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

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

-like

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

. 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, 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.e. ...

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 Proarti ...

''. It is from the Late

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and t ...

deposit of

Ediacara Hills Ediacara 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. The hills are known for being the location of significant fossils, and have given their nam ...

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

unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (a ...

becoming progressively larger as they approach 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 ...

-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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

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

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is est ...

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 has usually referred to a pictorial signature of a deity or spirit. In modern usage, especially in the context of chaos magic, sigil refers to a symbolic representation of the practitioner ...

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 bilaterian 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 fo ...

, 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 qui ...

' 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. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest o ...

and do not share common characteristics with any Praecambridium fossil specimens.

Description

''P. sigillum'' represents a 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

Trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s or

Chelicerate The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

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

Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal that lived during the late Ediacaran period in what is now Australia, China, Russia and Ukraine. The individual ''Dickinsonia'' typically resembles a bilaterally symmetrical ribbed oval. Its a ...

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

Arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s and

Annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

s. The sculpture of the head shield of ''Praecambridium siggilum'' has been proposed as being the expression of a underlying digestive/distribution system which is also seen in Agnostid Trilobites.

References

Cephalozoa Sprigginidae Yorgiidae Prehistoric invertebrates of Australia Prehistoric bilaterian genera {{Precambrian-animal-stub

Etymology

Classification and interpretations

Description

See also

References