This blog was kind of an accident. I was writing two other blogs about the polypterus and lungfish, and I wanted to take a big step back and look at the evolutionary placement of these fish. This kind of took me down a rabbit hole of phylogenetic history that I didn’t really intend to fall into, but I did anyway.
And surprisingly, I actually enjoyed it. I felt very much like a detective while I was doing this literature research. Rather than include all this info in one of the fish-focused blogs (and have it be a rather long-winded tangential section), separating this rollercoaster ride into its own post seemed like the best idea.
If you enjoy learning about the history of modern things, how they got to be this way and why, then you might enjoy this too. 🙂
Side note: If you are not familiar with phylogenies or are confused as to what the drawings are below, read this short primer.
Back before genetics and molecular testing were a thing, most animals were grouped together simply based on morphology. “If it looks like a duck and quacks like a duck, then it must be a duck.”
This is why we have so many fish are named ‘eels’ that are not eels. The electric eel [a type of knifefish] is more closely related to catfish, and the spiny eels and swampeels are more closely related to tuna . The hagfish is called a ‘slime eel’, although it developed almost 180 million years [ish] before eels even evolved.
Polypterus and lungfish have had a wild ride. When polypterus was originally described by Lacepède in 1797, it was grouped with osteichthyans. This was when fish had two main groups: Chondrichthyes (cartilaginous fish: the sharks, rays, and skates) and Osteichthyes (‘bony fish’: almost everything else). It was easy to place it with Osteichthyes – there weren’t many other choices.
The only group in Osteichthyes was Teleostei (which makes up 96% of all fish species currently). There were no osteichthyans that weren’t teleosts. So when gars (1758), bowfins (1766), paddlefish (1797), lungfish (1837 for the South American lungfish, 1870 for the Queensland lungfish, 1839 for African lungfish), reedfish (1865), and polypterus (1798) were identified, they were all classified as Teleosts, despite being morphologically unique in their own ways. Different from the rest of the Teleosts.
Enter Louis Agassiz stage right. (If the name sounds familiar, check out this blog post.) He ran Harvard’s Natural History Museum for a while and was a very influential white dude in biology. He was the first one to take a crack at these weird fish that weren’t quite teleosts.
In 1833, Agassiz devised the order of Ganoidei. One thing all these fish shared was a specific type of scale that wasn’t always shared with Teleosts: the ganoid scale.
Agassiz pulled gars, polypterus, paddlefish, lungfish, and even a few catfish species out of Teleostei and gave them their own group. He also pulled the sturgeons out of chondrichthys and placed them with Ganoidei. (Sturgeons have a cartilaginous skeleton, and based on this morphological characteristic, were placed with chondrichthyans, the cartilaginous fish.) But besides having a common-ish scale type, there were no other characteristics that really bound this group together, and there should have been. This was a problem.
Through the next 50 or 60 years, many attempted to solve this problem. In 1845, Müller created the groups Holostei (gars, bowfins, polypterus, reedfish, and lungfish and coelacanths, though I forgot them on the drawing) and Chondrostei (sturgeons and paddlefish, both have cartilaginous skeletons), recognizing these fish as being distinct groups.
Next, T.H. Huxley (aka “Darwin’s Bulldog”) made up an entirely new group of fish: Crossopterygians. In this new group were the polypertus, fossilized coelacanths, and lungfishes. Hindsight is 20/20, and even I’m still confused about this. Weird move.
However, in 1871, Edward Drinker Cope (that middle name tho) started to make sense of things. He redistributed the Ganoidei fish into two groups: Crossopterygii (polypterus, lungfish, fossilized coelacanths, and reedfish) and Actinopteri (which was composed of Chondrostei [sturgeon and paddlefish], Holostei [gar and bowfin], and Teleostei).
Then, in 1955, Romer divided osteichthyes into two main groups: lobe-finned fishes (sarcopterygians) and ray-finned fished (actinopterygians). It’s an important difference.
The left image shows the different fin anatomy between the two main classes of bony fish. The main difference you should take away is that in the actinopts, the end of their fins are not made of bony bits, but of cartilaginous ray-like elements called lepidotrichia. (The right image is a view of the lepidotrichia in the caudal [tail] fin of a fish.)
Sarcopts’ fins still have fin-rays, but their fins are much more bony and muscular, almost like a limb. In fact, they are the ancestral condition to limbs. That’s right, our closest fishy relative was a sarcopterygian fish.
Some other changes: Actinopterygii was coined in 1891 to describe both Actinopteri and Crossopterygii. Crossopterygii was renamed Cladistia in 1981. And with that, along with molecular testing to confirm when groups evolved and branched off relative to the other groups, you essentially have the phylogeny used today.
To sum up, polypterus went from being in teleostei, to ganoidei, to holostei, to crossopterygii, and finally to cladistia. Lungfish kind of had a similar trip, but from crossopterygii, it went to sarcopterygii in 1955.
Hopefully this will help set the stage for when I talk about polypterus and lungfish, and you’ll have a idea about where they fall respective to other actinopterygian and sarcopterygian fishes.
Much of this history came from: Patterson, C. Morphology and Interrelationships of Primitive Actinopterygian Fishes. American Zoology, 22: 241-259. 1982.