Th is chapter provides an account on the homologies between the head muscles of dipnoans and the muscles of all the other major extant vertebrate groups, which is based on our own dissections of several vertebrate taxa and on an extensive review of the literature, both old and recent. Our observations and comparisons indicate that 13 mandibular, hyoid, branchial and hypobranchial muscles are present in adult dipnoans such as Lepidosiren (not including the branchial muscles sensu stricto), six are present in adults of at least some of the non-osteichthyan taxa listed in our tables (intermandibularis, A2, interhyoideus, protractor pectoralis, coracomandibularis and sternohyoideus), six are present in adults of at least some of the actinopterygian groups listed in these tables (intermandibularis, A2, A3’, interhyoideus, protractor pectoralis and sternohyoideus), and eight are present in adults of at least some of the tetrapod taxa included in these Tables (A2, A2-PVM, interhyoideus, depressor mandibulae, protractor pectoralis, dilatator laryngis, constrictor laryngis and sternohyoideus). Th is shows that dipnoans may provide an important link to compare the anatomical structures of these three latter groups and
thus to discuss the homologies and evolution of these structures within the vertebrates as a whole. Importantly, dipnoans share some unique muscular synapomorphies with tetrapods, thus supporting the idea that these fi shes are in fact the closest living relatives of the clade including extant amphibians and amniotes (e.g., presence of an adductor mandibulae A2-PVM; presence of a levator hyoideus in at least some developmental stages; absence of a recognizable adductor operculi in adults; absence of a recognizable adductor arcus palatini; and possibly presence of a depressor mandibulae in at least some developmental stages). Th is stresses that comprehensive comparative studies of muscles provide not only useful information for functional, evolutionary and ecomorphological studies, but also crucial data to disclose the relationships between major vertebrate groups. Keywords: Dipnoi, cephalic muscles, evolution, homologies, vertebrates
Th e head muscles of dipnoans have been studied by anatomists since the 19th century. Authors who have provided information about these muscles are Owen (1841), Humphry (1872a,b,c), Bischoff (1840), Albrecht (1876), Bridge (1898), Edgeworth (1911, 1923, 1926c, 1935), Luther (1913, 1914), Adams (1919), Lightoller (1939), Kesteven (1942-1945), Säve-Soderbergh (1944), Jarvik (1963, 1980), Fox (1965), McMahon (1969), Wiley (1979a,b), Bemis (1982, 1986), Jollie (1982), Bemis and Lauder (1986), Forey (1986), Miyake et al. (1992), Bartsch (1994), and Wilga et al. (2000). In this chapter, we provide a detailed list of the head muscles that are found in dipnoans, in Tables 1-12, as well as illustrations of most of these muscles, in Figures 1-4. Th e aim of this chapter is however not to restate all the information that has already been provided in the literature about attachments, innervation, confi guration, ontogenetic development and function of each of these muscles. Instead, here we focus on a subject that in our opinion has been, unfortunately, somewhat neglected by anatomists, particularly over the most recent decades: the homologies between the dipnoan muscles and the muscles of other vertebrates. Because of their phylogenetic position (Fig. 5), extant sarcopterygian fi shes such as dipnoans are eff ectively a key group to clarify the homologies and evolution of muscles within vertebrates and namely to help comparing the muscles of other bony fi shes, of tetrapods, and of non-osteichthyan vertebrates (e.g., Diogo 2007, 2008). Diogo (2007, 2008) and Diogo et al. (2008a,b) have recently discussed homologies between the head muscles of dipnoans and the muscles of other osteichthyans, but they did not compare these muscles with those of other vertebrates such as lampreys, elasmobranchs and holocephalans (see Fig. 5). In this chapter we will thus provide an account of the homologies between the head muscles of dipnoans and the muscles of all the other major extant vertebrate groups, which is based on our own dissections of several vertebrate taxa and on an extensive review of the literature, both old and recent. To our knowledge the type of information provided in Tables 1-12 about the detailed homologies of the mandibular, hyoid, branchial
and hypobranchial head muscles (see below) of representatives of all the major extant groups of vertebrates has never been integrated in a single book chapter. In addition to its value for anatomists and functional morphologists, we hope this comparative anatomical information will be useful to researchers working in developmental biology, genetics and/or evolutionary developmental biology for it should help them determine the wider homologies of these structures in dipnoans and non-dipnoan vertebrate model organisms.