Bone tissue, microanatomy, and growth are studied in humeri of thepachypleurosaurs Dactylosaurus from the early Anisian of Poland and ofaff. Neusticosaurus pusillus from the Lettenkeuper (early Ladinian) ofsouthern Germany. Histology and modeled growth curves are compared toalready published data of other pachypleurosaurs. Therefore, we herein established growth curves for Anarosaurus from the middle Anisian ofWinterswijk (the Netherlands) and for pachypleurosaurs from the Anisian/Ladinian ofthe Alpine Triassic (i.e., Neusticosaurus spp. andSerpianosaurus). Humeri of Dactylosaurus,Anarosaurus, and aff. N. pusillus, all from the GermanicBasin, usually display an inner ring of (pre-)hatchling bone tissue. In somesamples this tissue is surrounded by a layer of perpendicularly oriented finefibers, which could indicate the start of active locomotion for foraging ormight be related to viviparity. However, pachypleurosaurs from the AlpineTriassic do not show this tissue. This in turn could be related to overalldifferences in the environments inhabited (Germanic Basin vs. AlpineTriassic). Histological comparison revealed distinct taxon-specificdifferences in microanatomy and bone tissue type between Anarosauruson the one hand and Dactylosaurus and theNeusticosaurus–Serpianosaurus clade on the other hand.Microanatomical differences imply a different degree in secondaryadaptation to an aquatic environment.

Life-history traits derived histologically and obtained from modeling growthwere in general rather similar for all studied pachypleurosaurs. Onset ofsexual maturation was within the first third of life. Asymptotic ages(maximum life span) considerably exceeded documented and modeled ages atdeath in all pachypleurosaur taxa. All traits modeled (more or less) matched values seen in similar-sized extant reptiles. Growth curves revealeddifferences in growth and maturation strategies within taxa that couldindicate sexual dimorphism expressed in different adult sizes and a differentonset of sexual maturation. Differences in gender size and morphology is welldocumented for the Chinese pachypleurosaur Keichousaurus and forNeusticosaurus spp. from the Alpine Triassic. Birth-to-adult sizeratios of herein studied pachypleurosaurs were consistent with those seen inother viviparous Sauropterygia, other viviparous extinct taxa as well as extantviviparous reptiles. Anarosaurus had the highest maximum growthrates of all pachypleurosaurs studied, which best conformed to those seen intoday's similar-sized reptiles and is expected from its bone tissue type. Theother pachypleurosaur taxa had lower rates than the average seen insimilar-sized extant reptiles.

We hypothesize from our data that the considerably higher asymptotic agescompared to ages at death, early onset of maturation compared to asymptoticage, and viviparity reflect that pachypleurosaurs lived in predator-dominatedenvironments.