Eunotosaurus

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Eunotosaurus
Temporal range:
Ma
Fossil specimen, on display at
Karoo National Park
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Reptiliomorpha
Clade: Amniota
Clade: Sauropsida
Genus: †Eunotosaurus
Seeley, 1892
Species:
†E. africanus
Binomial name
†Eunotosaurus africanus
Seeley, 1892

Eunotosaurus (

synapsid.[6]

Description

Eunotosaurus reached up to 30 cm (12 in) in total body length.

cervical vertebrae.[8][7]

intercostal muscles, which are muscles that assist in breathing. The lack of Sharpey's fibers on the anterior side of the ribs of Eunotosaurus suggests that it lacked functional intercostal muscles. Turtles also lack intercostal muscles and instead have muscles that connect to the undersides of the ribs for the purpose of locomotion. If Eunotosaurus is close to the ancestry of turtles, it may have had similar sets of muscles.[8]

Even though Eunotosaurus has been traditionally considered an anapsid, it is considered to possess a lower temporal fenestra, though without the temporal bar. Moreover, a juvenile specimen also shows upper temporal fenestrae, meaning the skull demonstrates a fully diapsid condition. In the adult, the upper fenestra is covered by the supratemporal bone.[9]

History of study

Eunotosaurus was named in 1892, but it was not until 1914 that it was proposed to be an ancestor of

cotylosaurs (now referred to as captorhinids) and Chelonia.[10] He compared it to "Archichelone", a name he devised for a hypothetical chelonian ancestor, noting that its ribs appeared to be intermediate between those of turtles and other tetrapods. Watson's "Archichelone" had a pelvic girdle that was pushed back on the vertebral column and placed under the shell. However, fossils of Eunotosaurus show that the pelvis is in the normal tetrapod position and is placed over the ribs rather than within them, as in modern turtles.[11] Many fossils have been found showing a semi-rigid, turtle-like rib cage, one which presumably necessitated a tortoise-like fashion of walking.[12]

Eunotosaurus was considered the ancestor of turtles up until the late 1940s. In his 1956 book Osteology of the Reptiles, American paleontologist

Anapsida in its own order incertae sedis.[11][13]

Over a century after its naming, Eunotosaurus was known from less than a dozen specimens, with very little material known from the skull. Despite the paucity of material, it was well described. Two additional skeletons were unearthed from the Karoo Supergroup and described in 1999. They are now housed in the

Bernard Price Institute for Palaeontological Research in Johannesburg and the National Museum, Bloemfontein. While relatively rare, Eunotosaurus is common enough in the Karoo to be used as a biostratigraphic marker. It is present in the upper Tapinocephalus Assemblage Zone and in all parts of the succeeding Pristerognathus Assemblage Zone.[14]

Classification

The ribs of Eunotosaurus were very wide and flat, touching each other to form broad plates similar to the

evolutionary convergence.[15] However, the discovery of Pappochelys, a prehistoric species whose fossil remains show a mixture of features found in Eunotosaurus and the toothed stem-turtle Odontochelys, helped to resolve the issue. Though an analysis which included data from Pappochelys found weak support for the idea that Eunotosaurus was a parareptile, it found stronger support for the hypothesis that Eunotosaurus was itself a diapsid closely related to turtles, and that its apparently primitive, anapsid skull was probably developed as part of the turtle lineage, independently of parareptiles.[16][7]

Eunotosaurus was assigned to its own family,

sister taxon of Milleretta and thus within the family Millerettidae.[21]

Eunotosaurus was incorporated in a recent 2010 phylogenetic analysis that sought to determine the origin of turtles.

plastron.[22]

The following

phylogenetic position of the Eunotosaurus, from Ruta et al., 2011.[23]

Life restoration
Parareptilia 
 
Mesosauria
 

Brazilosaurus sanpauloensis

Mesosaurus tenuidens

Stereosternum tumidum

Eunotosaurus africanus

 Millerettidae 

Milleretta rubidgei

Broomia perplexa

"Millerosaurus" nuffieldi

Milleropsis pricei

Millerosaurus ornatus

 Procolophonomorpha 

Australothyris smithi

Microleter mckinzieorum

Ankyramorpha

The cladogram below follows the most likely result found by another analysis of turtle relationships, published by Rainer Schoch and Hans-Dieter Sues in 2015. This study found Eunotosaurus to be an actual early stem-turtle, though other versions of the analysis found weak support for it as a parareptile.[16]

Archelosauria

The following cladogram is adapted from a 2022 study by Simões et al. Here, Eunotosaurus was recovered as neither a parareptile or a stem-turtle, but as a basal neodiapsid located outside the reptilian crown group.[5]

Sauropsida
Traditional Parareptilia

References

  1. ISSN 0038-2353
    .
  2. ^ Cox, C.B. (1969). "The problematic Permian reptile Eunotosaurus". Bulletin of the British Museum of Natural History. 18: 167–196.
  3. hdl:1983/6d1a4f9b-a768-4b86-acb1-b3ad1f7ee885
    .
  4. ^ "The trouble with turtles: Paleontology at a crossroads". EARTH Magazine. Naomi Lubick. Retrieved 19 March 2014.
  5. ^
    PMID 35984885
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  6. S2CID 233454789
    .
  7. ^
    ISBN 9781421428680
    .
  8. ^
    PMID 23727095
    .
  9. ISSN 1476-4687
    .
  10. ^ Watson, D.M.S. (1914). "Eunotosaurus africanus Seeley and the ancestors of the Chelonia". Proceedings of the Zoological Society of London. 11: 1011–1020.
  11. ^
    doi:10.1093/icb/31.4.616
    .
  12. doi:10.1093/icb/41.3.586
    .
  13. ISBN 978-0-89464-985-1
    .
  14. ^ Rubidge, B.S.; Modesto, S.; Sidor, C.; Welman, J. (1999). "Eunotosaurus africanus from the Ecca–Beaufort contact in Northern Cape Province, South Africa – implications for Karoo Basin development" (PDF). South African Journal of Science. 95: 553–555. Archived from the original (PDF) on 16 July 2011. Retrieved 1 August 2010.
  15. ^ "Facts About Turtles: Eunotosaurus And Turtle Evolution". All-About-Reptiles.com. Archived from the original on 12 September 2010. Retrieved 1 August 2010.
  16. ^
    S2CID 205243837
    .
  17. ^ Haughton, S.H.; Brink, A.S. (1954). "A bibliographical list of Reptilia from the Karoo Beds of South Africa". Palaeontologia Africana. 2: 1–187.
  18. ^ Cox, C.B. (1969). "The problematic Permian reptile Eunotosaurus". Bulletin of the British Museum (Natural History), Geology Series. 18 (5): 167–196.
  19. doi:10.1111/j.1096-3642.1995.tb00932.x
    .
  20. ^ Modesto, S.P. (2000). "Eunotosaurus africanus and the Gondwanan ancestry of anapsid reptiles". Palaeontologia Africana. 36: 15–20.
  21. S2CID 73723455
    .
  22. ^
    PMID 20534602
    .
  23. S2CID 83693335
    .

External links
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