Saturday, July 24, 2010
The Juvenile Hominoid from Butterfly Ridge
Over the last month I've been reviewing the study of Lufengpithecus lufengensis recently published by Xu and Lu (2008). It is the 3rd publication in the ponderous sounding series of monographs entitled "State Key Project of the 9th Five Year Plan -- Origin of Early Humans and Environmental Background." The 2nd monograph in the series deals with a number of sites in the Yuanmou Basin of Yunnan that have produced prolific fossil hominoids. Long thought to be younger than Lufeng, these sites are now known to be somewhat older, between 7-8 mya. Lufeng has been dated perhaps a million years younger. The most important hominoid fossil excavated from the Yuanmou basin is a little heralded juvenile skull which is finally described in detail in the recent monograph (Qi and Dong 2004).
There has been considerable terminological confusion surrounding the hominoid remains from Yuanmou. Some consider them conspecific with the poorly known and somewhat older material from Kaiyuan (Harrison et al., 2002), and they've been subsequently dubbed L. keiyuanensis (the difference in spelling is due to the romanization method employed in China during the 1950s when the site was first discovered). He (1997) in an initial review of the Yuanmou hominoids, attributes the remains to a new hominoid species, L. yuanmouensis. The consensus seems to be building, however, that the proper scientific name, based on nomenclatural precedence, should be L. hudienensis (Qi et al., 2006). This is a very poetic name as it's derived from the fossil bearing locality in the Yuanmou Basin that produced many hominoid fossils called Hudieliangzi, which translated means Butterfly (hudie) Ridge (liangzi). The hominoid bearing localities in the Yuanmou Basin have yielded hundreds of isolated teeth, several upper and lower jaw fragments with teeth intact and a single post-cranial bone (phalanx). The localities and the taxonomy of the hominoid remains are reviewed by Harrison et al. (2002) and Qi et al. (2006). The most important specimen from Butterfly Ridge is, however, the face and palate of a young juvenile hominoid that is the same dental age as the famous Taung Baby from South Africa which launched the careers of both Raymond Dart and Australopithecus africanus.
The only cranial specimen of a fossil hominoid from the Yuanmou basin is the juvenile skull YV0999. The specimen preserves a relatively complete face and a portion of the frontal bone. The areas immediately above the orbits match and can be pieced together. Most of the frontal squama is broken away but a portion of the frontal that extends to the coronal suture is preserved on the left side. There are 10 adult upper jaw fragments but they only give a small amount of information regarding the anatomy of the adult hominoid skull. We compare the juvenile skulls of orangutans, chimpanzees, gorillas and australopiths of roughly the same developmental age. A comparison of the characteristics of YV0999 follows:
(1) The temporal line and sagittal crest. The temporal line on the skull of Lufengpithecus hudienensis is poorly expressed. There is a weak projection of the temporal line arising on the left zygomatic process at the lateral margin of the orbit. In chimpanzees and gorillas, the temporal lines are well developed; in orangutans the development of the temporal line is weak. There is an obvious temporal line in the juvenile skull of Lufengpithecus lufengensis (PA828). It is also evident in the skull of human children. The starting location and characteristics of the temporal line in the Taung skull are similar to the Yuanmou specimen. The temporal line in YV0999 is most similar to the condition in orangutans. In the course of human evolution, the temporal ridge gradually evolved into the form of the temporal line, which is due to the expansion of skull and cranial vault and the increased weakening of the temporalis muscle, the bone does not require a high ridge for the attachment of a strong temporal muscle. The expansion of the skull also provides a large attachment surface for the temporalis muscle.
(2) Supraorbital ridge and supraorbital sulcus. The supraorbital ridge is very weak, and the frontal bone is continuous with the slightly elevated supraorbital portion of the bone surface. There is no supraorbital sulcus, the bone surface is smooth. In the orangutan the supraorbital region has a thin ridge-like projection. This forms a well expressed superaorbital ridge in adults. It is very much like the supraorbital ridge in the Sivapithecus GSP1500 skull, which lacks a supraorbital sulcus. Chimpanzees and gorillas have an obvious supraorbital ridge and sulcus. In the two juvenile Australopithecus skulls the supraorbital ridge and sulcus are not well-expressed. YV0999 differs from the modern apes and is most similar to Australopithecus.
(3) Glabellar region. Relatively broad and flat forehead. While observing the juvenile L. hudienensis skull we discovered that it had bubble-shaped holes in the glabella cavity, which was suspected development of the frontal sinus. Orangutans have a very narrow and shallow concave area between the eyebrows, with no development of a frontal sinus. Chimpanzees and gorillas have a glabellar area that is significantly elevated, and there is a very large frontal sinus. The two australopithecines have a relatively protruding glabellar region. It is broad with a very large frontal sinus. Lufengpithecus has a broad but deeply concave glabellar region with a developed and obvious frontal, sinus. CT scan results also confirmed the presence of frontal sinus. There is a great difference between YV0999 and orangutans.
(4) Interorbital region, comparatively broad, close to that seen in the African apes much broader than in orangutans (see Table 2.30). The interorbital region in the Taung skull is also relatively broad. The Inter-orbital region width in juvenile human skull is also larger. YV0999 and orangutans differ significantly.
(5) Lateral side of the orbits. Broad and flat surface anterolaterally, the lateral margin has a marginal tubercle, it rises in an arc. In this point YV0999 is most similar to the orangutan. In the chimpanzee and gorilla the lateral side of the orbit is relatively narrow and swollen. This area in Australopithecus is also arched.
(6) Position of the lacrimal fossa. In YV0999 it is positioned relatively high, its shape is relatively broad. The position of the posterior lacrimal crest is also somewhat high but not very sharp. In orangutans the lacrimal fossa is positioned relatively low but is relatively broad in shape. The posterior lacrimal crest is positioned very low. In chimpanzees and gorillas the lacrimal fossa and posterior lacrimal crest are positioned relatively high. In gorillas the lacrimal fossa is relatively broad and the posterior lacrimal crest is not that sharp, but in chimpanzees the lacrimal fossa is relatively narrow and the posterior lacrimal crest is tall and sharp and clearly projects. In YV0999 the position of the lacrimal fossa is more similar to that of the African apes.
(7) Orbital shape. Height is slightly greater than width, approximately elliptical in shape, the inner half of the superior margin of the orbit is relatively flat, the lower margin and lateral margins are relatively sharp. The shape of the orangutan’s orbits and its relatively sharp inferior and lateral orbital margins is similar to YV0999 but the perimeter of its orbital margins projects resembling the rim of eye glasses. This condition not seen in L. hudienensis or the African apes. The shape of the orbits in chimpanzees and gorillas does not differ greatly from YV0999 but their lower and lateral margins are relatively blunt.
(8) Zygomaticofacial foramen. One, positioned between the zygomaticofrontal suture and the zygomaticomaxillary suture. Narrow, long and slit-like in shape. The zygomaticofacial foramen in orangutans is situated in the same location and is also long, narrow and slit-like, but there are often more than one and they are slightly longer. Sivapithecus GSP15000 preserves the foramen on the left side, it is also relatively long, narrow and slit-like. The zygomaticofacial foramen of gorillas and YV0999 are relatively close seems to be somewhat more narrow. Chimpanzees have relatively small, rounded foramina.
(9) Infraorbital foramen, One, positioned just below the junction of the zygomaticomaxillary suture with the lower margin of the orbit. This is similar to the position in chimpanzees and gorillas. It is also similar to the position in humans. It is shifted more to the center in orangutans.
(10) Zygomatic arch. The zygomatic arch originates between the DP4 and M1, the location of the starting point is very low, the lower margin is very thin. In orangutans the root of the zygomatic arch begins at the level of M2 but the lower margin of its root is very thin. In chimpanzees and gorillas the origin is positioned at the level of M1 and the root is positioned low but has a thick lower margin. In PDYA2 adult maxilla lateral aspect retains the base of the zygomatic arch, Its leading edge positioned at M1, indicating that in adult individuals the base of the zygomatic arch is more forwardly placed and the upper jaw protrusion is less.
(11) Nasal piriform aperture and nasal bones. The piriform aperture is long, narrow and oval, The anterior nasal gutter is weakly expressed. Nasal bones are relatively broad. The nasal aperture of the orangutan is close to that of YV0999 but the nasal bones are narrow. The nasal aperture of African apes is relatively broad and the nasal bones are broad. The anterior nasal gutter is weakly expressed in both orangutans and gorillas. Chimpanzees do not have a structure like the anterior nasal gutter. Both Australopithecus and humans have broad nasal bones
(12) Nasal alveolar clivus. This area has been damaged beyond recognition. From what remains it trends towards being stepped, thus differing from the orangutan.
(13) The hard palate and dental arcade. The palate is relatively deep, two incisive foramina, Dental arcade narrow, anterior portion of the arcade is arched, Both sides extend evenly posterolaterrally. The greater palatine foramen is narrow and deep. It is positioned near the alveolar margin. In both the young and adult orangutan there is only one, very small and narrow slit-like incisive foramen. The tooth rows project laterally and then turn inwards, Gorillas and chimpanzees both have two incisive foramina located close to the alveolar margin similar to YV0999. The greater palatine foramen in orangutans and chimpanzees is located closer to the median plane.
(14) Canine fossa. Maxillary samples from the Yuanmou Basin show a relatively weak canine fossa. Thee canine fossa is very well expressed in orangutans but lacking in chimpanzees. During research it was observed that In gorillas some young specimens have it and others don’t. Its lost in adults.
(15) Facial index. Based on the values shown in the table African great apes have relatively narrow faces while orangutans have relatively broad faces. The Yuanmou specimen is relatively close to the Asian ape.
(16) Face in toto. Tthe degree of prognathism in the YV0999 snout is very weak. The facial planum beneath the orbits is close to the vertical, only slightly slanted backwards. In the Orangutan the jaw projects to a significant extent; in side view the eyes are sunken inward. In the African Great Apes and Australopithecus prognathism is weak, the infraorbital plane is vertical similar to YV0999.
In short, the juvenile skull of L. hudienensis in its weak supraorbital ridge, lack of supraorbital concavity, form of the lateral orbital and orbital area, very thin base of the zygomatic arch, pear-shaped nasal aperture shape and facial index is relatively similar to like aged orangutans and differs from like aged chimpanzees and gorillas. The breadth of the interorbital distance and nasal bones and relatively high position of the lacrimal fossa, etc. is relatively similar to chimpanzees and gorillas of similar age. In addition the short width of its hard palate and the arched anterior of the dental arcade is clearly different from orangutans of similar age. The single infraorbital foramen and the position of the infraorbital foramen is very similar to juvenile Austarlopithecus.In summary, the pongine like traits of YV0999 relate primarily to the morphology of the orbits and the supraorbital region, and the shape of the nasal aperture. In all other respects, including traits that are usually considered diagnostic of hominines such as 1) interorbital distance, 2) number and placement of facial foramina, 3) shape and configuration of the hard palate and maxilla, 4) size and morphology of the post-canine teeth (to be discussed later), etc. the Yuanmou juvenile sorts with the African Great apes and early and later hominans. So at 8 mya do we have the third chimpanzee, the second orangutan or the "first" hominan?
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