''Squamacula'' is an extinct
artiopodan
arthropod from the
Cambrian Series 2
Cambrian Series 2 is the unnamed 2nd series of the Cambrian. It lies above the Terreneuvian series and below the Miaolingian. Series 2 has not been formally defined by the International Commission on Stratigraphy, lacking a precise lower boundary ...
. The type species ''S. clypeata'' was described in 1997 from the
Chengjiang biota
The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
. At the time of description there were only two known specimens of ''S. clypeata'', but now there are at least six known specimens. In 2012 a second species ''S. buckorum'' was described from the
Emu Bay Shale of Australia.
Etymology
The
genus ''Squamacula'' is derived from the Latin word ''squama'', meaning scale, and the
diminutive suffix -''culus'', indicating that the animal is relatively small. The
species ''clypeata'' is derived from the Latin word ''clypeatus'', meaning shield-shaped. It was named this in reference to its shield-like outline.
Description
''Squamacula clypeata'' is flattened (dorsoventrally). It has 11 segments in total: the
cephalon
Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...
(the head), nine thoracic
tergites (each of which covers a
somite), and one
pygidium.
It has a
doublure, a piece of exoskeleton that covers part of the underside of the animal. Its doublure is, on average, about twice as long as the length of its cephalon. It has been hypothesized that this large doublure could have aided the animal in digging through sediment to find food, as it was thought to have been both a predator of small animals and a scavenger.
''Squamacula clypeata'' has one pair of antennae attached to its cephalon, but no other appendages or auditory or visual structures.
No mouth is present in the specimens, but the gut is present, so ''S. clypeata'' must have had a mouth, which is thought to have been between the cephalon and doublure on the underside.
''Squamacula clypeata'' has one pair of
biramous appendages (an appendage which branches into two) per thoracic tergite.
The
endopod (inside branch) has seven segments, the first six of which are roughly even in size and shape, with small spines on the underside, and the last of which terminates with a claw and appears to have been used for walking and eating, as the claw could be useful in grabbing hold of and tearing up food.
The exopod (outside branch) was not segmented, but flap-like, with
setae (bristles) near the tip, which may have been used for both locomotion and respiration.
Taxonomy
In phylogenetic analyses, ''Squamacula'' has consistently been placed as the earliest diverging and most primitive member of
Artiopoda, a clade that includes
trilobites and a variety of morphologically similar soft-bodied forms.
References
{{Taxonbar, from=Q85802772
Artiopoda