Evolution, development, and homology of the amniote dermal skeleton and pectoral apparatus.
Date
2007
Authors
Vickaryous, Matthew K.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
The dermal skeleton and pectoral apparatus have long been recognized as major determinants of vertebrate morphology. Until recently however, details of tissue development and diversity, particularly among arrmiotes, have been lacking. Furthermore, the homology of various elements remains uncertain. This research is broadly divided into two sections. First I investigated the development of the pectoral apparatus and determined that the long held interpretation of coracoid homology is incorrect. I find that the reptilian coracoid is homologous with the coracoid process of therian mammals and not, as previously suggested, the procoracoid of monotreme mammals. Using this information, I devised a new scenario for the evolution of the coracoid involving changes in the aggregation, proliferation, and differentiation of pre-skeletal cell condensations. I also re-explored the historic subject of furcula (wishbone) homology. Employing Colin Patterson's three tests of homology, I find that each of the interclavicle and the furcula pass the observational tests of similarity (similar topographic position, and pattern and mode of development) and conjunction (neither co-exists with the other). My test of congruence examined several competing hypotheses, and determined that each of the neomorphic origin and furcula as the homologue of the interclavicle to be equally parsimonious, although the results only differ by a single evolutionary step from the predominant furcula-equals-clavicles hypothesis. Hence, I find the homology of the furcula to be ambiguous.
Second, I investigated dermal skeleton development in Alligator mississippiensis. I document the first report of chondroid bone in a non-avian reptile. I also find that A. mississippiensis lacks secondary cartilage. My study further demonstrates that the rugose pattern of bony ornamentation across the skull (and osteoderms) of this taxon develops from the localized concentrations of bone deposition by osteoblasts; osteoclasts, while present, are restricted to deep bone surfaces. Similar to many amniotes, A. mississippiensis develops osteoderms, bony inclusions of the dermis. I compared the development of osteoderms in A. mississippiensis with the mammal Dasypus novemcinctus and observed that all osteoderms shared a relatively late onset and asynchronous pattern of development. For D. novemcinctus, I find that osteoderms begin as condensations of osteoblasts secreting osteoid. This mode of development is consistent with intramembranous ossification. For A. mississippiensis, I find that osteoderms develop in the absence of osteoblastic cells and osteoid; bone develops via the direct transformation of preexisting dense irregular connective tissue. I have identified this mode of bone formation as metaplasia. My studies clearly demonstrate that osteoderms are not histologically uniform but may involve a range of tissues including uncalcified and calcified dense connective tissues. Between taxa, not all osteoderms develop by homologous processes. However, I conclude that osteoderms may share a deep homology, connected by the structural and skeletogenic properties of the dermis.
Thesis (Ph.D.)--Dalhousie University (Canada), 2007.
Second, I investigated dermal skeleton development in Alligator mississippiensis. I document the first report of chondroid bone in a non-avian reptile. I also find that A. mississippiensis lacks secondary cartilage. My study further demonstrates that the rugose pattern of bony ornamentation across the skull (and osteoderms) of this taxon develops from the localized concentrations of bone deposition by osteoblasts; osteoclasts, while present, are restricted to deep bone surfaces. Similar to many amniotes, A. mississippiensis develops osteoderms, bony inclusions of the dermis. I compared the development of osteoderms in A. mississippiensis with the mammal Dasypus novemcinctus and observed that all osteoderms shared a relatively late onset and asynchronous pattern of development. For D. novemcinctus, I find that osteoderms begin as condensations of osteoblasts secreting osteoid. This mode of development is consistent with intramembranous ossification. For A. mississippiensis, I find that osteoderms develop in the absence of osteoblastic cells and osteoid; bone develops via the direct transformation of preexisting dense irregular connective tissue. I have identified this mode of bone formation as metaplasia. My studies clearly demonstrate that osteoderms are not histologically uniform but may involve a range of tissues including uncalcified and calcified dense connective tissues. Between taxa, not all osteoderms develop by homologous processes. However, I conclude that osteoderms may share a deep homology, connected by the structural and skeletogenic properties of the dermis.
Thesis (Ph.D.)--Dalhousie University (Canada), 2007.
Keywords
Biology, Zoology.