Dramatic Ordovician Alalcolmenaeus Opabinid Arthropod


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

Geological Time: Lower Ordovician

Size: Fossil 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

Fossil Site: Upper Fezouata Formation, Zagora, Morocco

Code: 16126

Price: $795.00

Alalcolmenaeus FossilDescription: The Fezouata Formation contains a diverse fauna from the Lower Ordovician, 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. The taxon 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 appendages.

Alalcolmenaeus is now among a growing list of Cambrian Explosion animals found to persist past the end-Cambrian extinction and into the 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. We list several taxonomies that have appeared over the years.

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