Written by
C. David Kreger
The discovery of KNM-WT 17000 (the "Black
Skull") in 1986 proved to be an important part
of the australopithecine puzzle. Very little is known about
Australopithecus aethiopicus, since so few specimens
have been attributed to the species, but the features that are
known provide important insights into the possible evolutionary
history between the "robust" and "gracile"
australopithecines. In general, aethiopicus shows a mixture
of both primitive and derived features, and dates to a time
that makes it a significant addition into the hominid phylogenetic
tree.
The
first specimen attributed to this species group is an edentulous
mandible (Omo 18) found in southern Ethiopia, west of the
Omo River, in 1967. The specimen was discovered by a French
expedition led by Camille Arambourg and Yves Coppens. This
2.5 million-year-old mandible was placed into a new species
by its discoverers, who named the species Paraustralopithecus
aethiopicus. They believed that the specimen deserved
a new species designation because its v-shaped jaw (among
other features) distinguished it from the robust australopithecus
forms known in the area. Generally, the discovery and designation
was ignored by the majority of paleoanthropologists.
Not
until the Black Skull discovery was there much interest in
the specimen, but once KNM-WT 17000
was discovered, interest was renewed in the Omo mandible.
The genus name was dropped in favor of the more traditional
Australopithecus designation, but Arambourg and Coppens'
species designation of aethiopicus was taken as the
species name. This species designation is still debated.
Diagnostic Features
The better known aethiopicus specimen is KNM-WT 17000,
a nearly complete skull sans the mandible. The specimen is
known as the "Black Skull"
because mineral uptake during fossilization gave the specimen
a blue-black color. The specimen was discovered in a 2.5 myr
deposit west of Lake Turkana, and threw a wrench in many evolutionary
schemes accepted by many researchers. The specimen is similar
to a male A. afarensis, but with a very small cranial
capacity (410 cc), and an even more powerful nuchal musculature
and very developed masticatory apparatus. The well-developed
masticatory features are indicated by:
- A
large palate with a thick roof.
- The
broken roots of large rooted (and thus probably large crowned)
molars and 4th premolar.
- The
very anterior zygomatic process of the maxilla whose size
and orientation creates a flat, flaring face (this puts
the masseter attachment far forward and lateral, where its
leverage in producing grinding motions is best).
- The
skull does not retain any teeth, but the size of the roots
and the palate indicate that the anterior and postcanine
teeth were very large.
Other
specimens attributed to aethiopicus show thickened
molar enamel (similar to afarensis.) The combination
of a very small brain and enlarged masticatory apparatus leads
to the development of a well-developed sagittal crest that
meets the nuchal crest to form a compound temporonuchal crest
similar to A. afarensis at the rear of the vault.
Other
features that resemble A. afarensis include:
- Large
anterior tooth sockets.
- A
flattened cranial base (and flattening of everything on
it such as the flat, nonprojecting articular eminence of
the mandibular fossa and the shallow palate).
- A
posterior foramen magnum position and more sagittal orientation
of the petrous pyramid of the temporal bone.
- Extreme
development of the nuchal muscles and the more vertical
orientation of their attachment of the occiptal bone.
- Extreme
facial prognathism.
Other
remains were attributed to aethiopicus following KNM-WT
17000, including Omo 338y-6. This is the 2.39 myr remains
of a 10-year-old juvenile vault that lacks the frontal bone
and the face. This juvenile specimen shows the early formation
of a powerful masticatory system. Features that indicate this
include:
- The
temporal lines meet at the middle in an anterior position
(there would have been a sagittal crest if the individual
had lived long enough).
- A
marked development of the superior nuchal line with a strong
downward projecting inion at its center.
- A
large overlap of the temporal onto the parietal bone.
The
earliest known aethiopicus material is probably the
2.7 myr L55s-33 mandible fragment from level C6 in the Omo
deposits, north of Lake Turkana. L55s-33 is the front portion
of a mandible that retains most of a worn P4 crown. This specimen
was generally not attributed to a species until the Black
Skull find, since the diagnostic tooth was too small to be
A. boisei (83% the size of the smallest known boisei
tooth), but did not fit into any other species well. G. Suwa,
who has specialized in the analysis of australopithecine premolars,
reported that the premolar was unusually thick-enameled and
had the squared-off shape of the hyper-robust species. He
was unable to attribute the premolar to a species, and placed
it in an unspecified robust australopithecine other than boisei.
It now seems that he was right and the material belongs to
A. aethiopicus.
Conclusions
Australopithecus aethiopicus became important in phylogenetic
considerations soon after the discovery of the Black Skull.
The species is generally accepted to have shown that the genus
designation Paranthropus is polyphyletic and invalid,
though some still vocally argue against that fact. One of
the earliest important cladistic analysis was by Walker and
Leakey (1988), which they claim shows aethiopicus is
at the base of the boisei lineage, is more primitive
than robustus, and that aethiopicus is not ancestral
to robustus. However, Strait et al. says that this
phylogeny requires 39 extra steps above the most parsimonious
tree, and most cladists do not favor this phylogeny. Skelton
and McHenry (1992) and Lieberman et al. (1996) both came to
the same conclusions regarding aethiopicus and Paranthropus
using different character traits. Both see aethiopicus
as a dead-end side branch and Paranthropus as polyphyletic
and invalid. On the other side of the coin, Strait et al.
(1997) see all the robusts sharing a recent common ancestor
(aethiopicus), with Paranthropus monophyletic.
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