The axial anatomy of monitor lizards (Varanidae)

Abstract

Because the musculoskeletal anatomy of the trunk is the framework for the behaviors of locomotion, ventilation, and body support in lepidosaurs, comparative study of trunk anatomy in this group is critical for unraveling the selective pressures leading to extant diversity in axial form and function among vertebrates. This work uses gross dissection and computed tomography to describe the muscular and skeletal anatomy of the trunk of varanid lizards (Varanidae, Anguimorpha). Gross muscle dissections were conducted to investigate the axial muscular anatomy of Varanus exanthematicus, Varanus giganteus, Varanus rosenbergi, and Varanus panoptes. Computed tomography scans of these and additional varanid lizards from the Varanus and Odatria subgenera were conducted to investigate rib and vertebral number and gross morphology. The number of vertebrae differs between species, with 27-35 presacral and 47-137 postsacral vertebrae. Although the number of floating and abdominal ribs in varanids is variable, most species examined have three to four cervical ribs and three true ribs. Attachment and insertion points of the epaxial and hypaxial musculature are detailed. The body wall has four main hypaxial layers, from superficial to deep: oliquus externus, intercostalis externi, intercostalis internii, and transversus. Varanids differ from other investigated lepidosaurs in having supracostalis dorsus brevis (epaxial) and levator costae (hypaxial), which independently connect each rib to the vertebral column. Although more basic muscle descriptions of the body wall in reptiles are needed, comparisons with the condition in the green iguana (Iguana iguana) can be made.

Breathing with floating ribs: XROMM analysis of lung ventilation in savannah monitor lizards

“…xiphisternal; Ve, 3 true, 0 xiphisternal) (Carrier, 1988;Cieri, 2018;Brainerd et al, 2016;Hoffstetter and Gasc, 1969). The motion of a vertebral element of a true rib, as opposed to a floating rib, should be constrained to bucket rotation by its connection to the sternum because pump handle motion would naturally either pull the sternal rib dorsally, dislocate the dorsal from the sternal rib, or force the sternal rib to bend in order to prevent this dislocation.…”

“…In contrast, a floating rib, subject to no sternal restrictions, is comparatively freer to rotate along a lateral axis. Pump motion in the true, dorsal ribs of V. exanthematicus can also be explained by this mechanism -pump motion of the floating ribs will also cause pump motion in the true ribs, because of the muscular linkages consisting of the internal and external intercostals (Cieri, 2018) running between each successive rib (Young and Kardong, 2010).…”

“…The observed differences in rib kinematics may also be related to differences in the muscular anatomies of I. iguana and V. exanthematicus. Carrier (1988) reports ligaments connecting the ribs of I. iguana to the vertebrae that restrict motion of the ribs in all but a cranial or caudal direction, but these ligaments are not present in V. exanthematicus (Cieri, 2018) and it is possible to rotate the dorsal ribs in all three planes. Electromyography of the hypaxial muscles during ventilation in iguanas shows that the external and internal intercostals are active during inspiration while the retrahentes costarum and transversalis are active during expiration.…”

“…Costal anatomy varies greatly among squamates, with species showing variable numbers of both true and floating ribs consisting of one to three different segments (Hoffstetter and Gasc, 1969). The axial skeleton in the trunk region of the savannah monitor Varanus exanthematicus (Bosc 1792) consists of the vertebral column, with 28 presacral vertebrae, sternum, and between 18 and 22 pairs of ribs (Cieri, 2018). The cranial-most four ribs are cervical ribs, followed by 18-22 dorsal ribs, starting with three true ribs with vertebral portions (hereafter referred to as vertebral ribs), connected directly to the sternum by sternal portions (Fig.…”

“…1D) (hereafter sternal ribs). The remaining 11-15 ribs are floating ribs, with only a vertebral portion (Cieri, 2018). By comparison, green iguanas (Iguana iguana) possess 23 presacral vertebrae, four cervical ribs, four true ribs, four xiphisternal ribs and nine floating ribs, of which the cranial four are considerably longer than the others (Carrier, 1988;Brainerd et al, 2016).…”