Joss Jean M P
Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia.
Gen Comp Endocrinol. 2006 Sep 15;148(3):285-9. doi: 10.1016/j.ygcen.2005.10.010. Epub 2005 Dec 7.
The first vertebrates recognizable as tetrapods appeared in the mid-Devonian. It is generally agreed that their ancestors were lobe-finned fish. What is not agreed is how close either of the extant groups of lobe-finned fish, lungfish or coelacanths, is to the actual ancestor of the tetrapods. The soft anatomy of living lungfish shares many similarities with that of living amphibians. Many of these similarities are not present in either coelacanths or any members of the other extant bony fish group, the ray-finned fishes. Many very well preserved lungfish from the Devonian possess specialized features that would appear to exclude them from being ancestral to tetrapods. I am hypothesizing that lungfish in the Devonian may have included metamorphosis in their life cycle and that neoteny in some species may have been an early corollary. These reproductively mature neotenous lungfish would not have had the specialised features of metamorphosed adults. Fossils of these neotenous forms may have more closely resembled the tetrapod ancestral lobe-finned fish, currently believed to be a panderichthiad fish. Living lungfish have a number of larval features, which suggest paedomorphosis. Also of significance is the very large genome of living lungfish, which, in urodele amphibians, is a feature correlated with neoteny. Our current knowledge of the thyroid axis in the lungfish, Neoceratodus forsteri, is consistent with neoteny in amphibians, but the only Devonian fossil considered to be a larval lungfish bears no resemblance to living lungfish or to panderichthiads. The enigmatic phylogenetic relationship of lungfish with the first tetrapods remains, but the hunt for other forms of larval Devonian lungfish is on!
最早可被确认为四足动物的脊椎动物出现在泥盆纪中期。人们普遍认为它们的祖先是肉鳍鱼。但目前尚未达成共识的是，现存的两类肉鳍鱼，即肺鱼和腔棘鱼，哪一类与四足动物的实际祖先关系更近。现存肺鱼的软组织解剖结构与现存两栖动物有许多相似之处。而这些相似之处在腔棘鱼或其他现存硬骨鱼群（辐鳍鱼）的任何成员中都不存在。许多来自泥盆纪保存完好的肺鱼具有一些特殊特征，这些特征似乎表明它们并非四足动物的祖先。我推测泥盆纪的肺鱼在其生命周期中可能包括变态发育，并且某些物种的幼态持续可能是一个早期的必然结果。这些生殖成熟的幼态持续肺鱼不会具有变态成年个体的特殊特征。这些幼态持续形态的化石可能与目前被认为是潘氏鱼的四足动物祖先肉鳍鱼更为相似。现存肺鱼具有许多幼体特征，这表明存在幼态发育。同样重要的是现存肺鱼非常大的基因组，在有尾两栖动物中，这是一个与幼态持续相关的特征。我们目前对澳洲肺鱼甲状腺轴的了解与两栖动物的幼态持续一致，但唯一被认为是幼体肺鱼的泥盆纪化石与现存肺鱼或潘氏鱼没有相似之处。肺鱼与最早的四足动物之间神秘的系统发育关系仍然存在，但对泥盆纪其他幼体肺鱼形态的寻找仍在继续！
