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An ethmoid exposure (os planum) in the orbit of Indri indri (Primates Lemuriformes).

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An Ethmoid Exposure ( 0 s Planum) in the Orbit of
lndri indri (Primates, Lemuriformes)
Departments of Anatomy and Anthropology, Duke liniuersity, Durham, North Carolina
2771 0 and Museum of Paleontology, Uniuersity of Michigan, Ann Arbor,
Michigan 48109
Indri . Indriidae
0 s planum
In 6 of 17 skulls of Indri indri retaining distinct sutures in the
medial orbital wall, a small separate bony element intervenes between the frontal and the maxillolacrimal suture. This element is demonstrably continuous
with the ethmoid on one such skull. The occurrence of an ethmoid exposure in
the orbit of Indri suggests that this trait is not a simple function of orbital size
and convergence. Since such an exposure is found in non-cheirogaleine lemuriforms, its distribution provides no support for the hypothesis that lorises and
galagos are derived from cheirogaleines.
J. F. Meckel, G. Cuvier, and other early students of the comparative anatomy of primates
proposed that the 0s planum, or periorbital
surface of the ethmoid, is a distinctive feature
of higher primates which does not appear in
prosimians. Major ('Ola,b) identified and
figured an 0 s planum in the medial orbital
wall of Tarsius, all the lorisiform prosimians,
and the lemuriforms Microcebus, Lemur, Lepilemur, Hapalemur, Avahi, and Propithecus.
Jones ('17) examined skulls of fetal and adult
Lemur and concluded that the bone which
Major had identified as the planum in Lemur
catta was in reality the inflated presphenopalatine lamina of the palatine bone. Seeking
support for his "tarsioid theory" of human origins, Jones mistakenly extended his findings
to all strepsirhine prosimians (Jones, '29: p.
123), concluding that the strepsirhines uniformly lack an 0s planum and should be excluded from the order Primates by virtue of
this and other respects in which they differ
from the tarsier-anthropoid assemblage
(Jones, '29: p. 144).
A more nearly correct account was provided
by Kollman ('20, '25), who used sectioned fetal
material to prove that Major had correctly
identified an 0s planum in the lorisiforms and
Microcebus, but had mistaken the palatine for
the ethmoid in Lemur, Lepilemur, and Avahi.
Kollman was unable to find an 0s planum in
juvenile or fetal specimens of Lemur fuluus,
AM. J. PHYS. ANTHROP. (1978)48: 535-538.
Hapalemur griseus, Lepilemur mustelinus,
Phaner furcifer, Propithecus verreauni, Avahi,
Zndri, or Daubentonia. He concluded that the
0s planum was a primitive primate feature
that had been secondarily lost in all the
Madagascar lemurs except Cheirogaleus and
Microce bus.
More recent authors have accepted Kollman's account as authoritative, although interpretations of his findings vary. Some
authors follow Kollman in regarding the
shared occurrence of t h e 0 s planum in
lorisiforms and cheirogaleines as a primitive
retention, reflecting the generally primitive
character of these small nocturnal primates
(Charles-Dominique and Martin, '70; Cartmill, '71). Others regard the planum as a convergence between cheirogaleines and lorisiforms (Simons and Russell, '60; Szalay,
'71; Gingerich, '761, or as a shared specialization arguing for a recent derivation of Lorisiformes from a cheirogaleine ancestor
(Cartmill, '75). These interpretations, however, presuppose that the 0s planum is unique
to the cheirogaleines among Madagascar
lemurs. We here report the occasional occurrence of an ethmoid element in the medial
orbital wall of the indriid Zndri indri.
We have examined 53 skulls of Indri in 7
museums in the United States and Europe. Of
these, 17 (32%) retained unfused sutures in
the medial orbital wall. In six of these 17
Fig. 1 Medial wall of left orbit of Indri indri, Leiden No. “H.” E, presumed ethmoid (0s planum); F, frontal; L, lacrimal; M, maxilla; P, palatine; Z, zygomatic.
Fig. 2 Right orbital region of immature Indri indri, American Museum of Natural History No. 100509,
lateral view. N, nasal; S, squamosal; other lettering as in figure 1. Scale indicates 5 mm.
Fig. 3 Indri indri, American Museum of Natural History No. 100509, ventral view. E, ethmoturbinal I; P,
planum; R, roof of orbit (frontal bone)
skulls (35x1, the frontal was separated from rated orbits (Cartmill, '70) tends to refute the
the maxillo-lacrimal suture by a small dis- conjecture that the presence or absence of an
crete bone (fig. 1). Damage to a specimen ethmoid element in the orbital mosaic de(No. 9.811-16) in the Cleveland Museum of pends wholly on the size and convergence of
Natural History reveals this to be an exposure the orbits (Simons and Russell, '60; Cartmill,
of a larger bony lamina directly underlying, '71; Gingerich, '76). The 0s planum's interest
but not fused with, the orbital wing of the to systematists is correspondingly increased.
frontal. This underlying element was most ex- However, its presence in Indri, coupled with
tensively exposed in the orbit of an infant the occurrence of an apparently homologous
Indri, American Museum of Natural History element in the orbital mosaic of some Adupis
No. 100509, in which the orbital processes of (Gingerich and Martin, '78, in preparation),
the maxilla and frontal remain partly unossi- suggests that a n 0s planum may be an anfied (fig. 21, although ossification has pro- cestral lemuroid character. If so, its occurceeded far enough to demonstrate that the rence in cheirogaleines and lorisiforms proupper end of the underlying element's exposed vides no support for the hypothesis that these
part was in contact with the periorbita. Dam- two groups of prosimians share a more recent
age to the left side of this skull reveals that common ancestry than either does with nonthis element is continuous with the ethmotur- cheirogaleid Madagascar lemurs (Szalay and
binals (fig. 3). We conclude that the discrete Katz, '73; Tattersall and Schwartz, '74; Cartelement seen in some adult skulls a t this point mill, '75).
We thank P. Napier (British Museum,
also represents the ethmoid, and is therefore a
true 0s planum. In most adult specimens, the Natural History); J. Lessertisseur (Institut
planum has either fused with, or disappeared d'Anatomie Comparee, Paris) 1 L. B. Holthuis
(Rijksmuseum van Naturlijke Historie,
beneath the surrounding membrane bones.
The occurrence of an 0s planum in Zndri, Leiden); F. A. Jenkins, Jr., D. McClearn, M. E.
which has relatively small and widely-sepa- Rutzmoser, and A. C. Walker (Museum of
Comparative Zoology, Harvard) ; S. Anderson
(American Museum of Natural History); C.
Howe, D. C. Johanson, and W. Kimbel (Cleveland Museum of Natural History); and H. W.
Setzer and R. W. Thorington, Jr. (U. S.
National Museum), for facilitating our access
to t h e collections of their respective museums.
The senior author’s travel and research was
made possible by NIH Grant l-KO4-HD0008301; t h a t of t h e junior author was aided by t h e
Turner Fund of t h e University of Michigan’s
Department of Geology. We thank K. Brown,
R. F. Kay, and R. D. Martin for their help and
Cartmill, M. 1970 The Orbits of Arboreal Mammals: A
Reassessment of the Arboreal Theory of Primate Evolution. Ph.D. thesis, University of Chicago.
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1975 Strepsirhine basicranial structures and
the affinities of the Cheirogaleidae. In: Phylogeny of the
Primates. W. P. Luckett and F. S. Szalay, eds. Plenum,
New York, pp. 313-354.
Charles-Dominique, P., and R. D. Martin 1970 Evolution of
lorises and lemurs. Nature, 227: 257-260.
Gingerich, P. D. 1976 Cranial anatomy and evolution of
early Tertiary Plesiadapidae (Mammalia, Primates).
Papers on Paleontol., Univ. Mich. Mus. Paleontol., 15:
Gingerich, P. D., and R. D. Martin (1978, in preparation)
The Cambridge skull of Adapis parisiensis.
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region of the skull of Lemur. Proc. Zool. Soc. London,
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(Zool.), 1: 1-20.
Major, C. I. F. 1901a On some characters of the skull in
lemurs and monkeys. Proc. Zool. SOC.London, 1901:
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anatomy of Necrolemur. Breviora Mus. Comp. Zool. (Harvard), 127: 1-14.
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52: 139-192.
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indri, primate, exposure, planum, ethmoid, lemuriform, orbit
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