Tritylodontidae

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Tritylodontidae
Temporal range: Late Triassic – Early Cretaceous Rhaetian–Aptian
Oligokyphus BW.jpg
Life restoration of Oligokyphus triserialis
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Superfamily:† Tritylodontoidea
Family:† Tritylodontidae
Cope, 1884
Genera

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 details of the skeleton. [1] They were the last-known family of the non-mammaliaform synapsids, persisting into the Early Cretaceous. [2]

Contents

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. [3] Tritylodontid fossils are found in the Americas, South Africa, and Eurasia—they appear to have had an almost global distribution, including Antarctica. [4]

Description

Skull of Kayentatherium wellesi Kayentatherium wellesi.jpg
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 [1] —the retention of the joint is one of the reasons they are technically regarded to not be mammals, but are instead non-mammalian mammaliamorphs. [5] 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. [1] 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, though unlike rodents tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front). [1] 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. [3] [ dubious – discuss ]

Like mammaliaforms, tritylodontids had epipubic bones, a possible synapomorphy between both clades, [6] 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. [7] Tritylodonts were active animals that were likely warm-blooded and possibly burrowed. [1] 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. [8]

Discovery

The first tritylodontid named was Stereognathus , from teeth found in the Middle Jurassic Great Oolite Group of England [9] [10] and the family name was erected by Cope in 1884. [11] Shortly after, another tritylodontid was discovered in the Upper Triassic rocks of South Africa. [12]

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. [13]

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. [14] 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. [2] [15] The morphology of Fossiomanus indictates it had a specialised fossorial (burrowing) lifestyle. [15]

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. [16]

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. Haughton and 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 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 Liu and Olsen (2010) places Tritylodontidae very closely to Mammalia, as the sister taxon of the clade formed by Brasilodontidae and Mammalia. [17] 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 from this analysis. [18]

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. [5]

Probainognathia  

Genera

Kayentatherium Kayentatherium.JPG
Kayentatherium

See also

Related Research Articles

<span class="mw-page-title-main">Synapsida</span> Clade of tetrapods

Synapsida is one of the two major clades of vertebrate animals in the group Amniota, the other being the Sauropsida. The synapsids were the dominant land animals in the late Paleozoic and early Mesozoic, but the only group that survived into the Cenozoic are mammals. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

<span class="mw-page-title-main">Therapsida</span> Clade of tetrapods including mammals

Therapsida is a clade composing of a major group of eupelycosaurian synapsids that includes mammals and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more underneath the body, as opposed to the sprawling posture of many reptiles and salamanders.

<span class="mw-page-title-main">Cynodontia</span> Clade of therapsids

Cynodontia is a clade of eutheriodont therapsids that first appeared in the Late Permian, and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advanced probainognathian cynodonts during the Late Triassic.

Oligokyphus is an extinct genus of herbivorous tritylodontid cynodont known from the Late Triassic to Early Jurassic of Europe, Asia and North America.

<i>Tritylodon</i> Extinct genus of mammaliamorphs

Tritylodon is an extinct genus of tritylodonts, one of the most advanced group of cynodont therapsids. They lived in the Early Jurassic and possibly Late Triassic periods along with dinosaurs. They also shared many characteristics with mammals, and were once considered mammals because of overall skeleton construction. That was changed due to them retaining the vestigial amniote jawbones and a different skull structure. Tritylodonts are now regarded as non-mammalian synapsids.

<span class="mw-page-title-main">Mammaliaformes</span> Clade of mammals and extinct relatives

Mammaliaformes 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 common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium.

Sinoconodon is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic traits with other non-mammaliaform cynodonts, it possessed a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which in more derived taxa would replace the primitive tetrapod one between the articular and quadrate bones. The presence of a dentary-squamosal joint is a trait historically used to define mammals.

<span class="mw-page-title-main">Theriodontia</span> Clade of therapsids

The theriodonts are a major group of therapsids which appeared during the Middle Permian and which includes the gorgonopsians and the eutheriodonts, itself including the therocephalians and the cynodonts.

<span class="mw-page-title-main">Evolution of mammals</span> Derivation of mammals from a synapsid precursor, and the adaptive radiation of mammal species

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

<i>Brasilodon</i> Extinct genus of mammaliamorphs

Brasilodon 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 found, the length of Brasilodon has been estimated at 12 centimetres (4.7 in). Its dentition shows that it was most likely an insectivore. The genus is monotypic, containing only the species B. quadrangularis. Brasilodon belongs to the family Brasilodontidae, whose members were some of the closest relatives of mammals, the only cynodonts alive today. Two other brasilodontid genera, Brasilitherium and Minicynodon, are now considered to be junior synonyms of Brasilodon.

Dinnebitodon is an extinct genus of advanced herbivorous cynodonts of the early Jurassic period. It has only been found in the Kayenta Formation in northeastern Arizona. It closely resembles the related genus Kayentatherium from the same formation. It is set apart by differences in the dentition, while resembling in most other respects.

<i>Kayentatherium</i> Extinct genus of mammaliamorphs

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.

<span class="mw-page-title-main">Prozostrodontia</span> Clade of cynodonts

Prozostrodontia is a clade of cynodonts including mammaliaforms and their closest relatives such as Tritheledontidae and Tritylodontidae. It was erected as a node-based taxon by Liu and Olsen (2010) and defined as the least inclusive clade containing Prozostrodon brasiliensis, Tritylodon langaevus, Pachygenelus monus, and Mus musculus. Prozostrodontia is diagnosed by several characters, including:

<i>Stereognathus</i> Extinct genus of mammaliamorphs

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. hebridicus, was named after the Hebrides in Scotland, where it was found; it was synonymized with S. ooliticus in 2017.

Xenocretosuchus is an extinct genus of tritylodont therapsids from the Aptian Ilek Formation of Siberia, in the Russian Federation. The type species, X. sibiricus, is known only from dental elements, as is X. kolossovi, described from the Batylykh Formation in 2008. Some authors have treated these species as part of the genus Stereognathus, otherwise known from the Middle Jurassic of Britain, but this is rejected by other authors.

Montirictus is an extinct genus of tritylodonts known from the Early Cretaceous Kuwajima Formation of Japan. It was among the latest surviving tritylodontids, and is closely related to the earlier Xenocretosuchus from mainland Asia, and the Jurassic Stereognathus from the UK. It may be a species of the genus Stereognathus, but resolution of its affinities conditions upon the discovery of additional material.

<i>Borealestes</i> Extinct genus of mammaliaforms

Borealestes is a genus of docodontan from the Middle Jurassic of Britain, first discovered on the Isle of Skye near the village of Elgol. It was the earliest mammaliaform from the Mesozoic found and named in Scotland. A second species and was later found in other Middle Jurassic sites in England, but is now shown to be a different genus. A new species, B. cuillinensis was named in 2021, also from Skye.

<i>Pseudotherium</i> Extinct genus of cynodonts

Pseudotherium is an extinct genus of prozostrodontian cynodonts from the Late Triassic of Argentina. It contains one species, P. argentinus, which was first described in 2019 from remains found in the La Peña Member of the Ischigualasto Formation in the Ischigualasto-Villa Unión Basin.

Fossiomanus is an extinct genus of tritylodontid mammaliamorphs from the Early Cretaceous of China. It includes one species, F. sinensis, which is known from a single nearly complete skeleton from the Aptian Jiufotang Formation. Features of its limbs and vertebrae indicate that Fossiomanus was adapted towards a fossorial lifestyle.

Michael Waldman is a British palaeontologist known for his work on fossil fish, mammals, and reptiles. He also discovered the globally important fossil site of Cladach a'Ghlinne, near Elgol on the Isle of Skye, Scotland. This site exposes the Kilmaluag Formation and provides a valuable record of Middle Jurassic ecosystems. During the 1970s he visited the site several times with fellow palaeontologist Robert Savage. The fossil turtle Eileanchelys waldmani was named after Michael in recognition of his notable contribution to palaeontology.

References

  1. 1 2 3 4 5 Kemp T (2004). The Origin and Evolution of Mammals. Oxford University Press. ISBN   978-0-19-850761-1.
  2. 1 2 Matsuoka H, Kusuhashi N, Corfe IJ (July 2016). "A new Early Cretaceous tritylodontid (Synapsida, Cynodontia, Mammaliamorpha) from the Kuwajima Formation (Tetori Group) of central Japan". Journal of Vertebrate Paleontology. 36 (4): e1112289. Bibcode:2016JVPal..36E2289M. doi:10.1080/02724634.2016.1112289. S2CID   130588924.
  3. 1 2 Hu Y, Meng J, Clark JM (2009). "A New Tritylodontid from the Upper Jurassic of Xinjiang, China". Acta Palaeontologica Polonica. 54 (3): 385–391. doi: 10.4202/app.2008.0053 .
  4. ↑ Hammer WR, Smith ND (2008). "A Tritylodont Postcanine from the Hanson Formation of Antarctica". Journal of Vertebrate Paleontology. 28: 269–273. doi:10.1671/0272-4634(2008)28[269:atpfth]2.0.co;2. S2CID   130101582.
  5. 1 2 Luo ZX, Kielan-Jaworowska Z, Cifelli R (2002). "In quest for a phylogeny of Mesozoic mammals" (PDF). Acta Palaeontologica Polonica. 47 (1): 1–78.
  6. ↑ Reilly SM, White TD (January 2003). "Hypaxial motor patterns and the function of epipubic bones in primitive mammals". Science. 299 (5605): 400–2. Bibcode:2003Sci...299..400R. doi:10.1126/science.1074905. PMID   12532019. S2CID   41470665.
  7. ↑ Hoffman EA, Rowe TB (September 2018). "Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth". Nature. 561 (7721): 104–108. Bibcode:2018Natur.561..104H. doi:10.1038/s41586-018-0441-3. PMID   30158701. S2CID   205570021.
  8. ↑ Kemp T, Smith R, Rubidge B, Huttenlocker A, Rega E, Ray S, et al. (November 2011). Forerunners of Mammals: Radiation• Histology• Biology. Indiana University Press. ISBN   978-0-253-00533-5.
  9. ↑ Charlesworth E (1854). "Title unknown". Rept Brit Assoc. Liverpool Abstracts. 80.
  10. ↑ Panciroli E, Walsh S, Fraser NC, Brusatte SL, Corfe I (2017). "A reassessment of the postcanine dentition and systematics of the tritylodontid Stereognathus (Cynodontia, Tritylodontidae, Mammaliamorpha), from the Middle Jurassic of the United Kingdom". Journal of Vertebrate Paleontology. 37 (5): e1351448. Bibcode:2017JVPal..37E1448P. doi: 10.1080/02724634.2017.1351448 . hdl: 10138/230155 .
  11. ↑ Cope ED (1884). "The Tertiary Marsupialia". American Naturalist. 18 (7): 686–697. doi: 10.1086/273711 .
  12. ↑ Owen R (1884). "On the skull and dentition of a Triassic mammal (Tritylodon longaevus) from South Africa". Quarterly Journal of the Geological Society of London. 40: 146–152. doi:10.1144/gsl.jgs.1884.40.01-04.07. hdl: 2027/hvd.32044107348633 . S2CID   131059991.
  13. ↑ Canon, Gabrielle (13 October 2023). "'Extremely rare' Jurassic-era fossils discovered in Lake Powell". The Guardian.
  14. ↑ Abdala, Fernando; Gaetano, Leandro C. (2018), Tanner, Lawrence H. (ed.), "The Late Triassic Record of Cynodonts: Time of Innovations in the Mammalian Lineage", The Late Triassic World, vol. 46, Cham: Springer International Publishing, pp. 407–445, doi:10.1007/978-3-319-68009-5_11, ISBN   978-3-319-68008-8 , retrieved 2021-05-24
  15. 1 2 Mao, Fangyuan; Zhang, Chi; Liu, Cunyu; Meng, Jin (April 2021). "Fossoriality and evolutionary development in two Cretaceous mammaliamorphs". Nature. 592 (7855): 577–582. Bibcode:2021Natur.592..577M. doi:10.1038/s41586-021-03433-2. ISSN   1476-4687. PMID   33828300. S2CID   233183060.
  16. ↑ Kalthoff DC, Schulz-Kornas E, Corfe I, Martin T, McLoughlin S, Schultz JA (2019-07-25). Viriot L (ed.). "Complementary approaches to tooth wear analysis in Tritylodontidae (Synapsida, Mammaliamorpha) reveal a generalist diet". PLOS ONE. 14 (7): e0220188. Bibcode:2019PLoSO..1420188K. doi: 10.1371/journal.pone.0220188 . PMC   6658083 . PMID   31344085.
  17. ↑ Liu J, Olsen P (2010). "The Phylogenetic Relationships of Eucynodontia (Amniota: Synapsida)". Journal of Mammalian Evolution. 17 (3): 151–176. doi:10.1007/s10914-010-9136-8. S2CID   40871206.
  18. ↑ Ruta M, Botha-Brink J, Mitchell SA, Benton MJ (October 2013). "The radiation of cynodonts and the ground plan of mammalian morphological diversity". Proceedings. Biological Sciences. 280 (1769): 20131865. doi:10.1098/rspb.2013.1865. PMC   3768321 . PMID   23986112.
  19. ↑ Szczygielski, T.; Van den Brandt, M. J.; Gaetano, L.; Dróżdż, D. (2024). "Saurodesmus robertsoni Seeley 1891—The oldest Scottish cynodont". PLOS ONE. 19 (5): e0303973. doi: 10.1371/journal.pone.0303973 . PMID   38809839.
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