Catopithecus is an early fossil catarrhine that post dates the platyrrhine – catarrhine divergence but predates the hominoid – cercopithecoid divergence. It is known from more than 16 specimens of a single species, Catopithecus browni, found in the Jebel Quaranti Formation of the Fayum Province, Egypt.1 The Jebel Quaranti Formation has been divided into two main faunal zones based on the fact that the fauna found in the lower portion of the quarry appear to be more primitive that fauna found in the upper section of the quarry .2 The upper zone has been dated to older than 31 ± 1 myr based on the dating of a basalt layer that lies immediately above the formation and Nicolas Steno’s Law of Superposition.1 The lower zone contains the late Eocene green shale unit called Locality-41 (L-41) in which all the specimens of Catopithecus browni have been found. The relative dating of L-41 based on paleomagnetic correlations places it at 36 Myr.3 This date indicates that Catopithecus lived during the Eocene epoch, making Catopithecus the earliest known anthropoid.4

Etymology

From the Greek “cato” meaning “below” and “pithekos” meaning “one who plays tricks” (an ape), therefore Catopithecus means “below (before) ape.”1
The species name, browni, is form the discoverer of the type specimen, Mark Brown.1

Morphology

The type specimen of C. browni, CGM 41885, is a right mandible discovered in 1987 by Mark Brown. The mandible was found with intact Molars1-3, and Premolars3-4, and alveoli are present for a canine tooth and Incisors1-2, indicating a mandibular (lower?) dental formula of 2.1.2.3. This dental formula was demonstrated to reflect the upper (maxillary) dental formula in specimen DPC 8701 which was discovered in L-41 in 1988.2 At least 17 specimens, including 6 almost intact skulls, have been described and are listed below:

Skulls: DPC 8701, CGM 42222, DPC 11388, DPC 11594, DPC 12367, and CGM 41900
Mandibles and other fragments: DPC 7339, 7340, 7341, 7342, 8772, 9869, 11434, 11541, 11638, and DPC 11943

Analyses of the skull specimens show that C. browni had post orbital closure developed to the degree seen in extant anthropoids.4 The orbit to skull size ratio of the C. browni skulls were compared to ratios of modern nocturnal and diurnal anthropoids in Rasmussen and Simmons (1992) and demonstrated that C. browni was most likely diurnal. The interorbital distance of the skulls was also compared to five taxa of modern primates in Rasmussen and Simons(1992), and demonstrated that C. browni had an interorbital distance range comparing it most closely to those of modern prosimians and callitrichids.4

In contrast to extant anthropoids that express a fused mandibular symphysis, the mandibular symphysis of C. browni was observed to be unfused but covered in small rugose features in at least seven specimens.5
The dentition of C. browni shows well developed crests on the bucchal side (nearest to the throat) of the tooth which is indicative of a foliverous and/or insectivorous diet, both of which require teeth expressing the cutting edges seen on the molars of C. browni.5 The three molars of C. browni decrease in size posteriorly, meaning that M1>M2>M3.1 Allometric molar size regressions were used to calculate an estimated weight range of 600-900g for C. browni.4 Both upper and lower canines were observed to be relatively large and long compared to the adjacent spatulate, and vertically emplaced incisors which are all features of modern anthropoids.6

The size of the olfactory bulb was measured as 4-5mm long and 4mm wide from specimen DPC 11434, a piece of fragmented frontal bone, using a latex mold of the outline of the imprinted braincase.4 Compared to the size of its cranium, this large olfactory bulb indicates a higher reliance on olfactory than visual senses, a feature not shared with modern anthropoids. The same speceimen preserved a cross section of the ectotypanic ring, showing it to be an annular structure which was fused to the margin of the bulla similar in structure to modern platyrrhines.4

CGM 42222 is the specimen in which the outline of the braincase was best preserved. After adjusting for potential distortion caused by crushing, a brain model was prepared that determining the approximate volume of the brain of CGM 42222 to be 3.1cm3 ± 10%.4 Using the estimations of brain volume and body weight an encephalization quotient (EQ) was calculated that showed the brain volume to body size ratio of C. browni is small compared to extant anthropoids of similar weight.4

Habitat

Floral and Faunal fossils associated with oligopithecine specimens suggest that Catopithecus and other oligopithecines inhabited wet, warm, forested and swampy, tropical environments.5

Conclusions

C. browni is a morphologically primitive anthropoid which occurred near the base of the catarrhine radiation. Its primitive features include an unfused mandibular symphysis, relatively large olfactory bulbs, small brain size, and large dentition compared to face and braincase. C. browni expresses notable derived anthropoidean and catarrhine traits including an “anthropoidean-like” auditory region, a reduction in the number of premolars per quadrant of both the maxilla and mandible, and the degree of observed postorbital closure.4 These features have led to a general agreement on the phylogenetic affinity of Catopithecus as a sister taxon of propliopithecine catarrhines .3
Claims of “substantial sexual dimorphism” have been made by several scientists but it is an extremely difficult claim to confirm as a result of the small sample size analyzed and conclusions based on sexual dimorphism should be acknowledged as possibilities, not as concrete fact.

Works cited

1 Simons, Elwyn L. "Description of two genera and species of Late Eocene Anthropoidea from Egypt."
Proceeding of the National Academy of Science 86 (1989): 9956-960. Print.

2 Simons, Elwyn L. "Discovery of the Oldest Known Anthropoidean Skull from the Paleogene of Egypt."
Science 247.4950 (1990): 1567-1569. Print.

3 Simons, Elwyn L., Michael J. Plavcan, and John G. Fleagle. "Canine sexual dimorphism in Egyptian
Eocene anthropoid primates: Catopithecus and Proteopithecus." Proceedings of the National
Academy of Science 96 (1999): 2559-562. Print.

4 Simons, Elwyn L., and Tab D. Rasmussen. "Skull f Catopithecus browni, an Early Tertiary Catarrhine."
American Journal of Physical Anthropology 100 (1996): 261-92. Print.

5 Rasmussen, Tab D., and Elwyn L. Simons. "Paleobiology of the Oligopithecines, the Earliest Known
Anthropoid Primates." International Journal of Primatology 13.5 (1992): 477-508. Print.

6 Simons, Elwyn L. "Skulls and anterior teeth of Catopithecus (Primates: Anthropoidea) from the Eocene
and anthropoid origins." Science 268.5219 (1995): 1885-888. Print.