Introduction

Humanity has always been fascinated by the abundance of beautiful forms in nature. Throughout our history, myths, religions and science have tried to give explanations for the great diversity in the living world. Darwin (1859) still had to admit ‘our ignorance of the laws of variation is profound’. In recent decades, research has brought some light into the molecular mechanisms of development that produce that variation and which are ultimately responsible for morphological diversity. Today's biodiversity, however, is only an estimated two to four per cent of the diversity that ever existed on earth (Benton 2009), the rest being extinct, in most cases leaving no traces of DNA or proteins for analyses. Is it possible to get insights into the molecular underpinnings of the diversity of organisms that have been extinct for millions of years?

To deal with this question I will first review how, in general, inferences on the molecular basis of morphological diversification in geological time can be drawn and I will assess the foundations of these inferences. The issues of reconstruction, evidence and predictability of evolution will be discussed. Second, I will present a case study on the Triassic ray-finned (actinopterygian) fish Saurichthys. Based on research on extant organisms reported in the literature I will develop inferences on the molecular underpinnings of the most prominent features of this fish, namely body shape and fin position, jaw elongation and scale reduction.