One of my ground rules for evaluating a claim that creature B
is creature A on its way to becoming creature C (i.e., a transi-
tional form) is that the features of creature B be either the same
as those of creature A or intermediate between those of creatures
A & C. In other words, if creature B shows specialization in a
direction away from creature C, then creature B cannot be accepted
as both a descendant of creature A and an ancestor of creature C.
Glenn labels this an unreasonable expectation and complains that it
requires one to ignore the adaptational needs of the transitional
animal.
The ground rule does not ignore the adaptational needs of
transitional animals but requires one to take the unique adapta-
tional features of creature B as an indication that creature B is
not in the direct lineage of creature C. This is not a radical
proposal. As I mentioned before, it is one of the reasons that
creatures are rarely, if ever, placed in direct lineages. Taking
an example from our previous discussion, panderichthyids are not
placed in the direct lineage of tetrapods because "[i]n addition to
their tetrapod-like features, panderichthyids possess some unique
characters (for example, the vertebral construction) which indicate
that they form a small clade rather than a paraphyletic segment of
the tetrapod stem lineage" (Ahlberg and Milner, 508). (A clade is
a monophyletic group, meaning one which contains all the descen-
dants of a common ancestor.)
William Fix's comments on this subject in _The Bone Peddlers_
(New York: Macmillan, 1984), p. 84 are worth quoting. In discuss-
ing Lucy's (_Australopithecus afarensis's_) long arms he writes:
Given the larger context of paleontological evidence and the
pattern of specialization it illustrates, the only way Lucy or
afarensis could be ancestral to man would be if they somehow
managed to reverse those trends. I know of no other case in
the fossil record where such a reversal has been documented.
It would be unscientific to say that it would be impossible in
the case of Lucy and afarensis, but it is unlikely in the
extreme. Specialization seems to be an inexorable tendency,
almost a law of nature. Once it is flowing in a certain
direction, it is reversed with about the same frequency that
rivers are found flowing uphill.
I could accept modifying this ground rule in those cases where
a reversal of the specialization trend was alleged if the proponent
identified the point of reversal and had a cogent explanation for
it. If, for example, one had a persuasive explanation for why in
the evolution of creature A into creature G there was a reversal of
the specialization trend at creature D, that could be factored into
the analysis. What one would then expect to see is uniform
movement in features from creatures A to D and then again from
creatures D to G. What cannot be accepted are assertions that
specialization trends reverse whenever the evolutionist finds it
convenient. As I've noted, evolutionists generally tend to be
properly conservative in this regard.
The impression that was given in the diagram in the Berta
article in _Science_ 263:180 (which arranged drawings of _Mesonyx
obtusidens_, _Ambulocetus natans_, an archaeocete, an odontocete,
and a mysticete in descending order) is that the higher creatures
were ancestors of the lower. So seductive was this impression that
Glenn was led to appeal to the drawings as evidence of the
transitional status of _Ambulocetus_. I objected to his using the
drawing of _Mesonyx obtusidens_ as an ancestor of _Ambulocetus_
because the genus _Mesonyx_ does not appear in the fossil record
until the middle Eocene, several million years after the appearance
of _Ambulocetus_. Glenn has taken me to task for this claim,
arguing that _Mesonyx_ *could have* existed in the Paleocene but
not left any fossil indications of that existence.
(For the record, Glenn has rightly corrected my charge that
the readers of _Science_ were not told that the drawing was of a
member of the genus _Mesonyx_. I either did not notice or did not
recall the reference to fn. 16 that the drawing was _Mesonyx_.
[That, by the way, is not how I learned of the source of this
reconstruction.] The essence of my complaint, however, is that the
readers were not told that this genus does not appear until the
middle Eocene; that is what makes it potentially deceptive. The
impression given by the diagram is that _Mesonyx_ is found in a
chronologically appropriate position to be an ancestor of _Ambuloc-
etus_, and that is incorrect.)
As I have previously pointed out, the notion that _Mesonyx_
existed in the Paleocene is purely an assumption that is made to
harmonize the data with a preexisting conviction that _Mesonyx_ was
ancestral to _Ambulocetus_. The fossil evidence is that _Mesonyx_
appears in the middle Eocene and continues until the upper Eocence.
There is nothing in the fact that _Mesonyx_ appears *later* than
_Ambulocetus_ to suggest that _Mesonyx_ actually lived *before*
_Ambulocetus_. Indeed, the inference from that data would be to
the contrary. If one can simply assume that the genus _Mesonyx_
lived long before there is any fossil evidence of it, then what is
to prevent another from assuming that _Ambulocetus_ is even older?
If the data do not constrain one's analysis, it's a free-for-all.
Contrary to Glenn's view, it is perfectly appropriate to
consider the relative positions of creatures in the fossil record
in assessing the claim that one was ancestral to the other. Robert
Carroll, for example, states that "_Hapolodectes_ [a mesonychid
known from the lower Eocene of North America and possibly the upper
Paleocene of Asia - ALC] *is probably too late* to be an ancestor
of whales, which are known from the lower Eocene, but according to
Szalay (1969a) they may share a close common ancestry with this
genus" (emphasis mine) (Robert L. Carroll, _Vertebrate Paleontology
and Evolution_ [New York: W. H. Freeman, 1988], 522). If that is
true of _Hapolodectes_, it applies with even greater force to the
middle-Eocene _Mesonyx_. I know of no one who proposes that
_Mesonyx_ is in the lineage of whales.
In the Nehm and Geary study, none of the alleged ancestors
appeared in the fossil record after their descendants. Rather, the
study is used to make the claim that *if* erosion had wiped out the
record of the alleged ancestor's (_P. con.'s_) earlier existence,
then a false impression would be created that _P. con_ was not old
enough to be the ancestor of _P. chris_. This would only be true,
of course, if the evidence of _P. con's_ prior existence was
eliminated from *every locale*, not simply the locale in which it
allegedly gave birth to _P. chris_. If that occurred, it would
indeed be bad luck for the evolutionist because he could not use
the fossil record to argue that _P. con_ was the ancestor of _P.
chris_.
As long as we're being creative with the fossil record, what
if what really happened is that _P. chris_ gave rise to _P. con._?
_P. chris_ existed in levels -2 and -1 and _P. chris.-P. con._ (the
alleged transitional) existed in level 0. Unfortunately, these
levels were eroded away leaving no trace of their existence. _P.
con_ in level 1 was actually the descendant of _P.chris-P. con._,
which in turn, was the descendant of _P. chris._ _P.chris-P.con_
simply avoided fossilization until level 3 and then went extinct,
and in a string of bad luck for those committed to the ancestral
status of _P. chris_, _P. chris_ skipped fossilization until level
4. If we start making arguments like this, we might as well stop
using the fossil record in the debate.
As for the boat and paddle illustration, we know (or presume
to know) that the paddle from 7500 B.C. was created for use in
boats (as opposed to fanning royalty). Thus, the existence of the
paddle implies the contemporaneous existence of boats. We have no
similar knowledge about alleged evolutionary relationships. In
other words, the existence of _Ambulocetus_ does not imply the
prior existence of _Mesonyx_. I thus fail to see the point. If
you are simply saying that it is *possible* for the first in
existence to appear second in the record, I have already explained
why that is not relevant to my position.
The mere fact that "Mesonychids walked on the earth before
whales swam" does not establish that mesonychids were ancestral to
_Ambulocetus_ or to whales. One needs some details about an
alleged ancestor in order to evaluate the claim of descent. When
one compares what is known about paleocene mesonychids (_Dissacus_)
to what is known about _Ambulocetus_, there are significant
differences at every point where comparisons are available (leg,
forearm, front and hind feet, and dentition). As you acknowledge,
Szalay agrees that _Dissacus_ is not an ancestor of archaeocetes,
but you do not make clear that he has no identifiable ancestor to
put in its place. He does *not* believe that hapalodectines (a
Mesonychid subfamily dating from the lower Eocene) are ancestral.
Indeed, I have already pointed out that Carroll believes hapalodec-
tines appear too late to qualify as archaeocete ancestors. Rather,
Szalay suggests that both hapalodectines and archaeocetes probably
"derived from either early or middle Paleocene mesonychids,
*species more primitive than known mesonychines*" (emphasis mine).
As I stated in my previous post on this topic, my conclusion
that the hind feet of _Ambulocetus_ are tetradactyl is based on the
drawing of Thewissen, et. al., the fact the first metatarsal was
not found, and the fact that only two proximal, three intermediate,
and three distal phalanges were found. The references by Thewis-
sen, et. al., to a "fifth digit" do not mean that the hind foot of
_Ambulocetus_ possessed five digits. The "fifth digit" is an
anatomical not a numerical reference, just like the term little
finger. This foot of _Ambulocetus_ apparently had only digits II-V
(index finger through little finger).
My point about the lost digit was simply that it seems
difficult to square with the alleged semi-aquatic lifestyle of
_Ambulocetus's_ ancestor. I understand that one can spin whatever
story one wants to spin to account for the loss, but it just
strikes me as strained. It seems that an extra digit would provide
a broader surface area for swimming. It is no answer to say that
the other four toes enlarged; would not five enlarged toes be even
better?
The illustration of the lines of pennies and nickels would
seem more appropriate if there was some uncertainty as to whether
bags of coins with holes in them could produce the pattern of coins
seen on the street. (That would parallel the uncertainty that
exists regarding a genetic mechanism that is adequate to account
for macroevolution.) In that case, one would be left with just a
pattern of coins, and the question would be how the coins got into
that configuration. The fact that the pattern of coins could
possibly be explained by the theory that the coins fell from a pair
of porous coin sacks would not mean, of course, that the theory
corresponded to reality. It may well be that the theory is a
clever but incorrect explanation. If a man of impeccable integrity
said that he had placed the coins on the street, I would be most
hesitant to reject that declaration just because that man did not
say enough about how he did it to explain the pattern.
I acknowledged that _Ambulocetus_ may well have been an
aquatic mammal and raised the question of whether that fact had
been established simply to highlight the circumstantial nature of
the evidence. (In doing so, by the way, I overlooked the fact that
_Ambulocetus_ was discovered in a shallow marine environment. That
also is not definitive, but it is a significant piece of evidence.)
One problem with deducing dorsoventral motion from the position of
the foot when the femur was retracted from the hip is that no hip
was found. That is why Berta states "since the pelvic girdle is
not preserved, there is no direct evidence in _Ambulocetus_ for a
connection between the hindlimb and the axial skeleton. This
hinders interpretations of locomotion in this animal, since many of
the muscles that support and move the hindlimb originate in the
pelvis" (p. 180). In the opinion of the writers of _Science News_,
this means that "paleontologists will need to find more fossils to
decipher how _Ambulocetus_ swam" ("Fossil Whale Feet: A Step in
Evolution," _Science News_ 145:36 [1/15/94]).
As for the shape of the one lumbar vertebra, no explanation is
provided as to why it demands the conclusion that _Ambulocetus_
swam by means of dorsoventral undulations. It is therefore
difficult to evaluate. It certainly appears to differ from the
vertebrae of the dorsoventral archaeocetes and later whales, and I
think Berta's caution on the matter supports a skeptical attitude.
But again, this whole issue was essentially an aside.
Finally, my reason for citing Berta's questioning of the use
by Thewissen, et. al., of unorthodox whale characters to define
_Ambulocetus_ as a cetacean was to show that any conclusion based
in whole or in part on the cetacean status of _Ambulocetus_ (e.g.,
that it was aquatic) was less sure than was advertised. If part of
the case that _Ambulocetus_ was aquatic is that it was a cetacean,
then the certainty of that cetacean determination affects the
certainty that it was aquatic.
Ashby