The Succession of Faunas

THE SUCCESSION OF FAUNAS Pre-Cambrian deposits of the age of the Algonkian and Torrid onian often exhibit traces of animal life in the form of tracks and burrows similar to those made by living marine "worms," but determinable remains of animals are exceedingly rare in them. C. D. Walcott has, however, described a small fauna from North America, which is remarkable for its resemblance to that which is found in the succeeding Lower Cambrian.

Cambrian.

In the Cambrian characteristic representatives of nearly all the phyla of the animal kingdom are found. Protozoa are represented by typical radiolarian skeletons, but Foraminifera are as yet unknown. Sponges are represented by a lithistid in pre-Cambrian rocks, although not actually in the Lower Cam brian. Protospongia is a hexactinellid found rather abundantly in Cambrian rocks in North America and Europe. The peculiar group Archaeocyathidae, which is usually considered to belong to the Coelenterata, but appears to differ fundamentally in struc ture from all representatives of that phylum, is entirely restricted to Cambrian rocks and its members occur all over the world. The normal coelenterates are, however, represented by impressions of jelly-fish in the Cambrian of British Columbia and in Upper Cambrian times the first graptolites appear in the form of Dictyo nema and its allies, together with a few representatives of the higher group, Axonolipa.

The Echinodermata are represented in Cambrian times by three classes, the Cystidea, the Crinoidea and the Holothuria. The Chaetopoda are represented by perfectly characteristic ani mals from the Middle Cambrian of Canada, in addition to many worm-castings, and one form from the same rocks is plausibly attributed to the Gephyrea, whilst another appears to be related to the living arrow-worm Sagitta, which belongs to the phylum Chaetognatha. No Cambrian representatives of the Platyhelmia, Nemertina or Nematoda are known, but these phyla are unrepre sented as fossils at any period. The Brachiopoda provide perhaps the most abundant of all Cambrian fossils and certain groups are restricted to rocks of that age. During Cambrian times, however, the articulate brachiopods are rare and the Telotremata, the most advanced of these, are completely absent. Cambrian Bryozoa

are unknown, and the Podaxonia are unrepresented as fossils at any time. Certain of the classes of the phylum Mollusca appear in the Cambrian. The Lamellibranchia are perhaps represented by forms belonging to the primitive division Taxodonta, but are in any case very rare. The Scaphoda, a small and inconspicuous group, are not known before Silurian times, and the Polyplaco phora, always rare, are of later introduction.

Amongst the Gastropoda several forms, the primitive and purely Palaeozoic Bellerophontidae, the Euomphalidae and the still-living Pleurotomariidae occur in the Cambrian. The limpets are also represented and the group Pteropoda is abundantly present if the Hyolithidae be correctly placed amongst them. The highest di vision of the Mollusca, the Cephalopoda, is apparently repre sented in Cambrian times by members of the sub-order Nauti loidea, but there is perhaps some doubt as to the correctness of the identifications on which this statement rests. The more ad vanced division of the cephalopods, the order Dibranchiata, is clearly of much later introduction. The group of the Arthropoda is well represented, but the majority of the forms known belong to the special Palaeozoic group of the Trilobita (q.v.). The Phyl lopoda are represented by a few forms from the Middle Cambrian Burgess shale of British Columbia, and by other Apus-like creatures from other deposits, but the morphology of these animals is still so incompletely known that their relationships are uncer tain. The higher group Phyllocarida is represented by many in completely known animals widely distributed in the Cambrian, but its more advanced groups are certainly of later introduction. The group of the Arachnoidea, which includes spiders and king-crabs, has several Cambrian forms, grouped in the order Merostomata, which are clearly far more primitive in structure than the later forms. The air-breathing myriapods and insects are absent from Cambrian rocks, and the vertebrate phylum is unrepresented.