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Hylonomus

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Hylonomus
Temporal range: Bashkirian[1]
Model
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Eureptilia
Genus: †Hylonomus
Dawson, 1860
Type species
†Hylonomus lyelli
Dawson, 1860

Hylonomus (/haɪˈlɒnəməs/; hylo- "forest" + nomos "dweller")[2] is an extinct genus of reptile that lived 318 million years ago during the Bashkirian stage of the Late Carboniferous. It is the earliest known crown group amniote and the oldest known unquestionable reptile, with the only known species being Hylonomus lyelli.[3][1] Despite being amongst the oldest known reptiles, it is not the most primitive member of the group, being a eureptile more derived than either parareptiles or captorhinids.

Discovery and naming

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This animal was discovered by John William Dawson in the mid-19th century.[4] The species' name was given it by Dawson's teacher, the geologist Sir Charles Lyell. While it has traditionally been included in the group Protorothyrididae, it has since been recovered outside this group.[5][6]

Description

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Skull reconstruction
Life restoration

Hylonomus was 20–25 centimetres (8–10 in) long (including the tail). Most of them are 20 cm long and probably would have looked rather similar to modern lizards. It had small sharp teeth and it likely ate small invertebrates such as millipedes or early insects.[7] Specimens of Hylonomus indicate that their bodies are covered with horny scales.[8] They are also described as having slender and lightweight leg and arm bones, long and slim hands and feet, a narrow and tongue-shaped part in the roof of the mouth, a deep groove on a certain bone in the skull, a bumpy structure on the back bones, changes in the height of certain back bone parts, a hole in a specific place on the skull, arm and leg bones that are the same length, a short fourth toe bone compared to the shin bone, a short fifth toe bone compared to the fourth toe bone, long neck bones, and a well-developed opening below the eye.[9]

Size comparison

Fossils of the basal pelycosaur Archaeothyris and the basal diapsid Petrolacosaurus are also found in the same region of Nova Scotia, although from a higher stratum, dated approximately 6 million years later.[10]

Fossilized footprints found in New Brunswick have been attributed to Hylonomus, at an estimated age of 315 million years.[11]

Paleoecology

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Fossils of Hylonomus have been found in the remains of fossilized club moss stumps in the Joggins Formation, Joggins, Nova Scotia, Canada. It is supposed that, after harsh weather, the club mosses would crash down, with the stumps eventually rotting and hollowing out. Small animals such as Hylonomus, seeking shelter, would enter and become trapped, starving to death. An alternative hypothesis is that the animals made their nests in the hollow tree stumps.[12]

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Hylonomus lyelli was named the Provincial Fossil of Nova Scotia in 2002.[13]

References

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  1. ^ a b Marjanović, D. (2021). "The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates". Frontiers in Genetics. 12. 521693. doi:10.3389/fgene.2021.521693. PMC 8149952. PMID 34054911.
  2. ^ Genus Hylonomus Etymology
  3. ^ Carpenter, D. K.; Falcon-Lang, H. J.; Benton, M. J.; Grey, M. (2015). "Early Pennsylvanian (Langsettian) fish assemblages from the Joggins Formation, Canada, and their implications for palaeoecology and palaeogeography". Palaeontology. 58 (4): 661–690. doi:10.1111/pala.12164.
  4. ^ J. W. Dawson. 1860. On a Terrestrial Mollusk, a Chilognathous Myriapod, and some New Species of Reptiles, from the Coal-Formation of Nova Scotia. Quarterly Journal of the Geological Society of London 16:268-277
  5. ^ Muller, J.; Reisz, R.R. (2006). "The phylogeny of early eureptiles: Comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade". Systematic Biology. 55 (3): 503–511. doi:10.1080/10635150600755396. PMID 16861212.
  6. ^ Ford, David P.; Benson, Roger B. J. (January 2020). "The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae". Nature Ecology & Evolution. 4 (1): 57–65. doi:10.1038/s41559-019-1047-3. ISSN 2397-334X. PMID 31900445. S2CID 209673326.
  7. ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 62. ISBN 978-1-84028-152-1.
  8. ^ J.W. Dawson (1882). "On the Results of Recent Explorations of Erect Trees Containing Animal Remains in the Coal-Formation of Nova Scotia". Philosophical Transactions of the Royal Society of London. 173: 621–668. doi:10.1098/rstl.1882.0013.
  9. ^ Muller, J.; Reisz, R. R. (2006). "The phylogeny of early eureptiles: Comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade". Systematic Biology. 55 (3): 503–511. doi:10.1080/10635150600755396. PMID 16861212.
  10. ^ van Tuinen, Marcel; Hadly, Elizabeth A. (2004). "Error in Estimation of Rate and Time Inferred from the Early Amniote Fossil Record and Avian Molecular Clocks" (PDF). Journal of Molecular Evolution. 59 (2): 267–276. Bibcode:2004JMolE..59..267V. doi:10.1007/s00239-004-2624-9. PMID 15486700. S2CID 25065918. Archived from the original (PDF) on June 28, 2011. Retrieved 2009-09-02.
  11. ^ Falcon-Lang, H.J., Benton, M.J. & Stimson, M. (2007): Ecology of early reptiles inferred from Lower Pennsylvanian trackways. Journal of the Geological Society, London, 164; no. 6; pp 1113-1118. article
  12. ^ McGhee, George R. Jr. (12 November 2013). When the Invasion of Land Failed: The Legacy of the Devonian Extinctions. Columbia University Press. pp. 254–5. ISBN 978-0-231-16057-5. Retrieved 10 June 2015.
  13. ^ "Provincial Fossil Act". Office of the Legislative Counsel, Nova Scotia House of Assembly. Retrieved 9 November 2018.
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