Odontochelys
Encyclopedia
Odontochelys semitestacea is an extinct turtle
species, the oldest known one. It is the only known species in the genus Odontochelys and the family Odontochelyidae. O. semitestacea was first described from three 220 million year-old specimens excavated in Triassic
deposits in Guizhou
, China
.
Odontochelys differed grossly from modern turtles. Modern turtles possess a horny beak
without any teeth in their mouth. In contrast, Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. One of the most striking features of turtles, both modern and prehistoric alike, are their dorsal shells, forming an armored carapace over the body of the animal. Odontochelys only possessed the bottom portion of a turtle's armor, the plastron. It did not yet have a solid carapace
as most other turtles do. Instead of a solid carapace, Odontochelys possessed broadened ribs like those of modern turtle embryos that still have not started developing the ossified
plates of a carapace.
Aside from the presence of teeth and the absence of a solid carapace, a few other skeletal traits distinguish Odontochelys as basal
compared to other turtles, extant and otherwise. The point of articulation between the dorsal ribs and the vertebrae are decidedly different in Odontochelys than in later turtles. In a comparison of skull
proportions, the skull of Odontochelys is far more elongated pre-orbitally (in front of the eyes) compared to other turtles. The tail of the prehistoric turtle was longer in proportion to its body than other turtles. In addition, the transverse processes found in the tail are not fused such as in later turtles. Also, the scapula
e of the examined specimens were identified to lack acromion process
es. Taken together, these anatomical differences have been interpreted by the discoverers to mean that Odontochelys has some of the most primitive features ever seen in a turtle and is somewhat of a transitional fossil
. Some scientists, however, are skeptical of this idea. Reisz and Head propose that Odontochelys does not represent a turtle with a partly evolved shell, but that it is a descendant of an older, land–living turtle, and that the shell became reversed. Such shells can be seen on modern turtles, like the leatherback turtles (the genus
Dermochelys).
It is likely that Odontochelys was aquatic, as the fossil
specimens were found in marine deposits rife with conodont
s and ammonite
s. It is theorized that the primitive turtle frequented shallow marine waters close to shore.
The species' name, Odontochelys semitestacea literally means "toothed turtle with a half-shell" - an apt description of its most striking physical characteristics.
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
species, the oldest known one. It is the only known species in the genus Odontochelys and the family Odontochelyidae. O. semitestacea was first described from three 220 million year-old specimens excavated in Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
deposits in Guizhou
Guizhou
' is a province of the People's Republic of China located in the southwestern part of the country. Its provincial capital city is Guiyang.- History :...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
Odontochelys differed grossly from modern turtles. Modern turtles possess a horny beak
Beak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
without any teeth in their mouth. In contrast, Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. One of the most striking features of turtles, both modern and prehistoric alike, are their dorsal shells, forming an armored carapace over the body of the animal. Odontochelys only possessed the bottom portion of a turtle's armor, the plastron. It did not yet have a solid carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
as most other turtles do. Instead of a solid carapace, Odontochelys possessed broadened ribs like those of modern turtle embryos that still have not started developing the ossified
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
plates of a carapace.
Aside from the presence of teeth and the absence of a solid carapace, a few other skeletal traits distinguish Odontochelys as basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
compared to other turtles, extant and otherwise. The point of articulation between the dorsal ribs and the vertebrae are decidedly different in Odontochelys than in later turtles. In a comparison of skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
proportions, the skull of Odontochelys is far more elongated pre-orbitally (in front of the eyes) compared to other turtles. The tail of the prehistoric turtle was longer in proportion to its body than other turtles. In addition, the transverse processes found in the tail are not fused such as in later turtles. Also, the scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
e of the examined specimens were identified to lack acromion process
Acromion process
The acromion process, or simply the acromion , is an anatomical feature on the shoulder blade , together with the coracoid process extending laterally over the shoulder joint.-In humans:...
es. Taken together, these anatomical differences have been interpreted by the discoverers to mean that Odontochelys has some of the most primitive features ever seen in a turtle and is somewhat of a transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
. Some scientists, however, are skeptical of this idea. Reisz and Head propose that Odontochelys does not represent a turtle with a partly evolved shell, but that it is a descendant of an older, land–living turtle, and that the shell became reversed. Such shells can be seen on modern turtles, like the leatherback turtles (the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Dermochelys).
It is likely that Odontochelys was aquatic, as the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
specimens were found in marine deposits rife with conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
s and ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s. It is theorized that the primitive turtle frequented shallow marine waters close to shore.
The species' name, Odontochelys semitestacea literally means "toothed turtle with a half-shell" - an apt description of its most striking physical characteristics.
External links
- Figures and Tables from the above-referenced article An ancestral turtle from the Late Triassic of southwestern China in the Journal Nature
- Oldest Turtle Found; May Crack Shell-Evolution Mystery
- First known turtle swam on the half shell