Foramina in plesiosaur cervical centra indicate a specialized vascular system

Abstract

Abstract. The sauropterygian clade Plesiosauria arose in the Late Triassic and survived to the very end of the Cretaceous. A long, flexible neck with over 35 cervicals (the highest number of cervicals in any tetrapod clade) is a synapomorphy of Pistosauroidea, the clade that contains Plesiosauria. Basal plesiosaurians retain this very long neck but greatly reduce neck flexibility. In addition, plesiosaurian cervicals have large, paired, and highly symmetrical foramina on the ventral side of the centrum, traditionally termed "subcentral foramina", and on the floor of the neural canal. We found that these dorsal and the ventral foramina are connected by a canal that extends across the center of ossification of the vertebral centrum. We posit that these foramina are not for nutrient transfer to the vertebral centrum but that they are the osteological correlates of a highly paedomorphic vascular system in the neck of plesiosaurs. This is the retention of intersegmental arteries within the vertebral centrum that are usually obliterated during sclerotome re-segmentation in early embryonic development. The foramina and canals are a rare osteological correlate of the non-cranial vascular (arterial) system in fossil reptiles. The adaptive value of the retention of the intersegmental arteries may be improved oxygen transport during deep diving and thermoregulation. These features may have been important in the global dispersal of plesiosaurians.

Taxonomic affinities of the putative titanosaurs from the Late Jurassic Tendaguru Formation of Tanzania: phylogenetic and biogeographic implications for eusauropod dinosaur evolution

“…floor of the neural canal), towards the posterior end of the neural canal, is excavated, forming a moderately deep, elliptical fossa. This fossa is present in subsequent caudal vertebrae too, and it might represent an opening for the basivertebral veins (Manning, Egerton & Romano, 2015) or possibly the arterial channels (Wintrich, Scaal & Sander, 2017). A similar fossa has also been described in the anterior caudal vertebrae of two Middle Jurassic eusauropods: an isolated vertebra from the Aalenian of the UK (Manning et al, 2015), and material from the Malagasy Bathonian taxon 'Bothriospondylus madagascariensis' .…”

Microstructure of Vertebrae, Ribs, and Gastralia of Triassic Sauropterygians—New Insights into the Microanatomical Processes Involved in Aquatic Adaptations of Marine Reptiles

“…The presence of large cavities in the core of vertebral centra involves complete reorganization and redistribution of the mass in the vertebrae and is in this extend unique among tetrapods (Dumont et al, ; Houssaye et al, ). Early Cretaceous choristoderes also show central cavities in the vertebral centrum but they are smaller (with one exception: see Skutschas and Vitenko, : Fig.9J) as is the central cavity in a cervical vertebra of a Jurassic plesiosaur (Wintrich et al, ). The retainment of a notochordal canal can be interpreted as an adaptation to an aquatic lifestyle (paedomorphosis).…”

New plesiosaurid material from the Maastrichtian type area, the Netherlands

“…Dorsally there are three large nutritive foramina. Such have recently been reinterpreted as blood vessels that went through the centrum (Wintrich et al, 2017). Two of these are connected at a deeper-lying level within the vertebral centrum, but are divided by a shaft at the surface.…”

“…2). Two other foramina, also of large size, are located ventrally (vf), which is a characteristic feature of elasmosaur centra (Wintrich et al, 2017). These foramina are often referred to as 'foramina subcentralia' (e.g.…”