The Alligatoridae (Alligators and Caimans), is now known almost exclusively from the Americas, with a single Asian example, the Chinese Alligator, Alligator sinensis, which molecular clock studies have suggested diverged from its closest living relative, the North American Alligator, Alligator mississippiensis, in the Eocene or Early Oligocene. The fossil record of Alligators in Asia has been poorly studied, which makes it difficult to assess the origin of the Chinese Alligator, and under what circumstances its ancestors migrated from America to Asia.
To date, only a single extinct species of Alligator has been described from Asia, Alligator luicus from the Miocene of Shandong Province in northeast China. However, a range of specimens have been assigned to Alligator sinensis, from the Late Miocene onwards, from Thailand, Japan, and Taiwan, which may hide a greater diversity of extinct Alligators in the region.
In a paper published in the journal Scientific Reports on 13 July 2023, Gustavo Darlim of the Department of Geosciences at Eberhard Karls Universität Tübingen, Kantapon Suraprasit of the Department of Geology at Chulalongkorn University, Yaowalak Chaimanee of Laboratory PALEVOPRIM at the University of Poitiers, Pannipa Tian, Chotima Yamee, Mana Rugbumrung, and Adulwit Kaweera of the Thai Department of Mineral Resources, and Márton Rabi, also of the Department of Geosciences at Eberhard Karls Universität Tübingen, and of the Central Natural Science Collections at Martin-Luther University, redescribe the Thai example of 'Alligator sinensis', and make new assessments about its taxonomy, age, and ecology, as well as its implications for the distribution of modern Alligator sinensis populations.
The specimen, DMR-BSL-2011–2, the nearly complete cranium of an Alligator, was uncovered in 2005 by a group of villagers excavating a pond at Ban Si Liam in Nakhon Ratchasima Province, northeastern Thailand. It was found within a layer of yellowish medium- to fine-grained sands about 2 m thick, which lay immediately beneath a 30 cm thick, organic-rich topsoil layer. Beneath the yellowish sand was a layer of indurated iron oxide, and then a yellowish clay. As this was an agricultural project carried out by local people, rather than a palaeontological dig, no attempt at dating the material was made at the time.
The topsoil layer is reported to have yielded a number of pottery fragments, while the sand layer also produced a fragment of Turtle shell, a Crocodylian vertebra, and fragments of a Wild Water Buffalo, Bubalus arnee, and a Sambar Deer, Rusa unicolor. It has previously been suggested that the yellowish sand and its faunal contents should be regarded as Late Miocene to Middle Pleistocene in age, by Darlim et al. disagree with this, noting that Miocene deposits in the Mun River Valley are typically found 10-20 m beneath the surface, and that the presence of Wild Water Buffalo and Sambar Deer suggests a date no older than the late Middle Pleistocene, with these Animals being known from the faunas of Tam Wiman Nakin (considered to be more than 169 000 years old, and Khok Sung, which has been separately dated as either 217 000 years old or 130 000 years old). Furthermore, since these species are still extant in the region, a Holocene date for the deposit cannot be excluded.
Examination of DMR-BSL-2011–2 led Darlim et al. to conclude that it represents a new, undescribed, species of Asian Alligator, which they describe as Alligator munensis, where 'munensis' means 'from Mun' in reference to the Mun River Valley, where the specimen was found. The skull of Alligator munensis is compressed anteroposteriorly, with external nares (nostrils) far back on the dorsal portion of the snout compared to other Alligators. The maxillary has no remaining teeth, but has only 12 alveoli (sockets), a significant reduction compared to other Alligator species.
Darlim et al. intend to publish a full taxonomic review of Alligator munensis elsewhere, but note that they believe it to be closely related to Alligator sinensis. Northern Thailand is a considerable range extension for Alligators, with Alligator sinensis being known only from the Yangtze and Xi river systems of China. These rivers come close to the Mekong and Chao Phraya systems of Thailand only in their upper reaches in the high Himalayas, an environment quite unsuitable for Alligators. However, the Himalayas are a relatively young mountain range, and it is possible that a Miocene ancestor of Alligator munensis and Alligator sinensis could have occupied the upper and lower reaches of both river systems, becoming separated into separate populations as the Himalayas rose, and eventually evolving into two different species.
The nares of Alligator munensis are far back, and located in a dorsal position on the snout, something not seen in any living Crocodilian. This state is seen in living Cetaceans, and extinct groups such as Ichthyosaurs, Plesiosaurs, Mesosaurs, and Metriorhynchid Crocodylomorphs, and in all cases appears to be an adaptation to a marine environment. However, this seems unlikely in Alligator munensis, not just because the single known specimen comes from a river sands deposit, not thought to have been laid down close to the sea, but also because all known species of Alligator lack lingual salt-excreting glands, which limits their ability to survive in marine waters.
Alligator munensis also has robust, deep skull, wide posterior process of the maxilla lateral to the suborbital fenestra, robust ectopterygoids, wide palatinal bridge between the suborbital fenestrae, and large and extended pterygoid flanges. This is a morphology consistent with a strong bite force and crushing dentition, something thought to have been the ancestral state in Alligatorines. This does not necessarily imply a particularly specialised diet in a Crocodilian, it is quite possible that such dentition would be found in an omnivorous species able to take tougher food, such as Molluscs or Turtles, but not dependent on such. The diet of Alligator sinensis has not been extensively studied, but Snails are thought to form an important part of its diet.
Virtual representation of Alligator munensis holotype (DMR-BSL-2011-2). (a) Digital
reconstruction and (b) artistic reconstruction, both in lef lateral views. Scale bar is 10 cm. Márton Szabó in Darlim et al. (2023).See also...
