How to identify a primate skull

The Key

Primate skulls can be identified through a few key characteristics.

1. Large cranial vault. The part of the skull where the brain would sit is relatively large in comparison to other mammals.

2. Forward facing eyes. Primates have front facing eyes, with the sockets positioned on the front of the face. Other mammals have their orbits on the sides of their skulls.

3. Post orbital bar. On primate skulls, the zygomata (cheekbones) and frontal bones surround the eye socket completely. This is called a post orbital bar.

Key for Asian primate cranial material

How to use the key

The keys works by guiding the user through a set of steps to establish a cranial specimen to genus level for any given extant Asian primate skull. Initially, it needs to be established whether the specimen is actually primate, before progressing to differentiate between haplorhines and strepsirrhines, tarsiiformes and simiiformes, and the superfamilies, families and subfamilies of Asian primates. Once the specimen has been identified to family level, all genera within that family are listed with their defining characteristics. As a worked example, genus Pongo is identified to species level for the three extant species; Pongo pygmaeus, Pongo abelii, and Pongo tapanuliensis. Please note that the drawings are not to scale.

Pathologies

Before beginning the key, carefully examine the skull for any damage inflicted pre- or postmortem, and make a note of it to ensure that it doesn’t influence your decision making when moving through the key. Examples of such pathologies include gunshot wounds and metabolic bone disease.
(Farrell et al., 2015) (Figure a; Figure b).

Figure 1: Two skulls from Mandrillus sphinx. The specimen on the left was a captive individual suffering from a metabolic bone disease, likely osteomalacia. The specimen on the right was an individual from the wild, not suffering from any metabolic bone disease. (Taken from Farrell et al., 2015: 404).

Figure 2: Examples of gunshot wounds to the cranium. a. shows a gunshot wound which produced a radial fracture, and b. shows a wound which did not. (Taken from Klepinger, 2006: 109).

Identifying a Primate Skull

All primates share a number of common cranial features, such as forward facing orbits, permitting visual field overlap, enabling stereoscopic vision (Gebo, 2014). The frontal and zygomatic bones completely surround the orbits, forming a post orbital bar, as can be seen in comparison to other mammals in Figure [3] (Gebo, 2014). The primate brain is also enlarged relative to other mammals, which is reflected in the size of the cranial vault, and the foramen magnum is under the base of the skull (Gebo, 2014; Swindler, 1998). The angle of the face should also be noted; the majority of primate faces are positioned beneath the cranium, whereas the faces of most other mammals are in front of it (Swindler, 1998).

Figure 3: Comparison in cranial morphology between a. dog (Canis lupus familiaris), b. cat (Felis catus), c. squirrel (Sciuridae) and d. Nycticebus. (Drawn by author).

Figure 4: Labelled lateral view of a generalised primate skull (Drawn by author).

Figure 5: Inferior view of a generalised primate skull (Drawn by author).

Figure 6: Anterior view of a generalised primate skull (Drawn by author).

Section 1:

Haplorhine or Strepsirrhine?

Relatively short cranial vault (Gebo, 2014). In the nuchal region, there is cresting posteriorly on the occipital, and on the base of the skull the position of the foramen magnum is quite posterior. Orbits oriented obliquely with postorbital bar evident. Unfused frontal bone with metopic suture running horizontally through the middle. Mandibles unfused anteriorly at symphysis; middle mandibular process lower than coronoid process; tooth row aligned with mandibular condyle. Tooth comb present- thin, blade like lower incisors extending away from the mandible; very small upper incisors with large gap separating them (Gebo, 2014)

Post-orbital closure at least to some extent; short, vertical naso-lacrimal duct (Fleagle, 2013). Fused mandibular symphysis (Gebo, 2014)

Figure 7: Comparison between a. strepsirrhine skull and b. haplorhine skull (Drawn by author).

Section 2:

Tarsiiforme or Simiiforme?

Mandibles unfused anteriorly at symphysis; middle mandibular process lower than coronoid process (Gebo, 2014)

Complete post-orbital closure. Mandible tall, particularly across ascending ramus, condyloid process similar height to coronoid process, mandible fused anteriorly at symphysis, angular process absent. Peg-like lower incisors and procumbent upper incisors (Gebo, 2014)

Figure 8: Comparison between a. tarsiifome skull and b. simiifome skull (Drawn by author).

Section 3:

Cercopithecoidea or Hominoidea?

Bilodonty present; dental formula 2-1-2-3 for upper dentition and 2-1-2-3 for lower dentition (Gebo, 2014). Relatively narrow nasal pyriform aperture and narrow tooth rows; narrow nasal region and palate (Fleagle, 2013)

Broad nasal region; broad palate; larger neurocranium; simple molars (Fleagle, 2013)

Figure 9: A comparison between a. the Cercopithecoidea and b.
Hominoidea. (Taken from Fleagle, 2013: 120)..

Figure 10: A diagram showing bilophodont molars. (Taken from Gebo, 2014: 99).

Section 4:
Superfamily Hominoidea- Family Hylobatidae or Family Hominidae?

Broad palate and nasals (Fleagle, 2013). Five cusps on lower molars, rounded molar cusps; upper premolars bicuspid; upper molars have anterior trigon and posterior hypocone (Swindler, 1998)

Relatively orthaganous, shallow faces; large orbits with protruding rims, wide interorbital distance. Globular braincase with no nuchal cresting; shallow mandible with broad ascending ramus. Simple molars with low, rounded cusps and broad basins; relatively short but board incisors; long, daggerlike canines (Fleagle, 2013). Dental formula 2-1-2-3 for upper dentition and 2-1-2-3 for lower dentition (Gebo, 2014)

Figure 11: Comparison between a. Hominidae skull and b. Hylobatidae skull (Drawn by author).

Section 5:

Superfamily Cercopithecoidea- Subfamily Cercopithecinae or Subfamily Colobinae?

 Lower incisors lack lingual enamel, nasal bones elongated, low cranial vault, elongated face (Strasser and Delson, 1987). P3 is a sectorial premolar, possessing a long cutting edge to sharpen the upper canine (Gebo, 2014). Narrow interorbital region; broad incisors; shallow mandible; low cusps on teeth (Fleagle, 2013)

Shortened trigonid, posteriorly deepened mandibular corpus (Strasser and Delson, 1987). Broad interorbital region; narrow incisors; high cusps on teeth (Fleagle, 2013), hypoconulids on lower M3s (Gebo, 2014)

Figure 12a: A comparison between a. the Cercopithecines and b. Colobines. (Taken from Fleagle, 2013: 121).

Figure 12b: Comparison between a. three cercopithecine monkeys and b. three colobine monkeys. (Taken from Fleagle, 2013: 121.)

Section 6:

Family Lorisidae- Genus Loris or Genus Nycticebus?

Lorisidae is the only strepsirrhine family with genera in Asia, both of which share the following features; ectotympanic ring attached outside auditory bulla; orbits less frontated, face slightly upward when viewed from the side. Dental formula is 2-1-3-3 for upper dentition and 2-1-3-3 for lower dentition; reduced upper M3s (Gebo, 2014).

Anteriorly prolonged maxilla, lower molar buccal cingulids, large third molar, submolariform fourth premolar, paracondid shelf on P4 (Groves, 2001).

Genus Loris (Figure 13)

Inflated anterior rostrum, robust zygomata; more bunodont molar and premolar cusps, pit lingual to the hypocone on M3, more robust anterior dentition (Groves, 2001).

Genus Nycticebus (Figure 14)

Figure 13: Anterior and lateral view of Loris skull. (Drawn by author).

Figure 14: Anterior and lateral view of Nycticebus skull (Drawn by author).

Section 7:

Family Tarsiidae

Family characterised by very large orbits (Osman Hill, 1973). The frontal bone and alisphenoid have expanded the postorbital bar with extra bony flanges. Dental formula is 2-1-3-3 for upper
dentition and 1-1-3-3 for lower dentition; loss of lower incisors; pointed upper incisors (Gebo, 2014). Orbits partially closed posteriorly; tympanic ring is external to auditory bulla and extends laterally to form the external auditory meatus. Large upper central incisors, small lower incisors, large canines (Fleagle, 2013).

Section 7.1

Incomplete infraorbital margin (Osman Hill, 1955). Dentition as a whole relatively small; occipital condyle is prominent; elongated foramen magnum; auditory bullae narrow and tympanic tube elongated. Cranial vault flatter than both other genera in family; metopic suture visible. Broad masseter scar on underside of zygomatic arch; narrow palate; rostrum sits behind premolars; shallow sigmoid notch; orbits less dorsally extended and less laterally flaring with inferolateral margins either complete or very slightly notched; nasals long and sloping (Groves and Shekelle, 2010).

Genus Tarsius

Rounded foramen magnum; generally large dentition; metopic suture obliterated; arched cranial vault; coronoid process is high and vertical; wide palate and rostrum; flared orbits, projecting dorsally (Groves, 2001; Groves and Shekelle, 2010)

Section 7.2

Occipital condyle is rim-like; tympanic tube very elongated. Sigmoid notch deep; orbits only slightly flaring; interorbital space relatively wide; nasals medium in length (Groves and Shekelle, 2010).

Genus Carlito

Mandibular notch very shallow; crowns of incisors lower than Tarsius spp. and P2 not reduced; orbits very large with a very narrow interorbital margin and interrupted lateral edge; narrow nasal aperture (Groves, 2001). Auditory bullae enlarged; tympanic tube short; occipital condyle not particularly prominent. Narrow masseter scar underneath zygomata; sigmoid notch relatively shallow; orbital flaring pronounced; interorbital space very narrow; nasals short and high on the face with vertical, parallel sides (Groves and Shekelle, 2010).

Genus Cephalopachus

Figure 15: Anterior and lateral view of Cephalopachus skull (Drawn by author).

Section 8:

Family Hylobatidae

 Lack of ‘simian shelf’ in mandible. Long canines with little or no sexual dimorphism; upper premolars bicuspid; lower premolars heterodont; hypoconulid present on all lower molars (Swindler, 1989)

Section 8.1

Low vault; sinuous facial profile; elongated ramus and deep corpus (Groves, 2001)

High and rounded vault; straight facial profile (Groves, 2001)

Section 8.2

Upper incisors heteromorphic (Swindler, 1989). Cranial length short; thick orbital rim (Groves, 2001)

Genus Hylobates

Figure 16: Anterior and lateral views of Hylobates skull. (Drawn by author).

Overall size of skull is large,; orbital rims slightly raised (Groves, 2001). Deep corpus and long ramus (pers. obs)

Genus Symphalangus

Figure 17: Anterior and lateral view of Symphalangus skull (Drawn by author).

Section 8.3

High frontal; orbital rims flattened. M3 almost equal to M1; bulky upper canine with long and deep medial groove (Groves, 2001). Longer upper canines; large pyriform aperture; wide, rounded orbits with pronounced orbital rim; short ascending ramus (pers. obs)

Genus Nomascus

Figure 18: Anterior and lateral view of Nomascus skull. (Drawn by author).

Nasals are convex, long and pointed (Groves, 2001). Upper rim of orbits flattened; wider maxilla (pers. obs)

Genus Hoolock

Figure 19: Anterior and lateral view of Hoolock skull (Drawn by author).

Section 9

Genus Pongo is the only representative of Family Hominoidea native to Asia. All members of the genus share highly sloping or dished profiles, oblong-shaped orbits. Dental formula 2-1-2-3 for upper dentition and 2-1-2-3 for lower dentition; tall and sharp upper canines; five cusps on lower molars, with hypoconulid; upper incisors move up and away from maxilla; bicuspid upper premolars (Gebo, 2014)

Section 9.1

Rounded pyriform aperture; relatively small orbits; relatively small incisors, premolars and molars; narrow upper canine; elongated tympanic tube; widened ascending ramus; deep face (Groves, 2001; Nater et al., 2017)

Maxillary incisor row narrow, narrow distance across palate between first molars; shorter temporomandibular joint; shorter horizontal length of the mandibular symphysis; smaller inferior transverse torus. Triangular pyriform aperture and angled facial profile; long nuchal surface; wide rostrum, which is posterior to the canines; narrow and low orbits; shorter and narrower foramen magnum; narrower bicondylar breadth; narrower mandibular incisor row and greater mesio-distal length of the upper canine. Almost straight zygomaxillary suture (Nater et al., 2017)

Pongo tapanuliensis

Figure 20: Anterior and lateral view of Pongo tapanuliensis skull (Drawn by author).

Section 9.2

Face more prognathous, foramen magnum short; infraorbital pillar prominent; suborbital fossa present (Groves, 2001)

Pongo pygmaeus

Figure 21: Anterior and lateral view of Pongo pygmaeus skull (Drawn by author).

More orthaganous face, inter orbital pillar flatter and foramen magnum longer; suborbital fossa not present (Groves, 2001)

Pongo abelii

Figure 22: Anterior and lateral view of Pongo abelii skull (Drawn by author).

Section 10:

Subfamily Cercopithecinae

Section 10.1: 

Genus Macaca

Oval to rounded, dorso-ventrally flattened neurocranium; prominent supraorbital ridges; concave facial profile; convex frontal; orbits almost quadrangular; rounded pyriform aperture, the top of which is below the orbits (Osman Hill, 1974). Moderately prognathic faces; high-crowned molars with very low cusps, and long third molars (Fleagle, 2013)

Figure 23: Anterior and lateral view of Macaca skull (Drawn by author).

Section 11:

Subfamily Colobinae

Facial skeleton long and narrow (Groves, 2001; Whittaker et al., 2006)

Facial skeleton short and wide (Groves, 2001)

Section 11.1

Elongated nasal aperture; strong supraorbital ridges

Visible muscle attachment sites. Mandible generally large with deep corpus (Groves, 2001; Harding 2010; 2010)

Section 11.2

Predominance of an underbite in the incisor occlusion, and the incisors are homodont; the skull vault is rounded; cranial base strongly flexed; relatively weak brow ridges; upper margin of the orbit is horizontal; convex nasal profile; pyriform aperture joins inferiorly into a point; well marked arch under malar-maxillary suture; a swelling behind the lamboid suture; mandible has a shallow corpus and the angle not enlarged, very robust genial region and ascending ramus; the M3 usually lacks a hypoconulid, and has no functional occlusion to the distal surface of the distal loph (Groves, 2001)

Genus Presbytis

Figure 24: Anterior and lateral view of Presbytis skull (Drawn by author).

Facial skeleton is short and wide with strong supraorbital torus; rounder orbits; interorbital pillar present (Groves, 2001; pers. obs)

Section 11.3

Skull is long and narrow, with elongated nasals and strong supraorbital ridges; narrow interorbital pillar; back-tilted ascending ramus (Delson, 1994; Groves, 2001; Jablonski, 1998; Pan and Groves, 2004). Maximum occipitonasal length is 143mm (Fitch, 2000).

Genus Nasalis

Long and narrow nasal bones (Whittaker et al., 2006). Overall quite similar to Nasalis spp. but smaller (Groves, 2001)

Genus Simias

Section 11.4

Skull is strongly prognathic, well developed supraorbital torus and temporal and nuchal lines for muscle attachment; basicranial flexion reduced; pyriform aperture evenly narrow; no marked arch under malar-maxillary suture; no infralamboidal swelling (Groves, 2001)

Genus Semnopithecus

Cranial vault is low but gracile; supraorbital torus not strong, and is rounded above each orbit and depressed at the glabella; quite orthognathous; large mandible with deep corpus; muscular attachments less developed than seen in Semnopithecus (Groves, 2001; Harding, 2010; 2011)

Genus Trachypithecus

Figure 25: Anterior and lateral view of Trachypithecus skull. (Drawn by author).

Section 11.5

Orbits slightly flattened under supraorbital torus; interorbital pillar is broad; marked suborbital fossa (Groves, 2001; pers. obs)

Genus Pygathrix

Figure 26: Anterior and lateral view of Trachypithecus skull. (Drawn by author).

Overall is quite similar to Pygathrix spp., but when compared Rhinopithecus has a wider and more prognathic facial skeleton, in addition to a longer palate. Also, orbits are rounder, the supraorbital torus is more pronounced, interorbital pillar slightly broader, the canines are larger and the zygomata are straighter (Groves, 2001; pers. obs)

Genus Rhinopithecus