{{short description|Extinct family of cynodonts}} {{Distinguish|Tritheledontidae}} {{Automatic taxobox | fossil_range = Late Triassic – Early Cretaceous {{fossilrange|Rhaetian|Aptian}} | image = Oligokyphus BW.jpg | image_upright = 1.1 | image_caption = Life restoration of ''Oligokyphus triserialis'' | display_parents = 2 | taxon = Tritylodontidae | authority = Cope, 1884 | subdivision_ranks = Genera | subdivision = See below }}
'''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, with several mammalian traits including erect limbs, endothermy, and some details of the skeleton.<ref name=Kemp>{{cite book | last = Kemp | first = T. | year = 2004 | title = The Origin and Evolution of Mammals | publisher = Oxford University Press | isbn = 978-0-19-850761-1 | url = https://books.google.com/books?id=Nr1WP8j5VtAC |via = Google }}</ref> They were the last-known family of the non-mammaliaform synapsids, persisting into the Early Cretaceous.<ref name=Matsuoka>{{cite journal | vauthors = Matsuoka H, Kusuhashi N, Corfe IJ | date = July 2016 | title = A new Early Cretaceous tritylodontid (Synapsida, Cynodontia, Mammaliamorpha) from the Kuwajima Formation (Tetori Group) of central Japan | journal = Journal of Vertebrate Paleontology | volume = 36 | issue = 4 | article-number = e1112289 | doi = 10.1080/02724634.2016.1112289 | bibcode = 2016JVPal..36E2289M | s2cid = 130588924 }}</ref>
Most tritylodontids are thought to have been herbivorous, feeding on vegetation such as stems, leaves, and roots, although at least one may have had a more omnivorous diet.<ref name=Huetal>{{cite journal | vauthors = Hu Y, Meng J, Clark JM | year = 2009 | title = A new Tritylodontid from the Upper Jurassic of Xinjiang, China | journal = Acta Palaeontologica Polonica | volume = 54 | issue = 3| pages = 385–391 | doi = 10.4202/app.2008.0053 | doi-access = free }}</ref> Tritylodontid fossils are found in the Americas, South Africa, and Eurasia – they appear to have had an almost global distribution, including Antarctica.<ref name=Hammer>{{cite journal | vauthors = Hammer WR, Smith ND | year = 2008 | title = A Tritylodont Postcanine from the Hanson Formation of Antarctica | journal = Journal of Vertebrate Paleontology | volume = 28 | pages = 269–273 | doi = 10.1671/0272-4634(2008)28[269:atpfth]2.0.co;2 | s2cid = 130101582 }}</ref>
==Description== [[File:Kayentatherium wellesi.jpg|thumb|left|Skull of ''Kayentatherium wellesi'']] The skull of tritylodontids had a high sagittal crest. They retained the primitive condition of the joint between the quadrate bone of the skull and the articular bone of the lower jaw<ref name=Kemp/> – the retention of the joint is one of the reasons they are technically regarded to not be mammals, but are instead non-mammalian mammaliamorphs.<ref name=Luo2002>{{cite journal |vauthors = Luo ZX, Kielan-Jaworowska Z, Cifelli R |year=2002 |title=In quest for a phylogeny of Mesozoic mammals |journal = Acta Palaeontologica Polonica |volume=47 |issue=1 |pages=1–78 |url=http://agro.icm.edu.pl/agro/element/bwmeta1.element.agro-article-a106466e-5e6b-4025-89a5-360d94a7eb83/c/app47-001.pdf |via=agro.icm.edu.pl }}</ref> The back of the skull had huge zygomatic arches for the attachment of its large jaw muscles. They also had a very well-developed secondary palate. The tritylodont dentition differed from that of most other cynodonts: They did not have canine teeth, and the front pair of incisors were enlarged and were very similar to those of modern-day rodents.<ref name=Kemp/> Tritylodontids had a large gap, called a diastema, 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 between the teeth in somewhat the same way as a modern rodent, although tritylodontids had a palinal jaw stroke (front-to-back), unlike rodents which have a propalinal jaw stroke (back-to-front).<ref name=Kemp/> The teeth were well suited for shredding plant matter; however, there is evidence that some tritylodontids had more omnivorous diets, much in the same vein as modern mammals with "herbivore dentitions" like modern rats.<ref name=Huetal/>{{dubious|date=July 2018}}
Like mammaliaforms, tritylodontids had epipubic bones, a possible synapomorphy between both clades,<ref>{{cite journal | vauthors = Reilly SM, White TD | date = January 2003 | title = Hypaxial motor patterns and the function of epipubic bones in primitive mammals | journal = Science | volume = 299 | issue = 5605 | pages = 400–402 | pmid = 12532019 | doi = 10.1126/science.1074905 | bibcode = 2003Sci...299..400R | s2cid = 41470665 }}</ref> and this suggests they may also have laid eggs like modern monotremes, or produced undeveloped fetus-like young like modern marsupials. A recent ''Kayentatherium'' shows that they indeed produced undeveloped young, but at litter sizes much larger than any monotreme or marsupial, at around 38 perinates.<ref>{{cite journal | vauthors = Hoffman EA, Rowe TB | date = September 2018 | title = Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth | journal = Nature | volume = 561 | issue = 7721 | pages = 104–108 | pmid = 30158701 | doi = 10.1038/s41586-018-0441-3 | bibcode = 2018Natur.561..104H | s2cid = 205570021 }}</ref> Tritylodonts were active animals that were likely warm-blooded and possibly burrowed.<ref name=Kemp/> The small early tritylodontid ''Oligokyphus'' has been compared to a weasel or mink, with a long, slim body and tail. In ''Kayentatherium'' the burrowing adaptations seen in the skeleton have been re-interpreted as possibly suggesting a semi-aquatic ecology.<ref>{{cite book | vauthors = Kemp T, Smith R, Rubidge B, Huttenlocker A, Rega E, Ray S, Botha J, Jasinoski S, Green JL, Abdala F, Hurum JH | display-authors = 6 | date = November 2011 | title = Forerunners of Mammals: Radiation, histology, biology | publisher = Indiana University Press | isbn = 978-0-253-00533-5 }}</ref>
==Discovery== The first tritylodontid named was ''Stereognathus'', from teeth found in the Middle Jurassic Great Oolite Group of England<ref name=Charlesworth>{{cite journal | vauthors = Charlesworth E | year = 1854 | title = {{grey|[title not cited]}} |journal=Rept. Brit. Assoc. Liverpool Abstracts | volume = 80 }}</ref><ref name=Panciroli>{{cite journal | vauthors = Panciroli E, Walsh S, Fraser NC, Brusatte SL, Corfe I | year = 2017 | title = A reassessment of the postcanine dentition and systematics of the tritylodontid ''Stereognathus'' (Cynodontia, Tritylodontidae, Mammaliamorpha), from the Middle Jurassic of the United Kingdom | journal = Journal of Vertebrate Paleontology |volume=37 |issue=5 | article-number = e1351448 | doi = 10.1080/02724634.2017.1351448 | hdl = 10138/230155 | doi-access = free | bibcode = 2017JVPal..37E1448P | hdl-access = free }}</ref> and the family name was erected by Cope in 1884.<ref name=Cope1884>{{cite journal | vauthors = Cope ED | year = 1884 | title = The Tertiary Marsupialia | journal = American Naturalist | volume = 18 | issue = 7 | pages = 686–697 | doi = 10.1086/273711 | bibcode = 1884ANat...18..686C | doi-access = free }}</ref> Shortly after, another tritylodontid was discovered in the Upper Triassic rocks of South Africa.<ref name=Owen1984>{{cite journal | vauthors = Owen R | author-link = Richard Owen | year = 1884 | title = On the skull and dentition of a Triassic mammal (''Tritylodon longaevus'') from South Africa | journal = Quarterly Journal of the Geological Society of London | volume = 40 | pages = 146–152 | doi = 10.1144/gsl.jgs.1884.40.01-04.07 | hdl = 2027/hvd.32044107348633 | s2cid = 131059991 | hdl-access = free }}</ref>
In 2023 skulls and teeth from dozens of tritylodonts were discovered at Lake Powell on the Colorado River in the United States, with more discoveries expected which may help to understand the history and evolution of mammals.<ref>{{cite news | last=Canon | first=Gabrielle | date=13 October 2023 | title='Extremely rare' Jurassic-era fossils discovered in Lake Powell | newspaper=The Guardian | place=London, UK | url=https://www.theguardian.com/science/2023/oct/13/rare-jurassic-era-fossils-discovered-lake-powell-utah }}</ref>
==Evolutionary history== Tritylodontids first appeared during the Rhaetian, the last stage of the Triassic, and were abundant during the Jurassic, with several records from the Early Cretaceous. 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.<ref>{{cite book |last1=Abdala |first1=Fernando |last2=Gaetano |first2=Leandro C. |year=2018 |section=The Late Triassic record of Cynodonts: Time of innovations in the Mammalian lineage |editor-last=Tanner |editor-first=Lawrence H. |title=The Late Triassic World |volume=46 |pages=407–445 |place=Cham |publisher=Springer International Publishing |lang=en |doi=10.1007/978-3-319-68009-5_11 |isbn=978-3-319-68008-8 |section-url=http://link.springer.com/10.1007/978-3-319-68009-5_11 |access-date=2021-05-24 }}</ref> ''Xenocretosuchus,'' ''Montirictus'' and ''Fossiomanus'' are the latest known tritylodontids, from the Barremian-Aptian aged Ilek Formation of Siberia, Kuwajima Formation of Japan, and Yixian Formation of China respectively.<ref name=Matsuoka/><ref name=Mao-Zhang-etal-2021/> The morphology of ''Fossiomanus'' indictates it had a specialised fossorial (burrowing) lifestyle.<ref name=Mao-Zhang-etal-2021>{{cite journal |last1=Mao |first1=Fangyuan|last2=Zhang |first2=Chi |last3=Liu |first3=Cunyu |last4=Meng |first4=Jin |date=April 2021 |title=Fossoriality and evolutionary development in two Cretaceous mammaliamorphs |journal=Nature |volume=592 |issue=7855 |pages=577–582 |doi=10.1038/s41586-021-03433-2 |pmid=33828300 |bibcode=2021Natur.592..577M |s2cid=233183060 |issn=1476-4687 |url=https://www.nature.com/articles/s41586-021-03433-2 |lang=en |url-access=subscription }}</ref>
== 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.<ref>{{cite journal | vauthors = Kalthoff DC, Schulz-Kornas E, Corfe I, Martin T, McLoughlin S, Schultz JA | date = 25 July 2019 | title = Complementary approaches to tooth wear analysis in Tritylodontidae (Synapsida, Mammaliamorpha) reveal a generalist diet | journal = PLoS One | volume = 14 | issue = 7 | article-number = e0220188 | pmid = 31344085 | pmc = 6658083 | doi = 10.1371/journal.pone.0220188 | doi-access = free | bibcode = 2019PLoSO..1420188K <!-- | veditors = Viriot L --> }}</ref>
==Phylogeny== Because of their mammal-like appearance, tritylodontids were originally placed within Mammalia. Starting with the work of British paleontologist D.M.S. Watson in 1942, a close relationship was favored between tritylodontids and cynodonts. Watson and other paleontologists noted that tritylodontids lacked the dentary and squamosal jaw articulation that was characteristic of early mammals. {{harvp|Haughton|Brink|1954}} were the first to classify tritylodontids within Cynodontia. Later studies identified close similarities between the teeth of tritylodontids and traversodontids, 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. The name Tritylodontoidea was previously used for the group, which traditionally included the families Diademodontidae, Trirachodontidae, Traversodontidae, and Tritylodontidae.
More recently, tritylodontids have been reinterpreted as close relatives of mammals. Beginning with {{harvp|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 analysis performed by {{harvp|Liu|Olsen|2010}} places Tritylodontidae very closely to Mammalia, as the sister taxon of the clade formed by Brasilodontidae and Mammalia.<ref name=LO10>{{cite journal | vauthors = Liu J, Olsen P | year = 2010 | title = The phylogenetic relationships of Eucynodontia (Amniota: Synapsida) | journal = Journal of Mammalian Evolution | volume = 17 | issue = 3 | pages = 151–176 | s2cid = 40871206 | doi = 10.1007/s10914-010-9136-8 }}</ref> {{harvp|Ruta|Botha-Brink|Mitchell|Benton|2013}}'s phylogenetic analysis which is partially based on {{harvp|Liu|Olsen|2010}} places Tritylodontidae in a more derived position than Brasilodontidae. Below is a cladogram from this analysis.<ref name=RBMB13>{{cite journal | vauthors = Ruta M, Botha-Brink J, Mitchell SA, Benton MJ | date = October 2013 | title = The radiation of cynodonts and the ground plan of mammalian morphological diversity | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 280 | issue = 1769 | article-number = 20131865 | pmid = 23986112 | pmc = 3768321 | doi = 10.1098/rspb.2013.1865 }}</ref>
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.<ref name="Luo2002"/>
{{clade| style=font-size:85%;line-height:85% |label1=Probainognathia |1={{clade |1=''Lumkuia'' |2={{clade |1=''Ecteninion'' |2={{clade |1={{clade |1=''Aleodon'' |2=''Chiniquodon''}} |2={{clade |1=''Probainognathus'' |2={{clade |1=''Trucidocynodon'' |2={{clade |1=''Therioherpeton'' |2={{clade |label1=Tritheledontidae |1={{clade |1=''Riograndia'' |2={{clade |1={{clade |1=''Chaliminia'' |2=''Elliotherium''}} |2={{clade |1=''Diarthrognathus'' |2=''Pachygenelus''}} }} }} |2={{clade |label1=Brasilodontidae |1={{clade |1=''Brasilitherium'' |2=''Brasilodon''}} |2={{clade |label1='''Tritylodontidae''' |1={{clade |1=''Oligokyphus'' |2={{clade |1=''Kayentatherium'' |2={{clade |1=''Tritylodon'' |2=''Bienotherium''}} }} }} |label2=Mammaliaformes |2={{clade |1=''Sinoconodon'' |2=''Morganucodon''}} }} }} }} }} }} }} }} }} }} }}
==Genera== [[File:Kayentatherium.JPG|thumb|right|''Kayentatherium'']] *''Bienotherium'' *''Bienotheroides'' *''Bocatherium'' *''Dianzhongia'' *''Dinnebitodon'' *''Fossiomanus'' *''Kayentatherium'' *''Lufengia'' *''Montirictus'' *''Nuurtherium'' *''Oligokyphus'' *''Polistodon'' *''Saurodesmus''?<ref name=Szczygielski2024>{{cite journal |last1=Szczygielski |first1=T. |last2=van den Brandt |first2=M. J. |last3=Gaetano |first3=L. |last4=Dróżdż |first4=D. |year=2024 |title=''Saurodesmus robertsoni'' Seeley 1891—The oldest Scottish cynodont |journal=PLoS One |volume=19 |issue=5 |article-number=e0303973 |doi=10.1371/journal.pone.0303973 |doi-access=free |pmid=38809839 |pmc=11135747 |bibcode=2024PLoSO..1903973S }}</ref> *''Shartegodon'' *''Stereognathus'' *''Tritylodon'' *''Xenocretosuchus'' *''Yuanotherium'' *''Yunnanodon''
== See also == * Embryonic diapause
== References == {{reflist|25em}}
{{Cynodontia|P.}} {{Taxonbar|from=Q134746}}
Category:Tritylodontidae Category:Prehistoric therapsid families Category:Late Triassic first appearances Category:Early Cretaceous extinctions Category:Taxa named by Edward Drinker Cope