古低等脊椎动物研究室知识库

The pelvic morphology, and whether the pelvic fin is present or absent in the earliest jawed vertebrates are key in interpreting the origin of vertebrate paired fins. Parayunnanolepis xitunensis, an antiarch placoderm from the Early Devonian of Yunnan, South China, was previously described to possess the earliest evidence of both dermal and endoskeletal pelvic girdles, presumably for the attachment of the pelvic fins. Here, we redescribe the pelvic region of the holotype based on high-resolution computed tomographic data. Instead of having two large plates previously designated as dermal pelvic girdles, Parayunnanolepis possesses three pairs of lateral pelvic plates, and one large oval median pelvic plate. The paired pelvic plates are flat ventral plates, and differ from other dermal pelvic girdles in lacking a dorsal extension. There is no definitive evidence for the presence of an endoskeletal pelvic girdle in Parayunnanolepis, although the possibility cannot be ruled out. A comparison of the dermal pelvic plates in various jawed stem-gnathostomes suggests the presence of both paired and median pelvic plates is shared by different lineages and might be plesiomorphic. The jawed stem-gnathostomes may have recruited the ventral dermal skeleton of the post-thoracic body into different functional units.

腰带部位的形态，以及腹鳍在最早的有颌脊椎动物中是否存在，是解答脊椎动物附肢如何起源的关键。我国云南曲靖下泥盆统西屯组中的西屯副云南鱼(Parayunnanolepis xitunensis)过去被描述为具有外骨骼和内骨骼的腰带。基于高精度CT数据重新描述了西屯副云南鱼正型标本的腰带部位形态。结果显示副云南鱼并没有像之前描述的那样，同时保存一对大的膜质腰带和对应内骨骼腰带，而是在腰带部位拥有三对成对的膜质骨片，以及一块较大的中线膜质骨片。这些成对的膜质骨片是平坦的，位于躯体腹面，区别于其他早期有颌类拥有背侧叶的膜质腰带。目前仍没有决定性证据证明副云南鱼是否具有内骨骼腰带。比较早期有颌类各大类群中腰带部位膜质骨骼形态，可以看到腰带部位同时存在成对和中线膜质骨片这一型式在这些大类群中也有发现，可能是有颌类的祖征。有颌类在早期演化中可能将腰带部位的膜质骨片整合入不同的功能结构单元中。