| Cynodontia | |
|---|---|
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| Examples of cynodonts. 1st row:Dvinia prima,Trirachodon berryi; | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Eutheriodontia |
| Clade: | Cynodontia Owen, 1861 |
| Clades | |
Cynodontia (from Ancient Greek κύων (kúōn) 'dog' and ὀδούς (odoús) 'tooth') is aclade ofeutheriodonttherapsids that first appeared in theLate Permian (approximately 260mya), and extensively diversified after thePermian–Triassic extinction event.Mammals are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advancedprobainognathian cynodonts during the Late Triassic.
Non-mammalian cynodonts occupied a variety ofecological niches, both as carnivores and as herbivores. Following the emergence of mammals, most other cynodont lines went extinct, with the last known non-mammaliaform cynodont group, theTritylodontidae, having its youngest records in theEarly Cretaceous.
The name cynodont ("dog tooth") comes from their cheek teeth, that can resemble a puppy'sincisors, which havemamelons.[1]
Early cynodonts have many of the skeletal characteristics ofmammals. The teeth were fully differentiated and the braincase bulged at the back of the head. Outside of somecrown-group mammals (notably thetherians), all cynodonts probably laid eggs. Thetemporal fenestrae were much larger than those of their ancestors, and the widening of thezygomatic arch in a more mammal-like skull would have allowed for more robust jaw musculature. They also have thesecondary palate that other primitivetherapsids lacked, except thetherocephalians, who were the closest relatives of cynodonts. (However, the secondary palate of cynodonts primarily comprises themaxillae andpalatines as in mammals, whereas the secondary palate of the therocephalians primarily comprises the maxillae and thevomer.) Thedentary was the largest bone in their lower jaw.
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The cynodonts probably had some form ofwarm-blooded metabolism. This has led to many reconstructions of cynodonts as havingfur. Being endothermic they may have needed it forthermoregulation, but fossil evidence of their fur (or lack thereof) has been elusive. Modern mammals haveHarderian glands secreting lipids to coat their fur, but the telltale imprint of this structure is only found from theprimitive mammalMorganucodon and onwards.[2] Nonetheless, recent studies onPermian synapsidcoprolites show that more basal therapsids may have had fur,[3] and at any rate fur was already present inMammaliaformes such asCastorocauda andMegaconus.
Early cynodonts had numerous smallforamina on their snout bones, similar to reptiles. This suggests that they had immobile, non-muscular lips like those of lizards, and lacked muscular cheeks.[4] As a muscular, mobile face is necessary to perform whisking movements and to avoid damage towhiskers, it is unlikely that early cynodonts had whiskers.[5][6] Inprozostrodontian cynodonts, the group that includes mammals, the foramina are replaced by a single large infraorbital foramen, which indicates that the face had become muscular and that whiskers would have been present.[7]
Derived cynodonts developedepipubic 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 highlyaltricial young as in modernmarsupials andmonotremes. Onlyplacentals, and perhapsMegazostrodon andErythrotherium, would lose these.[8][9] A specimen ofKayentatherium does indeed demonstrate that at least tritylodontids already had a fundamentally marsupial-like reproductive style, but produced much higher litters at around 38perinates or possibly eggs.[10]
Cynodonts are the only known synapsid lineage to have produced aerial locomotors, with gliding being known inharamiyidans[11] and various mammal groups, and placental mammals having developed flight.[12]
The largest known non-mammalian cynodont isScalenodontoides, atraversodontid, which has been estimated to have a maximum skull length of approximately 617 millimetres (24.3 in) based on a fragmentary specimen.[13]
The closest relatives of cynodonts aretherocephalians, with which they form the cladeEutheriodontia.[14]
The earliest cynodonts are known earlyLopingian (earlyWuchiapingian) aged sediments of theTropidostoma Assemblage Zone, in theKaroo Supergroup of South Africa, belonging to the basal familyCharassognathidae. Fossils of Permian cynodonts are relatively rare outside of South Africa, with the most widely distributed genus beingProcynosuchus, which is known from South Africa, Germany, Tanzania, Zambia, and possibly Russia.[15]
Cynodonts expanded rapidly in diversity after thePermian–Triassic extinction event. Peak disparity in cynodonts occurred from the Induan to the Carnian and in the middle Norian.[16] Post-Early Triassic cynodonts were dominated by members of the advanced cladeEucynodontia, which has two main subdivisions, the predominantly herbivorousCynognathia and the predominantly carnivorousProbainognathia. During the Early and Middle Triassic, cynodont diversity was dominated by members of Cynognathia, and members of Probainognathia would not become prominent until the Late Triassic (earlyNorian).[17] Almost all Middle Triassic cynodonts are known from Gondwana, with only one genus (Nanogomphodon) having been found in the Northern Hemisphere. Among the most dominant groups of Middle and Late Triassic cynodonts is the herbivorousTraversodontidae, predominantly in Gondwana, which reached a peak diversity in the Late Triassic.Mammaliaformes originated from probainognathian cynodonts during the Late Triassic.[18] Early Mammaliaformes were small bodied insectivores.[19] Only two groups of non-mammaliaform cynodonts existed beyond the end of the Triassic, both belonging to Probainognathia. The first is the insectivorousTritheledontidae, which briefly lasted into the Early Jurassic. The second is the herbivorousTritylodontidae, which first appeared in the latest Triassic, which were abundant and diverse during the Jurassic, predominantly in the Northern Hemisphere, persisted into the Early Cretaceous (Barremian-Aptian) in Asia, at least until around 120 million years ago, as represented byFossiomanus from China.[18][20]
During theirevolution, the number of cynodontjaw bones reduced. This move towards a single bone for the mandible paved the way for other bones in the jaw, thearticular andangular, to migrate to the cranium, where they function as parts of the mammalian hearing system.
Cynodonts also developed asecondary 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.
Richard Owen named Cynodontia in 1861, which he assigned toAnomodontia as a family.[21]Robert Broom (1913) reranked Cynodontia as an infraorder, since retained by others, including Colbert and Kitching (1977), Carroll (1988), Gauthieret al. (1989), and Rubidge andCristian Sidor (2001).[22] Olson (1966) assigned Cynodontia toTheriodontia, Colbert and Kitching (1977) to Theriodontia, and Rubridge and Sidor (2001) toEutheriodontia.William King Gregory (1910), Broom (1913), Carroll (1988), Gauthieret al. (1989), Hopson and Kitching (2001) and Bothaet al. (2007) all considered Cynodontia as belonging to Therapsida. Bothaet al. (2007) seems to have followed Owen (1861), but without specifying taxonomic rank.[23][24]
Below is acladogram from Ruta, Botha-Brink, Mitchell and Benton (2013) showing one hypothesis of cynodont relationships:[17]
| Cynodontia | |
Non-mammalian cynodonts have been found in South America, India, Africa, Antarctica,[25] Asia,[26] Europe[27] and North America.[28]
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