- Title
- Osteological Correlates of Cephalic Skin Structures in Amniota: Documenting the Evolution of Display and Feeding Structures with Fossil Data
- Author
- Hieronymus, Tobin L.
- Degree
- Doctor of Philosophy (PhD), Ohio University,
Biological Sciences (Arts and Sciences), 2009.
- Advisor
- Lawrence M. Witmer PhD (Advisor)
- Pages
- 254p.
- Abstract
- The research presented here is an examination of the morphology and histology of several broad categories of skin structures in living amniotes, together with analyses of the osteological correlates associated with each skin category. The epidermal horn and armor-like dermis of extant rhinoceros are examined in detail, and the evolution of both of these skin structures is reconstructed in phylogenetic context from fossil evidence. The evolution of rhinoceros dermal armor is strongly associated with the evolution of shearing tusks used in fighting behaviors, and precedes the evolution of epidermal horns by ~20 Ma. The distribution and morphology of cephalic scales, rhamphothecal plates,and feathers in Sauropsida is then examined in an analysis of evolutionary modularity.Two distinct regions of skin, one around the mouth and another on the skull roof, show independent patterns of morphological evolution, suggesting that skin features in these regions are interconnected as modules. Rhamphotheca in neornithine birds are one possible expression of this modularity. In a separate analysis, plates of compound rhamphotheca (e.g., in albatross) are shown to be homologous with regions of simple rhamphotheca. Rhamphotheca occupy a topographically similar area of skin in nearly all neornithine birds, and the variable expression of softer grooves leads to several homoplastic occurrences of compound rhamphotheca. Several adaptive scenarios have been proposed for novel skin structures in non-avian dinosaurs, but the lack of direct fossil evidence for skin in these animals and the ambiguity in available reconstructions has made it difficult to evaluate these scenarios. Detailed reconstructions for cephalic skin structures drawing on gross morphology and paleohistology are presented for the lineage of centrosaurine dinosaurs leading to Pachyrhinosaurus and for the abelisaurid theropod Majungasaurus. The transition from tall horn cores to gnarled pachyostotic bosses in centrosaurine dinosaurs closely resembles the morphology and evolution of the frontal boss in muskox. The rugose bone on Majungasaurus skulls closely resembles the attachment of dermal armor in rhinoceros. In both cases, agonistic behaviors associated with similar skin structures in extant animals cast doubt on the idea that they functioned only in visual display. The evolution of these novel structures was most likely driven by social selection.
- Subject Headings
- Biology; Paleontology; Zoology
- Keywords
- osteological correlates; rhinoceros horn; rhamphotheca; pachyrhinosaurus; majungasaurus; evolutionary modularity
Document number: ohiou1237491191.
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