''Kayentachelys'' ("Kayenta turtle") is an
extinct genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
known only from the "silty facies" of the
Lower Jurassic
The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...
Kayenta Formation
The Kayenta Formation is a geological formation in the Glen Canyon Group that is spread across the Colorado Plateau province of the United States, including northern Arizona, northwest Colorado, Nevada, and Utah. Traditionally has been suggested ...
in northeastern
Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
on the lands of the
Navajo Nation
The Navajo Nation ( nv, Naabeehó Bináhásdzo), also known as Navajoland, is a Native American reservation in the United States. It occupies portions of northeastern Arizona, northwestern New Mexico, and southeastern Utah; at roughly , the ...
.
History of discovery and significance
The earliest mention of turtles from the Kayenta Formation in the literature is within the description of the
ornithischian dinosaur ''
Scutellosaurus
''Scutellosaurus'' ( ) is a genus of thyreophoran ornithischian dinosaur that lived approximately 196 million years ago during the early part of the Jurassic Period in what is now Arizona, USA. It is classified in Thyreophora, the armoured di ...
'' by
Colbert (1981).
These first specimens were collected during the 1970s and early 1980s by field parties from the
Museum of Northern Arizona
The Museum of Northern Arizona is a museum in Flagstaff, Arizona, United States, that was established as a repository for Indigenous material and natural history specimens from the Colorado Plateau.
The museum was founded in 1928 by zoologist ...
(MNA) and the
Museum of Comparative Zoology
A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make thes ...
(MCZ) at
Harvard University
Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of high ...
. Specimens from these field excursions were used to establish the taxon ''Kayentachelys'' in 1987, and it is an MNA specimen (MNA V1558) which was designated as the holotype of the taxon.
During the early 1980s, addition turtle specimens from the Kayenta Formation were collected by field parties from the
University of California Museum of Paleontology
The University of California Museum of Paleontology (UCMP) is a paleontology museum located on the campus of the University of California, Berkeley.
The museum is within the Valley Life Sciences Building (VLSB), designed by George W. Kelham and ...
(UCMP), which were reported by Clark & Fastovsky (1986) prior to the naming of ''Kayentachelys''.
[Clark, J.M. and Fastovsky, D.E. (1986) Vertebrate biostratigraphy of the Glen Canyon Group in northern Arizona. ''In'' Padian, K. (ed.) ''The Beginning of the Age of Dinosaurs: Faunal Change Across the Triassic-Jurassic Boundary'' Cambridge University Press.] Many additional specimens of ''Kayentachelys'' were collected by the
Texas Memorial Museum
The Texas Memorial Museum, located on the campus of the University of Texas at Austin in Austin, Texas, USA, was created during preparations for the Texas Centennial Exposition held in 1936. The museum's focus is on natural history, including pal ...
(TMM; now the Texas Vertebrate Paleontology Collections) at
The University of Texas at Austin
The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...
between 1997 and 2000.
That work by the TMM remains the most recent large-scale collecting of vertebrate fossils from the Kayenta Formation in Arizona.
''Kayentachelys'' is significant because it is the first truly abundant turtle in the fossil record, and its unique phylogenetic position documents the transition from early turtles to the common ancestor of the
crown
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
. Additionally, as of 2018, ''Kayentachelys'' is the most abundantly known vertebrate taxon from the Kayenta Formation.
Diagnosis and description
''Kayentachelys'' is known from several dozen specimens preserving elements from both the
cranial
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
and
postcranial Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated s ...
skeleton. Its shell is about in length.
As originally diagnosed,
''Kayentachelys'' possesses pterygoid teeth and a ventrally-exposed prootic, both of which are ancestral
amniote features. Ancestral turtle features of the shell include nine costal scutes and an epiplastron with a dorsal process. Derived features shared with early turtles like ''Proterochersis'' and cryptodirans include an antrum postoticum of the
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...
, a fused basipterygoid articulation, and 11 peripheral osteoderms. Gaffney ''et al.'' united ''Kayentachelys'' with cryptodiran taxa based upon featured of the
palate
The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity.
A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly separ ...
and
braincase.
Sterli and Joyce (2007)
emended the diagnosis of ''Kayentachelys'' using a unique combination of ancestral and derived cranial character states, including prefrontals which do not contact one another at the midline, the absence of lacrimals, frontals which contribute to the orbit, an unpaired vomer, pterygoid teeth, the absence of palatal teeth, and a retroarticular process, among many other features.
Systematics
The genus ''Kayentachelys'' is
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
, comprising only one species. The type species ''Kayentachelys aprix'' was defined based upon the holotype specimen MNA V1558 and the referred specimens MNA V1559-V1570 and MCZ 8914-8917. The
specific epithet ''aprix'' (Greek: ''tight'') refers to the fused basicranial articulation.
When ''Kayentachelys'' was first described by
Gaffney et al. (1987), it was classified as the oldest and earliest branching
cryptodira
The Cryptodira ('' el, hidden neck'') are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the ...
n turtle and the sole taxon within a new clade called Kayentachelyidae.
''Kayentachelys'' was noted to possess a unique combination of
derived character states which united it with all cryptodiran turtles and
ancestral character states which excluded it from a clade comprising all other cryptodiran taxa that the authors called Selmacryptodira. Below is a cladogram depicting the initial classification of ''Kayentachelys'' within Cryptodira by Gaffney ''et al.'' (1987):
In 2007, a competing hypothesis regarding the phylogeny of ''Kayentachelys'' arose.
Sterli and Joyce (2007) Sterli and Joyce (2007) argued that ''Kayentachelys'' was not a member of Crytodira and instead represented an early
testudinatan outside of
Testudines
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
. The placement of ''Kayentachelys'' outside of Cryptodira has been upheld by most subsequent phylogenetic analyses of early turtles.
Below is a cladogram depicting the phylogenetic hypothesis from Sterli and Joyce (2007):
Occurrence and geologic setting
''Kayentachelys'' is known only from the "silty facies" of the Kayenta Formation in northeastern Arizona. The Kayenta Formation is a stratigraphic unit within the
Triassic
The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
-
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
Glen Canyon Group
The Glen Canyon Group is a geologic group of formations that is spread across the U.S. states of Nevada, Utah, northern Arizona, north west New Mexico and western Colorado. It is called the Glen Canyon Sandstone in the Green River Basin of C ...
, which crops out across the
Colorado Plateau. Despite extensive outcrop of the "typical facies" of the Kayenta Formation in
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
,
Colorado
Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...
, and northern Arizona, vertebrate fossils are rare in these regions. Vertebrate fossils from the "silty facies" of the Kayenta Formation are most abundantly known from Ward Terrace along the Adeii Eechii Cliffs southeast of
Tuba City
Tuba City ( nv, ) is an unincorporated town in Coconino County, Arizona, on the Navajo Nation, United States. It is the second-largest community in Coconino County. The population of the census-designated place (CDP) was 8,611 at the 2010 cen ...
, AZ on the lands of the
Navajo Nation
The Navajo Nation ( nv, Naabeehó Bináhásdzo), also known as Navajoland, is a Native American reservation in the United States. It occupies portions of northeastern Arizona, northwestern New Mexico, and southeastern Utah; at roughly , the ...
. In particular, specimens of ''Kayentachelys'' have been recovered near an area called Gold Spring, which has recently been dated to the
Pliensbachian
The Pliensbachian is an age (geology), age of the geologic timescale and stage (stratigraphy), stage in the stratigraphic column. It is part of the Early Jurassic, Early or Lower Jurassic epoch (geology), Epoch or series (stratigraphy), Series an ...
-
Toarcian
The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian.
The Toarcian ...
ages of the
Early Jurassic Epoch based upon
uranium-lead ages acquired using
laser ablation inductively coupled mass spectrometry.
Paleoecology
''Kayentachelys'' was initially interpreted as an aquatic turtle, based upon the sharp tapered edges of the low-domed shell and the lack of both limb armor and sculpturing on the
carapace.
''Kayentachelys'' is one component of a diverse assemblage of vertebrate taxa preserved within the Kayenta Formation. ''Kayentachelys'' lived alongside
hybodontoid chondrichthyans
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. C ...
; the early
anuran
A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
''
Prosalirus''; the
gymnophiona
Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics o ...
n ''
Eocaecilia
''Eocaecilia'' is an extinct genus of gymnophionan amphibian from the early Jurassic Kayenta Formation of Arizona, United States. One species is described, ''Eocaecilia micropodia''.
''Eocaecilia'' shared some characteristics with salamanders a ...
'';
sphenodontid diapsids
Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...
; several species of
crocodylomorph
Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction.
During Mesozoic and early Cenozoic times, cro ...
, including ''
Kayentasuchus
''Kayentasuchus'' (meaning "Kayenta Formation crocodile") is a genus of sphenosuchian, a type of basal crocodylomorph, the clade that comprises the crocodilians and their closest kin. It is known from a single skeleton found in rocks of the Si ...
'', ''
Eopneumatosuchus'', ''
Protosuchus'', and the
goniopholidid ''
Calsoyasuchus
''Calsoyasuchus'' (meaning " r. KyrilCalsoyas' crocodile") is a genus of crocodylomorph that lived in the Early Jurassic. Its fossilized remains were found in the Sinemurian-Pliensbachian-age Kayenta Formation on Navajo Nation land in Coconino ...
''; the
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
''
Rhamphinion
''Rhamphinion'' (meaning "beak nape"; Greek ''rhamphos'', "beak" is a common part of "rhamphorhynchoid" names, and the remains of this animal came from the rear of the skull, i.e. "nape" or ''inion'') is a genus of pterosaurs from the Sinemurian ...
''; the
sauropodomorph
Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lon ...
dinosaur
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 ...
''
Sarahsaurus
''Sarahsaurus'' is a genus of basal sauropodomorph dinosaur which lived during the Early Jurassic period in what is now northeastern Arizona, United States.
Discovery and naming
All specimens of ''Sarahsaurus'' were collected from the Lower J ...
''; the
theropod
Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
dinosaurs ''
Dilophosaurus
''Dilophosaurus'' ( ) is a genus of theropod dinosaurs that lived in what is now North America during the Early Jurassic, about 193 million years ago. Three skeletons were discovered in northern Arizona in 1940, and the two best preserve ...
'' and ''
"Syntarsus" kayentakatae''; the
ornithischian
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...
dinosaur ''Scutellosaurus''; the
tritylodontid
Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details ...
synapsids ''
Oligokyphus
''Oligokyphus'' ("few cusps") is an extinct genus of advanced herbivorous cynodonts of the late Triassic to early Jurassic periods. Originally considered to be an early mammal, it is now classified as a Mammaliamorph (nearly a mammal) because ''O ...
'', ''
Dinnebitodon'', and ''
Kayentatherium
''Kayentatherium'' is an extinct genus of tritylodontid cynodonts that lived during the Early Jurassic. It is one of two tritylodonts from the Kayenta Formation of northern Arizona, United States.
''Kayentatherium'' means "Kayenta Beast", and is ...
''; the
mammaliaform
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent comm ...
synapsid ''Dinnetherium''; and several other yet-unnamed taxa.
The paleoenvironment of the Kayenta Formation in which ''Kayentachelys'' lived has been reconstructed as a floodplain drained by low-energy, sediment-rich streams.
The presence of aquatic and moisture-reliant taxa including ''Kayentachelys'', ''
Prosalirus'', and ''
Eocaecilia
''Eocaecilia'' is an extinct genus of gymnophionan amphibian from the early Jurassic Kayenta Formation of Arizona, United States. One species is described, ''Eocaecilia micropodia''.
''Eocaecilia'' shared some characteristics with salamanders a ...
'' indicates that water was likely abundant. This abundance of water is also supported by the presence of
mudstones within "silty facies" the Kayenta Formation on Ward Terrace.
Petrified wood
Petrified wood, also known as petrified tree (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of '' fossilized wood'', the fossilized remains of terrestrial vegetation. ' ...
is common in the Kayenta Formation on Ward Terrace, suggesting abundant
gymnosperm
The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
plantlife.
References
{{Taxonbar, from=Q1393547
Mesozoic turtles of North America
Early Jurassic reptiles of North America
Prehistoric turtle genera
Taxa named by Eugene S. Gaffney
Testudinata