A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum and its evolutionary implications
A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum and its evolutionary implications
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DOI：
10.5194/fr-20-87-2017
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发表时间：
2017-02-21
期刊：
影响因子：
1.4
通讯作者：
Froebisch, Joerg
中科院分区：
文献类型：
--
作者：
Brocklehurst, Neil;Froebisch, Joerg
Phreatophasma aenigmatium是一种神秘的四足动物，来自俄罗斯早二叠纪中期，以一根股骨为代表。从最初的描述开始，在不同的时期，它被认为是一种兽尾纲的突触动物，一种来自Caseidae科的pelycosauria级的突触动物，最近被认为是一种舌形两栖动物。利用最新的颅后形态学和早期突触类动物的进化知识，标本被重新评估并进行分支分析。semouriamorph和therapsid亲和性被拒绝，而caseid亲和性基于深转子间窝得到支持；宽间隔的远端髁；短而结实的股骨干；而且转子间窝的后缘周围没有纵向脊。当包括在两个分支基质中，第一个是对基底突触的整体分析，第二个是对caseids的分析，发现Phreatophasma在caseids中占据基础位置，保留了非纯形特征，如股骨前髁缺乏压迫，两个髁的远端范围几乎相同。认识到phatophasma是一个基础病例，对该科的进化和生物地理学具有重要意义。这只是来自俄罗斯古温带地区的第二个病例，而且它与第一个（翼龙构造）并不密切相关，这意味着至少有两次不同的从古赤道到温带纬度的扩散事件。它还表明，caseids的许多半形似特征，包括小的体型和相对较长的股骨，一直保留到中二叠纪，而在这个时期，caseids以大型草食性类群为代表。
Phreatophasma aenigmaticum is a mysterious tetrapod from the earliest middle Permian of Russia, represented by a single femur. At various times since its original description it has been considered a therapsid synapsid, a pelycosaurian-grade synapsid from the family Caseidae, and most recently a seymouriamorph amphibian. Using upto- date knowledge of the postcranial morphology and evolution of early synapsids, the specimen is re-evaluated and subjected to cladistic analysis. Seymouriamorph and therapsid affinities are rejected, and a caseid affinity is supported based on the deep intertrochanteric fossa; the widely spaced distal condyles; the short, robust femoral shaft; and the lack of a longitudinal ridge enclosing the posterior margin of the intertrochanteric fossa. When included in two cladistic matrices, the first a global analysis of basal synapsids and the second devoted to caseids, Phreatophasma is found to occupy a basal position within caseids, retaining plesiomorphic characters such as the lack of compression of the anterior condyle of the femur and the almost identical distal extent of the two condyles. The recognition of Phreatophasma as a basal caseid has great implications for the evolution and biogeography of this family. This is only the second example of a caseid from the palaeo-temperate region of Russia, and it is not closely related to the first (Ennatosaurus tecton), implying at least two distinct dispersal events from the palaeoe-quatorial to temperate latitudes. It also implies that a number of plesiomorphic characteristics of caseids, including small body size and a relatively long femur, were retained as late as the middle Permian, a time when caseids were otherwise represented by large herbivorous taxa.