Odontochelys

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Odontochelys
Temporal range: Late Triassic, 220  Ma
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Odontochelys-Paleozoological Museum of China.jpg
Cast of the paratype specimen (IVPP V 13240), Paleozoological Museum of China
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Pantestudines
Family:† Odontochelyidae
Li et al. 2008
Genus:† Odontochelys
Li et al. 2008
Species:
†O. semitestacea
Binomial name
†Odontochelys semitestacea

Odontochelys semitestacea (meaning "toothed turtle with a half-shell") is a Late Triassic relative of turtles [1] before Pappochelys was discovered and Eunotosaurus was redescribed, [2] Odontochelys was considered the oldest undisputed member of Pantestudines (i.e. a stem-turtle). It is the only known species in the genus Odontochelys and the family Odontochelyidae.

Discovery

Odontochelys semitestacea was first described from three 200-million-year-old specimens excavated in Triassic deposits in Guizhou, China. The locale of its discovery at one time was the Nanpanjiang Trough basin, a shallow marine environment surrounded on three sides by land. These deposits preserve an ecosystem known as the Guanling biota, which was dominated by marine reptiles. [1] [3]

Description

Life reconstruction as a marine reptile, a controversial ecological hypothesis Odontochelys semitestacea.jpg
Life reconstruction as a marine reptile, a controversial ecological hypothesis

Odontochelys differed grossly from modern turtles. Modern turtles have a horny beak without teeth in their mouth. In contrast, Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. [4] 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. [1] It reached nearly 40 cm (16 in) in total body length. [4]

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 is 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 scapulae of the examined specimens were identified to lack acromion processes. 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. [1]

Evidence that the plastron evolved before the carapace, as indicated by the lack of carapace in Odontochelys semitestacea, is often viewed as an indication of the aquatic origin of turtles. [1] The fossil was found in marine deposits, further supporting that the primitive turtle frequented shallow marine water. Since it is generally accepted that the shell arose to provide protection against predators, the semi-aquatic nature of turtles and the development of the plastron complement each other. Ancestral turtles with protection on their underside are more protected from predators that attack from below. [5] Based on this interpretation, the development of the carapace was likely driven in a land animal.

Reisz and Head (2008), however, have a different interpretation on the same specimen. Instead, they suggest that the carapace on O. semitestacea was in fact present; it just lacked ossification of some of its dermal components. With this interpretation, the authors suggest that either turtle shells originally evolved in aquatic environments, or this fossil represents the earliest turtle transferring from terrestrial environments to marine habitats. [3]

Paleoecology

Even though the Odontochelys specimens were found in marine deposits, there is uncertainty over whether it was a primarily aquatic reptile, in transition from aquatic to terrestrial habitats, or fully terrestrial. Li et al. (2008) argued that Odontochelys lived in coastal or freshwater environments based on its hand proportions, which were similar to those of modern turtle species which lived in small, slow-moving bodies of water. [1]

Joyce (2015) supported a terrestrial origin for turtles based on his study of Eunotosaurus. His study noted that the hands of Odontochelys were not comparable to any modern turtles due to the retention of a plesiomorphic ("primitive") feature: four phalanges (finger bones) in the third and fourth fingers, rather than three. While the total relative finger lengths of Odontochelys do line up with those of semiaquatic freshwater turtles, the individual phalanges (finger bones) themselves were short and stout, much more similar to those of modern tortoises. Aquatic turtles achieved long hands by lengthening their phalanges, while Odontochelys retained long hands due to its plesiomorphic phalangeal count, similar to that of other reptiles (including terrestrial ones). In addition, Joyce (2015) argued that even if Odontochelys was semiaquatic, it probably would have avoided open marine waters due to a lack of efficient adaptations for swimming. [2]

On the other hand, a pathological study of Odontochelys performed by Rothschild & Naples (2015) discovered that both the left and right humeri (forearm bones) of the paratype specimen (IVPP 13240) of Odontochelys had been degraded near the shoulder sockets. The study rejected explanations such as weathering or a wound-induced bone infection, arguing that it would not have made sense for the shoulder area to degrade before the rest of the forelimbs, since the shoulder was more well-protected during life and after death. Instead, the study argued that decompression sickness was responsible for the injury. This condition has been observed in diving animals which are forced to make rapid ascents within a deep marine environment. Similar injuries have been reported in other fossilized marine reptiles, and their presence in Odontochelys supports the idea that it lived in an open marine environment. Modern sea turtles utilize behavioral tactics to avoid rapid ascension within water, which may also indicate that Odontochelys had not yet acquired the same behaviors to defend against decompression sickness. [6]

Related Research Articles

<span class="mw-page-title-main">Turtle</span> Order of reptiles characterized by a shell

Turtles are reptiles of the order Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira and Cryptodira, which differ in the way the head retracts. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins. They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. Like other amniotes they breathe air and do not lay eggs underwater, although many species live in or around water.

<i>Dinocephalosaurus</i> Extinct genus of reptiles

Dinocephalosaurus is a genus of long necked, aquatic protorosaur that inhabited the Triassic seas of China. The genus contains the type and only known species, D. orientalis, which was named by Chun Li in 2003. Unlike other long-necked protorosaurs, Dinocephalosaurus convergently evolved a long neck not through elongation of individual neck vertebrae, but through the addition of neck vertebrae that each had a moderate length. As indicated by phylogenetic analyses, it belonged in a separate lineage that also included at least its closest relative Pectodens, which was named the Dinocephalosauridae in 2021. Like tanystropheids, however, Dinocephalosaurus probably used its long neck to hunt, utilizing the fang-like teeth of its jaws to ensnare prey; proposals that it employed suction feeding have not been universally accepted. It was probably a marine animal by necessity, as suggested by the poorly-ossified and paddle-like limbs which would have prevented it from going ashore.

<i>Proganochelys</i> Extinct genus of turtles

Proganochelys is a genus of extinct, primitive stem-turtle. Proganochelys was named by Georg Baur in 1887 as the oldest turtle in existence at the time. The name Proganochelys comes from the Greek word ganos meaning 'brightness', combined with prefix pro, 'before', and Greek base chelys meaning 'turtle'. Proganochelys is believed to have been around 1 meter in size and herbivorous in nature. Proganochelys was known as the most primitive stem-turtle for over a century, until the novel discovery of Odontochelys in 2008. Odontochelys and Proganochelys share unique primitive features that are not found in Casichelydia, such as teeth on the pterygoid and vomer and a plate-like coracoid.

<i>Desmatochelys</i> Extinct genus of turtles

Desmatochelys is an extinct genus of sea turtles belonging to the family Protostegidae. This genus contains two known species, D. lowii and D. padillai. D. lowii was first discovered in 1895, followed by D. padillai in 2015. Having been estimated at over 120 million years old, D. padillai is currently the oldest known species of sea turtle.

<span class="mw-page-title-main">Turtle shell</span> Shield for the ventral and dorsal parts of turtles

The turtle shell is a shield for the ventral and dorsal parts of turtles, completely enclosing all the vital organs of the turtle and in some cases even the head. It is constructed of modified bony elements such as the ribs, parts of the pelvis and other bones found in most reptiles. The bone of the shell consists of both skeletal and dermal bone, showing that the complete enclosure of the shell likely evolved by including dermal armor into the rib cage.

<i>Acerosodontosaurus</i> Extinct genus of reptiles

Acerosodontosaurus is an extinct genus of neodiapsid reptiles that lived during the Late Permian of Madagascar. The only species of Acerosodontosaurus, A. piveteaui, is known from a natural mold of a single partial skeleton including a crushed skull and part of the body and limbs. The fossil was discovered in deposits of the Lower Sakamena Formation. Based on skeletal characteristics, it has been suggested that Acerosodontosaurus individuals were at least partially aquatic.

<i>Eunotosaurus</i> Extinct genus of reptiles

Eunotosaurus is an extinct genus of amniote, possibly a close relative of turtles. Eunotosaurus lived in the late Middle Permian and fossils can be found in the Karoo Supergroup of South Africa. Eunotosaurus resided in the swamps of southern Africa. Its ribs were wide and flat, forming broad plates similar to a primitive turtle shell, and the vertebrae were nearly identical to those of some turtles. Accordingly, it is often considered as a possible transitional fossil between turtles and their prehistoric ancestors. However, it is possible that these turtle-like features evolved independently of the same features in turtles, since other anatomical studies and phylogenetic analyses suggest that Eunotosaurus may instead have been a parareptile, an early-diverging neodiapsid unrelated to turtles, or a synapsid.

<i>Henodus</i> Extinct species of reptile

Henodus is an extinct placodont of the Late Triassic period during the early Carnian age. Fossils of Henodus chelyops were found in the Estherienschichten Member of the Grabfeld Formation, near Tübingen, Germany. It was around 1 metre (3.3 ft) in length. The single species within the genus is H. chelyops.

Psephosauriscus is an extinct genus of placodont reptile from the Middle Triassic of Israel and Egypt. It is known from bony armor plates that have been found from Makhtesh Ramon in Israel's Negev desert and Araif en Naqua on Egypt's Sinai Peninsula. The genus was erected in 2002 as a replacement name for several species of the genus Psephosaurus, which was named in 1957. It includes the species P. mosis, P. ramonensis, P. sinaiticus, and a possible fourth species, P. rhombifer. All species, with the exception of P. ramonensis, were once assigned to the genus Psephosaurus. Remains of P. mosis and P. ramonensis were found in Makhtesh Ramon, while P. sinaiticus and P. rhombifer were found in Araif en Naqua.

<i>Diandongosuchus</i> Extinct genus of reptiles

Diandongosuchus is an extinct genus of archosauriform reptile, possibly a member of the Phytosauria, known from the Middle Triassic of China. The type species Diandongosuchus fuyuanensis was named in 2012 from the Zhuganpo Formation of Yunnan Province. It is a marine species that shows similarities with another Chinese Triassic species called Qianosuchus mixtus, although it has fewer adaptations toward marine life. It was originally classified as the basal-most member of the pseudosuchian clade Poposauroidea. However, a subsequent study conducted by Stocker et al. indicated it to be the basalmost known phytosaur instead.

Jaxtasuchus is an extinct genus of armored doswelliid archosauriform reptile known from the Middle Triassic of the Erfurt Formation in Germany. The type species, Jaxtasuchus salomoni, was named in 2013 on the basis of several incomplete skeletons and other isolated remains. Like other doswelliids, members of the genus were heavily armored, with four longitudinal rows of bony plates called osteoderms covering the body. Jaxtasuchus is the first doswelliid known from Europe and is most closely related to Doswellia from the Late Triassic of the eastern United States. However, it was not as specialized as Doswellia, retaining several generalized archosauriform characteristics and having less armor. Jaxtasuchus fossils have been found in aquatic mudstones alongside fossils of temnospondyl amphibians, crustaceans, and mollusks, suggesting that Jaxtasuchus was semiaquatic like modern crocodilians.

Chinlechelys is an extinct genus of stem-turtle belonging to Testudinata. It lived in the Norian age of the Late Triassic and is the oldest turtle known from North America. Among turtles it is unique, mostly because of its very thin shell. The type and only species, C. tenertesta, was named and described with the genus by Walter G. Joyce et al. in 2009. It was probably terrestrial, and was found by Joyce et al. to be closely related to Proganochelys, another terrestrial testudinatan.

<span class="mw-page-title-main">Pantestudines</span> Clade of reptiles

Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles and all of their extinct relatives. Pantestudines with a complete shell are placed in the clade Testudinata.

<i>Pappochelys</i> Extinct genus of reptiles

Pappochelys is an extinct genus of diapsid reptile possibly related to turtles. The genus contains only one species, Pappochelys rosinae, from the Middle Triassic of Germany, which was named by paleontologists Rainer Schoch and Hans-Dieter Sues in 2015. The discovery of Pappochelys provides strong support for the placement of turtles within Diapsida, a hypothesis that has long been suggested by molecular data, but never previously by the fossil record. It is morphologically intermediate between the definite stem-turtle Odontochelys from the Late Triassic of China and Eunotosaurus, a reptile from the Middle Permian of South Africa.

<i>Basilemys</i> Extinct genus of turtles

Basilemys is a large, terrestrial nanhsiungchelyid turtle from the Upper Cretaceous of North and Central America. Basilemys has been found in rocks dating to the Campanian and Maastrichtian subdivisions of the Late Cretaceous and is considered to be the largest terrestrial turtle of its time. In an analysis made by Sukhanov et al. on a nansiunghelyid turtle from the Upper Cretaceous of Mongolia, it was demonstrated that Asian nanhsiungchelyids gave rise to the North American nanhsiungchelyids.

Litorosuchus is a genus of armored, semiaquatic archosauriform reptile from the Middle Triassic of China, closely related to the morphologically similar Vancleavea. It contains one species, L. somnii.

<i>Pectodens</i> Extinct genus of reptiles

Pectodens is an extinct genus of archosauromorph reptile which lived during the Middle Triassic in China. The type and only species of the genus is P. zhenyuensis, named by Chun Li and colleagues in 2017. It was a member of the Archosauromorpha, specifically part of the unnatural grouping Protorosauria. However, an unusual combination of traits similar and dissimilar to other protorosaurs initially led to confusion over its evolutionary relationships. In 2021, it was placed in a newly-established group, Dinocephalosauridae, along with its closest relative Dinocephalosaurus.

Eorhynchochelys is an extinct genus of stem-turtle from the Late Triassic Xiaowa Formation of southwestern China.

The Xiaowa Formation is a Carnian-age geological formation found in southern China. It is a sequence of limestone and marls from the Carnian stage of the Triassic. Its lower section was previously known as the Wayao Formation or Wayao Member of the Falang Formation. In 2002, the Wayao Member was renamed and raised to the Xiaowa Formation to prevent confusion with an Eocene unit of the same name. Crinoids and marine reptiles are abundant in the Xiaowa Formation, forming a lagerstätte known as the Guanling biota. Ammonoids and conodonts found in the formation constrain its age to the early Carnian. Reptiles of the Guanling biota include ichthyosaurs, thalattosaurs, placodonts, and Odontochelys. Sedimentary events within this formation have been tied to the Carnian Pluvial Event.

<i>Drazinderetes</i> Extinct genus of turtles

Drazinderetes is a large bodied genus of soft shell turtle from the Middle Eocene Drazinda Formation of Pakistan. Its presence in the shallow marine deposits of the Drazinda Formation suggests that Drazinderetes may have been a partially or fully marine animal. Indetermined trionychine remains from the same formation may suggest that Drazinderetes could have been among the largest known turtles, with one entoplastron indicating a potential length of 1.5 to 2.1 meters. Drazinderetes currently consists of only a single species: Drazinderetes tethyensis.

References

  1. 1 2 3 4 5 6 Li, Chun; Wu, Xiao-Chun; Rieppel, Olivier; Wang, Li-Ting; Zhao, Li-Jun (27 November 2008). "An ancestral turtle from the Late Triassic of southwestern China". Nature. 456 (7221): 497–501. Bibcode:2008Natur.456..497L. doi:10.1038/nature07533. PMID   19037315. S2CID   4405644.
  2. 1 2 Joyce, Walter G. (2015). "The origin of turtles: A paleontological perspective" (PDF). Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 324 (3): 181–193. doi:10.1002/jez.b.22609. ISSN   1552-5015. PMID   25712176.
  3. 1 2 Reisz, Robert R.; Jason J. Head (27 November 2008). "Palaeontology: Turtle origins out to sea". Nature. 456 (7221): 450–451. doi:10.1038/nature07533. PMID   19037315. S2CID   4405644.
  4. 1 2 Hans-Dieter Sues (August 6, 2019). The Rise of Reptiles. 320 Million Years of Evolution. Johns Hopkins University Press. pp. 50–51. ISBN   9781421428680.
  5. ↑ Gilbert, S.F. (2007). "How the turtle gets its shell". Biology of Turtles (The Structures to Strategies of Life).
  6. ↑ Rothschild, Bruce M.; Naples, Virginia (2015). "Decompression syndrome and diving behavior in Odontochelys, the first turtle" (PDF). Acta Palaeontologica Polonica. 60 (1): 163–167. doi: 10.4202/app.2012.0113 .
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