Origin of Micromorph Faunas in the Geologic Record

Stunting, transportation, juvenility and paedomorphosis are the principal causes of micromorph faunas. Stunting may be a result of high or low salinity, low food supply or low oxygen. Transportation can winnow and concentrate smaller members of an assemblage and create a sorted micromorph fauna. Juvenility is the result of large-scale seasonal fluctuations, and paedomorphosis is an evolutionary response to environmental factors such as instability or substrate fluidity. Macrofaunal characteristics potentially diagnostic of these causes are: 1) population dynamics including growth and mortality rates, age of attainment of sexual maturity, and age and size distributions, 2) diversity, 3) trophic structure, 4) preservational condition and orientation of the individuals in the fossil assemblage, and 5) functional morphological adaptations, behavioral characteristics and environmental tolerances. Other characteristics include trace fossils, microfaunal diversity and composition, sedimentary structures, texture and geochemistry, and stratigraphic evidence of paleogeographic isolation. Models defined by expected conditions for each of these diagnostic criteria are established for each of the principal causes. Although all the above criteria are useful in determining the probable specific cause of micromorphs, population dynamics is the criterion which best permits the distinction between micromorph and normal faunas and the recognition of the principal cause of the micromorph fauna.