Top rare filter-feeding appendage of the anomalocaridid Aegirocassis benmoulai
Aegirocassis is an extinct genus of anomalocarid arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician. It is known by a single species, Aegirocassis benmoulai. A fossil of A. benmoulai from the Fezouata biota, Morocco was discovered by and named after Mohamed Ben Moula, a fossil collector who recognized its rare characteristics and brought it to the notice of a professional paleontologist, Peter Van Roy, at the Ghent University in Belgium.
The conservation of soft parts in the fossil record is somewhat exceptional and only occurs under certain and exclusive taphonomic conditions. For example, anoxic substrates without oxygen, where the organic matter has difficulty decomposing.
The Fezouata Formation occurs in the lower part of the Lower to lower Middle Ordovician Outer Feijas Group, in the western, central and eastern Anti-Atlas
Mountains, southern Morocco (Destombes et al.1985; Gutierrez-Marco & Martin 2016).
It comes from The Konservat-Lagerstätten of Lower Fezouata Fm that spans the entire Tremadoc stage of the Ordovician system and is only present in the Moroccan Anti-Atlas. The discovery of the Fezouata biota in the latest Tremadocian of southeastern Morocco has significantly changed our understanding of the early Phanerozoic radiation. The shelly fossil record shows a well-recognized pattern of macroevolutionary stasis between the Cambrian Explosion and the Great Ordovician Biodiversification Event, but the rich soft-bodied Fezouata biota paints a different evolutionary picture. The Fezouata assemblage includes a considerable component of Cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the Early Ordovician. Study of the Fezouata biota is in its early stages, and future discoveries will continue to enrich our view of the dynamics of the early Phanerozoic radiation and of the nature of the fossil record.
The Fezouata specimens of the anomalocaridid Aegirocassis benmoulai revealed important new morphological information.
It is the first anomalocaridid to show definitive evidence of both dorsal and ventral flaps. Anomalocaridids are potentially critical to understanding the origin of the arthropod biramous limb, sitting, as they do, stemward of the euarthropods but above lobopodians (Budd 1996; Zhang & Briggs 2007; Daley et al. 2009; Kühl et al. 2009; Cong et al. 2014; Vinther et al. 2014; Ortega-Hernández 2015; Van
Roy et al. 2015). Prior to the discovery of A. benmoulai, anomalocaridids had been reconstructed with just one row of flaps along each side of the trunk (e.g. Whittington & Briggs 1985; Daley et al. 2009).
A. benmoulai reveals an additional dorsal set of flaps to which setal blades, which probably functioned as gills, are attached (equivalent to the exite or outer branch of the Cambrian biramous limb: Wolff & Scholtz 2008). Although the flaps of the anomalocaridid trunk are very different from modern arthropod limbs, even inserting at different places on the body wall rather than branching from the same limb base, it is clear that they are equivalent to the two branches typical of living aquatic arthropods. A. benmoulai shows a stage prior to fusion of the limb branches to form the biramous limb (Van Roy et al. 2015).
The ecology of Aegirocassis benmoulai is equally remarkable. The animal, which is preserved near complete in three dimensions in giant concretions and reached a size in excess of 2 m, has the paired anterior appendages characteristic of anomalocaridids. Unlike the head limbs of the great majority of anomalocaridids,
however, which are clearly for grasping large animals, those of A. benmoulai are equipped for filter feeding on small prey in the plankton (Van Roy et al. 2015). The only other anomalocaridid known to be similarly equipped is Tamisiocaris borealis from the lower Cambrian Sirius Passet locality of Greenland (Vinther et al.
2014) but to date it is known only from its anterior appendages. The
filter-feeding appendage of Aegirocassis is considerably more complex and sophisticated than that of Tamisiocaris and may herald a shift in the Early Ordovician to large-scale filter feeding in the oceans, associated with a massive plankton radiation at the start of the Great Ordovician Biodiversification Event (GOBE) (Servais et al. 2010). Aegirocassis benmoulai foreshadows the much later appearance of giant filter-feeding bony fishes, sharks and whales
(Friedman et al. 2010; Marx & Uhen 2010), and provides an early example of the origin of massive filter-feeders within a macro-predatory clade at a time of large-scale diversification in the plankton.
