Primary coarse cancellous bone in <i>Grippia longirostris</i> and another ichthyosauriform from the Lower Triassic of Svalbard

The morphology, osteohistology, and microanatomy of two humeri, one assigned to Grippia longirostris and one left in open nomenclature (Ichthyosauriformes indet.), are described. The humerus of Grippia longirostris was collected (by us in August 2022) from the bonebed at the base of the ‘Grippia Niveau’ interval and the other humerus about 3 m above, at the bonebed level of the the ‘Grippia Niveau’ interval (late Olenekian, Lower Triassic). Both humeri share in general a fast deposited tissue and a primary coarse cancellous structure (i.e. low bone compactness). In the humerus of Grippia, the only secondary structures are some large erosion cavities. In the larger humerus (Ichthyosauriformes indet.), the innermost cortex has some secondary trabeculae. Although depicting the typical microanatomy and fast deposited tissue implying a high metabolic rate, as was described for other ichthyosauriformes and ichthyosaurs, both humeri differ from each other. The humerus of Grippia is similar to that of Utatsusaurus from Japan but the other humerus shows a unique morphology and microanatomy. This humerus cannot be assigned confidently to one of the already described taxa from Svalbard and it might represent a new taxon. In any case, these two samples document again a high degree of adaptation to the aquatic environment shortly after the origin of the group.