Presentation Information
[S1-O-10](Invited) Terrestrialization and diversification of invertebrates in the Paleozoic
*Yume IMADA1 (1. Department of Biology, Graduate School of Science, Kyoto University)
Keywords:
terrestrialization,ichnofossil,arthropods
This presentation reviews the terrestrialization and subsequent diversification of invertebrates, with particular emphasis on arthropods. For animals that evolved in marine environments, the transition onto land was not merely an expansion of habitat but a major evolutionary transformation. It required adaptations to desiccation, gravity, temperature fluctuations, and aerial respiration, including changes in locomotion, osmoregulation, behavior, and reproduction. Animal terrestrialization also established decomposers, herbivores, and predators on land, creating terrestrial nutrient cycles and food webs. Reconstructing this history is therefore essential for understanding when and how modern terrestrial ecosystems were assembled. However, invertebrates are extraordinarily diverse, and competing hypotheses have long existed for relationships among animal phyla and major arthropod lineages. Although phylogenomics has revised these relationships, few syntheses have integrated this framework with paleontological, developmental, functional, and ecological evidence.
This history is reconstructed by combining body fossils, trace fossils, molecular phylogenetics, and comparative genomics. Body fossils can preserve anatomy and gut contents, but small early terrestrial animals had low preservation potential and are known mainly from a few exceptional deposits. Trace fossils, including trackways, trails, burrows, coprolites, and feeding damage preserved on fossil plants, provide important evidence of locomotion, feeding, and expansion into new terrestrial habitats even where the animals themselves are absent. In Japan, discoveries including one of the oldest reliable leaf-mine records in East Asia have provided insights into the behavioral evolution of herbivorous insects (Imada et al., 2022). Molecular phylogenies and clocks help fill gaps in the fossil record and estimate when arthropod lineages independently colonized land (Lozano-Fernandez et al., 2016). Comparative studies test how pre-existing systems for respiration, excretion, and cuticle formation were co-opted during terrestrialization.
Arthropods were major components of the earliest terrestrial ecosystems. Fossils indicate that, before terrestrial vertebrates diversified, arthropods already occupied several trophic levels as detritivores, fungivores, and predators. Terrestrialization occurred independently in several lineages, probably through intertidal, coastal, and freshwater environments. By the Silurian, terrestrial chelicerates and myriapods had appeared, and the Early Devonian Rhynie chert preserves arthropods together with plants, fungi, and microorganisms. The expansion and vertical structuring of vascular plants created new resources and habitats, extending plant–arthropod interactions from detritivory and fungivory to feeding on living tissues (Labandeira and Wappler, 2023). During the Carboniferous, forest expansion accompanied the diversification of hexapods and the emergence of groups specialized for flight, herbivory, and predation.
Major uncertainties remain concerning the timing and pathways of terrestrialization, the environments in which aerial respiratory organs evolved, and discrepancies between fossil and molecular-clock estimates. The identity of animals responsible for plant-feeding traces and coprolites, distinctions among trophic modes, and the reconstruction of terrestrial food webs also remain unresolved. Consequently, the origins of soil-faunal and herbivore communities are still poorly understood.
Recent advances include synchrotron X-ray imaging, three-dimensional analysis, integration of trace and body fossils, phylogenomics, and ecological network modeling (Otoo, 2025). Future work should expand exploration in undersampled regions, including Asia, and connect fossils with the functional morphology, genomes, and development of living semiterrestrial lineages. This will shift the central question from when animals colonized land to which functions were acquired, in what sequence, and how these changes drove the assembly and diversification of terrestrial ecosystems.
This history is reconstructed by combining body fossils, trace fossils, molecular phylogenetics, and comparative genomics. Body fossils can preserve anatomy and gut contents, but small early terrestrial animals had low preservation potential and are known mainly from a few exceptional deposits. Trace fossils, including trackways, trails, burrows, coprolites, and feeding damage preserved on fossil plants, provide important evidence of locomotion, feeding, and expansion into new terrestrial habitats even where the animals themselves are absent. In Japan, discoveries including one of the oldest reliable leaf-mine records in East Asia have provided insights into the behavioral evolution of herbivorous insects (Imada et al., 2022). Molecular phylogenies and clocks help fill gaps in the fossil record and estimate when arthropod lineages independently colonized land (Lozano-Fernandez et al., 2016). Comparative studies test how pre-existing systems for respiration, excretion, and cuticle formation were co-opted during terrestrialization.
Arthropods were major components of the earliest terrestrial ecosystems. Fossils indicate that, before terrestrial vertebrates diversified, arthropods already occupied several trophic levels as detritivores, fungivores, and predators. Terrestrialization occurred independently in several lineages, probably through intertidal, coastal, and freshwater environments. By the Silurian, terrestrial chelicerates and myriapods had appeared, and the Early Devonian Rhynie chert preserves arthropods together with plants, fungi, and microorganisms. The expansion and vertical structuring of vascular plants created new resources and habitats, extending plant–arthropod interactions from detritivory and fungivory to feeding on living tissues (Labandeira and Wappler, 2023). During the Carboniferous, forest expansion accompanied the diversification of hexapods and the emergence of groups specialized for flight, herbivory, and predation.
Major uncertainties remain concerning the timing and pathways of terrestrialization, the environments in which aerial respiratory organs evolved, and discrepancies between fossil and molecular-clock estimates. The identity of animals responsible for plant-feeding traces and coprolites, distinctions among trophic modes, and the reconstruction of terrestrial food webs also remain unresolved. Consequently, the origins of soil-faunal and herbivore communities are still poorly understood.
Recent advances include synchrotron X-ray imaging, three-dimensional analysis, integration of trace and body fossils, phylogenomics, and ecological network modeling (Otoo, 2025). Future work should expand exploration in undersampled regions, including Asia, and connect fossils with the functional morphology, genomes, and development of living semiterrestrial lineages. This will shift the central question from when animals colonized land to which functions were acquired, in what sequence, and how these changes drove the assembly and diversification of terrestrial ecosystems.
Comment
To browse or post comments, you must log in.Log in