Restoring dinosaur cephalic vascular anatomy and thermophysiology using osteological correlates and anastomotic connections

Abstract

The restoration of dinosaur cephalic blood vessels is critical to understanding physiological thermoregulatory strategies in dinosaurs. Cephalic blood vessels of extant diapsids are known to support physiological thermoregulatory capabilities that modulate head and neurosensory tissue temperatures. Given their close phylogenetic relationship to extant diapsids, dinosaurs are hypothesized to have had similar vascular arrangements and physiological abilities. To test this hypothesis, the cephalic vascular anatomy of extant diapsids (turtles, squamates, crocodilians, and birds) was investigated using CT‐scanning, vascular injection, and gross dissection with special attention given to blood vessels within known sites of thermal exchange (oral, nasal, and orbital regions). Vascular osteological evidence in skull bones of both extant and extinct specimens was collected to test hypotheses of vascular anatomy within the heads of dinosaurs. CT data were segmented in Avizo and vascular restorations were made using Maya. The large blood vessels that supply and drain diapsid heads displayed a conserved branching pattern, as the common carotid artery reliably bifurcates into internal and external carotid arteries. Further evidence for the subsequent branching patterns of these blood vessels adds support for a highly conserved diapsid vascular pattern. An important detail for identifying not only the blood vessels forming osteological correlates, but also identifying the blood vessels that passed between soft‐tissues, is the anastomotic connections that each blood vessel shared. For example, within the orbit, evidence for anastomoses between the supraorbital, ophthalmotemporal and branches of the cerebral carotid arteries was found in all three extant clades sampled. This evidence indicates the location of each participating artery and subsequently highlights the course of all three blood vessels back to their parent vessel. Within the narial region, near the maxilla‐premaxilla suture, evidence for anastomotic connections between the maxillary and nasal vessels was found. This anastomosis often included the palatal vessels, indicated by a canal that opened onto the palate. The connection between these vessels would have allowed blood to pass from the palate to the nasal region, or vice‐versa, serving as a potential collateral blood flow pathway to both of these regions. Additionally, some of the blood vessels to the nasal region are branches of the cerebral carotid arteries, which in large dinosaurs do not appear to be emphasized and seem inadequately sized to supply a large nasal region. This indicates that collateral blood flow was important for supplying emphasized sites of thermal exchange in large dinosaurs. When several dinosaur taxa were compared, different foramina sizes highlighted not only the potential diameter of blood vessels supplying sites of thermal exchange, but also which blood vessels supplied a majority of the blood to physiologically relevant regions of the head. Enabled by an understanding of the vascular anatomy of dinosaurs, we can begin to investigate the physiology of sites of thermal exchange and what important role they served in dinosaur thermoregulatory strategies.Support or Funding InformationUnited States National Science Foundation (IOB‐0517257, IOS‐1050154), Ohio University Heritage College of Osteopathic Medicine, Ohio University Student Enhancement Award, Jurassic Foundation Grant‐in‐aid of Research, Ohio Center for Ecological and Evolutionary Studies Research Fellowship, Ohio University Graduate Student Senate Grant–in‐aid of Research, Sigma Xi Grant‐in‐aid of Research, and the University of California Welles Fund.

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

“…Gross dissections revealed that only blood vessels passed through the foramen (Porter and Witmer, ; Porter et al, ). Saurischian dinosaurs had a similar osteological correlate, which is also hypothesized to only contain blood vessels (Porter, ). Likewise, only vascular components (the palatine artery and vein) traverse the suborbital fenestra in extant diapsids, and there is no evidence from the fossil record of extinct dinosaurs to indicate that they were any different in this regard.…”

“…The palatomaxillary vessels gave off the palatine vessels (which then pass through the small suborbital fenestra) dorsal to the ectopterygoid and then entered the dorsal alveolar canal as the maxillary vessels in the rostroventral aspect of the orbit (Fig. ; Porter, ). The dorsal alveolar canal passed ventral to the rostral loop of the airway (Witmer and Ridgely, ), where the roof of the canal was not detectable.…”

“…In crocodilians (and saurischians), a canal within the maxilla–premaxilla suture transmits an anastomotic branch between the lateral nasal, palatine, and maxillary vessels (Fig. ; Porter, ; Porter et al, ). The ornithischian anastomosis is similar to iguanid lizards, where branches of the nasal and palatine vessels pass dorsally through the rostral aspect of the premaxilla (Fig.…”

“…This finding highlights the differences between each species' thermoregulatory strategies. By extending this approach to investigations into the vasculature of nonavian dinosaurs, which have been shown to conform to broad diapsid vascular patterns (Porter, ), this study detected vascular osteological correlates that allowed the comparison of not only dinosaur vascular patterns, but also their thermoregulatory strategies.…”

“…This study was aided by our previous anatomical investigations of extant diapsids and vasculature within three known sites of thermal exchange, that is, the nasal, oral, and orbital regions (Porter, ; Porter and Witmer, , ; Porter et al, ). By comparing the vasculature of extant species within these physiologically relevant sites of evaporative cooling, it was found that blood vessels can supply different amounts of blood (based on the size of the vessels) to different sites of thermal exchange.…”