Article: The development of the loop in the Jurassic brachiopod Zeilleria leckenbyi
An ontogenetic series of Z. leckenbyi has been obtained from a locality in the mid-Cotswolds. Serial sectioning has enabled determination of the microstructure and development regime of the loop. The work reveals that a loop of adult character is formed by the time the brachial valve is about 4-0 mm long, confirming Elliott's suspicion that in zeilleriids the early stages of loop development were passed through very quickly. During early development phases the loop is connected to a septal pillar rising from the floor of the brachial valve. The general pattern of loop development appears to combine terebratellid and dallinid characters. It is found that the descending elements play only a subsidiary role during loop development. That they become relatively massive fairly early appears to be due to the fact that they are required to support the ascending complex after resorption of the connection with the median septum has occurred. The growth pattern of the median septum indicates that it may be regarded as a secondary character and therefore makes only a very limited contribution to loop development in the Zeilleriidae. Attention is focused on the gross inadequacy of our knowledge of the actual growth of juvenile loops during the recognized stages passed through during ontogeny. Analysis of the development regime illustrates some of the dangers of recording growth stages which are essentially momentary phenomena in what must necessarily be a cumulative process. The current work indicates that a cryptacanthiinin of Glossothyropsis type may be ancestral to the Zeilleriidae. It is concluded that the possession of spinose ascending and descending elements is a more important ancestral character than the absence of a median septum and that the microstructure of developing loops will provide the key to the solution of the complex phylogeny of the Terebratulida.