Ocepechelon
Ocepechelon | |
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Skull and size comparison of Ocepechelon | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Order: | Testudines |
Suborder: | Cryptodira |
Clade: | Panchelonioidea (?) |
Family: | †Protostegidae |
Genus: | †Ocepechelon Bardet et al, 2013 |
Type species | |
†Ocepechelon bouyai Bardet et al, 2013
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Ocepechelon is an
Description
Ocepechelon is known solely from an isolated yet complete skull, 70 centimetres (28 in) in length. From the size of the skull, it must have been one of the largest turtles that ever lived, with estimated carapace length of 130–150 centimetres (4.3–4.9 ft) and total length of 240 centimetres (7.9 ft).[1] The skull of this genus was not only exceptional in size, but also in appearance: large at the back, it narrowed at the front to form a sort of elongated, narrow, bony funnel.[1] The lower jaw and hyoid remain unknown.
The skull roof of Ocepechelon is dorsoventrally compressed, with a wide cerebral cavity and a lateral wall of the cranial cavity that is bowed anteromedially.[1] Its longirostrine snout has been compared to a pipette, and is unique among all tetrapods. Its flattened, streamlined skull, as well as the dorsal position of its orbits and nares, suggest it was a seagoing species which hunted close to the water's surface. This hypothesis is supported by sedimentological analysis and its skull anatomy. Several aspects of Ocepechelon's skull anatomy suggest it may have been a suction feeder, namely the tube-like upper jaw. In general, the skull must have looked something like a toothless crocodile: there were no teeth, but the eyes were upturned, as were the nostrils, which were set far back. It is the only tetrapod to have developed an evolutionary convergence with both syngnathid fishes and beaked whales, since it possessed a large head but a long, narrow, toothless snout.[1]
Like other genera in the Panchelonioidea, it likely possessed flippers, allowing an active swimming lifestyle. Material potentially referable to Ocepechelon supports this hypothesis.[1]
Phylogeny
Due to the limited fossil material available, Bardet et al. (2013) used numerous methods to assess the phylogenetic position of the animal. The removal of outgroup taxa, except for the hypothetical taxon, deliberated some relationships and showed that Ocepechelon and Bouliachelys could be basal dermochelyoids, forming a polytomy with the Dermochelyidae and Protostegidae. The inclusion of Chelomacryptodira resolved this polytomy, and suggested that Ocepechelon was the most basal dermochelyid known.[1]
Below is a cladogram from the description of Ocepechelon, by Bardet et al. (2013).
However, Scavezzoni & Fischer (2018),[2] in their phylogenetic analysis, showed that Dermatochelyidae and Protostegidae did not form a clade, and classified Ocepechelon within Protostegidae, as a sister taxon to Alienochelys. Below is a subsequent phylogeny based on Scavezzoni & Fischer's cladogram and Evers et al. (2019)[3] who both recovered it as a protostegid.
Panchelonioidea |
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Paleobiology
The remarkable similarities between Ocepechelon and