Late Devonian Amazicthys enters the LRT close to little Millerosteus

Amazicthys trinajsticae (Jobbins et al 2022; Late Devonian) was preserved with a body outline indicating a deep caudal and dorsal fin.

Otherwise
this placoderm was a larger and later version of Middle Devonian Millerosteus in most respects, but with a relatively taller thorax shield. The cheek bones remain unknown and are restored here.

We can no longer have it both ways. If gnathostomes are not extinct, placoderms are not extinct. Gnathostome jaws developed within Placodermi, at Bianchengichthys, to be exact.

“As placoderms have a poorly ossified axial skeleton, preservation of the mainly cartilaginous axial and fin elements is extremely rare, contrary to the heavily mineralized bones of the skull and thoracic armor. Therefore, the gross anatomy of the animals and body shape is only known from a few taxa, and reconstructions of the swimming function and ecology are speculative.

Not always speculative. Enough is known of several taxa to understand swimming function and ecology. Early forms were armored bottom dwellers. Some were giant open sea predators. Others (e.g. Shenacanthus) were tiny predators the size of inch worms and these were the first to develop teeth.

“Here, we describe articulated specimens preserving skull roofs, shoulder girdles, most fins, and body outlines of a newly derived arthrodire.

Two specimens of this new taxon were presented.

“Specimens of the selenosteid Amazichthys trinajsticae gen. et sp. nov. display a skull roof with reticular ornamentation and raised sensory lines like Driscollaspis, a median dorsal plate with a unique sharp posterior depression, the pelvic girdle, the proportions and shape of the pectoral, dorsal, and caudal fins as well as a laterally enlarged region resembling the lateral keel of a few modern sharks and bony fishes.

Driscollaspis has not yet been tested in the LRT.

“Our new phylogenetic analyses support the monophyly of the selenosteid family and place the new genus in a clade with Melanosteus, Enseosteus, Walterosteus, and Draconichthys.

These taxa have not yet been tested in the LRT.

The shape of its body and heterocercal caudal fin in combination with the pronounced “lateral keel” suggest Amazichthys trinajsticae was an active macropelagic swimmer capable of reaching high swimming speeds.”

Indeed.

Here’s a YouTube video
“A Fast Devonian Swimmer from Morocco” featuring lead author Melina Jobbins discussing her team’s find in a spirited discussion.

References
Jobbins M, Rücklin M, Ferrón HG and Klug C 2022. A new selenosteid placoderm from the Late Devonian of the eastern Anti-Atlas (Morocco) with preserved body outline and its ecomorphology. Frontiers in Ecology and Evolution 10:969158.
doi: 10.3389/fevo.2022.969158