False Ammonite


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Matting color

Another group of hypothetical anomalocarids from the Devonian. The large creature in the center is the False Ammonite (Pseudammonites ptilobrachiones). Its great appendages have evolved into massive “brooms” that sweep plankton from the water as the animal glides along, propelled slowly by its spade-shaped flippers. Much like a hermit crab (and its own distant relative, the Blue Squid-crab) the creature’s soft body is protected inside a discarded ammonite shell. Unlike the squid-crab, however, the False Ammonite’s shell actually floats thanks to a symbiotic siphonophore (Megaera deformibaccata) that inhabits the inner chambers. The striped pink tentacles dangling from the aperture of the shell are Megaera’s own fishing tentacles, while the orange, pear-shaped bodies around the rim are its reproductive zooids. (Meagaera, by the way, is named after the titular monster from the Hammer Horror film, The Gorgon).

Algae readily colonizes the shell and body of the False Ammonite, evident here in the rough brownish-yellow coat and green “toupee”, as well as the green cyanobacterial fuzz on the fin. There're a lot of creatures living in this algae "forest". You can read more about them here.
To the right of the False Ammonite is another hermit anomalocarid, the Sun Drifter (Medusocola silvadorsum). Drifters inhabit the stomachs of jellyfish, where they gain protection and free transportation. Like P. ptilobrachiones, they too are filter-feeders that use their elongated great appendages to snare plankton. If necessary, Drifters can leave their jellyfish hosts and swim freely, as the one on the left is doing.

Most Sun Drifters harbor a small colony of Gardner Crabs (Demeter hortulanus), an arthropod distantly related to the Cambrian “Lace Crab” Marella. Gardner Crabs feed on cyanobacteria that they cultivate on the back of their Drifter. Like a human gardener, they constantly tend and prune their harvest, brushing it clean with their furry antennae and keeping it generally free of other herbivores (although some do get through, as evidenced by the chiton that is feasting in the lower right).

Those strange, vertically-oriented creatures are highly-modified placoderms called Thief Fish (Scutoculus longinares). They hang out under the False Ammonite, using their large, fan-like tail fins to keep themselves vertically-oriented, and steal larger bits of food from the anomalocarid’s brooms. S. longinares have transparent bubbles of hard skin covering their eyes to protect them from the fine, stinging hairs that cover the False Ammonite’s arms and from the siphonophore’s tentacles.

Floating at the surface are a couple of By-the-Wind Sailors (another kind of siphonophore) and a few pelagic barnacles similar to the modern-day Buoy Barnacle Dosima fascicularis.


False Ammonite ©2011 John Meszaros.  All Rights Reserved