Trilobites (meaning "three lobes") are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites also form one of the earliest known groups of arthropods.

The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except Proetida died out.



Largest Known Trilobite Species

Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among one the most successful of all early animals, roaming the oceans for over 270 million years. The group Trilobita existed from early in the Cambrian Period (520 million years ago) until the end of the Permian Period (250 million years ago).

The name Trilobita is derived from the three (tri-) lobed structure of the exoskeleton, which has a raised central lobe (or axis) and a pair of side lobes, called pleurae. The trilobite body is also divided lengthwise into three regions or tagmata: a head or cephalon, a middle region (thorax) composed of several to many articulated segments, and a tail plate called a pygidium, which consists of fused segments.

Despite a quarter billion years since their extinction, the zoological affinities of trilobites can be determined from structures preserved in fossils. Before the first trilobite with its legs fossilised was described in 1870, it was already well established that Trilobita belonged to the Arthropoda. Arthropods are the most species-rich animal phylum living today, and have been the most diverse group of multicellular animals since the Early Cambrian.


Immense diversity of the Trilobites

Their main subgroups are the crustaceans (like prawns and crabs), the chelicerates (including spiders, scorpions, and mites), and the wholly terrestrial groups Insecta and Myriapoda (the latter including millipedes and centipedes). Trilobita is the most species-rich entirely extinct arthropod group. In trilobites, the hard exoskeleton covering the dorsal body surface and its well marked segmentation (for example, the jointed segments of the thorax) are classic arthropod features. The trilobite exoskeleton was mineralised, constructed of calcite. The trilobite hypostome, a plate attached to the lower side of the head just in front of the mouth opening, corresponds to a similar structure (the labrum) in other arthropods.

A pair of compound eyes is developed in most trilobites, with the arrangement of their units (ommatidia) being typical of Arthropoda (e.g., compound eyes in horseshoe crabs, crustaceans and insects). Trilobites periodically shed their exoskeleton to accommodate growth; trilobite fossils sometimes preserve so-called moult configurations that show various stages in the release of the old exoskeleton and escape of the then soft-bodied animal. Moulting is another diagnostic feature of the arthropods. In most trilobites moulting was accomplished by splitting the head shield along lines of weakness (called facial sutures) that run along the visual surface of the eye.

That trilobites are arthropods is beyond doubt, but the exact position of Trilobita in the evolutionary tree of the arthropods is more controversial. Early workers took the geological antiquity of trilobites as evidence that they were the most primitive kind of arthropod, and may have included the ancestors of crustaceans and chelicerates. A single pair of antennae is likely a primitive feature for all arthropods, and the similarity of leg structure along the trilobite body (e.g., without the specialised leg-derived mouthparts of crustaceans or insects) can also be interpreted as primitive. Most recent workers consider that, among living arthropods, the closest relatives of trilobites are the chelicerates. The similarity of these groups may not be obvious when we make comparison with the land-dwelling spiders, mites, or scorpions, but becomes more apparent when we examine the most primitive living chelicerates, the horseshoe crabs.

Trilobites, horseshoe crabs and sea scorpions have similar spine rows along the inner margin of their legs. The lamellae on the outer leg branch of trilobites are similar (and thought to have the same evolutionary origin) as the filaments of the book gills of horseshoe crabs and book lungs of arachnids.

Trilobites had many life styles and species; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where there are still scientific debates). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

Recent fossil evidence found out more about their behavior whereas; the trilobites brutally biting each other on the Cambrian seafloor. New research has revealed that these armored predators didn't only hunt smaller and weaker animals for food, but would occasionally take bites out of their trilobite comrades of the same species. This finding represents the earliest evidence of cannibalism in the fossil record to date.

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In popular culture[]

  • Trilobite appeared in ARK: Survival Evolved.
  • Trilobite ponyo

    Trilobites on a sumbarine in Ponyo

    Trilobites make an appearance as background animals in the Ghibli movie Ponyo.