The Non-Jawed Chordate Lineages of the World Series Part I

It is is true that the majority of animals in the Phylum Chordata are jawed vertebrates of the Infraphylum clade Gnathostomata. This is where we, along with other mammals, birds and reptiles, amphibians, and jawed fishes are a part of. It is no surprise that these are often considered to be crowd favorites and among the charismatic species. There is a biases for this as we are gnathostomes like ourselves.

Polycarpa aurata. Photo by Nick Hobgood.

Of course there is more to Chordata than just jawed vertebrates. In fact Chordata has some pretty interesting non-jawed animals. For starters we have the jawless fish, hagfish and lamprey (which sounds like the name of a band if you ask me!) which are surprisingly successful in their own way. As we go further down the evolutionary tree we meet some really cool, alien-like creatures, lancelets and tunicates. These marine invertebrates come in a dazzlingly variety of colors, shapes, and astonishing behavior! It is a shame that these chordates are often in the shadows of us jawed vertebrates, and until recently there has not been much information about them.

This will be the first series presented here on the blog and it will cover the lancelets, the tunicates, the hagfish and the lampreys of the world. We will begin this series with characteristics that unite these non-jawed chordates with their jawed chordate relatives.

The Synapomorphies of Chordata

A diagram of a chordate with the synapomorphies. By Miss Buchheit.

What defines an animal as a chordate is the following that were present at some point in their development (Mallatt, 2009):

1. A segmented, muscular post-anal tail
2. An endostyle
3. A notochord
4. A dorsal hollow neural tube
5. Pharyngeal slits

In adult individuals in Craniata/Vertebrata modified Traits 2-5 into various homologous structures.

• Endostyle becomes the thyroid gland
• Notochord becomes the spine
• Dorsal hollow neural tube becomes the spinal chord
• Pharyngeal slits become the gills (which in jawed vertebrates some gill arches become the jaws) 

Regardless of the physical changes that took place, embryological work has shown that the embryos of vertebrates is very much the same in terms of anatomy with the other chordate subphyla before the changes as mentioned above.

A Brief Discussion of the Evolution and Interrelationships among Chordates

Drawing of the hemichordate Ptychodera flava. By A. Wiley.

According to the fossil record and molecular clocking, Chordata originated from a common ancestor of the Terreneuvian series of the Cambrian (~540 million years ago). We chordates as a whole are part of the Superphylum Deuterostomia which also includes marine invertebrates like the Phylums Echinodermata (e.g. sea stars and urchins) and Hemichordata (e.g. acorn worms).  The latter phylum was thought of as the fourth Subphyla of Chordata due to having a stomochord which was previously thought as a homologous structure to the notochord (Bateson, 1886) and having a similar muscular system (Hildebrand & Goslow, 2001). However recent molecular studies have shown that the stomochord is not homologous to the notochord and that hemichordates are more related to the echinodermates in the clade Ambulacraria (Satoh, 2014). Regardless chordates and ambulacrarians share a common ancestor and share the feature of the blasterspore becoming the anus in development and gill slits.

Within Chordata there are three subphyla:

• Cephalochordata - Lancelets
• Tunicata/Urochordata - Tunicates and Sea Squirts
• Craniata/Vertbrata - Cranium chordates (majority of which are vertebrates)

Both tunicates and lancelets were initially classified in the clade Acraniata. Both groups lack a true cranium and head as seen in the other chordate clade, Craniata. However it has been proven that Acraniata is paraphyletic in respect to Craniata. This lead to the debate over the placement of the tunicates. The traditional theory is that tunicates are a basal branch in Chordata and the sister group to Notochordata (the lancelets and vertebrate clade) based on apomorphies such as extension of the notochord across the body and differentiation in head and tail. Indeed one author went further and classified lancelets as members of the vertebrate Subphylum Vertebrata (Haeckel, 1894). It is commonly thought that chordates are descended from a sessile, tunicate-like ancestor (Pough et. al, 2005).

A more recent theory, however, based on the molecular data is that it is the tunicates, not the lancelets, that are related to the vertebrates in the clade Olfactores (Delsuc et. al, 2006). The evidence for this is largely based on genetic material, although both have neuromast cells. This raises all sorts of question in regarding the evolution of modern chordates and the bio-ecological nature of this common chordate ancestor. I find this to be equally possible that the divergence and relationships among the chordates is trifurcation in origin as we have both evidences supporting in Notochordata and Olfactores. At least one author suggests that the three subphyla should be best be recognized as phyla due to their distinctive anatomy and uncertainty of the sister taxon to the cranium vertebrates (Satoh, 2014) although it is redundant to do so in my opinion honestly as the synapomorphies as listed above are enough to support a single phylum.

The current consensus of the phylogeny of Vertebrata. Diagram by Joseph Keating.

Lastly there is also the issue of the placement of the hagfishes and the lampreys. Despite being similar creatures of habit, both of these jawless fishes share mixed characteristics with each other as (both are jawless fishes with gill pouches) and vertebrates (hagfish have a similar nervous system to vertebrates but the lamprey has a true vertebrate; Hildebrand & Goslow, 2001). As a result there are two theories with equal amount of evidences of physiological data and molecular work. There is Cyclostomata a clade containing our hagfish and lamprey (mostly by the molecular work), and there is Craniata where essentially lampreys are more related to the gnathostomes (mostly the morphological work; Pough et. al, 2005). This is why Craniata and Vertebrata have been used interchangeably in the literature. It is still debated to this day, although as of now the evidence seems to suggest that Cyclostomata is more likely than a lamprey-gnathostome clade as the larval development of both hagfish and lampreys are extremely similar and their "adenohypophysis arises ectodermally" (Oisi et. al, 2013). In other words, this is one of the few morphological evidences that supports the otherwise molecular supported Cyclostomata.

What is Next?
In the next part of the series I will be discussing about the lancelets. As there is only one class of these animals, it will be a short post discussing the bio-ecology of these animals. From there we will work our way to the tunicates and finally the cyclostomes. 

References

• Bateson W. (1886). The ancestry of the Chordata. Q. J. Microsc. Sci. 26, 535–571.
• Delsuc, F., Brinkmann, H., Chourrout, D., & Philippe, H. (2006). Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature, 439(7079), 965-968.
• Haeckel E. (1894). Systematische Phylogenie. Berlin, Germany: Verlag von Georg Reimer.
• Hildebrand, M. & Goslow, G. (2001). Analysis of Vertebrate Structure. John Wiley & Sons, 24-27; 309.
• Mallatt, J. (2009). Evolution and phylogeny of chordates. In Encyclopedia of Neuroscience (pp. 1201-1208). Springer Berlin Heidelberg.
• Pough, F. H., Janis, C. M., & Heiser, J. B. (2005). Vertebrate Life. Pearson/Prentice Hall, 19-23.
• Oisi, Y., Ota, K. G., Kuraku, S., Fujimoto, S., & Kuratani, S. (2013). Craniofacial development of hagfishes and the evolution of vertebrates. Nature, 493(7431), 175-180.
• Satoh, N., Rokhsar, D., & Nishikawa, T. (2014). Chordate evolution and the three-phylum system. Proceedings of the Royal Society B: Biological Sciences, 281(1794), 20141729.