刺鲛是早已灭绝的化石鱼，接近软骨鱼和硬骨鱼的分叉点。它们的形态学研究有可能揭示关于有颚类脊椎动物早期演化的很多秘密。问题是人们对它们的解剖特征很不了解，它们大部分都局限于Acanthodes，这是生活在二叠纪的一种可能没有代表性的属，处在刺鲛历史的晚期。

现在，Martin Brazeau介绍了对Ptomacanthus的脑壳首次所做的详细描述，它是生活在早泥盆纪刺鲛鼎盛时期的一种刺鲛，距今大约4.15亿年前。这些结果表明，刺鲛可能不是一个自然的类群：Ptomacanthus要么是鲨鱼的一个非常早的近亲，要么接近所有现代有颚类脊椎动物的共同祖先。

推荐原始出处：

Nature 457, 305-308 (15 January 2009) | doi:10.1038/nature07436

The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins

Martin D. Brazeau1

Subdepartment of Evolutionary Organismal Biology, Department of Physiology and Developmental Biology, Evolutionary Biology Centre, Uppsala University, Norbyv?gen 18A, SE-752 36 Uppsala, Sweden

Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era1, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic 'acanthodians' are possibly the earliest2, 3 gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution1. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418–412 Myr bp) acanthodian, Ptomacanthus anglicus 4, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes 5, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes 5 in exhibiting several plesiomorphic features shared with placoderms6, 7 and some early chondrichthyans8, 9. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters.