Sanctacaris
Name:
Sanctacaris
(Saintly shrimp).
Phonetic: San-tah-ka-riss.
Named By: Briggs & Collins - 1988.
Classification: Arthropoda, Chelicerata.
Species: S. uncata (type).
Diet: Carnivore.
Size: Individuals measure between 46 and 93
millimetres long.
Known locations: Canada, British Columbia -
Burgess Shale.
Time period: Drumian of the middle Cambrian.
Fossil representation: At least 5 individual
specimens.
Given the informal name of ‘Santa Claws’, this arthropod was formerly named as Sanctacaris in 1988 which translates as ‘saintly shrimp’ (alternatively sometimes called ‘saintly crab’). Exact classification of Sanctacaris however has been hard to establish as opinions differ amongst researchers. Although usually attributed as a member of the Chelicerata, Sanctacaris has also been included with the arachnates, a specific group within the Chelicerata. Either way Sanctacaris is a representative of the group of arthropods that would go on to produce arachnids such as scorpions and spiders.
As
with the other creatures from the Burgess Shale, Sanctacaris
was an
aquatic creature that lived in the Cambrian era oceans (life would
not progress onto land for many millions of years to come). The
body of Sanctacaris can be dived into three parts
of head, trunk and
telson (for lack of a better word the tail). The head was mainly
a single concave (curved outwards) plate, while the trunk was
made up of eleven segmented sections that allowed the hard shelled body
to flex and move. The telson at the end of the trunk was short but
broad to provide for a large surface area. Sanctacaris
is thought to
have swum by moving a pair of appendages that were connected to the
trunk and ended in paddle shaped ends for locomotion, while the
telson steered like a rudder on a boat, something that was made all
the more efficient by the wide surface area.
On
the underside of the head were a further six appendages and combined
with the two from the trunk this produced a total of eight (part of
the reason for the arachnid lineage connection). Whereas the
appendages on the trunk ended with paddles however, the appendages
upon the underside of the head possess sensory antennae and have
raptorial ends for grasping things. These are clear signs of a
predatory adaptation and with the inclusion of two eyes that faced
forward, palaeontologists are comfortable describing Sanctacaris
as a
predator.
As
far as hunting behaviour goes, Sanctacaris
probably swam through the
water looking for smaller prey with its eyes. Once located
Sanctacaris would most likely swim into a position
where its head was
above is prey and then crash down on top of it. With the eyes facing
forwards however, Sanctacaris would not have been
able to rely upon
its eyes while feeding and it is here where the antennae on the forward
six legs would have helped it to feel around its prey as it pulled it
apart piece by piece so that it could fit its prey into its mouth.
Further reading
- A Middle Cambrian chelicerate from Mount Stephen, British Columbia. -
Palaeontology 31 (3): 779–798 - Derek E. G. Briggs & Desmond
Collins - 1988.
- A critical reappraisal of the fossil record of the bilaterian phyla.
Biological Reviews of the Cambridge Philosophical Society 75 (2):
253–95. - G. E. Budd & S. Jensen - 2000.
- Sanctacaris uncata: the oldest chelicerate (Arthropoda). - Die
Naturwissenschaften. 101 (12): 1065–73. - D. A. Legg - 2014.
- Mandibulate convergence in an armoured Cambrian stem chelicerate. -
BMC Evolutionary Biology. 17 (1): 261. - C. Aria & J. B. Caron
- 2017.
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