Peltocephalus maturin

Peltocephalus maturin was described in 2024 on the basis of specimen MERO.PV.H 007, a partial lower jaw discovered by gold miners within the Taquaras Quarry of Rondônia, Brazil. Since the Rio Madeira Formation is the only geological unit exposed at this quarry, the fossil is subsequently assumed to be Late Pleistocene to Early Holocene in age. ==Description==

Although the type material of Peltocephalus maturin is restricted to an incomplete mandible, a lot of information has been derived from this material. The mandibular symphysis are fused, clearly placing the animal within Pelomedusoides. The labial and lingual ridges, which are located to either side of the triturating (cutting or grinding) surface of the lower jaw, are straight, which sets them apart from the curved ridges seen in species of Podocnemis, the South American river turtles. It further differs from species of that genus by having a U-shaped outline of the midline of the lingual ridges and a lingual platform that is located towards the back on the mandible, ventrally (below) to the triturating surface and the symphyseal fossa. This fossa contains two pits, which can be used to differentiate the mandible of Peltocephalus maturin from the extant Madagascan big-headed turtle of the genus Erymnochelys. Impressions left by the rhamphotheca, the outermost keratinous layer of the beak, indicate that its back edge sloped dorsoventrally (from the top to the bottom of the jaw), whereas in both Podocnemis and Erymnochelys it slopes posteroventrally, meaning both down and towards the back of the jaw. A striking feature of Peltocephalus maturin is the greatly enlarged symphyseal hook that is formed at the tip of the lower jaw. The fact that this hook ends above the coronoid process, together with the lingual platform and the presence of two pits located within the symphyseal fossa, clearly identify it as a relative of the Big-headed Amazon River turtle, thus its placement within the genus Peltocephalus. ==Phylogeny==

Phylogenetic analysis confirmed the relationship between Peltocephalus maturin and Peltocephalus dumerilianus, both of which forming a clade that is deeply nested within Podocnemididae. The levels of resolution do differ between the two analysis conducted, with the strict consensus being poorly resolved though still supporting the clade formed by the two Peltocephalus species. The Majority Rule Tree, depicted below, yielded better resolved results that are in line with the internal relationships that were recovered by previous studies. Within this tree, Peltocephalus species are the basalmost off-shoot of a branch that also includes Stupendemys and the various species of Bairdemys, together forming one of two distinct branches within Erymnochelyinae. }}}} ==Paleobiology==

Diet in turtles can generally be inferred based on the height of the skull as well as the proportions and complexity of the triturating surface, which is used to cut and grind food. In this regard, Peltocephalus maturin bears close resemblance to its modern relative. Overall, podocnemidids are omnivores that consume varying levels of plant material, with Peltocephalus dumerilianus incorporating the greatest amount of other animals into its diet including apple snails. This is expressed through a much more simple triturating surface than what is seen in more herbivorous podocnemidids. The narrow triturating surface of Peltocephalus maturin is not especially well suited for durophagy nor herbivory, however at the same time the midline ridge that separates the triturating surfaces from another does suggest slightly less carnivorous habits than those of P. dumerilianus. The great size of this species might have been a factor that allowed it to fill a different nische from other podocnemidids, something that has been a proposed factor in the coexistence of the large-bodied Caninemys and Stupendemys during the Miocene and could also be responsible for the coexistence of today's Amazonian podocnemidids. The evolution of such a large bodysize further matches certain patterns within reptile evolutions, with prior size increases being noted to have taken place during the Paleogene in podocnemidids and snakes (like Titanoboa) as well as during the Miocene with podocnemidids and crocodilians (like Purussaurus). The discovery of Peltocephalus maturin matches the great size seen in certain Quaternary squamates like the varanid megalania (Varanus priscus), the teiid Tupinambis uruguaianensis, and the madtsoiid Wonambi. Other testudines reached massive sizes during this time as well, such as Megalochelys atlas. ==Extinction==

Ferreira and colleagues speculate that the extinction of Peltocephalus maturin may have been tied to exploitation by humans. Part of their reasoning is based on the extinction of various island species such as the turtles of the Turks and Caicos Islands as well as the meiolanids of Vanuatu and New Caledonia which are often thought to have been driven to extinction by human overhunting. Some of the earliest evidence for humans arriving in South America dates to roughly 12.6 to 9.8 thousand years BP and thus overlap with the presence of Peltocephalus maturin. Furthermore, sites that show signs of human habitation also preserve the remains of both tortoises and podocnemidids, with even modern humans still hunting turtles for their meat. At the same time, there is no evidence that humans and Peltocephalus maturin ever actually met, meaning that the reason for this species' extinction remains uncertain until the discovery of better materials provides more clues. ==References==