Vascular Patterns in Iguanas and Other Squamates: Blood Vessels and Sites of Thermal Exchange

Abstract

Squamates use the circulatory system to regulate body and head temperatures during both heating and cooling. The flexibility of this system, which possibly exceeds that of endotherms, offers a number of physiological mechanisms to gain or retain heat (e.g., increase peripheral blood flow and heart rate, cooling the head to prolong basking time for the body) as well as to shed heat (modulate peripheral blood flow, expose sites of thermal exchange). Squamates also have the ability to establish and maintain the same head-to-body temperature differential that birds, crocodilians, and mammals demonstrate, but without a discrete rete or other vascular physiological device. Squamates offer important anatomical and phylogenetic evidence for the inference of the blood vessels of dinosaurs and other extinct archosaurs in that they shed light on the basal diapsid condition. Given this basal positioning, squamates likewise inform and constrain the range of physiological thermoregulatory mechanisms that may have been found in Dinosauria. Unfortunately, the literature on squamate vascular anatomy is limited. Cephalic vascular anatomy of green iguanas (Iguana iguana) was investigated using a differential-contrast, dual-vascular injection (DCDVI) technique and high-resolution X-ray microcomputed tomography (μCT). Blood vessels were digitally segmented to create a surface representation of vascular pathways. Known sites of thermal exchange, consisting of the oral, nasal, and orbital regions, were given special attention due to their role in brain and cephalic thermoregulation. Blood vessels to and from sites of thermal exchange were investigated to detect conserved vascular patterns and to assess their ability to deliver cooled blood to the dural venous sinuses. Arteries within sites of thermal exchange were found to deliver blood directly and through collateral pathways. The venous drainage was found to have multiple pathways that could influence neurosensory tissue temperature, as well as pathways that would bypass neurosensory tissues. The orbital region houses a large venous sinus that receives cooled blood from the nasal region. Blood vessels from the nasal region and orbital sinus show anastomotic connections to the dural sinus system, allowing for the direct modulation of brain temperatures. The generality of the vascular patterns discovered in iguanas were assessed by firsthand comparison with other squamates taxa (e.g., via dissection and osteological study) as well as the literature. Similar to extant archosaurs, iguanas and other squamates have highly vascularized sites of thermal exchange that likely support physiological thermoregulation that “fine tunes” temperatures attained through behavioral thermoregulation.

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.…”

“…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.…”

Comparative morphology of snake (Squamata) endocasts: evidence of phylogenetic and ecological signals

“…; Ekdale, , , ; Willis et al. ), the vascular system (Porter & Witmer, ), the cranial nerves (George & Holliday, ) and pneumatic sinuses (Bona et al. ).…”

“…; Rieppel & Maisano, ; Comeaux et al. ), the vascular patterns (Porter & Witmer, ), and the lacrimal system (Souza et al. ).…”

Basic Science Model of Blood as an Electron-Delivery Circuit Describes the Evolution of the Human Circulation

“…Ambient oxygen levels influence growth in animals with a mixed circulation such as mudskippers and alligators [44,45]. The rete moribile, a prominent feature in some fish and reptiles, may allow greater circulatory redox variance and hysteresis by acting as a counter-current sink [46].…”