In my earlier post, I made the point that the importance of
transitional forms in the evolution-creation debate is due to the
assertion of Darwinian theory that every species that has ever
existed is the product of an unbroken chain of species stretching
back to the first cell. To use the fossil record in support of
this theory, the evolutionist must identify a section of a chain of
descent that crosses a significant morphological distance. Each
interior link in the section (transitional forms) must be morpho-
logically similar enough to its proposed ancestor to make the claim
of descent plausible (to a skeptic), and the ends of the section
(exterior links) must be morphologically different enough to
suggest that natural descent can produce the kind of massive
changes proposed by the theory.
Glenn has identified a section of a chain of descent that
certainly crosses a significant morphological distance. He alleges
that modern whales (suborders Mysticeti and Odontoceti) descended
from a land mammal, specifically a Mesonychid condylarth. In his
view, the ends of this section of chain are adequately connected by
two interior links or transitional forms: _Ambulocetus natans_ and
some kind of archaeocete. As he put it, "The presumed order of
evolution has Mesonychid as the ancestor, Ambulocetus as an
extremely early whale transition, the archaeocetes as the earliest
true whales and Mysticeti and Odontoceti as the modern groups."
I also mentioned in my former post that to evaluate the claim
that a particular fossil animal (creature B) constitutes a
transitional form, one needs to know the identity of the animals it
is alleged to connect (creatures A & C) and the anatomy and dates
of existence of each of the animals in question (creatures A, B &
C). If creature B is indeed creature A on its way to becoming
creature C, then one would expect the features of creature B to be
either the same as those of creature A or intermediate between the
features of creatures A & C (the original post mistakenly read A &
B). In other words, one would not expect the differences between
creatures B and A to be in any direction other than toward the
features of creature C.
To aid in this analysis, Glenn referred the reader to drawings
of the skeletons of these various creatures which appeared in the
article by Annalisa Berta in _Science_ 263:180 (1994). In his
opinion, these drawings show _Ambulocetus_ to be a morphological
intermediate between the mesonychid and an archaeocete. He was
particularly impressed by the direction of change in the skulls and
ribs.
Unfortunately, the readers of _Science_ are not told that the
mesonychid shown in the series is _Mesonyx obtusidens_, the drawing
having been taken from Plate 5 of W. B. Scott's "On Some New and
Little Known Creodonts," _Journal of the Academy of Natural
Sciences of Philadelphia_ 9:155-85 (1888). This is a crucial
omission because the genus of which that creature is a member
(_Mesonyx_) first appears in the fossil record in the Middle Eocene
(See, Frederick S. Szalay and Stephen Jay Gould, "Asiatic Mesony-
chidae [Mammalia, Condylarthra]," _Bulletin of the American Museum
of Natural History_ 132:168-70 [1966]; see also, Robert L. Carroll,
_Vertebrate Paleontology and Evolution_ [New York: W. H. Freeman,
1988], 643). This makes it too late to be the ancestor of
_Ambulocetus_, which its discoverers place in the Lower to Middle
Eocene (J. G. M. Thewissen, et. al., "Fossil Evidence for the
Origin of Aquatic Locomotion in Archaeocete Whales," _Science_
263:210 (1994). One suspects that a comparable omission in crea-
tionist literature would be viewed by evolutionists as deceptive.
The fact of the matter is that we know very little about the
morphology of paleocene mesonychids. According to Szalay and Gould
(p. 168-70), the only mesonychid genus that has been identified in
the paleocene is _Dissacus_. The most complete specimen of this
genus is of the species _Dissacus saurognathus_ (originally called
_Dissacus carnifex_) described by Henry Fairfield Osborn and
Charles Earle in "Fossil Mammals of the Puerco Beds," _Bulletin of
the American Museum of Natural History_ 7:31-39 (1895). The
specimen consists of some lower teeth and parts of a front limb, a
manus, a pelvis, a hind limb, a pes, and some vertebrae. The skull
of _Dissacus_ is not known.
(Note to Glenn - I have information that in the 1970's some
findings in China from the paleocene were assigned to the family
_Mesonychidae_, but I do not have access to the relevant journal.
If you are able, check the following volumes and pages in _Verte-
brata PalAsiatica_ : 11:31-35 [1973]; 14:252-58 [1976]; 16:77-85
[1978]; 18:138-41 [1980]. Given the fact that these findings have
received little attention in the literature and have not yielded
any new reconstructions [hence reliance on that of Scott from
1888], I suspect that they are quite fragmentary. If I am wrong,
I trust that you will let me know.)
The little that we do know about _Dissacus_ makes it very
unlikely that it was an ancestor of _Ambulocetus natans_. In
_Dissacus_, both the front and hind feet are pentadactyl, but in
_Ambulocetus_ the hind feet appear to be tetradactyl (judging from
the drawing, the fact that the first metatarsal was not found, and
the fact that only two proximal, three intermediate, and three
distal phalanges were found). It is hard to believe that a semi-
aquatic creature, as the ancestor of _Ambulocetus_ is hypothesized
to have been, would gain a reproductive advantage by losing one of
its hind toes. If anything, one would imagine that the additional
toe would provide a broader surface area to aid in swimming.
As for the front feet, in _Dissacus_ the first digit (thumb)
is large and the fifth digit (little finger) is reduced, but in
_Ambulocetus_, the thumb is "short and slender" relative to the
little finger (Thewissen, 211). Again, it is hard to take
seriously the claim that the latter configuration would provide a
selective advantage over the former.
Osborn and Earle (p. 35) note that the femur of _Dissacus_ is
"flattened transversely, this widening of the shaft being in strong
contrast to the rounded femora of recent Carnivora." There is no
mention of flattening in the description of the femur of _Ambulo-
cetus_. On the other hand, Thewissen, et. al. (p. 210), state that
the forearm of _Ambulocetus_ "was fixed in a semipronated
position, as a result of the triangular shape of the radial head."
There is no mention of such a feature in the description of the
radius of _Dissacus_.
The dentition of _Dissacus_ and _Ambulocetus_ is quite
different. Thewissen, et. al. (p. 212), state that the fourth
premolar of _Ambulocetus_ "has a single high labial cusp." This is
not true of _Dissacus_, as the drawing by Osborn and Earle (p. 31)
makes clear; P4 in _Dissacus_ has three cusps (actually the third
"cusp" is a cristid obliqua - see below). Thewissen, et. al. (p.
212) also state that the first and second lower molars of _Ambulo-
cetus_ have "a high connate para- and metacone, and a protocone
that is on a broad and low lingual shelf." As depicted by
Frederick Szalay, the lower molars of _Dissacus_ have a small,
rounded paraconid and are marked by a feature analogous to
carnassial notches and a crest (called cristid obliqua) formed by
the buccal wall of the talonid (Frederick S. Szalay, "Origin and
Evolution of Function of the Mesonychid Condylarth Feeding
Mechanism," _Evolution_ 23:704 [1969]). These latter features are
not present in _Ambulocetus_. In addition, the lower canine is
much larger in _Dissacus_ than in _Ambulocetus_, the premolars of
_Dissacus_ have a backward slant which is not present in _Ambuloce-
tus_, and there is a significant gap between the first and second
premolars of _Dissacus_ which is not present in _Ambulocetus_
(compare Szalay, 718 with photo and drawings in Thewissen, et. al.,
211).
The claim that _Ambulocetus_ was ancestral to the archaeocetes
cannot be correct because, as Thewissen, et. al., point out,
_Ambulocetus_ was found 120 meters *higher* than the oldest
archaeocete, _Pakicetus inachus_. In other words, _Pakicetus
inachus_ appears before _Ambulocetus_ in the fossil record. Since
the little that we know about _inachus_ (it is known only from an
incomplete skull and jaw fragments) suggests that it was very
similar to the archaeocetes of the early Middle Eocene, _Ambulo-
cetus_ is too late to be ancestral to that group.
In fact, _Pakicetus inachus_ is thought to be so similar to
the Middle Eocene archaeocete, _Pakicetus attocki_, that _attocki_
was used to derive the outline of the lower jaw of _inachus_
(Philip Gingerich, et. al., "Origin of Whales in Epicontinental
Remnant Seas: New Evidence from the Early Eocene of Pakistan,"
_Science_ 220:404 [1983]). The skull structure of _Ambulocetus_,
on the other hand, is very different from that of _Pakicetus
inachus_ and later archaeocetes. Just compare the back of the
craniums of the skulls of _inachus_, _Ambulocetus_, and later
archaeocetes. A diagram of _inachus_ can be found in the article
by Gingerich, and Berta (p. 180) provides diagrams of the last two.
The notion that the skull of _inachus_ evolved into the shape of
that of _Ambulocetus_ and then reverted to its original shape in
the skulls of later archaeocetes, all within a couple of million
years (if that long), cannot be taken seriously.
Accepting that _Ambulocetus_ is too late to be an ancestor of
the Middle Eocene archaeocetes, the question arises as to the
relationship between those archaeocetes and Mesonychid condylarths.
The theory that archaeocetes descended from mesonychids was first
proposed by Leigh Van Valen in "Deltatheridia, A New Order of
Mammals," _Bulletin of the American Museum of Natural History_
132:90-93 (1966). He argued, on the basis of certain dental
similarities between _Dissacus navajovius_ and some archaeocete
specimens, that Mesonychidae are the most likely candidates for
ancestors of the archaeocetes. His rather cautious conclusion is
worth quoting:
To my knowledge the family of Mesonychidae is one of the
relatively few groups of mammals (and even of reptiles) that
has not been specifically suggested as ancestral to the
whales, but in my opinion the preceding argument establishes
them as at least the most likely candidate. . . . _Dissacus
navajovius_ is possibly directly ancestral, but little is
known of the early history of the mesonychids, especially
outside North America (p. 92).
In a more extensive dental analysis published three years
later, Frederick Szalay suggested that both hapalodectines (a
Mesonychid subfamily dating from the Lower Eocene) and archaeocetes
probably "derived from either early or middle Paleocene mesony-
chids, *species more primitive than known mesonychines*" [emphasis
mine]. (Frederick S. Szalay, "The Hapolodectinae and a Phylogeny
of the Mesonychidae [Mammalia, Condylarthra]," _American Museum
Novitates_ 2361:25 [1969]; for application of statement to
archaeocetes, see figure 19, p. 24). In other words, the theory of
mesonychid ancestry, as modified by Szalay, does not propose that
any known mesonychids qualify as archaeocete ancestors. Rather,
the theory suggests that archaeocetes branched off from some
earlier, as yet undiscovered, Paleocene mesonychid.
Edwin Colbert's assessment of the theory of mesonychid
ancestry for archaeocetes captures well its limitations. He
writes: "In general this [archaeocete] skull appears as if it might
have been derived from a mesonychid type, but there is little
beyond certain general resemblances to support such a relationship"
(Edwin H. Colbert, _Evolution of the Vertebrates_ 3rd ed. [New
York: John Wiley & Sons, 1980], 329).
Before examining the alleged archaeocete to modern whale
transition, a word needs to be said about the claim that _Ambulo-
cetus_ was an aquatic mammal. This claim is based on its alleged
cetacean affinities, its presumed capacity to swim via dorsoventral
undulation, and the fact it was discovered in a region in which
whales are believed by evolutionists to have originated. While
_Ambulocetus_ may have been an aquatic mammal, none of these
reasons is sufficient to prove the point.
While Thewissen, et. al., boldly declare that "_Ambulocetus_
is clearly cetacean," a more cautionary note is sounded in the
article by Berta (cited above). Regarding the characters that
Thewissen, et. al., used to establish _Ambulocetus_ as a whale, she
states: "Before these purported whale characters can be used in a
phylogenetic definition of whales, however, the possibility that
some of them may have a broader distribution (for example, in
mesonychids) needs to be examined" (p. 181).
The equally bold declaration that "_Ambulocetus_ swam by means
of dorsoventral undulations of its vertebral column" (Thewissen,
et. al., 211) is based solely on the shape of the one lumbar
vertebrae that was found. As Berta points out, "since the pelvic
girdle is not preserved, there is no direct evidence in _Ambulo-
cetus_ 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 other words,
"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]).
The fact that _Ambulocetus_ was discovered in a region in
which whales are believed by evolutionists to have originated
(i.e., evolved) obviously has no probative value for a creationist.
One must accept the story of evolution for this to be of any help,
and even then it seems to be putting the philosophical cart before
the analytical horse.
Finally, a word about the alleged archaeocete to modern whale
transition. This view has its supporters, but it also has major
problems and a significant number of detractors. That is why
Szalay could say that "a probable link between the archaeocetes and
the modern odontocete and mysticete whales has never been substan-
tiated" (Szalay, "Origin and Evolution . . .," 704). As George
Gaylord Simpson concluded in "The Principles of Classification and
a Classification of Mammals," _Bulletin of the American Museum of
Natural History_ 85:214 (1945):
Thus the Archaeoceti, middle Eocene to early Miocene, are
definitely the most primitive of cetaceans, but they can
hardly have given rise to the other suborders. The Odonto-
ceti, late Eocene to Recent, are on a higher grade than the
Archaeoceti and, on the average, lower than the Mysticeti,
middle Oligocene to Recent, but apparently were not derived
from the former and did not give rise to the latter.
The fullest development of the arguments against archaeocetes
being ancestral to modern whales is by A. V. Yablokov in "Conver-
gence or parallelism in the evolution of cetaceans," _International
Geology Review_ 7:1461-1468 (1965). In his opinion, "It is now
obvious to most investigators that the Archaeoceti cannot be
regarded as direct ancestral forms of the modern cetaceans" (p.
1463). Yablokov points out that there are major differences
between archaeocetes and modern whales in the shape of their
bodies, structure of their thoracic fins, and arrangement of their
skulls. His overall conclusion is that "[t]he similarity between
the living suborders is the result of convergence, and the
Archaeoceti are not ancestral to the other two" (p. 1461).
This view is reflected in the phylogeny of the cetacea in
Robert L. Carroll's _Vertebrate Paleontology and Evolution_ (New
York: W. H. Freeman, 1988), 526. He depicts the mysticetes and
(less certainly) the odontocetes as having a common ancestry *above
the level of the known archaeocetes*.
Food for thought, I hope.
Ashby
Copyright (C) 1995 by Ashby L. Camp. All rights reserved. Anyone
wishing to copy this article and distribute it may do so, but only
if the entire text is distributed without alterations and free of
charge.