Fish—Their Origin and History (Part 1)

Part 1

1. An examination of the present basis of fossil classification.
2. How the fossil record is interpreted.

Part 2

1. Does the method or interpretation have any limitation?
2. An examination of the evolutionary interpretation.
3. A criticism of the evolutionary interpretation.

Part 3

1. Are there any alternatives to evolution?
2. How viable are such alternatives—a look at Creation Science and the Fish.

Introduction

Dogmatic statements such as “the vertebrates evolved from the invertebrates,” “the jawed fishes from the non jawed fishes,” “all major life groups have a common ancestor,” are common to many teachers, texts and students. Yet, how factual are such claims? Are they based on observed evidence? How well do such statements survive through scientific scrutiny? Are these statements viable hypotheses or merely dogmatic philosophical beliefs? This series of articles will examine the evidence for the reconstruction of the history of the fishes and look at the conclusions which can be drawn from such evidence. The series is arranged so that it can be used as an introductory teaching series on Fish History.

Fossil Classification

Since all phylogenetic histories (family trees) of the fishes are derived from the classification of either live or fossil groups, true understanding of the nature and any limitations of these family trees can only be gained from a clear insight into how the present classification schemes are constructed, what their aims are, and what deficiencies if any, they may possess. One modern biologist views present classification schemes this way, “Modern schemes of classification differ in the way in which they express the relationships of hemichordates, tunicates, acraniates, and vertebrates; often all are accorded equal rank as sub-phyla of the Phylum Chordata. Other schemes place the hemichordates in a phylum of their own, and retain only the acraniates and tunicates in the Chordata. This is certainly rather confusing when we examine different books, and confusion is made worse because the same group of animals may be given different names, for example Tunicata or Urochordata, Acrania or Cephalochordata. But of course, exactly how we associate large groups of like forms in these higher taxonomic units, or what we call them, merely reflects our view on the way in which the units are related and different writers have different views?” (Q. Bone, “Oxford Biology Readers,” The Origin of the Chordates).

The Basis of Classification

Taxonomers realized long ago that any system of classification based on only a limited number of observable characteristics depended too heavily on interpretation to be relied upon. For example, which of the observed characteristics were important? Were they all important? Were none of them important? Such questions just don“t seem to have answers that zoologists have been able to agree upon. For this reason, regardless of how easy and convenient this “observable” system is, it is presently regarded as being artificial or over simplified and is used to label things but not to discuss relationships.

Present day taxonomers have attempted to develop what is called a Natural system of classification, i.e. one which endeavors to show any true relationship that exists in the animal kingdom. The Natural system in common use has as its basis two major concepts or assumptions:

1. That true relationships can be determined by comparing the whole range of similarities and differences. Those animals which have many similarities are regarded as being closely related while those with few similarities are regarded as being only distantly related.
2. That all animals are related, i.e. they have developed or evolved from a first life form.

Using the assumption that similarity represents relationship, is however, not as easy as it seems, regardless of how exhaustive the list of similarities is. While it is easily proved via breeding experiments that related animals produce generations of offspring which are similar to each other and to the parental type, it does not necessarily follow that all similarities owe their origin to relationship. It has become the rather arduous task of many natural classifiers to distinguish those similarities which do indicate relationship from those similarities which do not.

While most natural classifiers use the assumption that all animals are related, there are two pitfalls which must be avoided: (a) there is an alternate assumption about relationship which cannot be overlooked, i.e. all animals are not related but have completely separate origins, and (b) avoid cyclic logic in the use of classification schemes: Step 1. assume all animals are related. Step 2. devise a classification scheme to show this. Step 3. use the resulting classification scheme as a proof that animals are related. Note that the conclusion in Step 3 is the assumption of Step 1. The logic has gone in a circle. It is important to remember that classification scheme regardless of its structure can ever be used as a proof. If a classification scheme has a non-tested assumption as a basis, then this factor must be taken into consideration in assessing its usefulness and truthfulness. Despite such criticism, however, it remains a necessary task of the biologist to seek to identify any true and natural relationships which exist either present or past in the animal kingdom.

For discussion

• Is it feasible to use a classification scheme to predict what sort of fossil groups are yet to be found, i.e. to describe a missing link or predict a gap?

Fossils and Natural Classification

In a “natural scheme” all facets of an animal’s existence are taken into account—internal and external structure, behavior, etc.—before it is classified. While this is feasible for living animals, most fossils can only provide us with a fraction of the information required for natural classification. This leads to significant restrictions or limitations on the degree of accuracy we can ever hope to achieve for many fossil classifications. Since we cannot hope to achieve a truly “natural scheme” for fossils, we are limited to an “artificial” type of classification on those often few (and hopefully important) features that have been preserved for our examination. In recognition of this uncertainty, a new term, “paleospecies,” has been introduced to distinguish a fossil species classification from the natural or present day usage of species. Concerning the nature of paleospecies, Professor Cain from Manchester University states: “The paleospecies is an uncomfortable compromise” between recognition of a fossil’s theoretical relationships and the necessity of incorporation of fragmentary fossil evidence into as natural as possible a system.

Phylogenetics (Family Trees)

In any reconstruction of an animal’s history based on fossil evidence, a minimum of four steps are usually involved. These are:

1. Find the fossil.
2. Give the fossil a position in the geologic column and an estimated age.
3. Reconstruct the fossil from its preserved parts.
4. Classify the fossil in an attempt to indicate its relationship to other groups, fossil or living.

Note: Only step 1 above can ever claim to be free of interpretation.

For discussion

1. Can you suggest times or fossil types when even step 1 is an interpretation?
2. A group of different fossils is found in one strata. Did the fossils live together, die together or simply get buried together? How would you prove your answer?

To illustrate steps l-4 let us consider the “find” of a fossil fish by Dr Ritchie, Curator of fossils at the Australian Museum in Sydney:

Step 1, Discovery:

This fossil was discovered by a research group from Bureau of Mineral Resources near Alice Springs (Amadeus Basin, Northern Territory, Australia). The fossil was stored in the Adelaide museum—no significance was attached to it at that date or for several years thereafter. Dr Ritchie visited the museum 1976-77. His analysis suggested it was significant and led to public reports such as that which appeared in the Australian newspaper (Saturday, April 2, 1977) under the headlines “Earth’s oldest backbone unearthed.” Shown above is a sketch (side view) of the fossil as found [Ed. note: sketch is not available in on-line article].

Step 2, Fossil position and estimated age:

1. The fossil was found in the lower Ordovician rocks of the Amadeus Basin.
2. Uniformitarian geologists gave the estimated age of these rocks as 480 million years via correlation of strata and radioactive dating of samples from the area.

Step 3, Reconstruction:

Shown below is a reconstruction by Ritchie and associated workers [Editor’s note: picture not available in on-line article]. It is important to note the amount of interpretation required for a reconstruction depends upon the nature and amount of material fossilized. It can range from almost 0% interpretation in the case of mammoths preserved in ice, to near 100% for poorly preserved materials.

Step 4, Classify the fossil and indicate its relationship to other fishes:

Classification Data:

1. The animal showed evidence of vertebrae (back bones) and was placed in the Phylum Chordata, Sub Phylum Vertebrata.
2. No trace of jaws were found and the anterior structures of the animal show distinct similarities to other known jawless fishes—Class Agnatha (no jaws).
3. The anterior end of the specimen possessed a shield-like covering and was therefore placed in the Sub Order Arandaspis (aspis shield).
4. Indications of body outline suggests a length of 1 Scm, but no further conclusions could be reached on the basis of such scanty evidence.

Relationships: General

Dr Ritchie’s interpretation: “It is very significant because we all come from something like this—man, fish, birds, etc. The age of the fossil is a key to a major period of evolution—the point when back boned animals evolved.”

The comment by Dr Ritchie is, of course, a very general statement and an obviously evolutionary one. In the next article we will take a very broad look at a total arrangement of fish fossils and see what conclusions are possible, and what weaknesses or strengths any such conclusions might have.