RECONSTRUCTING DEVELOPMENT IN FOSSIL VERTEBRATES USING VIRTUAL PALEONTOLOGY

The development of organisms has been an important aspect of evolutionary biology since the time of Haeckel and even more so since the evodevo revolution. Arguments derived from the development of a whole organism or individual organs are important to decipher homology and evolution of structures. In this research concept fossils are vital as they record the assembly of new structures.

Traditionally subsequent ontogenetic stages are studied making this approach difficult to apply to fossils as they are rare, often fragmentarily preserved and the timing between stages is uncertain. Complete conservation of ontogenetic stages, as found in the shells of Foraminifera or Cephalopoda facilitates this approach and makes these taxa classic examples. In vertebrates description of successive stages is mainly based on the gross morphology and the development is inferred, as histological methods are destructive and therefore rarely applicable.

We apply virtual paleontology using Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM) and virtually dissect the sclerochronology of mineralized vertebrate organs. With synchrotron tomography using 360º scans we almost double the width of the field of view and visualize even cm large specimens preserving highest resolution. 3D visualisation programs and powerful computing enable us to label micron-sized structures and reconstruct successive ontogenetic stages.

We test hypotheses on the development of teeth and jaws in the earliest jawed vertebrates (placoderms) and bony fishes (osteichthyans). The gnathal of derived placoderms is a composite of distinct teeth that developed in succession comparable to tooth families. The teeth are composed of dentine and bone, and exhibit a distinct pulp cavity that is infilled centripetally as development proceeds. This pattern is repeated in other placoderms and gnathals of stem osteichthyans.

We interpret our evidence to indicate that teeth did not evolve convergently among the extant and extinct classes of early jawed vertebrates but, rather, successional teeth evolved within the gnathostome stem-lineage soon after the origin of jaws. We conclude that virtual methods are ideal to reconstruct development and essential to test important evolutionary scenarios and homologies.