The Age of Fishes Part 1.

First in a long series.

There’s good reason that the Devonian is called “the age of fishes”, since this is the first major radiation of aquatic vertebrates. Although nearly all of the creatures swimming the waters around Opitz Station are unlike anything you’d see in Present Time.

I should point out that the term “fish” is actually not as scientifically precise as you’d think. Really it’s more of a type term, designating aquatic vertebrates that obtain oxygen by filtering it from the water with gills. It’s kind of like the term “vehicle”, which could refer to a car, bus, airplane, helicopter, boat, tank or really any device that transports people and goods from one place to another.

See, many creatures that we’d call “fish” are only distantly related. For example, a Present Time salmon is actually more closely related to a human being than a hammerhead shark. This is because both humans and salmons are part of the class Osteichthyes—fish with skeletons made of bone (and yes, that does mean that, technically, we Homo sapiens could be considered a highly, highly derived type of “fish”) whereas sharks and their relatives-- skates, rays and chimaeras-- are part of the class Chondrichthyes, which are fish with skeletons made of cartilage.

Yet even sharks and salmon are more closely related to each other than either is to lampreys (Petromyzontiformes) or hagfish (Myxini)—the latter of which are not even considered proper vertebrates since they don’t actually have hardened spinal columns (though they do have spinal nerve cords).

As if that weren’t complicated enough, the Devonian snarls up the concept of “fish” even further with all manner of delightful oddballs such as Thelodonts, Galeaspids, Osteostaracans, Heterostracan Acanthodian “Spiny Sharks”, armor-headed Placoderms and the bizarre, trap-jawed Conodonts.

I’ll have a detailed chart of all these fish-groups in a later post once I talk with Dr. Alonso and tease out all the groups. But for now enjoy a few of of the weirdo Devonian fishes from around our Station.

SPINY SHARK
Climatius reticulatus

Acanthodians or “spiny sharks” (which are only distantly related to true sharks—the name comes from their overall shape) are the most abundant schooling fish around Opitz Station. As their name suggests, the bodies of spiny sharks are studded with up to fifteen stout spines to protect them against predators and to act as stabilizing fins. Spiny sharks are believed to have been the first vertebrates to evolve jaws—which derived from the bony arches supporting its ancestors’ first set of gills.

Climatius reticulates, our most common acanthodian, congregate in massive schools to feast upon plankton. They’re rather bold, curious fish and will frequently swim up to check out a diving researcher. We haven’t yet figured out what the massive eyespot on the gill cover is for, though Dr. Alonso suspects it may be used to distract smaller predators away from the animal’s sensitive eyes.

GREEN ARROWNOSE
Pteraspis queeni

Heterostracans are tadpole-shaped fish covered in a mosaic of large armored plates and smaller pebble-like scales. These provide ample protection from predators such as placoderms, eurypterids, squids, ammonites, nautiloids, the more aggressive trilobites and the dreaded Emperor Death Shrimp-- a critter that even we humans are a little afraid of (more on that in a future post).

In addition to their head armor, heterostracans are also distinguished by the possession of a large gill opening on either side of the body. Most groups can shoot water from these openings to create a kind of jet propulsion.

Heterostracans don’t have any jaw bones. Their mouths are just armored openings that can’t be closed, though they do have muscles that seal off the end of the throat to prevent prey from escaping.

Pteraspis is a genus of heterostracans distinguished by a long snout or rostrum (though a few comparatively snub-nosed species are known) and a large spine on the back that serves as a dorsal fin. All of the species we encountered around Opitz station were open-water swimmers that hunted the huge schools of crustaceans and trilobites hanging out near the ocean surface. Pteraspis' heavy armor coupled with their lack of swim bladders means they are very negatively buoyant. Thus they must swim constantly to stay up in the water column.

Since many Present Time fossils are fragmentary, our researchers frequently have a hard time matching up fossil species with the creatures swiming through our Devonian waters. In such cases, they give the living specimens their own distinct species name. The creature pictured here, for example, is Pteraspis queeni, the Green Arrownose. Dr. Alonso named it. Apparently its emerald coloring reminds him of some sort of super Robin-Hood character from a comic he reads.

FRYING-PAN FISH
Drepanaspis gemuendensis

So nicknamed due to its flattened body, this heterostracan hangs out on the sea bottom camouflaged amongst the rocks and debris. It uses its upturned eyes to spy out small invertebrates swimming just above the ocean floor, which it slurps up with a quick lunge of its equally upturned mouth.

At Opitz station, the Drepanaspis are highly attracted to the overhead lights on the docks. Well, more accurately, they’re attracted to the abundant swarms of trilobites, ostracods and other arthropods that gather under the lights.

As you may have noticed, our Expedition Seal is a Drepanaspis. Specifically, this is our team pet, “Salty”, who Marisa and Healani rescued from a tide pool in our first few days at the Station.

Salty was more than half-dead when they found him , having suffered through several hours of excruciating heat, depleted oxygen levels and high salinity (hence the name) in the shallow, sun-exposed, rapidly-evaporating pool. Luckily they were able to get him to the Station in time and nurse him back to health in one of our sea-tables. He recovered quite well, though we think his brain got a little fried since he developed a weird habit of hanging out upside-down at the surface of his tank (not an easy task, mind you, since Drepanaspis are negatively-buoyant and bottom-heavy). Oddly, Salty was avocado-green instead of the usual mottled-brown of his fellow frying-pans. We haven’t been able to figure out if this was just a random mutation or an effect of his experience in the tide pool.