Ashby wrote:
> 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. <
This was the main article I was wanting to get. There are several things
wrong with Warren's et. al claim. First, the track way was found on a slab
in the courtyard of a house built in 1873. (above ref. p. 183) They did an
admirable job of trying to find where the rock came from, but they were
unsuccessful They state," We have located several quarries in the Mt. Bepcha
region, but at the time of writing have not been successful in determining
the source of the courtyard flagstones."
Thus, while they believe that the flagstones came from the Grampians Group,
there remains a small doubt. This means that there is some uncertainty in
the age.
Secondly, there are no digit impressions preserved. No toes. Thus we can not
be certain that a tetrapod made the track. They state,
"No digit impressions are preserved and there is no tail or body trace.
Because of this there is no certainty that the marks form a tetrapod track,
but their highly regular nature indicates that they do." p. 184.
They ascribe the lack of digit impressions to the slumping of the wet sand
into the track. But I could give an alternative explanation. Remember the
Panderichthyids had paired fins, Ahlberg and Milner (1994, p. 508)
Such a fish using fins in a manner similar to a tetrapod could have made
tracks similar to those seen in the courtyard stone. (see Warren et al, p.
185. The drawing shows what I mean).
In this regard, such behavior has been observed. Coates and Clack (1990, p.
68) note:" But the development of digits while retaining an aquatic habit has
long been known to occur. Limb-like fins with digit like rays are used for
grasping, STALKING, or as supplementary sensory organs (for example, the
pseudo-digited fins of triglids (gurnards), Lophius(anglers), Histrio
(sargassum frogfishes), and ogcocephalids (bat-fishes)). Digits in early
tetrapods may have functioned similarly." (Capitalization emphasis mine)
What they are suggesting is that legs may have formed for use in water and
that the earliest legs were not able to completely support the fish out of
water.
Ashby Camp writes:
> 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. <
The panderichthyids don't have to do anything to advance this situation. The
Acanthostega, advances the case quite nicely. It essentially has a half
evolved leg! A glance at figure 2 of Coates and Clack (1990, p. 68) shows an
amazing similarity of its humerus, radius and ulna and that of the fish fin
of Eusthenopteron
Ahlberg states: (1991, p. 298)
"Note that the forelimb of Acanthostega is more fish-like than the hindlimb
and could probably not be brought into a weight-bearing position."
Coates and Clack (1990, p. 67) state,
"In Acanthostega the terminal radial and ulnal condyles, and the lack of an
olecranon process on the ulna, suggest that the forelimb could never have
flexed from the elbow to lie in a fully load-bearing posture, and thei it was
probably held horizontally."
This is not a fully functional terrestrial leg! It makes one wonder about
statements like
"There are no true transitional forms (that is, in the sense of forms
containing incipient, developing or transitional structures - such as
half-scales/half feathers, or half-legs/half wings) anywhere among all the
billions of known fossil forms." (Morris and Parker, 1987, p. 11)
or
"Not a single transitional form has ever been found showing an intermediat
stage between the fin of the crossopterygian and the foot of the
ichthyostegid." (Gish, 1989, p. 79)
I would suggest that Acanthostega with its not quite functional leg which
must have been held horizontally, is a good candidate. (Coates and Clack,
1990, p. 67 at bottom)
Ashby Camp wrote:
> 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). <
These types of statements which are always quoted seem to indicate that the
amphibian leg popped into existence with the modern morphology. This is not
the case. Tetrapods today are based upon a 5-digit form. The earliest
tetrapods were polydactylous. Acanthostega had 8 digits on fore and back
limbs. Ichtyostega had 7 on the hind limb. Tulerpeton had six digits on
both fore and hind leg. (Ahlberg and Milner, 1994, p. 509).
They state, "However, it now appears that neither the pentadactyl condition
nor the canonical wrist pattern is primitive for tetrapods, which invalidates
such earlier theories substantially if not completely." p. 509
Thus the earliest tetrapods do not have wrists and legs like ours. If the
modern tetrapod form is fully evolved, then they are half evolved.
While Ashby does not like the Panderichthyids as an intermediate form,
consider this,
"Mobility between the front and back of the skull table is absent in all land
vertebrates, and in fact is absent in one family of rhipidistian fish, the
Panderichthyidae." (Carroll, 1992, p. 51) This is one more possible
transitional feature.
References.
Per E. Ahlberg, "Tetrapod or near-tetrapod fossils from the Upper Devonian of
Scotland," Nature, Nov. 28, 1991, p. 298-301.
Per E. Ahlberg and Andrew R. Milner, "The origin and early
diversification of tetrapods, Nature, 368, April 7, 1994 507-514
Robert L.Carroll, " The Primary Radiation of Terrestrial Vertebrates" Ann.
Rev. Earth Planet. Sci. 1992, p. 45-84.
M. I. Coates and J. A. Clack, "Polydactyly in the Earliest Known tetrapod
limbs." Nature 347, Sept 6, 1990, p. 66-69
Henry Morris and Gary E. Parker, _What is Creation Science?_ (El Cajon:
Master Books, 1987),
Anne Warren, Robert Jupp and Barrie Bolton,
"Earliest tetrapod trackway," _Alcheringa_ 10: 183-86 [198