Brachial supporting structure of Spiriferida (Brachiopoda)

The filter-feeding organ of some extinct brachiopods is supported by a skeletal apparatus called the brachidium. Although relatively well-studied in Atrypida and Athyridida, the brachidial morphology is usually neglected in Spiriferida. To investigate the variations of brachidial morphology in Spiriferida, 65 species belonging to eight superfamilies were analyzed. Based on the presence/absence of the jugal processes and normal/modified primary lamellae of the spiralia, four types of brachidium are recognized. Type-I (with jugal processes) and Type-II (without jugal processes), both having normal primary lamellae, could give rise to each other by losing/re-evolving the jugal processes. Type-III, without jugal processes, originated from Type-II through the evolution of the modified lateral-convex primary lamellae, and it gave rise to Type-IV subsequently by evolving the modified medial-convex primary lamellae. Besides, the evolution of brachidia within individual evolutionary lineages mus...

部分已灭绝腕足动物的滤食器官由名为腕骨（brachidium）的骨骼装置支撑。尽管无洞贝目（Atrypida）和轴腕贝目（Athyridida）中的腕骨形态已得到较为充分的研究，但螺旋贝目（Spiriferida）内的腕骨形态通常被忽视。为探究螺旋贝目内腕骨形态的变异规律，研究人员对隶属于8个超科的65个物种开展了分析。基于轭状突（jugal processes）的有无以及腕螺（spiralia）初级瓣片的正常/特化状态，研究人员识别出四种腕骨类型：兼具正常初级瓣片的I型（具轭状突）与II型（无轭状突）腕骨可通过轭状突的丢失或重新演化实现相互转化；无轭状突的III型腕骨起源于II型，其通过演化出特化的侧凸型初级瓣片形成，随后又通过演化出特化的中凸型初级瓣片衍生出IV型腕骨。此外，单个演化支内腕骨的演化过程必须……