Cretaceous Amber: A New Source of Bird Remains and of Palaeoecological Information in Dinosaur Bonebeds

Abstract

In order to reconstruct Cretaceous ecology and environments, this thesis investigates amber deposits in Myanmar and Western Canada, through chemical analyses of amber and study of the inclusions. As part of this work, I described a bird foot and a large series of feather inclusions from the amber of Myanmar. In the bird specimen, a series of osteological features and the morphology of the claws, support a systematic placement among Enantiornithes. I was able to describe delicate integumentary structures associated with the foot. This included scutellate scale feathers, a type of feather that has only been documented in Burmese amber specimens. Their distribution and relative size on the longest digit of the foot suggest a mechanosensory tactile function, and may allow us to draw parallels to some modern birds with tactile feathers on their feet. This finding strongly supports the longest digit acting as part of the feeding strategy of the bird, and fits well with previous interpretations of habitat use by enantiornithines. This dissertation demonstrates that even isolated vertebrate remains in amber are of high scientific interest. Through a detailed study of more than 180 Burmese amber samples preserving feathers, a diverse assemblage of Cretaceous feathers has been recovered. This provides the opportunity to document and investigate several structural types of feathers, ontogenetic stages, and pigmentation patterns. Based on recent work that associates feather types with skeletal remains, I was able to attribute many of the feathers to putative source animals. Although a few feathers may belong to non-avian dinosaur taxa, this survey confirmed earlier hypotheses that Enantiornithes were the dominant source of feathers in this ecosystem. The structure and pigmentation patterns reported added to larger-scale hypotheses regarding the groups present in the Cretaceous amber forest. One particular subset of ornamental feathers warranted detailed study because of its unusual structure. I determined that rachis-dominated feathers (RDFs) form a distinct morphotype characterized by a ventrally open rachis and a flexibility that was not documented previously. These results allow us to refine our evolutionary-developmental (evo-devo) models for integumentary structures in dinosaurs by providing additional data on structures preceding modern feathers. The structural features observed support a particular pathway for evolutionary development in feather subcomponents, addressing a major question in evo-devo models of feathers which have been based predominantly on modern material. A research deficit was addressed by focusing on Upper Cretaceous bonebed amber deposits of Western Canada. I established that amber can provide paleoecological, paleoenvironmental, paleoclimatic, and paleogeographic data, thus improving our knowledge of dinosaur habitats and their biota. Stable isotope analyses of amber refined our knowledge of the actual position and extant of the Western Interior Seaway (WIS) during the Late Cretaceous. A marine signal indicates the presence of remnants of the WIS near the deposits studied. Fourier-transform Infrared spectra showed that the dominant tree in the forests belonged to the Cupressaceae group. Feather fragments in Pipestone Creek amber, reveal the presence of aquatic birds (e.g., Hesperornithes and Ichthyornithes) 73 Ma, in Pachyrhinosaurus habitats. Insect inclusions yielded new species of Mymarommatidae wasp and Psocoptera which add to the fossil record and support the ecological interpretations based on amber chemistry. Ultimately, this line of research further supports paleoenvironmental hypotheses based on other sources of data. It also provided a new source of information on Cretaceous terrestrial habitats at high latitude, and a source for comparisons to other amber deposits during the latter part of the Cretaceous.