Animals : an Open Access Journal from MDPI · 6 authors, 5 centres
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This study provides the first detailed anatomical atlas of the rhinoceros iguana head using CT, MRI, and gross-sections, identifying key bone and soft tissue structures. The findings reveal unique features such as three nasal horns, an extracranial olfactory bulb, and a 40° encephalic angulation. These imaging references can aid veterinary diagnosis of head pathologies like abscesses, metabolic bone disease, fractures, and neoplasia in this endangered species.
**Background:** The rhinoceros iguana (Cyclura cornuta cornuta) is an endangered species native to Hispaniola, protected under CITES and classified as vulnerable by the IUCN. Despite growing interest in reptiles as companion animals, detailed anatomical descriptions of this species are lacking, particularly for the head. Advanced imaging techniques like computed tomography (CT) and magnetic resonance imaging (MRI) offer non-invasive, high-resolution visualization of internal structures, but no prior study has applied these to the rhinoceros iguana head. This study aimed to create an anatomical atlas of the head using CT, MRI, and macroscopic anatomical sections to provide a diagnostic reference for clinical evaluation of conditions such as abscesses, osteodystrophy secondary to nutrient imbalances, skull malformations, fractures, and neoplasia.
**Methods:** Two adult female rhinoceros iguana carcasses (94 cm and 91 cm snout-to-tail length; 5 kg and 4.2 kg weight) from Rancho Texas Lanzarote Park (Canary Islands, Spain) were used. Both died from natural causes with no abnormalities on physical exam. CT was performed using a 16-slice helical scanner (Toshiba Astelion) with a standard clinical protocol (100 kVp, 80 mA, 512×512 matrix, 1 mm slice thickness). Bone window (WW=1500, WL=300) and soft tissue window (WW=350, WL=40) settings were applied, and multiplanar reconstructions (MPR) in dorsal and sagittal planes were obtained. MRI was performed with a 1.5 T scanner (Canon Vantage Elan) using T1W (TR 634 ms, TE 10, thickness 2 mm) and T2W sequences (TR 4769–5271 ms, TE 120, thickness 2–2.9 mm) in transverse, dorsal, and sagittal planes. After imaging, carcasses were frozen at −80°C and sectioned transversely at 1 cm intervals using an electric band saw from snout to first cervical vertebra. Sections were cleaned, photographed, and matched to CT/MRI images. Anatomical identification was aided by reference texts.
**Key Results:** The study produced five transverse levels (I–V) from maxillary bone to brainstem, plus dorsal and sagittal images. CT excellently delineated bony structures of the neurocranium (prefrontal, frontal, postfrontal-postorbital, parietal, squamosal, quadrate, epipterygoid, pterygoid, basioccipital, exoccipital, otoccipital, parabasisphenoid) and splanchnocranium (nasal, premaxilla, maxilla, septomaxilla, vomer, jugal, palatine, nasal septum). The medial horn (pseudo-horn) was identified dorsal to the nasal bone with soft-tissue attenuation and a thin mineralized border. Nasal glands appeared as symmetric bilateral soft-tissue structures. Gas-filled structures (nasal conchal recess, oral cavity, nasopharyngeal duct, trachea, adductor fossa, otic cavity) showed vacuum effect. MRI provided superior soft tissue contrast, clearly visualizing the brain (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon), eyeball structures (vitreous humor, lens, retina, sclera, cornea, iris, scleral ossicles), tongue, masticatory muscles (geniohyoideus, hyoglossus, intermandibularis, pterygoideus, omohyoideus, sternohyoideus, ceratohyoideus, adductor mandibulae externus medialis and superficialis), and laryngeal structures (corniculate tubercles of arytenoid cartilage, thyroid cartilage, laryngeal ventricle). Notably, the olfactory bulb was found in an extracranial location, extending rostromedially between the eyeballs—a finding not previously reported in this species. The encephalon showed a 40° angulation relative to the horizontal axis of the myelencephalon (Figure 9), differing from the 28° reported in some other reptiles. The eyeballs were proportionally large, nearly the size of the entire encephalon.
**Clinical Implications:** This atlas provides the first comprehensive anatomical reference for the rhinoceros iguana head using CT, MRI, and gross-sections. The detailed identification of bone and soft tissue structures enables accurate interpretation of imaging studies in clinical practice, facilitating diagnosis of common head pathologies such as abscesses, metabolic bone disease (osteodystrophy), fractures, and neoplasia. The extracranial location of the olfactory bulb and the 40° encephalic angulation are unique anatomical features that may influence surgical planning and interpretation of neurological signs. The study also highlights the utility of combining CT (for bone) and MRI (for soft tissue) in reptile imaging, offering a non-invasive alternative to dissection for teaching and research. Limitations include the small sample size (n=2) and use of carcasses rather than live animals, but the findings are consistent with prior work in related species and provide a foundation for future clinical and comparative studies.