Exceptionally Preserved Jellyfishes from the Middle Cambrian

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (∼505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period

Photograph and interpretive drawing of Middle Cambrian cnidarian jellyfish in oblique lateral view possibly referable to the order Coronatae, class Scyphozoa.

<p>Specimen UU07021.05, scale bar equals 5 mm. Structure marked <i>Cg</i> in drawing interpreted as a coronal groove; structures marked <i>L</i> interpreted as lappets; structures marked <i>T</i> interpreted as tentacles.</p

Photograph of Middle Cambrian cnidarian jellyfish in oblique subumbrellar view possibly referable to the family Narcomedusae, class Hydrozoa.

<p>Specimen UU07021.04, scale bar equals 5 mm.</p

Stratigraphic column for Utah emphasizing Cambrian strata in the region where the new jellyfishes were collected.

<p>The new fossils come from an interval approximately 41–48 meters above the base of the Marjum Formation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001121#pone.0001121-Briggs1" target="_blank">[26]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001121#pone.0001121-2" target="_blank">[27]</a>; approximate ages of some stratigraphic horizons are shown.</p

Paleogeographic reconstruction for the Middle Cambrian emphasizing the position of North America.

<p>Approximate positions of locality in Utah yielding fossil jellyfishes and the site of the famous Burgess Shale are marked as indicated. Map derived using <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0001121#pone.0001121-Ross1" target="_blank">[41]</a>.</p

Photograph and interpretive drawing of Middle Cambrian cnidarian jellyfish in lateral view possibly referable to the family Narcomedusae, class Hydrozoa.

<p>Specimen UU07021.03, scale bar equals 5 mm.</p