Mixosaurus

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Mixosaurus is an extinct genus of Middle Triassic (Anisian to Ladinian, about 250-240 Mya) ichthyosaur. Its fossils have been found near the Italy–Switzerland border and in South China.[1]

The genus was named in 1887 by George H. Baur. The name means "Mixed Lizard", and was chosen because it appears to have been a transitional form between the eel-shaped ichthyosaurs such as Cymbospondylus and the later dolphin-shaped ichthyosaurs, such as Ichthyosaurus. Baur named Mixosaurus as a new genus because its forefin was sufficiently different from that of Ichthyosaurus.[4]

Numerous species have been described within this genus, which was once considered as the most common genus of Triassic ichthyosaurs.[4] The type species is M. cornalianus, and the definitive number of species is debated among researchers.[3]

Description

File:Mixosaurus Scale V2.svg
Mixosaurus with a human hand to scale

Mixosaurus was a small ichthyosaur, measuring Template:Cvt long and weighing Template:Cvt.[5] The largest specimens would have measured over Template:Cvt in length.[6] It possessed a long tail with a low fin, suggesting it wasn't as fast as Jurassic Ichthyosaurs, but also possessed a dorsal fin for stability in the water.[7] The paddle-like limbs were made up of five toes each, unlike the three toes found in later Ichthyosaurs. Noteworthy, however, is that each toe had more individual bones than is usual in reptiles, and the front limbs were longer than the back limbs, both adaptations typical of later ichthyosaurs. The jaws were narrow, with several sharp teeth, that would have been ideal for catching fish.[8] They had relatively large skulls compared to their bodies, unlike the basal ichthyosaurs, but resembled fish-shaped ichthyosaurs that appeared later. They had around 50 vertebrae in front of the pelvic girdle, around twice as many as terrestrial diapsids.[9] Recent studies suggest that genus Mixosaurus may have lived near shore or in a shelf-like habitat as it possesses more compact spongy bone within its long bones than other Ichthyosaurs.[10]

Species

A 2024 study which described the new species M. luxiensis showed that the identity of species within Mixosaurus is debated among researchers except for M. cornalianus and M. kuhnschnyderi.[3] M. xindianensis has been suggested as species inquirenda,[2] and it may more likely belong to Phalarodon.[3] Known species of Mixosaurus share many similar characteristics throughout the cranial and post cranial with the main differences morphology occurring in the dental region. Examples of the dental variation are the extent of the dental groove in the upper jaw, the shape and size of the teeth and the number of rows of teeth [4]

Previous authors assigned other species to the genus, including M. atavus (Quenstedt, 1852), M. callawayi Schmitz et al., 2004, M. panxianensis Jiang et al., 2006 and M. yangjuanensis Liu & Yin, 2008. These are now included in Contectopalatus, Phalarodon, Barracudasauroides and Nothosaurus, respectively.[11]

Mixosaurus species declared as nomina dubia, meaning the description was insufficient to fully classify them as species, are M. maotaiensis, M. helveticus, M. timorensis, M. major, and M. nordenskioeldii.[12][13][14]

Mixosaurus cornalianus

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File:Mixosaurus cornalianus Skeletal.png
Skeletal reconstruction of M. cornalianus showing soft tissue outline
File:Mixosaurus cornalianus SW 1.png
Life restoration of M. cornalianus

Many specimens of M. cornalianus have been found from the Middle Triassic of Monte San Giorgio and the Tessin areas on the border of Italy and Switzerland. M. cornalianus is the only Triassic ichthyosaur for which completely articulated skeletons have been found. Many specimens have been collected, but M. cornalianus is not well studied because all of the known specimens have been compressed during the preservation process. M. cornalianus has a sagittal crest associated with the expansion of the upper temporal fenestra. This indicates that it had exceptionally strong jaw muscles.[4] The current name-bearing specimen is the neotype (PIMUZ T 2420), which was approved by the ICZN in 2001 after a 1999 petition.[15][16]

Mixosaurus luxiensis

File:Mixosaurus Life Restoration.jpg
Life restoration of M. luxiensis

In 2024, Ye-Wei Fang and coauthors named a new species, M. luxiensis, with HFUT HL-21-08-002 being the holotype and only known specimen. This specimen, a largely complete but disarticulated skeleton, was found in the Upper Member of the Guanling Formation in Luxi County, Yunnan Province, China; the species name refers to this county.[3]

Mixosaurus panxianensis

File:Contectopalatus1DB.jpg
Life restoration of M. panxianensis

M. panxianensis was discovered in the Middle Triassic of the Guizhou Province, China. The specimens have been found in the Guanling Formation, which consists of thinly bedded bituminous limestones and marls. The specimens found have important mixosaurid characteristics such as a long sagittal crest along the top of the skull but is seen as a different species because there is no external contact between the jugal and the quadratojugal. Articulated skeletons have been found and the centra of the vertebrae are higher than they are long. This is evidence for its transitional position between basal early Triassic ichthyosaurs and more derived Jurassic and Cretaceous species; who have disc shaped circular centra of the vertebrae.[12] This species has been moved to its own genus, Barracudasauroides.[11][17] The 2024 study included M. panxianensis as a species of Mixosaurus based on phylogenetic analyses.[3]

Classification

File:Mixosaurus.jpg
M. cornalianus in Milan
File:Mixosaurus cornalianus Besano 3454.JPG
M. cornalianus

In recent years the taxonomy and phylogeny of mixosaurid ichthyosaurs has been a controversial topic. Most recently, Mixosauridae has been separated into Mixosaurinae and the sister group Phalarondontinae. Mixosaurus contains M. cornalianus, M. kuhnschnyderi and M. xindianensis, Barracudasauroides contains B. panxianensis, Phalarodon contains P. fraasi and P. callawayi, and Contectopalatus contains C. atavus.[2][11][17]

Mixosaurids are characterised by a relatively short and wide humerus and Phalarodon are characterised by the lack of a dental groove in the upper jaw. Phalarodon fossils are found in every major Mixosaur locality.

It was suggested that Tholodus schmidi should be included in Mixosauridae but only dental material has been found so it is difficult to assign it to a genus.[12]

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See also

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References

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  1. ^ a b Page Module:Citation/CS1/styles.css has no content.Cheng Ji; Da-Yong Jiang; Ryosuke Motani; Olivier Rieppel; Wei-Cheng Hao; Zuo-Yu Sun (2016). "Phylogeny of the Ichthyopterygia incorporating recent discoveries from South China". Journal of Vertebrate Paleontology. 36 (1) e1025956. Bibcode:2016JVPal..36E5956J. doi:10.1080/02724634.2015.1025956. S2CID 85621052.
  2. ^ a b c Page Module:Citation/CS1/styles.css has no content.Jun Liu; Ryosuke Motani; Da-Yong Jiang; et al. (2013). "The first specimen of the Middle Triassic Phalarodon atavus (Ichthyosauria: Mixosauridae) from South China, showing postcranial anatomy and peri-Tethyan distribution". Palaeontology. 56 (4): 849–866. Bibcode:2013Palgy..56..849L. doi:10.1111/pala.12021.
  3. ^ a b c d e f Page Module:Citation/CS1/styles.css has no content.Fang, Y.-W.; Wolniewicz, A. S.; Liu, J. (2024). "A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China". Acta Palaeontologica Polonica. 69 (2): 263–280. doi:10.4202/app.01133.2024.
  4. ^ a b c d Page Module:Citation/CS1/styles.css has no content.Motani R. (1999). "The skull and taxonomy of Mixosaurus (Ichthyopterygia)". Journal of Paleontology. 73 (5): 924–935. Bibcode:1999JPal...73..924M. doi:10.1017/S0022336000040750. S2CID 130661969.
  5. ^ Page Module:Citation/CS1/styles.css has no content.Sander, P.M.; Griebeler, E.M.; Klein, N.; Juarbe, J.V.; Wintrich, T.; Revell, L.J.; Schmitz, L. (2021). "Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans". Science. 374 (6575) eabf5787. doi:10.1126/science.abf5787. PMID 34941418. S2CID 245444783.
  6. ^ Page Module:Citation/CS1/styles.css has no content.Motani, R. (1999). "Phylogeny of the Ichthyopterygia". Journal of Vertebrate Paleontology. 19 (3): 472–495. Bibcode:1999JVPal..19..473M. doi:10.1080/02724634.1999.10011160.
  7. ^ Page Module:Citation/CS1/styles.css has no content.Renesto, Silvio; Dal Sasso, Cristiano; Fogliazza, Fabio; Ragni, Cinzia (2020). "New findings reveal that the middle Triassic ichthyosaur Mixosaurus cornalianus is the oldest amniote with a dorsal fin". Acta Palaeontologica Polonica. 65. doi:10.4202/app.00731.2020. ISSN 0567-7920. S2CID 222285117.
  8. ^ Page Module:Citation/CS1/styles.css has no content.Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 79. ISBN 978-1-84028-152-1.
  9. ^ Page Module:Citation/CS1/styles.css has no content.Motani, R.; et al. (1996). "Eel like swimming in the earliest ichthyosaurs". Nature. 382 (6589): 347–388. Bibcode:1996Natur.382..347M. doi:10.1038/382347a0. S2CID 4323012.
  10. ^ Page Module:Citation/CS1/styles.css has no content.Houssaye, Alexandra; Scheyer, Torsten M.; Kolb, Christian; Fisher, Valentin; Sander, P. Martin (April 2014). "A new look at ichthyosaur long bone microanatomy and histology: implications for their adaption to an aquatic life". PLOS ONE. 9 (4) e95637. Bibcode:2014PLoSO...995637H. doi:10.1371/journal.pone.0095637. PMC 3994080. PMID 24752508.
  11. ^ a b c Page Module:Citation/CS1/styles.css has no content.Michael W. Maisch (2010). "Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art" (PDF). Palaeodiversity. 3: 151–214.
  12. ^ a b c d Page Module:Citation/CS1/styles.css has no content.Jiang, D.; et al. (2006). "A new mixosaurid ichthyosaur from the Middle Triassic". Journal of Vertebrate Paleontology. 26: 60–69. doi:10.1671/0272-4634(2006)26[60:anmift]2.0.co;2. S2CID 55171917.
  13. ^ Page Module:Citation/CS1/styles.css has no content.Schmitz.; et al. (2010). "The taxonomic status of Mixosaurus nordenskioeldii". Journal of Vertebrate Paleontology. 25 (4): 983–985. doi:10.1671/0272-4634(2005)025[0983:ttsomn]2.0.co;2. S2CID 84214133.
  14. ^ Page Module:Citation/CS1/styles.css has no content.Prof. Dr. H.D Sues, ed. (2003). Handbook of Paeoherpetology, Part 8, Ichthyopterygia. Munchen: Friedrich Pfeil. p. 175.
  15. ^ Page Module:Citation/CS1/styles.css has no content."Opinion 1977 – Ichthyosaurus cornalianus Bassani, 1886 (currently Mixosaurus cornalianus; Reptilia, Ichthyosauria): neotype designated". The Bulletin of Zoological Nomenclature. 58 (2): 158. June 2001.
  16. ^ Page Module:Citation/CS1/styles.css has no content.Brinkmann, W. (1999). "Case 3122 – Ichthyosaurus cornalianus Bassani, 1886 (currently Mixosaurus cornalianus; Reptilia, Ichthyosauria): proposed designation of a neotype". The Bulletin of Zoological Nomenclature. 58 (2): 247–249. doi:10.5962/bhl.part.23086.
  17. ^ a b Page Module:Citation/CS1/styles.css has no content.Klein, Nicole; Schmitz, Lars; Wintrich, Tanja; Sander, P. Martin (2020). "A new cymbospondylid ichthyosaur (Ichthyosauria) from the Middle Triassic (Anisian) of the Augusta Mountains, Nevada, USA". Journal of Systematic Palaeontology. 18 (14): 1167–1191. Bibcode:2020JSPal..18.1167K. doi:10.1080/14772019.2020.1748132. S2CID 219078178.
  18. ^ Page Module:Citation/CS1/styles.css has no content.Moon, B.C. (2017). "A new phylogeny of ichthyosaurs (Reptilia: Diapsida)" (PDF). Journal of Systematic Palaeontology. 17 (2): 1–27. doi:10.1080/14772019.2017.1394922. S2CID 90912678. Archived from the original on 2020-03-18. Retrieved 2023-06-09.{{cite journal}}: CS1 maint: bot: original URL status unknown (link)

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