Vascular patterns in the heads of crocodilians: blood vessels and sites of thermal exchange

Abstract

Extant crocodilians are a highly apomorphic archosaur clade that is ectothermic, yet often achieve large body sizes that can be subject to higher heat loads. Therefore, the anatomical and physiological roles that blood vessels play in crocodilian thermoregulation need further investigation to better understand how crocodilians establish and maintain cephalic temperatures and regulate neurosensory tissue temperatures during basking and normal activities. The cephalic vascular anatomy of extant crocodilians, particularly American alligator (Alligator mississippiensis) was investigated using a differential-contrast, dual-vascular injection technique and high resolution X-ray micro-computed tomography (μCT). Blood vessels were digitally isolated to create representations of vascular pathways. The specimens were then dissected to confirm CT results. Sites of thermal exchange, consisting of the oral, nasal, and orbital regions, were given special attention due to their role in evaporative cooling and cephalic thermoregulation in other diapsids. Blood vessels to and from sites of thermal exchange were studied to detect conserved vascular patterns and to assess their ability to deliver cooled blood to neurosensory tissues. Within the orbital region, both the arteries and veins demonstrated consistent branching patterns, with the supraorbital, infraorbital, and ophthalmotemporal vessels supplying and draining the orbit. The venous drainage of the orbital region showed connections to the dural sinuses via the orbital veins and cavernous sinus. The palatal region demonstrated a vast plexus that comprised both arteries and veins. The most direct route of venous drainage of the palatal plexus was through the palatomaxillary veins, essentially bypassing neurosensory tissues. Anastomotic connections with the nasal region, however, may provide an alternative route for palatal venous blood to reach neurosensory tissues. The nasal region in crocodilians is probably the most prominent site of thermal exchange, as it offers a substantial surface area and is completely surrounded by blood vessels. The venous drainage routes from the nasal region offer routes directly to the dural venous sinuses and the orbit, offering evidence of the potential to directly affect neurosensory tissue temperatures. The evolutionary history of crocodilians is complex, with large-bodied, terrestrial, and possibly endothermic taxa that may have had to deal with thermal loads that likely provided the anatomical building-blocks for such an extensive vascularization of sites of thermal exchange. A clear understanding of the physiological abilities and the role of blood vessels in the thermoregulation of crocodilians neurosensory tissues is not available but vascular anatomical patterns of crocodilian sites of thermal exchange indicate possible physiological abilities that may be more sophisticated than in other extant diapsids.

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

“…The cerebral carotid artery supplies tissues within the endocranial cavity but is also a major blood supply to the nasal region (Oelrich, ; Burda, ; Baumel, ; Sedlmayr, ; Porter and Witmer, , ; Porter et al, ). An increase in the surface area of the nasal region should hypothetically lead to a proportional increase in the diameter of the cerebral carotid canal, up to the point where it impinges on neural or other tissues.…”

“…In extant taxa, the cerebral carotid arteries are likely of sufficient size to supply the endocranial cavity and nasal region simultaneously. The upper limit for cerebral carotid canal size in extant archosaurs is likely found in crocodilians, as they modify encephalic blood supply during development (Burda, ) and enlarge the caudal cerebral artery so that it can supply more blood to the nasal region, instead of the smaller rostral cerebral artery as is present in birds and squamates (Porter and Witmer, , ; Porter et al, ).…”

“…Head volume (Table ) was measured by segmenting CT data and wrapping the skull in a “skin” (the outer surface envelope of the head) and then subtracting the estimated volumes of the nasal and oral air spaces (Witmer and Ridgely, ; Snively et al, ). Diapsid 3D models (Porter and Witmer, , ; Porter et al, ) made in Maya (Autodesk, San Rafael, CA) were used to visualize blood vessels. We also collected body‐mass estimates from the literature (see Table ) as another measure of body size.…”

The Frontoparietal Fossa and Dorsotemporal Fenestra of Archosaurs and Their Significance for Interpretations of Vascular and Muscular Anatomy in Dinosaurs

“…pseudotemporalis superficialis and adductor mandibulae externus profundus) and communicate with the orbit via the supraorbital, infraorbital, and ophthalmotemporal vessels (and potentially any ophthalmic rete located between these branches), the brain ( via the vena capitis dorsalis and trigeminal vessels), and the skin of the dorsal surface of the head ( via the occipital, superficial temporal, supraorbital, and facial arteries (Figs. and ; Baumel, ; Sedlmayr, ; Holliday et al, ; Holliday, ; Porter et al, ; Porter and Witmer, ). In extant diapsids, many of these structures participate in a complicated vascular circuitry associated with regulating brain and eye temperature (e.g., Midtgård, , 1984b; Arad et al, ; Sedlmayr, ; Porter et al, ; Porter and Witmer, , this volume).…”

Pulmonary anatomy and a case of unilateral aplasia in a common snapping turtle (Chelydra serpentina): developmental perspectives on cryptodiran lungs

“…; Porter et al. ). For diagnostic purposes in veterinary medicine, volume rendered models (an automatic setting in most dicom viewing software) are suitable (Valente et al.…”