A Study of the Head during Prenatal and Perinatal Development of Two Fetuses and One Newborn Striped Dolphin (Stenella coeruleoalba, Meyen 1833) Using Dissections, Sectional Anatomy, CT, and MRI: Anatomical and Functional Implications in Cetaceans and Terrestrial Mammals

SIMPLE SUMMARY: The head region of the dolphin has been studied widely to identify its anatomical structures and to compare it with other marine and terrestrial mammals. In this study, specimens stranded off the Spanish coast were used. Our study analyzes four dolphin heads during fetal and perinatal development. All specimens were scanned using modern imaging techniques to study their internal organs and to preserve the specimens, which are difficult to obtain. Only one fetus was transversely cross-sectioned to help us to identify critical organs. The developmental study shows several anatomical structures that are compared with cetaceans and terrestrial mammals. During development of the oral cavity, it was observed that the rostral maxillary and mandible teeth (incisive area) had not completely erupted, in contrast with the rest of teeth, which have done so. Also, the main chewing muscle (masseter) was not observed. In addition, we describe the absence of major salivary glands during these developmental stages. Furthermore, we explain the characteristics of the orbit and its relation to the eyeball. In addition, the fetal dolphin’s ear is connected with pharynx in a way similar to that in horses. We conclude that these developmental studies will help cetacean conservation. ABSTRACT: Our objective was to analyze the main anatomical structures of the dolphin head during its developmental stages. Most dolphin studies use only one fetal specimen due to the difficulty in obtaining these materials. Magnetic resonance imaging (MRI) and computed tomography (CT) of two fetuses (younger and older) and a perinatal specimen cadaver of striped dolphins were scanned. Only the older fetus was frozen and then was transversely cross-sectioned. In addition, gross dissections of the head were made on a perinatal and an adult specimen. In the oral cavity, only the mandible and maxilla teeth have started to erupt, while the most rostral teeth have not yet erupted. No salivary glands and masseter muscle were observed. The melon was well identified in CT/MRI images at early stages of development. CT and MRI images allowed observation of the maxillary sinus. The orbit and eyeball were analyzed and the absence of infraorbital rim together with the temporal process of the zygomatic bone holding periorbit were described. An enlarged auditory tube was identified using anatomical sections, CT, and MRI. We also compare the dolphin head anatomy with some mammals, trying to underline the anatomical and physiological changes and explain them from an ontogenic point of view.