Trilobite

Trilobites are an exclusive form of marine animals that are a member of a group of extinct fossil arthropods (a large group of invertebrate animals in the greek language means joint-legged)  they are recognized from their distinctive feature as a three-lobed three-segmented form.

What is a Trilobite and how is There Only left of Trilobite Fossils?

The diversification of trilobites suggest that there were some trilobites which are active predators and others were scavengers and some of them ate plankton (algae, bacteria, protozoans, crustaceans, mollusks, and coelenterates) too.

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The trilobites Three-body lobes (lobe means clear anatomical division or extension of an organ that can be determined without the use of a microscope) from which two of them lay on each side of a longitudinal axial lobe.

The Trilobite Body is divided into three Distinctive Sections: 

1. Cephalon (head)

2. Thorax (body)

3. Pygidium (Tail region)

 Like all other arthropods, trilobites also had an external skeleton which is called the exoskeleton.

Trilobite Time Period: From dominating marine life to their extinction

At the beginning of the Cambrian period which was about 542 million years ago, trilobites started dominating the sea until they became less abundant in incoming geological periods. There is a trace of them until the upper Permian which is 251 million years ago. After that they went missing together with 90% of all species of earth.

Trilobites are highly diversified and geographically spread animals. Due to this, they left an extensive fossil record that has contributed a lot in understanding the evolution throughout the existence of the earth.

Let’s start with the Cambrian period when trilobites appeared as fully developed, from this we can say that their ancestors originate in Precambrian times during the Ediacaran period which was 630 million to 542 million years ago.

Ancestors to trilobites and other arthropods can be represented by Spriggina; their Fossils are known from the late Ediacaran period which means they existed in Precambrian times in a shallow-water marine deposit in Australia.

There is enough evidence that Spriggina which were present in the Ediacaran period almost 550 million years ago has Morphological similarities with trilobites Trilobite fossil.

Redlichiids and Ptychopariida are the earliest trilobite fossils found to belong to 540 to 520 million years ago. Most of the trilobites seem to emerge from present-day Siberia (a vast region of Russia and northern Kazakhstan).

They were the long-lasting animals with incredible diversification taken place over time. They fascinated everybody due to their evolutionary history and their adaptation towards nature to diversify themselves according to the extinct events that happen.

Started in the Cambrian and continued in the Ordovician periods and then they started declining in the Devonian period and then finally started extinction at the end of the Permian period.

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Pre-Cambrian

Early Cambrian suggest that a fauna related to trilobite existed before the tectonic breakup of subcontinent pannotia which was between 600 to 500 million years ago.

This breakup allowed the diversification and radiation which signify the antedates as the first appearance of trilobites in the fossil record.

Cambrian

The Cambrian was the period where trilobite was fully developed and start diversifying and they start developing with surviving orders that have thicker cuticles which allow them to become better at protecting themselves from predators so by this we can say that they were the animals which were very adaptable to change and crisis.

Genera of Trilobite in the Cambrian include:

• Abadilla (Lower Cambrian)

• Buenellus (Lower Cambrian)

• Judomia (Lower Cambrian)

• Olenellus (Lower Cambrian)

• Ellipsocephalus (Middle Cambrian)

• Elrathia (Middle Cambrian)

• Paradoxides (Middle Cambrian)

• Peronopsis (Middle Cambrian)

• Xiuqiella (Middle Cambrian)

• Yiliangella (Middle Cambrian)

• Yiliangellina (Middle Cambrian)

• Olenus (Late Cambrian)

Ordovician

The early years of this period were marked by vigorous radiation as many groups started appearing in the fossil for the first time. The peak of intra-species when trilobites diversified during the Cambrian and due to this trilobites were still active participants in the Ordovician radiation event.

Trilobites in the Ordovician period were successful in taking advantage of the new environments but still, some previously existing trilobites were successful in most of the Ordovician environment become extinct and with this, the Ordovician was the last period for trilobites where great diversification was taken place and after that post-Ordovician new pattern was seen and after that their declining phase was started.  

Genera of Trilobites Appearing in the Ordovician Period

• Cyclopyge (Early to Late Ordovician)

• Selenopeltis (Early to Late Ordovician)

• Parabolina (Early Ordovician)

• Cheirurus (Middle Ordovician)

• Eodalmanitina (Middle Ordovician)

• Trinucleus (Middle Ordovician)

• Triarthrus (Late Ordovician)

Silurian and Devonian

In the early Silurian, most of the late Ordovician trilobites survived. There was a lot of similarity between Silurian and Devonian trilobites and Ordovician trilobites.

They do not extend far in the Devonian and most of them were wiped out in the late Devonian extinction and at the end of the Devonian period, just a single order of Proetida Survived.

Genera of Trilobites Appearing in Silurian and Devonian Periods include

• Dalmanites (Early to Late Silurian)

• Calymene (Silurian)

• Encrinurus (Silurian)

• Exallaspis (Middle to Late Silurian)

• Paralejurus (Early Devonian)

• Lioharpes (Middle Devonian)

• Phacops (Middle to Late Devonian)

Carboniferous and Permian

It is still unknown how Proetida survived for millions of years even alone in Devonian and continued the survival in the Carboniferous period and lasted also at the end of the Permian period when most of the species were wiped out.

Proetida were diverse faunas both into deep and shallow water that maintained throughout the Carboniferous and they have no trouble living millions of years after that too.

Genera of Trilobites Appearing in the Carboniferous and Permian Periods

• Archegonus (Early to Middle Carboniferous)

• Hessle Rides (Middle Carboniferous)

• Endops (Middle Permian)

• Triproetus (Late Carboniferous to Early Permian)

• Ditomopyge (Late Carboniferous to Late Permian)

Although there is no clear evidence that trilobites become extinct after living millions of years and surviving and evolving so drastically and occupying and dominating the sea finally before ending the Permian period Trilobites become extinct. The fact that so many species evolved during this period and trilobites become extinct after surviving 300 successful years were unexpected.

Studies have shown that there were no direct descendants of trilobites. The closest we can get to trilobites was Chelicerata.

Trilobite Development

How trilobite was in its embryonic stage is unknown, as most of the arthropods trilobite was developed accomplishing various series of instars that are separated by molts but to take their new form they must shed their previous exoskeleton.

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Morphology: What is Trilobite Made of?

When the fossils of trilobites are found, their exoskeleton is the only part that was preserved and that too in an incomplete state but there were very few locations that could preserve soft body parts which were identifiable like legs, gills, musculature & digestive tract, and there were traces of other structures which indicates fine details of eye structure too.

Ranging from 3 millimeters to 30 centimeters fossils are found of trilobites with an average size range of 3–10 cm.  

The exoskeleton was made up of calcite and calcium phosphate minerals which cover the upper surface of the trilobite and it also covers the lower edge which produces a small fringe called the doublure (Lining)

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Cephalon or Head Section 

A trilobites cephalon is highly variable and complex. It looks like a heat shield composed of segments that contain sensory organs, eyes, antennae, and mouth and there is a special ventral plate which is called hypostome which functions as a mouth. The top three or four legs are associated with Cephalon.

Thorax

The thorax presented in the trilobite body is to protect the inner soft organs and it is articulated segments that lie between cephalon and pygidium as trilobites are very diverse creatures the segments can vary between 2 to 103.

Each segment in a trilobite body consists of the axial ring in the center and outer pleura  (thin layer of tissue that covers the lung area and interior wall of the chest cavity).

Helps to protect limbs and gills and they sometimes extend and take the form of long spines.

At many times it was hard to differentiate thorax and pygidium and many segment counts suffer from this issue. 

Volvation 

Most of the fossils found are curled up (enrolled) which they used to protect themselves. Research suggests that enrollment helps trilobites to protect against inherent weakness. Some of the trilobites have diversified themselves into achieving fully closed capsule and other have long pleural spines

Pygidium

Pygidium has several segments which are similar to thoracic segments but have not been articulated 

We can distinguish trilobites based on the pygidium and categories them as :

• pygidium smaller than cephalon (micropygous)

• pygidium subequal to cephalon  (subisopygous)

• pygidium equal in size to cephalon (isopygous)

• pygidium larger than cephalon (macropygus)

Soft Parts of Trilobites includes

• Appendages

• Digestive tract

• Internal organs

• Musculature

Sensory Organs of Trilobites includes

• Antennae

• Antennae

• Sensory pits

Trilobites are a very well-known fossil ever found and also the second famous fossil after dinosaurs. They were very abundant and dominating in marine life during the Cambrian period and survived through all those millions of years and finally got extinct in the Permian period. Their fossils are found worldwide with thousands of species and that helped geologists to date the age of the rock. 

Trilobites that have survived the Cambrian period developed the ability to roll up to defend themselves from predators. At the end of the Ordovician period where trilobites were on the verge of extinction, then at that time they had to restrict themselves into deeper water only.

From the above, it is clear that trilobites have survived long enough to prove that they might be extinct creatures now but they are adaptable throughout so many mass extinction events that’s what made them remarkable. They have survived more than dinosaurs which were more than 250 million years ago and they have dominated the seafloor with so many diverse varieties.

That’s not it if you found a rock from the Cambrian period which is 500 million years ago and almost every fossil is of trilobite that they have managed to fossilize themselves very well due to their exoskeletons.

The moment trilobites fossils were found they have fascinated the human mind and answered a lot of evolutionary concepts that in order to survive one needs to adapt to the changing earth as trilobites did.