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The osteology, taxonomy, and phylogenetic placement of Seeleyosaurus guilelmiimperatoris (Plesiosauroidea, Microcleididae) from the Lower Jurassic Posidonia Shale of Germany

Sachs, Sven ; Madzia, Daniel ; Marx, Miguel LU orcid ; Roberts, Aubrey J. ; Hampe, Oliver and Kear, Benjamin P. (2025) In The anatomical record
Abstract

Seeleyosaurus guilelmiimperatoris is an early-diverging plesiosauroid from the lower Toarcian (Lower Jurassic) of the Posidonienschiefer Formation (Posidonia Shale) of Holzmaden, southwestern Germany. It is known from almost complete skeletons of two osteologically mature individuals, the holotype and a referred specimen that was largely destroyed during World War II. Although well-preserved and substantially complete, the anatomy of Seeleyosaurus and its taxonomic and phylogenetic significance remain insufficiently understood. Here, we provide a complete osteological description of the taxon. Seeleyosaurus guilelmiimperatoris can be diagnosed based on a unique combination of characters, including one autapomorphy: the tall and basally... (More)

Seeleyosaurus guilelmiimperatoris is an early-diverging plesiosauroid from the lower Toarcian (Lower Jurassic) of the Posidonienschiefer Formation (Posidonia Shale) of Holzmaden, southwestern Germany. It is known from almost complete skeletons of two osteologically mature individuals, the holotype and a referred specimen that was largely destroyed during World War II. Although well-preserved and substantially complete, the anatomy of Seeleyosaurus and its taxonomic and phylogenetic significance remain insufficiently understood. Here, we provide a complete osteological description of the taxon. Seeleyosaurus guilelmiimperatoris can be diagnosed based on a unique combination of characters, including one autapomorphy: the tall and basally constricted neural spines of the posterior cervical, pectoral, and dorsal vertebrae which have a sinusoidal shape in lateral view. Our study supports the validity of the plesiosauroid taxon Plesiopterys wildi, which was considered a junior synonym of Seeleyosaurus in a previous joint assessment of the taxa. Our phylogenetic evaluation places S. guilelmiimperatoris among Microcleididae, in congruence with previous studies. However, in contrast to earlier phylogenetic reconstructions, our analyses, which take into account numerous changes to the character matrix, reconstruct S. guilelmiimperatoris as falling within the Microcleidus clade. While we admit that Seeleyosaurus might potentially be considered a species of Microcleidus, we refrain from promoting this nomenclatural change pending an osteological and taxonomic reassessment of Microcleidus spp. as well as other, potentially closely related forms, such as Lusonectes sauvagei.

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author
; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
in press
subject
keywords
Germany, Lower Jurassic, Microcleididae, Plesiosauria, Seeleyosaurus, Toarcian
in
The anatomical record
pages
64 pages
publisher
John Wiley & Sons Inc.
external identifiers
  • scopus:85218680577
  • pmid:39981975
ISSN
1932-8486
DOI
10.1002/ar.25620
language
English
LU publication?
yes
additional info
Publisher Copyright: © 2025 American Association for Anatomy.
id
71716540-fedb-4589-b5b4-867e03cb6d10
date added to LUP
2025-03-11 12:03:19
date last changed
2025-05-06 16:18:35
@article{71716540-fedb-4589-b5b4-867e03cb6d10,
  abstract     = {{<p>Seeleyosaurus guilelmiimperatoris is an early-diverging plesiosauroid from the lower Toarcian (Lower Jurassic) of the Posidonienschiefer Formation (Posidonia Shale) of Holzmaden, southwestern Germany. It is known from almost complete skeletons of two osteologically mature individuals, the holotype and a referred specimen that was largely destroyed during World War II. Although well-preserved and substantially complete, the anatomy of Seeleyosaurus and its taxonomic and phylogenetic significance remain insufficiently understood. Here, we provide a complete osteological description of the taxon. Seeleyosaurus guilelmiimperatoris can be diagnosed based on a unique combination of characters, including one autapomorphy: the tall and basally constricted neural spines of the posterior cervical, pectoral, and dorsal vertebrae which have a sinusoidal shape in lateral view. Our study supports the validity of the plesiosauroid taxon Plesiopterys wildi, which was considered a junior synonym of Seeleyosaurus in a previous joint assessment of the taxa. Our phylogenetic evaluation places S. guilelmiimperatoris among Microcleididae, in congruence with previous studies. However, in contrast to earlier phylogenetic reconstructions, our analyses, which take into account numerous changes to the character matrix, reconstruct S. guilelmiimperatoris as falling within the Microcleidus clade. While we admit that Seeleyosaurus might potentially be considered a species of Microcleidus, we refrain from promoting this nomenclatural change pending an osteological and taxonomic reassessment of Microcleidus spp. as well as other, potentially closely related forms, such as Lusonectes sauvagei.</p>}},
  author       = {{Sachs, Sven and Madzia, Daniel and Marx, Miguel and Roberts, Aubrey J. and Hampe, Oliver and Kear, Benjamin P.}},
  issn         = {{1932-8486}},
  keywords     = {{Germany; Lower Jurassic; Microcleididae; Plesiosauria; Seeleyosaurus; Toarcian}},
  language     = {{eng}},
  publisher    = {{John Wiley & Sons Inc.}},
  series       = {{The anatomical record}},
  title        = {{The osteology, taxonomy, and phylogenetic placement of Seeleyosaurus guilelmiimperatoris (Plesiosauroidea, Microcleididae) from the Lower Jurassic Posidonia Shale of Germany}},
  url          = {{http://dx.doi.org/10.1002/ar.25620}},
  doi          = {{10.1002/ar.25620}},
  year         = {{2025}},
}