The only fin Sacabambaspis had on its body was the caudal fin, or tail fin. Its relative Astraspis didn't even have that. Scientists used to think that Sacabambaspis had a shark-like tail, but now they know that it had a tail similar to modern jawless fish, a sort of eel-like tail.
The most striking feature about Sacabambaspis was the shell that covered its head. The armor that covered placoderm heads was more like a suit of armor. But Sacabambaspis had armor that was more like a clamshell. Placoderms had different plates of armor all over their head, which allowed different parts of their heads to move. Ostracoderms, armored agnathans, or jawless fish, had shell-like armor, which was usually two plates, one of the top of the head and one on the bottom. The shell on Drepanaspis, from the Devonian, gradually got smaller as Drepanaspis evolved, to form individual plates on the head, more like a placoderm, except it still had no jaws.
Sacabambaspis had both of its eyes facing forward, which meant that it had 3D vision. Most other jawless fish did not have this feature.
Sacabambaspis on a reef with a trilobite, an orthocone, a crinoid,
and rugose corals encrusted with bryozoans.
Sacabambaspis lived on reefs that were home to creatures such as trilobites, crinoids, orthocones, rugose coral, eurypterids, and bryozoans. At the time, any fish on a Bolivian reef almost had to be Sacabambaspis. There are no other known fish from the same place and time, in Bolivia 460 million years ago, when Sacabambaspis was found.
Sacabambaspis, accompanied by rugose corals and bryozoan-encrusted rocks,
viewed from above.
The arandaspid family includes Astraspis, Arandaspis, and Sacabambaspis. They were all from the Ordovician. Astraspis was from North America, Arandaspis was from Australia, and Sacabambaspis was from South America.
The arandaspid family: (from top) Astraspis, Arandaspis, Sacabambaspis
Arandaspis was more flattened on the sides than other arandaspids, and it was probably unstable and tilty. It had a rigid shell and was about four or five inches long.
Astraspis was also about four or five inches long. It had no fins, not even a caudal fin, and a very bumpy shell. I hypothesize that with no caudal fin, it probably mostly slithered across the bottom, and from time to time it could have squirmed around and swam up into midwater to feed on plankton. It could then drift down again to feed on algae and scraps of leftover carcass, which is what it mostly ate. Astraspis is the oldest known North American vertebrate.
The tail of the Ordovician fish Sacabambaspis
Super Little Giant Book of Prehistoric Creatures