Early crest animals and the insight they provide into the evolutionary origin of craniates.
The Cristozoa (also known as crest animals) are established as representing the animals containing neural crest and its derivatives, constituting all known craniates and their immediate precraniate precursors. The precraniate crest animals all are extinct and preserved only in Early Cambrian strata of Yunnan (southwestern China). Fine anatomical details of Haikouella and the slightly more advanced species Yunnanozoon exhibit many characters unique to vertebrates, but they lack a skull and most other elaborated placodal and neural crest derivations, throwing novel light onto the previously missing history at the very beginning of cristozoan evolution. Comparative study with the cephalochordate amphioxus suggests that precraniate evolution is marked by a series of innovations including: muscular ventilation with gill-bearing and jointed brachial arches, paired head sensorial organs including paired eyes and nostrils, relatively large, slightly differentiated brain, protovertebrae, and some derivatives of neural crest cells. But, they lack ears, a clear telencephalon, and a skull. Furthermore, comparison of the brains of amphioxus and craniates suggests that the tripartite brain with telencephalon was not an older structure but a novelty of the craniates.
Chen
Jun-Yuan
JY
LPS of Nanjing Institute of Geology and Paleontology, Institute of Evo/Developmental Biology, and State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing, China. chenjunyuan@163.net
D000717:Anatomy, Comparative; D000818:Animals; D005075:Biological Evolution; D043344:Chordata / Q000033:anatomy & histology* / Q000235:genetics*; D005580:Fossils