Early crest animals and the insight they provide into the evolutionary origin of craniates

Abstract

Summary: 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. genesis 46:623-639,

Genetic lineage labeling in zebrafish uncovers novel neural crest contributions to the head, including gill pillar cells

“…Recent paleontological analysis of cristozoan fossils such as Haikouella lanceolata and Yunnanozoon lividum suggests a precraniate history of 'crest animals'. These fossils are characterized by paired gill rays and jointed, widely spaced branchial arches, but do not possess a skull (Chen, 2008;Chen et al, 1999;Holland and Chen, 2001;Mallatt and Chen, 2003). Although many structures directly involved in prey capture have been shown to derive from NC, the embryonic origin of the new, expanded respiratory organ, which according to the recent paleontological interpretation seems to predate the appearance of the skull, has not been investigated.…”

“…These paired bones accommodating the inner ear are considered to be a novelty of craniates and are absent in early crest animals (Chen, 2008;Gross and Hanken, 2008a;Mallatt and Chen, 2003). Studies in birds and mammals have shown a NC contribution to the otic capsule (Le Lièvre, 1978;O'Gorman, 2005;Noden, 1983).…”

The Middle Cambrian fossil Pikaia and the evolution of chordate swimming

“…These are considered by many to be chordates or close to them [31,32], but there are other possibilities [33-35]. Yunnanozoans have a dorsal sail-like structure that is apparently cuticularized and at least partially articulated.…”

Evolution of vertebrate central nervous system is accompanied by novel expression changes of duplicate genes

“…During the transition from invertebrates to vertebrates, the central nervous system (CNS) has undergone a series of striking changes including the dramatic increase of neuron number and the emergence of the tripartite brain with telencephalon (Williams and Herrup, 1988;Holland and Holland, 1999;Chen, 2008;Holland and Short, 2008). These changes are thought to be founded upon novel gene functions and the expansion of regulatory networks (Davidson and Erwin, 2006).…”