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Because of the genetic and developmental information available, Danio rerio stands out as a vertebrate model system in which significant progress in the areas of development and evolution can be made. Despite its increasing popularity, little research has been done on skeletal development. In this report, we provide developmental information on the structure and composition of the zebrafish skull, pectoral, and pelvic girdle. We describe the sequence of ossification of the skull and paired fins from a large series of cleared and Alizarin red-stained specimens at larval and adult stages. The most commonly followed developmental sequence in Danio rerio is described. Chondrocranial development is noted from Alcian blue-stained specimens. General trends in ossification patterns are examined from developmental, phylogenetic, and functional contexts. No clear pattern in ossification order of dermal versus cartilage bones is evident. Ossification sequence conforms to functional need in a general way, but there are inconsistencies in the details of order. Selected phylogenetic comparisons of ossification sequence within cranial regions are made among Danio rerio, Betta splendens, Oryzias latipes, and Barbus barbus. Greater sequence conservation is apparent between D. rerio and Barbus barbus, the ostariophysans, than among other taxon pairs. Intraspecific variation in ossification order is apparent, most of which involves small adjustments in timing. © 1996 Wiley-Liss, Inc., (Copyright © 1996 Wiley-Liss, Inc.)

The development of the chondrocranium and the relative timing of ossification of the osteocranium is described in the teleost fish Betta splendens from a large series of cleared and differentially stained specimens. General trends in ossification patterns are examined from developmental, phylogenetic, and functional contexts. As in many other vertebrates, dermal bones form before cartilage bones. Ossification sequence conforms to functional need in a very general way, but there are many inconsistencies in the details of order. For example, some bones that are directly involved in feeding ossify no earlier than bones more indirectly involved. Comparisons of ossification sequence within specific cranial regions are made among Betta splendens, Oryzias latipes (Atherinomorpha), and Barbus barbus (Ostariophysi) within a phylogenetic framework. Many evolutionary changes in relative sequence of ossification are evident within regions among these taxa, yet many other sequences are conserved. The logistic difficulty of comparing entire cranial ossification sequences (vs. regional sequences) makes evident the need for new methods for identifying and quantifying sequence changes. Intraspecific variation in order of ossification is described for the first time in teleost fishes. To the extent that ossification sequence varies intraspecifically, conclusions drawn from previous interspecific comparisons are compromised. Understanding the importance of changes in ossification order within and among taxa will require experimental, functional, and evolutionary work. © 1996 Wiley-Liss, Inc., (Copyright © 1996 Wiley-Liss, Inc.)