Phylum Arthropoda, Clade Arachnomorpha, Class Megacheira (Megacheira
after Hou & Bergstrom, 1997)
Some alternative taxonomies:
- Phylum Arthropoda, Subphylum Chelicerata, Class
Megacheira, Family Alalcomeneidae
- Phylum Arthropoda, Class Dinocaridida,
Order Radiodonta, Family Opabiniidae
Time: Lower Ordovician
is 20 mm long by 8 mm wide (with limbs) on a 170 mm by 110 mm and
55 mm by 115 mm matrix pair
Upper Fezouata Formation, Zagora, Morocco
The Fezouata Formation contains a diverse fauna from the Lower
and some are thought to be relict examples of the more well-known
Cambrian Explosion of the Buress Shale of Canada and the Chengjiang
Biota of China. This unusual arthropod is known as Alalcomenaeus.
The specimen is a member of the “great appendage arthropods” known
as the opabinids after Opabinia from the Burgess Shale Fauna.
Other members from Chengjiang are Jianfengia and Leanchoilia.
was hitherto unknown outside the Chengjiang Biota. This is a
truly exceptional specimen, showing detail in both part and counterpart,
particularly to the segmentation of the body and the swimming
is now among a growing list of Cambrian Explosion animals found
to persist past the end-Cambrian extinction and
lower Ordovician (often called the great Ordovician radiation).
This, in turn, suggests Alalcolmenaeus was subject to considerable
selective pressure during and after the great Cambrian metazoan
radiation, thereby acquiring adaptive traits for longer survival.
several taxonomies that have appeared over the years.