Vertebrates and Their Closest Relatives

When I teach my course in Vertebrate Paleontology (see tweets at #UREES270), one of the first questions that arises is what is a vertebrate? What makes a vertebrate distinct from all other forms of life?

One interesting thing about vertebrates is that they don’t all have vertebrae. This one thing that seems like it should be the obvious thing that all vertebrates share, isn’t shared by all (although if you have an animal with vertebrae, it is most definitely a vertebrate!).

To make matters worse, you’d think that a big important group like the Vertebrata (the scientific name for the vertebrates) would get to have its own Phylum, like the mollusks, the cnidaria (jellyfish and kin), and the echinoderms (sea stars and sea urchins and kin), but no. The Vertebrata is relegated to Subphylum status within the Phylum Chordata, which means exactly nothing to most people.

The chordates (members of the Phylum Chordata) are a really interesting group, however. There are some chordates that are not vertebrates, including the lowly sea squirt, and the lancet (Amphioxus). What these chordates have that is shared with all vertebrates (hence grouping them together) is a notochord.

No doubt, the average reader has not heard of a notochord, so I expect that you might be surprized to know that you had one, just like the sea squirt, way back when you were an embryo. As an adult, your notochord is gone, though remnants remain in your intervertebral discs.

So next time someone you know has a disc rupture, tell them it’s their body trying to remember its sea squirt origins. Or not. Actually, you shouldn’t do that. Nevermind.

What makes a chordate a chordate? What distinguishes it from everything else? Here you go. A list.

1. Notochord – You knew this would come up. The Notochord is a rod of stiff material, like cartilage, that runs from nose to tail in chordates, either in an embryonic state (sea squirts and us), or in adults (like Amphioxus) What it does is keeps the animal from shortening up when muscles contract, like a worm will. The notochord acts as a flexible stiffening rod, allowing for side to side motions with coordinated muscular contractions.
2. Segmented muscles – There are little batches of muscle on both sides of the chordates body. You can see these segments if you filet a fish. These muscles cause the body to bend side to side when muscles on one side of the body contract at the opposite time as muscles on the other side of the body. This is how you get a fish-like swimming style.
3. Dorsal Hollow Nerve Cord (DHNC – which if you try to pronounce sounds rather Klingon) – This is the spinal cord in us. Ours was hollow embryologically, owing to its origins as a thickened and rolled up part of the outer layer of the embryo.
4. Podocytes – these cells work to clean and excrete unwanted metabolic wastes from the blood, much like our kidneys do. The difference is that they are not orgainzed into kidneys.
5. Pharynx with pharyngeal slits – Water passes into the chordate through the mouth and then back out again through pharyngeal slits. Sometimes the slits are mistakenly called ‘gill slits,’ but gills are not necessary. This a a condition that in some chordates is seen only in the embryo. We had pharyngeal slits that have since closed. The pharynx still exists, however, from the back of our mouths to where the esophagus begins. The pharynx in people is where the mouth, the nasal cavity, the lungs, and the esophagus connect.
6. Post anal tail – All chordates have tails. And in all cases the tail extends beyond where the digestive system outlets waste (aka, the anus).

All of these conditions apply to us as humans. But there is a further set that sets us apart from just ordinary chordates. We’re vertebrates. What makes us more betterer?

1. We have a head and brain and sensory organs – Chordates don’t have brains, despite having a dorsal hollow nerve chord, nor do they have eyes, ears, or noses.
2. We have a heart – We have blood vessels and a heart that circulates blood throughout the body.
3. We have kidneys – Instead of podocytes spread throughout the body, we have defined kidneys for ridding our body of nitrogenous waste.

Notice that not once have I mentioned bones nor vertebrae. Those things come later in vertebrates. The earliest vertebrates that we know of, the conodonts, only had tooth-like structures. The rest of their body was likely soft, which is why we tend to only find isolated conodont ‘teeth’.

I suspect you’re wondering (and if you aren’t, you should be) what is the closest living thing to chordates. (Really, you want to know!) This may be a surprize!

Meet your invertebrate cousins:

Yes, echinoderms. These are our closest living relatives. How that works is the topic of another blog post.