The first diverse assemblages of unquestionable animal fossils at the Proterozoic-Phanerozoic transition, which marks the change from a predominantly microbial biosphere to a modern type of biosphere abundant with multicellular life. The name derives from the Early Cambrian Tommotian Stage in Siberia, where the significance of this fauna of early skeletal fossils (often referred to as “small shelly fossils”) was first realized. However, the concept goes beyond the geographical and temporal boundaries of the Tommotian Stage. It can be traced back to the low-diversity assemblages of skeletal animal fossils appearing near the end of the Neoproterozoic, continuing into the Cambrian, in Siberia first in moderate diversity in the Manykaian (Nemakit-Daldynian) Stage, then burgeoning in the Tommotian Stage itself. See also: Cambrian; Fossil
Traditionally, the fauna was considered to predate the earliest trilobites and was therefore often referred to as “pre-trilobitic”; however, recent stratigraphic studies suggest that at least some of the Tommotian Stage correlates with trilobite-carrying beds elsewhere. A number of the characteristic Tommotian taxa are now also known to continue into post-Tommotian strata. Thus, freed of its tight stratigraphic constraints, the Tommotian fauna is known to have radiated from all continents, but it is particularly diverse and abundant on the Siberian Platform in Russia, in Australia, and in the belt of phosphorite-rich deposits that extends from the South China Platform through Mongolia, Kazakhstan, the Himalayas, and Iran. See also: Trilobita
The Tommotian fauna, being mainly restricted to animals with hard parts, does not yield as complete a picture of animal anatomy and diversity as the more well-known Cambrian Burgess Shale and Chengjiang (South China) faunas or the Upper Cambrian orsten fauna, all of which are characterized by frequent preservation of soft tissues. Nonetheless, it has been instrumental in forming our understanding of the Cambrian explosion (dramatic evolutionary radiation of animals beginning about 545 million years ago), because its preservation is generally not dependent on extraordinary conditions and is therefore less spotty. Also, fossilized embryos of Tommotian animals make it possible to understand the complete life cycles of some of the most basal members of the metazoan evolutionary tree. See also: Animal evolution; Burgess Shale
Characteristic morphologic features of the Tommotian fauna include mineralized tubes, spicules, sclerites, and shells, often belonging to animals of unknown affinities. The minerals involved are opal (a hydrated gel of silica), apatite (calcium phosphate), and aragonite/calcite (calcium carbonate)—the same minerals that are common in animal skeletons today.
The tubes represent many different kinds of animals that built protective sheaths reinforced with minerals. One of the earliest known such forms, the late Neoproterozoic Cloudina, had a multilayered cone-in-cone structure consisting of thin calcitic laminae (Fig. 1a). Already in these early mineralized animal skeletons there is evidence of attacks from shell-boring predators, suggesting that exoskeletons initially arose as a response to predation. Cloudina is now known to be associated with several other forms of skeletal fossils. The triradially symmetrical tubes of anabaritids (Fig. 1b) partly overlap Cloudina in stratigraphic range and are a particularly characteristic component of the Proterozoic-Cambrian boundary beds. Anabaritids may be related to cnidarians, although triradial symmetry is not a characteristic feature of later cnidarians. Associated embryos show definitive cnidarian features, although the connection between these embryos and adult anabaritids is only implied from their joint occurrence. See also: Cnidaria
Other possible cnidarians are represented by forms such as Olivooides, which shows similarities with modern scyphozoans (jellyfish). A rich embryonic material of Olivooides shows that it developed directly within the egg, without going through a free planula stage (Fig. 2).
Other tube-dwelling forms are even more difficult to put in their proper phylogenetic place because of the lack of anatomically significant characters. These include the hyolithelminths (Fig. 1c) and other taxa that similarly reinforced their tubes with calcium phosphate, as well as forms that used calcium carbonate for the same purpose. There were also a number of animals that constructed their tubular sheaths of sclerotized organic matter or strengthened them with agglutinated sedimentary particles.
Spicules (spikelike supporting structures) of various composition are widespread in the Animal Kingdom today. Although a number of mineralized spicules have been reported from the late Neoproterozoic, all of these reports are contested. Sponge spicules, however, are already considered a conspicuous component of the Tommotian fauna, and there are a number of spicules probably belonging to animals other than sponges.
The cup-shaped archaeocyathans (Fig. 3) long were of uncertain affinities and regarded by some as representing an extinct phylum. The presence of calcareous basal skeletons in several groups of recent sponges, however, has led to the now generally accepted idea that archaeocyathans are a group of calcified sponges that proliferated in the Early Cambrian and died out before the Ordovician. They are common in reefs but do not appear to have had the capacity of constructing reef frameworks. See also: Archaeocyatha
A conspicuous element of Tommotian fossil assemblages is sclerites (hardened plates) belonging to composite exoskeletons, scleritomes. In many cases, the body shape of the bearer and the distribution of sclerites on the body are not known, but finds of complete scleritomes or even bodies in shale deposits give occasional and crucial insights. Thus the star-shaped composite sclerites of the chancelloriids are known to belong to a cactuslike animal that in its organization seems closest to sponges: sedentary, sac-shaped bodies with an apical orifice and no evidence of internal organs. These sclerites belong to a type called coelosclerites, consisting of a mineralized envelope around a space originally filled with soft tissue and showing no evidence of accretionary growth. The halkieriids are characterized by scale-shaped coelosclerites, and finds of complete specimens show the animal to have been slug-shaped, with two large anterior and posterior shell plates in addition to the sclerites (Fig. 4a). Embryos of a segmented animal co-occurring with such sclerites have been proposed to belong to halkieriids. It is not clear whether coelosclerites are a convergent feature, independently evolved in several groups, or whether they were inherited from a common ancestor of the various groups of coeloscleritophorans. nature.com
Other sclerites were growing by stepwise accretion and often used calcium phosphate rather than calcium carbonate as shell mineral. The most widespread group of such phosphatic sclerite bearers are the tommotiids (Fig. 4c, d), of which so far no complete skeletons have been found. Tommotiids show a variety of sclerite shapes and ultrastructures and may in fact represent a polyphyletic assemblage of lineages that independently acquired a phosphatic scleritome. Other phosphatic sclerites include tooth-or hook-shaped objects as well as a variety of platelike types, most of which are of unknown nature. The earliest known brachiopod is represented in the Tommotian fauna by a phosphatic shelled form.
Mollusk-like forms are a common component of the Tommotian fauna, although their phylogenetic placement continues to be contentious. A number of gastropodlike shells (Fig. 4e) occur, but it is not clear whether the soft parts did in fact possess the torsion characteristic of crown-group gastropods. Laterally flattened shells showing what appears to be an incipient dorsal hinge (Fig. 4f) have been interpreted as ancestral to the later rostroconchs and bivalves. A number of Tommotian taxa have been assigned to polyplacophorans, but all of these suggestions are controversial. Examples are the halkieriids (Fig. 4a) and siphogonuchitids (Fig. 4b), which have been interpreted alternatively as stem-group polyplacophorans, stem-group mollusks, and stem-group lophotrochozoans (a phylum grouping including mollusks, annelids, and brachiopods). See also: Annelida; Bivalvia; Brachiopoda; Gastropoda; Mollusca; Polyplacophora
The phylogenetic relationships of the Tommotian animals are very incompletely understood. Although some forms have with reasonable confidence been interpreted to belong to the crown group or stem group of a living phylum, others are more problematic. To some extent this is because many of them are incompletely known, perhaps only from disarticulated sclerites and no soft parts. More pertinent, however, may be the observation that the Tommotian fauna represents the first major adaptive radiation of animals and so is likely to include a number of short-lived lineages that do not share anatomical features to any greater extent than do living phyla.
The appearance in the fossil record of the Tommotian fauna near the base of the Cambrian has often been interpreted as being primarily a biomineralization event rather than a major radiation. This has then been attributed to extrinsic (for example, seawater chemistry) as well as intrinsic (for example, evolution of biomineralization pathways) factors. However, biomineralization not involving skeleton formation can be shown to be much older than the Cambrian. In addition, independent lines of evidence (trace fossils, soft-body preservation, organic microfossils) show the Cambrian explosion to have involved nonskeletalized animals and nonanimals as well. Thus, the evolution of biomineralized skeletons assumes the same general significance as the evolution of various other kinds of animal tissues during this radiation event.