Seeleyosaurus

Seeleyosaurus; Temporal range: Early Jurassic, Toarcian PreꞒ Ꞓ O S D C P T J K Pg N
	Cast of Seeleyosaurus guilelmiimperatoris fossil with reconstructed parts based on the holotype
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Superfamily:	†Plesiosauroidea
Family:	†Microcleididae
Genus:	†Seeleyosaurus; White, 1940
Type species
	†Seeleyosaurus guilelmiimperatoris; (Dames, 1895)
Synonyms
	Microcleidus guilelmi imperatoris Menner, 1992; Plesiosaurus guilelmi imperatoris Fraas, 1910 non Dames, 1895; Plesiosaurus imperatoris guilelmi Menner, 1992; Plesiosaurus guilelmiimperatoris Dames, 1895; Seeleyosaurus holzmadensis White, 1940;

Seeleyosaurus is an extinct genus of plesiosaur from Germany and possibly also Russia.

Two species were known: the type, S. guilelmiimperatoris, and the now obsolete species S. holzmadensis,^[1] which has since been absorbed into S. guilelmiimperatoris.

Discovery and naming

The holotype is MB.R.1992, a large almost complete skeleton from the Upper Lias (Toarcian) Lias Group Formations of Württemberg, Germany. It was offered for sale by Bernard Hauff and it was purchased by the State Museum of Natural History Stuttgart in August 1893.^[2] It preserved soft tissue that was painted over around ten years later^[3] and a cast of the holotype was described in by Ketchum & Benson (2011), who determined that it seems to preserve the impression of a rhomboidal flap of skin in a vertical plane, suggesting that many other plesiosaurs may have been equipped in this way.^[4] The holotype was destroyed in 1945.^[4]

Seeleyosaurus was initially described as Plesiosaurus guilelmiimperatoris by Dames (1895)^[2] before White (1940) moved P. guilelmiimperatoris to its own genus.^[1] White (1940) also assigned a second species to Seeleyosaurus as S. holzmadensis which was later determined to be the same animal as S. guilelmiimperatoris.^[4]

A second specimen (SMNS 12039), preserved in 3D, was initially the holotype of S. holzmadensis.^[1] It was discovered within a chalkstone and shale quarry located between Holzmaden and Zell unter Aichelberg, and it intersected an extremely hard forty centimetres thick layer of Stinkstein chalk, which was deemed worthless. The rocks were dumped in a ravine and the holotype was discovered there in November 1906 by fossil trader Bernhard Hauff. Alongside the holotype of Meyerasaurus victor, it was offered for sale, and financial support by D. Landauer and Victor Fraas allowed the Stuttgarter Königliche Naturalienkabinett to obtain both specimens.^[5]

A fragmentary specimen of Seeleyosaurus guilelmiimperatoris has also been identified from the Middle Jurassic of Siberia by Menner (1992).^[6]

Description

Seeleyosaurus was a relatively small plesiosaur, measuring 2.9–3.6 m (9.5–11.8 ft) long.^[7]^[8]

Classification

The following cladogram follows an analysis by Ketchum & Benson, 2011.^[4]

Pistosauria

"Pistosaurus postcranium"

Augustasaurus hagdorni

Bobosaurus forojuliensis

Thalassiodracon hawkinsii

Hauffiosaurus spp.

	Attenborosaurus conybeari

	advanced pliosaurids

Rhomaleosauridae

Anningasaura lymense

advanced rhomaleosaurids

"Plesiosaurus" macrocephalus

	Archaeonectrus rostratus

	Macroplata tenuiceps

Plesiosauroidea

Stratesaurus taylori

Plesiosauridae

Seeleyosaurus guilelmiimperatoris

	OUMNH J.28585

	Plesiosaurus dolichodeirus

Elasmosauridae and Cryptoclidia

Microcleidus homalospondylus

	Hydrorion brachypterygius

	Occitanosaurus tournemiensis

A redescription and a study on the affinities of Seeleyosaurus guilelmiimperatoris published by Sachs et al. (2025) interpreted Plesiopterys wildi as a taxon distinct from S. guilelmiimperatoris.^[9]

Gallery

Restoration
Holotype of S. guilelmiimperatoris in 1895^[2]
Seeleyosaurus (left) and Thaumatosaurus, now Meyerasaurus (right) as depicted in Water reptiles of the past and present (1914)

References

^ ^a ^b ^c White, T. E. (1940). Holotype of Plesiosaurus longirostris Blake and classification of the plesiosaurs. Journal of Paleontology 14(5):451-467
^ ^a ^b ^c Dames, H. W. (1895). Die Plesiosaurier der süddeutschen Liasformation. Abhandlungen der Königlichen Akademie der Wissenschaften zu Berlin 2:1-83
^ Wilhelm, B. (2010). Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. M.S. thesis, Marshall University, Huntington, West Virginia, 76 pp.
^ ^a ^b ^c ^d Ketchum, Hillary F.; Benson, Roger B.J. (2011). "A new pliosaurid (Sauropterygia, Plesiosauria) from the Oxford Clay Formation (Middle Jurassic, Callovian) of England: evidence for a gracile, longirostrine grade of Early-Middle Jurassic pliosaurids". Special Papers in Palaeontology. 86: 109–129.
^ Fraas, Eberhard (1910). "Plesiosaurier aus dem oberen Lias von Holzmaden" (PDF). Palaeontographica. 57 (3–4): 105–140.
^ Menner, V. V. (1992). "Remains of Plesiosaurs from Middle Jurassic Deposits of Eastern Siberia". Russian Academy of Sciences, Department of Geology, Geophysics, Geochemistry and Mining Sciences, Order of the Red Manner of Labour Geological Institute: 74–91.
^ Peggy Vincent (2011). "A re-examination of Hauffiosaurus zanoni, a pliosauriod from the Toarcian (Early Jurassic) of Germany". Journal of Vertebrate Paleontology. 31 (2): 340–351. Bibcode:2011JVPal..31..340V. doi:10.1080/02724634.2011.550352. S2CID 84743241.
^ Valentin Fischer; Nikolay G. Zverkov; Maxim S. Arkhangelsky; Ilya M. Stenshin; Ivan V. Blagovetshensky; Gleb N. Uspensky (2020). "A new elasmosaurid plesiosaurian from the Early Cretaceous of Russia marks an early attempt at neck elongation". Zoological Journal of the Linnean Society. 192 (4): 1167–1194. doi:10.1093/zoolinnean/zlaa103. hdl:2268/251614.Supplementary Information
^ Sachs, S.; Madzia, D.; Marx, M.; Roberts, A. J.; Hampe, O.; Kear, B. P. (2025). "The osteology, taxonomy, and phylogenetic placement of Seeleyosaurus guilelmiimperatoris (Plesiosauroidea, Microcleididae) from the Lower Jurassic Posidonia Shale of Germany". The Anatomical Record. doi:10.1002/ar.25620. PMID 39981975.

[White40-1] White, T. E. (1940). Holotype of Plesiosaurus longirostris Blake and classification of the plesiosaurs. Journal of Paleontology 14(5):451-467

[Dames95-2] Dames, H. W. (1895). Die Plesiosaurier der süddeutschen Liasformation. Abhandlungen der Königlichen Akademie der Wissenschaften zu Berlin 2:1-83

[3] Wilhelm, B. (2010). Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. M.S. thesis, Marshall University, Huntington, West Virginia, 76 pp.

[desc-4] Ketchum, Hillary F.; Benson, Roger B.J. (2011). "A new pliosaurid (Sauropterygia, Plesiosauria) from the Oxford Clay Formation (Middle Jurassic, Callovian) of England: evidence for a gracile, longirostrine grade of Early-Middle Jurassic pliosaurids". Special Papers in Palaeontology. 86: 109–129.

[Fraas1910-5] Fraas, Eberhard (1910). "Plesiosaurier aus dem oberen Lias von Holzmaden" (PDF). Palaeontographica. 57 (3–4): 105–140.

[:0-6] Menner, V. V. (1992). "Remains of Plesiosaurs from Middle Jurassic Deposits of Eastern Siberia". Russian Academy of Sciences, Department of Geology, Geophysics, Geochemistry and Mining Sciences, Order of the Red Manner of Labour Geological Institute: 74–91.

[7] Peggy Vincent (2011). "A re-examination of Hauffiosaurus zanoni, a pliosauriod from the Toarcian (Early Jurassic) of Germany". Journal of Vertebrate Paleontology. 31 (2): 340–351. Bibcode:2011JVPal..31..340V. doi:10.1080/02724634.2011.550352. S2CID 84743241.

[8] Valentin Fischer; Nikolay G. Zverkov; Maxim S. Arkhangelsky; Ilya M. Stenshin; Ivan V. Blagovetshensky; Gleb N. Uspensky (2020). "A new elasmosaurid plesiosaurian from the Early Cretaceous of Russia marks an early attempt at neck elongation". Zoological Journal of the Linnean Society. 192 (4): 1167–1194. doi:10.1093/zoolinnean/zlaa103. hdl:2268/251614.Supplementary Information

[9] Sachs, S.; Madzia, D.; Marx, M.; Roberts, A. J.; Hampe, O.; Kear, B. P. (2025). "The osteology, taxonomy, and phylogenetic placement of Seeleyosaurus guilelmiimperatoris (Plesiosauroidea, Microcleididae) from the Lower Jurassic Posidonia Shale of Germany". The Anatomical Record. doi:10.1002/ar.25620. PMID 39981975.

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Seeleyosaurus Temporal range: Early Jurassic, Toarcian PreꞒ Ꞓ O S D C P T J K Pg N

Cast of Seeleyosaurus guilelmiimperatoris fossil with reconstructed parts based on the holotype
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Superfamily:	†Plesiosauroidea
Family:	†Microcleididae
Genus:	†Seeleyosaurus White, 1940
Type species
†Seeleyosaurus guilelmiimperatoris (Dames, 1895)
Synonyms
Microcleidus guilelmi imperatoris Menner, 1992 Plesiosaurus guilelmi imperatoris Fraas, 1910 non Dames, 1895 Plesiosaurus imperatoris guilelmi Menner, 1992 Plesiosaurus guilelmiimperatoris Dames, 1895 Seeleyosaurus holzmadensis White, 1940