''Proganochelys'' is an extinct, primitive stem-turtle that has been hypothesized to be the sister taxon to all other turtles creating a monophyletic group, the ''Casichelydia''. ''Proganochelys'' was named by

Georg Baur Georg Baur (1859–1898) was a German vertebrate paleontologist and Neo-Lamarckian who studied reptiles of the Galapagos Islands, particularly the Galápagos tortoises, in the 1890s. He is perhaps best known for his subsidence theory of the o ...

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 A century is a period of 100 years. Centuries are numbered ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is sometimes abbreviated as c. A centennial or ...

, 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 The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxill ...

and a plate-like coracoid. ''Proganochelys'' is the oldest stem-turtle species with a complete shell discovered to date, known from fossils found in Germany,

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

, Greenland, and Thailand in strata from the late Triassic, dating to approximately 210 million years ago. The location of these fossils suggest that ''Proganochelys'' was active throughout the continent of

Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...

. There are only two known species of ''Proganchelys'', with little information as a result of a small fossil record. All ''Proganochelys quentesti'' fossils were discovered in Germany, while ''Proganochelys ruchae'' fossils were found in Thailand. ''Psammochelys'', ''Stegochelys'', and ''Triassochelys'' are junior synonyms of ''Proganochelys''. ''Chelytherium'' von Meyer, 1863 has been considered a synonym of ''Proganochelys'' by some authors, but Joyce (2017) considers it a ''nomen dubium'' given the fragmentary nature of the syntype material. Joyce (2017) also considered North American genus ''

Chinlechelys ''Chinlechelys'' ( meaning ''Chinle turtle'') 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 b ...

'' to be a junior synonym of ''Proganochelys'', though the author maintains the type species of the former genus, ''C. tenertesta'', as a distinct species within the genus ''Proganochelys''.

Description and paleobiology

''Proganochelys'' was once considered to be the oldest known stem-turtle until the description of '' Odontochelys'' and '' Eorhynchochelys'', a slightly earlier genera that lived in the Carnian stage of the Triassic. In had a fully developed shell long. A total length of ''Proganochelys'' was about . Its overall appearance resembled modern turtles in many respects: it lacked teeth on the upper and lower jaw, likely had a beak and had the characteristic heavily armored shell formed from bony plates and ribs which fused together into a solid cage around the internal organs. ''Proganochelys'' had a semi-beak like structure along with teeth fused to its

. The plates comprising the

carapace A carapace is a Dorsum (biology), dorsal (upper) 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 tor ...

and plastron were already in the modern form, although there were additional plates along the margins of the shell that would have served to protect the legs. Also unlike any modern species of turtle, its long tail had spikes and terminated in a club, its head could not be retracted under the shell and its neck may have been protected by small spines. While it had no teeth in its jaws, it did have small denticles on the palate. The beak like structure suggests that the Triassic stem-turtles evolved from

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...

stem-turtles to herbivorous as the loss of teeth and gain of the beak would benefit the crushing of plants in these stem-turtles.

Synapomorphies and autapomorphies

''Proganochelys'' possess a few chelonian

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

including: a bony shell containing fused ribs, neural bones with fused thoracic segments, and a carapace and plastron that enclose the pelvic and shoulder girdle. ''Proganochelys'' was also known for its autapomorphy features which included a tail club and a tubercle on the basioccipital. The tail of ''Proganochelys'' was noticeably long and is hypothesized to have been used as a club for protection against predators. Although evolution of the shell has been clearly defined, the mechanisms behind the movement of the neck has been a subject of debate for ''Proganochelys''. It has been hypothesized that ''Proganochelys'' were able to retract their necks by tucking in their skull under the front of their shell when needed.

Shell

The broadened ribs on ''Proganochelys'' show "metaplastic ossification of the

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

". The enlarged ribs suggest that the

endochondral Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is produced. Unlike intramembranous ossification, the other process by which bone tissue is produced, ...

rib ossifications were joined by a second ossification instead of having expanded ribs. The 220-million-year-old stem-turtle ''Odontochelys'' only has a partially formed shell. ''Odontochelys'' is believed to only possess the underside element of a shell known as a plastron. The 5-million-year difference that distinguish ''Odontochelys'' from ''Proganochelys'' tell us that the evolution of the shell occurred relatively quickly in time. ''Proganochelys'' possess both a carapace, the upper formation of the shell, and the plastron, the lower. The shell is believed to be used for protection an enhanced feature for survival. ''Proganochelys'' fits well into the order as a turtle, as the shell of ''Proganochelys'' is in agreement with the evolution of other stem-turtles.

Skull

The dermal roofing elements of ''Proganochelys'' include a large

nasal Nasal is an adjective referring to the nose, part of human or animal anatomy. It may also be shorthand for the following uses in combination: * With reference to the human nose: ** Nasal administration, a method of pharmaceutical drug delivery ** ...

, a fully roofed skull, a flat squamosal, and an absent pineal foramen. Palatal characteristics include paired vomers, and a dorsal process containing premaxilla. An open interpterygoid vacuity along with a prominent elongated quadrate are notable basicranial elements. Overall, ''Pragonchelys'' is characterized by having few chelonian features and having a relatively generalized amniote skull. The skull of ''Proganochelys Quenstedti'' from Trossingen, West Germany, retains a number of well-known amniote features not found in any other turtle. For instance, the lacrimal bone, supratemporal bone, and

lacrimal duct The lacrimal canaliculi, (sing. canaliculus), are the small channels in each eyelid that drain lacrimal fluid, from the lacrimal puncta to the lacrimal sac. This forms part of the lacrimal apparatus that drains lacrimal fluid from the surface of ...

are notable structures that are kept. Furthermore, some traits that are present in modern turtles are not present in ''Proganochelys'' and therefore must have come after the evolution of the shell. For instance, jaw differentiation, the fusion of the vomer, and the loss of the lacrimal are clear examples of traits that evolved after the evolution of the shell in ''Proganochelys''. Proganochelys quenstedti 1

Discovery

The earliest fossils of ''Proganochelys'' were discovered in Germany in the rural towns of Halberstadt, Tübingen, and Trossingen. The fossils were found in an elaborate formation of shales, sandstones, and some limestone piles, with the formation believed to be between 220 and 205 million years old. Consensus among Geologists placed the fossils in the middle of the Norian, around 210 million years ago, although this is largely an estimate. In addition to ''Proganochelys'', the rock formations in Germany have also given fossils for the stem-turtle ''

Proterochersis ''Proterochersis'' is an extinct genus of turtle from the Late Triassic period ( Norian stage) of Europe. It is known from a large number of fossils uncovered in Germany and Poland.Fraas E (1913)"''Proterochersis'', eine pleurodire Schildkröte aus ...

''. Fossils have also been found the

Klettgau Formation The Klettgau Formation is a geological formation in Switzerland. It is Late Triassic in age, covering most of the mid to late Norian, the Carnian, and into the Rhaetian, spanning a period of 26-30 million years.ecology of the Late Triassic stem-turtles has been disputed and a major point of disagreement for many years among scientists. Triassic stem-turtles, including ''Proganochelys'', appear to have been both aquatic and terrestrial. Shell proportions are believed to be correlated to the environment in which a turtle lives in, seen in modern turtles today. Using this concept, scientists were able to infer on the habitat in which ''Proganochelys'' may have lived in. A comparison between modern turtles and ''Proganochelys'' found that it was not likely that stem-turtles had differentiated into specialized ecologies such as open water swimmers or solely terrestrial turtles in the Late Triassic period. If this is the case, a freshwater habitat would be the most likely environment for ''Proganochelys'' to have lived in. On the other hand, it is noted that some believe ''Proganochelys'' were solely terrestrial. Shell bone histology of extant turtles revealed congruence with terrestrial turtles for the earliest basal turtles, including ''Proganochelys'', taxa in one study. The common ancestry of all living turtles is believed to be aquatic, while the earliest turtles are believed to have lived in a terrestrial environment.

Environment and forelimbs

Forelimbs are believed to be a physical feature that reflects the preferences and adaptations to a specific environment, indicating the environment a turtle would be most likely to reside in. Based on morphological data, ''Proganochelys'' is believed to have lived in a semi-aquatic environment, though a 2021 study groups it with tortoises and other terrestrial taxa. Turtles possessing short hands are believed to be most likely terrestrial, while turtles with long limbs are more likely to be aquatic. The majority of all testudines are short-handed and terrestrial, while all cheloniods are long-handed and aquatic.

Classification

''Proganochelys'' belongs to the group of tetrapods with a shell known as

Testudinata Testudinata is the group of all tetrapods with a true turtle shell. It includes both modern turtles (Testudines) and many of their extinct, shelled relatives (stem-turtles). Though it was first coined as the group containing turtles by Jacob The ...

and is the oldest primitive stem turtle. The group does not include ''Odontochelys''. The cladogram below follows an analysis by Jérémy Anquetin (2012).

Habitat

''Proganochelys'' is considered to have lived in the giant continent

during the Triassic period. The fossil records show that ''Proganochelys'' might have lived anywhere in between Thailand and Germany. During the Triassic period,

was primarily dry and warm, especially in arid areas. ''Proganochelys'' shared their environment with a variety of

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

. ''Proganochelys'' lived in small water bodies such as ponds, but it was mainly earthbound.

References

External links

belated questions about beaks
{{Taxonbar, from=Q1193456 Monotypic prehistoric reptile genera Reptile taxonomy Norian first appearances Extant Late Triassic first appearances Late Triassic reptiles of Europe Late Triassic reptiles of Asia Prehistoric reptile genera Fossil taxa described in 1887 Taxa named by Georg Baur Testudinata