Research Article |
Corresponding author: Sita Manitkoon ( sita.m@msu.ac.th ) Academic editor: Johannes Müller
© 2022 Sita Manitkoon, Uthumporn Deesri, Komsorn Lauprasert, Prapasiri Warapeang, Thanit Nonsrirach, Apirut Nilpanapan, Kamonlak Wongko, Phornphen Chanthasit.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Manitkoon S, Deesri U, Lauprasert K, Warapeang P, Nonsrirach T, Nilpanapan A, Wongko K, Chanthasit P (2022) Fossil assemblage from the Khok Pha Suam locality of northeastern, Thailand: an overview of vertebrate diversity from the Early Cretaceous Khok Kruat Formation (Aptian-Albian). Fossil Record 25(1): 83-98. https://doi.org/10.3897/fr.25.83081
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The Khok Pha Suam locality in the province of Ubon Ratchathani, northeastern, Thailand, is known as “the last home of Thai dinosaurs”, because it belongs to the Lower Cretaceous Khok Kruat Formation (Aptian-Albian) which is currently the youngest Mesozoic vertebrate fossil producing formation in the Khorat Group. Here, we describe a diverse vertebrate assemblage, including hybodonts, ray-finned fishes, turtles, crocodyliforms, pterosaurs, and dinosaurs from the Khok Pha Suam locality. The updated data on the Khok Kruat fauna provides a better understanding of the variety and distribution of Early Cretaceous continental ecosystems, which are useful for palaeoenvironmental reconstruction. In addition to consolidating unincorporated data on fauna, this study also provides the palaeontological data necessary to illustrate the palaeoecosystem to the general public, as well as improving the academic value of the Pha Chan-Sam Phan Bok Geopark.
Aptian-Albian, Khorat Group, Lower Cretaceous, Pha Chan-Sam Phan Bok Geopark, Vertebrates
The Mesozoic Khorat Group is composed of non-marine sedimentary rocks ranging from the Upper Jurassic to Lower Cretaceous in northeastern Thailand. Three of the formations (Phu Kradung, Sao Khua, and Khok Kruat) have yielded rich vertebrate remains including selachians, actinopterygians, sarcopterygians, temnospondyl amphibians, turtles, crocodyliformes, pterosaurs, non-avian dinosaurs, and birds (Fig.
Khok Pha Suam is part of the Pha Chan-Sam Phan Bok Geopark. It is the third national geopark that has been created after the Satun Global Geopark (Satun Province) and the Khorat Geopark (Nakhon Ratchasima Province). This geopark has many outstanding geological sites and contains three main spots: ‘land of the last Thai dinosaurs’ in the Khok Pha Suam locality; unique natural places such as Pha Chan (high cliffs above the Mekong River), the Sam Pan Bok (known as the ‘Grand Canyon of Thailand’, which consists of extensive bedrock with many potholes outcropping in the Mekong River), Pha Taem (ancient cliff paintings in the Pha Taem National Park); and the two-color river viewpoint where the blue water of the Mun River mixes with the brown water of the Mekong River (
PRC Palaeontological Research and Education Centre, Mahasarakham University, Thailand;
SM Sirindhorn Museum, Kalasin Province, Thailand.
Khok Pha Suam is located in the Na Kham Subdistrict, Si Muang Mai District, Ubon Ratchathani Province. The outcrop was discovered in 1993 by villagers near the forested area maintained by the Na Kham Subdistrict Administrative Organization. The site has been extensively eroded by water into a badlands-type landscape (
The thickness of the lithostratigraphic section is about 4 meters (Fig.
The bonebed presents lithostratigraphic and sedimentary structures composed of fining-upward sequences, carbonate caliche horizon, micro cross-bedding, load-cast and rip-up clasts, micaceous fine sand and silt which also form scattered thin lenticular beds and laminated carbonaceous shale in the sequences. These features indicate low-energy current, floodplain deposits. Fossil remains consist of isolated or fragmentary elements indicating transport under high energy conditions and deposition on floodplains. It could correspond to an arid or semi-arid subtropical climate, as indicated by the caliche pebble conglomerate (Fig.
The specimens were collected from Khok Pha Suam locality, Na Kham Subdistrict, Si Muang Mai District, Ubon Ratchathani Province (Fig.
Locality map and Lithostratigraphic section of Khok Pha Suam locality. a. Map of Thailand, showing the location of the Khorat Plateau; b. Distribution of the Khok Kruat Formation in northeastern Thailand (green color; modified from DMR, 2004), location of Khok Pha Suam locality (red star); c. Stratigraphic column of Khok Pha Suam, d, a photograph of the excavation site; e. Logo of The Pha Chan-Sam Phan Bok Geopark.
The vertebrates found at the Khok Pha Suam locality comprise five taxa of hybodont sharks, at least two taxa of ginglymodians, a sinamiid fish, carettochelyid and adocid turtles, neosuchian crocodyliforms, pterosaurs, dinosaurs (iguanodontians, sauropods, and at least two taxa of theropods). The faunal diversity described in this article together with additional data from other localities allows us to propose a preliminary reconstruction of the Early Cretaceous Khok Kruat Formation ecosystem (Fig.
Class Chondrichthyes Huxley, 1880
Euselachii Hay, 1902
Hybodontiformes Patterson, 1966
Family Thaiodontidae Cuny, Suteethorn, Khamha & Buffetaut, 2008
Thaiodus ruchae
Cappetta, Buffetaut, & Suteethorn, 1990 (Fig.
Isolated microremains from Khok Pha Suam locality. Thaiodus ruchae tooth (a. PRCMR301) in labial view, Khoratodus foreyi tooth (b. PRCMR302) in lingual view, “Hybodus” aequitridentatus tooth (c. PRCMR303) in labial view, Heteroptychodus steinmanni tooth (d. PRCMR304) in apical view, Acrorhizodus khoratensis tooth (e. PRCMR305) in mesio-lingual or disto-lingual view, ginglymodian external side scales with ganoin, uncovered field and bone on the anterior margin (f–g. PRCMR305-306) in dorsal view, ginglymodian external side of the dermal bone (h. PRCMR307) in dorsal view, sinamiid centrum (i–j. PRCMR308) in anterior (i.) and dorsal (j.) views, carettochelyid shell fragment (k. PRCMR309) in dorsal view, adocid shell fragment (l. PRCMR310) in ventral view, neosuchian osteoderm (m. PRCMR311) in dorsal view, neosuchian tooth (n. PRCMR312) in lingual view, theropod tooth (o. SM2016-1-155), spinosaurid tooth morphotype I (p. PM2016-1-003) in anterior view, spinosaurid tooth morphotype II (q. PM2016-1-006) in anterior view, sauropod tooth (r. PRCMR315) in lingual view, iguanodontian tooth (s. SM2021-1-121), pterosaur tooth (t. PRCMR317), and bivalve mold right valve of articulated shell (u. PRCMR318) in external view. Scale bars: 0.5 cm (a, c–s); 1 cm (b).
T. ruchae
possesses asymmetric teeth with an occlusal crest displaced lingually. Its serrated teeth are strongly interlocked, which usually indicates preference in hunting large prey and possibly occasional scavenging via the ability to cut tough meat (
Khoratodus foreyi
Cuny, Suteethorn, Khamha & Buffetaut, 2008 (Fig.
K. foreyi
teeth are very elongated, flattened, and rod shaped. Thaiodus and Khoratodus teeth show common features in addition to their asymmetry, such as the ornamentation restricted to the upper half of the crown and they also show a common vascularization pattern of the root and an interlocking system between the teeth which is unusual among hybodonts. These genera are included in the family Thaiodontidae, which appear to be restricted to Asia (
“ Hybodus ” aequitridentatus Cuny, Suteethorn, Khamha, and Buffetaut., 2008 (Fig.
“H.” aequitridentatus teeth are probably adapted towards an opportunistic feeding method, but the low and blunt cusps indicate some specialization towards hard-shelled preys (
Family incertae sedis
Heteroptychodus steinmanni
Yabe & Obata, 1930 (Fig.
H. steinmanni
is quite abundant at Khok Pha Suam. Its teeth are typically broad, with a low crown densely and are strongly ornamented, which indicates specialization towards a durophagous diet (
Besides the Khok Kruat Formation, H. steinmanni was also discovered from various localities of the Sao Khua Formation (
Family incertae sedis
Acrorhizodus khoratensis
Cappetta, Buffetaut, Cuny & Suteethorn, 2006 (Fig.
A. khoratensis
teeth possess a high root with a U-shaped longitudinal crest, except in the posterior teeth. The crown is broadly rectangular in apical view. A blunt cusp is also observed on the labial side that is almost as wide as the crown. The tooth morphology indicates adaptations towards various food source, in a way probably similar to H. aequitridentatus (
Isolated remains of actinopterygians (ray-finned fishes) preserved in the Khok Pha Suam locality usually consist of vertebral centra, fragments of skull bones, fragments of jaws, isolated teeth, and numerous scales.
Holostei Müller, 1844 sensu Grande, 2010
Ginglymodi Cope, 1872 sensu Grande, 2010
Lepisosteiformes Hay, 1929 sensu López-Arbarello, 2012
Family incertae sedis (Fig.
Two taxa of ginglymodians can be separated by the ornamentation of their ganoid scales (
Lanxangichthys sp. Cavin, Deesri, Veran, Khentavong, Jintasakul, Chanthasit & Allain, 2018
So far, only one genus of ginglymodian, Lanxangichthys, has been identified on the basis of fossil material from the Khok Kruat Formation and from the Grès supérieurs Formation in Laos. Isolated skull remains from Ban Sapan Hin and Khok Pha Suam localities present strong ornamentation of ganoin forming radiating and tuberculate patterns similar to the ornamentation of non-weathered bones of a single articulated skull of the holotype of L. alticephalus from the Savannakhet Basin in Laos. As the Grès supérieurs Formation from the Savannakhet Basin are regarded as an equivalent to the Khok Kruat Formation, the isolated ornamented cranial remains from Khok Kruat Formation are referred, with caution, to Lanxangichthys sp. (
Amiiformes sensu Grande & Bemis, 1998
Halecomorphi Cope, 1872
Family Sinamiidae Berg, 1940
cf. Siamamia Cavin, Suteethorn, Buffetaut, Claude, Cuny, Le Loeuff & Tong, 2007 (Fig.
Vertebral centra are referred to a sinamiid together with fragments of dentaries, a premaxilla, possible fragments of maxillae and many scale types from the Khok Pha Suam locality. The material shows similarities with Siamamia naga from the older Sao Khua Formation (
Testudines Cope, 1868
Cryptodira Cope, 1868
Trionychoidae Fitzinger, 1826 sensu Gaffney & Meylan, 1988
Family Adocidae Cope, 1870 (Fig.
Family Carettochelyidae Boulenger, 1887 (Fig.
Two different families of trionychoids have been collected from the Khok Pha Suam locality, each identified by the ornamentation pattern of their shell fragments: the carettochelyid fragment is covered with strong ornamentation (Fig.
Crocodylomorpha Walker, 1970
Crocodyliformes Hay, 1929
Neosuchia Benton & Clark, 1988
Family incertae sedis (Fig.
Osteoderms (Fig.
Family Goniopholididae Cope, 1875 (Fig.
A nearly complete left mandible of goniopholidid (SM2021-1-112: Fig.
This left mandible of SM2021-1-112 cannot be compared with the slender-snouted neosuchian crocodyliform Khoratosuchus jintasakuli (
On the contrary, SM2021-1-112 shows a combination of characters on the dentary that have been described in “G.” phuwiangensis, which are: 1) anterior portion of the tooth row with no marked angulation; 2) absence of a strong outward protrusion of the lateral margin at the level of the third and the fourth dentary alveoli and 3) presence of dentary curvatures in both lateral and vertical planes. However, based on the strong undulation of its lateral margin on a vertical plane, SM2021-1-112 can be distinguished from “G.” phuwiangensis. The first and the second convexities of SM2021-1-112 are about twice as high as its concavity, whereas in “G.” phuwiangensis, the proportion between the maximum height of convexity and concavity is significantly less than that of SM2021-1-112. Additionally, the ornamentation on the lateral surface of SM2021-1-112 is faintly sculptured whereas that of “G.” phuwiangensis is heavily sculptured. These two characters, therefore, are sufficient to validate SM2021-1-112 as a species different from “G.” phuwiangensis.
Based on the obscured relationships of the Thai, European and North American Goniopholis as well as the absence of the lower jaw of Siamosuchus, it should be suitable for the time being to consider SM2021-1-112 as an uncertain genus in the family Goniopholididae until further studies can accurately evaluate the internal relationships of Thai goniopholidids.
Dinosauria Owen, 1842
Saurischia Seeley, 1888
Sauropoda Marsh, 1878
Neosauropoda Bonaparte, 1986
Macronaria Wilson & Sereno, 1998 (Fig.
Sauropod remains are rare in Khok Pha Suam. A small femur approximately 40 cm in length of a probable juvenile sauropod is an exhibit at the local museum under the supervision of Na Kham Subdistrict Administrative Organization. Some isolated teeth have been found but are very fragile. A peg-shaped tooth (PRCMR315, Fig.
Theropoda Marsh, 1881
Superfamily Allosauroidea Marsh, 1878 (Fig.
Several teeth of theropod dinosaurs differing in size have been collected and can be divided into Allosauroidea and Spinosauridae. The allosauroid teeth (Fig.
Family Spinosauridae Stromer, 1915 (Fig.
Spinosaurid teeth exhibit conical crown and extremely reduced serrations, a morphology reminiscent of crocodilian teeth. The teeth of Khok Kruat spinosaurids can be categorized into two sub-morphotypes (
The spinosaurid Ichthyovenator laosensis (
Onithischia Seeley, 1888
Neornithischia Cooper, 1985
Ornithopoda Marsh, 1881
Iguanodontia
Dollo, 1888 (Fig.
Teeth of iguanodontian dinosaurs are common and show a heavy degree of wear. SM2021-1-121 (Fig.
Although Khok Pha Suam vertebrates are mostly known from microremains and fragmentary larger remains, some isolated postcranial bones belonging to iguanodontians were discovered (Fig.
Three taxa of hadrosauroids have been described from the Khok Kruat Formation in Nakhon Ratchasima Province, including Siamodon nimngami (
Pterosauria Kaup, 1834
Pterodactyloidea
Plieninger, 1901 (Fig.
Pterosaur teeth in Thailand are very scarce. An isolated Ornithocheirid tooth has been reported from the Sao Kua Formation in northeastern Thailand (
The sedimentology of Khok Kruat localities shows that the vertebrate fossils were deposited in a fluvial system including flood plains and channels of meandering rivers. At Ban Saphan Hin, Nakhon Ratchasima Province, various vertebrate remains were found scattered and fragmented in a medium to thick-bedded reddish-brown conglomerates. The conglomerates are locally cross-bedded. These elements indicate that the fossils were transported with medium to high energy and deposited in the channel. Carbonate clasts are commonly found and well cemented by calcite. These shreds of evidence suggest a semi-arid environment. This locality has yielded Thaiodus ruchae, Heteroptychodus steinmanni, Lanxangichthys sp., Shachemys sp., Kizylkumemys khoratensis, Khoratosuchus jintasakuli, Eusuchian indet., Sirindhorna khoratensis, Siamraptor suwati (
Many groups of vertebrates have been found in the Sam Ran locality, Khon Kaen Province, including teeth of two hybodont taxa (incl. H. steinmanni, and T. ruchae), ganoid scales of actinopterygians (Ginglymodi type II), turtle shell fragments, teeth of crocodyliforms, teeth of large theropods and partial postcranial bones of spinosaurid indet. The presence of sedimentary structures such as fining upward, planar cross-bedding, load casted, rip-up clasts indicate meandering channel deposits and crevasse splay sequences. The very good preservation of the vertebrate fossils, notably the partial skeleton of a spinosaurid indet. is likely a result of a low-energy current system. The calcisol with pedogenetic carbonates is an indication of a semi-arid climate (
Lam Pao Dam locality in Kalasin Province has poor fossil preservation. This locality has yielded the teeth of hybodonts (incl. H. steinmanni, T. ruchae and possibly K. foreyi), teeth and scales of ginglymodians, teeth of crocodyliforms and theropods. Trace fossils, such as theropod footprint, Lockeia, Phycodes, Planolites, and Skolithos indicate a moderately to well-drained floodplain. It could have been arid or semi-arid in a subtropical climate, as indicated by the caliche-siltstone granule calcareous sandstone deposits. The presence of lithostratigraphic and sedimentary structures such as stacked fining-upward sequences, small scale cross-bedding, rip-up clasts, and conglomerates at the base of sequences indicate high-energy current, meandering channel conglomerate deposits or point-bar deposits (
The fossil remains from Ban Pha Nang Sua locality, Chaiyaphum Province are found in reddish-brown sandstones and siltstones in the lower part of the sequence which is interpreted as a crevasse splay deposit. Most of the specimens belong to a giant titanosauriforms indet., associated with some teeth of hybodonts, crocodyliforms, and theropods (
The vertebrate fauna from Khok Pha Suam represents the richest assemblage at the generic level within the Khok Kruat Formation (Table
Hundreds of isolated teeth and many dorsal fin spine fragments of hybodonts representing up to five different genera have been recovered so far. Pattern and form variation of these shark teeth show adaptations towards various diets: cutting (Thaiodus), crushing (Khoratodus, “Hybodus” and Acrorhizodus), and grinding (Heteroptychodus) (
Among the thousands of dinosaur bones from the Sao Khua Formation that belong to sauropods and theropods, there is so far no evidence of any ornithischians. In the Khok Kruat Formation, the diversity of sauropods appears to be greatly diminished with the appearance of basal ceratopsians and advanced iguanodontians (
The teeth of Khok Kruat spinosaurids can be categorized into two morphotypes (
It is worth noting that the material of psittacosaurids seems to be the only group of Khok Kruat animals that have never been discovered in Khok Pha Suam (Table
Another noteworthy point is the absence of amphibians from the Khok Kruat Formation (Fig.
List of vertebrate faunas from Khok Pha Suam locality with other taxa from the Khok Kruat Formation (*The Grès Supérieurs Formation of southern Laos is laterally considered an equivalent to the Khok Kruat Formation).
Taxa | Khok Pha Suam | References | Khok Kruat (other localities) | References |
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Ornithopods | Iguanodontian indet. | ( |
Mandschurosaurus laosensis (Laos*) | ( |
Siamodon nimngami (Nakhon Ratchasima) | ( |
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Ratchasimasaurus suranareae (Nakhon Ratchasima) | ( |
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Sirindhorna khoratensis (Nakhon Ratchasima) | ( |
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Ceratopsians | Psittacosaurus sattayaraki (Chaiyaphum) | ( |
||
Psittacosaurid indet. (Khon Kaen & Laos*) | ( |
|||
Spinosaurids | Spinosaurid type I | ( |
Ichthyovenator laosensis (Laos*) | ( |
Spinosaurid type II | ( |
Spinosaurid indet. (Khon Kaen, Kalasin & Chaiyaphum) | ( |
|
Allosauroids | Carcharodontosaurian indet. | ( |
Siamraptor suwati (Nakhon Ratchasima) | ( |
Sauropods | Titanosauriforms indet. | ( |
Tangvayosaurus hoffeti (Laos*) | ( |
Titanosauriforms indet. (Chaiyaphum & Khon Kaen) | ( |
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Pterosaurs | Pterodactyloid indet. | In this paper | Pterosaur indet. teeth (Nakhon Ratchasima) | |
Crocodyliforms | Goniopholidid indet. | ( |
Khoratosuchus jintasakuli (Nakhon Ratchasima) | ( |
Eusuchian indet. (Nakhon Ratchasima) | ( |
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Neosuchian indet. | Neosuchian indet. (Khon Kaen, Kalasin & Chaiyaphum) | ( |
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Turtles | Carettochelyid indet. | ( |
Kizylkumemys khoratensis (Nakhon Ratchasima) | ( |
Carettochelyid indet. (Kalasin, Khon Kaen) | ( |
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Adocid indet. | ( |
Shachemys laosiana (Laos*) | ( |
|
Shachemys sp. (Nakhon Ratchasima & possibly Khon Kaen) | ( |
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Halecomorphs | cf. Siamamia indet. | ( |
Sinamiidae indet. (Nakhon Ratchasima) | ( |
Ginglymodians | Lanxangichthys sp. | ( |
Lanxangichthys alticephalus (Laos*) | ( |
Ginglymodi type I | ( |
Lanxangichthys sp. (Nakhon Ratchasima) | ( |
|
Ginglymodi type II | ( |
Ginglymodi type I (Kalasin) | ( |
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Ginglymodi type II (Kalasin & Khon Kaen) | ( |
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Hybodontiforms | Heteroptychodus steinmanni | ( |
Heteroptychodus steinmanni (Kalasin, Khon Kaen, Nakhon Ratchasima & probably Chaiyaphum) | ( |
Thaiodus ruchae | ( |
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“Hybodus” aequitridentatus | ( |
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Khoratodus foreyi | ( |
Thaiodus ruchae (Nakhon Ratchasima,Khon Kaen, & Kalasin) | ( |
|
Acrorhizodus khoratensis | ( |
The Khok Pha Suam locality has yielded vertebrates from the Aptian-Albian stages. It represents one of the most diverse vertebrate assemblages in the Khok Kruat Formation of Thailand and the laterally equivalent Grès Supérieurs Formation of Laos. The site is characterized by the dominance of hybodont teeth and iguanodont postcranial material. This study underlines the palaeontological value of this site, which is an essential feature of the Pha Chan-Sam Phan Bok Geopark. The locality improves our knowledge of the diversity of Early Cretaceous vertebrate faunas and provides a useful point of comparison with other East and Southeast Asian taxa.
We would like to thank many colleagues for useful suggestions and comments; Clive Burrett, Bouziane Khalloufi and Thitiwoot Sethapanichsakul for their help with the English language. Sukboworn Tumpeesuwan, Suchada Khamha, Wilailuck Naksri, Sasa-On Khansubha, Kantanat Trakunweerayut, and Tapanan Sunipan for providing data on bivalves, hybodonts, turtles, sauropods, maps, and the history of Khok Pha Suam respectively. Nareerat Boonchai, Paul Grote, and Marc Phillipe for discussion about plants and palaeoenvironmental reconstruction. Witaya Nimgnam for data on hybodonts from the Ban Sapan Hin locality. The Khok Pha Suam ecosystem is illustrated by Sakka Weerataweemat. The stratigraphic column of Khok Pha Suam is from Wongwech Chowchuvech. We are grateful to the staff of the Palaeontological Research and Education Centre of Mahasarakham University, Sirindhorn Museum, and Na Kham Subdistrict Administrative Organization, who took part in our fieldwork and helped during visits to the collection. This research project has been supported by Mahasarakham University (First International Publication 2021). Sita Manitkoon was financially supported by the Mahasarakham University Development Fund for the international symposium presentation.
Mesozoic vertebrate fauna from the Indochina Terrane of Thailand and additional photos
Data type: Images
Explanation note: Fig. S1. Mesozoic vertebrate fauna from the Indochina Terrane of Thailand. Figs S2–S5. Additional photos were mentioned in the article.