Cooper and Chapleau: Monophyly and intrarelationships of the family Pleuronectidae 
687 
phylogenetic constraint versus environmental influ- 
ence could provide a more informed understanding 
of observed changes in life history traits. 
The objectives of this study are to clarify the mono- 
phyletic status of the Pleuronectidae, to offer an hy- 
pothesis of relationships within the group based on 
adult morphology, and to establish a phylogenetically 
informative nomenclature. To attain these objectives, 
a reassessment of morphological evidence in the lit- 
erature and new morphological characters were com- 
piled in a matrix analyzed within a cladistic frame- 
work. A new classification based on the phylogenetic 
information is offered. 
Historical classification and 
diagnosis of Pleuronectidae 
The Pleuronectidae was regarded by early ichthy- 
ologists to represent all known flatfishes (Norman, 
1934). For example, Cuvier (1816) subdivided the 
Pleuronectidae into five subfamilies: Hippoglossinae, 
Pleuronectinae, Platessinae, Soleinae, and Cynoglos- 
sinae; as did Jordan and Goss ( 1889) who also added 
the subfamilies: Samarinae and Oncopterinae. 
Changes to the early classification in flatfish focused 
on revisions that accommodated new species with- 
out a complete revision of the entire scheme. Newly 
discovered species, thought to represent distinct 
morphological groups, were classified into new sub- 
groups; but original species, and those that did not 
have special morphologies, were to remain within the 
Pleuronectidae. Thus, the Pleuronectidae became a 
“garbage” group. 
Jordan and Evermann (1898) raised flatfishes to 
the suborder Heterosomata with two distinct fami- 
lies: Pleuronectidae and Soleidae. The Pleuro- 
nectidae, with three subfamilies: Hippoglossinae, 
Pleuronectinae, and Psettinae, were characterized 
by “a more or less distinct preopercular margin (i.e. 
not hidden by the skin and scales of the head); eyes 
large, well separated; mouth moderate or large; teeth 
present” (Jordan and Evermann, 1898). The Soleidae 
were subdivided into two subfamilies, Soleinae and 
Cynoglossinae, and were characterized by “an adnate 
preopercular margin, hidden by the skin and scales 
of the head; eyes small, situated close together; 
mouth very small, much twisted; teeth rudimentary 
or wanting” (Jordan and Evermann, 1898). 
Regan (1910) proposed a new classification that 
raised the Heterosomata to the level of order with 
two suborders: Psettodoidea and Pleuronectoidea. 
Within the second suborder, the Pleuronectidae now 
contained three subfamilies; Pleuronectinae, 
Samarinae, and Rhombosoleinae. The family was 
characterized by “having eyes on right side of head, 
nerve of left eye always dorsal, olfactory lamellae 
slightly raised, parallel without central rachis and 
eggs without oil globules” (Regan, 1910). 
This classification was adopted by Norman ( 1934), 
who incorporated minor revisions from Regan ( 1920, 
1929) and Jordan ( 1923). The Pleuronectidae, at this 
point containing five subfamilies (Pleuronectinae, 
Samarinae, Rhombosoleinae, Poecilopsettinae, and 
Paralichthodinae) were characterized by Norman 
(1934) as “having eyes on the right side; optic chi- 
asma monomorphic, the nerve of the left eye always 
dorsal; dorsal fin extending forward on the head at 
least to above the eye; all the fin-rays articulated; 
pelvic of from 3 to 13 rays; mouth usually terminal, 
with the lower jaw more or less prominent; maxil- 
lary without a supplemental bone; palatines tooth- 
less; lower edge of urohyal deeply emarginate, so that 
the bone appears forked; preoperculum with free 
margin; nasal organ of blind side usually near edge 
of head, but sometimes nearly opposite that of ocu- 
lar side; vertebrae never fewer than 30; on each side 
a single post-cleithrum; ribs present; egg without an 
oil-globule in the yolk.” Later classifications removed 
the genera Br achy pleura and Lepidoblepharon from 
the Pleuronectidae and placed them in the Citharidae 
(Hubbs, 1945) but essentially agreed with the clas- 
sification proposed by Norman (1934). 
Nelson ( 1984) listed the Poecilopsettinae, Rhombo- 
soleinae, Samarinae, and Pleuronectinae as subfami- 
lies in Pleuronectidae on the basis of two character- 
istics: eyes almost always dextral and no oil globule 
in yolk of egg. Sakamoto’s (1984a) hypothesis of 
pleuronectid intrarelationships assumed that the 
Pleuronectinae, Samarinae, Rhombosoleinae, 
Poecilopsettinae, and Paralichthodinae were mono- 
phyletic because both eyes were on right side of the 
body, optic nerve of the left eye was always dorsal, 
preopercle had a free margin and fin rays were with- 
out spines. Hensley and Ahlstrom ( 1984), in a review 
of flatfish classification, indicated that the evidence 
for monophyly of Pleuronectidae (sensu Norman, 
1934) was not convincing. The diagnostic characters 
reviewed in Norman (1934) were found to be 
plesiomorphic for the order or had distributions that 
were unknown for many pleuronectiform taxa 
(Hensley and Ahlstrom, 1984). 
Subsequent cladistic analysis of major taxa within 
the order supported the hypothesis that the Pleuro- 
nectidae was not monophyletic and suggested that 
the subfamilies Pleuronectinae, Samarinae, Rhombo- 
soleinae, and Poecilopsettinae should be elevated to 
the family level (Chapleau and Keast, 1988; 
Chapleau, 1993). This new interpretation of taxo- 
nomic ranks in right-eyed flounders was recognized 
