Conodonts (including paraconodentes and euconodontes) are characterized by dental structures of calcium phosphate with dentin-like tissue, and taken as the first chordate, approximately 510 Ma.
Figure 3.6 – Conodont on the left, fossilized teeth on the right. Fossil image obtained from commons.wikimedia.org/ There was probably a selection advantage against predators that drove primitive evolution in many clades (for providing protection from predators, the energy spent on the production of mineralized structures varied over time as a response to predatory pressure), the irradiation during the cambrian era began with the bilateral skeletonized species, like the protoconodonts (Figure 3.6). Protoconodonts represent, possibly, the oldest skeletal remains attributed to bilateral animals. They represent calcified structures (calcium phosphate) associated with teeth – the leading hypothesis suggests the these structures were mineralized modifications of part of a set of gills – their significance, among other things was to increase the number of and mechanical force of “shell-grinding” predators, which helped stimulate the natural selection of harder, more ornate and regenerative shells, along with anti-predator behavior (e.g., burying). At least three times in the last 600 million years the increase in predator pressure pushed evolutional changes in the phylogenetic distribution and in biological function of mineralized skeletons.
The environmental conditions during the Later Cambrian made the cost of carbonatized skeletons prohibitive, and the change of these conditions may have contributed to their dissemination during the Ordovician period. In any event, the wide phylogenetic evolution of robust skeletons, as part of the general increase in biological diversity, again suggests that an increasing outbreak of predation pressure contributed to the evolutionary pattern observed: new predators of major importance including nautiloids, cephalopods and asteroideos echinoderms. The diversification of the Ordovician occurs in a context of tectonic growth activity, implying an increase in the flow of nutrients to the oceans. The condition would help explain the increase in body size and in the predatory mechanisms in the ordovician oceans.
