Invertebrate Department

Phylogeny is the study of the relationships between the living organisms. It is one of the most important disciplines in modern systematic work because it can help us creating hypothesis about the origin and evolution of life. The phylogenic work is based on the comparison of structures in different organisms. These structures can either be morphological traits, like organs, or it can be DNA or RNA. The fundamental idea is that closely related groups share some unique, advanced traits that not are present in other groups. Such traits are referred to as synapomorphies. For instance, an important synapomorphy for vertebrates is their vertebra. It developed at the base of the vertebrates and is not present in any other groups, thus we conclude that all animals with a vertebra is closely related. All other animal groups, on all different levels, is supported by similar synapomorphies. Thus, the first step in the phylogenetic work is to point out as many potential synapomorphic characters as possible. This often ends up with a very large dataset and it requires a computer to handle it further. When the dataset is coded in the computer program, the computer starts to calculate how these character traits could have evolved in the most simple way, which means with as few evolutionary changes as possible. The analysis results in on or several evolutionary trees showing the relationship between the animal groups and the synapomorphic characters that support the groups.

The finding of Limnognathia maerski has given new important  information about the relationships in the Animal Kingdom. Transmission electron microscopic (TEM) investigations of the jaws have shown that they are composed of densely packed rod-like structures. Jaw-like structures are found in several other animal groups but only two groups, Gnathostomulida and Rotifera, have jaws with a similar ultrastructure. Thus, the presence of jaws composed of these rods is a synapomorphy for Micrognathozoa (L. maerski), Gnathostomulida and Rotifera, and together these groups are united in the superphylum Gnathifera, meaning the "jaw bearers". Gnathifera also contains a fourth phylum, the Acanthocephala. Acanthocephala are highly modified parasites that live in the intestine of vertebrates, thus their feeding biology is so different that their alimentary system, including pharynx and jaws, is totally reduced. However, they have other traits that link them closely to the rotifers.

The discovery of L. maerski was phylogenetically important because it supports the close relationship between the gnathiferan groups. Both Gnathostomulida and Rotifera-Acanthocephala have earlier been difficult to place in the Animal Kingdom, so the gathering of the gnathiferan animals in one group has shed new light on the most basal splits in the Animal Kingdom.

 

 

Phylogeny

Click on the picture to read more about the relationship within Gnathifera

 

 

 

 

 

 

Click on the picture to read about the position of Gnathifera in the Animal Kingdom

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Micrognathozoa main page Reinhardt M. Kristensen Martin V. Sørensen

Last update: 14 december 2005

 

Responsible editor for this page: Martin V. Sørensen