Cynodont

The cynodonts ("dog teeth") (clade Cynodontia) are therapsids that first appeared in the Late Permian (approximately 260 Ma). The cynodonts include modern mammals (including humans) as well as their extinct ancestors and close relatives, but not all cynodonts were mammals.

Cynodonts spread throughout southern Pangea and are represented by fossils from South America, Africa, India, and Antarctica. In the northern continents, fossils have been found in eastern North America as well as in Belgium and northwestern France. Cynodontia is one of the most diverse groups of therapsids.

Cynodonts
Temporal range: Late Permian-Recent, 260–0 Ma
Non-mammalian synapsids died out 17.5 million years ago
Belesodon magnificus
Fossil of Chiniquodon in the Staatliches Museum für Naturkunde Stuttgart
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Order: Therapsida
Clade: Eutheriodontia
Suborder: Cynodontia
Owen, 1861

Taxonomy

Richard Owen named Cynodontia in 1861, which he assigned to Anomodontia as a family.[1] Robert Broom (1913) reranked Cynodonia as an infraorder, since retained by others, including Colbert and Kitching (1977), Carroll (1988), Gauthier et al. (1989), and Rubidge and Cristian Sidor (2001).[2] Olson (1966) assigned Cynodontia to Theriodonta, Colbert and Kitching (1977) to Theriodontia, and Rubridge and Sidor (2001) to Eutheriodontia. William King Gregory (1910), Broom (1913), Carroll (1988), Gauthier et al. (1989), Hopson and Kitching (2001) and Botha et al. (2007) all considered Cynodontia as belonging to Therapsida. Botha et al. (2007) seems to have followed Owen (1861), but without specifying taxonomic rank.[3][4]

In 2001, James Allen Hopson e.a. defined a clade Cynodontia as the most inclusive group containing Mammalia but excluding Bauria.[5]

Evolutionary history

Thrinaxodon BW
Thrinaxodon from the Early Triassic of South Africa

Together with the extinct gorgonopsians and the therocephalians, the cynodonts themselves are part of a group of therapsids called theriodonts.

The oldest and the most basal cynodont yet found is Charassognathus (Late Permian). Other basal cynodonts were the procynosuchids, a family that includes Procynosuchus and Dvinia. Cynodonts were among the few groups of synapsids that survived the Permian–Triassic extinction event and had a slow recovery after the extinction.

The most derived cynodonts are found within the clade Eucynodontia, which also contains the members of Mammalia. Representative genera of nonmammalian cynodonts include the large carnivorous cynognathids, the equally large herbivorous traversodonts, and the small mammal-like tritylodontids and ictidosaurs. The presence of respiratory turbinates suggests a rapid metabolism and possibly endothermy.

During their evolution, the number of cynodont jaw bones reduced. This move towards a single bone for the mandible paved the way for other bones in the jaw, the articular and angular, to migrate to the cranium, where they function as parts of the mammalian hearing system.

Cynodonts also developed a secondary palate in the roof of the mouth. This caused air flow from the nostrils to travel to a position in the back of the mouth instead of directly through it, allowing cynodonts to chew and breathe at the same time. This characteristic is present in all mammals.

Characteristics

Bienotherium yunnanense
Bienotherium yunnanense

Early cynodonts have many of the skeletal characteristics of mammals. The teeth were fully differentiated and the braincase bulged at the back of the head. Outside of some crown-group mammals (notably the therians), all cynodonts probably laid eggs. The temporal fenestrae were much larger than those of their ancestors, and the widening of the zygomatic arch in a more mammal-like skull would have allowed for more robust jaw musculature. They also have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts. (However, the secondary palate of cynodonts primarily comprises the maxillae and palatines as in mammals, whereas the secondary palate of the therocephalians primarily comprises the maxillae and the vomer.) The dentary was the largest bone in their lower jaw.

The cynodonts probably had some form of warm-blooded metabolism. This has led to many reconstructions of cynodonts as having fur. Being endothermic they may have needed it for thermoregulation, but fossil evidence of their fur (or lack thereof) has been elusive. Modern mammals have Harderian glands secreting lipids to coat their fur, but the telltale imprint of this structure is only found from the primitive mammal Morganucodon and onwards.[6] Nonetheless, recent studies on Permian synapsid coprolites show that more basal therapsids had fur,[7] and at any rate fur was already present in Mammaliaformes such as Castorocauda and Megaconus.

Marks in the upper and lower jaw of cynodonts have been interpreted as channels that supplied blood vessels and nerves to whiskers.[8][9] Whiskers may have been typical of cynodontia as a whole, or have evolved in this group.

Derived cynodonts developed epipubic bones. These served to strengthen the torso and support abdominal and hindlimb musculature, aiding them in the development of an erect gait, but at the expense of prolonged pregnancy, forcing these animals to give birth to highly altricial young as in modern marsupials and monotremes. Only placentals, and perhaps Megazostrodon and Erythrotherium, would lose these.[10][11] A specimen of Kayentatherium does indeed demonstrate that at least tritylodontids already had a fundamentally marsupial-like reproductive style, but produced much higher litters at around 38 perinates or possibly eggs.[12]

Cynodonts are the only known synapsid lineage to have produced aerial locomotors, with gliding and flying being known in haramiyidans[13] and various mammal groups.

Phylogeny

Below is a cladogram from Ruta, Botha-Brink, Mitchell and Benton (2013) showing one hypothesis of cynodont relationships:[14]

Cynodontia 

Charassognathus

Dvinia

Procynosuchus

 Epicynodontia 

Cynosaurus

Galesaurus

Progalesaurus

Nanictosaurus

Thrinaxodon

Platycraniellus

 Eucynodontia 

Cynognathia

Probainognathia

→ †Cynognathia 

Cynognathus

 †Gomphodontia 

Diademodon

 †Trirachodontidae 

Beishanodon

†Sinognathus

Trirachodon

Cricodon

Langbergia

 †Traversodontidae 

Andescynodon

Pascualgnathus

Scalenodon

Luangwa

Traversodon

"Scalenodon" attridgei

Mandagomphodon

Nanogomphodon

Arctotraversodon

Boreogomphodon

 †Massetognathinae 

Massetognathus

Dadadon

Santacruzodon

 †Gomphodontosuchinae 

Menadon

Gomphodontosuchus

Protuberum

Exaeretodon

†Scalenodontoides

→ Probainognathia 

Lumkuia

Ecteninion

Aleodon

Chiniquodon

Probainognathus

Trucidocynodon

Therioherpeton

 †Tritheledontidae 

Riograndia

Chaliminia

Elliotherium

Diarthrognathus

Pachygenelus

 †Brasilodontidae 

Brasilitherium

Brasilodon

 †Tritylodontidae 

Oligokyphus

Kayentatherium

Tritylodon

Beinotherium

 Mammaliaformes 

Mammalia

Sinoconodon

Morganucodon

See also

References

  1. ^ Classification of R. Owen 1861.
  2. ^ Classification of B. S. Rubidge and C. A. Sidor 2001
  3. ^ R. Broom. 1913. A revision of the reptiles of the Karroo. Annals of the South African Museum 7(6):361–366
  4. ^ S. H. Haughton and A. S. Brink. 1954. A bibliographical list of Reptilia from the Karroo Beds of South Africa. Palaeontologia Africana 2:1–187
  5. ^ James A. Hopson and James W. Kitching, 2001, "A Probainognathian Cynodont from South Africa and the Phylogeny of Nonmammalian Cynodonts" pp 5-35 in: PARISH A. JENKINS, JR., MICHAEL D. SHAPIRO, AND TOMASZ OWERKOWICZ, EDITORS, STUDIES IN ORGANISMIC AND EVOLUTIONARY BIOLOGY IN HONOR OF A. W. CROMPTON Bullettin of the Museum of Comparative Zoology. Harvard University 156(1)
  6. ^ Ruben, J.A.; Jones, T.D. (2000). "Selective Factors Associated with the Origin of Fur and Feathers". American Zoologist. 40 (4): 585–596. doi:10.1093/icb/40.4.585.
  7. ^ "Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia". Lethaia. doi:10.1111/let.12156.
  8. ^ Brink, A.S. (1955). "A study on the skeleton of Diademodon". Palaeontologia Africana. 3: 3–39.
  9. ^ Kemp, T.S. (1982). Mammal-like reptiles and the origin of mammals. London: Academic Press. p. 363. ISBN 0-12-404120-5.
  10. ^ Michael L. Power, Jay Schulkin. The Evolution of the Human Placenta. pp. 68–.
  11. ^ Jason A. Lillegraven, Zofia Kielan-Jaworowska, William A. Clemens, Mesozoic Mammals: The First Two-Thirds of Mammalian History, University of California Press, 17 December 1979 – 321
  12. ^ Eva A. Hoffman; Timothy B. Rowe (2018). "Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth". Nature. 561 (7721): 104–108. doi:10.1038/s41586-018-0441-3. PMID 30158701.
  13. ^ Qing-Jin Meng; David M. Grossnickle; Di Liu; Yu-Guang Zhang; April I. Neander; Qiang Ji; Zhe-Xi Luo (2017). "New gliding mammaliaforms from the Jurassic". Nature. in press. doi:10.1038/nature23476.
  14. ^ Ruta, M.; Botha-Brink, J.; Mitchell, S. A.; Benton, M. J. (2013). "The radiation of cynodonts and the ground plan of mammalian morphological diversity". Proceedings of the Royal Society B: Biological Sciences. 280 (1769): 20131865. doi:10.1098/rspb.2013.1865. PMC 3768321. PMID 23986112.

Further reading

  • Hopson, J.A.; Kitching, J.W. (2001). "A probainognathian cynodont from South Africa and the phylogeny of non-mammalian cynodonts". Bull. Mus. Comp. Zool. 156: 5–35.
  • Davis, Dwight (1961). "Origin of the Mammalian Feeding Mechanism". Am. Zoologist, 1:229–234.

External links

Abdalodon

Abdalodon is an extinct genus of late Permian cynodonts, known by its only species A. diastematicus. Abdalodon together with the genus Charassognathus, form the clade Charassognathidae. This clade represents the earliest known cynodonts, and is the first known radiation of Permian cynodonts.Abdalodon diastematicus is known from one crushed fossil skull from the Karoo Basin of South Africa. Of all Permian Therapsids, cynodonts are among the most rare (Biarmosuchians being the only therapsids being of comparable rarity). The fossil record of Permian cynodonts is characterized by a long ghost lineage. Abdalodon has been important for discerning the early evolution of cynodonts. Abdalodon, and its sister taxa Charassognathus are both small bodied animals, Abdalodon having a skull around six centimeters in length and the Charassognathus skull being slightly smaller. This suggests that early cynodont evolution occurred at small body size, which could explain the rarity of Permian cynodont fossils, because there is an inherent taphonomic bias against the fossilization of small bodied animals.

Alemoatherium

Alemoatherium is an extinct genus of chiniquodontid cynodont of the late Triassic of Brazil. It contains a single species, A. huebneri, named in 2017 by Agustín Martinelli and colleagues based on a left lower jaw.

Bolotridon

Bolotridon is an extinct genus of epicynodontian cynodont. It was renamed from its original genus designation of Tribolodon (Harry Govier Seeley, 1895), which was already occupied by a genus of cyprinid fish named in 1883 by Sauvage. The name Bolotridon was coined by Brian W. Coad in a 1977 publication as an anagram of Tribolodon.

Charassognathus

Charassognathus (meaning 'notched jaw') is an extinct genus of Late Permian cynodonts. Described in 2007 from a locality near Fraserburg, South Africa, Charassognathus is the earliest and most basal cynodont. It is known only from the holotype, which dates from the upper Permian Period. The type and only species is C. gracilis. The holotype (SAM-PK-K 10369) is made up of a crushed skull, partial lower jaw and one leg.

Charassognathus was a quadrupedal predator. It was named for a notch on its coronoid process which most likely was the insertion point for a chewing muscle, the adductor mandibulae externus. Charassognathus was a tiny animal, with a skull only 5 centimeters in length. Since the body of Charassognathus hasn't been discovered, its full length remains unknown, but estimates have been made at 50 centimeters.

Chronoperates

Chronoperates (meaning "time wanderer" in Greek) is an extinct genus of mammal whose remains have been found in a late Paleocene deposit in Alberta, Canada. It is represented by the type species Chronoperates paradoxus and known only from a partial left lower jaw. It was first identified in 1992 as a non-mammalian cynodont, implying a ghost lineage of over 100 million years since the previously youngest known record of non-mammalian cynodonts, which at that time was in the Jurassic period (some non-mammalian cynodonts are now known to have persisted until the Early Cretaceous). Subsequent authors have challenged the cynodont interpretation, particularly as the teeth do not resemble any known cynodonts. Chronoperates is now generally considered to be more likely to be a late-surviving symmetrodont mammal. This would still infer a ghost lineage for symmetrodonts, but a more plausible one, as symmetrodonts persisted into the Late Cretaceous.

Cromptodon

Cromptodon is an extinct genus of cynodonts from the Triassic of Cerro Bayo de Portrerillos, Cerro de las Cabras Formation, Argentina, South America. It is known only from PVL 3858, a mandible.

Dvinia

Dvinia is an extinct genus of cynodonts of the family Dviniidae found in Sokolki on the Northern Dvina River near Kotlas in Arkhangelsk Oblast, Russia. Its fossil remains date from the Late Permian and were found with Inostrancevia, Scutosaurus and Vivaxosaurus.

The species was small omnivore containing an extremely large temporal opening typical of advanced therapsids, with a thin bone separating the eye and muscle attachment. It is very close in the evolutionary line to mammals, but more analysis of the Cynodontia is needed. The teeth contain small incisors followed by 2 canines and 10-14 molar teeth following.

Eucynodontia

Eucynodontia ("true dog teeth") is a clade of cynodont therapsids including mammals and most non-mammalian cynodonts. The oldest eucynodonts are known from the Early Triassic and possibly Late Permian. Eucynodontia includes two major subgroups, Cynognathia and Probainognathia.The clade was named by Thomas Kemp in 1982.In 2001, James Allen Hopson e.a. defined the clade Euynodontia as the least inclusive group containing Mammalia and Exaeretodon.

Irajatherium

Irajatherium is an extinct genus of cynodonts, known only of the type species Irajatherium hernandezi. It is named in honor of Irajá Damiani Pinto.

Luangwa (cynodont)

Luangwa is an extinct genus of traversodontid cynodonts. The species Luangwa drysdalli was discovered 1963 in the valley of the Luangwa river in Zambia, Africa. Luangwa lived in the Triassic period 240 Million years ago.

In July 2008, a skull of Luangwa sudamericana was found in the Brazilian town of Dona Francisca (Rio Grande do Sul), which is part of the Geopark Paleorrota. The discovery was made by a team of the ULBRA.

Madysaurus

Madysaurus (Madygen reptile) is an extinct genus of cynodonts which existed in Kyrgyzstan. It was first named by Leonid Petrovich Tatarinov in 2005. Madysaurus is known from the Madygen Formation, a Triassic Lagerstätte that also includes well-preserved remains of insects and small reptiles like Sharovipteryx and Longisquama. Madysaurus is one of the most primitive cynodonts and is placed in its own family, Madysauridae.

Novocynodon

Novocynodon is an extinct genus of thrinaxodontid cynodonts from the Middle Permian of Russia. Fossils have been found in Alexandrovsky District, Orenburg Oblast. The type and only species is Novocynodon kutorgai.

Platycraniellus

Platycraniellus is an extinct genus of non-mammalian synapsid. It is a cynodont from the Lystrosaurus Assemblage Zone.

P. elegans was from the Triassic of South Africa.

Procynosuchus

Procynosuchus (Greek: "Before dog crocodile") is an extinct genus of cynodonts from the Late Permian. It is considered to be one of the earliest and most basal cynodonts. Remains of Procynosuchus have been found in Germany, Zambia and South Africa. It was 60 cm (2 ft) long.

Progalesaurus

Progalesaurus is an extinct genus of non-mammalian therapsid.

Siriusgnathus

Siriusgnathus is a traversodontid cynodont from the Carnian channel sandstones and mudstones of the Candelária Formation, belonging to the Santa Maria Supersequence of the Paraná Basin in southeastern Brazil. It includes one species, Siriusgnathus niemeyerorum and was described in 2018. The species epithet refers to the Niemeyer locality in Agudo, Rio Grande do Sul. The cynodont was found together with archosauromorphs, dinosauromorphs and other therapsids of Probainognathia and Prozostrodontia. The formation has provided other fossils as Brasilitherium riograndensis, Brasilodon quadrangularis, Irajatherium hernandezi, Prozostrodon brasiliensis.

Sludica

Sludica is an extinct genus of procynosuchid cynodont from the Late Permian of Russia. Fossils have been found within Velikoustyugsky District in Vologda Oblast. The type and only species is Sludica bulanovi.

Thrinaxodon

Thrinaxodon is an extinct genus of cynodonts, most commonly regarded by its species T. liorhinus which lived in what are now South Africa and Antarctica. Thrinaxodon has been dated between the Permian–Triassic boundary and the mid-Triassic. Its survival of the extinction may have been due to its burrowing habits.Similar to other synapsids, Thrinaxodon adopted a semi-sprawling posture, an intermediary form between the sprawling position of pelycosaurs (not unlike current Crocodylia) and the more upright posture present in current mammals. Thrinaxodon is prevalent in the fossil record in part because it was one of the few carnivores of its time, and was of a larger size than similar cynodont carnivores.

Major groups of therapsids
Basal therapsids
Eutherapsida
Related categories
Cynodonts

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