Name:
Aegirocassis
(Aegir’s helmet).
Phonetic: Ae-ge-r-o-cas-sis.
Named By: Peter Van Roy, Allison C. Daley
& Derek E. G. Briggs - 2015.
Classification: Arthropoda, Dinocaridida,
Radiodonta, Anomalocarida, Hurdiidae.
Species: A. benmoulae
(type).
Diet: Filter feeder.
Size: Largest specimens indicate that Aegirocassis
grew at least up to 2.1 meters long.
Known locations: Morocco.
Time period: Ordovician.
Fossil representation: Several specimens, some
three dimensionally preserved.
Out
of all the Cambrian era arthropods so far named, Aegirocassis
has
certainly been one of the most remarkable. Obviously the thing that
most people notice about Aegirocassis is the huge
size, at just over
two meters in length for the largest recorded specimen, Aegirocassis
was in 2015 the largest anomalocaridid
so far named, as well as
amongst the largest arthropods ever discovered.
Far
more important than the size however, the three dimensional
preservation of some of the Aegirocassis specimens
has proven that
Aegirocassis had two sets of swimming flaps, and
not one as had been
previously assumed for other anomalocaridids. Before, it had been
assumed that anomalocaridids had ‘lost’ their lower sets in favour
of one for easier swimming. Aegirocassis however
has proven that the
lower set, which originally would have been legs (as in
lobopods), were simply adapted into additional swimming flaps. In
addition to this comparisons to specimens of earlier named genera such
as Peytoia
and Hurdia
seems to indicate that these other genera also
had two sets of flaps and not just one as had been previously assumed.
This two branched limb arrangement is something that is seen in modern
arthropod types such as shrimps.
The
authors of the 2015 description of Aegirocassis
also noted ribbon
like structures believed to have been gills. Sometimes seen in other
anomalocaridids, these have in the past assumed to have been fixed to
the back, but the authors of Aegirocassis noted
that these possible
gills attached to the upper row of flaps.
Aegirocassis
was one of the later anomalocaridids living in the Ordovician period
whereas most other currently known anomalocaridids seem to have been
Cambrian. Earlier anomalocaridids are generally still perceived as
predators (though some detritivore theories exist). Aegirocassis
however is noted as having frontal spines with a very fine mesh of
further spine-like appendages that could have filtered planktonic
organisms from the water. This likely makes Aegirocassis
a planktonic
filter feeder, not an active predator. This may also in part
explain how Aegirocassis could grow so big, as
not only is plankton
abundant and nutritious, it requires very little energy expenditure
to catch, meaning more calories can be diverted to growth and
maintenance of large physical size.
The
idea of Aegirocassis as a filter feeding
anomalocaridid also goes to
show that nature has a habit of repeating itself. Anomalocaridids
almost certainly started out as carnivores, with some then growing to
truly gigantic size when making the switch to filter feeding. Fish
too mostly started out as predators, but some like Leedsichthys
which
lived much later during the Jurassic attained massive sizes simply by
filter feeding. In today’s oceans two of the largest sharks, the
whale shark (Rhincodon typus) and basking shark
(Cetorhinus
maximus) are both filter feeders. Whales too began their
early
evolutionary history as predators, but today the largest whales alive
are all filter feeders. What Aegirocassis
shows us is that this
pattern of growing giant by switching to filter feeding is one that
goes back at least as far as the Ordovician period and with
arthropods. Earlier in 2010 another anomalocaridid named
Tamisiocaris
was described, and this genus is also now speculated
to have been a filter feeder.
The
name Aegirocassis means ‘Aegir's helmet'. In
Norse mythology
Aegir was a giant who lived in the sea, and was chosen for the name
in reference to the large size of Aegirocassis.
The helmet part is a
reference to the size and structure of the head carapace of
Aegirocassis. The type species name is in honour
of Mohamed Ben Moula
who discovered the first specimens.
Further reading
- Anomalocaridid trunk limb homology revealed by a giant
filter-feeder with paired flaps. - Nature. - Peter Van Roy,
Allison C. Daley & Derek E. G. Briggs - 2015.