lobe-finned fish – Nix Illustration

Allenypterus

Allenypterus montanus was an unusual early coelacanth that lived during the late Carboniferous, around 324 million years ago, in a tropical bay covering what is now central Montana, USA.

Up to about 15cm long (~6″), its tapering tadpole-like body plan somewhat resembled that of modern knifefishes and featherbacks, with the top part of its tail fin highly elongated into a ribbon-like shape and the rest of its tail fins being vestigial. The distinctive humped shape of its back was also much more pronounced in larger, more mature individuals.

It was probably a fairly slow swimmer, and preserved gut contents suggest it mainly ate small soft-bodied prey.

Its closest known relative seems to have been the eel-like Holopterygius – but since around 60 million years and different continents separated them both, this suggests the existence of a whole ghost lineage of other tapering coelacanths yet to be discovered.

Holopterygius

Coelacanths are represented today by just two surviving species, one in East Africa and one in Indonesia, both very similar in appearance and ecology to each other.

For a long time their lineage was thought to be all “living fossils“, retaining the same basic body plan for the last 400 million years – but more recent discoveries have revealed that these fish were actually much more diverse over the course of their evolutionary history.

Holopterygius nudus was a fairly early member of the group, living during the mid-Devonian about 385 million years ago. The only known fossil specimen was discovered in Germany in the 1970s, but it was originally thought to be a different type of fish entirely and wasn’t identified as being a coelacanth until over 30 years later.

And compared to its living relatives it was tiny, just 7cm long (2.75″), with a distinctive tapering eel-like tail. Its convergent close resemblance to modern cusk-eels suggest it may have occupied a similar ecological niche, living near the sea floor and hiding in tight spaces like crevices and burrows.

Rebellatrix

Coelacanths are famous for being “living fossils”, completely disappearing from the fossil record at the end of the Cretaceous but then being rediscovered alive just 80 years ago. But although they’re often thought to have physically changed very little over the last 300 million years or so, more recent discoveries are starting to show that coelacanth body forms and lifestyles were actually more varied in the distant past.

Meet the wonderfully-named Rebellatrix divaricerca, from the Early Triassic of British Columbia, Canada (~251-247 mya). Measuring around 1.3m long (4′3″), its body shape and large symmetrical forked tail suggest it was adapted for fast swimming. Unlike its slow-moving deep-water modern relatives this coelacanth was a speedy oceanic active predator, convergently similar to tuna or some sharks.

Since it lived in the immediate wake of the end-Permian “Great Dying” mass extinction, Rebellatrix may have rapidly evolved from more standard-looking coelacanths to take advantage of a suddenly vacant ecological niche – or it might be part of a more extensive unusual lineage whose other members simply haven’t been discovered yet.