Trilobites

Introduction to Trilobites

Trilobites were among the earliest members of the Phylum Arthropoda, a group that includes a vast array of animals with hard exoskeletons and segmented bodies. Emerging during the Cambrian Explosion, a period of rapid evolution and diversification of multicellular life, trilobites are now extinct, but their legacy lives on in their modern arthropod relatives. These relatives include crustaceans like crabs and lobsters, chelicerates such as horseshoe crabs and spiders, and insects. By studying trilobites and their lifestyles, paleontologists can gain insights into the evolution of arthropod biology and ecology, and conversely, understanding modern arthropods can shed light on the lives of ancient trilobites.

Like many of their modern marine counterparts, trilobites were likely opportunistic scavengers, feeding on a wide range of organic matter found on and within seafloor sediments. Exceptionally preserved fossil specimens have revealed that trilobites preyed upon a variety of animals, providing a glimpse into their feeding habits. Notably, trilobites lacked mouthparts, suggesting that they may have been limited to consuming soft-bodied or thin-shelled prey. It is possible that some trilobite species used their legs to crush their food, similarly to some modern chelicerates and crustaceans. The presence of multi-chambered guts and strong digestive enzymes in trilobites indicates that they were well-adapted to digesting a diverse range of food sources. By studying trilobite ecology, we can gain a better understanding of the evolution of arthropod feeding strategies and the complex interactions between ancient organisms and their environments.

Trilobite Morphology

The name "Trilobite," meaning "three-lobed," refers to the trilobite body's distinctive shape, which is divided into three longitudinal lobes. These lobes consist of a central axial lobe that runs along the midline of the trilobite and two pleural lobes, one on each side.

Similar to other arthropods, trilobites have a segmented body that is divided into three main sections, or tagmata: the cephalon (head), the thorax (body), and the pygidium (tail).

The cephalon, or head, of the trilobite can be further subdivided into three parts:

The Glabella: The central part of the cephalon.
The Fixigena (Fixed Cheek): A fixed cheek area.
The Librigena (Free Cheek): A free cheek area.

**Figure 1:** An annotated illustration of Trilobite morphology. Labels: 1: Cephalon 2: Thorax 3: Pygidium 4: Left Pleural Lobe 5: Axial Lobe 6: Right Pleural Lobe. Image credit: Sam Gon III, derivative by Obsidian Soul on Wikimedia Commons.

Trilobites also possessed rigid, compound eyes that featured crystalline lenses, which often fossilized clearly. By studying changes in trilobite eye shapes throughout the fossil record, paleontologists have gained insights into trilobite lifestyle and evolution.

Although rarely preserved, the ventral (underside) view of a trilobite reveals features such as antennae and limbs. In rare instances, exceptionally well-preserved trilobite fossils may display these ventral features, providing a more comprehensive understanding of trilobite anatomy.

**Figure 2:** A photo and an ultra-macro photo of *Dalmanites vigilans*, found in the Silurian of Illinois, IUPC 101798. The lenses of this trilobite’s compound eye (magnified in inset) have been well-preserved.

**Figure 3:**The smallest trilobite specimen in the IUPC collection (*Peronopsis intestricta*, IUPC 101570, from the Cambrian of Utah, left) measures 3.51 mm long and 1.79 mm wide. It is next to the one of the largest Trilobite specimens in the IUPC collection (*Isotelus iowaensis*, IUPC 18400-7 from the Silurian of Missouri, right) which measures 120.39 mm long, and 72.65 mm wide. The cephalon of *Isotelus iowaensis* is facing the top-left of the image.

Trilobites exhibit a remarkable range of morphological diversity, encompassing various shapes and sizes. One of the largest trilobite specimens in the IUPC collection is Isotelus iowaensis, which measures 120.39 mm in length and 72.65 mm in width. In contrast, the smallest trilobite in the collection is Pronopsis interstricta, measuring a mere 3.51mm in length and 1.79 mm in width.

The proportions of the trilobite body parts, or tagmata, can also vary significantly. For instance, some trilobites have relatively small pygidia (tail sections) compared to their cephala (head sections), while others have pygidia that are equal in size or even larger than their cephala.

In addition to size and proportion variations, trilobites also display a range of morphological differences in other features. These include:

Genal (cheek) spines, which can vary in length, position, and number
Compound eyes, which exhibit a wide range of morphological variability, including:
- Lens separation and size
- Eye size
- Eye shape

These variations can be used to determine the age of certain trilobite specimens. Interestingly, some trilobite groups evolved to be eyeless, lacking any visual organs altogether. This diversity of morphology makes trilobites a fascinating group of ancient arthropods.

**Figure 4:** Mass extinction events are marked with stars; the majority of trilobite orders go extinct during a mass extinction. 1-Redlichiida, IUPC 8744-2 Olenellus sp., 2-Corynexochida, IUPC 101746 *Ogygopsis typicalis*, 3-Ptychoparriida, IUPC 101527 *Asphiscus wheeleri*, 4-Asaphiida, IUPC 18400-7 *Isotelus iowaensis*, 5-Odontopleurida, No specimen, illustration based off *Chlustina* sp., 6-Lichida, IUPC 13983-674 *Arctinurus boltoni*, 7-Angostida, IUPC 101570-101591 *Peronopsis interstricta*, 8-Harpetida, No specimen, illustration based off *Eoharpes* sp., 9-Phacopida, IUPC 16540 *Phacops* (tana?), 10-Proetida, No specimen, illustration based off Proetidae sp. Figure and illustrations by Alfredo Siqueiros-Gonzalez and Sierra Lopezalles.

The Orders of Class Trilobita

Trilobites were incredibly diverse and prolific during their time, with a success story that spanned the Paleozoic Era. As a class under the phylum Arthropoda, they were as distinct as classes Insecta and Arachnida. Class Trilobita comprises 10 traditionally accepted orders, which further branch out into 180 families, 5000 genera, and over 15,000 species. The following sections elaborate on these 10 orders, each with unique characteristics and morphologies that varied across different times.

1. Redlichiida (Cambrian)

- Characterized by large cephala with pronounced genal spines and segmented glabellas.
- Eyes are crescent-shaped.
- The thorax can have up to 90 segments, most of which are spiny, with a notable large spiny segment pair.
- The pygidium is small, typically with only one or two segments.

2. Corynexochida (Early Cambrian - Late Ordovician)

- Features a large cephalon with a large, long glabella and large eyes.
- Thorax typically restricted to about 7-8 segments, often spiny.
- Pygidium is shield-like, equal in size to the cephalon or slightly smaller, with diverse morphologies including plain and spinous types.

3. Ptychopariida (Early Cambrian - Late Ordovician)

- A large early group with unclear connections to other orders.
- Cephalon is broad with small genal spines; glabella has 3 small furrow pairs.
- Thorax typically has over eight segments.
- Pygidia are small in Cambrian members but larger in later members.

4. Asaphiida (Middle Cambrian - Late Ordovician)

- Represents a large proportion of trilobite diversity, with 1 out of every 5 species.
- Cephalon often includes large eyes and faintly furrowed glabella.
- Thorax usually has between 5 and 12 segments but can have as few as 2 or as many as 30.
- Pygidium is largely flat and broad, either slightly smaller than or exceeding the size of the cephalon.

5. Odontopleurida (Middle Cambrian - Late Devonian)

- Very spiny trilobites with cephala featuring large glabella and complex lobes.
- Eyes are small, and some suborders have additional frontal spines on the cephalon.
- Thorax segments number 8-13 and are very spiny.
- Pygidia are small but still spiny.

6. Lichida (Middle Cambrian - Late Devonian)

- Cephalon typically has a large broad glabella and genal spines; compound eyes are small.
- Thorax segments number 8-13 and are often spiny.
- Pygidia are typically either the same size or even larger than the cephala, ending with spinous tips.

7. Agnostida (Late Cambrian - Late Ordovician)

- Tiny trilobites, only a few millimeters long.
- Glabella has a wide base and tapers off; most genera are eyeless.
- Cephalon and pygidium are equal in size, both larger than the minute thorax, which has only 2-3 segments.

8. Harpetida (Late Cambrian - Late Devonian)

- Cephalon is large and semi-circular to ovular, often more than half of the trilobite’s surface area.
- Genal spines are large and broad, enveloping the thorax and pygidium.
- Typically more than 12 thorax segments.
- Pygidium is usually much smaller than the cephalon and short.

9. Phacopida (Early Ordovician - Late Devonian)

- A diverse order that originated in the Ordovician.
- Great diversity in cephalon glabellar morphology.
- Known for its unique schizochroal eyes (these contain many lenses with individualized cornea, allowing for much sharper vision in relation to holochroal eyes), and for holochroal eyes (these are found in all trilobites and consist of many lenses that share a cornea within a continuous membrane; detects light levels and movement).
- Thorax has 8-19 segments.
- Pygidia sizes are diverse, with various spine configurations.

10. Proetida (Early Ordovician - Late Permian)

- Smaller trilobites that survived until the Permian-Triassic Extinction Event.
- The only trilobites to survive the Late Devonian Mass Extinction.
- Cephalon has large glabella and genal spines; eyes were typically large.
- Thorax segments typically numbered 7-22.
- Pygidium is typically smaller than or nearly as big as the cephalon, spineless with 4-10 furrows.

The Emergence and Diversification of Trilobites

Trilobites made their debut in the global fossil record approximately 520 million years ago, during the Early Cambrian period. This marked the beginning of a significant surge in marine biodiversity, known as the Cambrian Explosion. Alongside trilobites, a plethora of new life forms emerged, including the first worms, chordates (such as Haikouichthyes), brachiopods, bivalves (clams), echinoderms, and other non-trilobite arthropods, including chelicerates and mandibulates.

In contrast, the preceding Ediacaran period (635-542 million years ago) was characterized by a dominance of soft-bodied invertebrates, such as sponges (Porifera) and jellyfish (Cnidaria). One theory proposed to explain the rapid diversification of complex marine organisms during the Cambrian period is an increase in atmospheric oxygen levels. This oxygenation may have enabled organisms, particularly invertebrates, to evolve more complex and larger body plans.

**Figure 5:**A map projection showcasing the surface of Earth during the Late Cambrian. Notice how the continents are highly disjointed, with some forming the supercontinent of Gondwana. The star on the figure indicates where the State of Indiana would be located at this time. Image credit: Scotese Map Project, Derivative by Antoine Archambault, The Burgess Shale Geoscience Foundation.

Trilobites exemplify this phenomenon, having developed a hard, mineralized shell and segmented legs that served as armor and facilitated movement, respectively. Additionally, they evolved compound eyes (a distinctive, although not unique, feature of trilobites) and segmented limbs that allowed them to crawl, swim, and interact with their environment. As a result, trilobites occupied a wide range of ecological niches, including:

Macrocarnivores: predators that fed on other large animals, including other trilobites
Detritivores: bottom-feeders that consumed detritus from the ocean floor
Scavengers: organisms that fed on dead and decaying matter
Filter-feeders: organisms that strained small particles from the water
Herbivores: plant-eaters

However, trilobites ultimately faced extinction, which occurred at the end of the Permian period, marking the end of the Paleozoic Era. Despite their demise, trilobites played a significant role in the evolution of marine ecosystems during the Cambrian and subsequent periods. Their diversification and eventual extinction provide valuable insights into the complex history of life on Earth.

**Figure 6:** A line graph displaying generic diversity of Class Trilobita through the Paleozoic Era (542-252 MYA). This figure is an approximation of true Trilobita biodiversity levels and is limited by the size of the dataset and taphonomic and collection biases. Source: Paleobiology Database (https://paleobiodb.org/navigator/) Restrictions: PALEOZOIC, TRILOBITA; Taxonomic level: GENUS; Temporal resolution: STAGES/AGES

Surviving the Paleozoic Mass Extinctions

Following their peak diversity during the Cambrian period, trilobites faced numerous mass extinction events throughout the Paleozoic era. These events included the Ordovician-Silurian, Devonian-Carboniferous (Mississippian), and Permian-Triassic extinctions. As illustrated in the graph, trilobite biodiversity declined from the Cambrian to the Ordovician, briefly rebounded during the Ordovician, and then continued to decrease during the Silurian.

The Ordovician-Silurian Extinction Event

Approximately 444 million years ago, the Ordovician-Silurian extinction event occurred, likely triggered by a period of global cooling. The formation of ice caps at the poles and a significant drop in sea levels led to the destruction of many shallow marine habitats of trilobites. This event marked a turning point in the history of trilobites, as they began a steady decline in their prevalence in Earth's oceans, never again achieving the levels of biodiversity seen during the Cambrian and Ordovician periods.

The Rise of Jawed Fishes and the Devonian Extinction

The Devonian period saw the emergence of jawed fishes, which further contributed to the decline of trilobites. The strong jaws of these fish allowed them to pierce through trilobite armor, as evidenced by bite marks on fossilized trilobites. Trilobites were also subject to another mass extinction event at the end of the Devonian period. A recent study by an interdisciplinary team at Indiana University Indianapolis found that this extinction was caused by a combination of high volcanic activity and the expansion of terrestrial plants (Smart et al., 2023). The increased volcanic activity raised CO2 levels in the atmosphere, while the growth of land plants led to soil runoff rich in elements like phosphorus and potassium into shallow coastal oceans. This resulted in decreased ocean pH and decreased oxygen levels, making it difficult for trilobites to survive.

Fossil Record Reflections

These trends in biodiversity are reflected in the fossil content of our collections. The majority of trilobite specimens in our collection are from Cambrian-aged sedimentary rocks, with relatively few specimens from subsequent Paleozoic periods. This mirrors the decline of trilobite diversity over time, as recorded in the fossil record.

The Demise of the Trilobites

Trilobites were a remarkably resilient and successful group of animals that had endured multiple extinction events throughout their existence. However, their reign ultimately came to an end during the devastating Permian-Triassic mass extinction event, which occurred approximately 252 million years ago. This catastrophic episode, often referred to as the "Great Dying," was triggered by intense volcanic activity caused by the initial breakup of the supercontinent Pangea. The resulting climate change had a profound impact on global marine ecosystems, with ocean water temperatures soaring by an average of 8°C (14°F) and a drastic decline in dissolved oxygen levels. These extreme conditions proved fatal for an estimated 80% of marine life, including the trilobites. Despite their impressive 270-million-year history of adapting to changing environmental conditions, the trilobites were unable to survive this catastrophic event. There are no fossil records of trilobites existing beyond the Permian period, marking the definitive end of Class Trilobita.

A Legacy of Trilobite Research: A History of the Collection

The trilobite collection in the IU Paleontology Collections features an impressive array of specimens, many of which were collected by Charles Frederick Deiss, Indiana State Geologist and chairman of the Indiana University Department of Geology from 1945 until his passing in 1959. Prior to his tenure at IU, Deiss conducted research and taught at Montana State University, focusing on Cambrian stratigraphy and trilobite paleontology in the Northern Rocky Mountains. As part of his research, Deiss amassed and cataloged hundreds of trilobite specimens, which were subsequently accessioned into the IUPC collection.

A significant portion of the collection consists of over 800 casts of Cambrian trilobites created by Deiss. These casts are replicas of specimens housed in museums across the United States, including the Smithsonian Institution (USNM) and the University of Montana Museum of Geology (UMPC), as well as international institutions. For more information on the trilobite inventory, please refer to our Trilobite inventory.

The collection also boasts a notable connection of Charles D. Walcott, a renowned Secretary of the Smithsonian Institution. Walcott's groundbreaking discovery of the Burgess Shale Formation, a site celebrated for its exceptional preservation of soft-bodied organisms, has significantly advanced our understanding of early arthropods, including trilobites. Although only a few fossils in our collection originate from the Burgess Shale, Walcott's contributions to trilobite research render these specimens particularly unique and valuable holdings.

Trilobites in Indiana

Indiana's Paleozoic rocks offer a treasure trove for paleontologists and fossil enthusiasts, particularly when it comes to trilobite fossils. These ancient creatures can be found throughout the state, with a notable concentration in Ordovician rocks. While less abundant, trilobite fossils can also be discovered in smaller numbers in Silurian, Devonian, and Mississippian rocks, providing a glimpse into Indiana's rich geological history.

Figure 7: A map of bedrock ages and Trilobite locations in Indiana. Base map from Indiana Geological and Water Survey. Trilobite occurrences were downloaded from the Paleobiology Database on November 10, 2024, with the following parameters: time intervals = Paleozoic, environment = marine, Map extent = ([38, -94], [42, -76]), taxon = Trilobita.

Paleoart: Erbenochile erbeni

**Figure 8:***Erbenochile erbeni* from the middle Devonian. Art by Abigail Smith.

The trilobite Erbenochile erbeni roamed the oceans during the Lower to Middle Devonian period. One of its most remarkable features was its compound eyes, which granted it nearly 360° vision on a horizontal plane. These eyes were composed of hundreds of hexagonal calcite lenses, similar in structure to those of a fly. Each eye could contain up to 500 lenses, although the exact number varied depending on the specimen and preservation type. To reduce glare from the water's surface and sun, shades covered the eye lenses, suggesting that trilobites with well-developed eyes like Erbenochile erbeni may have been active during the day.

The trilobite's body was characterized by free-standing spines along its back, accompanied by individual plates of the axial lobe. It measured between 2 to 6 inches in length, from the cephalon (head) to the pygidium (tail).

The paleoart scene depicts Erbenochile erbeni in a Devonian setting, surrounded by other animals of the time. The trilobite is shown fleeing from a juvenile Orothocone, an extinct cephalopod with a straight, conical shell. Another Orothocone is visible in the background, while a large Dunkleosteus fish, the apex predator of the Devonian oceans, looms nearby. The scene also features smaller trilobites, corals, and other creatures. This artwork illustrates the dangers that trilobites faced during their reign in the Paleozoic era.

Photogrammetry

Video 1: Photogrammetry model of Trimerus (Dipleura) dekayi, IUPC 101784, found in the Middle Devonian Hamilton Group.

Video 2:Photogrammetry model of Calymene celabra, IUPC 101799 found in the Late Silurian (Niagaran) of Illinois.

Acknowledgements and Sources

- Undergraduate co-authors: Matthew Sullivan, Abigail Smith, Ren McCormack, Reece Leach and Alfredo Siqueiros-Gonzalez
- Graduate author and CBRC research assistant: Samantha Hartzell
- IUPC Collections Manager: Sierra Lopezalles
- CBRC Director: Claudia C Johnson

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