Colours of Wildlife: Ancient Fishes and Fishoids
Created | Updated Apr 7, 2019
Ancient Fishes and Fishoids
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
From those old bison, which as you may remember only very recently disappeared, evolving into new species rather than completely going extinct, we now go way, way, way back in time. Here we see some ancestors of those very bison – not to mention of us and of a very large number of living things today. These fishy thingies you see here are some of the very earliest animals with backbones – vertebrates. Though perhaps not being direct ancestors of modern vertebrates, these ones are very close indeed to what those ancestors must have been like.
First we have something that is barely a fish, if at all. This is little Metaspriggina walcotii. It lived about 508 million years ago in the Cambrian period, close to the point at which complex multicellular life apparently sprung into being, the so-called Cambrian Explosion of around 541 million years ago. Actually, life had been evolving into complex forms for quite some time prior to that point, but the start of the Cambrian somehow marks a point where for some reason or another they suddenly started getting preserved so that we can find them as fossils. This may have had something to do with rock and ocean chemistry of the time. It might be around that point when critters 'learnt' to build hard internal and external skeletons to support their bodies, which also would 'suddenly' have made them more easily preserved as fossils.
Metaspriggina, however, was not one of the ones with the hard skeletons or shells. We know of it because of the shale in which it was found being very fine and having preserved even soft-bodied critters in excellent detail. It was small, reaching only about 10 cm/4" in length. It had a supporting rod called a notochord inside its body, stretching from head to tail. This later became the focus for the development of a backbone. In the case of Metaspriggina the chord was still probably made of cartilage. Cartilage also formed supporting bars for the gills, each gill opening having its own supporting bar or arch. Interestingly, two of the front ones of these bars did not have accompanying gills, and they might soon afterward have been co-opted to support a new structure, the jaws. But Metaspriggina didn't yet have proper jaws. It hardly had a head, in fact. It had a pair of eyes at the front, with nostrils between them, and a small, sucking mouth below them. It likely swam over the ocean floor by side-to-side undulations of its body, sucking in water with its mouth and perhaps using its gills to 'filter' small particles of food from it.
Metaspriggina was around very early in the story of complex, multicellular life. It shows us that our group, the vertebrates, does have quite a lengthy pedigree. From something very much like it, the first true vertebrates – the first fishes – soon evolved. True fish were likely around before the end of the Cambrian, and considerable evolution must have happened in the next period, the Ordovician. But we don't have many good fossils until the period coming after that, the Silurian.
Below Metaspriggina I illustrate a fish from the early Silurian. This is Jamoytius kerwoodi. It already looks more like a fish. It lacked fins on the side of its body, but it had a tail fin as well as fins on its back and belly corresponding to the dorsal and anal fins of modern fish. It had a definite head. It also had 'camera'-style eyes, that is to say where the light entered through a small opening, a pupil, to be focused into an image at the back of the eye. Presumably Jamoytius would have had the beginnings of a brain with which to process this sensory information. It still had no solid skull but it had a skeleton of cartilage. It also still had no jawbones, its sucker-mouth still functioning to suck in small food particles. On its body it had the beginnings of scales, though these were still quite soft. Later fish rapidly 'learnt' to deposit layers of minerals to strengthen these scales. Some also developed bony plates that sometimes entirely encased the body.
The next two illustrated fishies (below Jamoytius) are Cornovichthys blaauweri and Lasanius problematicus both from Scotland. Lasanius lived in the Late Silurian, while Cornovicthys lived in the next period, the Devonian. These were both a bit more evolved than Jamoytius; Cornovichtys was basal to the group to which Lasanius belonged, the Anaspids. The Anaspids were early jawless fish which started to really evolve into nice new forms. They lived mainly in the Silurian period, 443-419 million years ago, and went extinct early in the next period, the Devonian.
The three fish on the right are all Anaspids, of the order Birkeniiformes. They are characterized by having scales in neat rows covering their bodies; they have bony plates around their heads and they also sometimes have bony spikes on their backs or along their bellies or sides. They don't yet have proper pectoral fins along their sides. They likely were still not very maneuverable; they could merely sweep their tails from side to side to drive their bodies forward. The anaspids had a single nostril and also a third or 'pineal' eye between their proper eyes. Still without bony support for their jaws, they sucked or scooped up their food. Pharyngolepis oblongus and Pterygolepis nitidus are both known from the Silurian of Norway, and Birkenia elegans is known from the Silurian in Europe as well as North America (Canada).
The modern critter that is most similar to these very early fishes, is the Lamprey. This is nevertheless today a very specialized species, adapted to using its sucker mouth, lined with razor sharp 'teeth' (similar to the tooth-scales of sharks), to latch onto its prey, from which it sucks blood. But young lampreys still feed by sucking in water and filtering out tiny organisms with their gills. Lampreys resemble these ancient fishes in lacking fins on the sides of their bodies, in having a single nostril, in having a third 'pineal' eye between the real eyes, and in having seven separate individual gill openings on each side of its head. This doesn't mean that the ancient fishies described here were 'ancestral lampreys'; it simply means modern lampreys didn't evolve much beyond these physical features which were actually the original ancestral form for all the vertebrates. So we are all descended from them, lampreys included. But in the things other than the lampreys, lots and lots of new changes and novel bits of anatomy appeared, giving rise to all the other fishes – sharks, chimeras, bony fishes – and the latter soon in turn gave rise to amphibians and through them to all of the land-living backboned animals, as well as those among them who secondarily went back to living in the sea.
