THE SCLERAL OSSICLES OF OPISTHOCOMUS AND 
THEIR PHYLOGENETIC SIGNIFICANCE 
KEVIN DE QUEIROZ AND DAVID A. GOOD 
Museum of Vertebrate Zoology and Department of Zoology, University of California, 
Berkeley, California 94720 USA 
ABSTRACT.?Dried scleral rings of Opisthocomus, Galliformes, Cuculiformes, and other birds 
were examined to determine the pattern of ossicle overlap in Opisthocomus and its bearing 
on the phylogenetic relationships of this taxon. Although Opisthocomus shares a derived 
number of 12 scleral ossicles with cuculid Cuculiformes, the pattern of ossicle overlap differs. 
Nevertheless, fewer modifications are required to derive the number and pattern of ossicles 
in Opisthocomus from the conditions in cuculid or musophagid Cuculiformes than from the 
conditions in any galliform. Our findings also indicate that the scleral rings of birds, unlike 
those of lizards, often do not conserve overlap relations between adjacent ossicles during 
phylogeny. Received 2 March 1987, accepted 23 July 1987. 
THE phylogenetic relationships of the Hoa- 
tzin (Opisthocomus hoazin) have been the subject 
of a long-standing controversy in systematic or- 
nithology. An enigmatic inhabitant of the 
flooded forests of Amazonia, Opisthocomus has 
been regarded as most closely related to many 
avian taxa, but most commonly to either Cu- 
culiformes or Galliformes (Olson 1985; re- 
viewed by Sibley and Ahlquist 1972,1973). One 
character that has been used to support a close 
relationship betv^reen the Opisthocomus and Cu- 
culiformes is the morphology of the sclerotic 
ring, a ring of small bones that develops within 
the sclera in the corneal hemisphere of the eye 
in birds and many other vertebrates (Edinger 
1929). Lemmrich (1931) found that the scleral 
ring of Opisthocomus consists of 12 ossicles per 
eye, which is lower than the 13-16 ossicles he 
found in 10 species of cracid and phasianid Gal- 
liformes (all of which showed a modal number 
of 14) but is identical to that found in a single 
cucuiiform, Cuculus canorus. 
In addition to ossicle number, the pattern in 
which the scleral ossicles overlap varies among 
avian taxa (Lemmrich 1931, Curtis and Miller 
1938). Because the same number of ossicles may 
have been derived independently, simple counts 
can be misleading. The pattern of ossicle over- 
lap provides evidence about ossicle homologies 
and hence about cases in which the same num- 
ber of ossicles has been derived through the 
loss of different individual ossicles. 
Although the pattern of scleral ossicle over- 
lap is known for many birds, it has been un- 
known for Opisthocomus. We compared the 
numbers of ossicles and their patterns of over- 
lap in the scleral rings of larger samples of Opis- 
thocomus, Cuculiformes, Galliformes, and other 
relevant avian taxa. Our findings have impli- 
cations both for the relationships of Opis- 
thocomus and for the manner in which scleral 
rings evolve. 
MATERIALS AND METHODS 
Twenty-four scleral rings from 14 Opisthocomus in- 
dividuals were compared with rings from 42 galli- 
form and 36 cucuiiform species representing all of 
the recognized families, subfamilies, and tribes (Mo- 
rony et al. 1975) in those groups (Table 1). To un- 
derstand variation in scleral ossicle overlap patterns 
for a given number of ossicles, we also examined 
representatives of other avian groups with the same 
number of ossicles as Opisthocomus (Spheniscidae, Su- 
lidae, Psittacidae, and Todidae). 
We examined only the dried, detached scleral rings 
from museum skeletal specimens; no rings were ex- 
amined in situ. In many cases scleral ossicle patterns 
could be distinguished in these rings without further 
preparation, but in others adherent connective tissue 
was removed. This was accomplished by immersing 
the rings in water or ethanol to soften the adherent 
tissue, which was then removed with forceps. Because 
we used only detached scleral rings, the side of the 
head from which each ring came had to be inferred. 
We used the patterns in the same or closely related 
taxa observed by Lemmrich (1931) in conjunction with 
the fact that the widest ossicles occur at the tempo- 
rodorsal edge of the ring (best seen in Fig. ID) to 
determine whether a given ring was from the right 
or left side. 
We followed Lemmrich's (1931) conventions for 
numbering ossicles and describing patterns of ossicle 
overlap. Lemmrich recognized two basic patterns of 
ossicle overlap, which he designated Types A and B. 
29 The Auk 105: 29-35. January 1988 
30 DE QUEIROZ AND GOOD [Auk, Vol. 105 
1+     ^?'ij 
Fig. 1. Modal patterns of scleral rings in Opisthocomus, Galliformes, and Cuculiformes. (A) Opisthocomus 
hoazin (Opisthocomidae), (B) Alectura lathami (Megapodiidae), (C) Aburria aburri (Cracidae), (D) Nuntida meleagris 
(Phasianidae), (E) Crotophaga ani (Cuculidae), (F) Tauraco hartlaubi (Musophagidae). All rings are from the right 
side, seen in corneal view. Scale equals 5 mm. 
In Type A two ossicles in each ring overlap, and two 
others are overlapped by, the immediately adjacent 
ossicles; all others have one edge above and one be- 
neath the adjacent ossicles (Fig. lA, C-F), Lemmrich 
pointed out that the most dorsal ossicle in Type A 
rings always overlaps both its neighbors, and he re- 
ferred to such overlapping ossicles as "+" elements. 
Approximately opposite this dorsal "+" element is 
another "+" element that he designated ossicle num- 
ber 1. He counted ossicles around the ring postero- 
dorsally from this ventral "+" element, clockwise on 
the right ring and counter-clockwise on the left. He 
then recorded the positions of the "+" elements and 
those overlapped by both of their neighbors, the "?" 
elements, by listing the positions of the "+" elements 
separated by commas, then a semicolon, followed by 
the positions of the "-" elements separated by com- 
mas. The ossicles overlapped by one neighbor and 
overlapping the other ("imbricating" ossicles) are ex- 
cluded in this notation. In this way, the ring in Fig. 
lA would be recorded as 1,7;4,9. In Lemmrich's Type 
B pattern, only a single ventral "+" element and a 
single dorsal "-" element are present. Hence, the 
pattern in Fig. IB is designated 1;7. 
Hypotheses of phylogenetic relationship were based 
on the tenet that only shared derived characters con- 
tain useful information about recency of common an- 
cestry (Hennig 1966). Character polarity was deter- 
mined by outgroup comparison (e.g. Maddison et al. 
1984). 
RESULTS 
The distribution of scleral ossicle number and 
pattern in Galliformes, Cuculiformes, and Opis- 
thocomus is listed in Table 1. Opisthocomus (Fig. 
lA) showed a modal number of 12, v^rith a range 
from 11 to 13. Galliformes (Fig. IB-D) showed 
a modal number of 14, with only about 15% of 
the specimens having 13 or 15, often on a single 
side. No specimens were observed to have 16 
ossicles, a number seen rarely in Callus gallus 
(Lemmrich 1931). Lemmrich examined only a 
single cuculiform, Cuculus canorus, with 12 os- 
sicles; all 25 cuculid species we examined agreed 
with this finding (Fig. IE). All musophagids 
examined (Fig. IF) had 13 ossicles. 
Opisthocomus, Cuculiformes, and cracid and 
phasianid Galliformes all characteristically pos- 
sess Lemmrich's (1931) Type A pattern of ossicle 
overlap. The modal ossicle distribution pattern 
in Opisthocomus (Fig. lA) was 1,7;4,9, although 
five other patterns were observed (Table 1). 
Within Galliformes, three major patterns were 
January 1988] Scleral Ossicles of Opisthocomus 31 
TABLE 1. Numbers and patterns of scleral ossicles in representative species of Galliformes, Cuculiformes, 
and Opisthocomus. Pattern designations are described in the Methods. Numbers in parentheses indicate 
subsample size. Modal configurations for variable species are in boldface. 
Specimens/rings 
Taxon examined No. of ossicles Pattern 
Megapodiidae 
Aepypodius arfakianus 1/2 14 1;7 
Alectura lathami 1/2 14 1;7 
Macrocephalon maleo 1/2 14 1;7 
Megapodius freycinet 6/10 14 l;6(l) 
14 1;7 (9) 
Cracidae 
Aburria aburri 2/4 14 1,9;7,10 
A. pipile 4/6 14 1,9;7,10 
Chamaepetes goudotii 2/3 14 1,9;7,10 (2) 
15 1,9;7,11 (1) 
Crax daubentoni 1/2 15 1,9;7,10 
C. mitu 1/2 14 1,9;7,10 
C. pauxi 2/3 14 1,9;7,10 
15 1,9;7,10 
15 1,10;8,11 
C. rubra 2/4 13 1,8;6,9 (2) 
14 1,9;7,10(1) 
15 1,9;7,10 (1) 
Nothocrax urumutum 1/2 14 1,9;7,10 
Ortalis canicollis 3/3 14 1,9;7,10 
O. cinereiceps 1/2 14 1,9;7,10 
O. garrula 1/1 14 1,9;7,10 
O. motniot 2/4 14 1,9;7,10 
O. vetula 3/6 14 1,9;7,10 
Penelope albipennis 1/2 14 1,9;7,10 
P. jacquacu 6/10 14 1,9;7,10 
P. purpurascens 2/4 14 1,9;7,10 (3) 
15 1,9;7,11 (1) 
Phasianidae 
Meleagridinae 
Agriocharis ocellata 
Tetraoninae 
Bonasa umbellus 
Dendragapus obscurus 
Lagopus lagopus 
Tympanuchus cupido 
T. phasianellus 
Odontophorinae 
Colinus cristatus 
C. virginianus 
Dactylortyx thoracicus 
Callipepla californicus 
Philortyx fasciatus 
Phasianinae 
Perdicini 
Coturnix coturnix 
Perdix perdix 
Phasianini 
Gallus gallus 
111 14 1,9;640 
1/2 14 1,9;6,10 
3/4 14 1,9;6,10 
1/1 14 1,9;7,10 
1/1 15 1,10;8,11 
1/2 14 1,9;7,10 
15 1,10;8,11 
1/1 14 1,9;6,10 
1/2 14 1,9;6,10 
2/3 14 1,9;6,10 
6/29 14 1,8;6,10 (2) 
14 1,9;6,10 (23) 
14 1,9;6,11(1) 
15 1,9;6,10(1) 
15 1,9;7,11(2) 
1/2 14 1,9;6,10 
1/1 14 1,9;6,10 
1/1 14 1,9;6,10 
1/1 14 1,9;6,10 
32 DE QUEIROZ AND GOOD [Auk, Vol. 105 
TABLE 1.    Continued. 
Taxon 
Specimens /rings 
examined No. of ossicles Pattern 
G. sonneratii 
Lophura stvinhoei 
Phasianus colchicus 
Numidinae 
Acryllium vulturinum 
Guttera plumifera 
G. pucherani 
Numida meleagris 
N. mitrata 
Opisthocomidae 
Opisthocomus hoazin 
Musophagidae 
Corythaeola cristata 
Corythaixoides leucogaster 
Crinifer piscator 
C. zonurus 
Musophaga rossae 
Tauraco corythaix 
T. erythrolophus 
T. hartlaubi 
T. persa 
T. porphyreolophus 
T. schalowi 
Cuculidae 
Cuculinae 
Cacomantis pyrrhophanus 
Chakites basalis 
Clatnator jacobinus 
Cuculus palUdus 
C. soUtarius 
Misocalius osculans 
Phaenocophaeinae 
Coccyzus americanus 
C. melacoryphus 
Piaya cayana 
Crotophaginae 
Crotophaga ani 
C. sulcirostris 
Guira guira 
111 14 1,9;6,10 
1/2 14 1,9;6,10 
1/1 14 1,9;6,10 
4/6 13 1,9;6,10(1) 
14 1,9;6,10 (4) 
15 1,9;6,11 (1) 
3/4 14 1,9;6,10 (3) 
14 1;10(1) 
1/2 14 1,9;6,10 
15 1,9;6,10 
5/9 14 1,9;6,10 (8) 
14 1,9;7,10(1) 
2/2 14 1,9;6,10 
14/24 n 1,6;4,7(1) 
11 1,6;4,8 (3) 
11 1,7;4,8 (1) 
12 1,7;4,9(11) 
12 1,7;5,9 (1) 
13 1,7;4,9 (5) 
13 1,8;4,10 (2) 
2/3 13 1,7;4,9 
1/2 13 1,6;4,9 
13 1,7;4,8 
1/2 13 1,7;4,9 
14 1,7;4,10 
1/1 13 1,7;4,9 2/4 
13 1,7;4,9 
4/7 13 1,7;4,9 
2/3 12 1,7;4,9 (1) 
13 1,7;4,9 (2) 
6/7 13 1,7;4,9 (6) 
14 1,7;4,9 (1) 
1/2 13 1,7;4,9 
1/1 13 1,7;4,9 2/4 
11 1,6;4,8 (1) 
12 1,7;4,9(1) 
13 1,7;4,9 (2) 
1/1 12 1,8;5,10 2/2 
12 1,8;5,10 
3/6 12 1,8;5,10 
1/2 12 1,8;5,10 
1/1 12 1,8;5,10 
1/2 12 1,8;5,10 
3/4 12 1,8;5,10 
1/1 12 1,8;5,10 4/6 12 
1,8;5,10 
2/4 12 1,8;5,10 
5/9 12 1,8;5,10 
10/15 12 1,8;5,10 (13) 
13 1,6;4,10 (1) 
14 1,8;5,10(1) 
January 1988] Scleral Ossicles of Opisthocomus 33 
TABLE 1.    Continued. 
Specimens/rings 
Taxon examined           No. of ossicles Pattern 
Neomorphinae 
Dromococcyx pavoninus 1/2 12 1,8;5,10 
D. phasianellus 1/2 12 1,8;5,10 
Geococcyx californianus 3/6 12 1,8;5,10 
Morococcyx erythropygus 3/6 12 1,8;5,10 
Neomorphus geoffroyi 1/2 12 1,8;5,10 
Couinae 
Coua caerulea 2/3 12 1,8;5,10 
C. cristata 1/1 12 1,8;5,10 
Centropodinae 
Centropus bengalensis 1/1 12 1,8;5,10 
C. goliath 3/6 11 1,7;4,9 (2) 
12 1,8;5,10 (4) 
C. phasianius 1/1 12 1,8;5,10 
C. senegalensis 1/1 12 1,8;5,10 
C. sinensis 2/4 12 1,8;5,10 
C. superciliosus 3/5 12 1,8;5,10 
seen: cracids (Fig. IC) had primarily a 1,9;7,10 
pattern, phasianids (Fig. ID) had mostly 1,9;6,10, 
and megapodiids (Fig. IB), the only examples 
in the present study of Lemmrich's Type B pat- 
tern, had 1;7. Most musophagid Cuculiformes 
(Fig. IF) had 1,7;4,9 and most cuculids (Fig. IE) 
1,8;5,10. 
In comparisons within families whose mem- 
bers vary widely in body (or eye) size (e.g. Co- 
turnix vs. Agriocharis within Phasianidae), ossi- 
cle configuration did not correlate with size. 
DISCUSSION 
As pointed out by Lemmrich (1931), cuculids 
are more similar to Opisthocomus than is any 
group of Galliformes in having 12 ossicles per 
ring; however, other groups of birds share this 
number. Lemmrich found 12 ossicles in the 
spheniscid Catarrhactes (= Eudyptes) chrysocome 
(1,8;5,10), the sulid Sula bassanus (1,7;4,10), and 
the 11 psittacid species he examined (1,6;4, 
9/l,7;4,9/l,8;5,ll). There are 12 ossicles in the 
trochilid Selasphorus rufus (1,6;4,8) (Curtis and 
Miller 1938). In addition, we observed 12 os- 
sicles in Sula capensis (1,5;4,10), S. serrator 
(1,6,8;4,7,10), and S. abbotti (overlap patterns not 
determined), and in 1 out of 2 eyes in Todus 
todus (1,8;5,10). 
Birds probably are derived from within Di- 
nosauria, and thus the closest outgroups to birds 
are to be found among the nonavian members 
of this monophyletic taxon (e.g. Ostrom 1976, 
Gauthier 1984, Gauthier and Padian 1985). We 
know of no complete scleral rings among non- 
avian Saurischia, but complete rings with a range 
from 13 to 15 ossicles occur in Ornithischia 
(Edinger 1929, Russell 1940, Ostrom 1961, Gal- 
ton 1974). Heilmann (1926) reported 14 scleral 
ossicles in Archaeornis {=Archaeopteryx), al- 
though Wellnhofer (1974) counted "approxi- 
mately 12" in a different specimen. Because Cu- 
culiformes, Galliformes, and Opisthocomus are 
all neognaths, ratites plus tinamous constitute 
an even closer outgroup for the relationships 
under investigation in the present study (Pra- 
ger et al. 1976, Sibley and Ahlquist 1981, Stapel 
et al. 1984, Cracraft 1986). Struthio and Dromaius 
have 15 ossicles (Lemmrich 1931). We observed 
15-16 in the tinamou Crypturellus cinnamomeus 
and 15 in Tinamotis pentlandii. Twelve ossicles, 
as seen in Opisthocomus and Cuculidae, is a low 
number compared with that seen in the most 
significant outgroups and suggests that such a 
number is derived relative to the mode of 14 
seen in Galliformes. 
As noted above, various avian taxa possess 
what appears to be a derived number of 12 scler- 
al ossicles. The positions of the "+" and " ?" 
ossicles, however, often differ among taxa with 
the same total number of ossicles. For example, 
other taxa with a mode of 12 ossicles have the 
following modal patterns: Eudyptes (Lemmrich 
1931) and Todus (this study) 1,8;5,10; Sula (3 spp.) 
34 DE QUEIROZ AND GOOD [Auk, Vol. 105 
1,5;440/1,7;4,10/1,8;4,10/1,6,8;4,7,10 (Lemm- 
rich 1931, this study); Psittacidae (5 spp.) 1,6;4,9/ 
1,7;4,9/1,8;5,11 (Lemmrich 1931, Curtis and 
Miller 1938); and Trochilidae (1 sp.) 1,6;4,8 
(Curtis and Miller 1938). These differences might 
be taken to suggest that reduction to 12 ossicles 
has been achieved independently in each of the 
taxa with "+" and "?" ossicles in different po- 
sitions through the loss of different imbricating 
ossicles. In lizards, conservation of overlap pat- 
tern and loss of imbricating ossicles can account 
for most transformations between scleral rings 
with different overlap patterns (de Queiroz 1982, 
Underwood 1984). In birds, however, lack of 
conservation of overlap relationships between 
adjacent ossicles is supported by the occurrence 
of Type A and Type B rings with the same total 
number of ossicles within a single species (e.g. 
Guttera plumifera) or in closely related taxa (e.g. 
megapodes vs. other Galliformes), and by the 
occurrence within single species of rings con- 
forming to the same basic type (A or B) but 
having the "+" and "?" ossicles at different 
positions in the ring (e.g. Opisthocomus) (Table 
1). These differences presumably result from 
direct shifts in overlap between adjacent ossi- 
cles, with conservation of ossicle number. 
Given the preceding considerations, we de- 
termined the minimum number of evolution- 
ary events necessary to transform the scleral 
ring of one group into that of another, A cuclid- 
Opisthocomus transition requires only a single 
change, in which cuculid ossicle 1 becomes 
Opisthocomus ossicle 12 by shifting overlaps such 
that cuculid ossicle 1 no longer overlaps, but is 
instead overlapped by cuculid ossicle 2 (which 
then becomes Opisthocomus ossicle 1). Similarly, 
a musophagid-Opisthocomus transition requires 
only a single step, with an ossicle either added 
or lost between the "?" ossicle 9 and "+" os- 
sicle 1. On the other hand, at least three changes, 
involving both losses and shifts in overlap of 
ossicles, are necessary between Opisthocomus and 
any of the galliform patterns. 
Considerable individual variation was seen 
in ossicle overlap in certain taxa, notably Opis- 
thocomus (Table 1). We examined this variation 
to determine if any of the nonmodal variants 
might be more similar (i.e. fewer changes) to 
any of the cuculiform or galliform patterns than 
was the modal, suggesting a possible transfor- 
mation series. None of the Opisthocomus variants 
was any more similar to cuculids or musophag- 
ids than was the modal pattern, however, nor 
did any of the variants make a galliform rela- 
tionship more tenable; at least three changes 
were necessary to transform any of them to any 
of the galliform patterns. 
The shared possession of a derived reduced 
number of ossicles per ring in conjunction with 
the minimum number of steps necessary be- 
tween modal patterns suggests that Opisthoco- 
mus is more closely related to Cuculiformes than 
to Galliformes. As with an Opisthocomus-cucu- 
liform transition, however, only one step is 
necessary between Opisthocomus and 12-ossicled 
Spheniscidae, Sulidae, and Todidae, 13-ossicled 
Coraciidae and Alcedinidae (Lemmrich 1931), 
or 11-ossicled Columbidae (Lemmrich 1931). It 
is therefore just as reasonable on the basis of 
scleral ring morphology to infer that Opistho- 
comus is allied with one of those groups as with 
Cuculiformes. Further, some 12-ossicled psit- 
tacids share an identical pattern to that of Opis- 
thocomus. 
We do not mean to suggest that the Hoatzin 
is closely related to penguins, boobies, todies, 
rollers, kingfishers, doves, or parrots. Compar- 
isons with these taxa are provided only to il- 
lustrate that the evidence for the placement of 
Opisthocomus based on scleral ring morphology 
is equivocal. In general, limited conservation of 
overlaps between adjacent ossicles makes it dif- 
ficult to determine homologies in the avian 
scleral ring, and this limits the usefulness of 
overlap patterns as systematic characters. 
Nevertheless, given the current lack of evi- 
dence supporting a close relationship to some 
other group of birds (Sibley and Ahlquist 1972, 
1973), one might assume that Opisthocomus is 
allied either with Galliformes or Cuculiformes. 
Under this assumption, scleral ring morphol- 
ogy suggests a closer relationship to Cuculi- 
formes. 
ACKNOWLEDGMENTS 
We thank the following curators for allowing us to 
examine material under their care; G. F. Barrow- 
clough (AMNH), J. W. Fitzpatrick (FMNH), N. K. 
Johnson (MVZ), J. V. Remsen (LSU), C. G. Sibley 
(YPM), R. W. Storer (UMMZ), and R. L. Zusi (USNM). 
We thank D. A. Bell, J. G. Groth, N. K. Johnson, B. C. 
Livezey, J. I. Smith, D. W. Steadman, and D. B. Wake 
for critical attention to previous drafts of this paper. 
In addition, the senior author thanks P. Sereno for 
information about literature on ornithischian scleral 
ossicles. 
January 1988] Scleral Ossicles of Opisthocomus 35 
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