This isn't a crushed Oreo, it's a fish

Perhaps in the eyes of many people, fish all look the same, with only some being tasty and some being unpalatable. However, the word "fish" (very inaccurately) includes the group of vertebrates with the largest number of species and the longest history. If the evolutionary history of vertebrates is compared to a big tree, its roots and most of the branches close to the trunk are occupied by "fish", while amphibians, reptiles, birds and mammals are all crowded on an upper branch.

A taxonomic tree of vertebrates, with the animals we call “fish” taking up most of the space | Kei Yamamoto et al. / Development Growth and Regeneration (2017)

The mouth is not necessarily used for eating

Today's story starts with a special type of fish - the ancient jawless Agnatha. "Jawless", as the name suggests, they do not have a hinged jaw, so they cannot chew or bite. The original jawless fish rely on gills to filter food and absorb nutrients. Simply put, water flows into their bodies through the gill slits at the front, and then the round body expands and contracts to discharge the water from the gills on both sides, and the tiny food particles are collected by the gill cavity in the middle. Some modern fish also use gills to filter food.

Lamprey, one of the few jawless fishes that survive today | pixnio

However, as evolution progressed, in the late Middle Ordovician, armored fish Ostracoderms (this classification unit is not formal and includes multiple subclasses) appeared in the jawless superclass, and in the armored fish family, the first species appeared that used gills independently for breathing. They took food with their mouths and then filtered it through the gill sacs. "The mouth is for eating" seems to be a matter of course, but there are exceptions in fish, and eating with the mouth can even be regarded as an evolutionary innovation.

The Field Museum of Natural History in the United States has a collection of many paleontological fossils, among which is a fossil numbered FMNH PF 3895, which is the protagonist of today. It is an armored fish with the scientific name Cardipeltis bryanti, belonging to the subclass Heteropterygii.

Cardipeltis bryanti | James St. John / Wikimedia Commons

The Hardest Armor and the Strongest Spear

Armored fish, as the name suggests, have hard outer armor, and can be said to have spent most of their skill points on defense. Although armored fish are not large in size, their heads are covered with several unbreakable bone plates, just like the armor worn by ancient humans. There are only gill holes, eye sockets, and the pinal opening, a "skylight" that helps the pineal gland to sense light. Their bodies are like chain mail with bone scales connected together, which can not only twist the body to provide driving force, but also bring extraordinary overall defense effect.

Model of Eurypterus | Ryan Somma / Wikimedia Commons

There is an old saying that the only thing that never changes is change itself. The evolutionary pressure brought by armor has caused predators to begin to develop "armor-breaking" attack capabilities. For example, the ferocious-looking Eurypterida, it is speculated that large Eurypteridae can be more than 2 meters long, and the second pair of legs have evolved into giant pincers that can break through the hard armor of armored fish. Predators and prey have evolved together, and the morphology of armored fish has gradually diversified. Some have grown horn-like structures, and some have flat backs, allowing them to lurk on the seabed, like today's flounders, lying flat to avoid predators' searches.

Reconstruction of a genus of armored fish called Anglaspis | Nobu Tamura / Wikimedia Commons

Model of a Pteraspis genus armored fish, also known as finned turtle in Chinese | Ghedo / Wikimedia Commons

The strange and bulky appearance of armored fish gives people a very ancient feeling. In fact, they also have "advanced" technology - some armored fish have evolved paired fins (paired fins). In addition to the tail fin and dorsal fin, they also have a pair of pectoral fins that can propel the body movement, which can improve mobility and facilitate escape.

The rise and fall of a fish dynasty

More than 420 million years ago, in the late Silurian period, another branch of fish evolved from the jawless class, the jawed fish (Gnathostomata), which are fish with jaws. The armored fish finally withdrew from the stage of history at the end of the Devonian period, and only two families of the jawless class, the lampreys and hagfish, have survived to this day. The jawed fish spread throughout the oceans in the Devonian period, and from this family evolved the cartilaginous fish and bony fish that we are familiar with today. One of the bony fish, the Sarcopterygii, eventually made it to land and evolved into a variety of terrestrial vertebrates, including us humans.

Wow! Shuyu zhejianensis looks like an emoji. It belongs to the order Euarchosauridae. The armored fish may be the armored fish closest to the jawed fishes. | Zhikun Gai et al. / Nature (2011)

In the process of biological evolution, there are many seemingly insignificant but low-key creatures that are milestones, and there are also many colorful and popular trendsetters that eventually become yesterday's news. We still cannot fully understand the entire context of the evolution of life on Earth, and there are more mysterious stories waiting for us to dig and explore.