Rock Discoveries

Unlocking the Enigma of Agnostida Trilobites

Introduction to Trilobites

Trilobites are one of the most fascinating creatures to have ever existed. They roamed around the oceans during the Palaeozoic Era, alive for over 270 million years before abruptly going extinct.

Luckily, we can still discover these enigmatic creatures today through their fossils. In this article, we will delve into the exciting world of Trilobites, learning how to identify different species and how they are classified into orders.

Get ready for a journey back in time!

Trilobite Fossils

Fossils provide us with a gateway into the past, allowing us to see what once lived and breathed on our planet long before our time. Trilobites are no exception.

Their hard exoskeletons preserved in the fossil record allow us to see their impressive armour and the imprint of their delicate legs. These certain types of fossils are often found in shale and limestone, as these types of rocks have fine-grained sediment and are less prone to erode the fossils.

Species Identification

Trilobites come in many shapes and sizes, with over 20,000 species discovered so far. Their exoskeletons boast a wide variety of patterns, which present challenges for discerning different species.

One of the most evident ways to differentiate species is by looking at their eyes, which differ in size, quantity, and arrangement. Another feature that can help with species identification is the number of thoracic segments, each of which has a pair of legs.

Some traits that cannot be seen on the exoskeleton, however, must be distinguished from the internal anatomy or by the structure of the metamorphosis. Thus, the ID of Trilobites can be challenging for novice palaeontologists.

Orders Classification

To organize the vast number of Trilobite species, palaeontologists have sorted them into ten orders, each with distinct body shapes, leg structures, and features. The first three orders, called the


Corynexochida, and

Ptychopariida, are considered “primitive” types of Trilobites.

They were mostly found during the early Cambrian and have simpler exoskeletons. The next group, the

Asaphida, is characterized by large jaws.

The fifth group is called the Odontopleurida, and it stands out due to its backward-facing spines on the thorax. The following orders, the Harpetida, Calymenida, Lichida,

Phacopida, and

Proetida, were the most common and diverse in the Silurian and Devonian periods.

Their dissection and examination contributed a lot to the knowledge of morphogenesis and the evolution of the early life-forms.


Ptychopariida is a primitive order of trilobites distinguished by their convex or flat exoskeletons. The exoskeleton is made up of many plates, allowing the ptychopariid to thrive in different sea environments.

They were widespread and long-lived throughout the Cambrian period, but most ptychopariid fossils are found in North America. These creatures were small, with an average length of 1.5 cm, so it is not surprising that they had limited mobility.

Basic Body Plan

Ptychopariids have a basic body plan consisting of three lobes; the axial lobe down the centre called the axis, and two lateral lobes. The head shield is wider than the tail shield, with a row of paired spines or tubercles along the thorax.

The lobes present in the cephalon and pygidium have undifferentiated axial blades. They have a well-defined, strongly bulging glabellar lobe (or cranidium in other taxa) and the eye line has fewer than ten eyes.

Small free cheeks cover the thorax, which consists of nine segments. The pygidium also has three lobes, such as the axial lobe, and two lateral lobes with an anal shield anterior, and a posterior border.

The internal anatomy of ptychopariids remains mysterious, and no complete bodies have been found to date.

Common Species

Some of the most common species of ptychopariids include Ptychoparia and Olenellus. Olenellus has been found in rocks in both North America and Greenland, while Ptychoparia’s fossils have mainly been found in North America.

Scientists find identifying species tricky. For instance, there is some confusion about whether there are one or two Olenellus species, as O.

gilberti and O. thompsoni have slight differences in their anatomy.

Geological Areas

North America is the superior continent known for rich Trilobite concentrations, with many geological areas bearing fossil-rich sediments, such as the Montagne Noire in southern France and some parts of China. Despite the lack of documentation, we believe that they are quite significant for the study of ptychopariids.

The stipples of the mass extinction that wiped out the majority of Trilobites are the last traces of the creatures that once thrived on Earth. In conclusion, Trilobites present fascinating organisms to study, particularly regarding their vast variety of species, orders, and their ample body of scientific literature.

Understanding their morphology, functioning, and classification provides a deep dive into the prehistoric world to unravel a broader picture of the evolution of arthropods and life through time.


Phacopida is an order of trilobites that flourished from the Middle Ordovician, around 470 million years ago, to the end of the Devonian Period, around 350 million years ago. During this time period, the

Phacopida trilobites diversified into a wide range of species and were very successful, making up a significant proportion of the trilobite diversity in the Late Devonian.


One of the most distinctive characteristics of

Phacopida is their planar shape. This flattened shape was the ideal adaptation for life in shallow, muddy or sandy environments.

They grew to an average length of 8-12 cm and had large convex eyes that provided them with excellent vision. The exoskeletons of

Phacopida were covered in small bumps and lines, a characteristic known as pitting or reticulation.

These bumps and lines served to strengthen the exoskeleton and provide increased surface area for attachment of muscles.

Phacopida trilobites also had a pair of distinctive spines that protruded from the rear of their pygidiums, which are the tail segments.

Common Species

Phacopida trilobites are known to be found all over the world, but they are most commonly found in North America, Europe, and China. Some common species include Phacops rana, Phacops coryndon and Phacops latifrons.

P. rana is one of the most famous trilobite species of all time and is often referred to as the “Celebrity Trilobite” because of its ubiquity in collections worldwide.

P. coryndon is known to have a pygidium with an extended spine, often that is longer than the entire body.

P. latifrons has a distinctive nose-like projection that protrudes from the front of its head shield.

Collecting Considerations


Phacopida trilobites can be a thrilling experience, but there are a few things to take into consideration when starting your hunt. Firstly, you should make sure it’s legal to go fossil hunting in your chosen location.

Most public areas prohibit the collection of fossils; therefore, it’s essential to research the laws and regulations of the area in which you plan to collect. If you’re interested in finding your own

Phacopida fossils, rock quarries, shale pits, and outcrops are good places to start.

Make sure to bring a hammer, chisel, brush, and small containers to store your finds safely. Preserve the fossils by wrapping them in paper to protect them from jostling during transportation and storage.

Finally, label your fossils with their collection date, location, species name, and other relevant information.



Corynexochida is an order of trilobites that have been discovered in rocks dating back from the Early to Late Ordovician, which makes them one of the oldest orders of trilobites.

Corynexochida are recognized by their sharp spines that protrude from their exoskeletons, which varied in shape and size depending on the species.

Spiny Exoskeletal Shells

Corynexochida trilobites have incredibly sharp and intricate exoskeletons that feature spines covering most of the body. These spines provided both protection from predators and an opportunity to react to them by defensive motion.

The spines that protruded from

Corynexochida were a defining feature of the Order and provided essential defense mechanisms necessary for the species’ success. The spine also extended from a slightly curved pygidium than the cephalon, which was peculiar compared to most other trilobites.

Common Species

Corynexochida trilobites have an extensive range, and they are primarily found in rocks from the Ordovician, spanning from approximately 485 – 440 million years ago. Some common species from the

Corynexochida order include Corynexochus, Ceraurus, Glabrella, and Dimeropygiella.

C. cornutus is one of the most well-known species from the

Corynexochida Order and has well-defined and decorated spiny exoskeletons.


Trilobites were among the most diverse and successful groups of animals that lived on Earth, but they went extinct around 252 million years ago. The

Phacopida and

Corynexochida orders were among the most well-known and distinctive orders of trilobites, known for their flattened bodies and spiny exoskeletons, respectively.

They are both fascinating creatures that can teach us a lot about the evolution of life on earth. While collecting fossil samples, make sure to abide by the appropriate rules and regulations in the area, and always handle fossils with care, keeping track of the important information.


Asaphida is an order of trilobites that existed during the Late Cambrian to the Early Carboniferous Period. They have a unique feature – the ventral median suture, which is a deep groove that runs along the middle of their underside.

This trait distinguishes

Asaphida from other orders of trilobites, and its significance will be discussed in this article. We will also cover some common species of

Asaphida and learn briefly about the primitive trilobite species.

Ventral Median Suture

The ventral median suture of

Asaphida is a deep groove that separates the exoskeleton into two halves, left and right. This feature helped give

Asaphida trilobites greater flexibility when walking and allowed for greater mobility when burrowing.

The ventral median suture is also an essential diagnostic tool for identifying

Asaphida trilobites and has been used extensively by paleontologists to understand their anatomy.

Primitive Trilobite Species

Asaphida order includes some of the most primitive trilobites. They date back to the Late Cambrian period, around 500 million years ago, and are considered to be some of the first trilobites that evolved.

These species have primary morphology, with simple, oval-shaped exoskeletons lacking the complex ornamentation seen in later trilobites. Some of the primitive species in the

Asaphida order include Remopleuridoides, Isotelus, and Elrathina.

Common Species

Asaphida have a vast number of species, with some of the most common ones found in North America. They were most extensively distributed during the Ordovician to Devonian periods.

Some examples include Asaphus kowalewskii, Asaphus raniceps, and Greenops widderensis. Asaphus kowalewskii is known for its remarkable size, reaching up to 40 centimetres in length.



Redlichiida is one of the first orders of trilobites. They first appeared in the fossil record during the early Cambrian, approximately 541 million years ago.

They were one of the earliest and most successful groups of trilobites and are known to be quite diverse. Here are some features of the

Redlichiida order;

Early Appearance in Fossil Record


Redlichiida is the earliest known order of trilobites. They first appeared during the early Cambrian period, about 541 million years ago, and diversified rapidly during what is known as the Cambrian Explosion.

Their diversity decreased considerably during the later stages of the Cambrian period, and they eventually went extinct during the Ordovician.

Common and Large Species

The most common species of the

Redlichiida are found in the genus Paradoxides, which are known for their large size, elongated bodies, and distinctive shield. These trilobites would have grown to be over 60 centimetres long, making them some of the largest arthropods that ever existed.

Other common species of

Redlichiida include Olenellus, Kootenichelus, and Primaspis.

Beware of Fabrications

Due to their historical significance, many

Redlichiida trilobite fossils have been fabricated or altered over time. Fabricated specimens can be challenging to identify and require meticulous examination.

Paleontologists must study the fossil for minute details or inconsistencies that wouldn’t be present in naturally occurring specimens. Therefore, it’s essential to ensure the trilobites you purchase or collect come from a legitimate source and that you’re not contributing to the sale of fraudulent specimens.



Asaphida and

Redlichiida orders of trilobites played a significant role in the evolution of life on Earth. The presence of the unique ventral median suture makes

Asaphida easy to identify, while

Redlichiida is known for their large size and early appearance in the fossil record.

The availability of their fossils has allowed us to learn so much about the biodiversity and evolutionary history of these creatures. It’s important to take precautionary measures when collecting or purchasing their fossils, ensuring that they are legitimate specimens.


Proetida is a diverse order of trilobites that thrived from the early Ordovician to the end of the Devonian Period, about 471 to 359 million years ago. They were one of the most successful orders of trilobites, but unfortunately, they went extinct during one of the five mass extinctions that occurred in Earth’s history.

In this article, we will cover the extinction, characteristic and common species of



Despite their success,

Proetida began to decline during the Late Devonian period, which may have been caused by several factors. Changes in climate, ocean chemistry, and food availability, combined with increased competition from other marine arthropods, likely contributed to their eventual extinction.

The exact cause of their extinction is still unknown.


Proetida trilobites had a characteristic shield-like exoskeleton that was often heavily ornamented with spines or tubercles.

Proetida were a unique order of trilobites, with distinctive features that set them apart from other orders.

Their three-part exoskeleton featured a glabella, visible between the eyes on the head, followed by the thorax, which contained the legs, and a rounded or wedge-shaped pygidium, the tail end. Their exoskeleton was typically longer than it was wide, and they had few spines.

They grew to an average length of four to five centimeters, about the size of your palm.

Common Species

Proetida was a diverse order of trilobites, with many species known from all around the world. Some of the most common species include Phacops rana, Trinucleus fimbriatus, and Acanthopyge (Diacanthaspis) carinata.

Phacops rana was a widespread species that lived around four hundred million years ago, and its fossils are frequently found in the United States and Europe. Trinucleus fimbriatus is arguably one of the most distinctive and attractive trilobites, with long spines along the body.

Acanthopyge (Diacanthaspis) carinata is well known for its sharp and spiny body armour.


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