.org/10.1016/j.annpal.2011.08.001

52 E. Buffetaut, V. Suteethorn / Annales de Paléontologie 97 (2011) 51–62

du maxillaire. Siamodon vient s’ajouter à la liste déjà longue des taxons d’Iguanodontia évolués de la fin duCrétacé inférieur en Asie. La diversité et l’abondance de ces formes peuvent suggérer que les Iguanodontiaévolués apparurent d’abord en Asie avant de se répandre dans d’autres parties du monde.© 2011 Elsevier Masson SAS. Tous droits réservés.

Mots clés : Ornithopoda ; Iguanodontia ; Crétacé inférieur ; Aptien ; Thaïlande ; Nouveau taxon

1. Introduction

Ornithopod remains were first reported from the Cretaceous of Southeast Asia by Hoffet(1936), who mentioned various postcranial elements from the Cretaceous “Grès supérieurs” ofMuong Phalane, near Savannakhet in southern Laos. Later, Hoffet (1944) described the materialfrom Muong Phalane as belonging to a new species of Mandschurosaurus Riabinin, 1930, M.laosensis (see discussion in Buffetaut, 1991). More recently, remains of advanced ornithopodshave been discovered at various localities in the late Early Cretaceous Khok Kruat Formationof northeastern Thailand (Buffetaut and Suteethorn, 1998; Buffetaut et al., 2005). In the presentpaper, a new taxon of advanced iguanodontian is described on the basis of some of the cranialmaterial from the Khok Kruat Formation.

2. Geographical and geological setting

The specimens on which the new taxon is based come from a stone quarry at Ban SaphanHin, a village close to the city of Khorat (Nakhon Ratchasima), in Nakhon Ratchasima Province,northeastern Thailand.

The quarries at Ban Saphan Hin expose continental coarse sandstones and conglomerates ofthe Khok Kruat Formation, deposited in a fluvial environment. According to the nomenclature ofRacey and Goodall (2009), the Khok Kruat Formation is the uppermost unit of the Khorat Group.It is referred to the Aptian on the basis on vertebrates and palynomorphs (reviews in Buffetautet al., 2005; Racey and Goodall, 2009). At Ban Saphan Hin, the Khok Kruat Formation hasyielded remains of freshwater sharks, semionotiform fishes, turtles, crocodilians and dinosaurs(theropods, sauropods, ornithopods).

3. Institutional abbreviations

PRC: Palaeontological Collection, Palaeontological Research and Education Centre,Mahasarakham University.

4. Systematic description

Dinosauria Owen, 1842Ornithischia Seeley, 1887Ornithopoda Marsh, 1881Iguanodontia Sereno, 1986

Siamodon, n.g.

E. Buffetaut, V. Suteethorn / Annales de Paléontologie 97 (2011) 51–62 53

Diagnosis: as for species

Siamodon nimngami, nov. gen, nov. sep.

Diagnosis: an advanced iguanodontian showing a combination of plesiomorphic and apomor-phic features, as follows: maxilla shaped like an isosceles triangle, with the dorsal process locatedat about mid-length of the bone; tab-like jugal process; strong longitudinal bulge on the medialsurface of the maxilla; at least 25 maxillary teeth, which bear a prominent median primary ridge,one short weak subsidiary ridge or no subsidiary ridge at all, and mamillated denticles on thecrown margins.

Derivatio nominis: generic name from Siam, the ancient name of Thailand, and odous, Greekfor tooth. The spelling odon is used to denote similarities with Iguanodon. Specific name in honourof Mr Withaya Nimngam, who kindly donated the specimens.

Type specimen: a nearly complete left maxilla bearing several teeth (PRC-4).Referred specimens: an isolated maxillary tooth (PRC-5), a braincase (PRC-6).Locus typicus: Ban Saphan Hin, Nakhon Ratchasima Province, northeastern Thailand.Stratum typicum: reddish conglomerates of the Khok Kruat Formation, Aptian.

4.1. Holotype

The holotype of S. nimngami nov. gen, nov. sep. (Fig. 1) is a fairly well preserved left maxilla(PRC-4), although its anterior and posterior ends show signs of abrasion and may be slightlyincomplete. It shows a triangular outline, with the apex of the triangle (formed by the dorsalprocess of the maxilla) nearly equidistant from the anterior and posterior ends of the bone (aspreserved). The anterodorsal margin of the maxilla bulges slightly dorsally. The ventral margin ofthe maxilla is slightly concave in lateral or medial view, and nearly straight in ventral view, exceptin its posteriormost part, where it curves slightly laterally. The tooth row is only partly preserved.Only six tooth crowns are more or less clearly visible in lateral view. They are not visible inmedial view because the medial surface of the maxilla extends farther ventrally than the lateralsurface. The number of tooth positions is uncertain because of the relatively poor preservationof that area. There may have been as many as 25 tooth positions in the maxilla, if one assumesthat there is one special foramen (Edmund, 1957) per tooth position on the medial surface of thebone (see below). Tooth morphology is described below. The lateral surface of the bone is slightlyconcave anteroposteriorly. At about mid-height it shows a series of large oval foramina, up to5 mm in length. Above them, there is a longitudinal bulge, which terminates posteriorly in a well-defined jugal process, which juts out posterolaterally and overhangs a large, 15 mm long, foramen.This foramen has a dorsal opening medial to the jugal process. Dorsally, the dorsal process isoffset from the lateral face of the maxilla and curved medially. Its dorsal end is two-pronged andserrated. The posterior margin of the dorsal process is smoothly rounded and apparently formedthe anteroventral edge of the antorbital fenestra, which must have been relatively large, but mayhave been largely obliterated in lateral view by the jugal and lacrimal. The well-defined dorsalprocess is separated from the lateral surface of the bone by a deep longitudinal groove whichextends posteriorly to the level of the jugal process. This groove probably accommodated theanteroventral part of the jugal.

In dorsal view, the maxilla appears to be somewhat broader at its anterior and posterior endsthan in the middle. Anteriorly, there is a longitudinal triangular depression, with the apex locatedposteriorly, which probably accommodated a posterior process of the premaxilla. A well-marked


Fig. 1. Holotype of Siamodon nimngami nov. gen, nov. sep., left maxilla, PRC-4, Khok Kruat Formation (Aptian), BanSaphan Hin (Nakhon Ratchasima Province, Thailand). A. Lateral view. B. Medial view. C. Dorsal view. Scale bar: 50 mm.Holotype de Siamodon nimngami nov. gen, nov. sep., maxillaire gauche, PRC-4, Formation Khok Kruat (Aptien), BanSaphan Hin (Province de Nakhon Ratchasima, Thaïlande). A. Vue latérale. B. Vue médiale. C. Vue dorsale. Barre d’échelle :50 mm.

ridge separates this depression from the medial face of the bone. At the level of the dorsal process,the above-mentioned groove is visible in dorsal view. More posteriorly, the dorsal margin of thebone becomes much broader transversely and forms a complex sutural surface covered withoblique grooves, medially and posteriorly to the jugal process. This corresponds to the contactwith the ectopterygoid medially and the jugal laterally.

The medial surface of the maxilla bears a well-marked longitudinal bulge which extends fromend to end of the bone. Anteriorly, this bulge forms a prominent shelf-like ridge. More posteriorly itbecomes more rounded and less prominent and posteriorly becomes a striated sutural area. Abovethe bulge, the medial surface of the bone is smoothly concave. At the level of the dorsal processand posterior to it, it is pierced by two large elongate foramina. Ventrally to the longitudinalbulge, there is a series of slit-like foramina which are aligned along a regular curve (the “specialforamina” of Edmund, 1957). At the level of this line of foramina, there is a distinct “step”


Fig. 2. Close-up of teeth in the type maxilla of Siamodon nimngami nov. gen, nov. sep., PRC-4, labial view. Scale bar:10 mm.Agrandissement de dents du maxillaire type de Siamodon nimngami nov. gen, nov. sep., PRC-4, vue labiale. Barred’échelle : 10 mm.

between the dorsal area and a smooth, slightly inset sheet of bone which forms the medial wall ofthe tooth row. Each special foramen apparently corresponds to a tooth position (Edmund, 1957).Their exact number is not easy to determine, because in some areas their limits are not clear, butthere are at least 25 of them, possibly 27, so the number of tooth positions was probably 25 ormore.

Measurements:Length of maxilla: 430 mm.Maximum height of maxilla: 100 mm.Teeth: as mentioned above, several teeth are visible in medial view, so that the lingual surface of

the crown is exposed (Fig. 2). They are unworn replacement teeth, which do not protrude beyondthe jaw margin. The anteriormost of these teeth, at the level of the dorsal process of the maxilla, isrelatively well preserved, although the basal part of the crown is hidden. What can be seen of thecrown is tongue-shaped in outline. The mesial and distal margins of the tooth bear strong denticles,which become smaller toward the apex. There is a very prominent median ridge on the lingualface of the crown, ending at the apex. Anterior to this primary ridge, a much weaker subsidiaryridge is present. The next tooth position is occupied by a large tooth, which is more eruptedthan the first one, and therefore the crown is better exposed. It is relatively narrow relative to itsapicobasal height (maximum width 15 mm, total exposed height 22 mm), and slightly diamond-shaped. The mesial and distal margins bear distinct denticles. There is a prominent median ridge,but no subsidiary ridges are present. In the next tooth position, only the tip of a tooth is visible,showing denticulate margins and a median ridge. Farther along the tooth row, the tips of threepoorly preserved teeth are visible in some of the posteriormost tooth positions. They do not seemto differ noticeably from the more anterior teeth.

4.2. Referred material

An isolated maxillary tooth (PRC-5) from the same locality clearly shows the same charactersas the teeth present in the type maxilla and is therefore referred to S. nimngami nov. gen, nov. sep.(Fig. 3). It is apparently from a right maxilla. Being isolated, it shows some details not visible on


Fig. 3. Isolated maxillary tooth of Siamodon nimngami nov. gen, nov. sep., PRC-5, from the Khok Kruat Formation(Aptian) at Ban Saphan Hin. A. Labial view. B. Mesial view. Scale bar: 10 mm.Dent maxillaire isolée de Siamodon nimngami nov. gen, nov. sep., PRC-5, de la Formation Khok Kruat (Aptien) à BanSaphan Hin. A. Vue labiale. B. Vue mésiale. Barre d’échelle : 10 mm.

the teeth still embedded in the jaw, and is therefore described here. Only a small portion of thenarrow root is preserved. The base of the tooth appears “pinched”, or narrow mesiodistally becauseof mesial and distal depressions which accommodated adjacent teeth, showing that the tooth waspart of a dental battery. The crown is narrow and diamond-shaped in labial view. Enamel seems tobe restricted to the labial surface. The lingual face of the tooth is mesiodistally convex. The labialface bears a very prominent primary ridge in a median position. A short and weak subsidiaryridge is present in what is presumably the mesial half of the crown, in the apical part. Both themesial and distal surfaces of the labial face, separated from each other by the strong median ridge,are markedly concave. The margins of the crown are thickened and bear strong denticles, whichthemselves are “mammillate”, i.e. composed of smaller denticles aligned labiolingually.

Measurements:Total apicobasal height: 28 mm.Maximum width: 12 mm.A well-preserved ornithopod braincase (PRC-6) from the same locality as the type maxilla

probably belongs to Siamodon, and possibly to the same individual as the maxilla describedabove, although that cannot be demonstrated with complete confidence, as the bones were notfound in direct association. It consists of the basioccipital, part of the exoccipitals, supraoccipi-tal, parietals, laterosphenoid, opisthotic, prootic, orbitosphenoid and basisphenoid (Fig. 4). Thisbraincase shows the usual iguanodontian characters and will be described in a separate paper.

5. Comparisons

S. nimngami nov. gen, nov. sep. differs from more basal iguanodontians, such as Iguanodonand closely related forms, by the morphology of its maxillary teeth, which are narrower and


Fig. 4. Braincase referred to Siamodon nimngami nov. gen, nov. sep., PRC-6, from the Khok Kruat Formation (Aptian)at Ban Saphan Hin. Right lateral view. Scale bar: 50 mm.Boîte crânienne rapportée à Siamodon nimngami nov. gen, nov. sep., PRC-6, de la Formation Khok Kruat (Aptien) à BanSaphan Hin. Vue latérale droite. Barre d’échelle : 50 mm.

bear a strong median primary ridge, sometimes accompanied by a weak subsidiary ridge, insteadof a distally displaced primary ridge and several subsidiary ridges. In addition, the maxilla hasthe outline of an isosceles triangle, with the apex approximately at mid-length, whereas in morebasal forms the apex is in a more posterior position. The maxilla of S. nimngami nov. gen, nov.sep. differs from that of hadrosaurids in the articular area for the jugal, which forms a tab-likeprocess, whereas in hadrosaurids the expanded anterior end of the jugal contacts and overlaps alarge sutural area on the maxilla. The maxilla is also relatively low by comparison with that ofmost hadrosaurids, and the maxillary teeth are not as narrow (mesiodistally) as in hadrosaurids,including rather basal forms such as Telmatosaurus (Weishampel et al., 1993).

The combination of characters seen on the maxilla of S. nimngami nov. gen, nov. sep. indi-cates that it belongs to a group of advanced iguanodontians, more derived than Iguanodonand closely related forms but less derived than hadrosaurids, sometimes referred to as “basalhadrosauroids” (Godefroit et al., 2005), which was widespread in Asia in the late Early Cre-taceous, and includes such genera as Probactrosaurus, Equijubus, Altirhinus, Jinzhousaurus,Fukuisaurus, Shuangmiaosaurus and Peneloposaurus (see review in Carpenter and Ishida, 2010).Comparisons have been made mainly with these forms, although one character of Siamodon, viz.the dorsally bulging anterodorsal margin of maxilla, may be reminiscent of a true hadrosaurid, thelambeosaurine Aralosaurus tubiferus, from the Late Cretaceous of Kazakhstan (Rozhdestvensky,1968; Godefroit et al., 2004a). However, this bulge is much more developed, forming a wing-likeblade, in Aralosaurus. Jinzhousaurus yangi, from the Yixian Formation of Liaoning, in north-eastern China (Wang and Xu, 2001; Barrett et al., 2009) differs from S. nimngami nov. gen, nov.


sep. in having maxillary teeth with a distally displaced primary ridge and several well-definedsubsidiary ridges mesial to it, rather than a median primary ridge and absent or reduced sub-sidiary ridges; in addition, the total number of maxillary teeth appears to be lower than in the Thaiform. In Altirhinus kurzanovi, from the “Khukhtekian” (Aptian-Albian) of Mongolia (Norman,1998), the maxillary teeth resemble those of the Thai form, but the dorsal process of the maxillais in a posterior, rather than central, position, which gives the bone a markedly different outlinefrom that seen in S. nimngami nov. gen, nov. sep. Equijubus normani, from the Middle Grey Unit(Albian) of the Xinminbao Group of Gansu, in northwestern China (You et al., 2003a), differsfrom S. nimngami nov. gen, nov. sep. in that its maxillary teeth bear a primary ridge that is offsetdistally and two or three accessory ridges which rise from the base of the crown. In this respect,Equijubus is more basal than Siamodon, but it appears to be more advanced in that there is no jugalprocess on the maxilla. Fukuisaurus tetoriensis, from the Kitadani Formation (late Hauterivian toBarremian) of Japan (Kobayashi and Azuma, 2003), has a smaller number (20) of maxillary teeththan S. nimngami nov. gen, nov. sep., and these teeth bear a mesially offset, rather than median,primary ridge. In addition, the maxilla appears to be lower than in the Thai form. In Shuang-miaosaurus gilmorei, from the mid-Cretaceous Sunjiawan Formation of Liaoning (You et al.,2003b), the articulation of the jugal with the maxilla seems more advanced than in S. nimngaminov. gen, nov. sep. and there are more maxillary tooth positions (27). The maxillary teeth seemto be similar to those of the Thai form. No direct comparisons are possible with Peneloposaurusweishampeli, from the Bayan Gobi Formation (Albian) of Inner Mongolia (Godefroit et al., 2005),which is known only from a dentary. However, the relatively low dentary tooth count (20) sug-gests that the maxilla bore fewer tooth positions than in S. nimngami nov. gen, nov. sep. As toNanyangosaurus zhugeii, from the Early Cretaceous Sangping Formation of Henan, China (Xuet al., 2000), it is known only from postcranial remains, so that no comparisons are possible withSiamodon.

The closest similarities seem to be with Probactrosaurus, originally described by Rozhdestven-sky from the Dashuigou Formation (Barremian-Albian) of Inner Mongolia, China, with P.mongoliensis as type species (Rozhdestvensky, 1966; Norman, 2002). Probactrosaurus was alsodescribed from the Middle Grey Unit of the Xinminbao Group (Albian) of Gansu, northwesternChina, by Lü (1997), as Probactrosaurus mazongshanensis, but according to Norman (2002),this form differs from Probactrosaurus in the morphology of its dentary teeth and should notbe placed in that genus. The maxillary teeth of S. nimngami nov. gen, nov. sep. are very similarto those of “Probactrosauru” mazongshanensis, with a prominent median primary ridge, andstrongly denticulate margins. The maxillary teeth of P. mongoliensis are not very well known,but, according to Norman (2005, p. 125), they are “characterized by a single, very prominentprimary ridge and there is little evidence of subsidiary ridges adjacent to it” – which is similar tothe condition in S. nimngami nov. gen, nov. sep., in which there is a prominent median ridge and aweakly developed subsidiary ridge, or no subsidiary ridge at all. The maxillae of Probactrosaurusgobiensis and S. nimngami nov. gen, nov. sep. are similar, with a tab-like jugal process. However,in P. gobiensis, the apex of the triangular maxilla is offset posteriorly, unlike the condition inS. nimngami nov. gen, nov. sep., where the apex is more or less equidistant from the anterior andposterior ends of the bone. In addition, the number of tooth positions in P. gobiensis is estimatedby Norman to have been 22–23, whereas there are apparently more than 25 tooth positions inS. nimngami nov. gen, nov. sep. A further difference is that the medial wall of the maxilla isdescribed by Norman (2002, p. 118) as “vertical and planar” in P. gobiensis, whereas in the Thaispecimen it bears a strong longitudinal bulge-like ridge. This prominent ridge does not seem tobe common in iguanodontians, at least those in which the medial surface of the maxilla is visible.


The medial surface of the maxilla is described as flat in Iguanodon-grade forms (Hooley, 1925;Norman, 1986), as well as in various hadrosaurs, for instance Gilmoreosaurus and Bactrosaurus(Weishampel and Horner, 1986), and Amurosaurus (Godefroit et al., 2004b). There is, however, a“medial shelf”, which may be similar to the longitudinal bulge of the Thai form, in Iguanacolossusfortis, a basal iguanodontian from the Early Cretaceous (probably Barremian) of Utah (McDonaldet al., 2010).

Comparisons can also be attempted with advanced iguanodontians from the mid-Cretaceous ofNorth America. Eolambia caroljonesa, from the uppermost Cedar Mountain Formation (Albian-Cenomanian boundary) of Utah, was first described as a basal lambeosaurine (Kirkland, 1998),but was reinterpreted by Head (2001) as a non-hadrosaurid iguanodontian, possibly closely relatedto Probactrosaurus. Its maxillary teeth are similar to those of Siamodon. However, the number oftooth positions in the maxilla (32) is significantly higher than in the Thai form. As to Protohadrosbyrdi, from the Woodbine Formation (Cenomanian) of Texas, it was described by Head (1998)as a basal hadrosaurid, but Norman (2004) considers it as a non-hadrosaurid iguanodontian closeto the origin of hadrosaurids. Its maxilla differs from that of S. nimngami nov. gen, nov. sep. inhaving a distally offset dorsal process, and 36 tooth positions, but its maxillary teeth are similarto those of the Thai form, and there seems to be some evidence of a longitudinal bulge on themedial surface on the maxilla, as in Siamodon.

On the basis of these comparisons, the Thai form seems to be distinguishable from otherhitherto described advanced iguanodontians, and it is therefore considered as belonging to a newgenus and species. It seems to be relatively close to Probactrosaurus.

6. Additional material possibly referable to Siamodon

Various postcranial elements and abundant isolated teeth of advanced ornithopods have beenfound at several localities in the Khok Kruat Formation on the Khorat Plateau (Buffetaut andSuteethorn, 1998; Buffetaut et al., 2005), notably at Khok Pa Suam (Ubon Ratchathani Province).It is obviously difficult to decide whether the postcranial remains can be referred to Siamodon.Many of the teeth are heavily worn and show few distinctive characters. Some maxillary teethfrom Khok Pa Suam are very similar to those of S. nimngami nov. gen, nov. sep. and can be referredto this taxon. Dentary teeth exhibiting a broader crown and more numerous ridges (Buffetaut andSuteethorn, 1998) than the maxillary teeth may belong to Siamodon, too, since in many advancediguanodontians the maxillary teeth are narrower than the dentary teeth and bear fewer ridges. Thisis the case, for instance, in Probactrosaurus (Norman, 2002) and Equijubus (You et al., 2003a).However, in the absence of articulated material, one must be careful about attributing dentaryteeth to Siamodon. Evidence from other parts of Asia indicates that several taxa of iguanodontianssometimes coexisted in the same area during the late Early Cretaceous. For instance, the MiddleGrey Unit of the Xinminbao Group of the Gongpoquan area of Gansu Province (China) hasyielded remains of both “Probactrosaurus” mazongshanensis (Lü, 1997) and Equijubus normani(You et al., 2003a). From the lateral equivalent of the Khok Kruat Formation in Laos, Hoffet(1944) described two different types of ornithopod ilia, which may suggest the presence of morethan one taxon (see also Buffetaut, 1991). Therefore, it cannot be excluded that more than oneadvanced iguanodontian may be present in the Khok Kruat Formation of the Khorat Plateau. Inthe future, a detailed study of the material from various localities, notably Khok Pa Suam, mayshed some light on that question.

Comparisons between Siamodon and the ornithopod material from Laos described by Hoffet(1944) are not possible because all the specimens described from Laos are postcranial elements.


As noted by Buffetaut (1991), Hoffet’s material, notably the ilia, is suggestive of advanced iguan-odontians. Hoffet referred part of his material to a new species of Mandschurosaurus Riabinin,1930, M. laosensis, on the basis of similarities with Mandschurosaurus mongoliensis, describedby Gilmore (1933) from Iren Dabasu, in Inner Mongolia. This taxon from Iren Dabasu was laterplaced in the genus Gilmoreosaurus by Brett-Surman (1979). Gilmoreosaurus is generally con-sidered as a hadrosaurid (Horner et al., 2004), but whether the form from Laos really belongs tothat taxon is highly uncertain, to say the least. Pending further work on ornithopods from Laos,it seems advisable to consider M. laosensis as a nomen dubium. The occurrence of Siamodon inLaos would not be unexpected, as the iguanodontian remains from that country come from anequivalent of the Khok Kruat Formation (Buffetaut, 1991).

7. Conclusions

The new dinosaur from the Aptian Khok Kruat Formation is an addition to the relativelylong list of advanced iguanodontians from the late Early Cretaceous of Asia. As shown above, itdiffers from previously described taxa and can therefore be placed in a new genus and species. Itsclosest affinities seem to be with Probactosaurus, from northwestern China. Comparisons reveala notable amount of mosaic evolution among those advanced iguanodontians: the morphology ofthe maxilla and that of the maxillary teeth do not seem to have evolved at the same rate in differenttaxa. Equijubus normani, for instance, has maxillary teeth which are less derived than those ofSiamodon nimngami nov. gen, nov. sep., but its maxilla-jugal articulation is more hadrosaurid-like. Reconstructing the phylogeny of these forms is therefore difficult, as shown by the numberof different cladograms that have been published in recent years. There is no consensus amongthem, and it does not seem necessary to add one more phylogenetic analysis at this stage. Evidencefrom China suggests that several taxa of iguanodontians often coexisted in parts of Asia duringthe later part of the Early Cretaceous. Whether this was the case in Southeast Asia is still unclear,although there is some evidence to suggest such a coexistence in the assemblage from the KhokKruat Formation of Thailand and Laos.

Interestingly, iguanodontians (like ornithischians in general) are unkown from the Sao KhuaFormation of Thailand, which is now convincingly dated as Barremian (Tumpeesawan et al., 2010)and has yielded abundant remains of saurischian dinosaurs (Buffetaut and Suteethorn, 1999). Thissuggests a dispersal of iguanodontians into Southeast Asian in the Aptian, perhaps following theremoval of a barrier of some kind.

It should also be noted that in Asia advanced iguanodontians are present as early as the Aptian,as exemplified by Siamodon nimngami nov. gen, nov. sep., whereas similar forms from NorthAmerica, such as Protohadros and Eolambia are from the latest Albian and Cenomanian. Froma purely chronological point of view, therefore, advanced iguanodontians may well have firstappeared in Asia before they dispersed to North America. However, future discoveries in NorthAmerica may easily lead to revise this scenario.

Acknowledgments

Special thanks to Mr. Withaya Nimngam, who donated the specimens from Ban Saphan Hindescribed in the present paper.


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a new iguanodontian dinosaur from the khok kruat formation (early cretaceous, aptian) of...

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