Praecambridium sigillum is an extinct organism that superficially resembles a segmented trilobite-like arthropod.
Etymology
The generic name is a compound word, with the
Latin prefix
prae "before" and a reference to the
Cambrian mollusc genus
Cambridium, in reference to how the appearance of the various segments are reminiscent of the muscle-scars on the inner surface of the shells of
Stenothecoida.
The specific name is from Latin
sigillum "a sigil".
Classification and interpretations
Originally, Runnegar and M.A. Fedokin (1992) suggested that
P. sigillum should become a synonym of
Spriggina, while others have stated that the animal shares more in common with
Dickinsonia brachina. Gehling (2006) noted that there is 'no evidence that
Praecambridium is a juvenile
Dickinsonia,
Spriggina or
Marywadea and he pointed out the fact that all of the juvenile forms of these genera are only found in
South Australia and do not share common characteristics with any
Praecambridium fossil specimens.
Description
Praecambridium sigillum represents an organism which possessed 3-6 arcuate segment-like structures that have also been interpreted as appendages.
The appearance of structures which bare a resemblance to segments has led to a possible affinity -or being a close relative of
or
or possibly a close relative of both. Because the presumed intestinal caeca are present in some
Dickinsonia specimens, it has also been suggested that
Pracambridium also had them internally.
possessed a head shield that paired a branching pair of ribs with the ribs of the organism tapering towards their end and as they get further from the head shield, they point towards the end of the animal;
with Glaessner and Wade concluding that the segments (or lobes) increased in size during growth of
Praecambridium.
The ribs on either side of the body coalesce along a close midline. However, contradicting the comparison of
Praecambridium between the Trilobites and Chelicerates, it may be that
Praecambridium wasn't a truly segmented animal albeit was still compared in morphology to
and
.
The sculpture of the head shield of Praecambridium siggilum has been proposed as being the expression of an underlying digestive/distribution system which is also seen in Agnostida.
See also
