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The Middle Triassic Luoping Biota in south-west China represents the inception of modern marine ecosystems, with abundant and diverse arthropods, fishes and marine reptiles, indicating recovery from the Permian–Triassic mass extinction. Here we report a new specimen of the predatory marine reptile Diandongosaurus, based on a nearly complete skeleton. The specimen is larger than most other known pachypleurosaurs, and the body shape, caniniform teeth, clavicle with anterior process, and flat distal end of the anterior caudal ribs show its affinities with Diandongosaurus acutidentatus, while the new specimen is approximately three times larger than the holotype. The morphological characters indicate that the new specimen is an adult of D. acutidentatus, allowing for ontogenetic variation. The fang-like teeth and large body size confirm it was a predator, but the amputated hind limb on the right side indicate itself had been predated by an unknown hunter. Predation on such a large predator reveals that predation pressure in the early Mesozoic was intensive, a possible early hint of the Mesozoic Marine Revolution.

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 consists of a compact periosteal cortex surrounding an inner endosteal territory. Toward the distal end of the ribs, the periosteal thickness decreases whereas the endosteal territory increases. Despite a shift from periosteal versus 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 remodeling 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 remodeling or inhibition of bone resorption leads to osteosclerosis. Besides differences regarding the microanatomy, all studied bones of a taxon are consistent in their bone tissue type. Anat Rec, 302:1770-1791, 2019. © 2019 American Association for Anatomy.

Nothosauria and Pachypleurosauria inhabited shallow marine environments of the Tethys Ocean during the Early to Middle Triassic. They shared a similar elongated body shape including long necks and tails and dorsoventrally flattened skulls. Pachypleurosaurs are small marine reptiles whereas Nothosauria include small to large forms, all following an active predatory lifestyle. Pachypleurosaurs swam by tail-driven propulsion whereas nothosaurs used paraxial locomotion in which the forelimb was actively involved in propulsion. Generally, the main tissue deposited is parallel-fibered bone but both groups show a certain intraspecific variability and developmental plasticity. Pachypleurosaurs and nothosaurs grew in general with lamellar-zonal bone tissue that is low to moderately vascularized in most pachypleurosaurs and usually moderately vascularized in nothosaurs. The pachypleurosaur Anarosaurus heterodontus, however, shows well-vascularized incipient fibrolamellar bone tissue and some nothosaurs show local layers of fibrolamellar bone. Humeri of pachypleurosaurs are osteosclerotic with a medullary cavity reduced or filled in by endosteal bone. Nothosaurs show a wide spectrum of microanatomical patterns spanning from thick-walled osteosclerotic to thin-walled (almost bird-like) humeri, most likely reflecting differences in habitat and swimming style.

Having provided dozens of articulated, and even more disarticulated skeletons of marine reptiles with varying degrees of completeness, the Middle Triassic fossil deposit of Winterswijk, The Netherlands, might be considered the richest within the entire Germanic Basin concerning associated and partially articulated remains. Aside from these remains, a high number of isolated bones are also found. Among the various marine reptile taxa, the pachypleurosaur Anarosaurus and the nothosaur Nothosaurus are the most common. The skeletons and remains show diverse disarticulation patterns. These patterns range from mostly complete articulated skeletons to clustered and isolated bones. Interestingly, a large number of isolated articulated limbs is found, a pattern which is not known from other marine localities. Although numerous reports have been made on the taxa from Winterswijk, the taphonomy of the locality still remains poorly understood. Here, a series of methods are used to describe the taphonomy of 327 marine reptile specimens from the locality of Winterswijk in a qualitative manner. Furthermore, it is tested whether biofilms such as microbial mats could have had an influence on the preferential preservation of several body regions. Upon investigation, it is concluded that the taphonomy of Winterswijk is very complex and indeed depends on various factors such as: current activity, microbial activity, and anatomical differences between taxa. Finally, a total of 12 recurring patterns were identified within the dataset, which could largely be explained by the aforementioned factors.

Bone tissue, microanatomy, and growth are studied in humeri of the pachypleurosaurs Dactylosaurus from the early Anisian of Poland and of aff. Neusticosaurus pusillus from the Lettenkeuper (early Ladinian) of southern Germany. Histology and modeled growth curves are compared to already published data of other pachypleurosaurs. Therefore, we herein established growth curves for Anarosaurus from the middle Anisian of Winterswijk (the Netherlands) and for pachypleurosaurs from the Anisian/Ladinian of the Alpine Triassic (i.e., Neusticosaurus spp. and Serpianosaurus). Humeri of Dactylosaurus, Anarosaurus, and aff. N. pusillus, all from the Germanic Basin, usually display an inner ring of (pre-)hatchling bone tissue. In some samples this tissue is surrounded by a layer of perpendicularly oriented fine fibers, which could indicate the start of active locomotion for foraging or might be related to viviparity. However, pachypleurosaurs from the Alpine Triassic do not show this tissue. This in turn could be related to overall differences in the environments inhabited (Germanic Basin vs. Alpine Triassic). Histological comparison revealed distinct taxon-specific differences in microanatomy and bone tissue type between Anarosaurus on the one hand and Dactylosaurus and the Neusticosaurus–Serpianosaurus clade on the other hand. Microanatomical differences imply a different degree in secondary adaptation to an aquatic environment. Life-history traits derived histologically and obtained from modeling growth were in general rather similar for all studied pachypleurosaurs. Onset of sexual maturation was within the first third of life. Asymptotic ages (maximum life span) considerably exceeded documented and modeled ages at death in all pachypleurosaur taxa. All traits modeled (more or less) matched values seen in similar-sized extant reptiles. Growth curves revealed differences in growth and maturation strategies within taxa that could indicate sexual dimorphism expressed in different adult sizes and a different onset of sexual maturation. Differences in gender size and morphology is well documented for the Chinese pachypleurosaur Keichousaurus and for Neusticosaurus spp. from the Alpine Triassic. Birth-to-adult size ratios of herein studied pachypleurosaurs were consistent with those seen in other viviparous Sauropterygia, other viviparous extinct taxa as well as extant viviparous reptiles. Anarosaurus had the highest maximum growth rates of all pachypleurosaurs studied, which best conformed to those seen in today's similar-sized reptiles and is expected from its bone tissue type. The other pachypleurosaur taxa had lower rates than the average seen in similar-sized extant reptiles. We hypothesize from our data that the considerably higher asymptotic ages compared to ages at death, early onset of maturation compared to asymptotic age, and viviparity reflect that pachypleurosaurs lived in predator-dominated environments.

An eosauropterygian skeleton found in the Middle Triassic (upper Anisian) Gutenstein Formation of the Fatric Unit (Demänovská dolina Valley, Low Tatra Mountains, Slovakia) represents the earliest known occurrence of marine tetrapods in the Western Carpathians. The specimen represents a partly articulated portion of the postcranial skeleton (nine dorsal vertebrae, coracoid, ribs, gastral ribs, pelvic girdle, femur and one zeugopodial element). It is assigned to the Pachypleurosauria, more precisely to theSerpianosaurus–Neusticosaurusclade based on the following combination of features: (1) small body size; (2) morphology of vertebrae, ribs and femur; (3) tripartite gastral ribs; and (4) microanatomy of the femur as revealed by μCT. Members of this clade were described from the epicontinental Germanic Basin and the Alpine Triassic (now southern Germany, Switzerland, Italy), and possibly from Spain. This finding shows that pachypleurosaur reptiles attained a broader geographical distribution during the Middle Triassic, with their geographical range reaching to the Central Western Carpathians. Pachypleurosaurs are often found in sediments formed in shallow, hypersaline carbonate-platform environments. The specimen found here occurs in a succession with vermicular limestones in a shallow subtidal zone and stromatolitic limestones in a peritidal zone, indicating that pachypleurosaurs inhabited hypersaline, restricted carbonate ramps in the Western Carpathians.

The postcranial morphology of Nothosaurus from Winterswijk is described on the basis of three partially preserved skeletons. Because of the association with a diagnostic cranium, two of these can be confidently assigned to N. marchicus. Preserved humeri are recognized as morphotype II, which confirms the relationship between this humeral morphotype and N. marchicus. The occurrence of an additional nothosaur taxon in the Lower Muschelkalk, strong sexual dimorphism, or a broader size range of N. marchicus is evidenced again. The postcranial morphology of Nothosaurus is compared to published and new data on the basal pistosauroid (cf. Cymatosaurus) from the same locality. Numerous shared morphological characters of N. marchicus, Nothosaurus sp., and the basal pistosauroid from Winterswijk hamper assignment of isolated bones, which is presently only possible if a combination of features on multiple bones from a single individual can be assessed. Ontogenetic stage of the described skeletons is discussed as well. Differences between N. marchicus, the basal pistosauroid, and the morphologically similar, but smaller pachypleurosaur A. heterodontus are mainly related to the morphologies of the humerus, ulna, and the clavicle-interclavicle complex, and thereby indicate different modes of locomotion i.e., swimming styles. The pachypleurosaur and the basal pistosauroid were most likely anguilliform swimmers, whereas Nothosaurus also employed its forelimbs during swimming (paraxial swimming). Taphonomical observations on material of the Winterswijk locality indicate the presence of scavengers, although most disarticulated skeletons were transported by water and decayed and disintegrated in shallow water.

The Xingyi Fauna has been known widely for yielding abundant specimens of the pachypleurosaur sauropterygian Keichousaurus hui, the first Triassic marine reptile described from China (Young, 1958)....

Sauropterygia from the Muschelkalk are only found in lag deposits known as bone beds, and most of the material consists of isolated bones. Alpha taxonomy of Sauropterygia from the Germanic Basin which include Pachypleurosauria is thus based mainly on skull morphology of a few specimens. Articulated or associated postcranial material of pachypleurosaurs, associated with diagnostic skull material, is very rare in the Germanic Basin and currently occurs in larger numbers only in the Lower Muschelkalk of Winterswijk (Gelderland Province, The Netherlands), which continuously produces new material. For the first time, the morphology of several partially articulated skeletons of the pachypleurosaur Anarosaurus heterodontus is described and compared. Some of those specimens have skull material attached; others were identified as pachypleurosaurs on the basis of their long bone histology. The current study revealed that postcranial bones of A. heterodontus feature a diverse morphology reflecting differences during ontogeny. Thus, A. heterodontus specimens could be assigned to size classes (I–III). However, on the basis of morphology, histology, and maximal known size of isolated skulls and humeri, none of these specimens represent fully grown individuals. Growth mark counts of midshaft-femur samples, morphologically assigned to size class III, document that this size class was reached within the first year of life. Size class III continued into the second year of life, and then afterwards skeletal maturity was reached. Thus, a juvenile A. heterodontus grew very fast, which is also indicated by its bone tissue type, composed of a high number of radial vascular canals and a fast-deposited bone matrix. The assignment of isolated bones from Lower to Middle Muschelkalk localities to A. heterodontus is now possible with an extensive amended diagnosis of this taxon. This largely contributes to the understanding of taxonomical diversity and distribution. Morphological comparison of the postcranial skeleton of A. heterodontus with that of the two other valid pachypleurosaurs from the Germanic Basin, Anarosaurus pumilio and Dactylosaurus, supports their close phylogenetic relationship. Furthermore, the skeleton of A. heterodontus has no morphological or histological aquatic adaptation such as pachyostosis or pachyosteosclerosis and thus represents the least degree of aquatic adaptation within Pachypleurosauria.

© 2015 L. Xing et al. Keichousaurus hui is a small pachypleurosaur (Reptilia: Sauropterygia) from the Triassic of China. Many specimens of various growth stages are known, making them ideal for ontogenetic research. We report 22 new specimens from the Middle Triassic of Xingyi (Guizhou, south China), and combined their skeletal measurements with those from 85 published specimens to analyze the ontogenetic trajectory of sexual dimorphism. An Exploratory Factor Analysis suggests that the largest factors behind morphological disparity within the species are body size followed by gender. Sexual dimorphism is most clearly reflected in selected skeletal ratios that are more pronounced in males than in females. We found that the relative length of femur to body size was useful in gender identification, in addition to three ratios that are traditionally used, namely a distal expansion of the humerus relative to its shaft, humerus length relative to body size, and humerus length relative to femur length. Two distinctive patterns exist in allometric changes of these four ratios. The distal expansion of the humerus is exceptional in that it is equally pronounced in juvenile and adult males and therefore must have been fully established during embryonic growth. The other three features are not pronounced at birth size and subsequently become pronounced during postembryonic growth. However, males and females already show different growth trajectories at birth size even in these three. Therefore, the fate of sexually dimorphic features seems to have already been set during embryonic growth in K. hui.

A Middle Triassic vertebrate fauna from the Jilh formation in Saudi Arabia is described. Although the material is very fragmentary, the occurrence of a wide taxonomic variety of taxa can be documented, including hybodontiform sharks, actinopterygians, and sauropterygian reptiles such as a pachypleurosaur, Simosaurus, nothosaurs and cyamodontoid placodonts. The fauna compares well with other Middle Triassic faunas from the Germanic and Alpine Triassic, and from the northern Gondwanan shelf, in particular with the fauna from Makhtesh Ramon, Negev, Israel.

Sauropterygians form the largest and most diverse group of ancient marine reptiles that lived throughout nearly the entire Mesozoic era (from 250 to 65 million years ago)1,2. Although thousands of specimens of this group have been collected around the world since the description of the first plesiosaur in 1821 (ref. 3), no direct evidence has been found to determine whether any sauropterygians came on shore to lay eggs (oviparity) like sea turtles, or gave birth in the water to live young (viviparity) as ichthyosaurs and mosasauroids (marine lizards) did4,5,6. Viviparity has been proposed for plesiosaur, pachypleurosaur and nothosaur sauropterygians7,8,9,10, but until now no concrete evidence has been advanced. Here we report two gravid specimens of Keichousaurus hui Young from the Middle Triassic of China. These exquisitely preserved specimens not only provide the first unequivocal evidence of reproductive mode and sexual dimorphism in sauropterygians, but also indicate that viviparity could have been expedited by the evolution of a movable pelvis in pachypleurosaurs. By extension, this has implications for the reproductive pattern of other sauropterygians and Mesozoic marine reptiles that possessed a movable pelvis.

BackgroundSauropterygia is an abundant and successful group of Triassic marine reptiles. Phylogenetic relationships of Triassic Sauropterygia have always been unstable and recently questioned. Although specimens occur in high numbers, the main problems are rareness of diagnostic material from the Germanic Basin and uniformity of postcranial morphology of eosauropterygians. In the current paper, morphotypes of humeri along with their corresponding bone histologies for Lower to Middle Muschelkalk sauropterygians are described and interpreted for the first time in a phylogenetic context.Methodology/principal findingsNothosaurus shows a typical plesiomorphic lamellar-zonal bone type, but varying growth patterns and the occurrence of a new humerus morphotype point to a higher taxonomic diversity than was known. In contrast to the enormous morphological variability of eosauropterygian humeri not assigned to Nothosaurus, their long bone histology is relatively uniform and can be divided into two histotypes. Unexpectedly, both of these histotypes reveal abundant fibrolamellar bone throughout the cortex. This pushes the origin of fibrolamellar bone in Sauropterygia back from the Cretaceous to the early Middle Triassic (early Anisian). Histotype A is assigned to Cymatosaurus, a basal member of the Pistosauroidea, which includes the plesiosaurs as derived members. Histotype B is related to the pachypleurosaur Anarosaurus. Contrary to these new finds, the stratigraphically younger pachypleurosaur Neusticosaurus shows the plesiomorphic lamellar-zonal bone type and an incomplete endochondral ossification, like Nothosaurus.Conclusions/significanceHistological results hypothetically discussed in a phylogenetical context have a large impact on the current phylogenetic hypothesis of Sauropterygia, leaving the pachypleurosaurs polyphyletic. On the basis of histological data, Neusticosaurus would be related to Nothosaurus, whereas Anarosaurus would follow the pistosaur clade. Furthermore, the presence of fibrolamellar bone, which is accompanied with increased growth rates and presumably even with increased metabolic rates, already in Anarosaurus and Cymatosaurus can explain the success of the Pistosauroidea, the only sauropterygian group to survive into the Jurassic and give rise to the pelagic plesiosaur radiation.

Pachypleurosaur material from the Middle Triassic ‘ Grenzbitumen’-horizon (Anis-Ladin boundary) of Monte San Giorgio, Kanton Tessin, Switzerland, is described as a new genus and species,Serpianosaurus mirigiolensis.A detailed morphological description is followed by a quantitative analysis. The taxon differs from other pachypleurosaurids primarily by its relatively large skull and its straight lower jaw. Rib pachyostosis is absent. In most characters the taxon remains plesiomorphous with respect to other pachypleurosaurs from the Middle Triassic deposits of Monte San Giorgio, which accords well with its early stratigraphic occurrence at that locality. Sexual dimorphism is expressed by the size and shape of the humerus. A cladistic analysis shows the Pachypleurosauridae to constitute the sistergroup of all other Sauropterygia. The Sauropterygia and the Placodontia together form a monophyletic group, the Euryapsida, which is subordinated to the Diapsida and to the Neodiapsida, but which is classifed outside the archosauromorph-lepidosauromorph dichotomy within the Neodiapsida. A concluding paragraph discusses the status of the pachypleurosaurs within the Sauropterygia. Skull structure, and jaw mechanics in particular, suggest a relatively plesiomorph position of pachypleurosaurs within the Sauropterygia. Their adaptation to an aquatic mode of life was not carried to the degree observed in other sauropterygians, and the ability for rapid and deep dives seems to have been limited by middle-ear structure.