Article: Loop development and the classification of terebratellacean brachiopods

The classical separation of Cainozoic long-looped brachiopods into two groups of genera, dallinid and terebratellid, according to certain early developmental features of the loop, does not withstand critical examination. These studies confirm the high taxonomic value of developing loop patterns but consider features of the developing loop, used previously to separate families, to be invalid. The dallinid sequence as defined previously is shown herein to include two groups of genera differing in the loop patterns exhibited during intermediate phases of development. These two groups of genera differ from each other and from terebratellid genera in the manner of resorption of parts of the ring and the stage at which it is freed from the septum. These factors govern the presence in one group (Dallinidae) of descending branches with double anterior limbs and in the other group (Laqueidae) of vertical connecting bands, neither of these structures occurring in the development of the Terebratellidae. The growth phases of the loop are identified by a simpler terminology using descriptive in place of generic adjectives. Two subfamilies, the Dallininae and Nipponithyrinae, are retained in the Dallinidae and four subfamilies, the Kingeninae, Pictothyrinae, Macandreviinae, and Terebrataliinae (nov.), are included in the Laqueidae.