Ischnacanthiform dentitions and the origin and evolution of vertebrate teeth

• Author / Creator

  Blais, Stephanie A.

• Living jawed vertebrates can be readily assigned to two major well-supported clades: the cartilaginous Chondrichthyes (sharks and their kin) and the ‘bony fishes’, the Osteichthyes (which include tetrapods, and to which humans belong). Together, these make up the crown group Gnathostomata. Chondrichthyes and Osteichthyes shared a most recent common ancestor no less than 423 million years ago, allowing ample time for the living members of these groups to diverge and acquire new characters and character states, and resulting in a lack of clarity regarding the ancestral conditions of Gnathostomata as a whole. The assignment of fossil taxa to the osteichthyan, chondrichthyan, and gnathostome stem groups is necessary to understand the ancestral conditions and evolutionary origins of these vertebrates, but determining the phylogenetic relationships of Paleozoic gnathostomes presents a challenge, exacerbated by a relative dearth of well-preserved fossil material from the Silurian and Early Devonian. The Man On The Hill (MOTH) locality in the Northwest Territories of Canada has yielded beautifully preserved fossils of Early Devonian gnathostomes, providing a unique opportunity to investigate their diversity and adding new data to formulate and test hypotheses of evolutionary relationships. In particular, MOTH is one of the only fossil sites in the world to preserve articulated skeletons of an enigmatic group of fishes known as acanthodians. Recently, acanthodians have received increased attention, as they represent a likely sister group to the Chondrichthyes. In this phylogenetic context, acanthodian features may provide insight into the primitive characters of Chondrichthyes, and possibly the primitive conditions for Gnathostomata. This thesis provides a comprehensive study of acanthodian fossils, particularly those belonging to the order of acanthodians called Ischnacanthiformes. This group is particularly poorly known, being represented primarily by isolated jaw bones. As a part of this study, four new genera comprising six new species of ischnacanthiform acanthodians are described, greatly increasing our comprehension of the diversity of the group. The presence of several closely related species coexisting in a relatively restricted geographic area has been hypothesized to indicate trophic niche partitioning; this hypothesis is indirectly tested here through the use of three-dimensional reconstructions of articulated pairs of jaws, revealing different styles of occlusion and feeding mechanics in different species of ischnacanthiforms from MOTH. The differences in jaw occlusion as well as in patterns of tooth wear support the hypothesized trophic niche differentiation among ischnacanthiform species from MOTH, and suggests that rather than being indiscriminate generalist predators, at least some of these early jawed vertebrates may have specialized in capture and processing of preferred prey items. Previously unidentified morphological and histological structures are also described from MOTH acanthodians, with comments on the potential phylogenetic implications of these discoveries. A new hypothesis is proposed for the mechanism of jaw bone growth and tooth attachment in ischnacanthiform acanthodians. In providing some insight into the diversity, ecology, and evolutionary history of the Ischnacanthiformes, I hope to have provided a better picture not only of their phylogenetic affinities, but also of the ancient world in which these animals lived, and how they may have interacted with their environment and with each other.

• Subjects / Keywords

  • Ischnacanthiforms
  • Fossil fishes
  • Acanthodians
  • Early vertebrates
  • Evolution of teeth
  • Vertebrate teeth

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3C24R27V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.