Caninemys Skull

''Caninemys'' is an extinct genus of large freshwater side-necked turtle, belonging to the family Podocnemididae. Its fossils have been found in Brazil and Colombia, in rocks dating back from the middle to late Miocene.

Discovery and naming

The type specimen of ''Caninemys'', a well preserved skull (DNPM-MCT 1496-R), was found in 1962 in Acre State, Brazil by Llewellyn Ivor Price. Originally mentioned by Lapparent de Broin (1993) alongside several shell fragments similar to ''Stupendemys'', it was concluded that the data was not sufficient enough to refer the skull to the massive podocnemid, especially in the light of other similarly sized turtle remains from Miocene South America. In 2009 Meylan, Gaffney and De Almeida Campos described ''Caninemys'' on the basis of the skull alone, not including the shell fragments mentioned by de Broin due to the possibility of a chimeric combination. They do however consider it very probable that LACM 141498, a lower jaw, belongs to the same taxon. Both the holotype skull and the afforementioned lower jaw were used as a basis for the skull of the restored ''Stupendemys'' skeleton exhibited at the American Museum of Natural History. However, for the remains of the enormous ''Stupendemys'' to match the skull material of ''Caninemys'', the later had to be scaled up to twice its original size.

In 2020 Cadena and colleagues described new specimens of '' Stupendemys geographica'', in the process synonymizing both ''S. souzai'' and ''Caninemys'' with this genus on the basis of lower jaws found in the same area as ''Stupendemys'' fossils. Later finds of additional ''Caninemys'' material (shells and skulls) as well as new undisputed ''Stupendemys'' skull material helped re-establish ''Caninemys'' as a valid genus. The new material was discovered in the Colombian La Victoria Formation in the Tatacoa Desert.

''Caninemys'' is named for its bulldog-like appearance, with the maxillary processes found in the taxon mirroring the placement of the canine (tooth) in mammals. The species name derives from the tridentate ("three toothed") appearance of the skull, as most easily observed when viewed from the front.

Description

The skull of ''Caninemys'' is robust and roughly triangular in shape with thick and massive premaxilla. The eyes are dorsally oriented, facing slightly upward. No nasal bones are present as in other members of Pelomedusoides. Instead the prefrontals make up most of the anterior portion of the skull, including the area just above the nares. The prefrontals extend past the maxilla, giving it a somewhat stub-nosed appearance. The paired prefrontals, frontals and parietals all contact their respective second bone at the midline. The frontals lack the groove that is characteristic for ''Podocnemis''. Scales are preserved on the skull roof, especially over the parietals. Based on this ''Caninemys'' had triangular interparietal scales, similar to modern ''Podocnemis'' species. The premaxilla are paired, but only make up a small portion of the jaws just below the nares. The triturating (grinding) surface of ''Caninemys'' is notably more complex than in modern podocnemidids, with a sharp labial ridge that forms a small but distinct toothlike process at the midline contact of the premaxilla. The premaxilla also form a large recess to accommodate for a hooked lower jaw, as seen in the modern Alligator Snapping Turtle. The maxilla of ''Caninemys'' are noticeably enlarged and give the animal a superficially "bulldog-like" appearance. The maxilla also stands out for the large, tooth-like processes formed by the labial ridge and flanking the premaxilla on both sides. These processes, superficially resembling the canine teeth seen in mammals, are very noticeable when the skull is viewed head-on. These two processes, alongside the premaxillary process, account for the tridentate condition that gives ''Caninemys tridentata'' its species name. Even without these processes the labial ridge is very pronounced in ''Caninemys'', being thick and extending downward 2 centimers into a flattened edge. The ridge curves upwards close to the maxilla, creating a u-shaped notch. However, ''Caninemys'' does not possess a secondary palate like in '' Bairdemys'', '' Stereogenys'' and '' Shweboemys''.

The lower jaw LACM 141498 may represent the same taxon as the ''Caninemys'' holotype. The lower jaw possesses a pronounced symphyseal hook similar to those seen in modern alligator snapping and musk turtles. Although the jaw is slightly too large to belong to the exact same specimen as the holotype, the jaws line up well. While there is no direct link between the two fossils, they match closely enough to suggest they belong to at least closely related taxa, if not the same species. This would suggest that ''Caninemys'', besides having pronounced maxilla, also had a robust and hooked lower jaw. This combination of features leads Meylan ''et al.'' to compare ''Caninemys'' to a "pleurodiran snapping turtle".

Based on the head:shell ratio of extant podocnemidid species (including ''Podocnemis expansa''), ''Caninemys'' was likely much smaller than ''Stupendemys'', reaching an estimated length of 1.2 to 1.5 meters.

Phylogeny

''Caninemys'' shows multiple morphological characters that clearly identify it as a pelomedusoid pleurodire, specifically a podocnemidid. The phylogenetic tree below is based on the initial analysis conducted in 2009.

The following phylogenetic tree is a simplified version of the one recovered by Cadena ''et al.'' (2021), recovering ''Stupendemys'' and ''Caninemys'' as members of two separate branches within Podocnemididae. The scoring of this tree was significantly improved through the discovery of additional remains for both taxa. While ''Stupendemys'' claded with the extant Big-headed Amazon River turtle, ''Caninemys'' closest living relative was found to be the Madagascan big-headed turtle.

Paleobiology

The three tooth-like processes of ''Caninemys'' are similar to those seen in the living '' Claudius angustatus'', the Narrow-bridged musk turtle, a member of a group known to utilize a specialised hyoid apparatus to capture highly mobile prey via vacuum feeding. Such a feeding style in combination with the massive jaws and tooth-like processes may suggests that it would have been a predatory animal with a lifestyle similar to snapping turtles, feeding on fish, crocodilians and snakes respectively. This hypothesis was later contested by Cadena and colleagues in 2020, who instead proposed that ''Caninemys'' (by them treated as a synonym of ''Stupendemys'') had a much broader diet not exclusive to vertebrate prey. However, in their 2021 follow-up paper, Cadena and colleagues once again inferred a more carnivorous, vacuum-feeding strategy for ''Caninemys'' in opposition to a generalist/durophagous ''Stupendemys'', arguing that these dietary differences might explain the co-occurrence of two large-bodied turtles within the same ecosystems.

References

{{Testudines

Prehistoric turtles of South America
Podocnemididae
Prehistoric turtle genera
Extinct turtles
Miocene turtles
Miocene genus first appearances
Miocene extinctions
Miocene reptiles of South America
Neogene Brazil
Fossils of Brazil
Neogene Colombia
Fossils of Colombia
Fossil taxa described in 2009