In this series, we trace interesting phylogenetic relationships between clades of organisms. This is mostly a fun exercise in taxonomy, but it also helps demystify the complex relationships between various groups of organisms.
Today we are looking at the path that leads from Animalia, all the way to Thelodonti.
Follow along by using the Phylogeny Explorer. Click along the indicated links in the tree so you can eventually get to Animalia.
Note that the Phylogeny Explorer refers to this clade as the Anapsid-Thelodont clade. We will continue to refer to this as Thelodonti.
What is the Thelodonti clade? It includes all jawless fish with distinct scales instead of armour. The homo clade branches off this one. But with many, many subbranches between them.
We start with the Metazoa or Animalia clade, as we agreed we would. Only the most stupid person would deny that fish are all animals!
We must now head down the Eumetazoa clade. This includes all animals more advanced than sponges. It is safe to say that fish are more advanced than sponges. So, down the Eumetazoa branch we go!
Now, fish have bilateral symmetry as an embryo. That is, their left and right sides are mirror images of each other. This means that they also have head and tail as well as a belly and a back. This makes them members of the Bilateria clade.
Note that not all Bilateria maintain their bilateral symmetry as adults. Bilateria are bilateral symmetrically as embryos but might be as adults. For instance, the echinoderms are bilaterally symmetrical in the embryonic stage. But as adults, they achieve pentaradial symmetry instead of bilateral symmetry.
So, down the Bilateria branch we go!
During embryonic development of fish, the first opening, the blastopore, becomes the anus. Every fish is at some point nothing but an … well yes.
Anyway … this makes fish a deuterostome!
Next, we must head down the Chordate branch. This clade is defined as all organisms with the following at some stage of their life-cycle/during embryonic development:
A flexible rod formed of a material similar to cartilage called the notochord. In vertebrates, such as fish, this later develops into the spine.
The presence of a hollow nerve cord dorsal to the notochord, known as a dorsal nerve chord. In vertebrates, such as fish, this develops into the spinal cord.
The presence of pharyngeal slits. In the case of fish, these develop into gill arches, the bony or cartilaginous gill supports.
The presence of an endostyle. This is an organ that assists in filter-feeding and seems to suggest that the common ancestor of chordates was a filter-feeding organism.
The presence of a post-anal tail at some point in their embryonic development. Of course, unlike humans, fish maintain this feature in the form of their visible tail!
So, we have gone down the Chordate branch, where next?
Fish have well-defined heads so down the Craniata branch we head. This branch includes all organisms with well-defined heads with brains, sense organs, eyes and a skull.
Now, our fish have a backbone, so we go down the Vertebrata branch.
Yes, we just saw that apparently the olfactory nerve predates the development of distinct heads and backbones! The sense of smell predates skulls and backbones. Neat!
Next, we head down the Agnatha-Pteraspidomorphi branch. These are believed to be a parent clade which is possibly ancestral to all jawed vertebrates. And ancestral to Thelodonti.
And now we find the Anapsid-Theolodont branch. These are the jawless fish with distinct scales instead of armour and what we were looking for! Finally!
That took a while! But we got there in the end and took a good look at some of the most basal groups of animals.
What phylogenetic journey will we go on next time? I have not decided yet, but I am sure it will be interesting!