Tritylodontidae ("three-knob teeth", named after the shape of their
cheek teeth) is an extinct family of small to medium-sized, highly specialized
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
-like
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s, bearing several mammalian traits like erect limbs, endothermy and details of the skeleton.
They were the last-known family of the non-
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 com ...
synapsid
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
s, persisting into the
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
.
Most tritylodontids are thought to have been
herbivorous
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
, feeding on
vegetation
Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic character ...
, such as
stems,
leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
, and
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
s, although at least one may have had a more omnivorous diet.
Tritylodontid fossils are found in the
Americas
The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World.
Along with th ...
,
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, and
Eurasia
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...
—they appear to have had an almost global distribution, including
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
.
Description
![Kayentatherium wellesi](https://upload.wikimedia.org/wikipedia/commons/f/f5/Kayentatherium_wellesi.jpg)
The skull of tritylodontids had a high
sagittal crest
A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptiona ...
. They retained the primitive condition of the joint between the
quadrate bone
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper ...
of the skull and the articular bone of the lower jaw
—the retention of the joint is one of the reasons they are technically regarded to not be mammals, but are instead non-mammalian
mammaliamorph
Mammaliamorpha is a clade of cynodonts. It contains the clades Tritylodontidae and Mammaliaformes, as well as a few genera that do not belong to either of these groups. The family Tritheledontidae has also been placed in Mammaliamorpha by some ...
s.
The back of the skull had huge zygomatic arches for the attachment of its large jaw
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s. They also had a very well-developed
secondary palate
The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates.
In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medi ...
. The tritylodont dentition differed from that of most other cynodonts: they did not have
canines
Canine may refer to:
Zoology and anatomy
* a dog-like Canid animal in the subfamily Caninae
** ''Canis'', a genus including dogs, wolves, coyotes, and jackals
** Dog, the domestic dog
* Canine tooth, in mammalian oral anatomy
People with the surn ...
, and the front pair of
incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whe ...
s were enlarged and were very similar to those of modern-day rodents.
Tritylodontids had a large gap, called a
diastema
A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...
, that separated the incisors from their square-shaped cheek teeth. The cheek teeth in the upper jaw had three rows of cusps running along its length, with grooves in between. The lower teeth had two rows of cusps which fitted into the grooves in the upper teeth. The matching of the cusps allowed the teeth to
occlude more precisely than in earlier cynodonts. It would grind its
food
Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...
between the teeth in somewhat the same way as a modern rodent, though unlike rodents tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front).
The teeth were well suited for shredding
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
; however, there is evidence that some tritylodontids had more
omnivorous
An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nutri ...
diets, much in the same vein as modern mammals with "herbivore dentitions" like modern
rats.
Like
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 com ...
s, tritylodontids had
epipubic bone
Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them).
They first occ ...
s, a possible
synapomorphy
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 ...
between both clades, and this suggests they may also have laid eggs like modern
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
, or produced undeveloped fetus-like young like modern
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
s. A recent ''
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 ...
'' showcases that they indeed produced undeveloped young, but at litter sizes much larger than any monotreme or marsupial, at around 38 perinates.
Tritylodonts were active animals that were likely
warm-blooded
Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The onl ...
and possibly burrowed.
The small early tritylodontid, ''
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 ...
'' has been compared to a
weasel
Weasels are mammals of the genus ''Mustela'' of the family Mustelidae. The genus ''Mustela'' includes the least weasels, polecats, stoats, ferrets and European mink. Members of this genus are small, active predators, with long and slender bo ...
or
mink
Mink are dark-colored, semiaquatic, carnivorous mammals of the genera ''Neogale'' and '' Mustela'' and part of the family Mustelidae, which also includes weasels, otters, and ferrets. There are two extant species referred to as "mink": the A ...
, with a long, slim body and tail. In ''
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 burrowing adaptations seen in the skeleton have been re-interpreted as possibly suggesting a semi-aquatic ecology.
Discovery
The first tritylodontid named was ''
Stereognathus
''Stereognathus'' is an extinct genus of tritylodontid cynodonts from the Middle Jurassic of the United Kingdom. There is a single named species: ''S. ooliticus'', named after the Great Oolite deposits of England. A second species, ''S. hebrid ...
'', from teeth found in the Middle
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), 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 J ...
Great Oolite Group
The Great Oolite Group is a Middle Jurassic stratigraphic unit that outcrops in southern England. It consists of a complex set of marine deposits primarily mudstone and bioclastic ooidal and fine grained limestone, deposited in nearshore to she ...
of England
and the family name was erected by Cope in 1884.
Shortly after, another tritylodontid was discovered in the Upper
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
rocks of South Africa.
Evolutionary history
Tritylodontids first appeared during the
Rhaetian
The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...
, the last stage of the
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, and were abundant during the
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), 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 J ...
, with several records from the
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
. The records of the group are almost entirely confined to the Northern Hemisphere, with the only records outside this region being in the Early Jurassic of South Africa and Antarctica. ''
Xenocretosuchus,'' ''
Montirictus
''Montirictus'' is an extinct genus of Tritylodontidae, tritylodonts known from the Early Cretaceous Kuwajima Formation of Japan. It is currently the latest surviving tritylodontid species, and is closely related to the earlier ''Xenocretosuchus ...
'' and ''
Fossiomanus'' are the latest known tritylodontids, from the
Barremian
The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...
-
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
aged
Ilek Formation
The Ilek Formation is a Lower Cretaceous geologic formation in Western Siberia. Many different fossils have been recovered from the formation. It overlies the Late Jurassic Tyazhin Formation and underlies the Albian Kiya Formation.
The formation ...
of Siberia,
Kuwajima Formation
The Kuwajima Formation is an Early Cretaceous geologic formation in Japan. Its precise age is uncertain due to a lack of identifying fossils, and it was previously considered likely Valanginian to Hauterivian in age. However, it is now considered ...
of Japan, and
Yixian Formation
The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its ex ...
of China respectively.
The morphology of ''Fossiomanus'' indictates it had a specialised
fossorial
A fossorial () animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamanders, as well as many beetles, wasps, and bees.
Prehistoric eviden ...
(burrowing) lifestyle.
Ecology
The tooth morphology suggests that tritylodonts were primarily herbivorous, tooth microwear analysis indicates that tritylodonts ate food with low to moderate abrasiveness, and may have also consumed invertebrates.
Phylogeny
Because of their mammal-like appearance, tritylodontids were originally placed within Mammalia. Starting with the work of British paleontologist
D. M. S. Watson
Prof David Meredith Seares Watson FRS FGS HFRSE LLD (18 June 1886 – 23 July 1973) was the Jodrell Professor of Zoology and Comparative Anatomy at University College, London from 1921 to 1951.
Biography
Early life
Watson was born in the Highe ...
in 1942, a close relationship was favored between tritylodontids and cynodonts. Watson and other paleontologists noted that tritylodontids lacked the
dentary
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
and
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 ...
jaw articulation that was characteristic of early mammals. Haughton and Brink (1954) were the first to classify tritylodontids within Cynodontia. Later studies identified close similarities between the teeth of tritylodontids and
traversodontids
Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. ...
, and tritylodontids were eventually thought to be descendants of traversodontids. Under this classification, which was widely accepted in the following decades, Tritylodontidae was previous considered to be part of Gomphodontia, a larger group within
Cynognathia
Cynognathia ("dog jaw") is one of two major clades of cynodonts, the other being Probainognathia. Cynognathians included the large carnivorous genus '' Cynognathus'' and the herbivorous traversodontids. Cynognathians can be identified by several ...
. The name Tritylodontoidea was previously used for the group, which traditionally included the families
Diademodontidae,
Trirachodontidae
Trirachodontidae is an extinct, possibly paraphyletic family of cynognathian cynodonts from the Triassic of China and southern Africa. Trirachodontids appeared during the Early Triassic soon after the Permian-Triassic extinction event and quick ...
, Traversodontidae, and Tritylodontidae.
More recently, tritylodontids have been reinterpreted as close relatives of mammals. Beginning with Kemp (1983), Tritylodontidae has been proposed by numerous studies as a member of
Probainognathia, the cynodont group containing mammals and related taxa. Gomphodontia is still used for the cynognathian group containing traversodontids and is preferred over Tritylodontoidea now that tritylodontids are not part of it. A
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analysis performed by Liu and Olsen (2010) places Tritylodontidae very closely to Mammalia, as the
sister taxon
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
of the clade formed by
Brasilodontidae
''Brasilodon'' ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the Late Triassic epoch, about 225.42 million years ago. While no complete skeletons have been foun ...
and Mammalia.
Ruta ''et al.'' (2013) phylogenetic analysis which is partially based on Liu and Olsen (2010) places Tritylodontidae in a more derived position than Brasilodontidae. Below is a
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
from this analysis.
The exact position of Tritylodontidae in relation to Mammalia is still debated, but most researchers agree they are closely related, usually considering Tritylodontidae to be non-mammaliaform mammaliamorphs.
Genera
![Kayentatherium](https://upload.wikimedia.org/wikipedia/commons/3/3f/Kayentatherium.JPG)
*''
Bienotherium''
*''
Bienotheroides
''Bienotheroides'' is an extinct genus of tritylodontid mammaliamorphs from the Jurassic of China and Mongolia
Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, borde ...
''
*''
Bocatherium''
*''
Dianzhongia''
*''
Dinnebitodon''
*''
Fossiomanus''
*''
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 ...
''
*''
Lufengia''
*''
Montirictus
''Montirictus'' is an extinct genus of Tritylodontidae, tritylodonts known from the Early Cretaceous Kuwajima Formation of Japan. It is currently the latest surviving tritylodontid species, and is closely related to the earlier ''Xenocretosuchus ...
''
*''
Nuurtherium
''Nuurtherium'' is an extinct genus of tritylodontid cynodonts of the Late Jurassic Ulan Malgait Formation
The Ulan Malgait Formation is a Late Jurassic geologic formation in Mongolia. Dinosaur remains are among the fossils that have been rec ...
''
*''
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 ...
''
*''
Polistodon''
*''
Shartegodon''
*''
Stereognathus
''Stereognathus'' is an extinct genus of tritylodontid cynodonts from the Middle Jurassic of the United Kingdom. There is a single named species: ''S. ooliticus'', named after the Great Oolite deposits of England. A second species, ''S. hebrid ...
''
*''
Tritylodon''
*''
Xenocretosuchus''
*''
Yuanotherium''
*''
Yunnanodon''
See also
*
Embryonic diapause Embryonic diapause (from late 19th century English: dia- ‘through’ + pause- 'delay') (aka delayed implantation in mammals) is an evolutionary reproductive strategy used by several animal species across a number of kingdoms, including approximate ...
References
{{Taxonbar, from=Q134746
Prehistoric therapsid families
Late Triassic first appearances
Early Cretaceous extinctions