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Chaohusaurus

Chaohusaurusis anextinctgenusofbasalichthyosauriform,depending on definition possiblyichthyosaur,from theEarly TriassicofChaohuandYuanan,China.

Chaohusaurus
Temporal range:Early Triassic,251.3–247.2Ma[1]
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Specimen AGM CHS-5
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: †Ichthyopterygia
Genus: †Chaohusaurus
Young & Dong,1972
Type species
†Chaohusaurus geishanensis
Young & Dong, 1972
Species
  • †C. geishanensisYoung & Dong, 1972
  • †C. chaoxianensisChen, 1985
  • †C. zhangjiawanensisChen et al.,2013[2]
  • †C. brevifemoralisHuang et al., 2019

Discovery and naming

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Specimen AGM CH-628-22

Thetype speciesChaohusaurus geishanensiswas named and described byYang Zhong gianandDong Zhimingin 1972, based on a fossil found during the construction of a railway. The generic name refers to lake Chao Hu. Thespecific namerefers to theGeishanlocation. Theholotype,IVPP V 4001,was uncovered in a layer of theMa gian shan Limestone Formationdating from theAnisian.It consists of a partial skeleton, containing the skull and the front torso.[3]In 1985Chen Lizhunamed two additional species based on fossils found in the same formation:Anhuisaurus chaoxianensisandAnhuisaurus faciles.[4]However, the generic name had already been preoccupied by thelizardAnhuisaurusHou 1974. Therefore,AnhuisaurusChen 1985 was in 1991 renamed intoChensaurusbyJean-Michel Mazine.a.[5]In 1998,Ryosuke Motaniand Hailu You established that theChensaurusfossils represented remains of juveniles, with those ofC. facilesbeing the youngest,[6]and that these formed a growth series withChaohusaurus.[7]This implied that the known material ofChaohusauruswas increased with the specimens AGM P45-H85-25, the holotype ofChensaurus chaoxianensis;AGM P45-H85-20, the holotype ofChensaurus faciles;and the front flippers IVPP V 11361 and IVPP V 11362.[6]In 2001 a detailed description of theosteologyofChaohusauruswas published byMichael Maisch.[8]

Holotype specimen (AGB7401) ofC. brevifemoralis,Geological Museum of China

In 2014, three additional specimens were reported: AGM I-1, AGM CHS-5, and AGM CH-628-22. AGM I-1 also contains the remains of three embryo's. These finds are part of discoveries by a Chinese-Italian paleontological project of eightyChaohusaurusfossils.[9]

In 2013, a second species was named and described by Chen Xiaohong e.a.:Chaohusaurus zhangjiawanensis.The specific name refers to its provenance near the village ofZhangjiawan.The holotype, WHGMR V26001, was uncovered in a layer of theJialingjang Formationdating from theSpathian.It consists of a relatively complete skeleton with skull. A second skeleton lacking the skull, WHGMR V26002, was referred.[2]

In 2015, Ryosuke Motani and his colleagues reported a large adult forelimb specimen that resembles the immature specimens ofC. chaoxianensis,and argued thatC. chaoxianensisis not the juvenile ofC. geishanensisand is indeed a valid species within the genusChaohusaurus.[10]

Description

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C. geishanensisrestoration

Chaohusaurusis a basal ichthyopterygian. It thus shows traits that are typical for the direct ancestors of ichthyosaurs. As a result, basal ichthyosaurs likeCymbospondylusandMixosaurusare closer in build toChaohusauruswhile later genera likeIchthyosaurushave a more derived morphology.Chaohusaurusdid not have the dolphin-like form of later ichthyosaurs; it had a more lizard-like appearance with an elongated body. These proportions were not caused by a large number of vertebrae, the total of presacrals being about forty, but by an elongated build of each individual vertebra. The head is short, in adults having about the third the length of the trunk, with a narrow pointed beak and large eye-sockets. Its teeth are conspicuouslyheterodont,differing in shape: those of the upper jaws are pointed though blunt; those of the rear lower jaws are bulbous with a convex profile, a possible adaptation to adurophagousdiet, crushingshellfish.The neck is relatively long.Chaohusaurusdid have flippers, rather than webbed feet. The tail fin is wide-based in side-view and short.[citation needed]

C. geishanensiswith a human to scale.

Chaohusaurusis one of the smallest known ichthyopterygians, measuring about 0.7–1 m (2.3–3.3 ft) long and weighing 1.3–3.1 kg (2.9–6.8 lb).[11][12][13]

Being a basal ichthyopterygian,Chaohusaurusprovides important information about the early evolution of the group. It shows that some typical ichthyosaurian traits were probably already part of the original ichthyopterygianBauplan.It furthermore has a mix of basal traits that were subsequently lost, early derived traits indicating the genus is not the basalmost ichthyopterygian known and someautapomorphiesof its own.[citation needed]

An original ichthyopterygian trait is the fact that the skull roof is short and wide. The basisphenoid at the rear lower end of the braincase is not fused with the basioccipital, the lower bone of the rear skull. The parasphenoid at the front of the braincase has a long cultriform process. There is no separate ectopterygoid present. The lower jaw has a retroarticular process at its rear.[citation needed]

A conspicuous basal trait is the shortness of the head; in later forms it would be longer relative to the trunk. Also the snout is relatively short, only about twice as long as the part behind the eye-socket. The nasals do not reach further backwards than the eye-sockets. The suture between thenasal boneand the frontal is transversely oriented. From the prefrontal a flange overhangs the upper front edge of the eye-socket, perhaps to protect the eyes. The frontal is part of the top edge of the eye-socket and thus lacks a lateral buttress. The frontal is about as large as the parietal. In top view the rear edge of the skull roof is notched. Theparietal eye(or at least theforamen parietale) is located between the parietals, not (partly) between the frontals as with later forms. Thesquamosalis a large element, as large as thequadratojugalwith which it is firmly fused. The basisphenoid is narrow. The cultriform process of the parasphenoid at the rear gradually merges with the main body, not via a narrow waist as with later forms. The palate was not firmly attached to the basipterygoid, allowing the snout some vertical movement relative to the remainder of the skull. In most ichthyosaurs no such movement was possible. The opening between the pterygoids was narrow and slit-like, not wide. The pterygoids do not cover the rear underside of the braincase. The neck is relatively long. The tail is elongated, about as long as the head, neck and trunk combined. The vertebrae of the tail base are elongated also, about as long as they are tall. The longest tail spines are located rather to the front indicating that a possible tail fin must have been more horizontal than with later forms. Theclavicleshave a wide inner flange. Thescapulais short, wider than long. Thehumerusstill has a distinct head but not yet a dorsal trochanter. At its lower end, the humerus has a larger facet contacting the radius than contacting the ulna. The upper end of the ulna is narrower than the lower end, not equal in size to it. In general the bones of the lower arm, including the hand bones, are rather elongated, not transformed into discs. The ulna and radius still have a shaft as with land animals. In the wrist the pisiform is about as large as the ulnar carpal, not much smaller or absent. The fifth metacarpal has a convex rear edge and is longer than the fourth distal carpal. There are still five fingers present, without a reduction of the first finger.Polydactyly,an extra number of fingers, is thus lacking.Hyperphalangy,supernumerary phalanges, is likewise absent. The phalanx formula is 2-3-4-4-2. The upper phalanges are relatively elongated, longer than wide. The lower phalanges still show ossification below the cartilageperichondriumand have notches in their edges. Thepubic boneis perforated by aforamen obturatum,which closed in side view and located some distance from the rear edge of the body. The hindfin is about as large as the forefin, not smaller. In the thighbone, the facet contacting the tibia is as large and as far reaching downwards as the facet contacting the fibula. Between the tibia and fibula still a space is present, and both bones, though flattened are relatively elongated with a clear shaft. The same is true for the metatarsals, that are cylinder-shaped. There are still five toes present, again rather elongated and hourglass-shaped.[citation needed]

Skeletal restoration of adult (A) and juvenile (B)

An early derived trait is the relative shortness of the spines of the tail. The humerus has a flange at the front edge but it is not secondarily reduced. The flange has a small notch. The fifth metatarsal is shortened and the first is even shorter. The fifth toe is shorter than the first toe.[citation needed]

InChaohusaurusthe width of basisphenoid is about 63% of its length.Chaohusaurushas a combination of more pointed and bulbous teeth, that was probably separately evolved.[citation needed]

The second species,Chaohusaurus zhangjiawanensis,has some distinguishing traits. The skull is rather flat. The trunk vertebrae have well-developed transversal processes. Theprefrontaltouches thepostfrontal,excluding the frontal from the rim of the eye-socket, a derived trait. Thecalcaneumis larger. The first sacral rib has an expanded outer end. The second sacral rib has a tapered end.[2]

Classification

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Chaohusauruswas in the original description of 1972 assigned to theOmphalosauridae.[3]Both the 1999 phylogenetic analysis of Motani and the 2000 analysis of Maisch and Matzke foundChaohusaurusto by thesister taxonofGrippia,the two genera united in a group namedGrippidiaorGrippiidae.In Motani's study,Chaohusauruswas classified as a non-ichthyosaurian ichthyopterygian, while Maisch and Matzke, who used a more inclusive definition of Ichthyosauria, considered it to be a basal ichthyosaur.[14][15][16]

Thecladogrambelow follows Maisch and Matzke, 2000.[16]

Ichthyosauria

Acladisticanalysis published by Chen and colleagues in the 2013 description ofChaohusaurus zhangjiawanensisdid not find this taxon to be thesister speciesofChaohusaurus geishanensiswhich was more closely related toGrippia.Even though this would make the genusparaphyletic,C. zhangjiawanensiswas nevertheless placed inChaohusaurusbecause of the morphological similarity to the type species. The authors did not place the genus in a family, instead listing it asincertae sedis.[2]

The analysis of Motani and colleagues in 2015 foundChaohusaurusto be basal to all other ichthyopterygians.[17][18]Subsequent analyses have also supported such a placement.[18][19][20]Ji and colleagues attributed this change in phylogenetic position to a greater understanding ofChaohusaurusanatomy and additional specimens.[19]While sometimes still classified within Ichthyopterygia, Moon noted that as this group was not redefined to account for this newer topology,Chaohusaurustechnically falls outside of it.[18]

The cladogram below follows Huang and colleagues, 2019.[20]

Ichthyosauromorpha

Hupehsuchia

Ichthyosauriformes

Nasorostra

Chaohusaurus geishanensis

Chaohusaurus zhangjiawanensis

Chaohusaurus chaoxianensis

Chaohusaurus brevifemoralis

Ichthyopterygia

Reproduction

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Specimen AGM I-1 with three juveniles

One specimen among those reported in 2014, AGM I-1, also contained the remains of twoembryosand oneneonate.Chaohusaurusthus birthed its youngviviparouslyin the water, like later ichthyosaurs. However, from the orientation of both the embryos inside the maternal body and the neonate that had already been giving birth to, it was clear that the young exited the birth canal head-first. This differs from the method used by most extant marine viviparousAmniota,which expel the young tail-first to prevent them from suffocating. Many younger ichthyosaur specimens had earlier been found showing embryos in both positions, leaving it undecided which was the normal one. Motaniet alii(2014) concluded that, becauseChaohusaurusis a very basal form, this provided strong evidence that, at least originally, ichthyopterygian young were born with the head first. This early method might have later been changed because it resulted in a too high mortality. They also cited this as evidence for a terrestrial evolution ofviviparityin the land-dwelling ancestors of ichthyosaurs. AGM I-1 in 2014 represented the oldest reptile viviparous birth known.[9]

Palaeoecology

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C. geishanensiswas most likely a nearshore specialist that only occasionally wandered out into deep water environments.[21]

See also

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References

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  1. ^"Chaohusaurus at Fossilworks".Paleobiology Database.Fossilworks.Retrieved17 December2021.
  2. ^abcdXiaohong Chen; P. Martin Sander; Long Cheng; Xiaofeng Wang (2013)."A New Triassic Primitive Ichthyosaur from Yuanan, South China".Acta Geologica Sinica (English Edition).87(3): 672–677.Bibcode:2013AcGlS..87..672C.doi:10.1111/1755-6724.12078.S2CID140143030.
  3. ^abYoung, C.C.; Dong, Z. (1972). "[On the Triassic aquatic reptiles of China]".Memoirs of the Nanjing Institute of Geology and Paleontology.9:1–34.
  4. ^Liezhu, Chen (1985). "[Ichthyosaurs from the lower Triassic of Chao County]".Anhui Regional Geology of China.15:139–146.
  5. ^Mazin, J.-M.; Suteethorn, V.; Buffetaut, E.; Jaeger, J.-J.; Helmckeingavat, R. (1991). "Preliminary description ofThaisaurus chonglakmaniin. g., n. sp., a new ichthyopterygian (Reptilia) from the Early Triassic of Thailand ".Comptes Rendus de l'Académie des Sciences, Série II.313:1207–1212.
  6. ^abMotani, R.; You, H. (1998). "The forefin ofChensaurus chaoxianensis(Ichthyosauria) shows delayed mesopodial ossification ".Journal of Paleontology.72(1): 133–136.Bibcode:1998JPal...72..133M.doi:10.1017/S0022336000024069.S2CID55923016.
  7. ^Motani, R.; You, H. (1998). "Taxonomy and limb ontogeny ofChaohusaurus geishanensis(Ichthyosauria), with a note on the allometric equation ".Journal of Vertebrate Paleontology.18(3): 533–540.Bibcode:1998JVPal..18..533M.doi:10.1080/02724634.1998.10011080.
  8. ^Maisch, M.W. (2001). "Observations on Triassic ichthyosaurs. Part VII. New data on the osteology ofChaohusaurus geishanensisYOUNG & DONG, 1972 from the Lower Triassic of Anhui (China) ".Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen.219(3): 305–327.doi:10.1127/njgpa/219/2001/305.
  9. ^abRyosuke Motani; Da-yong Jiang; Andrea Tintori; Olivier Rieppel; Guan-bao Chen (2014)."Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils".PLOS ONE.9(2): e88640.Bibcode:2014PLoSO...988640M.doi:10.1371/journal.pone.0088640.PMC3922983.PMID24533127.
  10. ^Motani, R.; Jiang, D.-Y.; Tintori, A.; Rieppel, O.; Chen, G.-B.; You, H. (2015). "Status ofChaohusaurus chaoxianensis(Chen, 1985) ".Journal of Vertebrate Paleontology.35(1): e892011.Bibcode:2015JVPal..35E2011M.doi:10.1080/02724634.2014.892011.S2CID84863323.
  11. ^Stone, R. (2010). "Excavation Yields Tantalizing Hints of Earliest Marine Reptiles".Science.330(6008): 1164–1165.Bibcode:2010Sci...330.1164S.doi:10.1126/science.330.6008.1164-b.PMID21109639.
  12. ^Li, Qiang; Liu, Jun (2020)."An Early Triassic sauropterygian and associated fauna from South China provide insights into Triassic ecosystem health".Communications Biology.3(1): 63.doi:10.1038/s42003-020-0778-7.PMC7012838.PMID32047220.
  13. ^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.PMID34941418.S2CID245444783.
  14. ^Maisch, M.W. (2010). "Phylogeny, systematics, and origin of the Ichthyosauria - the state of the art".Palaeodiversity.3:151–214.
  15. ^Motani, R. (1999). "Phylogeny of the Ichthyopterygia".Journal of Vertebrate Paleontology.19(3): 473–496.Bibcode:1999JVPal..19..473M.doi:10.1080/02724634.1999.10011160.
  16. ^abMaisch, M.W.; Matzke, A.T. (2000)."The Ichthyosauria".Stuttgarter Beiträge zur Naturkunde.Serie B (Geologie und Paläontologie).298:1–159.
  17. ^Motani, R.; Jiang, D.Y.; Chen, G.B.; Tintori, A.; Rieppel, O.; Ji, C.; Huang, J.D. (2015). "A basal ichthyosauriform with a short snout from the Lower Triassic of China".Nature.517(7535): 485–488.Bibcode:2015Natur.517..485M.doi:10.1038/nature13866.PMID25383536.S2CID4392798.
  18. ^abcMoon, 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.hdl:1983/463e9f78-10b7-4262-9643-0454b4aa7763.S2CID90912678.Archived(PDF)from the original on 2020-03-18.Retrieved2024-06-22.
  19. ^abJi, C.; Jiang, D. Y.; Motani, R.; Rieppel, O.; Hao, W. C.; Sun, Z. Y. (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.S2CID85621052.
  20. ^abHuang, J.; Motani, R.; Jiang, D.; Tintori, A.; Rieppel, O.; Zhou, M.; Ren, X.; Zhang, R. (2019)."The new ichthyosauriformChaohusaurus brevifemoralis(Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China ".PeerJ.7:e7561.doi:10.7717/peerj.7561.PMC6741286.PMID31565558.
  21. ^Gu, Li-Ang; Wolniewicz, Andrzej S.; Liu, Jun (18 September 2024)."New information on the dentition of Chaohusaurus zhangjiawanensis (Reptilia, Ichthyosauriformes) from the Early Triassic of Yuan'an, Hubei Province, China".Swiss Journal of Palaeontology.143(1): 35.Bibcode:2024SwJP..143...35G.doi:10.1186/s13358-024-00331-8.ISSN1664-2376.
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