The remains of Mesozoic birds are relatively rare, but especially so from the earliest stages of the Cretaceous. Although in recent years a number of enantiornithine birds have been recovered from some Early Cretaceous localities in Europe and Asia, in general the remains of other archaic Mesozoic lineages remain insubstantial at this time. Notable exceptions to this trend are the Yixian and Jiufotang formations of Liaoning Province in northeastern China, and similar formations in nearby provinces, from where hundreds of specimens of the primitive bird Confuciusornis, as well as representatives of several other lineages have been discovered [9] and [22]. However, the fossil record of Early Cretaceous birds from other parts of Asia remains very imperfectly known.

In this paper, we present a description of the first Mesozoic avian material from Southeast Asia, a partial humerus from the Early Cretaceous Sao Khua Formation of Thailand.

The humerus fragment was surface collected in 1992 by one of us (H.T.) from an outcrop of red siltstones of the Sao Khua Formation [4] at the locality of Khok Kong, in Kalasin Province, northeastern Thailand. Abundant teeth of crocodilians and theropod dinosaurs have also been found at this locality.

Since the 1970s, remains of fossil vertebrates have been recovered from a number of sites within the Mesozoic non-marine sedimentary sequences (Khorat Group) of northeastern Thailand [3]. In particular, vertebrate fossils are especially abundant in the Sao Khua Formation, which over recent years has yielded the remains of chondrichthyans, actinopterygians, turtles, crocodilians, pterosaurs and dinosaurs (including the tyrannosauroid Siamotyrannus, the spinosaurid Siamosaurus, an ornithomimosaur, and the nemegtosaurid sauropod Phuwiangosaurus) [4].

The Sao Khua Formation outcrops in many parts of the Khorat Plateau in northeastern Thailand (Fig. 1). It is now generally considered to be non-marine, having been deposited in an extensive floodplain with low-energy meandering rivers [17]. Its age, however, has formed the subject of some controversy over the years; early studies suggested a Jurassic age, but it is now generally accepted that the Sao Khua Formation is Early Cretaceous. This assignment is mostly based on palynological studies [19] and [20]. The Phra Wihan Formation, which underlies the Sao Khua Formation, contains Early Cretaceous palynomorphs, suggesting a Berriasian to Barremian age [19] and [20], confirmed by fission-track dating on detrital zircons [1]. Therefore, the Sao Khua Formation cannot be older than Early Cretaceous. Its age is further constrained by dating of the Khok Kruat Formation, which overlies the Phu Phan Formation, which itself overlies the Sao Khua Formation. The presence of the freshwater hybodont shark Thaiodus suggests an Aptian–Albian age for the Khok Kruat Formation [5], which has also yielded palynomorphs indicating an Aptian age [21]. There is therefore no doubt that the Sao Khua Formation is Early Cretaceous, older than Aptian, in agreement with the vertebrates [4] it has yielded.

The humerus presented here from Khok Kong (K3-1) belongs to the Palaeontological Collection of the Department of Mineral Resources. The specimen is stored at the Sahat Sakhan Dinosaur Research Centre (Sahat Sakhan, Kalasin Province).

The Khok Kong fragment (K3-1; Fig. 2), 15.2 mm in total length, is from a very well-preserved left humerus. Although broken immediately proximal to the brachial fossa, its shaft is slightly curved proximally and dorsally. The dimensions of this bone suggest a bird comparable to a Mallard Duck (Anas platyrhynchos) in size. The shaft of K3-1 is hollow and thin walled; notably, the entire element is naturally flat craniocaudally. The entire distal margin of this element is angled ventrally and a process that may be homologous to the avian flexor process can be seen; this region is well-developed and projected as in all neornithines and some enantiornithines [8]. In cranial view (Fig. 2), the distal condyles are well-rounded, well-developed and are projected distally. The dorsal condyle is bulbous and has its long axis orientated at a high angle (almost transverse) with respect to the angle of the shaft. The ventral condyle is much shorter than the corresponding dorsal condyle, the brachial fossa is shallow and well defined. If present at all, the extensor process of the dorsal condyle would have been only very weakly developed (Fig. 3). On the caudal surface, grooves for the scapulotriceps and humerotricipitalis muscles are not present and the olecranon fossa is only weakly developed.

Although the Khok Kong specimen is the first Mesozoic bird to be described from Southeast Asia, it is not the first record of a fossil avian from Thailand – Miocene bird remains have been known from this region since the 1980s [6] and [7]. Despite the fact that the humerus from Khok Kong is incomplete, it does at least preserve two characters that may provide some information as to its likely phylogenetic position – the long axis of the dorsal condyle is orientated close to 90 degrees (transverse) with respect to the axis of the bone and the brachial fossa is developed as a flat scar (characters 120 and 124, listed in Clarke and Norell [10]). Of these features, a transversely orientated dorsal condyle is seen in some ornithurines (e.g., Apsaravis), while a flat brachial fossa is currently considered characteristic of Ichthyornis and basal Neornithes [10]. Most enantiornithine birds, for example, do not have a visible brachial fossa [8]. The remaining preserved features visible on this distal part of the humerus (the further eight characters of this region listed in Clarke and Norell [10]) are primitive and thus uninformative. On the whole, the few characters seen on the humerus fragment from Khok Kong suggest that it is not an enantiornithine, and may be an early ornithurine. This is interesting because a number of early ornithurines have been described from the Early Cretaceous Jehol Group of northeastern China, which is roughly coeval with the Sao Khua Formation.

Because Early Cretaceous birds are usually rare in the fossil record, any new descriptions of material of this age are significant. This is especially true of new specimens discovered in regions that continue to have a patchy record with regard to the palaeobiogeography of some groups (such as Southeast Asia). With the notable exception of Jehol Group sediments (Yixian and Jiufotang formations) in northeastern China [22] and [23], the Asian fossil record of Early Cretaceous birds is limited to just a handful of bones from Mongolia and former Soviet Central Asia [14] and [18]. The sole exception to this general trend from Asia outside of China, the enigmatic Ambiortus [13] from Mongolia is known from a partial skeleton, but remains of uncertain phylogenetic position [14].

Although incomplete, the new humerus from Khok Kong demonstrates that birds were constituents of the Early Cretaceous vertebrate fauna of northeastern Thailand. The Indochina Block, which includes the Khorat Plateau, apparently came into contact with Chinese blocks in the Late Permian [16], thus Mesozoic vertebrate faunas from northeastern Thailand show similarities with coeval assemblages from China [2]. The occurrence of a bird in the Sao Khua Formation is therefore not unexpected. The likelihood of more material from these deposits is high, simply because the Sao Khua Formation is similar in age to the Chinese Jehol Group, which has yielded several abundant and diverse bird assemblages [22], alongside some other previously described and tantalising Early Cretaceous fossil bird material from other parts of Asia [11], [12], [13], [14] and [15].