# Vertebral microstructure marks the emergence of pelagic ichthyosaurs soon after the End Permian Mass Extinction

Lene Liebe Delsett, Mathieu Gabriel Faure-Brac, Victoria Sjøholt
Engelschion, Alexandra Houssaye, Anusuya Chinsamy, Jørn Harald Hurum,
Benjamin P Kear

► To cite this version:

Lene Liebe Delsett, Mathieu Gabriel Faure-Brac, Victoria Sjøholt Engelschion, Alexandra Houssaye, Anusuya Chinsamy, et al.. Vertebral microstructure marks the emergence of pelagic ichthyosaurs soon after the End Permian Mass Extinction. Scientific Reports, 2025, 15 (1), pp.30221. 10.1038/s41598-025-14335-y. hal-05336777

HAL Id: hal-05336777

https://hal.science/hal-05336777v1

Submitted on 29 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

Check for updates

OPEN

# Vertebral microstructure marks the emergence of pelagic ichthyosaurs soon after the End Permian Mass Extinction

Lene Liebe Delsett<fnref n="1" />✉, Mathieu Gabriel Faure-Brac<fnref n="1" />, Victoria Sjøholt Engelschiøn<fnref n="1" />, Alexandra Houssaye², Anusuya Chinsamy³, Jørn Harald Hurum<fnref n="1" /> & Benjamin P. Kear⁴

Ichthyosaurs were the first fully marine tetrapods, and evolved a streamlined body, flippers, live birth, and endothermy-like physiology. However, the transition to these adaptations and how it relates to divergence into ocean environments is ambiguous. Here, we use vertebral bone microstructure to document the first ontogenetic series of two Early Triassic taxa that include the oldest ichthyosaur foetal fossils. One series is from *Grippia*, an early ichthyopterygian with a small body, and limbs with some plesiomorphic features. The other is a large, contemporaneous ichthyosaur, *Cymbospondylus*. Together, they phylogenetically bracket the ichthyopterygian-ichthyosaurian transition. *Grippia* has a unique microstructure with a distinctive compacted outer layer, whereas *Cymbospondylus* vertebrae are cancellous throughout, indicating a different ecology and swimming style. The dissimilar distribution of woven-parallel complex in the histology between the two taxa indicates that growth progressed at different speeds. We also document birth lines in ichthyosaurs for the first time. Pelagic, tail-propelled, rapid-growing ichthyosaurs were thus present less than five million years after the End Permian mass extinction, alongside more anguilliform ichthyopterygians. These data capture the ecological and evolutionary transition from reptiles with eel-like swimming to whale-like ichthyosaurs, implying a paradigm shift in ecology and physiology that paved the way for ichthyosaur radiation.

**Keywords** Ichthyopterygia, Ichthyosauria, Triassic, Microanatomy, Histology, Growth, Locomotion

Only a handful of land-living vertebrates have ever become fully pelagic, and these transitions are rapid in geological time<fnref n="1" />,². Ichthyosaurs were the first fully marine tetrapods, and evolved a streamlined body, flippers, live birth and endothermy-like body temperatures³⁻⁸. However, timing and ecology of their transition from land to water and their earliest diversification is poorly understood⁹,¹⁰. Fossil discoveries of the last fifteen years indicate either a rapid evolution of pelagic adaptations and larger body size following a land-water transition after the End Permian Mass Extinction (EPME), or an ichthyopterygian origin before the EPME⁷,¹¹,¹². Understanding the early evolution of key adaptations for pelagic top predators can illuminate ichthyosaur origins¹³ and mapping the timing and sequence of these adaptations will also increase our understanding of the evolution leading to the classical, thunniform taxa of the Jurassic and Cretaceous¹⁴.

Here, we use vertebral bone microstructure to pinpoint the earliest stages of pelagic adaptations for these top predators. Vertebral centra are common skeletal components in the fossil record, and provide an untapped resource for exploring morphological evolution, and for which development is scarcely studied in an evolutionary context¹⁵,¹⁵,¹⁷. Internal bone microstructure records signals of ontogeny, ecology, locomotion and metabolism¹⁸,¹⁸,²⁰ at microanatomical and histological scales. Microanatomical data reveal the inner architecture and compactness of bone, which is associated with locomotion and other physical forces¹⁷,²¹,²². Histological data such as vascularization, tissue types and osteocytes, on the other hand, can be used as proxies for physiological processes. Vertebral microstructure has been used to understand aquatic adaptation in various vertebrates¹⁷,¹⁹,²³,²³,²⁵ yet data is limited, with ichthyopterygians as a key example²⁶,²⁷. Ichthyopterygians were one of the most specialized and long lived clades, with all taxa having amphicoelous centra that were unfused to the neural arch²⁸ and that were separated by intervertebral disks²⁹.

<footnote n="1">¹Natural History Museum, University of Oslo, Sars' gate 1, Oslo 0562, Norway. ²Département Adaptations du vivant, UMR 7179 CNRS/Muséum National d'Histoire Naturelle, 57 rue Cuvier CP-55, Paris 75005, France. ³Department of Biological Sciences, University of Cape Town, Private Bag, Rondebosch 7700, South Africa. ⁴Department of Palaeobiology, Swedish Museum of Natural History, Stockholm 10405, Sweden. ✉email: lenelie@uio.no</footnote>
We compiled comprehensive ontogenetic series of fossil dorsal vertebral centra for two proxy taxa from the Early Triassic, phylogenetically bracketing the origin of Ichthyosauria as a pelagic reptile clade (Figs. 1 and 2A,B)$^{30}$. *Grippia* Wiman, 1928$^{31,32}$ was a small-sized (ca. 1.5 m) early ichthyopterygian$^{9,33,33,35}$ (Fig. 1) with elongated vertebrae, that possibly retained some plesiomorphic characteristics, such as relatively long limb bones$^{36,36,38}$. *Cymbospondylus* Leidy, 1868$^{39,40}$, on the other hand, on the lineage towards the classical dolphin-shape ichthyosaurs, attained whale-sizes, had short vertebrae, an elongated tail and probably a pelagic lifestyle$^{7,9,34,34,36,39,41}$. All the vertebrae in our study sample originate from two different well-documented faunal assemblages at Svalbard (Supplementary material)$^{39,42}$. Together, they chronostratigraphically span the early to late Spathian, with a time frame of approx. two million years, from *Grippia* at approx. 249 Ma, in a transgressive succession of increasing water depth.

We selected specimens (10 *Grippia* and 15 *Cymbospondylus*) from the entire available size range (Supplementary table 1), and because all centra are from the dorsal region, an increase in size is interpreted as an increase in body size and thus assumed to illustrate ontogenetic stage. The same approach was used in previous studies comparing bone microstructure at different ontogenetic stages$^{14,43}$. Using size as an indicator of ontogenetic stages is common, but can be inaccurate$^{44}$ and intraspecific and sexual dimorphism may influence the results.

For comparisons with existing literature, and because they are reliably identified based on anatomical criteria, our study focused on bone microstructure of centra from the dorsal region of the two ichthyopterygians, recovered from bonebeds at Spitsbergen (Supplementary table 1). The material was collected in 2014–2016 from the northern flank of Marmierfjellet in Central Spitsbergen$^{45}$. The *Grippia* specimens were collected from the *Grippia* bonebed (Early Triassic, Early Spathian), deposited in a mid to distal shelf setting$^{42}$. Ten centra were selected for our study based on the following criteria for assignment to the dorsal region of *Grippia's* vertebral column: (1) small size (dorsoventral height less than 40 mm)$^{27,32}(2)$ amphicoelous$^{28}(3)$ with para- and diapophyses$^{32,34}$ although the two are often connected, rather forming an upper and a lower confluent facet and (4) relatively long anteroposterior length compared to other ichthyopterygians$^{28,32,46}$, based on this, centra were selected where height, width and length were approximately equally long.

The *Cymbospondylus* material comes from the Lower Saurian niveau, first described by Wiman (1910) and interpreted as a distal shelf setting$^{39}$. The Lower Saurian niveau is a silty shale horizon rich in isolated, three-dimensionally preserved vertebrate remains. It is in the uppermost section of the Lower Triassic Vendomdalen Member, Vikinghøgda Formation and is of Spathian age$^{39,45}$. Fifteen centra from *Cymbospondylus* were selected based on a truncated diapophysis on the anterior margin of the centrum$^{39,41,47,48}$.

# Results

## Microanatomical and histological features

For both *Grippia* and *Cymbospondylus*, the largest centra have well finished bone surfaces (Fig. 2C), which is a sign of osteological maturity, and we interpret them as adult (sexually mature). The smallest centra are foetal, supported by several lines of evidence: Firstly, their miniature size, which is significantly smaller than

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Vertebral microstructure in *Grippia* (PMO 250.499) and *Cymbospondylus* (PMO 230.627) from the Early Triassic point to different locomotion and ecology. EPME End Permian Mass Extinction. Illustrations: Esther van Hulsen, Jakub Kowalski and Piotr Janecki.</figcaption></figure>
the adults of both *Grippia* (the smallest centrum in the sample is 4 mm high, the largest 16 mm, Fig. 2A1-10) and *Cymbospondylus* (smallest centrum is 6 mm, largest is 66 mm, Fig. 2B1-15). In a confirmed case, foetal centra associated with a gravid ichthyosaur were approximately 32% the height of those of the parent⁴⁹ which is far larger than our specimens. Secondly, the smallest *Cymbospondylus* centra have a porous external texture (Fig. 2C), often used as an osteological indicator of early ontogeny⁴⁴. Middle Triassic *Cymbospondylus* foetal vertebrae²⁷,⁴⁹ also share the same microanatomical features. The foetal *Grippia* centra in our series are among the oldest foetal ichthyopterygian material known (Fig. 2A1,2), while those of *Cymbospondylus* are the oldest ichthyosaurian foetal fossils (Fig. 2B1-4). The earliest ichthyopterygian foetuses are from a gravid *Chaohusaurus* (Spathian, approx. 248.8 Ma)⁴. Younger foetal material is known from Middle to Upper Triassic *Besanosaurus*⁵⁰ *Mixosaurus*⁵¹ and *Shonisaurus*⁵². Early to Middle Jurassic *Stenopterygius*⁵³,⁵⁴ and *Ichthyosaurus*⁵⁶,⁵⁷ to Cretaceous *Maiaspondylus*⁵⁸ and *Platypterygius*⁵⁹⁶⁰. Thus, our vertebral centra series capture ontogeny from an early foetal stage, through to juvenile to adulthood, providing a unique perspective on ichthyopterygian ontogeny in the early Triassic not covered in previous studies¹⁴,²⁷,⁴³.

Some centra preserve a birth (neonatal) line⁶¹: a distinct line separating the inner, trabecular area in two parts, i.e. pre- and post-birth, and gives an approximation of when birth occurred (e.g. Figures 2A5,A8, 2B6,B8, 3E). Importantly, similar birth lines are visible in published pictures of juvenile *Stenopterygius*²⁷ and have been identified in other marine vertebrates, among them phocid seals¹⁸,⁶². In *Cymbospondylus*, the smallest centra where this is observed is PMO 230.842 (Fig. 2B6, centrum height 24 mm), which means that the series might include five foetal centra from individuals that died during pregnancy, an interpretation that is supported by the abundant presence of calcified cartilage in these centra (Fig. 4D)²⁴. In a larger centrum (PMO 231.220, Fig. 2B8; height 38 mm) and for those larger in size, the outer layer is more organized than in the small centra and there are many signs of remodelling. These features likely represent the slowing down of growth when entering adulthood. In *Grippia*, the features are less clear, however the smallest centrum where remnants of a birth line are present is PMO 250.500 (Fig. 2A5, height 9 mm; Fig. 3E,E1,E2), indicating that larger centra are juvenile and adult specimens.

In addition, some larger (juvenile to adult) centra have growth marks (lines of arrested growth, LAGs) in the compacted parts of the outer layer. In *Grippia*, this is evident e.g. in PMO 230.499 (Fig. 3D, height 9 mm), and in *Cymbospondylus* PMO 229.743 (Fig. 4C, height 50 mm) a group of two or three LAGs are present, whereas in PMO 229.741 (height 57 mm), five LAGs occur. The presence of these periodic interruptions in growth indicates episodic growth in both taxa.

Rapid growth is indicated by the presence of woven-parallel (fibrolamellar) complex i.e., the combination of woven-fibered tissue (matrix with large and randomly oriented osteocyte lacunae, typical of static osteogenesis) and primary osteons, as well as enriched vascularization, only present in the outer layer and oriented longitudinally to the main axis for both taxa⁶³,⁶³⁻⁶⁶,⁶⁸. The *Grippia* centra have some sparse patches of woven-fibered matrix, associated with globular osteocyte lacunae, in trabeculae in the dorsal portion and in the ventral outer layer in some centra (Fig. 3C), while the main bulk of the vertebra is made of parallel-fibered complex (see Supplementary Results). Conversely, in *Cymbospondylus*, woven-parallel complex is consistently observed in both small and large centra (Fig. 4E,F).

In addition to histology, *Grippia* and *Cymbospondylus* centra display notable differences in microanatomical features (Fig. 2A,B), that reflect locomotion. Locomotion is the major function of the vertebral column in marine tetrapods⁶⁹,⁶⁹,⁷¹ and swimming patterns are reflected in the inner microanatomy and external morphology of the centra¹⁷,²¹,²². In sagittal section the *Grippia* centra display a narrower, outer endochondral region²⁷ and a wider, inner periosteal region with irregularly arranged trabeculae (Fig. 3B). In contrast, in *Cymbospondylus*, trabeculae are anteroposteriorly aligned (Fig. 4B).

In transverse section, both taxa show relatively cancellous centra throughout, and the centre of growth is located around the notochordal canal. The centrum microanatomy (Figs. 3A and 4A,G) is divided into (1) The notochordal ring surrounding the notochordal canal, with compacted bone; (2) the dorsocentral region supporting the neural canal (3) Paired dorsolateral cones supporting the neural arch, which in *Cymbospondylus* are more distinctive and cover a larger area, extending to the rib articulations. (4) The inner cancellous region is periosteal in origin. Its thickness varies depending on the extent of the cones. (5) The outer bone layer, which varies in thickness.

Overall bone compactness and the extent of the outer bone layer are main differences between the two genera. As statistical measures, calculated parameters were Compactness versus dorsoventral height (as a proxy for centrum size) and Mean outer layer thickness versus total area. To investigate the relationships, linear regressions (GLM) were conducted in PAST v4.13⁷² (Fig. 2D) (see also Supplementary results).

*Grippia* has the same overall microanatomy as found in other grippioids, with uneven trabeculae and relatively large intertrabecular spaces²⁷. However, *Grippia* centra studied here differ in important aspects from these other grippioids: The larger centra possess a three-layered structure (Figs. 2A and 3A) i. e. a compacted notochordal ring, inner periosteal region with large intertrabecular spaces and lastly, throughout ontogeny, a well-defined compacted outer layer. This outer layer is relatively thicker as compared to that in *Cymbospondylus* (Figs. 2B,D and 4A) and what was previously known for any ichthyopterygian²⁷,⁷³. For *Grippia*, the thickness of the outer layer remains similar compared to the total area of the centra, regardless of centrum size (Fig. 2D1). By contrast, in *Cymbospondylus*, the outer layer is thin and becomes relatively thinner compared to centrum area through ontogeny (Fig. 2D3).

In *Grippia*, relative bone compactness also remains similar throughout ontogeny (Fig. 2D2). In contrast, in *Cymbospondylus*, overall compactness decreases with size (Fig. 2D4), which means that in *Cymbospondylus*, vertebral centra became less compact through ontogeny. In contrast to *Grippia*, *Cymbospondylus* has a microanatomy more typical for a pelagic amniote and, like modern whales, shows a more cancellous structure, increased tightness and a virtually non-existent compact outer layer (Figs. 2B and 4A). Its microanatomy is more
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

consistent in larger specimens: dorsolateral cones with unorganized trabeculae, and a distinct circumferential organisation of the trabeculae in most of the periosteal region, corresponding to the trabecular organisation in the dorsocentral region (Fig. 4A).

Discussion

Bone microstructure implications for ontogeny

The presence of woven-parallel (fibrolamellar) complex implies rapid skeletal growth⁶⁴,⁶⁵ and is present in patches in *Grippia* centra and more consistently in *Cymbospondylus*. Woven-parallel complexes have been
► **Fig. 2.** (A, B) Vertebral microanatomy in transverse section of Early Triassic specimens used in this study.
(A) *Grippia*. To scale, scale bar = 5 mm. 1: PMO 250.504. 2: PMO 250.503. 3: PMO 250.502. 4: PMO 250.501. 5: PMO 250.500. 6: PMO 250.499. 7: PMO 250.498. 8: PMO 250.497. 9: PMO 250.496. 10: PMO 233.890.
(B) *Cymbospondylus*. To scale, scale bar = 10 mm. 1: PMO 229.744. 2: PMO 230.686. 3: PMO 231.342. 4: PMO 231.337. 5: PMO 230.738. 6: PMO 230.842. 7: PMO 230.870. 8: PMO 231.220. 9: PMO 230.627. 10: PMO 229.734. 11: PMO 229.743. 12: PMO 229.741. 13: PMO 229.740. 14: PMO 229.735. 15: PMO 229.745.
(C) *Cymbospondylus* centra: The smallest, foetal (PMO 229.744, top) and the largest, adult (PMO 229.745) with unfinished and finished external bone texture, respectively. (D) Quantitative results for microanatomy. 1 (*Grippia*) and 3 (*Cymbospondylus*) shows Mean outer layer (log) versus total area (log); 2 (*Grippia*) and 4 (*Cymbospondylus*) shows Compactness versus dorsoventral height. 1: significant linear relationship (GLM) *p* < 0.01. 2: No significant linear relationship. The outlier is PMO 250.498 (see text). 3: significant linear relationship (GLM) *p* < 0.01. 4: significant linear relationship (GLM) *p* < 0.01.

reported in Early Triassic *Omphalosaurus* and *Utatsusaurus*, Middle Triassic *Mixosaurus* and *Cymbospondylus*, as well as the Jurassic, parvipelvian taxa *Stenopterygius* and *Ichthyosaurus*<sup>14,27,43,74,75</sup>. Therefore, it appears to be a general ichthyopterygian feature. However, our data demonstrate that the rate of this growth changed during the transition from ichthyopterygian to ichthyosaurs, with *Cymbospondylus* and later ichthyosaurs having higher growth rates as compared to *Grippia*.

*Cymbospondylus* also displays circumferential trabeculae (Figs. 2B10–15, 4H) combined with cones with contrasting growth. Interestingly, a similar microanatomical organisation as in *Cymbospondylus* has been linked to rapid growth in temnospondyls and placodont sauropterygians<sup>23,65</sup>. It is also observed in some early whales<sup>76</sup>. Circumferential organisation of trabeculae is also observed in some ichthyosaurs from the Middle Triassic onwards<sup>27</sup>. Other features, such as the dorsolateral cones of *Cymbospondylus* (Fig. 4A) likely result from the influence of articulating skeletal elements, as they were situated adjacent to the neural arch and rib articulations and might reflect structural stress.

The bone compactness of *Cymbospondylus* decreases through ontogeny (47.3–27.6%; Fig. 2D), whereas this is not the case in *Grippia*. Larger differences in compactness have been associated with ecological differences in a study on plesiosaurs<sup>77</sup>; however the sampled elements may not have been conspecific<sup>16</sup> and the habitat differentiation is not the case for all plesiosaurs<sup>78</sup>. Here, we included several centra of varying sizes from the same area, and they do not indicate habitat partitioning between age groups, similar to hydropelvic mosasaurs, where microanatomy is adapted to the same environment through ontogeny<sup>24</sup>.

## Bone microstructure implications for locomotion

Microstructural differences between *Grippia* and *Cymbospondylus*, and a tandem shift in anatomy and microanatomy, has been noted before, but not attributed to swimming style and rather to evolution of disk-like centrum shapes<sup>27</sup>. However, we interpret the data from the ontogenetic series to clearly indicate differences in locomotion, making it possible to pinpoint the transition from axial undulation to caudal oscillation<sup>79</sup>. In *Grippia*, dorsal vertebrae were relatively anteroposteriorly long and spool-shaped<sup>32,37,80</sup>. This provided a greater range of motion<sup>81</sup> and much of the vertebral column was likely used in anguilliform swimming, which also involved a long tail as compared to the trunk length<sup>33,81,81,82,84</sup>. Conversely, *Cymbospondylus* had many dorsal vertebrae that were anteroposteriorly short<sup>7,9,39</sup> stiffening the trunk<sup>81</sup> with propulsion driven mainly by tail movements. Among modern dolphins and porpoises with analogous body form, longer centra are associated with coastal areas versus open ocean taxa<sup>82</sup>. Based on the differences between *Grippia* as compared to ichthyosaurs such as *Cymbospondylus*<sup>27</sup> the same evolutionary trajectory and ecological diversification seem to have taken place rapidly among early ichthyopterygians.

Ecological differentiation between taxa is further underlined by the consistent and remarkable differences in the vertebral microanatomy and histology between these two taxa. Microanatomically, *Cymbospondylus* shares an overall cancellous inner structure and a thin outermost layer with many pelagic amniotes, whereas *Grippia* has a relatively thicker compacted outermost layer throughout ontogeny, and together with other features of its vertebrae it does not resemble those of any other vertebrate. However, this outer layer is far from approaching the thickness observed in many perhaps more coastal early sauropterygians, corresponding to bone mass increase<sup>19</sup> or to that evident in the early cetacean *Basilosaurus*<sup>76</sup>. The compacted outer layer of *Grippia* to some extent resembles that of the manatee *Trichechus*<sup>22</sup> and the mosasaur *Dallasaurus*<sup>23</sup>. Previous studies have not found a direct relationship between the thickness of an outer layer of vertebrae and habitat, and among semi-aquatic amniotes, some have a thickened outer layer whereas other do not<sup>22</sup>. Except for *Basilosaurus* and *Remingtonosaurus*, other early, fully aquatic cetaceans have cancellous vertebrae without a compact outer layer<sup>76,85</sup>.

The bone compactness in *Grippia* centra varies from 46.0 to 61.1% (Fig. 2D2) and has no significant relationship to centrum size. This result contrasts with previous statements that all ichthyopterygian vertebrae had < 50% compactness<sup>73</sup> and confirms the possibility of higher compactness indicated by Houssaye et al.<sup>27</sup>.

One *Grippia* centrum (PMO 250.498) is clearly an outlier (Fig. 2A7). Its compactness is much higher (84.8%), which is visible in all parts of the centrum in transverse view, except the ventralmost portion. It however conforms to the overall *Grippia* internal organization in terms of the surrounding outer layer, and the increased compactness around the notochordal canal. The reason for the differences is unknown as there is no apparent difference from the other centra in outer morphology, taphonomy or infilling by sediments or minerals, but one possibility is pathology.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Cancellous skeletal elements have evolved repeatedly in pelagic marine tetrapods<sup>27,85,85,86,88</sup> including ichthyosaurs, therefore its presence in *Cymbospondylus* confirms that it had a microanatomy shared with later ichthyosaurs<sup>27</sup>. The adaptive value of compact and cancellous parts of vertebral centra in marine tetrapods is not fully understood. In whales, increased length of trabecular networks seem to be adaptive, to withstand the mechanical stress from locomotion<sup>17</sup>. Cancellous bone can also be an adaptation to increased lipid storage especially in deep divers<sup>89,89,91</sup>.

The anteroposteriorly directed trabeculae in *Cymbospondylus*, compared to the disorganized trabecular network in *Grippia* (Figs. 3A and 4A) also suggest differing locomotion styles. Trabeculae are commonly oriented in the direction of maximum stress<sup>27</sup> and in a comparative study, the trabecular orientation of coastal
► **Fig. 3.** *Grippia.* (A) Transverse section of PMO 233.890 with associated symmetrical axes and a schematic representation of the different areas on the right: (1) Notochordal ring, (2) Dorsocentral region, (3) Dorsolateral cone, (4) Inner cancellous periosteal region, (5) Outer bone layer; (B) Sagittal section of PMO 250.496 with associated symmetrical axes. Anterior and posterior directions are unknown, and orientation is only indicative. The dashed lines delineate the inner periosteal territories (on the top and bottom) and the outer endochondral territories (in between, on the right); (C) Magnification on one of the cones in the transverse section of PMO 233.890, with a mix of LPL and XPL. White arrowheads indicate WB osteocyte lacunae, and blue arrowheads indicate Sharpey's fibres. The circle is an artefact and does not indicate anything; (D) Growth lines in the transverse section of PMO 250.499. Yellow arrowheads indicate LAGs; (E) Notochordal ring and dorsocentral area in the transverse section of PMO 250.503, with the birth line indicated with yellow arrowheads. E1 and E2 are magnified pictures of pre- (E2) and post-birth (E1) histological deposition in XPL. Scale bars --- (A) 1 cm; (B) NA; C, E, E1. and E2. 100 µm; D. 200 µm. *D* dorsal, *V* ventral, *L* lateral, *LPL* linearly polarized light, *A* anterior, *P* posterior, *XPL* cross-polarized light.

and freshwater, slow-moving marine mammals were less anteroposteriorly structured as compared to more pelagic, faster moving taxa¹⁷. Based on this, our findings suggest that *Cymbospondylus* centra experienced more unidirectional and higher anteroposteriorly directed stress, than *Grippia*, which is consistent with *Grippia* being more anguilliform, whereas *Cymbospondylus* had a stiffened trunk and movement was powered by the tail.

## Implications for ichthyosaur evolution

We interpret the microanatomical differences in the outer layer and the compactness between the two taxa as stemming from differences in ecology. *Grippia* and *Cymbospondylus* inhabited the same seas, but used the habitat in different ways, with different locomotory styles. The former used large parts of its vertebral column involved for moving at slower speeds, and it might have dived in shallower areas. For *Cymbospondylus*, propulsion was more tail-driven, and the vertebral column was stiffened. It probably swam faster, dived deeper, and used the open ocean. By extension, these findings indicate that they used different food sources, which is supported by the substantial difference in body size as well as in tooth shape, size and replacement pattern⁹,⁹,³.

These findings are significant since they indicate that pelagic ichthyosaurs evolved as early as the Early Triassic (at least by the Spathian) which has been suggested previously¹¹. This supports the hypothesis that complex marine ecosystems with large, open ocean predators rapidly evolved soon after the End Permian Mass Extinction¹⁰,¹²,¹³. Our analyses further suggest that the shift from nearshore to pelagic environments for at least one ichthyosaur lineage happened rapidly in the evolution of ichthyosaurs during the recovery phase of the biggest mass extinction event. Furthermore, our results imply a deeper divergence of the grippidian versus ichthyosaurian morphotypes, and the explosive radiation into different habitats and swimming styles which fostered ecological niche partitioning among ichthyosaurs.

The adaptations observed in *Cymbospondylus* may have facilitated the evolution of giant body size in this genus⁷,³⁹,⁴¹especially in combination with rapid ontogenetic growth. Physiological signals indicating endothermy-like physiology and rapid growth challenge previous interpretations which assumed marine reptiles preferred warmer conditions¹³. All the large-sized Early Triassic ichthyosaurs are found in non-Tethys seas⁷,³⁹,⁹⁴,⁹⁵ and inherently accelerated growth and high metabolism were likely advantageous. Ichthyosaurs evolved rapid growth and pelagic adaptations soon after the End Permian mass extinction and it is evident that the adaptations discussed above were key to the successful explosive radiation and globalization of ichthyosaurs.

## Methods

### Thin section preparation

This study focuses both on microanatomy and histology, studied from thin sections of the vertebrae. Prior to thin sectioning, CT scans of the *Cymbospondylus* centra were performed, whereas for *Grippia* this was not executed due to a large amount of available material. The scans confirmed that they do not provide the necessary resolution making thin sectioning necessary. Note that the CT scans (that can be 3D printed) limit the damage by destructive sampling. Measurements taken before thin sectioning were: dorsoventral height; height from the middle of the notochordal canal to the dorsal margin; anteroposterior length at diapophysis; and maximum mediolateral width (Supplementary Table 1).

For thin sectioning, the ichthyopterygian vertebrae were impregnated using the epoxy EpoFix Resin using the methodology outlined in Chinsamy and Raath⁹⁶. Some were made at the Department of Geosciences, University of Oslo, some at NGU Trondheim, and the remaining at the Centre de Recherche en Paléontologie, at the Muséum national d'Histoire naturelle, Paris. At the former, CaldoFix was used for attachment to a glass plate. Coarse polishing was done using Logitech and Buehler Phoenix 4000 polishing machines before polishing with a Thorlag grinding and polishing automat. At MNHN Paris, the protocol outlined in Lamm 2013⁹⁷ was followed. All centra were sectioned in the transverse plane as close as possible to the anteroposterior midpoint, and some in addition in the sagittal plane, through the centre of the vertebra. The thickness of the final sections were 50–100 micrometres.

### Microstructure – qualitative analysis

Pictures were taken using a Leica DMLP microscope equipped with a Leica MC H170 HD camera and with the software Leica Application Suite EZ v3.4.0. Pictures were taken at various magnification using linear polarized light (LPL), cross polarized light (XPL), and XPL associated with a lambda wave plate (XPL + 1/4λ). Thin sections were scanned with a Nikon Super Coolscan 4000 using the VueScan64 software (9×64, version 9.7.02
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

Professional edition, hamrick.com). The terminology used is based on Buffrénil and Quilhac 2021⁶⁷. We use the term “outer layer” for the outermost portion of the centra, as this is not a fully compacted layer as might be implied by the term “compacta”. “Cortex” in histological terminology implies a certain growth process, which is also not the case for the part we are interested in here. We use presence or absence of woven-parallel complex as a qualitative proxy for rapid growth⁶⁵,⁶⁶.
► **Fig. 4.** *Cymbospondylus.* (A) Transverse section of PMO 229.734 with associated symmetrical axes and a schematic representation of the different areas on the right: (1) Notochordal ring, (2) Dorsocentral region, (3) Dorsolateral cone, (4) Inner cancellous periosteal region, (5) Outer bone layer; (B) Sagittal section of the dorsal half of PMO 230.627 with associated symmetrical axes. Anterior and posterior directions are unknown, and orientation is only indicative; (C) Growth lines in the transverse section of PMO 229.743. Yellow arrowheads indicate LAGs; (D) Inner edge of the notochordal ring in the transverse section of PMO 231.342, displaying calcified cartilage (red arrowhead pointing on large globular lacunae typical of this tissue) and ongoing endochondral ossification (orange arrowheads). (E) and (F). Woven parallel complex in outer layers in transverse section of PMO 229.743. (E) is from trabeculae in the outer layer, (F) is from the inner, periosteal region. White arrowheads indicate globular osteocytes, typical of WB. (G) Secondary osteon in the notochordal ring in PMO 229.743, with cementing lines. (H) Layering in transverse section of PMO 229.734, displaying the regular circumferential deposition progressively being remodeled in new trabeculae. Scale bars --- (A) 1 cm; (B) NA; (C) and H. 200 µm; D, E, F and G. 100 µm. *D* dorsal, *V* ventral, *L* lateral, *A* anterior, *P* posterior.

**Microstructure – quantitative analysis**

Using scans of the thin sections, black and white pictures were obtained with the “adjust threshold” function in ImageJ. The following measurements were taken for quantitative analyses: Total area using ImageJ v1.8.0_345⁹⁸; Compactness: ImageJ v1.8.0_345⁹⁸ on black and white pictures; Outer layer thickness. The latter was calculated from the mean of measurements from five sites, except the dorsal margin, because of the neural arch facets. One *Cymbospondylus* specimen (PMO 229.745) was too incomplete for total area or outer layer thickness to be measured but is included in the analysis of compactness versus dorsoventral height. Based on the other centra, dorsoventral height for this specimen is estimated to be 74 mm.

All measurements were log transformed prior to analysis. Calculated parametres were Compactness versus
dorsoventral height and mean cortical thickness versus total area. For comparing compactness, dorsoventral
height was used as a proxy for size.

To investigate whether the parametres are related to size only, centra from the two genera with the same
dorsoventral height were compared and the absolute thickness of the outer layer compared. *Grippia* PMO
250.501 and *Cymbospondylus* PMO 230.686 both have a height of 7 mm, whereas their outer layer thickness
are 0.41 mm and 0.18 mm, respectively. *Grippia* PMO 250.496 and *Cymbospondylus* PMO 231.342 both have a
height of 11 mm, whereas their outer layer thickness is 0.62 mm and 0.21 mm, respectively.

**Data availability**

All data generated or analysed during this study are included in the supplementary information files.

**Museum abbreviation**

PMO Natural History Museum, University of Oslo, palaeontological collection.

Received: 17 April 2025; Accepted: 30 July 2025

Published online: 05 September 2025

**References**

1. Kelley, N. P. & Pyenson, N. D. Evolutionary innovation and ecology in marine tetrapods from the Triassic to the Anthropocene. *Science* **348**, aaa3716-aaa3713. -aaa3716-3717 (2015).

2. Motani, R. & Vermeij, G. J. Ecophysiological steps of marine adaptation in extant and extinct non-avian tetrapods. *Biol. Rev.* **96**, 1769-1798. https://doi.org/10.1111/brv.12724 (2021).

3. Miedema, F. et al. Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis. *BMC Ecol. Evol.* https://doi.org/10.1186/s12862-023-02110-4 (2023).

4. Motani, R., Jiang, D., Tintori, A., Rieppel, O. & Chen, G. -b. Terrestrial origin of viviparity in Mesozoic marine reptiles indicated by Early Triassic embryonic fossils. *PLoS ONE*. **9**, e88640. https://doi.org/10.1371/journal.pone.0088640 (2014).

5. Bernard, A. et al. Regulation of body temperature by some Mesozoic marine reptiles. *Science* **328**, 1379-1382. https://doi.org/10.126/science.1187443 (2010).

6. Houssaye, A., Scheyer, T. M., Kolb, C., Fischer, V. & Sander, P. M. A new look at ichthyosaur long bone microanatomy and histology: implications for their adaptation to an aquatic life. *PLoS ONE*. **9**, e95637. https://doi.org/10.1371/journal.pone.0095637 (2014).

7. Sander, P. M. et al. Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans. *Science* **374**, eabf5787. https://doi.org/10.1126/science.abf5787 (2021).

8. Reassessment of body temperature and thermoregulation strategies in Mesozoic marine reptiles. Paleobiology Cambridge Core

9. Moon, B. C. A new phylogeny of ichthyosaurs (Reptilia: Diapsida). *J. Syst. Paleontol.* **17**, 129–155. https://doi.org/10.1080/14772019.2017.1394922 (2017).

10. Jiang, D. et al. Emergence and ecological transition of the Mesozoic marine reptiles: evidence from the Early Triassic Chaohu and the Middle Triassic Xingyi faunas. *Palaeogeogr., Palaeoclimatol. Palaeoecol.* **628**, 111750. https://doi.org/10.1016/j.palaeo.2023.111750 (2023).

11. Kear, B. P., Engelschiøn, V. S., Hammer, Ø., Roberts, A. J. & Hurum, J. H. Earliest Triassic ichthyosaur fossils push back oceanic reptile origins. *Curr. Biol.* **33**, R178-R179. https://doi.org/10.1016/j.cub.2022.12.053 (2023).

12. Moon, B. C. & Stubbs, T. L. Early high rates and disparity in the evolution of ichthyosaurs. *Commun. Biology.* **3**, 68. https://doi.org/10.1038/s42003-020-0779-6 (2020).

13. Motani, R., Jiang, D., Tintori, A., Ji, C. & Huang, J. Pre- versus post-mass extinction divergence of Mesozoic marine reptiles dictated by time-scale dependence of evolutionary rates. *Proc. Royal Soc. B: Biol. Sci.* https://doi.org/10.1098/rspb.2017.0241 (2017).

14. Kolb, C., Sanchez-Villagra, M. R. & Scheyer, T. M. The palaeohistology of the basal ichthyosaur *Mixosaurus* Baur, 1887 (Ichthyopterygia, Mixosauridae) from the Middle Triassic: Palaeobiological implications. *C.R. Palevol.* **10**, 403–411. https://doi.org/10.1016/j.crpv.2010.10.008 (2011).
15. Verrière, A., Fröbisch, N. B. & Fröbisch, J. Regionalization, constraints, and the ancestral ossification patterns in the vertebral column of amniotes. *Sci. Rep.* **12**, 22257. https://doi.org/10.1038/s41598-022-24983-z (2022).

16. Houssaye, A., Sander, M., Klein, N. & P. & Adaptive patterns in aquatic amniote bone microanatomy—More complex than previously thought. *Integr. Comp. Biol.* **56**, 1349–1369. https://doi.org/10.1093/icb/icw120 (2016).

17. Dumont, M. et al. Inner architecture of vertebral centra in terrestrial and aquatic mammals: A two-dimensional comparative study. *J. Morphol.* **274**, 570–584. https://doi.org/10.1002/jmor.20122 (2013).

18. Chinsamy, A. Palaeoecological deductions from osteohistology. *Biol. Lett.* **19**, 20230245. https://doi.org/10.1098/rsbl.2023.0245 (2023).

19. Klein, N., Canoville, A. & Houssaye, A. Microstructure of vertebrae, ribs, and gastralia of Triassic sauropterigians—New insights into the microanatomical processes involved in aquatic adaptations of marine reptiles. *Anat. Rec.* **302**, 1770–1791. https://doi.org/10.1002/ar.24140 (2019).

20. Ray, S., Mukherjee, D. & Bandyopadhyay, S. Growth patterns of fossil vertebrates as deduced from bone microstructure: case studies from India. *J. Biosci.* **34**, 661–672. https://doi.org/10.1007/s12038-009-0055-x (2009).

21. Houssaye, A. et al. Vertebral microanatomy in squamates: structure, growth and ecological correlates. *J. Anat.* **217**, 715–727. https://doi.org/10.1111/j.1469-7580.2010.01307.x (2010).

22. Houssaye, A., Tafforeau, P. & Herrel, A. Amniote vertebral microanatomy – what are the major trends? *Biol. J. Linn. Soc.* **112**, 735–746. https://doi.org/10.1111/bij.12311 (2014).

23. Houssaye, A. & Bardet, N. Rib and vertebral micro-anatomical characteristics of hydropelvic mosasauroids. *Lethaia* **45**, 200–209. https://doi.org/10.1111/j.1502-3931.2011.00273.x (2012).

24. Houssaye, A. & Tafforeau, P. What vertebral microanatomy reveals about the ecology of juvenile mosasaurs (Reptilia, Squamata). *J. Vert Paleo.* **32**, 1042–1048. https://doi.org/10.1080/02724634.2012.680999 (2012).

25. Buffrénil, V., Amson, E., Quilhac, A., Voeten, D. F. A. E. &amp; Laurin, M. Vertebrate Skeletal Histology and Paleohistology (eds V. Buffrénil, A. Ricqlès, L. Zylberber, &amp; Kevin Padian) Ch. 4, 59–80. (CRC Press Taylor and Francis Group, 2021).

26. Zverkov, N. G., Grigoriev, D. V., Wolniewicz, A. S., Konstantinov, A. G. & Sobolev, E. S. Ichthyosaur from the Upper Triassic (Carnian–Norian) of the new Siberian islands, Russian Arctic, and their implications for the evolution of the ichthyosaurian basicranium and vertebral column. *Earth Environ. Sci. Trans. Royal Soc. Edinb.* **113**, 51–74. https://doi.org/10.1017/S1755691021000372 (2022).

27. Houssaye, A., Nakajima, Y. & Sander, P. M. Structural, functional, and physiological signals in ichthyosaur vertebral centrum microanatomy and histology. *Geodiversitas* **40**, 161–170. https://doi.org/10.5252/geodiversitas2018v40a7 (2018).

28. McGowan, C. & Motani, R. *Ichthyopterygia* **8182** (Verlag Dr. Friedrich Pfeil, 2003).

29. Wintrich, T. et al. Palaeontological evidence reveals convergent evolution of intervertebral joint types in amniotes. *Sci. Rep.* **10**, 14106. https://doi.org/10.1038/s41598-020-70751-2 (2020).

30. Fang, Y. W., Wolniewicz, A. S. & Liu, J. A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi county, Yunnan province, South China. *Acta Palaeontol. Pol.* **69**, 263–280. https://doi.org/10.4202/app.01133.2024 (2024).

31. Wiman, C. Eine neue marine Reptilien-Ordnung Aus der Trias Spitzbergens. *Bull. Geol. Insititution Univ. Uppsala.* **22**, 183–196 (1928).

32. Wiman, C. Über *Grippia longirostris*. *Nova Acta Regiae Societatis Scientiarum Upsaliensis.* **9**, 1–20 (1933).

33. Mazin, J. M. *Grippia longirostris* Wiman, un Ichthyopterygia primitif du Trias inférieur du Spitsberg. *Bull. Muséum Natl. d'histoire Nat. Sect.* **C 3**, 317–340 (1981). (1929).

34. Ji, C. et al. Phylogeny of the Ichthyopterygia incorporating recent discoveries from South China. *J. Vert Paleo.* **36**, e10259561-e102595618. https://doi.org/10.1080/02724634.2015.1025956 (2016).

35. Huang, J. et al. The new ichthyosauriform *Chaohusaurus brevifemoralis* (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China. *PeerJ.* **7**, e7561. https://doi.org/10.7717/peerj.7561 (2019).

36. Motani, R. Evolution of fish-shaped reptiles (Reptilia: Ichthyopterygia) in their physical environments and constraints. *Annu. Rev. Earth Planet. Sci.* **33**, 395–420. https://doi.org/10.1146/annurev.earth.33.092203.122707 (2005).

37. Motani, R. Skull of *Grippia longirostris*: no contradiction with a diapsid affinity for the Ichthyopterygia. *Palaeontology* **43**, 01–14. https://doi.org/10.1111/1475-4983.00115 (2000).

38. Motani, R. First complete forefin of the ichthyosaur *Grippia longirostris* from the Triassic of Spitsbergen. *Palaeontology* **41**, 591–599 (1998).

39. Engelschön, V. S., Delsett, L. L., Roberts, A. J. & Hurum, J. H. Large-sized ichthysaurs from the Lower Saurian Niveau of the Vikinghögda Formation (Early Triassic), Marmierfjellet, Spitsbergen. *Norw. J. Geol.* **98**, 239–266 (2018).

40. Leidy, J. Notice of some reptilian remains from Nevada. *Proc. Acad. Nat. Sci. Phila.* **20**, 177–178. https://doi.org/10.2307/4059877 (1868).

41. Sander, P. M., Dederichs, R., Schaaf, T. & Griebeler, E. M. *Cymbospondylus* (Ichthyopterygia) from the Early Triassic of Svalbard and the early evolution of large body size in ichthyosaur. *PalZ* **98**, 275–280. https://doi.org/10.1007/s12542-023-00677-3 (2024).

42. Nakrem, H. A. & Orchard, M. J. Conodonts from the Grippia Niveau bonebed (Lower triassic, Spathian), Spitsbergen, Arctic Norway. *Lethaia* **56**, 1–10. https://doi.org/10.18261/let.56.4.7 (2023).

43. de Buffrénil, V. & Mazin, J. M. Bone histology of the ichthysaurs: comparative data and functional interpretation. *Paleobiology* **16**, 435–447 (1990).

44. Griffin, C. T. et al. Assessing ontogenetic maturity in extinct saurian reptiles. *Biol. Rev.* **96**, 470–525. https://doi.org/10.1111/brv.12666 (2021).

45. Hurum, J. H. et al. The history of exploration and stratigraphy of the Early to Middle Triassic vertebrate bearing strata of Svalbard (Sassendalen group, Spitsbergen). *Norw. J. Geol.* **98**, 165–174 (2018).

46. Ekeheien, C., Delsett, L. L., Roberts, A. J. & Hurum, J. H. Preliminary report on ichthyopterygian elements from the Early Triassic (Spathian) of Spitsbergen. *Norw. J. Geol.* **98**, 219–238 (2018).

47. Sander, P. M. The large ichthyosaur *Cymbospondylus buchseri*, sp. nov., from the Middle Triassic of Monte San Giorgio (Switzerland), with a survey of the genus in Europe. *J. Vert Paleo.* **9**, 163–173 (1989).

48. Merriam, J. C. Triassic Ichthyosauria, with special reference to the American forms. *Mem. Univ. Calif.* **1**, 1–193 (1908).

49. Klein, N., Schmitz, L., Wintrich, T. & Sander, P. M. A new cymbospondylid ichthyosaur (Ichthyosauria) from the Middle Triassic (Anisian) of the Augusta Mountains, Nevada, USA. *J. Syst. Paleontol.* https://doi.org/10.1080/14772019.2020.1748132 (2020).

50. Dal Sasso, C. & Pinna, G. *Besanosaurus leptorhynchus* n. gen. n. sp., a new shashtasaurid ichthyosaur from the Middle Triassic of Besano (Lombardy, N. Italy). *Paleontologia Lombarda.* **4**, 1–21 (1996).

51. Brinkmann, W. A mixosaur (Reptilia, Ichthyosauria) with embryos from the Grenzbitumenzone (Middle Triassic) of Monte San Giorgio (Switzerland, Canton Ticino). *Eclogae Geol. Helv.* **89**, 1321–1344 (1996).

52. Camp, C. L. Large ichthysaurs from the Upper Triassic of Nevada. *Palaeontographica Abteilung A: Paläozoologie - Stratigraphie.* **170**, 139–200 (1980).

53. McGowan, C. A revision of the Lower Jurassic ichthysaurs of Germany with descriptions of two new species. *Paleontographica Abt A.* **166**, 93–135 (1979).

54. Böttcher, R. Neue erkenntnisse Über die Fortpflanzungsbiologie der Ichthyosaurier (Reptilia). *Stuttgarter Beiträge Zur Naturkunde Serie B (Geologie und Paläontologie).* **164**, 1–51 (1990).

55. Lomax, D. R. & Massare, J. The first reported *Leptonectes* (Reptilia: Ichthyosauria) with associated embryos, from Somerset, England. *Paludicola* **8**, 263–276 (2012).
56. Deeming, D. C., Halstead, L. B., Manabe, M. & Unwin, D. M. An ichthyosaur embryo from the lower Lias (Jurassic: Hettangian) of Somerset, England, with comments on the reproductive biology of ichthyosaurs. *Mod. Geol.* **18**, 423–442 (1993).

57. Boyd, M. & Lomax, D. R. The youngest occurrence of ichthyosaur embryos in the UK: A new specimen from the Early Jurassic (Toarcian) of Yorkshire. *Proc. Yorks. Geol. Soc.* **62** 77–82 https://doi.org/10.1144/pygs2017-008 (2018).

58. Maxwell, E. E. & Caldwell, M. W. First record of live birth in Cretaceous ichthyosaurs: closing an 80 million year gap. *Proceedings of the Royal Society of London. Series B: Biological Sciences* **270**, 104–107. https://doi.org/10.1098/rsbl.2003.0029 (2003).

59. Kear, B. P. & Zammit, M. *In utero* foetal remains of the Cretaceous ichthyosaurian *Platypterygius*: ontogenetic implications for character state efficacy. *Geol. Mag.* **151**, 71–86. https://doi.org/10.1017/S0016756813000113 (2014).

60. Wade, M. A review of the Australian Cretaceous longipinnate ichthyosaur *Platypterygius* (Ichthyosauria, Ichthyopterygia). *Mem. Qld. Museum.* **28**, 115–137 (1990).

61. Nacarino-Meneses, C. & Köhler, M. Limb bone histology records birth in mammals. *PLOS ONE*. **13**, e0198511. https://doi.org/10.1371/journal.pone.0198511 (2018).

62. Woolley, M. R., Chinsamy, A., Govender, R. & Bester, M. N. Microanatomy and histology of bone pathologies of extant and extinct phocid seals. *Hist. Biol.* **33**, 1231–1246. https://doi.org/10.1080/08912963.2019.1689238 (2021).

63. Amprino, R. La structure du tissu osseux envisagée comme expression de différences dans la vitesse de l'accroissement. *Arch. Biol.* **58**, 315–330 (1947).

64. Marotti, G. Static and dynamic bone formation and the mechanism of collagen fiber orientation. *Bone* **25**, 123–126 (1999).

65. Marotti, G. Static and dynamic osteogenesis. *Italian J. Anat. Embryol.* **115**, 123–126 (2010).

66. Shapiro, F. & Wu, J. Y. Woven bone overview: structural classification based on its integral role in repair and pathological bone formation throughout vertebrate groups. *Eur. Cells Mater.* **38**, 137–167. https://doi.org/10.22203/eCM.v038a11 (2019).

67. Ferretti, M. Static and dynamic osteogenesis: two different types of bone formation. *Anat. Embryol.* **206**, 21–29 (2002).

68. Buffrénil, V. &amp; Quilhac, A. *Vertebrate skeletal histology and paleohistology* (eds V. Buffrénil, A. Ricqlès, L. Zylberberg, &amp; Kevin Padian) 147–182. (CRC Press Taylor and Francis Group, 2021).

69. Esteban, J. M. et al. The impact of the land-to-sea transition on evolutionary integration and modularity of the pinniped backbone. *Commun. Biology.* **6**, 1141. https://doi.org/10.1038/s42003-023-05512-8 (2023).

70. Jones, K. E. & Pierce, S. E. Axial allometry in a neutrally buoyant environment: effects of the terrestrial-aquatic transition on vertebral scaling. *J. Evol. Biol.* **29**, 594–601. https://doi.org/10.1111/jeb.12809 (2015).

71. Pabst, D. A. Intramuscular morphology and tendon geometry of the epaxial swimming muscles of dolphins. *J. Zool.* **230**, 159–176. https://doi.org/10.1111/j.1469-7998.1993.tb02679.x (1993).

72. Hammer, Ø., Harper, D. A. T. & Ryan, T. D. PAST: paleontological statistics software package for education and data analysis. *Palaeontologia Electronica*. **4**, 1–9 (2001).

73. Sander, P. M. *Vertebrate skeletal histology and paleohistology* (eds V. Buffrénil, A. Ricqlès, L. Zylberberg, &amp; Kevin Padian) Ch. **24** 458–466. (CRC Press Taylor and Francis Group, 2021).

74. Anderson, K. L., Druckenmiller, P. S., Erickson, G. M. & Maxwell, E. E. Skeletal microstructure of *Stenopterygius quadricissimus* (Reptilia, Ichthyosauria) from the Posidonienschiefer (Posidonia shale, Lower Jurassic) of Germany. *Palaeontology* **62**, 433–449. https://doi.org/10.1111/pala.12408 (2018).

75. Nakajima, Y., Houssaye, A. & Endo, H. Osteohistology of the Early Triassic ichthyopterygian reptile *Utatsusaurus hataii*: implications for early ichthyosaur biology. *Acta Palaeontol. Pol.* **59**, 343–352 (2014).

76. Houssaye, A., Tafforeau, P., de Muizon, C. & Gingerich, P. D. Transition of Eocene whales from land to sea: evidence from bone microstructure. *PLoS ONE*. **10**, e0118409. https://doi.org/10.1371/journal.pone.0118409 (2015).

77. Wiffen, J., de Buffrénil, V., de Ricqlès, A. & Mazin, J. M. Ontogenetic evolution of bone structure in Late Cretaceous Plesiosaurs from New Zealand. *Geobios* **28**, 625–640 (1995).

78. Liebe, L. & Hurum, J. H. Gross internal structure and microstructure of plesiosaur limb bones from the Late Jurassic, central Spitsbergen. *Norw. J. Geol.* **92**, 285–309 (2012).

79. Gutarra, S. & Rahman, I. A. The locomotion of extinct secondarily aquatic tetrapods. *Biol. Rev.* **97**, 67–98. https://doi.org/10.1111/brv.12790 (2022).

80. Maisch, M. W. & Matzke, A. T. The Ichthyosaurus. *Stuttgarter Beiträge Zur Naturkunde Serie B*, 1–159 (2000).

81. Buchholtz, E. A. Swimming styles in Jurassic ichthyosaurs. *J. Vert Paleo.* **21**, 61–73. https://doi.org/10.1671/0272-4634 (2001). (2001)021[0061:SSIJI]2.0.CO;2

82. Gillet, A., Frédérich, B. & Parmentier, E. Divergent evolutionary morphology of the axial skeleton as a potential key innovation in modern cetaceans. *Proc. Royal Soc. B: Biol. Sci.* **286**, 20191771. https://doi.org/10.1098/rspb.2019.1771 (2019).

83. Buchholtz, E. A. Vertebral osteology and swimming style in living and fossil whales (Order: Cetacea). *J. Zool.* **253**, 175–190. https://doi.org/10.1017/S0952836901000164 (2001).

84. Motani, R., You, H. & McGowan, C. Eel-like swimming in the earliest ichthyosaurs. *Nature* **382**, 347–348. https://doi.org/10.1038/382347a0 (1996).

85. van Vliet, H. J. et al. A vertebra of a small species of *Pachycetus* from the North Sea and its inner structure and vascularity compared with other basilaurid vertebrae from the same site. *PeerJ* **12**, e16541. https://doi.org/10.7717/peerj.16541 (2024).

86. Hua, S. & de Buffrénil, V. Bone histology as a clue in the interpretation of functional adaptations in the Thalattosuchia (Reptilia, Crocodylia). *J. Vert Paleo.* **16**, 703–717 (1996).

87. Houssaye, A. Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life? *Biol. J. Linn. Soc.* **108**, 3–21. https://doi.org/10.1111/j.1095-8312.2012.02002.x (2013).

88. de Ricqlès, A. & de Buffrénil, V. *Secondary Adaptation of Tetrapods To Life in Water*. 289–310 (eds Jean-Michel Mazin & Vivian de Buffrénil) (Verlag Dr. Friedrich Pfeil, 2001).

89. Sheldon, A. *Ancient marine reptiles* (eds Jack M. Callaway & Elizabeth L. Nicholls) Ch. 22, 333–354. (Academic Press, 1997).

90. Kihle, J., Hurum, J. H. & Liebe, L. Preliminary results on liquid petroleum occurring as fluid inclusions in intracellular mineral precipitates in the vertebrae of *Pliosaurus funkei*. *Norw. J. Geol.* **92**, 341–352 (2012).

91. Serafini, G. et al. Not entirely ichthyosaur: A mysterious lamniform and ichthyopterygian-fall association from the abyssal Upper Cretaceous of the Northern Apennines (Italy). *Palaios* **38**, 331–344. https://doi.org/10.2110/palo.2022.054 (2023).

92. Maxwell, E. E., Caldwell, M. W. & Lamoureux, D. O. Tooth histology, attachment, and replacement in the Ichthyopterygia reviewed in an evolutionary context. *Paläont Z.* **86**, 1–14. https://doi.org/10.1007/s12542-011-0115-z (2012).

93. Motani, R. Redescription of the dentition of *Grippia longirostris* (Ichthyosaurus) with a comparison with *Utatsusaurus hataii*. *J. Vert Paleo.* **17**, 39–44. https://doi.org/10.1080/02724634.1997.10010951 (1997).

94. Sander, P. M., de Villar, P., Furrer, P. R. & Wintrich, T. Giant Late Triassic ichthyosaurs from the Kössen Formation of the Swiss Alps and their paleobiological implications. *J. Vert Paleo* https://doi.org/10.1080/02724634.2021.2046017 (2022).

95. Delsett, L. L. & Pyenson, N. D. Early and fast rise of Mesozoic ocean giants. *Science* **374**, 1554–1555. https://doi.org/10.1126/science.abm3751 (2021).

96. Chinsamy, A. & Raath, M. A. Preparation of fossil bone for histological examination. *Palaeontologia Africana.* **29**, 39–44 (1992).

97. Lamm, E. T. *Bone Histology of Fossil Tetrapods* (eds Padian Kevin & Lamm Ellen-Thérèse) Ch. 4, 55–160. (University of California Press, 2013).

98. Schindelin, J. et al. Fiji: an open-source platform for biological-image analysis. *Nat. Methods.* **9**, 676–682. https://doi.org/10.1038/nmeth.2019 (2012).
## Acknowledgements

We would like to thank V. Rommevaux (MNHN, Paris), Salahalldin Akhavan (University of Oslo), and Frida R.Forsberg and Ana Banica (NGU) for thin section preparation. For preparation and technical support, Nelia Castro, Thomas Bang Holm, Øyvind Hammer, Hans Arne Nakrem. For fossil excavation the volunteers for field work at Spitsbergen. We would also like to thank Mike Benton and an anonymous reviewer for providing very helpful reviews that improved the manuscript significantly.

## Author contributions

LLD, VSE and BK designed the study. LLD, MGFB, VSE and JHH collected the data. LLD, MGFB, AH and AC analyzed the data. LLD wrote the main manuscript text and prepared Figs. 1 and 2. MGFB and LLD prepared Figs. 3 and 4. All authors gave input on and reviewed the manuscript.

## Declarations

### Competing interests

The authors declare no competing interests.

### Additional information

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1038/s41598-025-14335-y.

Correspondence and requests for materials should be addressed to L.L.D.

Reprints and permissions information is available at www.nature.com/reprints.

**Publisher's note** Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

© The Author(s) 2025