agnatha – Nix Illustration

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.

Eons Roundup 13

I haven’t posted any PBS Eons commissions here for quite a while, so let’s catch up a bit of the backlog:

An illustration of the extinct mammal Repenomamus robustus. It's a low-slung, stocky, badger-like animal with a thick tail, depicted as colored brown and white. It's shown baring a mouth full of pointed teeth. — *Repenomamus robustus*

An illustration of the extinct mammal Repenomamus giganticus. It's a low-slung, stocky, badger-like animal with a thick tail, depicted as colored yellow-brown and dark brown. It's shown walking with its head held down as if stalking something. — *Repenomamus robustus*

The Cretaceous mammals Repenomamus robustus and Repenomamus giganticus, from “When Mammals Only Went Out At Night”
https://www.youtube.com/watch?v=JqZONKXWPfw

An illustration of a decaying carcass of the extinct whale Borealodon. Its skin is tattered and torn as if scavengers have been picking at it, and part of its lower jaw bone is exposed. — *Borealodon osedax* (carcass)

A carcass of the whale Borealodon, from “How Ancient Whales May Have Changed the Deep Ocean”
https://www.youtube.com/watch?v=1vb00-gcdtA

An illustration of the extinct early vertebrate Haikouichthys. It's a small eel-like animal with large eyes, a circular mouth, and seven gill slits. It's depicted as pale blue colored, and it's swimming in dark blue water. — *Haikouichthys ercaicunensis*

An illustration of the extinct early vertebrate Myllokunmingia. It's a small eel-like animal with fairly large eyes, a circular mouth, and six gill slits. It's depicted as pale green colored, and it's swimming in dark blue water. — *Haikouichthys ercaicunensis*

And the early vertebrates Haikouichthys and Myllokunmingia, from “Why Sour May Be The Oldest Taste”
https://www.youtube.com/watch?v=XXgd_cNZSvk

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.

Doryaspis

While Doryaspis nathorsti here looked a bit like a weird prehistoric sawfish, it was actually an ancient jawless fish more closely related to modern lampreys and hagfish.

Measuring just 15cm long (6″), this odd little fish lived in the shallow seas of what is now the Arctic Svalbard archipelago, around 407 million years ago during the early Devonian period when the region was located in much more tropical latitudes.

It was part of a group called the heterostracans, a lineage of jawless fish with heavy armor covering the fronts of their bodies. They had no paired fins and relied solely on their powerful tails for propulsion, and some like Doryaspis also developed large stiff wing-like projections from the sides of their armor that acted like hydrofoils to provide extra lift while swimming.

But the strangest feature of Doryaspis is that pointy serrated saw-like “snout” – which wasn’t actually a snout at all, but instead formed from a part of its jawless mouth roughly equivalent to the lower lip and chin.

It’s unclear what the purpose of this appendage was, but it might have been used for prey detection, probing around the muddy seafloor in a similar manner to sawfish or the big-chinned porpoise Semirostrum.

Tridenaspis

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.

Tridenaspis 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.

Myllokunmingia

Myllokunmingia, from the Early Cambrian of China (~530 mya).

Just under 3cm long (or just over 1″), this tiny animal seems to have been a very close relative of the true vertebrates – almost a vertebrate itself but not quite there yet. A single known fossil specimen shows evidence of a cartilage skull and skeletal elements, five or six gill pouches, a large sail-like dorsal fin, and paired finfolds on its underside.