DINOSAUR FEATHERS, FISH FINGERS, AND LUCK OF THE MAMMALS IN EVOLUTIONARY PEDAGOGY

Darwinian evolutionary mechanisms, especially adaptive fitness and natural selection, are powerful explanations for much of the diversity of life, but do not sufficiently explain many major biological transformations in life history. This leads to challenges in evolutionary teaching such as the rhetorical “what good is half a wing (or leg) in survival of the fittest?” Mechanisms beyond natural selection that enhance traditional Darwinism are needed to better explain so-called “macroevolution” and biotic diversity. Fortunately, the vertebrate fossil record includes several major transitional and replacement sequences with examples of the important evolutionary mechanisms of exaptation (i.e. preadaptation), paedomorphosis (neoteny), and chaos (chance). The vertebrate fossil record tends to preserve sequences of morphological transitions because vertebrates are relatively large and most are skeletonized, both conditions favoring fossilization. Well-documented cases of exaption include evolution of the avian wing and feathers from non-avian, feathered theropod dinosaur hands, and the modification of lobe-finned fish's subaqueous walking limb into the terrestrial tetrapod (basal amphibian) leg. Retention of juvenile characters and changes in reproductive timing (paedomorphosis) may explain such diverse events as the evolution of chordates from larval echinoderm ancestry, and the infantile (e.g. naked, big-brained, small-snouted) morphology of our own species, compared with other apes. Hypothetically, the adaptive success of mammals in the post-Cretaceous world, with rapid evolution of many new body plans and lineages, may be attributed more to chaos than to general adaptive fitness of mammals over dinosaurs. Mammals actually appear before dinosaurs in the fossil record, yet were a small-bodied, low-diversity, subdominant clade for over 165 million years, until an extraterrestrial impact wiped out larger terrestrial tetrapods.