刺鮫是早已滅絕的化石魚(yú),，接近軟骨魚(yú)和硬骨魚(yú)的分叉點(diǎn)。它們的形態(tài)學(xué)研究有可能揭示關(guān)于有顎類(lèi)脊椎動(dòng)物早期演化的很多秘密,。問(wèn)題是人們對(duì)它們的解剖特征很不了解,，它們大部分都局限于Acanthodes，這是生活在二疊紀(jì)的一種可能沒(méi)有代表性的屬,，處在刺鮫歷史的晚期,。
現(xiàn)在，Martin Brazeau介紹了對(duì)Ptomacanthus的腦殼首次所做的詳細(xì)描述,，它是生活在早泥盆紀(jì)刺鮫鼎盛時(shí)期的一種刺鮫,，距今大約4.15億年前。這些結(jié)果表明,，刺鮫可能不是一個(gè)自然的類(lèi)群：Ptomacanthus要么是鯊魚(yú)的一個(gè)非常早的近親,，要么接近所有現(xiàn)代有顎類(lèi)脊椎動(dòng)物的共同祖先。（生物谷Bioon.com）
生物谷推薦原始出處：
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.
