Mesosaurus caudal vertebrae divisions reviewed
Most reptiles = amniotes
have tail vertebrae that consists of a cylindrical portion surrounding a spinal chord, topped without suture to a neural arch. A separate chevron sometimes appears between each caudal vertebra.
Mesosaurs are different. (Fig 1) Piñeiro et al 2025 described the difference.
Figure 1. Mesosaurus and Stereosternum tail vertebrae compared to Proterogyrinus caudals.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg?w=91″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg?w=311″ class=”size-full wp-image-91410″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg” alt=”Figure 1. Mesosaurus and Stereosternum tail vertebrae compared to Proterogyrinus caudals. ” width=”584″ height=”1923″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg?w=584&h=1923 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg?w=46&h=150 46w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg?w=91&h=300 91w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/stereosternumbhm999_caudal588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Mesosaurus and Stereosternum tail vertebrae compared to Proterogyrinus caudals.
The authors wrote,
“We present an alternative hypothesis that suggests the presence of multipartite caudal centra represented by pleurocentrum and intercentrum arranged in the way expected for a reverse embolomerous pattern, previously undescribed for early stegocephalians
or amniotes.”
“Our reinterpretation of the structure of mesosaur caudal vertebrae supports the
… suggested phylogenetic affinities of mesosaurs with respect to taxa that are close to, or part of the amniote stem under some phylogenies.”
This conclusion was reached by omission of related aquatic taxa recovered by the LRT.
“Cope (1886) … placed [mesosaurs] tentatively in the “Batrachia” [frogs + salamanders]. He based this relationship on the distinctive small size of the centrum of dorsal vertebrae relative to the swollen neural arch, a feature more commonly observed in seymouriamorphs and diadectomorphs rather than in reptiles and other basal amniotes.”
“Gauther et al 1988 … linked them to the stem sauropsids or to the parareptiles.”
The LRT does not recover a monophyletic clade ‘Parareptilia’, which also includes distinctively different pareiasaurs and other terrestrial taxa.
Figure 2. Aquatic younginiforms in the LRT include Sauropterygia, Ichthyopterygia, Mesosauria and Thalattosauria.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg?w=90″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg?w=307″ class=”size-full wp-image-88179″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg” alt=”Figure 2. Aquatic younginiforms in the LRT include Sauropterygia, Ichthyopterygia, Mesosauria and Thalattosauria.” width=”584″ height=”1949″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg?w=584&h=1949 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg?w=45&h=150 45w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg?w=90&h=300 90w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/thalattosaurialrt588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Aquatic younginiforms in the LRT include Sauropterygia, Ichthyopterygia, Mesosauria and Thalattosauria.
Instead
in the large reptile tree (LRT, 2336 taxa, subset Fig 2) mesosaurs nest with similar aquatic thalattosaurs, both basal to ichthyosaurs and all three derived from a series of aquatic basal sauropterygians = pachypleurosaurs.
Mesosaurs are among the few taxa present in Early Permian deposits, which are very rare. Nearly all other taxa tested here (Fig 2) are from the Triassic. That fact may have confused taxonomists, but then, these taxa have been tested together only in the LRT.
Piñeiro et al reject
the hypothesis of caudal autotomy = shedding of the tail tip as a defense. The authors provided a new interpretation for the ‘putative fracture planes’.
Piñeiro et al
noted the similarity of the Early Permian Mesosaurus tail to that of a much earlier and unrelated basal tetrapod, Proterogyrinus (Fig 1), which also splits each caudal vert into three parts (4 if you include the chevron).
“In the literature on marine mosasauroids, ichthyopterygians and sauropterygians, there is no mention of caudal vertebral structures (i.e., fracture planes) suggesting autotomy.”
“Mesosaurs have no known predators.”
“This [vertebral splitting] pattern is otherwise unknown among amniotes, but it has been described for Devonian and Early Carboniferous stem-tetrapods and may be plesiomorphic for stegocephalians.”
The authors remind the reader of
“the pachyosteosclerotic condition of most of the axial skeleton of mesosaurs, even from the earliest post-hatching stages,
The authors sought other examples
from a wide gamut of taxa, including drepanosaurs and captorhinids, but did not include the closest aquatic relatives of mesosaurs recovered by the LRT (Figs 2–4) in their study. So I took a look and found no weakening or sutures of the caudal vertebrae in published reports of those taxa. Stereosternum is an example mesosaur (Figs 1, 3), but the angles and drawings prevent observation of sutures or lines in that taxon.
Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg?w=161″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg?w=549″ class=”size-full wp-image-32463″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg” alt=”Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen.” width=”584″ height=”1090″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg?w=584&h=1090 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg?w=80&h=150 80w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg?w=161&h=300 161w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/08/mesosaurus_origins588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. From 2018. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen. Also see figure 4 for a late addition.
In a phylogenetic context
it may be worthy to note that mesosaur ancestors lacked deep chevrons. I only noticed this lack by lining them up (Fig 3) and looking.
Coincidence or causation or correlation?
Figure 1. Luopingosaurus from Xu et al 2023 shown full size at 72dpi.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif?w=106″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif?w=363″ class=”size-full wp-image-75463″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif” alt=”Figure 1. Luopingosaurus from Xu et al 2023 shown full size at 72dpi.” width=”584″ height=”1649″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif?w=584&h=1649 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif?w=53&h=150 53w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif?w=106&h=300 106w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2023/01/luopingosaurus_insitu588.gif 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Luopingosaurus from Xu et al 2023 shown full size at 72dpi. Note the lack of chevrons.
Figure 6. Serpianosaurus is a basal thalattosaur and a proximal outgroup for the Mesosauria. Note the lack of deep chevrons.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg?w=584″ class=”size-full wp-image-91425″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg” alt=”Figure 6. Serpianosaurus is a basal thalattosaur and a proximal outgroup for the Mesosauria. Note the lack of deep chevrons. ” width=”584″ height=”389″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg?w=584&h=389 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg?w=150&h=100 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg?w=300&h=200 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/01/serpianosaurus588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 5. Serpianosaurus is a basal thalattosaur and a proximal outgroup for the Mesosauria. Note the lack of deep chevrons.
The authors concluded,
“Whereas mesosaur phylogenetic affinities remain controversial, a putative relationship between mesosaurs and the basalmost amniote groups seems possible, even at the level of the amniote stem group.”
The LRT (subset Fig 2) refutes that possibility.
“Indeed, mesosaurs have always appeared as a very basal group in the main known phylogenetic trees on amniote relationships, either as basal sauropsids or as basal parareptiles (regarding that the later are also basal sauropsids), and that signal should be taken into account in future studies.”
Taxon exclusion is the reason behind the authors’ reasoning.
The LRT resolves all such phylogenetic issues by minimizing taxon exclusion, the number one problem in paleontology in 2025 and for decades earlier.
“Our analysis reveals that mesosaurs may display a previously undocumented vertebral type in their caudal vertebrae consisting of a multipartite centrum formed by an intercentrum partially fused to the pleurocentrum, which is surprisingly located anterior
(cranial) to it.”
That is a valid conclusion. Perhaps a genetic reversal without consequence.
But consider those deep chevrons.
References
Cope ED 1886. A contribution to the vertebrate paleontology of Brazil. Proceedings of the American Philosophical Society 23 (121): 1-21. https://www.jstor.org/stable/982910
Piñeiro G et al (4 co-authors) 2025. Caudal autotomy in Mesosaurus tenuidens Gervais, 1865 under scrutiny and a surprising new pattern of vertebral organization in the mesosaur tail. Geodiversitas 47 (2): 17-38. https://doi.org/10.5252/geodiversitas2025v47a2. http://geodiversitas.com/47/2
Source: https://pterosaurheresies.wordpress.com/2025/01/30/mesosaurus-caudal-vertebrae-divisions-reviewed/
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