Re: Palaeontology (1)

3 Aug 1995 13:43:42 GMT

Ralph L Holloway ( wrote:
: On Wed, 2 Aug 1995, Elaine Morgan wrote:

: > They seem to be saying: "The chimpanzee became quadrupedal because it
: > needed to increase its range. The hominid became bipedal because it
: > needed to increase its range."
: >
: > That is a perfectly tenable position, but only on the premise that the
: > animals were aleady very different before either of them set a foot on
: > the ground - eg maybe the prehominid ape was smaller-bodied, had a
: > different intramembral index, etc. But this is pure speculations,
: > isn't it? For all we know, at 4mybp all the African apes were
: > small-bodied and resembled one another very closely. It has been
: > suggested that the l.c.a. looked like a bonobo. The gorilla could have
: > become larger much later - as late as 2mybp or even 1mybp. We have no
: > fossils.
: >
: > If they did resemble one another closely, we would need a very good
: > reason why one lot would respond to the advent of savannah mosaic by
: > becoming quadrupedal and the other by becoming bipedal.
: >
: It might be a good idea to pick up a copy of Szalay and Delson's
: book on the fossil record for primates and read the chapters on pongids.
: There is the sense for most of us paleo types that the fossil record has
: a lot of different species, but as most of the finds are bits and pieces,
: it is almost impooissible to sort it out nice and neat into any picture
: approaching what we know about living species.

The degree of homoplasy seen in the primate fossil record is surprisingly
high, which makes the pure cladist position vulnerable. For example, my
character database associates the Chinese Giganthropus with Pongo, but the
early Gig. specimens Simons found in India look like large Sivapithecus.
The argument Holloway refers to here about Proconsul major is an example
of the same sort of issue. The consistency index for my database is about
0.45, which is disappointingly low if you're a cladist. What is even more
interesting is that the consistency indices for some of the characters
often used to sort out the primates are really low. E.g.,
substratum--0.17, diet--0.08 to 0.2, upper premolar count--0.17,
diastema--0.16, presence of a honing premolar--0.22, molar cusp
height--0.07, molar series type (which molars are bigger than
which)--0.17, protocone fold--0.14, lingual cingulum--0.15, molar enamel
thickness--0.11, hypoconulid enlargement--0.14, upper premolar root
number--0.19, jaw depth--0.12, locomotion preferences (as long as they are
behavioral alternatives)--0.11-0.20. On the other hand, some are higher:
male weight (0.56), most tooth counts (0.33-1.0), difficult-to-acquire
dental specializations (many), transverse tori states (0.33-0.4), face
evolution in general (many), cranial capacity (0.65) and EQ (0.78), sexual
dimorphism in cranial capacity (0.38), tympanic structure (0.33), limb
adaptions and some locomotor adaptions (brachiation, terrestrial adaptions
in general, vertical clinging and leaping--all around 0.5), and most of
Wood and Chamberlain's numerical characteristics (they do good work 8).

: Just trying to sort out
: Proconsul major (a thread present here over the past several days)
: regarding its affinities proves almost impossible. The impression I have
: is far different from Elaine's. I see a lot of diversity of pongids
: throughout the Miocene and probably into the Pliocene. I don't have any
: trouble at all believing that two different species of ape, living in
: roughly sympatric regions could show different adaptations to roughly
: similar environmental changes, i.e., one becoming more proficient at
: knuckle-walking, the other becoming more proficient at bipedalism.

That's exactly what I see in the character distributions. Crossing
synaptomorphies and ambiguous polarities all over the place.

: I
: think Elaine expects too much homogeneity both in the fossil record and
: in evolutionary responses to chaning environments via mutation,
: selection, and drift. Minor differences in habitat distribution, gene
: frequencies, etc, within a species can eventually lead to some profound
: adaptational differences and thus morphological components.
: There is a big difference between asserting that the fossil record
: for the pongids is unclear as to asserting that it is relatively
: homogeneous and small.
: Ralph Holloway

Harry Erwin
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PhD student in comp neurosci: "Glitches happen" & "Meaning is emotional"