Over 30 tetrapod groups have evolved a snake-like, elongate, limbless body plan. Studies of the
patterns of limb reduction have revealed a close relationship between the reduction of the limbs
and body elongation; however, the loss of the skeletal elements that support the limb, the
pectoral girdle, has been less thoroughly examined. Here, I use computed tomography to
generate three dimensional models of the pectoral girdles of an Australian skink genus, Lerista.
This group contains pentadactyl species, fully limbless species and many species with
intermediate morphologies. I employed a 3D geometric morphometric analysis to compare the
shapes of these structures and revealed that the reduction of the pectoral girdle is correlated with
the degree of loss in the limbs. The girdle in species with well developed limbs is longer and
more narrow than the short, broad girdle of species with highly reduced or absent limbs, but the
degree of reduction is only loosely correlated with the degree of reduction in the limbs. Certain
events appear to occur concurrently such as the losses of the coracoid foramen and the humerus,
but other events such as the loss of the epicoracoid are not consistently associated with any other
event. The extent to which limb reduction is associated with the degree of pectoral girdle
reduction and the morphology of the girdles appears to be closely associated with subclade, with
individuals from closely related clades showing dramatically different degrees of girdle
reduction despite similarities in the degree of limb loss. Despite these differences, the patterns in
Lerista are generally more similar to each other than to those of other lizard groups, and more
similar to those of other skinks than to those of other groups of lizards. Though some aspects of
limb loss are common to all lizard groups that have evolved a serpentine body plan, this study
shows that, even in closely related groups, this transition does not proceed in exactly the same
way. / Master of Science / Over 30 tetrapod groups have evolved a snake-like, elongate, limbless body plan. Many studies of the patterns of limb reduction (evolutionarily losing fingers and toes) have revealed a close relationship between the reduction of the limbs and body elongation; however, the loss of parts of the internal skeleton that support the limb (e.g. shoulder blade, collar bone) have been less thoroughly examined. Here computed tomography was used to generate three dimensional models of the pectoral girdle, the array of limb support bones, of the Australian lizard genus Lerista. This group contains species that have five fingers, species which are fully limbless (and thus have no fingers), and some that have intermediate numbers of fingers. I quantitatively compared the pectoral girdle shapes, which revealed that the reduction of the pectoral girdle occurs gradually and in a series of steps that are loosely associated with how many fingers are present. The pectoral girdle as a whole shows a more obvious reduction in length than in width, with this length reduction being loosely associated with the number of fingers present. Certain events appear to occur concurrently such as the loss of a small hole in one of the bones through which nerves that control the arm pass and loss of the upper arm bone. However other events, such as the loss of a cartilaginous structure called the epicoracoid is not consistently associated with any other event. The extent to which limb reduction is associated with the extent of pectoral girdle reduction is group dependent, with individuals from closely related groups showing dramatically different degrees of pectoral girdle reduction despite a similarity in the degree of digit loss. Despite these differences, the patterns in Lerista are more similar to each other than to those of other lizard groups, and more similar to those of other skinks than to those of other groups of lizards. Though some aspects of limb loss are common to all lizard groups that have evolved a snake-like body plan, this study reveals that even in closely related groups, this transition does not proceed in exactly the same way.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110823 |
| Date | 23 December 2020 |
| Creators | Koeller, Krista Leslie Marie |
| Contributors | Geosciences, Stocker, Michelle, Nesbitt, Sterling James, McGlothlin, Joel W. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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