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386 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 14.8 Exceptional arthropod-dominated faunas

Arthropods are common in a number of Lagerstätten deposits, suggesting that they were much more
diverse in the past than the regular fossil record suggests. More than 40% of the animals described
from the Mid Cambrian Burgess Shale are arthropods. Apart from typical trilobites such as Olenoi-
des there are also soft-bodied taxa, for example Naraoia and the larger Tegopelte. However the

commonest and first discovered Burgess arthropod is the elegant, trilobitomorph Marrella. The fauna
contains many other arthropods such as Canadaspis, probably the fi rst phyllocariid crustacean. There
are many unique arthropods in the fauna that are diffi cult to classify: Anomalocaris, Emeraldella,
Leanchoilia, Odaraia, Sidneyia and Yohoia are not easily aligned with established groups. The small
and bizarre Hallucigenia was probably an onychophoran, while Sanctacaris was a stem-group che-
licerate. The slightly older faunas at Chengjiang, South China, and Sirius Passet, North Greenland,
have also yielded a spectacular array of enigmatic arthropod faunas, further contributing to our
knowledge of the Cambrian explosion.
Calcareous concretions (or orsten) from the Upper Cambrian of the Baltic area have yielded a
phosphatized fauna dominated by stem- and crown-group crustaceans and ostracodes together with
agnostid trilobites. Many of these diverse forms were minute, living in microhabitats within or on
the muds of the Cambrian seas (Fig. 14.24). These faunas are quite distinct from the earlier Burgess
Shale-type faunas and provide a window on a habitat occupied by a wide range of body plans on
a microscopic scale, possibly adapted to life below the sediment–water interface. Recent work by
Dieter Walossek (Ulm Universität) on, for example, remarkably preserved complete ontogenetic series
of Rehbachiella from orsten has helped elucidate the life cycle, habits and functional morphology
of these animals. Moreover some of the most remarkable of all the arthropods, the pycnogonids, or
sea spiders, are now known from the Cambrian orsten banks, the Silurian Herefordshire fauna and
the Devonian Hunsrückschiefer (Budd & Telford 2005).
The Early Devonian faunas of the Hunsrückschiefer of the German Rhineland contain beautifully
preserved phyllocariid crustaceans such as Nahecaris, together with a number of other arthropods
apparently lacking living counterparts such as Cheloniellon (a large, ovoid creature with a pair of
antennae, nine segments and conical telson) or Mimetaster, which is similar to Marrella from the
Burgess Shale.

B C d

c D
a+A
b

12
Anoxic zone

Figure 14.24 Composite of Mid Cambrian and Late Cambrian forms and reconstructions. Lower
case letters (a–d), larvae; upper case letters (A–D), adult stages. Distance of sinking into the zone
of preservation: 1, short distance; 2, long distance. (Redrawn from Walossek, D. 1993. Fossils
and Strata 32.)