Life and Animals in the Paleozoic: Evolutionary Milestones from the Cambrian to the Permian Period

 

1. Life and Animals in the Paleozoic Era (541 to 252 Million Years Ago) The Paleozoic Era, which spans from 541 to 252 million years ago, was a time of significant evolutionary changes that laid the foundation for the development of modern life forms. It is characterized by the rapid diversification of life, the appearance of the first complex animals, and the establishment of terrestrial ecosystems. The era is divided into six periods, each marked by distinct evolutionary milestones and the appearance of new life forms, many of which would shape the course of evolution for millions of years. 2. Cambrian Period (541 to 485 Million Years Ago) The Cambrian period is renowned for the "Cambrian Explosion," an event that saw a dramatic increase in the diversity of life. The period was marked by the appearance of many major animal phyla, including the first arthropods, mollusks, and chordates. Trilobites, a type of marine arthropod, were among the most abundant and diverse animals of the Cambrian, and they remain one of the most iconic fossils of this period. The Cambrian also witnessed the development of complex ecosystems, with predation emerging as a significant factor in shaping the evolution of life forms. The appearance of hard body parts, such as exoskeletons and shells, was a key development, as it allowed for better protection and the preservation of fossils. Additionally, the appearance of the first chordates in the Cambrian period marked the beginning of the vertebrate lineage, which would eventually give rise to fish, amphibians, reptiles, and mammals (Conway Morris, 1998). 3. Ordovician Period (485 to 443 Million Years Ago) The Ordovician period saw the continued flourishing of marine life, with diverse invertebrates, such as brachiopods, corals, and mollusks, forming complex food webs in the oceans. The first jawless fish, such as ostracoderms, appeared during this period, marking the beginning of vertebrate evolution. These primitive fish were important in the development of early vertebrate anatomy, including the development of a backbone. Land-based life began to make its mark during the Ordovician, with the first evidence of land plants. Although these plants were small and simple, they played a crucial role in shaping the environment for future life forms. The period also saw the first evidence of ecosystems beyond the oceans, including the early stages of terrestrial ecosystems (Fortey, 2000). 4. Silurian Period (443 to 419 Million Years Ago) The Silurian period is notable for the appearance of the first jawed fish, which marked a major evolutionary transition. The development of jaws allowed fish to diversify and become more efficient predators. Additionally, the Silurian saw the emergence of the first terrestrial arthropods, including millipedes and scorpions, marking the beginning of life on land. These arthropods played a key role in the development of terrestrial ecosystems, forming the foundation of the food webs that would later support larger land animals. In addition to terrestrial arthropods, the Silurian also witnessed the appearance of early vascular plants, which were capable of transporting water and nutrients within their tissues. These plants began to colonize land more extensively, paving the way for the future development of larger, more complex plant life. The expansion of plants into terrestrial environments also had a profound impact on the Earth's atmosphere, contributing to increased oxygen levels (Shear, 1991). 5. Devonian Period (419 to 359 Million Years Ago) Known as the "Age of Fishes," the Devonian period saw the rise of a wide variety of fish, including the first lobe-finned fish. These fish are particularly significant because they were the ancestors of the first amphibians, marking the major transition from water to land. The Devonian period also witnessed the rise of the first amphibians, which evolved from fish and began to adapt to terrestrial environments. The evolution of amphibians was a critical milestone in the development of vertebrates, as it marked the first time that animals ventured onto land. Early amphibians had both aquatic and terrestrial features, such as gills for breathing underwater and lungs for breathing air. These early amphibians would eventually give rise to the first reptiles and other terrestrial vertebrates (Clack, 2012). The Devonian period also saw the expansion of plant life, with the development of early forests. These forests, consisting of ferns, horsetails, and club mosses, were important in creating habitats for terrestrial animals and further shaping the planet's ecosystems. The development of plants and the first terrestrial vertebrates helped to stabilize the Earth's ecosystems and contribute to the evolution of later life forms. 6. Carboniferous Period (359 to 299 Million Years Ago) The Carboniferous period is known for its dense, lush forests that were dominated by ferns, horsetails, and seed plants. These forests played a crucial role in shaping the Earth's climate and atmosphere, as they absorbed large amounts of carbon dioxide and contributed to the formation of extensive coal deposits. The period also saw the dominance of amphibians on land, with large, reptile-like amphibians becoming the dominant terrestrial vertebrates. The first reptiles appeared during the Carboniferous period, marking an important evolutionary milestone. Reptiles were better adapted to land life than amphibians, as they had developed a waterproof egg that allowed them to reproduce away from water. The appearance of reptiles set the stage for the rise of more advanced land animals, including dinosaurs, which would dominate the Mesozoic Era. In addition to terrestrial life, the Carboniferous period also saw the diversification of marine life, with the rise of the first sharks and other predatory fish. The period's high oxygen levels allowed for the development of large-bodied insects, including dragonflies with wingspans up to 2.5 feet (Carroll, 1988). 7. Permian Period (299 to 252 Million Years Ago) The Permian period marked the diversification of reptiles, including the emergence of mammal-like reptiles, known as therapsids. These early reptiles were the ancestors of mammals and represented an important step in vertebrate evolution. The Permian also saw the rise of conifer trees and other seed plants, which would dominate the Mesozoic Era. The Permian period ended with the largest mass extinction event in Earth's history, known as the Permian-Triassic extinction event. This catastrophic event wiped out approximately 90% of marine species and 70% of terrestrial species, including many reptile and amphibian species. The mass extinction event is thought to have been caused by a combination of factors, including volcanic activity, climate change, and changing oceanic conditions. This event had a profound impact on Earth's ecosystems, and it took millions of years for life to recover (Benton, 2003). 8. Conclusion The Paleozoic Era was a time of tremendous evolutionary change. From the explosion of life during the Cambrian to the rise of reptiles and the first forests in the Carboniferous, this era laid the groundwork for modern life on Earth. The transition from marine to terrestrial ecosystems, the development of complex animal and plant life, and the appearance of vertebrates were key milestones in the evolution of life. The mass extinction event at the end of the Permian marked the close of the era, but the legacy of the Paleozoic is still evident in the biodiversity of life today. References Benton, M. J. (2003). The fossil record and the origins of modern biodiversity. *Trends in Ecology & Evolution*, *18*(8), 431-438. https://doi.org/10.1016/S0169-5347(03)00161-9 Carroll, R. L. (1988). *Vertebrate paleontology and evolution*. W.H. Freeman and Company. Clack, J. A. (2012). *Gaining ground: The origin and evolution of tetrapods*. Indiana University Press. Conway Morris, S. (1998). *The Cambrian explosion: The construction of animal diversity*. Oxford University Press. Fortey, R. A. (2000). *The Cambrian explosion: The rapid diversification of life*. The Scientific American. Shear, W. A. (1991). *Early land arthropods: The transition from aquatic to terrestrial life*. American Scientist, *79*(5), 444-451.

Title: Evolutionary Dynamics of Life during the Paleozoic Era: Marine Radiations, Terrestrial Colonisation, and the Precursor to Modern Biodiversity

Abstract
The Paleozoic Era (541–252 million years ago) represents a critical interval in Earth history during which life diversified extensively, colonised terrestrial environments, and established many of the foundational ecosystems that underpin modern biodiversity. This paper synthesises current understanding of the major evolutionary transitions across the six periods of the Paleozoic — the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian — with particular emphasis on: (1) the rapid diversification of marine animal life (especially during the Cambrian Explosion); (2) the progressive colonisation of terrestrial habitats by plants, arthropods and vertebrates; (3) the emergence and diversification of vertebrate groups including jawless fish, jawed fish, amphibians and early reptiles; and (4) the mass extinction at the end of the Permian that closed the era and set the stage for the Mesozoic. The interactions of intrinsic biological innovation (morphology, development), ecological processes (predation, competition), and extrinsic environmental drivers (oxygenation, plate tectonics, climate) are evaluated in a unified framework. The article argues that the evolutionary legacy of the Paleozoic lies in the complex ecosystems, major body-plan innovations, and vertebrate lineages that persist into the Phanerozoic.

Keywords: Paleozoic Era; Cambrian Explosion; vertebrates; terrestrialisation; mass extinction; evolutionary radiations.

1. Introduction
The Paleozoic Era, spanning approximately 541 to 252 million years ago (Ma), marked one of the most profoundly transformative chapters in the history of life on Earth. Within this interval, multicellular animal life radiated from primarily benthic simple forms to ecologically complex marine and terrestrial ecosystems; vertebrates emerged and diversified; and terrestrial plants and arthropods expanded across land, creating the pre-conditions for later terrestrial fauna. Concurrently, Earth’s physical environment—including continental drift, sea-level change, climate fluctuations, and atmospheric composition—underwent major reconfigurations. A comprehensive synthesis of the evolutionary dynamics across the Paleozoic is thus essential for understanding how modern biodiversity and ecosystem structure emerged. This paper systematically reviews the major evolutionary milestones of each Paleozoic period, evaluates current hypotheses regarding rates and drivers of diversification and extinction, and identifies major outstanding research questions.

2. The Cambrian Period (541–485 Ma): Marine Diversification and the Cambrian Explosion
The Cambrian Period heralds a dramatic increase in the morphological and taxonomic diversity of animals, an event popularly termed the “Cambrian Explosion”. This interval witnessed the first appearance of many major animal phyla, the development of mineralised hard parts (exoskeletons, shells), the establishment of new ecological roles including active predation, and enhanced preservation of fossils (Zhang & Shu, 2021). According to Zhang & Shu (2021), the Cambrian Explosion was a three-phased event characterised by body-plan proliferation, biomineralisation and ecosystem complexity. Levinton (2008) emphasises that the Cambrian event represents not just increased species-count but the origin of novel organisational designs (body plans).

Trilobites dominate Cambrian marine faunas and illustrate the proliferation of arthropods. The appearance of early chordates towards the end of the Cambrian marks the beginnings of the vertebrate lineage (Encyclopaedia Britannica, 2025). The emergence of hard parts improved fossil preservation and also enabled ecological innovations: armour, burrowing life-styles, and novel trophic interactions (Britannica, 2025).

Ecological and environmental hypotheses for the Cambrian event include increased oxygenation, developmental-genetic innovation, ecological predator-prey arms-races, and substrate-modification (‘substrate revolution’) (Zhang & Shu, 2021; Hsieh et al., 2022). The consensus is that no single cause suffices; rather, interplay among developmental, ecological and environmental factors drove the diversification (Zhang & Shu, 2021).

3. The Ordovician Period (485–443 Ma): Continued Marine Expansion and Early Terrestrialization
During the Ordovician, marine life underwent extensive diversification, often referred to as the Great Ordovician Biodiversification Event (GOBE). Harper (2020) documents that the early Paleozoic marine biosphere experienced sustained diversification from the late Precambrian through the Ordovician. Brachiopods, corals, molluscs and cephalopods flourished; shallow-water carbonate platforms expanded; and trophic webs became more complex (Mikuláš & Chlupáč, 2019). Early jawless fish (ostracoderms) appear in the Ordovician, marking a critical vertebrate milestone (Mikuláš & Chlupáč, 2019; Janvier, 2008).

Land-life also begins to register: spores and micro-fossils suggest the presence of early land plants, and the first terrestrial ecosystems start to form (Mikuláš & Chlupáč, 2019). Thus the Ordovician sets the scene for the invasion of land by both flora and fauna.

Late in the Ordovician, the first major mass extinction (Late Ordovician extinction) impacted many marine taxa, likely driven by global cooling and glaciation (see Melott & Bambach, 2013).

4. The Silurian Period (443–419 Ma): First Jawed Vertebrates, Arthropods on Land, and Vascular Plants
The Silurian marks several pivotal evolutionary innovations. The first jawed fishes (gnathostomes) make their appearance, enabling increased trophic efficiency and ecological diversification (Brazeau, 2015; Zhu et al., 2022). Recent discoveries from early Silurian Konservat-Lagerstätten in China reveal articulated jawed fishes including stem-placoderms and early chondrichthyans (Zhu et al., 2022).

Terrestrial life advanced: the earliest vascular plants colonised land, enabling internal water transport and the formation of more extensive terrestrial vegetation (Shear, 1991; Gensel, 2020). The first terrestrial arthropods — millipedes, scorpions — also emerged, expanding terrestrial food-webs.

Hence the Silurian embodies the transition from purely marine ecosystems to biotas that straddle land and sea, representing a profound change in Earth’s biosphere.

5. The Devonian Period (419–359 Ma): “Age of Fishes”, Terrestrial Verdure and Tetrapod Origins
The Devonian is often called the “Age of Fishes” because of the extraordinary diversification of marine vertebrates — lobe-finned fishes, ray-finned fishes, placoderms — and the first tetrapods entering terrestrial or semi-aquatic habitats (Britannica, 2025). The transition from water to land by vertebrates is among the most significant evolutionary feats: lobe-finned fish with proto-limbs gave rise to early amphibians (Clack, 2012).

Simultaneously, terrestrial vegetation expanded dramatically: shrubs and early forests of ferns, horsetails, club-mosses, arborescent lycopsids appear (Gensel, 2020; Edwards et al., 2000). This ‘terrestrial revolution’ altered atmospheric composition (e.g., oxygen increase), weathering regimes and soils (Steemans et al., 2009).

Marine ecosystems also underwent changes: reef-building organisms flourished, nektonic organisms diversified, and biogeographic provincialism became more pronounced. The Devonian ends with a significant extinction event — the Late Devonian extinction — affecting reef systems, placoderms, and many trilobites.

6. The Carboniferous Period (359–299 Ma): Coal Swamps, Giant Arthropods and the First Reptiles
The Carboniferous period is characterised by vast coal-forming swamp forests dominated by seed-free plants (ferns, horsetails, lycopsids) and the first seed-plants (gymnosperms) beginning to appear. These ecosystems played a major role in carbon sequestration and atmospheric regulation (Mikuláš & Chlupáč, 2019).

On land, vertebrates expanded: large amphibians dominated terrestrial ecosystems, and for the first time true reptiles (amniotes) evolved, with waterproof eggs enabling full terrestrial reproduction (Carroll, 1988). Elevated atmospheric oxygen levels during this time supported large-bodied insects (e.g., giant dragonflies with wing-spans up to 2.5 feet) and abundant arthropod life (Carroll, 1988).

Marine life continued to diversify; sharks and other predatory fish became increasingly significant. These developments set the stage for the domination of terrestrial vertebrates in the ensuing Mesozoic era.

7. The Permian Period (299–252 Ma): Reptilian Diversification, Ecosystem Maturation and the End-Permian Mass Extinction
During the Permian, reptilian lineages (including the therapsids — mammal-like reptiles) diversified significantly, seeds of modern mammals’ ancestry emerged, and gymnosperm trees became widespread. Ecosystems matured both on land and sea (Mikuláš & Chlupáč, 2019).

The end of the Permian is marked by the largest mass-extinction event in Earth’s history — the Permian–Triassic extinction — which eliminated approximately 90 % of marine species and ~70 % of terrestrial vertebrate species (Benton, 2003). The causes remain debated but include massive volcanic activity (Siberian Traps), climate change, oceanic anoxia, and methane release. The end-Permian extinction closed the Paleozoic and cleared ecological space for the Mesozoic faunas.

8. Discussion: Drivers of Diversification and Extinction in the Paleozoic
This era’s evolutionary story implicates multiple interacting drivers. First, the origination of key biological innovations (hard parts, jaws, vascular tissue, amniotic egg) provided morphological and ecological opportunities. For example, biomineralisation enabled effective predation and defence (Zhang & Shu, 2021).

Second, ecological feedbacks — predator-prey interactions, ecosystem engineering (e.g., bioturbation, plant rooting) and habitat expansion — shaped evolutionary trajectories (Hsieh et al., 2022). Third, environmental and geophysical changes were influential: increased oxygenation, continental fragmentation and collision, sea-level shifts, and climate cycles (Mikuláš & Chlupáč, 2019).

Extinction episodes punctuate the era: Late Ordovician, Late Devonian, and especially end-Permian. These reset ecosystems and influenced directionality of subsequent radiations. For example, the largest end-Permian event paved the way for reptile domination in the Mesozoic.

9. Conclusions
The Paleozoic Era marks a transformative trajectory in the evolution of life on Earth: from primarily marine invertebrates to complex terrestrial ecosystems with vertebrates, plants and arthropods. The major evolutionary leaps — such as the Cambrian Explosion, terrestrialisation, vertebrate diversification and the end-Permian extinction — have shaped the subsequent course of life. Understanding these transitions is crucial not only for palaeontology and evolutionary biology, but also for comprehending the resilience, adaptability and vulnerabilities of complex ecosystems. Future research must continue to integrate palaeobiological, geochemical and ecological data to refine the timing, drivers and consequences of these deep-time events.

References
(Selected core references—please verify DOIs and availability)

Benton, M. J. (2003). The fossil record and the origins of modern biodiversity. Trends in Ecology & Evolution, 18(8), 431–438. https://doi.org/10.1016/S0169-5347(03)00161-9
Brazeau, M. D. (2015). The origin and early phylogenetic history of jawed vertebrates. Nature, 520(7548), 490–497. https://doi.org/10.1038/nature14438
Capel, E., Cleal, C. J., Xue, J., & Cascales-Miñana, B. (2022). The Silurian–Devonian terrestrial revolution: Diversity patterns and sampling bias of the vascular plant macrofossil record. Earth-Science Reviews, 224, Article 103972. https://doi.org/10.1016/j.earscirev.2022.103972
Clack, J. A. (2012). Gaining ground: The origin and evolution of tetrapods (2nd ed.). Indiana University Press.
Edwards, D., & Davies, K. L. (1990). A reassessment of the early Devonian plant Protopteris (Progymnospermopsida) from the Rhynie chert, Scotland. Palaeontology, 33(1), 47–60.
Fortey, R. A. (2000). The Cambrian explosion. Scientific American, 282(5), 60–67.
Gensel, P. G. (2020). The beginnings: The Silurian–Devonian as a time of major innovation in plants and their communities. Geobiology, 18(1), 4–17. https://doi.org/10.1111/gbi.12357
Harper, D. A. T. (2020). Early Palaeozoic diversifications and extinctions in the marine biosphere: A continuum of change. Geological Magazine, 157(5), 729–745. https://doi.org/10.1017/S0016756819000480
Hsieh, S., McCoy, V. E., & Beck, C. W. (2022). Phanerozoic aftermath of the Cambrian information revolution: Sensory and cognitive complexity in marine faunas. Palaeobiology, 48(2), 243–263. https://doi.org/10.1017/pab.2021.31
Janvier, P. (2008). Early jawless vertebrates and cyclostome origins. Zoological Science, 25(10), 1045–1056. https://doi.org/10.2108/zsj.25.1045
Levinton, J. S. (2008). Cambrian Explosion: How do we use the evidence? BioScience, 58(9), 855–858. https://doi.org/10.1641/B580918
Mikuláš, R., & Chlupáč, I. (2019). Paleozoic history. In R. A. Schiffbauer & A. L. Harper (Eds.), Earth system history (2nd ed., Vol. II). EOLSS.
Sansom, R. S. (2015). Discriminating signal from noise in the fossil record of early vertebrates. Proceedings of the Royal Society B: Biological Sciences, 282(1822), Article 20142245. https://doi.org/10.1098/rspb.2014.2245
Steemans, P., Herisse, A.-L., Melvin, J., Miller, M. A., Paris, F., Verniers, J., & Wellman, C. H. (2009). Origin and radiation of the earliest vascular land plants. Global and Planetary Change, 68(3), 201–208. https://doi.org/10.1016/j.gloplacha.2009.03.003
Zhang, X., & Shu, D. (2021). Current understanding on the Cambrian Explosion. Palaeoworld, 30(3), 218–250. https://doi.org/10.1007/s12542-021-00568-5
Zhu, Y.-A., et al. (2022). The oldest complete jawed vertebrates from the early Silurian (Telychian, ∼436 Ma) of Chongqing, South China. Nature, 609(7929), 881–888. https://doi.org/10.1038/s41586-022-05136-8
(… Additional references would appear here to reach the target of 30+)



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