Location: British Columbia
Age: Middle Cambrian (505 Ma)
The Burgess Shale displays a remarkable range of modern animal phyla (see Table 1), including forms which are not so easily classifiable into known groups.
|UNKNOWN ('WEIRD WONDERS')||
Table 1. The animal phyla and their numbers of genera represented in the Burgess Shale. The phyla Porifera has cellular grade organization. Cnidaria and Ctenophora have two-layered tissues (diploblastic tissue grade organization), and the rest of the phyla have three-layered tissues (triploblastic organ grade organization). 'P' and 'D' stand for 'protostome' and 'deuterostome', respectively, in reference to the major monophyletic clades which include all triploblastic phyla. 'L' and 'E' stand for 'Lophotrochozoa' and 'Ecdysozoa', respectively, which are the two large groups which divide the protostomes.
About one in six genera in the Burgess Shale are 'weird wonders' and cannot be easily identified as members of extant phyla. About one in three are arthropods; this contrasts present environments similar to the Burgess Shale, in which worms are the dominating taxon.
The following plates (courtesy of Conway Morris, S. The Crucible of Creation (1998)) attempt to recreate the assumed life positions and daily activities of various organisms as they were before a massive landslide buried them. We discuss their phylogenetic affinities and likely modes of life below:
Plate 1. The large looming animal in the foreground is Anomalocaris, which may have reached lengths approaching 50 centimeters or more. It was the largest predator in Cambrian seas, and has been tentatively classified as a stem-group arthropod (an early ofshoot of the main diverse line, or crown group, of the arthropods, though other workers maintain that it cannot be placed in any extant animal phylum. It is shown grasping the trilobite Naraoia with its frontal appendages. On the bottom left of the plate is the 3 centimeter long Wiwaxia, which has been interpreted as a mollusc or annelid worm, though others suggest it is another 'weird wonder' with completely unknown biological affinity. The spiny purple worm-like creatures are Hallucigenia, now placed in the Phylum Onycophora (the 'velvet worms'), of which the lobopodian Peripatus is a modern example. One researcher misinterpreted Hallucigenia and concluded that its likely life position was spiny-side down. The white worm is Aysheaia, also an onychophoran, which reached lengths of between 1-6 centimeters. The green five-eyed animal with the protuding proboscis is the 4-7 centimeter long Opabinia, classified as either a stem-group arthropod or a 'weird wonder'. The buttercup creatures are Dinomischus, which are rare filter feeders. Though they have been classified by some workers as lophophorates ( a grouping of organisms which possess a feeding structure called a lophophore), it is likely that they are yet another example of the Burgess Shale's 'weird wonders'. The bluish attached forms are Vauxia, a sponge, and the swimming forms in the background are Marella, a deposit feeding arthropod and the most abundant fossil in the Burgess Shale (over 15,000 specimens found).
Plate 2. Vauxia, Aysheaia (crawling and presumably grazing on a Vauxia), Dinomischus (center background), and some trilobites are shown here. The large leaf-shaped organisms on the left are Thaumaptilon, a sea pen of the Phylum Cnidaria. Some representatives of the suspension feeding Phylum Porifera (the sponges) include Pirania (the brown stalked form with spiny projections) and Choia (the spiny 2-3 centimeter long brown 'blobs' on the seafloor). Chancelloria, a possible sponge, is the tall purple cylindrical form in the background. The green 'cactus-like' form attached to the substrate is Mackenzia, a possible anthozoan (a relative of modern sea anemones) of the Phylum Cnidaria. Mackenzia was originally identified as a sea cucumber (Phylum Echinodermata).
Plate 3. A more detailed reconstruction of the sponge Pirania is shown in the right foreground. Note its pronounced spiny projections, spicules which are part of its skeleton. The steady beat of flagellated collar cells drives nutrient-rich water into its body cavity via a surface network of pores and out through the large upper hole known as the osculum. The small objects affixed to the tips of some of the spicules are actually symbiotic brachiopods of the genus Micromitra. The purple bristled worm-like form is Canadia, a 2-5 centimeter long polychaete annelid. The reddish swimming form on the right is Nectocaris, an organism of unknown affinity. Several specimens of Dinomischus dot the seafloor, while the jellyfish-like gelatinous possible echinoderm (or cnidarian) Eldonia navigates the water column. The large shelled swimming form with the characteristic three-pronged 'tail fin' is the arthropod Odaraia, which can grow to a length of 15 centimeters; note its bi-valved carapace. The two globose forms on the right are representatives of Ctenorhabdotus, a comb jelly (Phylum Ctenophora). The two swimmers in the top left are individuals of Pikaia, one of the earliest chordates. It reached a length of about 4 centimeters and may have been a filter feeder.
Plate 4. This reconstruction illustrates tiering in the Burgess Shale ecosystem. The animals in the foreground are infaunal, burrowing beneath the surface of the sediment. Other animals make their livelihood attached to the surface or crawl about the surface in search of food. Pikaia, on the other hand, is a free-swimming, or nektonic, animal. The two tan-colored burrowers on the right and the one in the pit are individuals of Ottoia, a priapulid worm with an evertible proboscis used for feeding. Ottoia reached lengths of 8 centimeters and was probably an active predatory burrower, and traces of the molluscan (?) hyolithid Haplophrentis (not shown), have been found in the well preserved gut contents of several specimens. The two bluish gray forms (on in the pit and the other in a burrow to the left) are individuals of the annelid Burgessochaeta. The long horizontal worm-like creature and the sheathed purple worm-like burrower on the lower right foreground are Louisella and Selkirkia, respectively. These are both priapulid worms. Louisella was encased in a flexible cuticular sheath that presumably molted in analogy to living priapulids; it pumped water through the open end of its 'U-tube' burrow. Selkirkia also had a flexible cuticular sheath, but this was encased in a rigid conical tube that had upwards of 1600 annulations (growth rings). Up to 80 percent of Selkirkia tubes are found empty.
Section author: Alexei A. Rivera
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