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.
Continue reading “Cambrian Explosion Month #16: Phylum Chordata – Vertebrata”
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.
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, 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.