Nicole Klein
email: nklein@posteo.de.

Aurore Canoville

Alexandra Houssaye

Microstructure of vertebrae, ribs, and gastralia of Triassic sauropterygians -New insights into the microanatomical processes involved in aquatic adaptations of marine reptiles

Keywords: pachyosteosclerosis, osteosclerosis, inhibition of remodelling, central cavity in centra, ribs with consistent global compactness

Isolated ribs and vertebrae of Middle Triassic sauropterygians are studied. The vertebrae have a well-defined large cavity in their centra, which is a unique feature and is without any modern analogue. The articular facets of vertebrae are made of endochondral bone including calcified as well as uncalcified cartilage. Vertebrae are pachyosteosclerotic in the pachypleurosaurs Neusticosaurus and Serpianosaurus from the Alpine Triassic, and osteosclerotic in the placodont, in the medium-sized Nothosaurus marchicus, and in the pachypleurosaur Anarosaurus. In large Nothosaurus specimens, the vertebrae are cavernous.

The ribs of all sampled specimens are osteosclerotic, which resembles the microanatomy of long bones in all studied taxa. The proximal to medial part of ribs mainly consist of a compact periosteal cortex surrounding an inner endosteal territory. Towards the distal end of the ribs, the periosteal thickness decreases whereas the endosteal territory increases. Despite a shift from periosteal vs. endosteal tissues, global rib compactness remains relatively constant.

Osteosclerosis in ribs and vertebrae is reached by the same processes as in the long bones: by a relative increase in cortex thickness that is coupled by a reduction of the medullary cavity, by the persistence of calcified cartilage, and by an inhibition of remodelling although some resorption may occur but without complete redeposition of bone. Processes differ from those observed in Permian marine reptiles and some mosasaurines, where either extensive remodelling or inhibition of bone resorption leads to osteosclerosis.

INTRODUCTION

During the Early and Middle Triassic, numerous tetrapod groups reinvaded the marine realm, thus developing convergent adaptations to an aquatic lifestyle. Sauropterygia was one diverse group of diapsid marine reptiles that existed from the late Early Triassic to the end of the Cretaceous [START_REF] Motani | The evolution of marine reptiles[END_REF][START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. Their Triassic radiation was restricted to near-shore habitats of the Tethys Ocean and connected epicontinental seas [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. It thus primarily involved coastal and shallow marine forms of Placodontia, Pachypleurosauria, Nothosauroidea, and Pistosauroidea; the latter three clades forming the Eosauropterygia [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. In the deposits of the Germanic Basin of Central Europe (i.e. Muschelkalk layers/units), individual numbers of Eosauropterygia are high. However, most skeletal remains are found in bonebeds and condensation horizons (e.g., Meyer, 1847-55; [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF][START_REF] Schröder | Wirbeltiere der Rüdersdorfer Trias[END_REF] and are thus usually highly disarticulated and/or isolated. Thus, the taxonomical assignment of these isolated bones beyond family level is often difficult (e.g., Klein et al., 2015aKlein et al., , 2016a;;[START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF].

In the course of secondary adaptation to an aquatic lifestyle, a given lineage will undergo numerous changes affecting, for example, its locomotion and buoyancy, sensors, salinity/osmoregulation, feeding strategy and diet, reproduction, as well as air regulation during This article is protected by copyright. All rights reserved.

diving (see [START_REF] Houssaye | Functional (secondary) adaptation to an aquatic life in vertebrates: an introduction to the symposium[END_REF][START_REF] Thewissen | Sensory Evolution on the Threshold[END_REF]. Besides grossmorphological changes affecting the skeleton, the microstructure (i.e., inner structure) of skeletal elements is also affected due to changing requirements in locomotion and buoyancy. Bone microanatomy, i.e., the amount and distribution of the osseous tissue in the bone, is strongly associated with lifestyle and bone biomechanics (e.g. Canoville andLaurin, 2009, 2010;[START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF][START_REF] Dumont | Inner architecture of vertebral centra in terrestrial and aquatic mammals: a two-dimensional comparative study[END_REF][START_REF] Houssaye | Vertebral microanatomy in squamates: structure, growth and ecological correlates[END_REF][START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF]Quemeneur et al., 2013) and can thus indicate ecological preferences (e.g. [START_REF] Buffrénil V De, Mazin | Caractères structuraux et mode de croissance du fémur d'Omphalosaurus nisseri, ichthyosaurien du Trias moyen du Spitzberg[END_REF]Buffrénil et al., , 1990;;[START_REF] Germain | Microanatomy of the radius and lifestyle in amniotes (Vertebrata, Tetrapoda)[END_REF][START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF][START_REF] Laurin | The evolution of long bone microstructure and lifestyle in lissamphibians[END_REF][START_REF] Laurin | A microanatomical and histological study of the paired fin skeleton of the Devonian sarcopterygian Eusthenopteron foordi[END_REF][START_REF] Laurin | Bone microanatomy and lifestyle: a descriptive approach[END_REF]. Further on, changes of microanatomical patterns during the evolutionary history of a clade can document processes of secondary aquatic adaptations (e.g. [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF][START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF].

Aquatic tetrapods display two main types of microanatomical specializations (e.g., [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF][START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Canoville | Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on palaeobiological inferences[END_REF][START_REF] Hayashi | Bone inner structure suggests increasing aquatic adaptations in Desmostylia (Mammalia, Afrotheria)[END_REF][START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF]Houssaye et al., , 2016;;[START_REF] Ricqlès A De | Bone histology, heterochronies and the return of tetrapods to life in water: where are we?[END_REF] resulting in bone mass increase (BMI) or bone mass decrease (BMD) as compared to the plesiomorphic terrestrial phenotype. BMD results in the acquisition of a spongious inner-architecture of skeletal elements and an overall lightening of the skeleton. This spongious organization is typical for active swimmers requiring high speed and manoeuvrability in open water habitats [START_REF] Buffrénil V De | Comparaison de la structure et du volume squelettiques entre un delphinidé (Delphinus delphis L.) et un mammifère terrestre (Panthera leo L.)[END_REF][START_REF] Buffrénil V De | Bone histology of the ichthyosaurs: comparative data and functional interpretation[END_REF][START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF][START_REF] Ricqlès A De | Bone histology, heterochronies and the return of tetrapods to life in water: where are we?[END_REF]. BMI acts as a ballast to counteract positive buoyancy and is typical for shallow marine forms being slow swimmers and This article is protected by copyright. All rights reserved. divers, and/or taxa at an initial stage of adaptation to an aquatic environment [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF][START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Houssaye | Pachyostosis" in aquatic amniotes: a review[END_REF][START_REF] Houssaye | Palaeoecological and morphofunctional interpretation of bone mass increase: an example in Late Cretaceous shallow marine squamates[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF]Houssaye et al., , 2016)). BMI is the outcome of two independent processes that can be coupled or not, general or localized in the skeleton: i) osteosclerosis that corresponds to an inner bone compaction of the skeletal element, accompanied by a reduction of the medullary cavity, as compared to the plesiomorphic terrestrial phenotype; ii) pachyostosis that is an hyperplasy of bone periosteal cortices (extended centrifugal growth of the periosteal cortex), affecting the gross-morphology (swollen aspect) of the skeletal elements [START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Houssaye | Pachyostosis" in aquatic amniotes: a review[END_REF]. However, these contrasting patterns are not mutually exclusive and can be variously combined between different elements of the skeleton [START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF]Houssaye et al., 2016). Inversions in trends of skeletal density can occur during the evolutionary history of a given clade and are usually associated to ecological changes (e.g., cetaceans, mosasaurs;[START_REF] Buffrénil V De | Bone histology of the ichthyosaurs: comparative data and functional interpretation[END_REF][START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF][START_REF] Ricqlès A De | Bone histology, heterochronies and the return of tetrapods to life in water: where are we?[END_REF]. Different skeletal regions (appendicular and axial) can exhibit similar microanatomical adaptations (e.g., [START_REF] Amson | Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure[END_REF][START_REF] Dewaele | Diversity of late Neogene Monachinae (Carnivora, Phocidae) from the North Atlantic, with the description of two new species[END_REF] and thus a congruent ecological signal, although this is not always the case (Houssaye et al., 2016).

Microstructural adaptations to an aquatic lifestyle have been extensively studied in the limb bones of various tetrapod clades (e.g., [START_REF] Canoville | Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on palaeobiological inferences[END_REF][START_REF] Ksepka | Bone histology in extant and fossil penguins (Aves: Sphenisciformes)[END_REF], and to a lesser extent in the axial skeleton (mostly vertebrae and ribs, [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF]Buffrénil et al., 1990;[START_REF] Houssaye | A preliminary report on the evolution of the vertebral microanatomy within mosasauroids (Reptilia, Squamata)[END_REF][START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF][START_REF] Dumont | Inner architecture of vertebral centra in terrestrial and aquatic mammals: a two-dimensional comparative study[END_REF]Gray et al., 2007;[START_REF] Houssaye | A preliminary report on the evolution of the vertebral microanatomy within mosasauroids (Reptilia, Squamata)[END_REF][START_REF] Houssaye | Rib and vertebral micro-anatomical characteristics of hydropelvic mosasauroids[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF]. For Triassic sauropterygians, previous This article is protected by copyright. All rights reserved.

works have primarily focused on the histology and microanatomy of long bones (Buffrénil and Mazin, 1992;[START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF][START_REF] Hugi | Long bone microstructure gives new insights into the life of pachypleurosaurids from the Middle Triassic of Monte San Giorgio, Switzerland/ Italy[END_REF][START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Griebeler, 2016, 2018;Klein et al., 2015bKlein et al., , 2016b;;[START_REF] Krahl | Evolutionary implications of the divergent long bone histologies of Nothosaurus and Pistosaurus (Sauropterygia, Triassic)[END_REF][START_REF] Sander | Skeletochronology in the small Triassic reptile Neusticosaurus[END_REF]. These studies have shown that bone tissue types of Triassic Sauropterygia can be generally categorized as lamellar-zonal or fibrolamellar.

However, these marine reptiles exhibit an enormous diversity of bone tissues as well as combinations of tissues, varying vascular densities, as well as diverse microanatomical patterns, which are to this extent unknown in any extant tetrapod group (see references above).

A comprehensive study of the vertebral and rib microanatomy of these groups is lacking.

The current study thus aims at describing the rib and vertebral microanatomy and histology of some Triassic sauropterygians. Further on, comparisons with their long bone microstructure and with other taxa are conducted, in order to better understand their skeletal adaptations to an aquatic lifestyle.

MATERIAL AND METHODS

The sampled ribs, gastralia, and vertebrae originate from the locality of Winterswijk (The Netherlands; Anisian), from classical Muschelkalk and Lettenkeuper localities in southern Germany (early and late Ladinian), and from the Alpine Triassic of Monte San Giorgio (Anisian/Ladinian) (Table 1).

All ribs and vertebrae were found in isolation except for the nothosaur rib SMNS 80266, which belongs to a disarticulated but nearly complete individual of Nothosaurus sp. (pers. com. D. Seegis, 2016/SMNS). All respective thin sections are curated under the collection numbers given in Table 1. Some thin sections were newly made for this study; others were already in the collections of the SMNS (unpublished master thesis of Weidemeyer, 2001 at the GPIT) or of the PIMUZ [START_REF] Hugi | Long bone microstructure gives new insights into the life of pachypleurosaurids from the Middle Triassic of Monte San Giorgio, Switzerland/ Italy[END_REF][START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF]Hugi unpublished material). If not mentioned otherwise, sampled ribs are usually mid-dorsal ribs and vertebrae represent dorsal vertebrae.

The morphology of eosauropterygian ribs and vertebrae is not diagnostic. An accurate taxonomical assignment based on morphology is thus not possible but also beyond the scope of this paper, which intends to focus on trends in histology and microanatomy of ribs and vertebrae for this group. However, based on characteristic tissue types and overall microanatomy, a taxonomical identification was sometimes possible (Table 1). Indeed, ribs and vertebrae are clearly pachyostotic in placodonts and in all Eosauropterygia from the Alpine Triassic: i.e., the pachypleurosaurs Neusticosaurus spp. and Serpianosaurus as well as the nothosaurs Lariosaurus/Ceresiosaurus (summarized in [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. Vertebrae and the proximal part of This article is protected by copyright. All rights reserved.

ribs appear usually somewhat swollen in other Eosauropterygia (i.e. Anarosaurus, Nothosaurus marchicus, pistosauroid) but are not pachyostotic when compared to those of the abovementioned taxa.

The ribs and vertebrae were photographed and measured before destructive sampling (Table 1). Transverse sections of the ribs were cut along their shaft (perpendicular to the long axis of the element) to document microanatomical variation along the length of the rib from the proximal to the distal end. Vertebrae were usually cut transversally except for one that was cut longitudinally. The thin sections were produced following standard petrographic methods [START_REF] Klein | Bone histology and growth of the prosauropod dinosaur Plateosaurus from the Norian bonebeds of Trossingen (Germany) and Frick (Switzerland)[END_REF], scanned with an Epson V740 PRO high-resolution scanner, and studied and photographed with a Leica1DM 750P compound polarizing microscope equipped with a digital camera (Leica1ICC50HD). The bone histological terminology follows [START_REF] Francillon-Vieillot | Microstructure and mineralization of vertebrate skeletal tissues[END_REF].

RESULTS

Vertebrae

Gross morphology

In Eosauropterygia, the neurocentral suture, i.e., the connection between the centrum and the neural arch, is typically not fused in adults (skeletal paedomorphosis), contrary to placodonts were both are usually fused (see [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. The neural arch pedicels are expanded at their This article is protected by copyright. All rights reserved.

base resulting in a characteristic butterfly-shaped or cruciform facet, which makes the identification of isolated sauropterygian centra and/or neural arches easy (Rieppel, 2000:fig. 5).

The vertebral centra are slightly constricted ventrally and laterally (except in some strongly pachyostotic taxa). Neural spines are low in pachypleurosaurs (e.g., [START_REF] Rieppel | A new pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland[END_REF][START_REF] Sander | The pachypleurosaurids (Reptilia: Nothosauria) from the Middle Triassic of Monte San Giorgio (Switzerland) with the description of a new species[END_REF] and some nothosaurs (Nothosaurus marchicus, N. giganteus) but high in others (N. mirabilis) [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]Klein et al., 2015a;[START_REF] Sander | Postcranial morphology of a basal Pistosauroidae (Sauropterygia) from the Lower Muschelkalk of Winterswijk, The Netherlands[END_REF]. However, the height of the neural arch depends also on the anatomical position of the vertebra.

Microanatomical features

The longitudinal section of the dorsal centrum (Wijk08-47) shows that the periosteal (dorsal and ventral cones) and endochondral (anterior and posterior cones) territories (see [START_REF] Buffrénil V De | Bone vascular supply in monitor lizards (Squamata: Varanidae): influence of size, growth, and phylogeny[END_REF] for details) are clearly distinct, the cones of periosteal origin being much more compact (Fig. 1A). The layer of compact cortex is thicker in the dorsal cone than in the ventral one.

Around the growth center, resorption occurred, resulting in large cavities scattered around the core of the centrum. Away from the sagittal plane, resorption is more important in the ventral cone of periosteal origin so that all this area becomes spongious, whereas the dorsal cone remains strongly compact.

In transverse section, the compact bone surrounds most of the periphery of the vertebra (i.e., the dorsal part of the neural arch, including the neural spine, the inner margin of the neural canal, and the ventrolateral margin of the centrum). The layer can be rather thin, like in the This article is protected by copyright. All rights reserved. centrum and neural arch of Wijk10-535 (Fig. 1C) or thick, like in the centrum of Wijk08-94 (Fig. 1D) and in the neural arch of Wijk13-88 (Fig. 1B). The zones of articulation (articular facets between neural arch and centrum as well as laterally where the ribs articulate) are spongious except in Serpianosaurus and Neusticosaurus (Figs. 1,2). The centrum comprises a large central cavity, most likely corresponding to the former notochord, except in Wijk10-535, SMNS 54418, and Serpianosaurus. The centrum is crossed by trabeculae in Wijk10-535 (Fig. 1C) and filled by endosteal bone and dense calcified cartilage in Serpianosaurus, resulting in an extremely compact vertebra in the latter (Fig. 1G). Some endosteal trabeculae reach into the centrum cavities of Wijk08-94, Wijk08-126, and in that of Neusticosaurus (Figs. 1D,E,H,I). Vertebrae of pachypleurosaur Neusticosaurus edwardsii are less compact when compared to that of Serpianosaurus due to a strong radial vascularization (Figs. 1H,I). Additionally, around the articular facets some resorption occurred in Neusticosaurus, resulting in some erosion cavities. In SMNS 54418, the centrum consists of a spongiosa made of small trabeculae surrounded by a layer of compact bone (Fig. 1F). The neural arch and spine are in this sample compact, too, except for a central cavity in mid-line with the neural spine. This specimen results from a natural break and not from true sectioning.

StIPB R 68a and StIPB R 52, both from large Nothosaurus (N. mirabilis or N. giganteus), differ from the smaller eosauropterygian vertebrae in being generally less compact. They show a large central cavity in the centrum as well as several cavities in the neural arch (Fig. 1J), This article is protected by copyright. All rights reserved.

resulting in an overall cavernous structure. A thin layer of compact bone surrounds the entire vertebrae as well as these inner cavities.

The placodont sample StIPB R 68b has a much thicker periosteal cortex around a relatively small central, triangular shaped cavity in the centrum with a small ring of endosteal bone in its center, forming a small pipe (Fig. 1K). One leg of the triangle reaches down to the ventral bone surface, the other extends only half way down, which might be an artifact of sampling location. Only one half of the neural arch is preserved that exhibits a large cavity, which is surrounded by two layers of periosteal bone, a thin one along the neural canal and a thicker one along the periphery.

Histological features

Bone tissue of the placodont vertebra (StIPB R68b) differs from that of the other described vertebrae by the occurrence of true fibro-lamellar bone with essentially radially oriented vascular canals organized in circumferential layers and a locally occurring primary trabecular structure (Figs. 2A-C), resulting in an overall spongious structure.

Primary bone tissue in the pachypleurosaurs Neusticosaurus and Serpianosaurus from the Alpine Triassic is made of well (Neusticosaurus) and low (Serpianosaurus) vascularized highly organized parallel-fibred bone. Vascular canals are simple and are arranged radially or longitudinally in circumferential rows (Figs. 1G-I).

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The primary periosteal bone tissue of Nothosaurus is essentially made of parallel-fibred bone with a moderate vascular density (Figs. 2D-F). Vascular canals are simple, incompletely lined or can appear locally as primary osteons [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Klein et al., 2016b). A longitudinal and radial orientation dominates. Locally, there is a tissue with much larger osteocyte lacunae that are also randomly shaped and sized, but that shows an extinction pattern in polarized light similar to that of parallel-fibered bone (Figs. 2H,I). This tissue thus is comparable to the unusual parallel-fibered bone evoked by [START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF] in mosasaurs. This kind of parallel-fibred bone probably represents an intermediate stage between parallel-fibered and fibrous/woven bone [START_REF] Klein | Bone histology, microanatomy, and growth of the nothosauroid Simosaurus gaillardoti (Sauropterygia) from the Upper Muschelkalk of southern Germany/Baden-Württemberg[END_REF]. Locally, in combination with the occurrence of primary osteons, it results in incipient fibro-lamellar bone sensu [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF].

The tissue of the regions of the articular facets (between neural arch and centrum as well as laterally where the ribs articulate) shows endochondral bone consisting essentially of calcified cartilage enclosed by endosteal trabeculae of varying thickness (thickest in the placodont and in large nothosaurs) or scattered with intertrabecular spaces lined by thin layers of endosteal bone (Figs. 1E, 2K-O). The placodont vertebra shows a network of endosteal trabeculae enclosing calcified cartilage, which is in a lesser extend also the structure of the articular regions in large nothosaur vertebrae. A trabecular structure of the endosteal tissue is not observed in small and medium sized eosauropterygians. The connective area between the endochondral territories of the centrum and the neural arch consists of uncalcified cartilage (Fig. 2O). Remodeling of periosteal bone was not observed in any of the samples. Only some resorption is documented in This article is protected by copyright. All rights reserved.

the placodont vertebra in form of enlargement of primary osteons. In the samples where the neural spines are preserved, they are avascular (Figs. 1B,C).

Growth marks are common in periosteal regions of the vertebral centra with the succession of zones and annuli, and sometimes the addition of a true LAG (Figs. 2A,D,H,K). At the (dorsolateral) limit of the regions of periosteal and endochondral origin in the centrum, there are long distinct Sharpey's fibers (Fig. 2P).

Ribs

Gross morphology

A dorsal rib can morphologically be divided into the rib head, the usually curved proximal part, the only slightly curved or straight medial part, and the flat but broad distal part (Fig. 3B). This division was followed for sampling and study of the inner structure. Rib cross sections document the shape changes during ontogeny as a result of elongation and a posterior extension of the rib (Fig. 3). Cervical ribs of Sauropterygia are double headed whereas dorsal and sacral ribs are holocephalous [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF].

Microanatomical features

Changes in the overall microstructure along the rib shaft are documented for all studied Sauropterygia (Figs. 3456), as was observed in other amniotes. The rib head and the proximal part of the rib shaft generally display a thick and compact layer of periosteal bone that surrounds a This article is protected by copyright. All rights reserved.

small medullary cavity or medullary region. From the medial part of the rib towards its distal end, the relative size of the medullary region increases whereas the relative thickness of the primary periosteal cortex decreases. The latter is reduced to a very thin layer in the distal part of all studied ribs. The structure and size of the medulla varies considerably between different samples, as well as along the shaft of a single rib (Table 1; Fig. 3). The medullary region is usually as compact as the periosteal region due to the persistence of calcified cartilage (Figs. 3,5).

Only a few samples have at the proximal part of the rib a medullary cavity containing no or only a few bone trabeculae (Fig. 3B1). Others exhibits a small, not centred medullary cavity (Figs. 3D1, E1, 5D, E,) to a moderately (Figs. 3F1,G1) or large-sized medullary region (Figs.

3A1, 5A, B, F-H). SMNS 54434 (Fig. 3C1) is the only sample that has proximally a de-central, round medullary region filled solely by calcified cartilage. In the medial part of the rib a medullary region made by endosteal bone and calcified cartilage is present in all samples (Figs.

3A2-3, C2, D3-5, E2-3, F2, G2-3, 5J), except for Wijk07-31 where a cavity is retained but surrounded and crossed by trabeculae (Figs. 3B3-6). In the distal ends of all sampled ribs, the periosteal compact bone tissue is reduced and most of the section consist of the medullary territory that is occupied by endosteal bone, sometimes forming trabeculae, and calcified cartilage (Figs. 3A4, B7-8, C3, D5-6, E4, F3, G4, 5C, K-O).

Histological features

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In all samples simple vascular canals and primary osteons are longitudinally and radially organized. The periosteal bone of the placodont rib (Wijk16-GG; Figs. 4A-C) consists of fibrolamellar bone tissue, which is however, less vascularized than in the vertebra (StIPB 68b, Figs. 1K, 2A-C) but much higher vascularized than in studied Eosauropterygia. Primary osteons in the placodont rib are locally enlarged by resorption (Figs. 4A,B).

Primary bone tissues in ribs of studied pachypleurosaurs are of variable nature. It consists of well, radially vascularized parallel-fibred bone tissue in Neusticosaurus (Fig. 6F), and of low to nearly avascular parallel-fibred and/or lamellar bone in Serpianosaurus (Figs. 6A-I).

The ribs of the large Nothosaurus (SMNS 80266, SMNS 84371, SMNS 84393; Figs. 4K-M) are well vascularized and fibrolamellar bone is locally deposited (Figs. 4K,L; Table 1). In Ceresiosaurus, parallel-fibred bone tissue is moderately vascularized (see also [START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF]; Figs.

6J-M).

The periosteal bone of medium-sized Eosauropterygia is essentially made of parallelfibred bone, whose vascular density and periosteal tissue organization is variable. However, vascular density is usually moderate. The bone tissue between the pachypleurosaur Anarosaurus (Figs. 4E,H) and Nothosaurus (Figs. 4F,I) differs mainly in its degree of tissue organization and vascular density. Tissue in Nothosaurus is less vascularized and more highly organized. In addition, Anarosaurus can display incompletely lined primary osteons and woven bone resulting in incipient fibrolamellar bone [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Klein | Growth patterns, sexual dimorphism, and maturation modeled in Pachypleurosauria from Middle Triassic of central Europe (Diapsida: Sauropterygia)[END_REF] This article is protected by copyright. All rights reserved.

Locally, mainly in nothosaur ribs, the periosteal tissue is interspersed or even overprinted by different types of fibres. Sharpey's fibres, short angled, or longitudinal fibres can occur (Figs. 4N,O). Generally, the amount of fibres is the highest in distal samples but other parts of the rib can also contain fibres, most likely depending on the anatomical position of the rib.

The medullary region of ribs consists in all studied taxa of a matrix of calcified cartilage scattered with erosion cavities that are lined or filled by endosteal bone (Figs. 3,5).

In some specimens, the medulla is lined by a sharp line with calcified cartilage along its inner margin (Figs. 5D,E,). This sharp line separates the endochondral territory from the periosteal territory and is also described for sauropterygian long bones [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Griebeler, 2016, 2018;Klein et al., 2016b). A sharp line is proximally visible in SMNS 84393, SMNS 80266, and SMNS 54434, medially in samples of SMNS 80266, SMNS 84393, SMNS 54434, Wijk09-557, N. edwardsii, and Serpianosaurus. It is visible in nearly all distal samples. Resorption occurs only in the inner cortex of Wijk09-382 (Fig. 3E) and locally in the placodont rib. In the latter, only primary osteons are affected by resorption whereas in Wijk09-382 the entire periosteal cortex is affected. Beyond this exception, remodelling of periosteal bone was in no sample observed and was also not extensive in the medullar territory.

Gastralia

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All studied gastralia (Neusticosaurus, Serpianosaurus, Nothosaurus spp., small Eosauropterygia indet.) share a thick periosteal cortex consisting of avascular lamellar bone. In most samples, the medullary territory is completely filled by endosteal bone. In medial samples, growth marks resemble morphological shape changes from triangular to oval or round cross-section (Figs. 6N-P).

DISCUSSION

Comparison with other tetrapod vertebrae and ribs

Vertebrae

An overview of microanatomy in amniote vertebrae is given in Houssaye et al. (2016), revealing a high morphological and microanatomical variability. Amniote vertebrae generally display a spongious inner organization with two thin walls of compact bone at the periphery and around the neural canal (see [START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF]. The tightness of the vertebral trabecular network strongly varies among taxa [START_REF] Dumont | Inner architecture of vertebral centra in terrestrial and aquatic mammals: a two-dimensional comparative study[END_REF][START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF]Houssaye et al., , 2016;;[START_REF] Skutschas | Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia[END_REF]. Most semi-aquatic taxa (shallow swimmers to deep divers) show an inner organization similar to that of terrestrial taxa whereas exclusively aquatic taxa display either i) an increase in the tightness of the spongiosa (more numerous and proportionally thinner trabeculae with reduced intertrabecular spaces) and a reduction in thickness of the surrounding compact layers (active swimmers; [START_REF] Dumont | Inner architecture of vertebral centra in terrestrial and aquatic mammals: a two-dimensional comparative study[END_REF]Houssaye et al., 2016;[START_REF] Skutschas | Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia[END_REF], or ii) an increase in vertebral compactness, from a thickening of the compact cortical layers, like in sirenians [START_REF] Hayashi | Bone inner structure suggests increasing aquatic adaptations in Desmostylia (Mammalia, Afrotheria)[END_REF], to an extreme thickening with almost completely This article is protected by copyright. All rights reserved.

compact vertebrae, like in some Late Cretaceous marine squamates (see [START_REF] Houssaye | Palaeoecological and morphofunctional interpretation of bone mass increase: an example in Late Cretaceous shallow marine squamates[END_REF]. Such compact vertebrae were so far known only in some varanoid lizards, pythonomorphs, hindlimbed snakes, and in mesosaurs (NK pers. obs.) and to a lesser extent in the diapsid Claudiosaurus [START_REF] Buffrénil V De | Bone histology of Claudiosaurus germaini (Reptilia, Claudiosauridae), and the problem of pachyostosis in aquatic tetrapods[END_REF]. Our study reveals that this extreme pachyosteosclerotic pattern also occurs in the pachypleurosaurs from the Alpine Triassic (Neusticosaurus, Serpianosaurus) where it is the result of an increase in cortical deposits and in inner bone compaction, strong inhibition of primary bone resorption, and the persistence of calcified cartilage (incomplete endochondral ossification). Conversely, there is a rather moderate osteosclerotic pattern in vertebrae of medium-sized eosauropterygians from Winterswijk locality, which is however, achieved by the same processes, except for the persistence of calcified cartilage at midshaft in the long bones. The placodont vertebra displays an increase in cortex thickness, which has however a spongious structure, due to a general high vascular density, local primary trabecular structure, and resorption of primary osteons. This contradicting signal (increase of periosteal thickness and increase of porosity) was also described for some placodont long bones (Klein et al., 2015b). Vertebrae of large Nothosaurus are not osteosclerotic due to the presence of large cavities in the neural arch and centrum, resulting in a cavernous structure.

The presence of large cavities in the core of vertebral centra involves complete reorganization and redistribution of the mass in the vertebrae and is in this extend unique among tetrapods [START_REF] Dumont | Inner architecture of vertebral centra in terrestrial and aquatic mammals: a two-dimensional comparative study[END_REF][START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF]. Early Cretaceous choristoderes also show central cavities in the vertebral centrum but they are smaller (with one exception: see Skutschas

This article is protected by copyright. All rights reserved. and Vitenko, 2017:fig. 9J) as is the central cavity in a cervical vertebra of a Jurassic plesiosaur [START_REF] Wintrich | Foramina in plesiosaur cervical centra indicate a specialized vascular system[END_REF]. The retainment of a notochordal canal can be interpreted as an adaptation to an aquatic lifestyle (paedomorphosis). However, such a cavity does not occur in vertebrae of the pachypleurosaur Serpianosaurus, were it is filled up with endosteal bone to achieve increase in bone mass. It is also not observed in the sacral vertebra (Wijk10-535), which might be related to its anatomical position, and is also not visible in SMNS 54418, which is most likely related to a non-central sampling location. In StIPB R68b the central cavity is also relatively small when compared to the condition in Eosauropterygia. Its shape is triangular, with two ventrally reaching legs, most likely representing nutrient foramina.

A further preliminary observation of our study reveals that dorsal centra of Eosauropterygia seem to be generally more compact than centra of cervical and sacral vertebrae (see Figs. 1B-F).

Ribs

Rib microanatomy was in the focus of a few studies (e.g., [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF]Buffrénil et al., 1990[START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF] but comparison is often difficult due to the lack of a homologous sampling location along the ribs and between ribs along the ribcage (Houssaye et al., 2016). [START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF] were the first who provided an overview of rib inner organization conducted on a large sample of extant amniotes from which standardized midshaft sections of mid-dorsal ribs were taken. They found that ribs exhibit an ecological This article is protected by copyright. All rights reserved.

signal. Thus, ribs of terrestrial taxa generally consist of a rather tubular structure with a peripheral compact cortical layer and either an open medullary cavity or an inner loose spongiosa. Amniotes inhabiting shallow waters globally show higher rib compactness than terrestrial relatives. To the contrary, flying amniotes generally possess relatively thinner cortices [START_REF] Amson | Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru[END_REF]Buffrénil et al., 1990[START_REF] Buffrénil V De, Canoville | Evolution of sirenian pachyosteosclerosis, a model-case for the study of bone structure in aquatic tetrapods[END_REF][START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF]. However, some marine mammals (cetaceans) display a great variability in rib compactness, reaching from osteoporoticlike ribs to ribs with thick compact cortices, which is however, in some taxa, related to a variable habitat [START_REF] Canoville | Microanatomical diversity of amniote ribs: an exploratory quantitative study[END_REF][START_REF] Houssaye | Biomechanical evolution of solid bones in large animals: a microanatomical investigation[END_REF]. Microanatomical variability of the ribs is also high in mosasaurs [START_REF] Houssaye | Rib and vertebral micro-anatomical characteristics of hydropelvic mosasauroids[END_REF].

In spite of some variability, the rib microanatomy of Triassic sauropterygians is relatively uniform and generally results in osteosclerosis. The latter is achieved by the persistence of calcified cartilage and by an inhibition of remodeling. Although the proportions in the amount of periosteal and endosteal tissue change from the rib head towards the distal end of the rib, global compactness of the rib does not change significantly along the shaft. This is because incomplete endochondral ossification results in rather compact medullary regions (Figs. 3,5; Table 1). The This article is protected by copyright. All rights reserved. Surmik et al. (2018: figs. 2, 3) thin sectioned the proximal part of a pachyostotic dorsal rib from the Anisian of Poland that probably belongs to a pistosauroid. The Gogolin Formation, from which this specimen originates, also represents a near coastal, shallow marine environment.

Besides differences in vascular system (dominance of small longitudinal ?simple vascular canals), the periosteal as well as the endosteal domains are significantly more compact than in any rib from our sample, including similar-sized nothosaur ribs. Differences are most likely related to taxonomy rather than to the anatomical position of the sampled element or the environment in which the animal lived. This again indicates the use of histology, at least in some groups, to distinguish taxa or support taxonomical assignment. An Upper Jurassic plesiosaur rib is, however, comparable in compactness (i.e., degree of osteosclerosis) and microanatomical organization along the shaft to our large nothosaur ribs [START_REF] Street | Evidence of Pachyostosis in the Cryptocleidoid Plesiosaur Tatenectes laramiensis from the Sundance Formation of Wyoming[END_REF].

Striking is the high amount of short fibers along the shaft in many of the rib samples, with a dominance in the medial to distal part of the rib and in general in large nothosaur samples (Table 1). In some samples, the entire periosteal tissue is overprinted by these short fibers (Figs. 4N,O). Sharpey's fibers, which are longer, seem to be restricted to the proximal part of the rib.

The high amount of fibers in some dorsal ribs might be related to their anatomical position (i.e. anterior) and their possible association with thorax musculature and involvement into costal breathing [START_REF] Carrier | The integration of ventilation and locomotion in archosaurs[END_REF][START_REF] Perry | Reconstructing the evolution of the respiratory apparatus in tetrapods[END_REF]. The placodont rib sections are completely free of any fibers, which is probably related to a different (i.e., more posterior)

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anatomical position (i.e., functional signal related to breathing) although taxonomical differences can also not be excluded.

Gastralia

All studied gastralia of Eosauropterygia consist of avascular lamellar bone tissue and are pachyosteosclerotic. They lack any fibres, which might be related to their anatomical position.

The Jurassic marine pleurosaurid Paleopleurosaurus shows a similar osteosclerotic gastralium [START_REF] Klein | Microanatomy and life history in Palaeopleurosaurus (Rhynchocephalia: Pleurosauridae) from the Early Jurassic of Germany[END_REF] as do some ichthyosaurs [START_REF] Kolb | The palaeohistology of the basal ichthyosaur Mixosaurus (Ichthyopterygia, Mixosauridae) from the Middle Triassic: Palaeobiological implications[END_REF]. Conversely, the gastralium of the Upper Jurassic plesiosaur [START_REF] Street | Evidence of Pachyostosis in the Cryptocleidoid Plesiosaur Tatenectes laramiensis from the Sundance Formation of Wyoming[END_REF] is pachyostotic but has a spongious inner structure. A gastralium of early Cretaceous choristoderes also exhibits compact avascular bone but has a small free medullary cavity (Skutschas and Vintenlo, 2017).

Comparison of sauropterygian ribs and vertebrae, with long bones

Placodontia indet.

The sampled placodont rib and vertebra have a comparable microstructure to that of the long bones of fast growing placodont taxa (Klein et al., 2015b(Klein et al., , 2015c) (Table 2). They all share fibrolamellar bone tissue type and a high vascular density. Primary osteons are arranged radially or longitudinally in circumferential rows. A primary mainly radial trabecular structure can locally occur. They also have in common that primary osteons are enlarged by resorption.

Mainly longitudinal primary osteons are sheathed by lamellar bone but the canal is never filled up, which represents a special form of incomplete remodeling (Klein et al., 2015b). Conversely, This article is protected by copyright. All rights reserved.

the thickness of the cortex is in these placodont bones increased by more deposits and an inhibition of resorption, resulting in clear osteosclerotic bones. However, high vascular density and resorption processes act towards the contrary: to bone mass decrease. Thus, bones of fast growing placodonts display a contradicting pattern of localized BMD and BMI in the same bones (Klein et al., 2015b, c).

Calcified cartilage is at midshaft of placodont long bones completely resorbed but persists in the medullary territory of ribs and vertebrae. The placodont rib shows a network of endosteal trabeculae enclosing calcified cartilage.

Considering bone tissue, vascular density, and organization, these placodonts have the highest growth rates among Triassic Sauropterygia, comparable to that of ichthyosaurs (Klein et al., 2015b), which have been sustained swimmers in the open sea [START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF].

Placodonts however, are regarded as slow swimmers or bottom walkers (summarized in Klein et al., 2015b), independently of their armoured or unarmoured body plan. They are all durophageous, feeding on hard shelled, largely sessile invertebrates [START_REF] Scheyer | Revised paleoecology of placodonts-with a comment on 'The shallow marine placodont Cyamodus of the central European Germanic Basin: its evolution, paleobiogeography and paleoecology[END_REF].

All samples of these fast growing placodonts show besides generally pachyostotic bones a similar osteosclerotic microanatomy (i.e. inner bone compaction that results in a small medullary cavity), independently of the environment in which they had been found: Some placodont samples originate from near coastal environments (rib from Winterswijk; long bones from Poland/Anisian) whereas others come from shallow marine environments (vertebra and long bones, Upper Muschelkalk of southern Germany/Ladinian). Beside a certain developmental This article is protected by copyright. All rights reserved. plasticity, the consistency in general histological and microanatomical characters over time (Anisian to Ladinian) is noticeable. It is not understood yet why some placodonts grew with fibrolamellar bone tissue types and others with lamellar zonal bone tissue types (Klein et al., 2015b).

Pachypleurosauria

Serpianosaurus and Neusticosaurus grew with uniform lamellar-zonal bone tissue type [START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF][START_REF] Klein | Growth patterns, sexual dimorphism, and maturation modeled in Pachypleurosauria from Middle Triassic of central Europe (Diapsida: Sauropterygia)[END_REF][START_REF] Sander | Skeletochronology in the small Triassic reptile Neusticosaurus[END_REF] this study). Bone tissue is generally better organized (well organized parallel-fibered bone and lamellar bone) and less vascularized in Serpianosaurus than in N. edwardsii. The latter presents relatively higher vascular density, which is dominated by radial vascular canals and its tissue is dominated by less organized parallel-fibred bone. The persistence of high amounts of calcified cartilage is documented for long bones (at midshaft), vertebrae, and ribs in both taxa (Table 2). Thus, osteosclerosis (BMI) is in Serpianosaurus and Neusticosaurus mostly the result of an incomplete resorption of calcified cartilage and thus incomplete endochondral ossification. The retainment of calcified cartilage at midshaft of long bones distinguishes these taxa from other Sauropterygia, which do not show calcified cartilage at midshaft in long bones. Osteosclerosis is in these taxa always combined to an obvious pachyostosis (summarized in [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. All studied bones of Serpianosaurus and Neusticosaurus (ribs, vertebrae and limb bones) are thus pachyosteosclerotic. Among This article is protected by copyright. All rights reserved.

Triassic Sauropterygia, they show the most numerous adaptive features and highest degree to an aquatic lifestyle.

Considering organization of bone tissue, vascular pattern and density as well as the deposition of regular annual growth marks, these pachypleurosaurs have the lowest growth rates among all Triassic Sauropterygia [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Griebeler, 2016, 2018). Serpianosaurus and Neusticosaurus are small animals (around 1 m) with an elongated body shape (long neck, trunk, and tail), and a small head. They are regarded as axial swimmers, hunting for invertebrates such as arthropods/decapods or small fishes [START_REF] Rieppel | Feeding mechanics in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas[END_REF]. They lived in shallow marine to open sea habitats (Alpine Triassic; [START_REF] Röhl | Microfacies, geochemistry and palaeoecology of the Middle Triassic Grenzbitumenzone from Monte San Giorgio (Canton Ticino, Switzerland)[END_REF].

Anarosaurus heterodontus

Some of the rib samples are assigned to the pachypleurosaur Anarosaurus heterodontus on the basis of its characteristic bone tissue type (Table 1). Bone tissue type as well as microanatomical pattern of Anarosaurus differ largely from those of the pachypleurosaur Dactylosaurus as well as from pachypleurosaurs from the Alpine Triassic (Serpianosaurus-Neusticosaurus clade) [START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF][START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Klein | Growth patterns, sexual dimorphism, and maturation modeled in Pachypleurosauria from Middle Triassic of central Europe (Diapsida: Sauropterygia)[END_REF]. Rib and long bones of Anarosaurus both display moderate osteosclerosis, achieved by an increase of cortical thickness through a reduction of the medullary cavity when compared to the ancestral terrestrial condition.

Anarosaurus is only known from coastal to near coastal environments of Winterswijk and central

Germany [START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. Anarosaurus was clearly less adapted to an aquatic lifestyle than other This article is protected by copyright. All rights reserved. pachypleurosaurs regarding its microanatomy and morphology but had a much higher growth rate as shown by its incipient fibrolamellar bone tissue [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Klein | Growth patterns, sexual dimorphism, and maturation modeled in Pachypleurosauria from Middle Triassic of central Europe (Diapsida: Sauropterygia)[END_REF].

Nothosaurus spp.

Bone tissue type, vascularization pattern and density are consistent between long bones, ribs and vertebrae in nothosaurs [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Klein et al., 2016b;this study). Nothosaur bones are dominated by parallel-fibred tissue that shows different degrees of organization (low to high) and includes even unusual parallel-fibred bone [START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF]Klein et al., 2016b).

Sometimes woven bone is deposited as well, forming-in combination with primary osteonslocally fibrolamellar bone or incipient fibrolamellar bone when primary osteons are only incompletely lined up by endosteal lamellar bone [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]. Vascularization is dominated by longitudinal and radial canals as in other Sauropterygia. Vascular density is in average moderate but can vary. Growth rate in nothosaurs is clearly higher than in modern reptiles and distinctly higher than in pachypleurosaurs from the Alpine Triassic but not as high as in the pachypleurosaur Anarosaurus or in fast growing placodonts [START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF][START_REF] Klein | Long bone histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin provides unexpected implications for phylogeny[END_REF]Griebeler, 2016, 2018;Klein et al., 2016b).

The microanatomical pattern in nothosaur long bones is however not as consistent as in other Sauropterygia (Table 2). Klein et al. (2016b) described five microanatomical categories for Nothosaurus spp. humeri, two of which are unique among amniotes. These categories range from extreme osteosclerotic to very thin-walled cortices and involve different processes (i.e. increase This article is protected by copyright. All rights reserved.

in cortical thickness through a reduction of the medullary cavity; reduction of the cortex due to an enlargement of the medullary cavity, spongy resorption pattern of periosteal bone). The microanatomy of nothosaur ribs is uniform and osteosclerotic, independent if they belong to small or large bodied nothosaur taxa. Vertebrae of small nothosaurs show a moderate osteosclerosis but in large Nothosaurus the structure is cavernous (large cavities in the neural arch and centrum).

In the Anisian, nothosaurs were small to medium sized animals (1.5 m -3 m), but some taxa became rather large during the Ladinian, reaching more than 6 m in body length (summarized in Rieppel and[START_REF] Rieppel | A revision of the genus Nothosaurus (Reptilia: Sauropterygia) from the Germanic Triassic, with comments on the status of Conchiosaurus clavatus[END_REF][START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. They all share a dorsoventrally flattened, elongated body. According to Caroll and Gaskill (1985) the forelimbs were used for propulsion, the hindlimbs for manoeuvring, and the tail helped to the propulsion by undulating.

Nothosaurs had a heterodont dentition, including large fangs that suggest piscivory, although stomach contents also include small-bodied marine reptiles [START_REF] Rieppel | Feeding mechanics in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas[END_REF]. Small-bodied nothosaurs (Nothosaurus marchicus) are known from Anisian near coastal to coastal environments (Winterswijk and Poland) whereas large bodied nothosaurs inhabited shallow marine and open marine environments but some forms may have stayed near coasts as well (Upper Muschelkalk; Klein et al. 2016b). Environmental differences and habitat preferences of individuals might be the main cause for microanatomical diversity as is observed in some modern marine mammals. However, taxonomic differences and developmental plasticity must be This article is protected by copyright. All rights reserved. considered, too. Additionally, it could well be that some large nothosaurs would exhibit osteosclerotic vertebrae but that they have simply not been sampled yet (sampling bias).

The rib Wijk07-31 shows a very highly organized avascular bone tissue, which does not evoke that of Nothosaurus marchicus or Anarosaurus. Size and high tissue organization might fit to bone tissue of Lariosaurus [START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF], whereas the limited osteosclerosis does not fit to Ceresiosaurus [START_REF] Hugi | The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy)[END_REF], the sister group of Lariosaurus (Rieppel;[START_REF] Rieppel | Sauropterygia I-Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea[END_REF]. It also differs from the microanatomical patterns of N. marchicus and Anarosaurus.

Other marine reptiles

Other marine reptiles, such as the Permian Claudiosaurus [START_REF] Buffrénil V De | Bone histology of Claudiosaurus germaini (Reptilia, Claudiosauridae), and the problem of pachyostosis in aquatic tetrapods[END_REF] and Mesosaurus (NK pers. obs.), some Late Cretaceous marine squamates [START_REF] Houssaye | Palaeoecological and morphofunctional interpretation of bone mass increase: an example in Late Cretaceous shallow marine squamates[END_REF]) also show strongly pachyosteosclerotic bones. However, they do not retain calcified cartilage in their medullary regions but rather achieved osteosclerosis through either intensive remodelling with excessive secondary bone deposits or through an inhibition of bone resorption, sometimes accompanied by an inhibition of calcified cartilage but only in the trabeculae of endochondral origin (see also Table 2). This article is protected by copyright. All rights reserved.

CONCLUSIONS

All gastralia are osteosclerotic with a thick avascular cortex, and the lack of a medullary cavity in most cases.

All sauropterygians share an inhibition of remodeling (see also Klein et al. 2015bKlein et al. ,c, 2016b)). Some resorption may occur (mainly in the placodont and some large nothosaur bones) but redeposition of bone does not occur at all or is incomplete. The processes leading to osteosclerosis in Sauropterygia are different to that observed in diapsid marine reptiles form the Permian (i.e. Claudiosaurus, [START_REF] Buffrénil V De | Bone histology of Claudiosaurus germaini (Reptilia, Claudiosauridae), and the problem of pachyostosis in aquatic tetrapods[END_REF]Mesosaurus, Ricqlès, 1976;NK pers. obs.) where an imbalanced remodeling with excessive secondary bone deposits leads to osteosclerosis. Ichthyosaurs differ in general due to the occurrence of a secondarily compacted spongiosa near the growth center in long bones conferring BMI in this region, whereas the rest of the bone organization remains spongious and rather evokes BMD [START_REF] Houssaye | A new look at ichthyosaur long bone microanatomy and histology: Implications for their adaptation to an aquatic life[END_REF].

Mosasaurs tendentially show BMD, except for the humerus of Dallasaurus [START_REF] Houssaye | Microanatomical and histological features in the long bones of mosasaurine mosasaurs (Reptilia, Squamata)-implications for aquatic adaptation and growth rates[END_REF].

However, the results of our work can only be preliminary because of taxonomical uncertainties, and due to small sample size, variations along the vertebral column and the rib cage cannot be addressed adequately. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved. Table 2. Overview about microanatomical patterns in Sauropterygia and other marine reptiles. Please note that all centra of Sauropterygia vertebrae share a large central cavity (except in the pachypleurosaurs Neusticosaurus and Serpianosaurus), which is so far unique among amniotes.

Information for the long bones are taken from other studies cited in the first column. Abbreviations: cc = calcified cartilage; po = primary osteons. This article is protected by copyright. All rights reserved.

  observed histological variability reflects differences in sampling location along the rib shaft as well as taxonomical differences and different anatomical positions and thus function of the ribs in the trunk region (e.g., presence of fibers). A correlation of the development of the spongiosa in the medullary region with size as Canoville et al. (2016) found, was not documented for Triassic Sauropterygia.

  The microanatomy and histology of studied long bones, ribs, and vertebrae are in general consistent within taxa. Except for some large Nothosaurus specimens, bones of Triassic Sauropterygia exhibit generally osteosclerosis and thus BMI. The pachypleurosaurs This article is protected by copyright. All rights reserved. Neusticosaurus and Serpianosaurus show the strongest osteosclerosis, coupled with pachyostosis. The pachypleurosaur Anarosaurus and Nothosaurus marchicus show moderate osteosclerosis. Placodonts also have pachyosteosclerotic bones due to the increase of cortex thickness but resorption of vascular spaces counteracts this on the histological level. In large Nothosaurus osteosclerosis is lowest or in some even not present, respectively.We identified three main processes in how osteosclerosis is achieved in Triassic sauropterygians: 1) In the pachypleurosaurs Neusticosaurus and Serpianosaurus osteosclerosis is in all bones (ribs, vertebrae, and long bones) the result of incomplete endochondral ossification, i.e. the persistence of high amounts of calcified cartilage. 2) In long bones of fast growing placodonts and of some large Nothosaurus, osteosclerosis is the result of a strong increase in cortical thickness through a reduction of the medullary cavity and an inhibition of bone resorption. Calcified cartilage can also occur but is only limited. 3) In the placodont rib and vertebra some calcified cartilage is retained but also not in the same amount as in Neusticosaurus and Serpianosaurus. The pachypleurosaur Anarosaurus and the small nothosaur Nothosaurus marchicus show a similar pattern: no calcified cartilage at the midshaft of long bones but in the ribs and vertebrae. Both taxa achieve osteosclerosis by a moderate increase in cortical thickness through the reduction of the medullary cavity and by an inhibition of remodeling. All large nothosaur ribs are osteosclerotic; they retain some calcified cartilage and show an increase in cortical thickness, whereas the vertebrae of large nothosaurs are cavernous.

Figure 1

 1 Figure 1 Microanatomical overview of sauropterygian vertebrae. A, Composite microscopic pictures of longitudinal section of dorsal centrum Wijk08-47, Eosauropterygia, depicting compact periosteal cones and less compact endochondral cones. B, Transverse section of complete cervical vertebra Wijk13-88, Eosauropterygia, depicting compact periosteal bone in the dorsal-lateral part of the neural arch and in the mediodorsal and ventral part of the centrum.

Figure 2

 2 Figure 2Vertebrae histology of Sauropterygia

Figure 3

 3 Figure 3 Overview of sauropterygian rib microanatomy. Serial cross-sections of dorsal ribs sampled from the proximal (rib head) to the distal end, as shown in A (in normal light). A1-4, Composite microscopic pictures of placodont rib Wijk16-GG. B1-8, Rib Wijk07-31 (Eosauropterygia). C1-3, Composite microscopic pictures of rib SMNS 54434 (Eosauropterygia) in normal light. D1-6, Composite microscopic pictures of rib Wijk09-557 (Eosauropterygia) in normal light except for D-2 which is in polarized light. E1-4, Composite microscopic pictures of rib Wijk09-382 (Eosauropterygia) in normal light. F1-3, Composite microscopic pictures of rib SMNS 84393 (Nothosaurus sp.) in normal light. G1-4, Composite microscopic pictures of rib SMNS 80266 (Nothosaurus sp.) in normal light.

Figure 4

 4 Figure 4

Figure 5 .

 5 Figure 5. Details of medullary regions and endosteal bone. A, Medial sample of the placodont rib (Wijk16-GG) in normal and B, polarized light. Note the large medullary region largely filled by trabeculae that enclose calcified cartilage. C, Detail of calcified cartilage in the placodont rib Wijk16-GG in normal (right side) and polarized (left side) light. D, Detail of medullary cavity largely filled by endosteal bone and surrounded by a sharp line and calcified cartilage in proximal rib sample of Wijk10-130 (aff. Anarosaurus) E, Detail of medullary region in the proximal rib of ?Nothosaurus marchicus (Wijk09-382) in polarized light. The cavity is here only partially filled by endosteal bone but also surrounded by a sharp line and calcified cartilage. F, Transition between medullary cavity lined by a thin layer of endosteal bone and periosteal bone in proximal rib sample of large Nothosaurus (SMNS 96928) in normal light. G, Medullary region consisting of large erosion cavities scattered into the periosteal bone in proximal rib sample of large Nothosaurus (SMNS 80266) in normal light. H, Medullary region consisting of large erosion cavities lined by endosteal bone in medial rib sample of large Nothosaurus (SMNS 80266) in normal light. I, Medullary region filled by calcified cartilage and erosion cavities lined by endosteal bone in medial sample of SMNS 54434 (Eosauropterygia) that is surrounded by a sharp line. J, Cross section of the medial part of the rib of ?Nothosaurus marchicus (Wijk09-382) in polarized light. Note the already reduced cortex and the medullary region filled by calcified cartilage and endosteal bone. K, Distal sample of large Nothosaurus (SMNS 81881) in polarized light. The medullary region is surrounded by a thin layer of periosteal cortex.

Figure 6 .

 6 Figure 6. A-M, Rib cross sections of the pachypleurosaurs Serpianosaurus and Neusticosaurus, and of the nothosaur Ceresiosaurus, all from the Alpine Triassic. N-P, Osteosclerotic gastralia of Eosauropterygia. Samples are figured in normal light if not mentioned otherwise. A-D, Samples of dorsal ribs of Serpianosaurus. A, Proximal part of a dorsal rib (PIMUZ T 1565). B, Proximal part of a dorsal rib PIMUZ T 1564). C, Medial part of a dorsal rib (PIMUZ T 131). D, Distal part of a dorsal rib (PIMUZ T 1564). E-I, Samples of dorsal ribs of Neusticosaurus edwardsii. E, Proximal part of a dorsal rib (PIMUZ T 4757). F, Medial part of a dorsal rib (PIMUZ T phz 14). G, Medial part of a dorsal rib (PIMUZ T 4748). H, I, Distal part of a dorsal rib (PIMUZ T 4757) in normal and polarized light. J-M, Samples of dorsal ribs of Ceresiosaurus. J, Rib head of a dorsal rib (PIMUZ T phz 296). K, Proximal part of a dorsal rib (PIMUZ T 5622). L, Medial part of a dorsal rib (PIMUZ T 5622). M, Distal part of a dorsal rib (PIMUZ T 5153). N1-2, Proximal and medial cross section of a gastralium (SMNS 54413).

(

  compact neural arch with large ventral cavities at each side; spongious pfb, unusual pfb, radial svc; moderate vascularity aff. Nothosaurus marchicus This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ar.24140

  medium sized cavity surrounded by compact osteosclerosis/BMI change in distribution of periosteal and moderate osteosclerosis /BMI This article is protected by copyright. All rights reserved.

Table 1 .

 1 Stratigraphic age, locality information, and measurements as well as histological and microanatomical information of sampled ribs and vertebrae. Abbreviations: Lb, lamellar bone; pfb, parallel-fibred bone; FLB, fibrolamellar bone tissue po, primary osteons; svc, simple vascular canals;

	Repository number/	locality & age	height x	microanatomy	histology	taxonomical
	anatomical position		width (cm)			assignment
	Vertebrae
	Placodontia
	StIPB R 68b	Bayreuth	>5.65 x 4.35	osteosclerosis; thick compact cortex but spongious	FLB with high radial vascular density	?Cyamodus
	incomplete dorsal	early Ladinian		by vascular system; triangular small centrum	(incl. primary trabeculae); resorption of
				cavity; spongious facet tissue	primary osteons
	Nothosauridae
	StIPB R 52	Bayreuth	2.85 x ~4.2	large centrum cavity surrounded by a thin layer of	mainly pfb, locally incipient FLB;	Nothosaurus sp.
	dorsal centrum	early Ladinian		compact bone; spongious facet tissue	radially svc, incompletely and
					completely lined po
	StIPB R 68a	Bayreuth	>4.55 x 4.3	cavernous neural arch; spongious facet tissue; large	mainly pfb, locally incipient FLB;	Nothosaurus sp.
	incomplete ?sacral	early Ladinian		centrum cavity, surrounded by a thin layer of	radially svc, incompletely and
				compact bone	completely lined po
	SMNS 54418	Gogolin/Poland	3.7 x ~2.0	compact neural arch with a central cavity; large	no thin section available	Nothosaurus sp.
	complete dorsal	Bithnyian		spongious medullary region (nearly empty cavity)
				surrounded by a thin layer of compact bone
	Wijk13-88	Winterswijk	2.3 x 2.4	moderate osteosclerosis; compact neural arch
	complete cervical	Anisian

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ACKNOWLEDGMENTS

We are grateful to E. Maxwell, R. Schoch (both SMNS), M. Sander (StIPB), and T. Scheyer (PIMUZ) for the permission to sample specimens or use existing samples under their care.

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Ch. Wimmer-Pfeil (SMNS) and O. Dülfer (StIPB) are acknowledged for the production of thin sections. We also would like to thank P. Skutschas (SPBU), D. Surmik (WNOZ), as well as an anonymous reviewer for their helpful comments and suggestions.

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O1-2, Proximal and medial cross section of a gastralium (Wijk09-27).

P, Cross section of a large gastral rib of Nothosaurus (SMNS 81882).

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Abbreviations

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