Taxonomic reassessment of Hydralmosaurus as Styxosaurus: new insights on the elasmosaurid neck evolution throughout the Cretaceous
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Otero, Rodrigo A.
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Taxonomic reassessment of Hydralmosaurus as Styxosaurus: new insights on the elasmosaurid neck evolution throughout the Cretaceous
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Abstract
Two extremely-long necked elasmosaurids, AMNH 1495, holotype of Hydralmosaurus
serpentinus, and AMNH 5835, previously referred to H. serpentinus, are here reviewed
in detail. Unique features of the cervical vertebrae, which are only present on elasmosaurids
from the Western Interior Seaway, are recognized based on these specimens
and by comparison with penecontemporaneous taxa with biogeographic affinities.
Phylogenetic analysis, bivariate graphic analysis of cervical vertebrae proportions,
comparisons of different cervical vertebral types, paleobiogeographic distribution and
study of the elasmosaurid axial evolution throughout the Cretaceous are here integrated.
As a result, at least two separate lineages within the Elasmosauridae are identified by
independently acquired extremely-long necks (over 60 cervical vertebrae). First, a still
scarcely known lineage is so far represented by the lower Cenomanian Thalassomedon
haningtoni, the Turonian Libonectes morgani and close relatives. A second lineage is
here defined as a new clade, the Styxosaurinae, which groups the Campanian genera
Terminonatator, Styxosaurus (D`Hydralmosaurus'), Albertonectes and Elasmosaurus,
the two latter forming a derived branch that includes the most extreme amniote
necks known to date (more than 70 cervical vertebrae). Phylogenetic analysis supports
AMNH1495 andAMNH5835 as being closely related to Styxosaurus snowii. Therefore,
the species Styxosaurus browni is re-validated, while AMNH 1495 is here referred to
Styxosaurus sp. This research also recognizes the `Cimoliasauridae' (nomen dubium) as a
paraphyletic group but informative of a plesiomorphic cervical vertebral morphology of
elasmosaurids which was persistent throughout the whole Cretaceous and from whom
aristonectines, styxosaurines and Thalassomedon and close relatives are derived. The
genus Hydralmosaurus is recommended for being abandoned.
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Richard Gilder Foundation, American Museum of Natural History (New York)
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URI: https://repositorio.uchile.cl/handle/2250/139324
DOI: DOI: 10.7717/peerj.1777
ISSN: 2167-8359
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PEERJ Volumen: 4 Número de artículo: e1777 (2016)
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