Vascular Patterns in the Heads of Dinosaurs: Evidence for Blood Vessels, Sites of Thermal Exchange, and Their Role in Physiological Thermoregulatory Strategies

Abstract

Body size has thermal repercussions that impact physiology. Large‐bodied dinosaurs potentially retained heat to the point of reaching dangerous levels, whereas small dinosaurs shed heat relatively easily. Elevated body temperatures are known to have an adverse influence on neurosensory tissues and require physiological mechanisms for selective brain and eye temperature regulation. Vascular osteological correlates in fossil dinosaur skulls from multiple clades representing different body‐size classes were identified and compared. Neurovascular canals were identified that differentiate thermoregulatory strategies involving three sites of evaporative cooling that are known in extant diapsids to function in selective brain temperature regulation. Small dinosaurs showed similarly sized canals, reflecting a plesiomorphic balanced pattern of blood supply and a distributed thermoregulatory strategy with little evidence of enhancement of any sites of thermal exchange. Large dinosaurs, however, showed a more unbalanced vascular pattern whereby certain sites of thermal exchange were emphasized for enhanced blood flow, reflecting a more focused thermal strategy. A quantitative, statistical analysis of canal cross‐sectional area was conducted to test these anatomical results, confirming that large‐bodied, and often large‐headed, species showed focused thermal strategies with enhanced collateral blood flow to certain sites of heat exchange. Large theropods showed evidence for a plesiomorphic balanced blood flow pattern, yet evidence for vascularization of the large antorbital paranasal air sinus indicates theropods may have had a fourth site of heat exchange as part of a novel focused thermoregulatory strategy. Evidence presented here for differing thermoregulatory strategies based on size and phylogeny helps refine our knowledge of dinosaur physiology. Anat Rec, 303:1075–1103, 2020. © 2019 American Association for Anatomy

The braincase of <em>Bissektipelta archibaldi</em> — new insights into endocranial osteology, vasculature, and paleoneurobiology of ankylosaurian dinosaurs

“…As noted earlier in the description of the endocast, the position of the median vessel in Bissektipelta generally corresponds to the position of the pineal complex in extant lepidosaurs and birds. In extant diapsids, the pineal complex is well vascularized, supplied by branches of the posterior cerebral artery and drained by the dorsal longitudinal sinus (Dendy, 1909;Ralph, 1970). We doubt that the canal housed the pineal/ parapineal organ itself but hypothesize that the median vessel and its branches may represent dorsal continuations of the pineal complex vasculature.…”

“…Notably, the pineal complex, optic chiasm, and the neurohypophysis are all diencephalic derivates. However, the external pineal (parietal) foramen was lost early in archosauriform evolution (Hopson, 1979;character 63 in Nesbitt, 2011); extant birds have a pineal that lies internally within the braincase, adjacent to the skull roof (Ralph, 1970). Thus, we doubt that the median canal of Bissektipelta contained a pineal/parapineal organ that was exposed on the dorsal surface of the skull roof and consider the structure a vascular canal.…”

“…The significance of cephalic vasculature for proper thermoregulation of the head has been well demonstrated in extant squamates, crocodylians, and birds Witmer, 2015, 2016a;Porter et al, 2016). Porter and Witmer (2019) recently conducted a thorough study on the thermoregulatory strategies and corresponding vascular patterns in various dinosaurs. In a re-assessment of the structure and homology of the frontoparietal and supratemporal fossae among archosaurs, Holliday et al (2019) hypothesized the presence of a vascular network interconnecting the supratemporal, orbital, and endocranial regions.…”

“…Bourke et al (2018) suggested that the convoluted nasal passages may have been the source of the cooled venous blood that circulated into the braincase region and maintained thermal homeostasis of the brain. Furthermore, Porter and Witmer (2019) found compelling evidence that both Euoplocephalus and Panoplosaurus had a focused thermoregulatory strategy with an unbalanced pattern of blood vessels. The authors proposed that the convoluted nasal passages of these ankylosaurs were the main sites of thermal exchange and that the modified blood flow sustained redistribution of blood from these sites to relevant organs (e.g., neurosensory tissues).…”

Paleobiology in the 21st Century

“…This work includes studies of vascularization patterns in the skulls of dinosaurs (Porter and Witmer, , in this issue), cranial myology in dinosaurs (Holliday et al, , in this issue; Nabavizadeh, , in this issue), the functional evolution of the dinosaurian fourth trochanter (Persons and Currie, , in this issue), and the relationship between dinosaurian size evolution and ecological dominance in the Late Triassic (Griffin and Nesbitt, , in this issue). For example, Porter and Witmer (, in this issue) took CT scans of both extant specimens (gecko, varanid, iguana, alligator) and 17 dinosaur skulls. Using these CT scans, they digitally measured cross‐sectional areas for various cranial vascular canals and quantified head volume, incorporating estimates of skin and muscle volume (Witmer and Ridgely, ; Snively et al, ).…”