DEWEL, R.A.*; CONNELL, M.U.; DEWEL, W.C.: Bridging morphological transitions to the Metazoa

Our grasp of the development of metazoan complexity has been impeded by several misconceptions regarding the evolution of the eukaryotes. One is the idea that living organisms can be divided into �kingdoms� such as the Protozoa and Animalia with the result that many eukaryotes are lumped together based on grades of organization rather than phylogenetic relationships. Another arises from molecular analyses suggesting that �crown� eukaryotes originated within a short a period of geologic time from unicells that varied little genetically or morphologically. This view now has been tempered by an awareness that molecular data may be of limited value in resolving deep relationships and by a geological record showing substantial intervals separating origins of taxa. The view is also contradicted by the existence characters shared by broader assemblages, suggesting that “crown” groups may not have originated simultaneously. The analysis here, based primarily on morphological characters, extends beyond merely evaluating branching patterns in terms of shared character distributions. Transitions are evaluated for mutational, selective, and developmental likelihood of transformations using characters that would appear to have evolved only once. The analysis indicates that the history of the Metazoa, when traced from early eukaryotes, encompassed the gain and loss primary and secondary photosynthetic endosymbionts with accompanying changes in trophic level. It also involved body reorganization including transformations from flagellate to amoeboid forms and increased levels of intracellular compartmentation. These changes may have provided the underlying complexity required for cell specialization, one of several trends evident in early metazoan evolution.