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Early hindbrain development in all major vertebrate lineages exhibits a shared anatomical blueprint of cranial motor nuclei and nerves originating in one or more serially arranged segments or rhombomeres [rhs; e.g., Lumsden and Keynes (1989), Gilland and Baker (2005)]. Here, we consider the development and evolution of hindbrain circuitry leading to novel innovations in social signaling, integrating information across behavioral, neurophysiological, and morphological levels of analysis. Two neural networks are the focus: the sonic–vocal basis of acoustic signaling (Fig. 10.1A) and pectoral control of anterior appendages, fins, and forelimbs (Fig. 10.1B). For context, we first briefly review vertebrate phylogeny and the ancestral “blueprint” for hindbrain motor phenotypes.

VERTEBRATE PHYLOGENY

Living craniates include jawless vertebrates or agnathans and jawed vertebrates or gnathostomes [Fig. 10.1B; reviewed in Nelson (2006)]. Fossil evidence indicates several lineages of extinct agnathans [e.g., Osteostracans; Fig. 10.1B; e.g., Forey and Janvier (1993)]. Chondrichthyes (i.e., cartilaginous fishes) are the most basal group of jawed vertebrates and include two subclasses, Elasmobranchii (i.e., sharks, skates, and rays) and Holocephali or chimaeras. Bony vertebrates, the sister group to Chondrichthyes, include Actinopterygii or rayfinned fishes and the Sarcopterygii or lobe-finned fishes. Sarcopterygians include the coelacanth (Latimeria), lungfish (Dipnoi), and tetrapods.

Here, we mainly review recent evidence showing that a caudal hindbrain (rh8)-spinal cord compartment is the developmental origin of premotor-motor circuitry for sonic–vocal and pectoral behavioral phenotypes. Actinopterygians, which include nearly half of living vertebrate species, were the focus of these studies. By integrating these new findings into a single framework, we aim to achieve a more complete understanding of the evolutionary origins of vocal and pectoral motor systems among vertebrates in general, including the more highly derived pectoral systems of tetrapods that serve a range of functions including nonvocal sonic and forelimb gestural signaling.

HINDBRAIN SEGMENTAL BLUEPRINT

Vertebrates have two functional series of hindbrain motor nuclei, somatic and branchiomeric (Lumsden and Keynes, 1989; Gilland and Baker, 2005), that were likely present in the earliest, pregnathostome vertebrates (Northcutt, 1985). Somatic nuclei innervate head muscle derived from unsegmented (i.e., prechordal plate) and segmented paraxial mesoderm (i.e., occipital somites); branchiomeric nuclei target derivatives of



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