As a general rule in paleontology any fossil found is considered (until shown otherwise) to being generally representative of the population it was part of. That is, it is not an oversize individual, or an unusually small example, or a malformed individual. And when you have a number fossils from the same species, you will have generally representative sample of both sexes, adults and juveniles. That is the general expectation but most paleontologists keep in the back of their minds that this may not always be true. It just might be biased.
The Ramapithecus Story: or the dangers of a biased fossil record
As an undergrad in the early 70’s, I was introduces to a fossil ape by the name of Ramapithecus punjabicus (Rama’s ape from Punjab). It was dated in the region of 15mya. Some paleontologists (i.e. David Pilbeam and Elwyn Simons) had proposed that that Ramapithecus was a human ancestor. This was based on several factors: it had small canines, a parabolic mandible, lack of canine diastema, and the dryopithcine Y-5 pattern.. First off, some of you may recognize that these features are all “dental.” This is because all the fossils attributed to Ramapithecus were in fact all parts of the mandible (lower jaw).
Small canines are a feature of hominines (humans and our ancestors). All of the other apes (and monkeys) have large canines. Well, that is, males have large canines and females have ones smaller than males. Both have smaller lower canines than upper. This is a sexually dimorphic trait of primates. A sexually dimorphic trait is one that differs between the sexes of a species. They may also be called secondary sexual characteristics, especially the traits that develop with sexual maturation. These traits are why men and women’s bodies have obviously different shapes.
Lower jaw and upper jaws of apes are said to have a U-shape. That is, the incisors form a nearly straight line, the canines are set back a little and the premolars and molars form nearly straight rows that are parallel to each other, left and right. In humans the tooth row curves from the center-line (between the central incisors and curving through the molars forming a parabolic-like curve. All the mandibles of Ramapithecus were incomplete, either left of right, broken at the chin. So determining the shape of the tooth row was a matter of speculation, and the use of a mirror.
All mammals, reptiles, birds, fish and amphibians are bilaterally symmetrical. the left side looks pretty much like the right side. Among the fossils fragments that were found of the austrolopithecine known as Lucy, were her left femur and the ilium (the hip bone). Because the right side is a mirror image of the left, we know what the right side looks like. By placing the half mandible next to a mirror, the reflected image creates an image of the other mandible and the shape of the intact jaw. That is, if the mandible fragment is oriented at the true angle in real life. Which is a matter of interpretation since the actual suture where the two halves meet is missing. The “reconstructed jaw” seemed to be that of a parabolic arcade, a hominine jaw shape. Again, this is only accurate if the angle is correct.
A canine diastema is a space in the lower jaw that allows the long upper canine to pass by the first premolar and the jaws to close. If you feel the tooth right behind your lower canine (or look at it in a mirror), you will feel or see that it has two cusps with a low spot between them. The tooth is called the first premolar or more commonly a bicuspid because of the two cusps. In humans, the upper canine fits in the valley between the two cusps. In the other primates with long canines (fangs) the first cusp is not there. When the male animal closes it mouth the upper canine slides behind the lower canine and against the premolar. It is believed that this forms a” honing complex” and sharpens the upper canine. Females have smaller upper canines and a tiny diastema. Also the lower canine slides in front the upper canine and there is a gap between the upper canine and the lateral incisor. Humans with neither a long upper canine or lower have no diastemata.
The dryopithecine Y-5 pattern refers to the fact the living great apes (this includes humans) have 5 cusps on the surface of their lower molars. There is a valley down the middle of the tooth with two pairs cusps on inside edge of the tooth and three cusps on the outside edge. The valley between the two inside cusps bifurcates as it passes on either side of the middle cusp looking like a “Y.” This pattern was first noticed in fossils of Dryopithecus, first found in the late 1800’s in Miocene deposits in Europe. This pattern also occurs in humans and indicates a relationship between dryopithecines and humans. Its presence in ramapithecine molars indicated that Ramapithecus was related to Dryopithecus and possibly a human ancestor. But not necessarily, since other apes, not our ancestors have the Y-5 pattern. All the Y-5 pattern indicates for sure is descent from Dryopithecus. Since we have the Y-5 pattern any ancestor of us has to also have it.
Not everyone accepted, based on these traits, that the hypothesis that Ramapithecus was an ancestor was correct. There was a a lot of discussion (or heated argument) about it. Added to the morphological argument, several molecular chemists said (based on their immunological work) that Ramapithecus was 10 million or more years to old to be a hominine.
Anyway, that is the background to the story of Ramapithecus. and why it is it was and is an important fossil. Ramapithecus was among a number of fossils found in the Siwalik Hills of India in the early 1900’s. Fossils of several ape genera and species were also found, e.g. Sivapithecus, Bramapithecus, and Sugrivapithecus (Shiva’s ape, Brahma’s ape, and Sugriva’s ape). These all tended to be rather fragmentary mandibles, maxilla, and teeth. Further study revealed an odd fact, Ramapithecus was known only from fragments of mandibles (the lower jaw) and Bramapithecus was known only from fragments of maxillae (the upper jaw). Both had small canines and teeth that looked very similar. Eventually it was realized that they were upper and lower jaw fragments of the same species. Since Ramapithecus was the first named, the senior name, Bramapithecus was “sunk,” as taxonomists say: included as a junior synonym of Ramapithecus.
Sivapithecus was recognized as an ancestral orangutan. The fragments of the maxilla included enough of the face to include the “nasal gutter” (depressed area on each side of the nasal opening) that is distinctively unique to the living orangutan species. The teeth were also very similar to the teeth of living orangutans. Males of which have large canines as most primates species do.
This was the way things stood for years. Sivapithecus was an ancestral orangutan, and Ramapithecus/Bramapithecus may be or may not be ancestral to humans. But probably not, since the immunological clock, joined to the DNA evidence (molecular clock) gained widespread acceptance that the last common ancestor of chimpanzees and humans existed about 7 million years ago and therefore Ramapithecus was about 8 million years to old.
Then a new Ramapithecus was found, a fragment of maxilla that extended far enough to the nasal cavity to show a nasal gutter. Ramapithecines were actually an array of species of Sivapithecus. To add to this, I recently read that all the ramapithecine fossils were all from females. So they would all have had small canines in the first place.
The point of all this, is that almost all fossils are fragments. Very rarely is a major intact part of a skeleton found, let alone a complete one. Things like age, gender, and just how representative of the entire population it is, is a big question. The beginning assumption is that all fossils are “ordinary.” It is not a particularly large individual or particularly small (if it is an adult). Lucy is a small individual (and that rarity a nearly complete skeleton) so there were and still are still questions about how representative she is of the Australopithecus afarensis female size range. The Hobbit from Flores raised all kind of questions about whether she small and her small brain were pathological or not. If the fossil is of a juvenile, is it developing at about normal speed. The Turkana Boy (AKA Nariokotome Boy or WT-15000), an rare nearly complete skeleton, is a tall (61 inches) 11-12 year old Homo ergaster (an African Homo erectus) would have been 73 inches tall at adulthood. Because the fossil is of an immature individual it is hard to be sure of the actual age and even sex. He may have been as young as 7 (still very tall) and may even have been a she. If the fossil is of a sexually reproducing species there is a 50-50 chance as to which sex you have. So if you have several fragments of different individuals of the same specie, there is a statistical likelihood that you have a sample of each sex. But the story of Ramapithecus is good example of how wrong the basic assumption can be: all the Ramapithecus mandibles were from female and all the maxillae, formerly known as Bramapithecus, were also female.
There is also the problem of identifying pathologies of a skeleton and not including them in the species definition. The classic example is Marcellin Boule’s reconstruction of the male Neandertal skeleton from La Chapelle-aux-Saints in France. His reconstruction provided the archetypal caveman image, the bent knee, stooped shouldered apish beast. When Boule examined the skeleton of the Old Man of La Chapelle-aux-Saints he recognized that he was seriously crippled by arthritis and injuries from a long hard life. While the reconstruction may have been accurate for that specific individual at his time of death, it was not at all accurate for how a Neandertal normally looked. Cave and Strauss reexamined Neandertals in the the 1950s and demonstrated that they walked upright like modern humans and were not so apish as Boule has interpreted the skeleton.
The fossil record is full of potential traps and must be interpreted with caution.