Alligator munensis
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| Alligator munensis Temporal range: late Middle Pleistocene (possibly Holocene) | |
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| Top view of Alligator munensis | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Order: | Crocodilia |
| Family: | Alligatoridae |
| Genus: | Alligator |
| Species: | †A. munensis |
| Binomial name | |
| †Alligator munensis Darlim et al., 2023 | |
Alligator munensis is an extinct species of alligator from the Quaternary of Thailand. After the skull of A. munensis was discovered, it was tentatively assigned to the Chinese alligator before being recognized as a distinct species. Although the two are still considered to be close relatives, the pronounced anatomical differences suggest that the two species split from one another long prior to the Pleistocene, possibly during the uplifting of the Tibetan Plateau during the Miocene. It had a short and robust skull and may have had globular back teeth possibly corresponding to a greater amount of hard-shelled prey items. The nostrils of A. munensis were positioned much further towards the back of the skull than in other alligators, but the function of this is unknown.[1]
The only currently known skull of Alligator munensis, specimen DMR-BSL2011-2, was discovered in 2005 in a layer of Quaternary alluvium in the village of Ban Si Liam, northeastern Thailand. The specimen was first discussed in a brief report by Claude et al. in 2011, who tentatively referred the material to the extant Chinese alligator (Alligator sinensis), although they already noted the distinct shape of this specimen. As the skull was in need of additional preparation, a detailed description was not possible until 2023, when Darlim and colleagues published their research on the skull. Using CT-scan data, Darlim and colleagues were able to compare the Thailand skull to material confidently assigned to Alligator sinensis, concluding that the material is different enough to warrant being placed in its own species. Furthermore, the team concludes based on other fossils from the same excavation that the locality that yielded these fossils dates to at least the middle Pleistocene, greatly narrowing down the timeframe from what was initially suggested by Claude et al. (who proposed it ranging from the Miocene to Pleistocene).
The species name of Alligator munensis references the Mun river, which is located near the site where the fossil of this species was found.[1]
Description
The skull of Alligator munensis is broadly triangular as in other alligators, but shows a remarkable level of compression that greatly shortens the overall length of the head. Furthermore, the rostrum was not especially flattened and still rather deep around the level of the nostrils, a condition otherwise known as altirostry. The region around the nose is raised thanks to an upturn of the nasal bones, which subsequently creates a marked ditch just behind the nares when observing the skull in profile view.[1]
Although the external nares are used to identify A. munensis as a member of the genus Alligator (the nares are separated from one another by the nasal bones), they are still unique compared to all other species of Alligator. They only cover about a third the length of the premaxilla, not two thirds like in other species. The bar that splits the nares is a lot broader in A. munensis, whereas it is thin in other species. Furthermore, while this internarial bar is typically formed mostly by the nasals, the premaxilla contribute heavily in the species from Thailand. The greater contribution of the premaxilla also extends to the back of the nares, where the bone wraps around and participates in the posterior border of these openings. The entire internarial bar is very high and partially separates the nasal cavity. Finally, the subcircular (rather than teardrop-shaped) nares are located much further back than in any other species of Alligator.[1]
The choana is also unique relative to all other species of the genus. It is almost circular with a sudden constriction towards the back, rather than elliptical, and it lacks the raised rim around the back of the opening.[1]
Several distinct ridges ornament the skull of Alligator munensis. One rostral ridge for example stretches across the maxilla and lacrimal bone, where it transitions to a further pair of ridges that span the prefrontal bones. This later pair of ridges, which is seen in all species except for A. mcgrewi, forms a structure referred to as spectacle. Additional ridges can be seen following the midlines of the nasals, the frontal bone and the parietal bone. Notably, the squamosals lack any distinct ridges or crests and are instead flat, whereas in today's Chinese alligator these bones are raised to form low horns.[1]
Alligator munensis had comparably reduced dentition. In addition to the typical five teeth in each premaxilla, it only possessed 12 teeth in either maxilla. Chinese alligators meanwhile possess 14 maxillary teeth. No teeth are known, but the 9th to 11th alveoli of the maxillae are clearly enlarged, which may correlate with the globular or blunt teeth. This could indicate a return to a more ancestral dentition, as today's generalist alligators evolved from more specialised ancestors.[1]
