Trilobite


Trilobites are extinct marine arthropods that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, existing in oceans for almost 270million years, with over 22,000 species having been described. Because trilobites had wide diversity and an easily fossilized mineralised exoskeleton made of calcite, they left an extensive fossil record. The study of their fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are placed within the clade Artiopoda, which includes many organisms that are morphologically similar to trilobites, but are largely unmineralised. The relationship of Artiopoda to other arthropods is uncertain.
Trilobites evolved into many ecological niches; some moved over the seabed as predators, scavengers, or filter feeders, and some swam, feeding on plankton. Some even crawled onto land. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism. Some trilobites are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. The largest trilobites were more than long and may have weighed as much as.
The first appearance of trilobites in the fossil record defines the base of the Atdabanian/Cambrian Stage 3 time period of the Early Cambrian around. Trilobites were already diverse and globally dispersed shortly after their origination, with trilobites reaching an apex of diversity during the late Cambrian–Ordovician, and remained diverse during the following Silurian and early Devonian. During the mid-late Devonian, their diversity strongly declined, being impacted by successive extinction events, including the Taghanic event, the Late Devonian mass extinction/Kellwasser event and the Hangenberg/end-Devonian mass extinction, wiping out most trilobite diversity and leaving Proetida as the only surviving order. Their diversity moderately recovered during the Early Carboniferous, before dropping to persistently low levels during the late Carboniferous and Permian periods, though they remained widespread until the end of their existence. The last trilobites disappeared in the end-Permian mass extinction event about 251.9million years ago, by which time only a handful of species remained.

Evolution

Trilobite relatives

Trilobites belong to the Artiopoda, a group of extinct arthropods morphologically similar to trilobites, though only the trilobites had heavily mineralised exoskeletons. Thus, other artiopodans are typically only found in exceptionally preserved deposits, mostly from the Cambrian period.
The exact relationships of artiopods to other arthropods is uncertain. Some scholars consider them closely related to chelicerates as part of a clade called Arachnomorpha, while others consider them to be more closely related to Mandibulata as part of a clade called Antennulata.
Cladogram of Artiopoda including trilobites after Berks et al. 2023.

Fossil record of early trilobites

The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to around 520million years ago. Contenders for the earliest trilobites include Profallotaspis jakutensis, Fritzaspis spp., Hupetina antiqua and Serrania gordaensis. Trilobites appeared at a roughly equivalent time in Laurentia, Siberia and West Gondwana.
All Olenellina lack facial sutures, and this is thought to represent the original state. The earliest sutured trilobite found so far, occurs almost at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils. Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on their origins.
Three specimens of a trilobite from Morocco, Megistaspis hammondi, dated 478million years old contain fossilized soft parts. In 2024, researchers discovered soft tissues and other structures including the labrum in well-preserved trilobite specimens from Cambrian Stage 4 of Morocco, providing new anatomical information regarding the external and internal morphology of trilobites, and the cause of such extraordinary preservation is probably due to their rapid death after an underwater pyroclastic flow.

Divergence and extinction

Trilobites saw great diversification over time. For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished, and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period.

Evolutionary trends

Principal evolutionary trends from primitive morphologies, such as exemplified by Eoredlichia, include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium, and development of extreme spinosity in certain groups. Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments. Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures, and hypostome specialization. Several morphologies appeared independently within different major taxa.
Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, and the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details can make the determination of phylogenetic relationships difficult.

Cambrian

Although it has historically been suggested that trilobites originated during the Precambrian this is no longer supported, and it is thought that trilobites originated shortly before they appeared in the fossil record. Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—Redlichiida, Ptychopariida, Agnostida, and Corynexochida. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian; surviving orders developed isopygius or macropygius bodies and developed thicker cuticles, allowing better defense against predators. The Late Cambrian marks the beginning of the apex of trilobite diversity. The end-Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida and most Late Cambrian stocks became extinct. A continuing decrease in Laurentian continental shelf area is recorded at the same time as the extinctions, suggesting major environmental upheaval.
Notable trilobite genera appearing in the Cambrian include:
  • Abadiella
  • Buenellus
  • Judomia
  • Olenellus
  • Ellipsocephalus
  • Elrathia
  • Paradoxides
  • Peronopsis
  • Xiuqiella
  • Yiliangella
  • Yiliangellina
  • ''Olenus''

    Ordovician

The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites, with many groups appearing in the fossil record for the first time. Although intra-species trilobite diversity seems to have peaked during the Cambrian, trilobites were still active participants in the Ordovician radiation event, with a new fauna taking over from the old Cambrian one. Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors. The Phacopida and other "new" clades almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly. Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician, allowing many families to continue into the Silurian with little disturbance. Ordovician trilobites were successful at exploiting new environments, notably reefs. The Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Telephinidae and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites: very few entirely new patterns of organisation arose post-Ordovician. Later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction, vigorous trilobite radiation has stopped, and gradual decline is foreshadowed. The Ordovician marks the apex of trilobite morphological and species diversity.
Some of the genera of Trilobites appearing in the Ordovician include:
  • Cyclopyge
  • Selenopeltis
  • Parabolina
  • Cheirurus
  • Eodalmanitina
  • Trinucleus
  • ''Triarthrus''

    Silurian and Devonian

Most Early Silurian families constitute a subgroup of the Late Ordovician fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida. Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida. The Silurian diversity of trilobites was high during the Llandovery and Wenlock, though there was a sharp drop during the Pridoli at the end of the period, followed by a diversification during the Early Devonian, reaching a high point of 180 trilobite genera during the Emsian stage.
The Middle-Late Devonian was a decisive turning point in trilobite history, with the Taghanic event during the Givetian sharply decreasing trilobite diversity, particularly in shallow water environments, which was followed by the Late Devonian mass extinction/Kellwasser event at the Frasnian–Famennian boundary, widely regarded as one of the most significant mass extinction events in Earth's history, decimating the group's diversity including the extinction of the orders Corynexochida, Harpetida and Odontopleurida, with the low trilobite diversity in its aftermath in the Famennian, consisting only of the orders Phacopida and Proetida, being again strongly impacted by the Hangenberg event at the end of the Devonian, with both shallow water and deep water trilobites being affected. Only a single order, the Proetida, survived into the Carboniferous.
Genera of trilobites during the Silurian and Devonian periods include:
  • Dalmanites
  • Calymene
  • Encrinurus
  • Exallaspis
  • Paralejurus
  • Lioharpes
  • ''Phacops''