| Caninemys | |
|---|---|
 | |
| Photographs and diagram of the skull | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Suborder: | Pleurodira |
| Family: | Podocnemididae |
| Genus: | Caninemys Meylanet al, 2009 |
| Type species | |
| †Caninemys tridentata Meylan et al, 2009 | |
Caninemys is anextinctgenus of large freshwaterside-necked turtle, belonging to the familyPodocnemididae. Itsfossils have been found inBrazil[2] andColombia,[3] in rocks dating back from the middle to lateMiocene.
The type specimen ofCaninemys, a well preserved skull (DNPM-MCT 1496-R), was found in 1962 inAcre State,Brazil byLlewellyn Ivor Price. Originally mentioned by Lapparent de Broin (1993) alongside several shell fragments similar toStupendemys, it was concluded that the data was insufficient 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 describedCaninemys 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 aforementioned lower jaw were used as a basis for the skull of the restoredStupendemys skeleton exhibited at theAmerican Museum of Natural History. However, for the remains of the enormousStupendemys to match the skull material ofCaninemys, the later had to be scaled up to twice its original size.[2]
In 2020 Cadena and colleagues described new specimens ofStupendemys geographica, in the process synonymizing bothS. souzai andCaninemys with this genus on the basis of lower jaws found in the same area asStupendemys fossils.[4] Later finds of additionalCaninemys material (shells and skulls) as well as new undisputedStupendemys skull material helped re-establishCaninemys as a valid genus. The new material was discovered in theColombianLa Victoria Formation in theTatacoa Desert.[3]
Caninemys is named for itsbulldog-like appearance, with the maxillary processes found in the taxon mirroring the placement of thecanine (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.[2]
The skull ofCaninemys is robust and roughly triangular in shape with thick and massive premaxilla. The eyes are dorsally oriented, facing slightly upward. Nonasal bones are present as in other members ofPelomedusoides. Instead theprefrontals 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 andparietals all contact their respective second bone at the midline. The frontals lack the groove that is characteristic forPodocnemis. Scales are preserved on the skull roof, especially over the parietals. Based on thisCaninemys had triangular interparietal scales, similar to modernPodocnemis species. Thepremaxilla are paired, but only make up a small portion of the jaws just below the nares. The triturating (grinding) surface ofCaninemys 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 modernAlligator Snapping Turtle. Themaxilla ofCaninemys 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 givesCaninemys tridentata its species name. Even without these processes the labial ridge is very pronounced inCaninemys, being thick and extending downward 2 centimeters into a flattened edge. The ridge curves upwards close to the maxilla, creating a u-shaped notch. However,Caninemys does not possess asecondary palate like inBairdemys,Stereogenys andShweboemys.[2]
The lower jaw LACM 141498 may represent the same taxon as theCaninemys 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 thatCaninemys, besides having pronounced maxilla, also had a robust and hooked lower jaw. This combination of features leads Meylanet al. to compareCaninemys to a "pleurodiran snapping turtle".[2]
Based on the head:shell ratio of extant podocnemidid species (includingPodocnemis expansa),Caninemys was likely much smaller thanStupendemys, reaching an estimated length of 1.2 to 1.5 meters.[2]
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.[2]
The following phylogenetic tree is a simplified version of the one recovered by Cadenaet al. (2021), recoveringStupendemys andCaninemys as being quite distant from one another within Podocnemididae. The scoring of this tree was significantly improved through the discovery of additional remains for both taxa. WhileStupendemys claded with the extantBig-headed Amazon River turtle,Caninemys was found to be much more basal, being recovered as the sister taxon to the clade that includes both theStupendemys branch as well as the lineage that leads to theMadagascan big-headed turtle.[3] This was later reaffirmed in the description ofPeltocephalus maturin, with the phylogenetic tree of said study being shown below.[5]
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The three tooth-like processes ofCaninemys are similar to those seen in the livingClaudius angustatus, the Narrow-bridged musk turtle, a member of a group known to utilize a specialisedhyoid 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.[2] This hypothesis was later contested by Cadena and colleagues in 2020, who instead proposed thatCaninemys (by them treated as a synonym ofStupendemys) had a much broader diet not exclusive to vertebrate prey.[4] However, in their 2021 follow-up paper, Cadena and colleagues once again inferred a more carnivorous, vacuum-feeding strategy forCaninemys in opposition to a generalist/durophagousStupendemys, arguing that these dietary differences might explain the co-occurrence of two large-bodied turtles within the same ecosystems.[3]
