Acellular bone was discovered over 100 years ago, and yet many fundamental qualities of its biology remain obscure. Acellular bone comprised the skeleton of the earliest vertebrates, "heterostracans," which are now extinct. Most living vertebrates possess cellular bone. However, one group of extant derived fishes, the Teleostei, includes many members with acellular bone. Although it has been assumed to be similar in both groups, there has never been a comparative study of this peculiar tissue in teleosts and "heterostracans." This study addresses the development of acellular bone in "heterostracans" and teleosts in order to determine whether this tissue is the same in these two distantly related groups. Growth series of heterostracan fossils will be compared to developmental series of a living fish with acellular bone, Geophagus brasiliensis. The study will focus on microscopic aspects of bone such as the amount and site of cell entrapment within the hard tissues, the relative size of tooth-like structures, and how these factors relate to the overall pattern of morphogenesis. The result will be the first comprehensive developmental study of earliest vertebrate hard tissues. Furthermore, it will elucidate some basic aspects of the biology of acellular bone.
