Aldabrachampsus

''Aldabrachampsus'' is an extinct genus of small horned crocodile known from fragmentary remains. It lived during the Pleistocene on Aldabra Atoll, Seychelles in the western Indian Ocean. The name ''Aldabrachampsus dilophus'' means "Two-crested crocodile from Aldabra". It was a small animal, reaching a length of , comparable in size to the smallest extant crocodilians.

Description

''Aldabrachampsus'' is only known from fragmentary cranial and mandibular remains including the premaxilla, frontal bone, parietal, dentaries and squamosals among others. Some postcranial bones are also known including several vertebrae and a femur. Like most other crocodilians ''Aldabrachampsus'' possessed five teeth situated in the premaxilla, however the second to fifth are all aligned in a straight line rather than following a convex path. Following the premaxillary teeth the rostrum of the crocodile constricted, creating a notch for one of the dentary teeth. There is a toothless region (diastema) present between the second and third alveoli of the dentary. The orbital margins of both the frontan and prefrontal bone are raised. The main body of the frontal is shaped like a pentagon with a distinctly V-shaped suture with the parietals, excluding the frontal from contributing to the supratemporal fenestrae. The anterior process of the frontal bone is slender and notably protrudes from the body following a sudden sharp constriction. Of the parietal only the posterior section is known, which shows a flat surface and a deep notch in its rear margin exposing the occipital region. The squamosal bone widens towards its suture with the postorbital and the bone bears a horn-like projection that faces outwards. Similar structures are known in the Malagasy crocodile ''Voay'', ''Crocodylus anthropophagus'' from Tanzania and even alligatoroids like ''Acresuchus'' and ''Ceratosuchus''. In modern species both Siamese crocodiles and Cuban crocodiles share similar squamosal horns. However ''Aldabrachampsus'' differs from these taxa in two ways. The horns are notably less pointed, showing a more rounded morphology, and their apex is located towards the halfway point of the bone, rather than the squamosal horns of other crocodilians which reach their apex towards the posterior.

The vertebrae show that the neural centra and neural arches were at least partly fused with the sutures beginning to close. This indicates that the animal's these bones belonged to had almost reached their adult size. With a length of the vertebrae suggest that the animal, assuming similar proportions to modern crocodiles, was relatively small in size, approximately long. Extrapolating the size based on the skull remains gives similar results ranging from . This size would be similar to the smallest extant crocodilians, the South American dwarf caimans and African dwarf crocodiles, alongside extinct dwarf mekosuchines like ''Volia'' and ''Mekosuchus''. Some crocodilian remains found later might indicate the presence of a larger bodied and more robust animal on the island, it is however not clear if these fossils belong to a large specimen or species of ''Aldabrachampsus'' or a taxon previously not known from the island like the Nile crocodile or an osteolaemine related to ''Voay''. These remains indicate a crocodile with a body length of .

Classification

The fragmentary nature of ''Aldabrachampsus'' has left it's exact placement within crocodilia somewhat ambiguous. Phylogenetic analysis suggests it is part of the family Crocodylidae, composed of ''Crocodylus'' and ''Osteolaeminae''. Although this relationship is recovered by the strict consensus tree, which results in a large polytomy rendering ''Aldabrachampsus'' exact position unknown, other trees suggest potentially different relationships. In some trees ''Aldabrachampsus'' is recovered as a sister taxon to ''Voay'', while others find it to be a sister to ''Crocodylus palaeindicus''. However both these positions are weakly supported, based on a single morphological character (the exposed supraoccipital), which is expressed differently between the different crocodiles in question. Brochu however argues that despite being based on the same character, a relationship to ''Voay'' (then ''Crocodylus robustus'') within Osteolaeminae was more likely due to the anatomy of the nares, palate and the squamosal horns, although they differ in shape.

Evolution

The first remains of ''Aldabrachampsus'' were found in Quaternary deposits of Point Hodoul at the eastern end of the Aldabra Atoll. The limestone cavities that yielded these remains also preserved the fossils of lizards, tortoises, birds and land snails and date to approximately 118.000 years ago. Just before the appearance of ''Aldabrachampsus'' the atoll was entirely submerged by water, which suggests that rather than being an endemic species affected by island dwarfism, this crocodile was already of small stature by the time it arrived on the island. The fragmentary nature and poorly resolved phylogenetic relationships however make it difficult to determine where ''Aldabrachampsus'' originated. It is possible that the genus arrived in the Aldabra Atoll from the east assuming close relationship with ''Crocodylus palaeindicus'', however Brochu again suggests that a relationship to ''Voay'' and thus an African or Malagasy origin is more likely. A Malagasy origin would be consistent with the proximity between the atoll and Madagascar, which has led to faunal exchanges between the islands, especially prominent in regards to tortoises. Given the small stature of ''Osteolaemus'', it is possible that minute bodysize is ancestral to this grouping and was simply reversed by ''Voay'' on Madagascar. Another hypothesis meanwhile suggests that smaller bodysize is a trait evolved by ''Osteolaemus'' and ''Aldabrachampsus'' independently of one another.

Paleoecology

Fossil discoveries made at a dried up pond on Grande Terre Island corresponding to the Late Pleistocene Aldabra Limestone show evidence of a predator-prey relationship between crocodilians and the native giant tortoise ''Aldabrachelys gigantea''. Several tortoise shell fragments from the site are known to bear distinct feeding marks in the form of unhealed puncture wounds inflicted to the nuchal armor, plastron and the pubis of the tortoise. It is possible that a crocodile attacked and killed a tortoise head on as it was drinking at a watering hole, puncturing the nuchal plate at the front of the shell rather than attacking the high, better defended sides of the turtle. Another possibility is that the tortoise was not killed by crocodiles and instead only scavenged on. Again the punctures on the nuchal shield are caused by the fact that this particular area allows for better access at the meat beneath the shell. However, as the crocodile remains found alongside the turtle bones are notably larger than any previously recorded ''Aldabrachampsus'' specimens and very fragmentary, it's not certain if they belong to the same genus or instead represent a second species of crocodile native to the island.

References

{{Taxonbar, from=Q4713415, from2=Q15041820

Crocodylidae
Pleistocene crocodylomorphs
Monotypic prehistoric reptile genera
Prehistoric pseudosuchian genera
Fossil taxa described in 2006