OSTARIOPHYSI

Datovo, Aléssio & Vari, Richard P., 2014, The adductor mandibulae muscle complex in lower teleostean fishes (Osteichthyes: Actinopterygii): comparative anatomy, synonymy, and phylogenetic implications, Zoological Journal of the Linnean Society 171 (3), pp. 554-622 : 610-612

OSTARIOPHYSI

The Ostariophysi is one the largest groups within the Teleostei and encompasses almost 70% of all freshwater fishes across the world ( Nelson, 2006). Amongst the large monophyletic groups within the Teleostei it is also amongst the best delimited in terms of numbers of morphological synapomorphies ( Rosen & Greenwood, 1970; Fink & Fink, 1981, 1996). Fink & Fink (1981) proposed the possession of a ventrolateral A1 section of the adductor mandibulae as a synapomorphy for the Ostariophysi , with a reversal to the absence of this muscle section in the Siluriformes . According to Fink & Fink (1981), the A1 section inserts on the maxilla in basal lineages in the Ostariophysi (gonorynchiforms, cypriniforms, and primitive characiforms), on what the authors termed the ligamentum primordium (= retrojugal lamina) in some characiforms, and on the lower jaw in gymnotiforms. On the basis of these descriptions, the term A1 apparently was applied by Fink & Fink (1981) to several different muscle sections: the rictalis in the case of the Gonorynchiformes ( Fig. 14 ), Gymnotiformes ( Fig. 19 ), and some lineages of the Characiformes ( Datovo & Castro, 2012) , and the ectorictalis in the case of the Cypriniformes ( Figs 15 , 16 ) and basal lineages in the Characiformes ( Fig. 17 ). Furthermore, none of the facial sections of the adductor mandibulae can be considered to be actually absent in the Siluriformes (present study; Datovo & Vari, 2013).

Diogo & Chardon (2000a), following Gosline (1989), also considered the ventrolateral facial section of the adductor mandibulae in the Ostariophysi to be unique to that lineage in the Teleostei and designated this section as the A1-OST, which would also be present in the Siluriformes . However, as demonstrated in Datovo & Vari (2013) and detailed in the muscle synonymies for the Ostariophysi herein, the name A1-OST of Diogo & Chardon (2000a) was also ambiguously applied by those authors to different portions of the adductor mandibulae (rictalis and ectorictalis) in various taxa. Moreover, none of the muscle sections that correspond to the A1 of Fink & Fink (1981) or the A1-OST of Diogo & Chardon (2000a) is, in fact, exclusive to the Ostariophysi . Regardless of their degrees of separation from the adjoining muscle sections, both the rictalis and ectorictalis are present across all of the Teleostei.

All gonorynchiforms ( Fig. 14 ; Howes, 1985a; Diogo & Doadrio, 2008; Diogo et al., 2008a; Diogo, 2008b), all cypriniforms ( Figs 15 , 16 ; Vetter, 1878; Takahasi, 1925; Edwards, 1926; Eaton, 1935; Van Dobben, 1935; Girgis, 1952; Munshi, 1960; Weisel, 1960; Alexander, 1966; Ballintijn et al., 1972; Wu & Shen, 2004; Hernandez et al., 2005; Diogo & Doadrio, 2008; Diogo et al., 2008a, b; Staab & Hernandez, 2010; Staab et al., 2012), most basal characiforms (most distichodontids and anostomoids; Vari, 1979; Datovo & Castro, 2012), and the basal-most gymnotiforms (gymnotids; Fig. 19 ; Aguilera, 1986) have the entire rictalis or, at least, its ventrolateral portion (= ectorictalis) inserted onto the posterolateral face of the buccopalatal membrane. Although the ectorictalis of these groups has often been cited as inserting on the maxilla via a tendon, these insertional tendons are clearly derived from the buccopalatal membrane (present study; Datovo & Castro, 2012; Datovo & Vari, 2013). Even some siluriforms (e.g. Heptapterus ; A. Datovo pers. observ.) and several derived characiforms ( Winterbottom, 1974: 232; Datovo & Castro, 2012: 102) have a tenuous attachment of the rictalis to the buccopalatal membrane, with the connection possibly representing a remnant of the putative ancestral condition for ostariophysans. However, it remains unclear whether the association of at least the ventrolateral portion of the rictalis with the buccopalatal membrane can be considered a synapomorphy for the Ostariophysi or whether it would define a much more inclusive group in the Teleostei. An association of the rictalis with the buccopalatal membrane also characterizes various taxa more or less proximate to the Ostariophysi , such as the basal osteoglossomorph Hiodon ( Fig. 10 ), some osmeroids, and, perhaps, also some argentinoids (present study; Williams, 1987, 1997; Sanford, 2000).

These issues aside, the Ostariophysi can apparently be defined by one synapomorphy involving the adductor mandibulae: [4] a rictalis differentiated into endorictalis and ectorictalis subsections ( Fig. 27A). This condition is present across all of the Gonorynchiformes ( Fig. 14 ; Howes, 1985a; Diogo & Doadrio, 2008; Diogo et al., 2008a; Diogo, 2008b) and Cypriniformes ( Figs 15 , 16 ; Vetter, 1878; Takahasi, 1925; Edwards, 1926; Eaton, 1935; Van Dobben, 1935; Girgis, 1952; Munshi, 1960; Weisel, 1960; Alexander, 1966; Ballintijn et al., 1972; Wu & Shen, 2004; Hernandez et al., 2005; Diogo & Doadrio, 2008; Diogo et al., 2008a, b; Staab & Hernandez, 2010; Staab et al., 2012), and basal groups in the Characiformes (most members of the Distichodontidae and Anostomoidea; Vari, 1979; Datovo & Castro, 2012). An undifferentiated rictalis is present as the primitive condition for the Teleostei, as well as in derived characiforms and all gymnotiforms and siluriforms. Amongst lower teleosts, only a few taxa in the Anguilliformes and Notacanthiformes (Elopomorpha) similarly have the ectorictalis and the endorictalis differentiated from one another, but within present concepts of higher-level relationships within the Teleostei, this condition in the Elopomorpha obviously evolved independently from that of the Ostariophysi . Therefore, under the traditional morphological hypothesis of relationships within the Ostariophysi ( Fink & Fink, 1981, 1996), the differentiation of the rictalis into ectorictalis and endorictalis sections may be optimized as appearing at the base of the Ostariophysi , with a reversal at the base of the Siluriphysi (= Siluriformes + Gymnotiformes ) and other secondary reversals occurring within the Characiformes ( Fig. 27A). It is difficult to estimate the number of reversals of this character within the Characiformes as a consequence of the many uncertainties as to the internal phylogenetic relationships in the order (compare the topologies of Ortí & Meyer, 1997; Buckup, 1998; Calcagnotto, Schaefer & DeSalle, 2005; Mirande, 2009, 2010; Oliveira et al., 2011). By contrast, phylogenetic analyses based solely on molecular data proposed several alternative hypotheses of relationships amongst the Characiphysae; none of which recovered the monophyly of the Siluriphysi ( Siluriformes plus Gymnotiformes ; see Ortí & Meyer, 1997; Lavoué et al., 2005; Nakatani et al., 2011; Near et al., 2012; Betancur-R et al., 2013). Under such schemes of characiphysan relationships, a rictalis differentiated into the endorictalis and ectorictalis (character 4) would be highly homoplastic across the Ostariophysi and recovered as a synapomorphy for this subcohort only under accelerated transformation (ACCTRAN) optimization ( Fig. 27B).

Figures

Figure 10. Adductor mandibulae and associated structures of Hiodon tergisus (Hiodontiformes: Hiodontidae), USNM 167970 (17.8 mm standard length). Left side in (A) lateral and (B) medial views. Abbreviations: AM, adductor mandibulae; BPM, buccopalatal membrane; IA, intersegmental aponeurosis; RMT, ramus mandibularis trigeminus nerve.

Figure 14. Adductor mandibulae and associated structures of Chanos chanos (Gonorynchiformes: Chanidae), USNM 173572 (147.3 mm standard length). Left side in (A) lateral, (B) medial, and (C) dorsal views; ramus mandibularis trigeminus nerve removed. Abbreviations: AM, adductor mandibulae; IA, intersegmental aponeurosis.

Figure 15. Adductor mandibulae and associated structures of Rasbora cephalotaenia (Cypriniformes: Cyprinidae), USNM 330848 (81.2 mm standard length). Left side in (A) lateral and (B) medial views. Abbreviations: AM, adductor mandibulae; IA, intersegmental aponeurosis; RMT, ramus mandibularis trigeminus nerve.

Figure 16. Adductor mandibulae and associated structures of (A) Raiamas senegalensis (Cypriniformes: Cyprinidae), USNM 271201 [93.0 mm standard length (SL)] and (B) Carassius auratus (Cypriniformes: Cyprinidae), MZUSP 112353 (44.2 mm SL). Left sides in lateral view. Abbreviations: AM, adductor mandibulae; RMT, ramus mandibularis trigeminus nerve.

Figure 17. Adductor mandibulae and associated structures of Xenocharax spilurus (Characiformes: Distichodontidae), AMNH 230302 (95.2 mm standard length). Left side in (A) lateral and (B) medial views. Ramus mandibularis trigeminus nerve removed. Abbreviations: AM, adductor mandibulae; BPM, buccopalatal membrane; IA, intersegmental aponeurosis.

Figure 19. Adductor mandibulae and associated structures of Gymnotus carapo (Gymnotiformes: Gymnotidae), MZUSP 69391 (145.0 mm standard length). Left side in (A) lateral and (B) medial views. Arrow indicates tendon fragment leading to subcutaneous layer of connective tissue (cut). Abbreviations: AM, adductor mandibulae; BPM, buccopalatal membrane; IA, intersegmental aponeurosis.