CAECILIANS


Of the three groups of modern amphibians, there is one group, the legless caecilians, which do not occur in the United States. Two caecilians are pictured in the following images.
Chtonerpeton indistinctum

caecilian

Siphnops paulensis

caecilian

The evolutionary and creationism models make different predictions on how caecilians should be represented in the fossil record. Which is supported by the actual fossil evidence? The evolutionary and creationism models make predictions on the type of groups that caecilian species can be divided into. The evolutionary, creationism, and intelligent design models make different predictions about the types of variations that can be observed within these groups. Which is supported by the actual evidence?

EVOLUTIONARY MODEL

If evolution has occurred, then the types of caecilian alive today are not expected to have been present throughout the early part of life's history. The relationships among modern caecilian should form a nested hierarchy showing degrees of varying relatedness. At each level of classification, populations can acquire new features as they adapt to their niche. The same types of modifications which separate one group from another may be observed within a group.

CREATIONISM MODEL

If the creationism model is correct, then every "kind" of modern caecilian has always existed. There should be no period of the fossil record and certainly no period during the fossil record of life on land, which lacks caecilian fossils. Every "kind" of caecilian would have been in existence since the first week of life on earth. While evolution may occur within a "kind", one "kind" cannot evolve from another. As a result, each "kind" has no relation by descent to any other "kind" and an examination should prove which organisms are completely unrelated to each other. The variations which occur within a "kind" should be minor compared to those which differ between "kinds".

INTELLIGENT DESIGN

If intelligent design has occurred, then some/substantial evolution could have taken place in caecilian lineages. However, each separate "design" should be impossible to develop over time. The variations on a specific "design" should not significantly alter the "design" or create a complex new "design".

 

WHICH OF THESE TWO PREDICTIONS IS SUPPORTED BY FOSSIL EVIDENCE?

Not only does the caecilian fossil evidence strongly support the evolutionary model, it contradicts the predictions of the creationism model. The fossil evidence does not indicate that caecilians have always existed. The earliest caecilians are transitional forms between modern caecilians and more primitive amphibians.

caecilian

Modern species have no limbs nor trace of limb girdles and may have more than 200 vertebrae. The earliest caecilian fossils (Eocaecilia) are known from the Jurassic Period and, unlike modern caecilians, possess legs. The skull of Eocaecilia was also closer to the ancestral condition than that of modern caecilians (Jenkins, 1993).

comparison of skull bones

ANATOMICAL AND MOLECULAR COMPARISONS OF FROGS SUPPORT A CLASSIFCATION SCHEME WHICH CREATES A NESTED HIERARCHY OF RELATED ORGANISMS

The three orders of modern amphibians form a clade with salamanders and frogs closer to each other than either group is to caecilians (Zardoya, 2001). All studies indicate that caecilians share a common ancestry and there are traits which all caecilians share such as ringed annuli around the body, scales (a feature which no other group of amphibians possesses), unique skeletal and muscular modifications of the jaws, a male copulatory organ which allows internal fertilization, and complex extendible sensory tentacles (Mattison, 1998; Pugh 1998).

Based on molecular and anatomical characteristics, the more than 160 species of modern caecilians (Order Gymnophiona) can be classified into six families after their separation from the fossil group which included Eocaecilia. The most primitive family (Rhinatrematidae) which forms a sister group to the others. The two families Ichthyophiidae and Uraetyphlidae form a clade as do the higher caecilians of the families Caecilidae, Typhlonectidae, and Scolecomorphidae. (The family Caecilidae may be paraphyletic because the family Typhlonecticae arose within it.) Additional branching is evident when comparing the genera in these families (San Mauro, 2004; Wilkinson, 2002; Pugh, 1998).

Caecilians do not support the creationist concept of "kinds". Caecilians do not form a group which is equally unrelated to all other amphibians (let alone all other organisms), the families of caecilians are not equally unrelated, and the genera are not equally unrelated.

WHY ARE THERE SO MANY VARIATIONS WITHIN CAECILIANS?

It could be argued that there an animal's anatomy and physiology that needs to be designed because of the complex interactions of its components. However, if such a complex "design" is indeed so "intelligent", then why did so many caecilians rework their designs? Since much of this variation occurs within families (and even smaller taxonomic units), most modern creationists would permit this type of evolution in their model. If the original organization in caecilians was created or designed by direct divine influence, why did it require subsequent evolutionary tinkering in order to fix it and make it workable?

There are significant differences between the familes of caecilians. In general, caecilians vary betweem 7 cm and 1.5 m. Some are slender (with a ratio of body length to diameter of 100:1) while others are stouter (with a ratio of 15:1). Some species may have more than 200 vertebrae. The earliest caecilian fossils are known from the Jurassic Period and, unlike modern caecilians, possess legs. (Jenkins, 1993). The most primitive caecilians posses tails with 4-12 vertebrae (Rhinatrematidae, Uraetyphlidae, and Ichthyophiidae) while more advanced caecilians lack tails. The most primitive species possess scales (which are more like fish scales than those of reptiles) which are reduced in some lineages and lost in the most advanced groups. The higher groups of Caecilidae and Scolecomorphidae have reduced the number of skull bones they possess. The number of rings per body segment can vary. The family Rhinatrematidae possesses tentacles which are more posterior than the position found in other caecilians and unlike most other caecilians, the mouth is located at the tip of the face rather than being located under the snout. They can also vary in the presence of a bony layer over the eye and a temporal opening in the skull roof. In some caecilians (family Scolecomorphidae), the skull bones no longer form an orbit for the eye and the vestigial eyes may be extended as part of the tentacles. While most caecilians have adapted to a burrowing lifestyle, the family Typhlonectidae is aquatic.

A number of variations occur within caecilian families. Within the family Caecilidae, species can vary between 100 cm and 1500 cm in length. There is variation in the number of lungs (some with two, some with one, and no lungs in one species in Typhlonectidae). The lungless typhlonectine species (Atretochoana eiselti) is the only tetrapod in which the internal nares do not open into the oral cavity.

One of the most significant variations in caecilians is in reproduction style. The most primitive caecilians lay eggs which hatch into aqatic larvae which mature into terrestrial adults. In some species, the larvae do not pass through an aquatic stage and undergo direct development. About half of modern species give live birth to young that were nourished inside the female's body through secretions from the oviducts. Some species of the family Caecilidae lay eggs while others give live birth. Parental care is known in both egg-laying an live bearing species (Pough, 1998; Mattison, 1998).

While most caecilians are burrowers, some can thrive on the surface, some are semi-aquatic, and some species of the family Typhlonectidcae are aquatic and possess a dorsal fin (Pough, 1998; Halliday, 1987).

Variation which is known within caecilians is far greater than that which would be needed to modify an ancestral form into a primitive caecilian.

Are caecilians a "kind"? No one who has studied caecilians has argued that evidence supports separating them into groups unrelated by descent because of the unique anatomical structures they share and the molecular evidence linking the groups. Creationists accept that evolution can occur within a "kind". If some or all of the above variation can evolve then what aspects of the organism are so complex that they could not evolve? If they are designed, then why didn't the original designs work?