jawless fish – Nix Illustration

Bataspis

Bataspis crux was an armored jawless fish that lived during the early Devonian, around 419 million years ago, in coastal marine waters covering what is now southwestern China.

It was a member of the galeaspids, a group characterized by massive headshields covered in a pattern of sensory canals, with a large opening on the front top surface between the eyes. While this hole often looked like a comical mouth, it was actually a single nostril that took in water for the gills, while the real mouth was located on the animal’s underside.

Galeaspids also had scaly bodies with three dorsal fins, a tail fin, and long low ribbon-like fins along the sides that may have been the precursors to paired fins in jawed fish.

Bataspis’ headshield was unusually shaped compared to its relatives, with sweeping side projections forming a distinct “batwing”. Hydrodynamic simulations indicate that it functioned as a hydrofoil, generating lift while swimming and making Bataspis an efficient underwater glider — and suggesting that galeaspids were much more ecologically diverse than previously thought.

Haikouichthys

Haikouichthys ercaicunensis was one of the earliest known vertebrates, living in what is now southwestern China during the Cambrian, around 518 million years ago.

About 2.5cm long (~1″), it had a streamlined body with a long fin along its back and tail. It lacked limbs and jaws, but it did have a flexible “upper lip” that could potentially have been used to manipulate food particles, either engulfing them on the seafloor or stirring them up into the water for filter-feeding.

And a recent study revealed a big surprise — it had four eyes! Along with its already-known large sideways-facing pair of eyes, it also had another smaller upwards-facing pair in the middle of its face, giving it a much wider field of vision and the ability to watch out for predators from multiple angles at once.

The extra pair of eyes may be the evolutionary precursors to the pineal complex in later vertebrates — represented in modern forms by the pineal gland and the parietal eye.

Tauraspis

Osteostracans were an ancient group of jawless fish, closely related to early jawed vertebrates, whose fossils are known from the mid-Silurian to the late Devonian of what is now North America, Europe, and Asia.

They were heavily armored, with bony head shields and rows of large scales covering their bodies. While their flattened shapes and upward-facing eyes have resulted in them traditionally being interpreted as mud-grubbing bottom-dwellers, their paddle-shaped pectoral fins, dorsal fins, and strong tails indicate they were also quite good swimmers – and their diverse hydrodynamic head shield shapes suggest they probably had a much wider range of ecologies than previously thought.

Although many osteostracans had large flaring spines on the sides of their heads, or long snout-like spikes at the front, Tauraspis rara here was unique in having two long front-facing horn-like projections.

Around 7.5cm long (~3″), it lived in brackish and freshwater environments in what is now northern Siberia during the early Devonian, about 410-407 million years ago. Like other osteostracans it had a small keyhole-shaped “nostril” opening, and large patches of sensory organs known as “cephalic fields” on the sides and top of its head shield.

The fields were covered with a mosaic of small bony plates, and their exact function is still a mystery – but they may have been involved in sensing vibrations in the water, or possibly even been electric organs.

Similarly, what Tauraspis used its unusual pair of “horns” for is also unknown.

Cambrian Explosion Month #16: Phylum Chordata – Vertebrata

Vertebrates are by far the most numerous and diverse group of chordates today, with over 65,000 known species including fish, amphibians, reptiles, birds, and mammals. Genetic studies show that they’re closely related to the weird bag-like tunicates, and their shared common ancestor was probably something lancelet-like.

And the earliest true vertebrates would have looked something like Haikouichthys ercaicunensis.

Tridensaspis

Although the only surviving agnathans in modern times are the lampreys and hagfish, back in the early-to-mid-Paleozoic these “jawless fish” were much more diverse. Many of them were heavily armored with large bony head shields – a feature eventually inherited by early jawed fish like the placoderms – which protected their heads, gills, and some of their internal organs.

And some of the oddest-looking of these armored agnathans was a lineage known as the galeaspids.

Known from southern China, Tibet, and Vietnam, these small fish were bottom-dwellers living in the shallow waters of lagoons and river deltas. Their most distinctive feature was a single large opening on the upper side of their head shields – and despite looking like a particularly goofy mouth this hole was actually a nostril, used for both a sense of smell and as a water intake for their gills. The actual mouth and the gill openings were on the underside of the head.

While early galeaspids had rounded head shields, later forms developed some more unusual shapes, with long spines sticking out to each side and pointed or spatula-shaped snouts.

Tridensaspis magnoculus here lived during the early Devonian in Southwest China, about 407-393 million years ago, and was only about 5cm long (2″). It wasn’t the most extremely pointy of its kind, but still had a weird kite-shaped head shield, a long vertical slit-shaped nostril opening, and rather large upwards-facing eyes.