Name: Orsten
Location: Sweden
Age: Upper Cambrian
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Fauna and Flora

The first small arthropods from the Orsten were found in 1975 by Müller and collegues (Tang, 2001). Müller discovered a rich fauna of tiny phosphatised arthropods including agnostids, trilobites and early crustaceans. These were preserved in fine detail in 3D and the preservation only occurred in fossil specimens that were less than 2 mm in length. The palaeontologists described dozens of genera and species, almost all of them new, and the Orsten sites have yielded the oldest well-documented examples of meiofauna in the fossil record.

The exceptional preservation means that not only the morphology of different groups of arthropods can be described, but also assumptions can be made about functional morphology and life habits. The detailed preservation allows comparisons of Orsten animals with extant arthropod orders. Most fossils represent early and previously unknown arthropod groups and the earliest known tardigrades and pentastomids, which are extant groups but were previously unknown in the fossil record (Tang, 2001).

The bodies and appendages indicate that the majority of the fauna were active swimmers. The flocculent bottom layer, which is the fine-grained unconsolidated layer above the seafloor, may have been a suitable habitat with a high content of nutrients and low currents (Tang, 2001).

In 1990, Walossek and Müller found three new arthropods, Martinssonia, Goticans, and Cambrypachycepe, in an uncompressed condition. These were recognised as descendants of early offshoots from the stem-lineage of Crustacea. Their morphology provided new insights into the evolutionary path and progressive development of ground plan characteristics along the stem-lineage and gives further support for the monophyletic origin of Crustacea (Müller, 1990).

Martinssonia was found in juvenile and adult forms. It is thought it was a bottom-dweller and fed on particles that it stirred up from the seafloor with its limbs and tail. This organism resembles a crustacean in some ways, but differs in others.

Skara, Bredocaris, Dala, and Rehbachiella are thought to be direct ancestors of modern crustacean lineages. Skara was a sweep-net feeder with a soft head shield. Out of 100 specimens of two species of Skara, only adult stages were represented (Müller, 1990). Dala had special bristles that it used as sensory organs and for effecient swimming, which the organism probably did upside down. Bredocaris admirabilis was most likely a suspension feeder.

The trilobite Agnostus pisiformis is represented by different growth stages. Calcareous exoskeletons are abundant, but phosphatised specimens are rare (Müller, 1990). This animal had one pair of antennae, three pairs of head limbs and five pairs of trunk limbs. It had head and tail shields which may have closed to form a protective shell from predators or adverse physical conditions (Tang, 2001).

The oldest members of Pentastomida, which is a worm-like internal parasite that inhabits most modern terrestrial vertebrates, were found at these sites. Studies also show conodonts and arthropods were suprisingly diverse, complex and advanced for this time period (Tang, 2001).

To find out more and for pictures of the fauna see Dieter Walossek's homepage.

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