Greetings to all.
After listening for several weeks to the discussion on this
listserv, I wanted to say a few (perhaps a few too many) words in
response to Glenn's post on the fish to amphibian transition and
the land mammal to whale transition. I appreciate his challenge
and his concern for the faith of young men and women who are
heading off to college. I admit to some hesitancy about entering
the discussion due to time demands and fear of the "tar baby
syndrome," but my desire to address this subject proved stronger.
On p. 74 of _Darwin on Trial_, Phillip Johnson provides the
following quote from Barbara J. Stahl's textbook _Vertebrate
History: Problems in Evolution_ (1985): "none of the known fishes
is thought to be ancestral to the earliest land vertebrates. Most
of them lived after the first amphibians appeared, and those that
came before show no evidence of developing the stout limbs and ribs
that characterized the primitive tetrapods." It seems clear that
her objection to the various fish which have been put forth as
tetrapod ancestors is not that they lack signs announcing their
ancestral status but that they lack evidence of developing the
structures which most distinguish the early tetrapods.
On pp. 74-75 Johnson points out that the internal organs of
the coelacanth, an ancient fish which was thought to have been long
extinct until one was caught in 1938, showed no signs of preadapta-
tion for use in a terrestrial environment. This was contrary to
the expectations of evolutionists. It was they, not Professor
Johnson, who pinned high hopes on the study of the coelacanth's
soft biology. This is evident from the comments of P. L. Forey in
_Proceedings of the Royal Society of London, B_ (1980) 208:369
(quoted in Michael Denton's _Evolution: A Theory in Crisis_, 179):
We had to wait nearly one hundred years before discovery of
the Recent coelacanth. During that time many fossil coela-
canths were described and, on the basis of osteological
features, their systematic position as near relatives of the
extinct rhipidistians and as tetrapod cousins had become part
of "evolutionary fact", perpetuated today in textbooks. Great
things were therefore expected from the study of the soft
anatomy and physiology of _Latimeria_. With due allowance for
the fact that _Latimeria_ was a truly marine fish, it was
expected that some insight might be gained into the soft
anatomy and physiology of that most cherished group, the
rhipidistians. Here at last was a chance to glimpse the
workings of a tetrapod ancestor. These expectations were
founded on two premises. First, that rhipidistians are the
nearest relatives of tetrapods, and secondly, that _Latimeria_
is a rhipidistian derivative.
I therefore think that the criticism of Johnson on this point
is unfounded. (For the record, according to Colbert's _Evolution
of the Vertebrates_, 3rd ed. [1980], 456, the coelacanth is *not*
a different order than the crossopterygian. Crossopterygii is an
order of the subclass Sarcopterygii; the order Crossopterygii
contains the suborders Rhipidistia [extinct] and Coelacanthini [the
surviving _Latimeria_]).
Before addressing the particular animals which were put forth
as transitional forms, some general comments on the subject may be
helpful. The question about the fossil record is not whether there
is a progressive appearance of features as one moves up the
geologic column but why that is the case. The evolutionist
contends that every species in the fossil record is the product of
an unbroken chain of ancestral species reaching all the way back to
the first living cell. The creationist, on the other hand,
contends that every species in the fossil record is the product of
either fiat creation or a chain of ancestral species reaching back
to one of many products of fiat creation (the "Genesis kinds"). In
other words, the difference between a theistic evolutionist and a
creationist is ultimately over the number of species that were
created by divine fiat (one versus many).
One can sympathize with the evolutionist's frustration over
the creationist's inability to identify the number or specific
features of the various species which he contends were created by
fiat. This "weasel room" makes the theory extremely difficult to
disprove, but being difficult to disprove is different than being
false. In fact, the theory of evolution has much the same quality.
Rather than predicting the path along which life would diversify,
the theory simply offers ad hoc explanations of the fossil record
as it stands. The theory is so flexible that any pattern of change
or stability can be accommodated by it. One is limited only by
one's ability to concoct some selective advantage for poorly known
animals living in even lesser known environments. Given the
elasticity of both evolution and creation theories, theological or
philosophical considerations often play a major role in one's
choice between them.
The importance of transitional forms in this 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.
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 & 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. (That is one reason why fossil creatures are rarely,
if ever, placed in a direct line of descent. The direction of
change is not uniform with regard to the various features of the
animals in question.)
Turning to the question of whether a crossopterygian possess-
ing a nasal passage, a lung, and infolded enamel on its teeth
qualifies as a transitional form, one needs to know the ancestor
and descendant it is alleged to connect. By labeling these fish
transitional, one implies that they descended from a fish (or some
other creature) that lacked a nasal passage, a lung, and labyrin-
thodont infolding of dentine. There is, however, no indication of
such an ancestry for crossopterygians in the fossil record.
The class Osteichthyes (bony fishes) is divided into two
subclasses: Actinopterygii (ray-finned fishes) and Sarcopterygii
(lobe-finned, air breathing fishes, which include the orders
Crossopterygii and Dipnoi [lungfishes]). Since both of these
widely divergent subclasses appear at the same time in the fossil
record (early Devonian), neither is believed to be the ancestor of
the other, and no plausible ancestor for either has been found. As
Romer acknowledges, "The common ancestor of the bony-fish groups is
unknown" (Alfred Sherwood Romer, _Vertebrate Paleontology_, 3rd ed.
[Chicago: University of Chicago Press, 1966], 53). As put by Errol
White, a former president of the Linnean Society and an expert on
lungfishes: "Whatever ideas authorities may have on the subject,
the lungfishes, like every other major group of fishes that I know,
have their origins firmly based in nothing" (quoted in W. R. Bird,
_The Origin of Species Revisited_ [Nashville: Regency, 1991;
originally published by Philosophical Library, 1987], 1: 62-63).
In this light, the issue is not whether crossopterygians
constitute a transitional form (since they have no known ancestor)
but whether they can be shown persuasively to be ancestral to
tetrapods (the beginning link of a chain that extends to tetra-
pods). Since Ahlberg and Milner ("The origin and diversification
of tetrapods," _Nature_ 368: 507-14) suggest that panderichthyids
are the closest crossopterygians (suborder rhipidistia, superfamily
osteolepidoidea) to the tetrapods, that is where the case is to be
made. (Note: This classification is from Robert L. Carroll,
_Vertebrate Paleontology and Evolution_ [New York: W. H. Freeman,
1988], 611. Ahlberg and Milner suggest that panderichthyids are no
longer classified as osteolepiforms, but I was unable to confirm
that.)
As Ahlberg and Milner point out (p. 507), the oldest pander-
ichthyids currently known date from the Lower Frasnian (Upper
Devonian - about 375 mya). There is solid evidence, however, for
the existence of tetrapods back to the Lower Devonian, some 25
million years earlier (Anne Warren, Robert Jupp and Barrie Bolton,
"Earliest tetrapod trackway," _Alcheringa_ 10: 183-86 [1986]).
This rather inconvenient piece of data was omitted from the Ahlberg
and Milner review. If tetrapods existed millions of years before
panderichthyids, then tetrapods either evolved in more than one
line or panderichthyids had nothing to do with their evolution.
Putting aside this chronological problem, the gap between the
panderichthyids and the first amphibians for which we have skeletal
remains is very wide. It has long been true that the most
conspicuous differences between rhipidistians and amphibians are in
the skeletal structures associated with locomotion. The real
skeletal trick to transforming a rhipidistian into an amphibian is
in drastically altering the pectoral girdle and detaching from the
head, drastically altering the pelvic girdle and attaching it to
the vertebral column, and drastically altering the bones of the
fins to function as weight-bearing limbs. The panderichthyids do
nothing to advance the case for evolution in this regard.
As Keith Stewart Thomson recently cautioned in "The origin of
tetrapods," _American Journal of Science_ (1993) 293-A:58:
The limbs, of course, occupy pride of place in any analysis of
tetrapod origins. The pattern of internal structure of the
osteolepiform limb as in _Eusthenopteron_ [cite omitted] and
_Sterropterygion_ [cite omitted] is clearly homologous with
that of tetrapods with respect to the humerus/femur or ulna
and radius/tibia or fibula, but little else. It would also be
a mistake to exaggerate the extent to which osteolepiform
fishes actually used their fins as arms and legs; the fins in
the forms that we know are all small and feeble (compared even
with the large fins of porolepiforms, coelacanths, and the
modern lungfish _Neoceratodus_, which have a different
internal structure). These fishes obviously could not live
out of water because they would suddenly be unsupported and
feel the force of gravity.
In contrast, the earliest land vertebrates (for which there
are skeletal remains, i.e., _Ichthyostega_, _Acanthostega_, and
_Tulerpeton_) "had short but massive limbs of the basic pattern of
subsequent tetrapods" (Robert L. Carroll, "The Primary Radiation of
Terrestrial Vertebrates," _Annual Review of Earth Planet Science_
[1992] 20:47). These differences are so significant that Dr.
Carroll states that "no fossils are known that can be considered
intermediate between these clearly aquatic [osteolepiform] fish and
genera that are unequivocally classified as terrestrial verte-
brates" (_Ibid_., 45). Similarly, Dr. Thomson declares (p. 39):
While we still do not have any really intermediate fossil
forms between fishes and tetrapods (we are getting closer,
with the description of _Panderichthys_ and _Elpistostege_;
see later) we are free to argue vociferously about the
identity of the group of fishes that must be the tetrapod
ancestor. (This is like the joke about the baseball player
who, although he was terrible at bat, couldn't field either.)
It is thus not only antievolutionists who recognize the chasm
that still exists between all proposed fish ancestors and the Upper
Devonian tetrapods. One need not be irrational to view this gap as
real.
One should note that Ahlberg and Milner are not contending
that tetrapods actually descended from panderichthyids. Rather,
they recognize that "[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 descendants of a common
ancestor.)
This is important because the theory of evolution is a theory
about direct descent. What is needed to support the theory is
chains of descent, but what one finds is an absence of such
lineages. The evolutionist is free to believe that the presence of
panderichthyids means the presence of some other similar species
which he believes would qualify as a tetrapod ancestor, but that is
a different matter than having the species in hand.
As for the earlier misidentifications of panderichthyid skulls
(assuming that they were misidentified), this is largely due to the
fragmentary nature of the specimens and the difficulty inherent in
interpreting fossilized material. Paleobiology is much more an art
than is commonly appreciated, and this provides ample room for
subjectivity to enter into the evaluation. As Meyer and Dolven
recently noted, most of the fossils are only available from
incomplete skeletons and the record is difficult to interpret.
"[T]he same features can be viewed differently by different
researchers." (Axel Meyer and Sarah I. Dolven, "Molecules,
Fossils, and the Origin of Tetrapods," _Journal of Molecular
Evolution_ (1992) 35:106.) One suspects that David Pilbeam's
reflections on paleoanthropology are applicable beyond that
subspecialty:
[P]erhaps generations of students of human evolution, includ-
ing myself, have been flailing about in the dark; . . . our
data base is too sparse, too slippery, for it to be able to
mold our theories. Rather, the theories are more statements
about us and ideology than about our past. Paleoanthropology
reveals more about how humans view themselves than it does
about how humans came about. (David Pilbeam, "Book Review of
Leakey's Origins," 66 _American Scientist_ (1978): 379 [cited
in Bird, 1:226]).
Later I hope to have something to say about the land mammal to
whale transition.
Ashby L. Camp
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
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