Introduction and biology

Trilobites were a very successful fossil group during the Palaeozoic Era. They appear in the fossil record at about 521 million years ago, and ultimately went extinct at 252 million years ago, having struggled since the Devonian.

We'll start by looking at the biology of the trilobites, what other animals they are related to, and when they lived.


Key points to take away from this video are:

  • Trilobites are arthropods (members of a group defined by having segmented bodies, and jointed limbs).
  • Arthropods are a megadiverse group, and trilobites were very speciose in the early palaeozoic.
  • They had a variety of modes of life, which you can identify through their morphology.
  • Trilobites grew by moulting their exoskeleton: a process called ecdysis.

Meet your first trilobite

Below you can find a 3D model of a trilobite.

Load the model and take a few minutes to look at its anatomy. In the next video we're going to be learning the terms for the different elements, so before starting that it would be good to make sure you know how the different bits and pieces relate to each other. Usefully a few of the basic ones are included in this model, so feel free to click on them, and check out the names.

Specimens of Homotelus bromidensis an Ordovician trilobite. These were found in the Bromide Formation of Carter County, Oklahoma. They are housed by the Paleontological Research Institution, Ithaca, New York (specimen number PRI 45505). Longest dimension of this piece of rock is ~14 cm.


Now we have an idea of what trilobites are, we can learn more about their anatomy. This is what we are going to cover in the next video. I've provided, on blackboard, an image of a trilobite. I suggest that as the video goes along you pause it when required, and label the bits on either a digital or physical copy of that image.

In particular, the terms I introduce are:

  • Cephalon
  • Thorax
  • Pygidium
  • Pleural lobe
  • Axial lobe
  • Glabella
  • Eyes
  • Fixed Cheek
  • Free Cheek
  • Facial suture


Key points to take away from this video are:

  • Trilobites are split into three major divisions of their body from front to back: cephalon, thorax and pygidium.
  • On the cephalon you find a raised area, the glabella, eyes, and a cephalic suture, to allow moulting.
  • The thorax comprises between 2 and ~100 segments that are similar in shape, but vary a little in size.
  • These articulate, allowing the trilobite to enroll.
  • The pygidium is made of a series of fused segments, and forms the posterior of the animal, but can vary greatly in size compared to the other bits.

Meet your second trilobite

Below you can find 3D models of two more trilobites.

Load the first model and take a few minutes to look at the animal's anatomy. Make sure you can differentiate all the features that I have mentioned. If it helps you, in terms of learning the features, please feel free to make sketches.

This trilobite is the species Flexicalymene meeki. It is from the Ordovician Ft. Ancient Formation of Brown County, Ohio, and is on display at the Museum of the Earth, Ithaca, New York (specimen number PRI 41460). It is approximately 3.5 cm in length.

This is the same species, but here is is enrolled. Close the above model, then open the one below. How might we tell this is the same species?

Enrolled Flexicalymene meeki also Ordovician, from Cincinnati, Ohio (PRI 70754). Specimen housed in the Paleontogical Research Institution, Ithaca, New York. Maximum diameter approx 2.5 cm.

I've got to be straight with you all, the question above was a bit mean. What a species means in the fossil record is actually a really difficult question, to which there isn't an easy answer.

A quick quiz

That was pretty cool, right? Do you think you spotted everything? In particular, one of the harder features to make out was the facial suture. If you didn't spot it, perhaps go and load the first model again and see if you can. If you want a pointer - or to check you're correct - expand the item below.

Show me that suture

Here you can see the suture marked in pink (right; image of Flexicalymene meeki by James St. John, Ohio State University).

To test your knowledge there is a quick quiz below.

Why should we care?

Because they were so diverse, and appear early in the fossil record, trilobites are of great use to geologists, in a wide range of contexts.

We finish by looking at what trilobites look like in a rock, and in section, and then highlighting why knowing about trilobites is important.


Key points to take away from this video are:

  • Trilobites can tell us about the age, and depositional environment (for example, the water depth) of a rock.
  • The same is true of trilobite traces, which are called Cruziana. The study of trace fossils is called ichnology.
  • Fossils are key pieces of evidence for dating sedimentary rocks. Using them to correlate rock ages is biostratigraphy, as we touched on in the introductory videos.
  • Trilobites are very important for Cambrian biostratigraphy, and also for understanding the spatial relationships of past continents (palaeobiogeography).

Now meet a trace fossil

This is what Cruziana looks like! This one is quite a big model, so it may take a small while to load.. But it should be worth the wait. These locomotion traces can be created by a range of different groups. But in the Palaeozoic we think many are the result of trilobite locomotion.

A specimen of Cruziana from the western Nefud Desert, Saudi Arabia, thought to be Cambrian. Courtesy of The University of Queensland's School of Earth and Environmental Sciences.