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A brief account is given of events and influences that preceded my being invited to lead the palaeobiological reinvestigation of the Burgess Shale. Joining these studies were D.L. Bruton and C.P. Hughes, subsequently D.E.G. Briggs and S. Conway Morris at Cambridge, in Oslo, and in the National Museum of Natural History, Washington, D.C. The geological setting of the Shale was first revealed by W.H. Fritz. Concomitant studies of Precambrian fossils, trace fossils, small shelly fossils of the early Cambrian, and new discoveries of soft-bodied biotas in China, Sweden and Greenland, have stimulated thought on evolutionary processes, and given rise to the concept of the Cambrian explosion.
In looking forward I comment on the importance of new investigations on taphonomy, and the promise they have for deeper understanding of what is preserved in the fossils, and of how they may be better prepared for study. The description of Anomalocaris revealed that predation by larger animals was a factor in palaeoecology from the early Cambrian onwards. Geochronology has shown the relative brevity of the Middle and Upper Cambrian, and I suggest that the transition from these times into the Ordovician may present as great a challenge in the understanding of evolutionary events as does the earlier Cambrian explosion. Despite the sporadic nature of the fossil record, new collections and new approaches give palaeontology a great opportunity to continue giving its unique contribution to the understanding of how evolution proceeded.
Origin of the eukaryotic organisms (including the multicellular animals or Metazoa) is commonly considered to be related to growing oxygen content in the atmosphere up to a level that allows aerobic metabolism. Here it is suggested that oxygenation of the biosphere was not a permissive condition but rather a forcing factor that drove evolution towards the formation of complex biological systems. Growing concentration of free oxygen in conjunction with other geohistorical trends acted to chemically impoverish the ocean and atmosphere and made many of the chemical elements immobile or unavailable for metabolic processes. Of particular importance in this connection was the decreasing concentration in sea water of the heavy metals that demonstrate high catalytic ability and make an active center in many enzymes. Increasing biological complexity and the eukaryotization of the biosphere (origin of the eukaryotic cell, growing role of heterotrophy, increasing biodiversity, rise of multicellular organisms, lengthening of trophic chains, acceleration of biological recycling of the chemical elements, etc.) can be considered as an evolutionary response to the geochemical deterioration of the environment.
Recent discoveries of the oldest megascopic eukaryotes, such as spiral Grypania (1.9 Ga), the necklace-like colonial organism of tissue-grade organization Horodyskia (1.5 Ga), vermiform Parmia (about 1.0 Ga) and Sinosabellidites (800 Ma ago) are consisitent with the “molecular clock” models on an early origin of animals; metazoans were, however, confined to relatively cold and well oxygenated basins beyond the carbonate belt of the ocean until the end of the Proterozoic. Large and diverse invertebrates of the Vendian Period are known mostly from siliciclastic marine basins. This fauna is characterized by high density of the benthic populations and well established clades both at the diploblastic (e.g., Phylum Trilobozoa) and triploblastic (e.g., Phylum Proarticulata) grades of organization as well as some taxa related to the Paleozoic phyla. An organic skeleton preceded the rise of the mineralized skeleton in some metazoan phyla. Low temperature of the habitats inhibited biomineralization. Almost simultaneous development of the phosphatic, carbonate and siliceous skeletons in different metazoan groups at the beginning of the Cambrian Period some 545 Ma ago could be related to the colonization of the warm carbonate basins by the metazoans. An additional factor for the rapid diversification of the biomineralized phyla could be the growing length of the trophic chains brought about by the rapidly increasing biodiversity and the need for detoxification at the top of the trophic pyramid. Being the byproduct of detoxification, sclerites and spicules, hard mineralized shells and carapaces immediately became an important factor of morphological evolution and growing biodiversity, as well as the object of intensive selection under the growing pressure of predators. Explosive growth of morphophysiological diversity in metazoans during the Vendian and Cambrian had an enormous impact on evolution of other groups of organisms and on the environment.
Functional, constructional, and preservational criteria led to a reinterpretation of seemingly complex trace fossils and the majority of assumed metazoan body fossils from Vendian lagerstatten. In the new scenario, Ediacaran biota were dominated by procaryote biomats and giant protozoa (Xenophyophoria and Vendobionta), which developed a great variety of shapes and lifestyles in the climatically controlled “golden age” that followed the Marinoan snowball earth. Contemporary metazoans (sponges; polyps; soft-bodied mollusks; possible echinoderms; worm-like burrowers) were adapted to this non-uniformitarian environment, but they remained scarce and relatively small. Some phyla (arthropods, brachiopods) appear to have still been absent. Our study also accentuates the Cambrian Explosion, which put an end to the peaceful “Garden of Ediacara”. Not only did the former rulers become extinct or restricted to less favorable environments, but the radiation of metazoan phyla was also accompanied by an ecological revolution that established a new and more dangerous world, which persists to the present day.
The Chengjiang fauna, an exceptionally well-preserved fossil lagerstatte, from the lower part of the Lower Cambrian Eoredlichia-Wutingaspis Biozone in the Kunming area, Yunnan Province, China is generally introduced, including the research history of the area, stratigraphy in the interval with soft-bodied fossils, geological setting, depositional environment, discovery, distribution, significance and faunal association. The Chengjiang lagerstatte yields various mineralized and nonmineralized skeletons and internal soft parts of organisms, as well as complete soft-bodied animals. The fauna includes virtually all animal phyla that were previously known from the Middle Cambrian and vividly reproduces the appearance of the oldest Phanerozoic animals.
‘Orsten’-type preservation, i. e., phosphatisation of cuticles without further diagenetic deformation, has yielded three-dimensional fossils at a scale of 0.1–2.0 mm. Such fossils, first described from Upper Cambrian limestone nodules found in Sweden, have been reported from several continents and from the early Cambrian (approx. 520 M. y. BP) to the early Cretaceous (approx. 100 M. y. BP). Fossils from Cambrian ‘Orsten’-type lagerstatten are mainly representatives of different euarthropod groups and also of different evolutionary levels. This allowed the reconstruction of the early phylogeny particularly of Crustacea in great detail and the recovery of major evolutionary traits within this group, i. e., in the progressive modification of the locomotory and feeding apparatus of the head region. More recently, derivatives also of the early stem lineage toward the Euarthropoda have been discovered. These include apparently parasitic larvae of stemlineage Pentastomida (tongue worms) today living in various tetrapods, a minute fossil related to the equally minute tardigrades (water bears), and fragments of a small tubular organism with segmental tubular limbs, interpreted as the first lobopodian in an ‘Orsten’-type preservation. Lobopodians are worm-like derivatives of the earliest phase in the evolution of arthropods before the development of a sclerotic, segmented dorsal cuticle (arthrodized tergum) and similarly segmented limbs (arthropodia), hitherto known only from the Lower to Middle Cambrian. The presence of these “pre-euarthropods,” which lack, or partly lack, characteristic features developed later in the arthropod evolutionary lineage, and the recent record of phosphatocopine Crustacea in the earliest Palaeozoic are regarded as a support for the view that the ancestry of Arthropoda lies much further back, possibly well in the late Pre-Cambrian. This does not support a “Cambrian explosion”.
In embryonic development of the vertebrate head, neural crest-derived ectomesenchyme contributes to a wide range of tissue types including oro-pharyngeal and ethmoidal cartilages. The evolution of the jaw, therefore, can be viewed as a change of developmental program for specification of the crest cells. Along the anteroposterior axis of the neural crest of amniote embryos, a series of homeobox genes are expressed in a nested pattern, and the jaw-forming mandibular arch receives crest cells expressing no Hox genes and midbrain-derived crest cells that express Otx2. Cognates of these regulatory genes are present in the lamprey, and are expressed in the comparable cell lineages of the embryo. Evolution of the jaw cannot be explained from such shared developmental mechanisms, but rather noncomparable elements have to be sought, if the jaw is truly an evolutionary novelty. By precise comparative morphology and gene expression analyses, a possibility was inferred that ammocoete lips may not be identical to gnathostome jaws.