''Praecambridium sigillum'' is an extinct organism that superficially resembles a segmented

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

-like

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

. It was originally described as being a trilobite-like arthropod, though the majority of experts now place it within the

Proarticulata Proarticulata is a proposed phylum of extinct, near-bilaterally symmetrical animals known from fossils found in the Ediacaran (Vendian) marine deposits, and dates to approximately . The name comes from the Greek () = "before" and Articulata, i. ...

as a close relative of the much larger ''

Yorgia ''Yorgia waggoneri'' is a discoid Ediacaran organism. It has a low, segmented body consisting of a short wide "head", no appendages, and a long body region, reaching a maximum length of . It is classified within the extinct animal phylum Proart ...

''. It is from the Late

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

deposit of

Ediacara Hills Ediacara Hills ( ), also known as Ediacaran Hills, are a range of low hills in the northern part of the Flinders Ranges of South Australia, around north of the state capital of Adelaide city centre, Adelaide. They are within the Nilpena Ediaca ...

, Australia, about 555 million years ago. On average, ''P. sigillum'' had at least 5 pairs of segments, with each

unit Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...

becoming progressively larger as they approach the

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

-like head.

Etymology

The generic name is a compound word, with the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

prefix ''prae'' "before" and a reference to the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

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 '' Cambridium''. The specific name is from Latin ''sigillum'' "a

sigil A sigil () is a type of symbol used in magic. The term usually refers to a pictorial signature of a spirit (such as an angel, demon, or deity). In modern usage, especially in the context of chaos magic, a sigil refers to a symbolic represen ...

Classification and interpretations

Originally, Runnegar and M.A. Fedokin (1992) suggested that ''P. sigillum'' should become a synonym of ''

Spriggina ''Spriggina'' is a genus of early animals whose relationship to living animals is unclear. Fossils of ''Spriggina'' are known from the late Ediacaran period in what is now South Australia. ''Spriggina floundersi'' is the official fossil emblem o ...

'', 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 ''Marywadea'' is a genus of Ediacaran biota shaped like an oval with a central ridge. It is a bilaterian organism as evidenced by its symmetry, vaguely resembling a very primitive trilobite. The fossil has an asymmetrical first chamber of the qu ...

'' and he pointed out the fact that all of the juvenile forms of these genera are only found in

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

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

s or

chelicerate The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks ...

s or possibly a close relative of both. Because the presumed intestinal caeca are present in some ''

Dickinsonia ''Dickinsonia'' is a genus of extinct organism that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It had a round, bilaterally symmetric body with multiple segments running along it. It could range f ...

'' specimens, it has also been suggested that ''Pracambridium'' also had them internally. ''P. sigillum'' 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

s and

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

s. 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 agnostid trilobites.

References

{{Taxonbar, from1=Q1448828 Cephalozoa Sprigginidae Yorgiidae Prehistoric invertebrates of Australia Prehistoric bilaterian genera

Etymology

Classification and interpretations

Description

See also

References