Skeletal microstructure of Stenopterygius quadriscissus (Reptilia, Ichthyosauria) from the Posidonienschiefer (Posidonia Shale, Lower Jurassic) of Germany

Abstract

Ichthyosaurians (Ichthyosauria) are a major clade of secondarily aquatic marine tetrapods that occupied several major predatory niches during the Mesozoic Era. Multiple lines of evidence including isotopic, body shape and swimming modality analyses suggest they exhibited elevated growth and metabolic rates, and body temperatures. However, applications of osteohistological methods to test hypotheses regarding their physiology are few. Previous studies focused on the humeri, vertebrae and ribs from a small number of taxa. Here, we use osteohistological methods to describe the bone microstructure of over 30 cranial and post‐cranial elements from a nearly complete, articulated individual of Stenopterygius quadriscissus from the Posidonienschiefer Formation (Posidonia Shale, Lower Jurassic) of Germany. The specimen shows highly vascularized primary bone and spongious secondary bone in its limbs, suggesting an overall shift to a lighter spongious‐structured skeleton was achieved through multiple developmental mechanisms. The modified perichondral ossification in elements of the limbs distal to the stylopodium informs our understanding of functional morphology, including hydrodynamic forces on the paddles. The ribs show variation in cortical thickness and trabecular organization along their length. Cyclical growth is inferred from changes in vascularization and osteocyte density as well as the presence of annuli in primary fibrolamellar bone. Cranial elements, due to their relative density and better preservation of growth marks, may prove to be of particular importance in future skeletochronological studies of post‐Triassic ichthyosaurians. We infer and corroborate hypotheses of elevated growth rates and metabolic rates in ichthyosaurians, and the potential for thermoregulation similar to extant homeothermic ectotherms.

Is the hyoid a constraint on innovation? A study in convergence driving feeding in fish-shaped marine tetrapods

“…Interestingly, the same overall structure is found in the other odontocete studied here, Lagenorhynchus albirostris . In one ichthyosaur, a Lower Jurassic Stenopterygius , rostral elements (dentary and premaxilla) were more compact than any of the sampled postcranial elements (Anderson et al 2018).…”

The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK

“…In both BRSMG-Cb-3870 [ 16 , 18 ] and our specimen, as well as in BRSMG Cg2488 R-101 [ 18 ] and BRSMG Cb3869 [ 16 , 18 ], the occurrence of multiple open periosteal canals and high vascularization in the outer cortex indicate that growth by periosteal apposition was not ceased at the moment of death. The few histological analysis carried on ichthyosaur rostra found them to be more compact than postcranial material [ 35 ]. We can therefore support that the high vascularization is a genuine indicator of active growth rather than being related to an increased osteoporotic specialization of the lower jaw.…”

Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis

“…5 Simplified phylogenetic bracketing of five ichthyopterygians on the basis of recent phylogenetic analyses and their preferred birth orientation. Early-diverging forms such as Chaohusaurus and Cymbospondylus both have one gravid specimen with head-first birth, Mixosaurus has a 66%/33% based on three specimens, and Merriamosauria (phylogenetic node denoted with M) likely have a slight preference for tail-first birth based on the sample of Stenopterygius and the fact that all other Merriamosauria show tail-first birth or are inconclusive (note: the relative phylogenetic position of Cymbospondylus and Mixosaurus is debated; compare e.g., [ 30 , 31 ] (silhouettes adapted from: Chaohusaurus [ 2 ]; Cymbospondylus [ 87 ]; Mixosaurus [ 29 ]; Besanosaurus [ 84 ]; Stenopterygius [ 89 ] …”