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Use and Abuse of the Fossil Record: The Case of the ‘Fish-ibian’

Penny Higgins

September 13, 2006

A common argument of creationism is that no transitional forms between major organismal groups (e.g. the classes of the vertebrates: fish, birds, mammals etc.) are seen in the fossil record. However, paleontologists have uncovered a great wealth of fossils that are clearly transitional forms. In this essay, I will specifically address the transition from aquatic fish to terrestrial amphibian. First, let’s review what a ‘transitional’ form is and how we might recognize it.

A simple phylogeny for vertebrates.

Above is a simple phylogeny for vertebrates. All vertebrates are united by having a spinal column. The red lines are the evolutionary relationships among various groups of vertebrates. As one moves upward along any of these evolutionary lines, one moves forward in time, toward more ‘modern’ or ‘advanced’ species (see my discussion of the term ‘advanced’ in my previous column here). The splits between groups occurred among ‘primitive’ members of the groups. For example, the first amphibians lie on the phylogeny at the point labeled “2” which represents the origin of terrestrial locomotion. These first amphibians, while classified as amphibians, are quite different from modern frogs and salamanders we see today. These first amphibians evolved from primitive fishes, which are also unlike any fish we have today.

Positions on the phylogeny where transitional forms are likely to occur.

Shown above are the positions on the phylogeny where the intermediate forms between the major classes would reside. Note that these are not at the tips of the evolutionary lines (modern forms) but lower on the phylogeny. In other words, evolution moves in the direction of the blue arrows (see below) and not the red arrows.

A cartoon phylogram illustrating how evolution progresses. Red arrows show the direction of evolution were transitions between groups to have occurred between modern members of the groups. Blue arrows show how evolution proceeds where transitions between modern groups occurred in the remote past.

The transition from fishes to amphibians

Is (or was) there such a thing as a “fish-ibian?” (The term ‘fishibian’ is borrowed from Huse, 1993, p. 60.) If so, what would it look like? How would you know the “fish-ibian” was neither a fish nor an amphibian or that it was both? If we were to look at the most familiar modern fish and amphibians, we might envision the transitional form to be half frog, half trout:

A cartoon concept of a fish-amphibian intermediate form, based upon modern fish and amphibians. Though obviously silly, such “intermediates” are frequently illustrated in Christian ‘science’ textbooks.

This is the result of following the red arrows. But the reality is that the transition did not occur between modern fish and amphibians, but rather between some very ancient vertebrates some 375 million years ago. Back then, the distinction between fish and amphibians was not as immediately clear as it is today.

Cartoon of fossil genera representing the transition from fish to amphibians, which occurred in the Devonian. Tiktaalik is a new genus recently described and fits the temporal and morphological gap between known Devonian fishes and Devonian amphibians.

To put this on a phylogram, it would might something like this:

A sketch of how modern fish and amphibians, and Devonian fish and amphibians lie on a phylogram

Recently, a new fossil touted as a transitional form was discovered and described from Ellesmere Island in Canada’s Northwest Territories. This new species is called Tiktaalik rosae (Daeschler et al., 2006; Shubin et al., 2006). To understand fully the status of Tiktaalik as a transitional form between fish and amphibians, we need first to consider what it is that defines a ‘fish’ and what defines an ‘amphibian,’ and whether or not there distinctions will be visible in the fossil record.

Terms

The previous installment of this column discussed various terms used, misused, and abused in the discussion of transitional fossils. For this discussion, it will be helpful to have the following terms and definitions at our disposal.

- Taxon (taxa): A group of organisms categorized together. E.g.: “Primates” is a taxon that includes humans, apes, and monkeys. The Order “Perrisodactyla” includes several taxa of hooved mammals.

- Definition: A summary of characteristics that describes a taxon. This is generally a looser description, where some characteristics may or may not be present in all members of the taxon. Often the definition of a taxon provides ranges of variation within or among taxa or individuals within the taxon.

What is a “Fish?”

Definition: In the simplest sense, a fish is an aquatic vertebrate that depends upon water as its primary environment for day-to-day living and reproduction. Fishes breathe oxygen dissolved in water through gills. Fish have limbs adapted as fins for swimming, with no ‘fingers’ or ‘toes.’ Fishes typically have bony scales covering their entire body.

As quoted from the Illinois State Museum website: http://www.museum.state.il.us/mic_home/schools98/pces/project/characters.htm

What is an “Amphibian?”

Definition: Amphibians are semi-aquatic vertebrates which divide their time between water and land for day-to-day living. Amphibians are dependent upon water for reproduction. Amphibians typically have paired limbs (arms and legs) that include ‘fingers’ and ‘toes’ for moving about on land. Amphibians also have strengthened pectoral and pelvic girdles adapted for walking on land. Amphibians begin life in water breathing through gills and later in life transition to being able to breathe air through lungs. Amphibians typically do not have bony scales on their bodies.

As quoted from the Illinois State Museum website: http://www.museum.state.il.us/mic_home/schools98/pces/project/characters.htm

Which Distinctive Characteristics Will Fossilize?

These definitions depend a great deal on processes, behaviors and features that would not fossilize. As paleontologists, what are we left with? What is readily evident in the fossil record? And which of these characteristics allow us to distinguish between Fishes and Amphibians?

Fish have backbones. Amphibians have backbones. Same
Fish are cold-blooded. Amphibians are cold-blooded. Same
Fish breathe with gills. Amphibians first breathe with gills, and then with lungs. Different in adults
Fish lay eggs in water. Amphibians lay eggs in water. Same
Fish have scales. Amphibians have smooth, moist skin. Different
Fish have fins. Amphibians have legs and feet for walking on land. Different

Based upon this, we see that the primary distinctions that may be made between fish and amphibians are with the breathing apparatus and skull (in adults especially), in the structure of the skin and scales, and in the structure of the limbs.

The Fish-ibian

Based upon the above characters, what would we predict the ‘fish-ibian’ to look like?

The Breathing Apparatus and the Structure of the Skull

In the Devonian, the general pattern of change from fish to amphibian was the loss of the gill cover (opercular bones) and reduction in size of the postparietal bones. The loss of the opercular bones makes sense when you consider the change from breathing through gills to breathing with lungs. It also turns out that many of the bones that formerly made up the gill cover were incorporated into the shoulder apparatus of terrestrial vertebrates.

The reduction of the gill cover bones provides the animal the flexibility to lift its head and look up without having to change the orientation of the entire body, which is a tremendous advantage when the body is lying on solid land rather than floating in water.

It is important to note that modern fishes also have a great reduction in the postparietal bones when compared to fossil fishes. This is not a good distinguishing character between modern fishes and amphibians.

Simplified drawings of the skulls of some modern fish and amphibians and the skulls of Devonian fish and amphibians highlighting important changes associated with the transition from life in water to life on land

The Structure of Skin and Scales

Modern amphibians lack scales. They utilize their smooth, moist skin as a respiratory organ. Since scales are oftentimes lost or displaced in fossils, this is a difficult characteristic to address with fossils alone. We can state merely that scales may or may not have been present in the earliest amphibians and would not be surprizing in a transitional form between fish and amphibians.

Limb Structure

Perhaps one of the most striking differences between fishes and amphibians — at least in the Devonian — was in the structure of the limbs. This makes sense because a fin used to paddle a neutrally-buoyant fish about in water is not capable of lifting and hauling this same fish about on dry land.

Simplified drawings of the forelimbs of some modern fish and amphibians and the forelimbs of Devonian fish and amphibians highlighting important changes associated with the transition from life in water to life on land

One character that makes a fish a FISH is the presence of fin rays or lepidotrichia. These are the tiny bones that support the flexible, almost clothlike, membrane of the fin itself. The lepidotrichia are movable, allowing the fish to alter the shape of the fin in behavioral displays or in locomotion. Lepidotrichia are “dermal bones,” meaning that they develop embryologically directly from the dermal layers of skin. Land-dwelling vertebrates lack lepidotrichia, as the fin membrane is lost, to be replaced with bony fingers. Finger bones, as well as all wrist and arm bones, are termed “endochondral bones,” which are bones that develop from a cartilage precursor. Thus, lepidotrichia and phalanges arise from different embryological origins, despite occupying a similar position on the limb.

What is Tiktaalik?

The skull of Tiktaalik resembles that of other Devonian vertebrates, whether considered fish or amphibian. It notably lacks the bony opercular bones that characterize Devonian fishes like Eusthenopteron and Panderichthyes, and resembles more closely Devonian vertebrates regarded as amphibians such as Acanthostega and Ichthyostega. In the absence of other information, it may be best to classify Tiktaalik as an ‘amphibian.’

Tiktaalik is known to possess scales on the outer surface of its body. Based upon this, it may be most appropriate to classify Tiktaalik as a ‘fish.’

Because Tiktaalik possesses lepidotrichia, it is best classified as a ‘fish.’ However, its limb structure includes several endochondral bones (basically all but the phalanges), that are typical only of terrestrial vertebrates.

Based upon these characteristics, the authors describing Tiktaalik (Daeschler et al., 2006) classified it as a ‘fish.’

The Diagnosis of Tiktaalik — Cladistics style.

What I have presented above is only the simplest picture of distinguishing between fish and amphibians. Below is a more detailed listing of the various characteristics used to distinguish Tiktaalik from other fish and amphibians (Data from Daeschler et al., 2006; Shubin et al., 2006). Based upon this, Tiktaalik appears to be more amphibian-like than fish-like.

Click for a full sized version of this table.

So then, what is Tiktaalik?

Let’s return to the definitions of ‘fish’ and ‘amphibians’ and the characteristics of each that can fossilize.

Breathing: Fish have gills — therefore they also have the opercular bones. Amphibians (adults anyway) lack opercular bones. Tiktalik also lacks opercular bones, therefore it is more like an AMPHIBIAN.

Scales: Fish have scales covering their bodies. Modern amphibians lack scales. Tiktaalik has scales along its back, making it more like a FISH. However, it is not certain that early amphibians lacked scales, and the presence or absence of scales is not considered definitive.

Limb structure: Fish have fins with lepidotrichia used for swimming. The most ‘primitive’ fish lack the limb bones characteristic of terrestrial vertebrates. However, some lobe-finned fishes have characteristic limb bones like (in the arm) the humerus, radius, and ulna, as well as some of the carpal bones. The finger bones (metacarpals and phalanges) are lacking in lobe-finned fish, which instead have lepidotrichia. Amphibians have the complete suite of limb bones required for terrestrial locomotion (humeri to phalanges), and no lepidotrichia. Tiktaalik has lepidotrichia, which is more like a FISH. However, it also has elaborations of the bones in the limb which may have been precursors of metacarpals and phalanges, making it more like an AMPHIBIAN.

It appears that Tiktaalik is both FISH and AMPHIBIAN. It is a transitional form between fish and amphibians.

How do we classify Tiktaalik?

This is the difficult part, and where it is important to remember the distinction between taxonomy (the classification system for organisms) and phylogeny (the actual relationships among organisms). The authors that described Tiktaalik (Daeschler et al., 2006) chose to classify it as a tetrapod-like FISH. This decision was made for many reasons, mostly due to tiny details in the structure of the skull and the limbs. The cumulative result of all these small details is that Tiktaalik clearly is NOT a terrestrial vertebrate. Amphibians ARE terrestrial vertebrates. It is clear from Tiktaalik’s body form and from the rocks that it came from that it likely spent most of its life in water — like FISH. So, it is classified as such.

Bibliography and Suggested Reading:

Penny Higgins

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Dr. Pennilyn (Penny) Higgins is a Research Associate in the Department of Earth and Environmental Sciences at the University of Rochester. Most of her research revolves around studying the chemistry of fossil mammal teeth to learn about the environments in which the animals lived and what they might have been eating while living there. She is particularly interested in episodes of rapid climate change in the geologic record. In addition to doing research and managing a geochemistry laboratory, Penny also teaches courses in introductory geology and paleontology at the University of Rochester. When she's not in the office or laboratory, Penny can be spotted writing fiction, practicing the western martial arts, or just screwing around on Twitter.