HENSLEY AND AHLSTROM: PLEURONECTIFORMES 



671 



Psettodidae 



Scophlhalmidae 



E 



Bothidae 



Citharidae 



V V 



Soleidae Cynoglossidae 



Pleuronectidae 



Pleuronectoidei 



Soleoidei 



Psettodoidei 



Fig. 358. 

 (1969). 



Current hypothesis for interrelationships of pleuronectiform fishes. Based on Norman (1934, 1966), Hubbs (1945), and Amaoka 



is presented. However, these authors present this as a tentative 

 hypothesis and admit that the interrelationships expressed are 

 still problematic. Most of the character states they use are re- 

 ductive, few characters were analyzed, and the authors were 

 understandably unaware of recent character surveys, since much 

 of this information is unpublished. 



We have made the assumption that the order Pleuronecti- 

 formes is monophyletic and the sister group is the remaining 

 percomorph fishes (sensu Rosen and Patterson, 1 969 and Rosen, 

 1973). Although the monophyly and origin of the group is still 

 open to question and hypotheses of multiple origins have been 

 proposed (e.g., Kyle, 1921; Chabanaud, 1949; Amaoka, 1969), 

 a monophyletic model with a percomorph sister group still ap- 

 pears to be the most parsimonious. In other words, with the 

 information available, there appears to be no need to hypoth- 

 esize multiple origins for flatfishes; to do so demands the inclu- 

 sion of a great deal of convergence. 



Relationships 



The following discussion of relationships within the pleuro- 

 nectiforms is cursory and preliminary. In fact, it asks more 

 questions than it answers and illustrates that more work (par- 

 ticularly osteological) is needed in certain groups before the 



order can be subjected to an in-depth cladistic analysis. Until 

 this work is completed, it is premature to offer a new hypothesis 

 of interrelationships for the entire order. 



Adult characters 



Several criteria were used for selecting characters for discus- 

 sion: (1) amount of information available on the distribution 

 of character states; (2) characters commonly used in the past to 

 define groups of pleuronectiforms; (3) those for which our 

 knowledge of distributions of states is limited, but appear to 

 indicate groupings different from those hypothesized in the 

 working classification and which need additional study; and (4) 

 characters which are well known in certain groups and are po- 

 tentially useful for elucidating relationships within these groups. 

 Characters and character complexes used in this study are dis- 

 cussed below. Characters and states are presented in Table 179. 



Optic chiasma.—The relationship between the optic chiasma 

 and ocular asymmetry of pleuronectiforms has been investigated 

 by several workers beginning mainly with the work of Parker 

 (1903). Hubbs (1945) examined this relationship further and 

 presented all data from previous studies. Parker found that most 

 fishes have a dimorphic optic chiasma, i.e., the nerve of the left 



