Entelognathus primordialis
Name: Entelognathus primordialis [Primordial complete jaw]
When: Late Silurian Period, about 424 million years ago
Where: Xiaoxiang Reservoir, Yunnan, China
Claim to fame: This is an ugly fish! Why have I bothered with it? Because Entelognathus is a hero. Let me explain.
The first, and the most evolutionarily primitive vertebrates with proper jaws were an odd assortment called placoderms - distant relatives of proper bony and cartilaginous fish, and their fossil relatives. Placoderms showed primitive features of the first jawed vertebrates including aspects of the shape of the skull and the presence of extensive bony armor. Despite this, we don't call them bony fish, because their bone was mostly external. Their vertebrae, ribs, and so on were cartilage, instead. But note: Proper bony fish also have bony armor, but the identities of the specific bones weren't thought to match up to those of placoderms, whereas within the bony fish, we are safe saying that a particular bone like the parietal of a goldfish is the equivalent to the parietal of a catfish, a frog, or a crow. So, paleontologists long assumed that bony armor in placoderms and bony fish was independently derived.
But that is so 2012! In 2013, Min Zhu and colleagues described a new placoderm - Entelognathus. Remarkably, the armor plates around its mouth corresponded to the bones of the mouth of bony fish. At lease some placoderm and bony fish armor did correspond! Armed with this new search-image, other researchers have identified other corresponding bones in several types of placoderms. This realization set off a cascade of other insights:
- Placoderms and bony fish apparently inherited their bony armor from a common ancestor.
- That implies that cartilaginous fish (sharks, etc.) started out with armor and secondarily lost it.
- An it's that revelation that made us realize the acanthodians like Climatius were all branches from the cartilaginous fish stem.
That's a lot of heavy lifting for a single ugly fish!