Data from: Vascular patterns in iguanas and other squamates: blood vessels and sites of thermal exchange

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.

有鳞类（Squamata）会借助循环系统在升温和降温过程中调节躯体与头部的体温。该循环系统的灵活性或许超越了恒温动物（endotherms），由此衍生出多种获取或留存热量的生理机制（例如提升外周血流量与心率、通过冷却头部以延长躯体的晒背时长），同时也具备散热的生理通路（调节外周血流量、暴露热交换位点）。有鳞类同样能够建立并维持与鸟类、鳄类及哺乳动物一致的头-躯体体温差，但它们并不具备独立的迷网（rete）或其他血管生理结构。有鳞类为推断恐龙与其他已灭绝主龙类（archosaurs）的血管结构提供了重要的解剖学与系统发育学证据，因为它们能够阐明基础双弓类（diapsid）的解剖生理特征。基于这一基础演化位置，有鳞类同样能够为推断恐龙可能具备的生理体温调节机制的范围提供参考并加以约束。遗憾的是，当前关于有鳞类血管解剖学的研究文献较为匮乏。本研究采用双血管注射差分造影（differential-contrast, dual-vascular injection, DCDVI）技术与高分辨率X射线显微计算机断层扫描（μCT），对绿鬣蜥（Iguana iguana）的头部血管解剖结构展开了探究。研究人员对血管进行了数字分割，以构建血管通路的表面可视化模型。研究重点关注了已知的热交换区域——包括口腔、鼻腔与眼眶区域——因为这些区域在脑部与头部体温调节中发挥着关键作用。本研究对连接热交换区域的血管进行了探究，以识别演化保守的血管模式，并评估其向硬脑膜静脉窦（dural venous sinuses）输送冷却血液的能力。研究发现，热交换区域内的动脉可通过直接通路与侧支通路输送血液。研究还发现，静脉引流存在多条可影响神经感觉组织温度的通路，同时也存在绕过神经感觉组织的引流通路。眼眶区域存在一个大型静脉窦，可接收来自鼻腔区域的冷却血液。来自鼻腔区域与眼眶静脉窦的血管与硬脑膜窦系统存在吻合连接，这使得脑部温度能够被直接调节。研究通过直接比对其他有鳞类类群（例如通过解剖与骨学研究）以及查阅相关文献，验证了在鬣蜥中发现的血管模式的普遍性。与现存主龙类类似，鬣蜥与其他有鳞类均拥有高度血管化的热交换区域，这类区域或许能够辅助生理体温调节，对通过行为体温调节获得的体温进行“精细化调控”。