Figure 17. Hypothetical phylogenetic relationships of Sauropterygia based on long bone histology.

Patterns of bone tissue types in Sauropterygia long bones contradict the current phylogenetic hypothesis based on morphology as is shown in figure 1. Solely on the basis of the bone histological data, pachypleurosaurs are polyphyletic because Neusticosaurus shows lamellar-zonal bone whereas Anarosaurus heterodontus has incipient fibrolamellar bone. Cymatosaurus already shows a radial vascular pattern and distinct layers of fibrolamellar bone within the parallel-fibered bone tissue. Placodonts have the most advanced bone tissue within Sauropterygia. LZB  =  Lamellar-zonal bone tissue type, FLB  =  Fibrolamellar bone tissue type. Data for pistosaurs and plesiosaurs are taken from the literature [24], [25]. This hypothesis is not yet supported by a data matrix or phylogenetic analysis and thus remains provisional.