Chapter 7: Fossils

Learning Objectives

The goals of this chapter are to:

  • Recognize specimens of the most common invertebrate fossils
  • Classify the phyla, order, and/or class associated with common fossils
  • Compare and contrast symmetry in fossil specimens

Chapter Notes: The main text and much of the imagery from this chapter come from the Digital Atlas of Ancient Life (CC BY-NC-SA), which is managed by the Paleontological Research Institution. Exercises in this chapter will heavily rely on sketching, so be sure to review the best practices for sketching from Chapter 0.

7.1 Introduction

Life on Earth has been around for a very long time, at least 3.5 byrs, and organisms have continually evolved and become extinct during this time. Ever since the first fossil was found, paleontologists have been studying them to help decipher Earth’s history. Early Greek philosophers, such as Xenophanes (570-480 BC) and Herodotus (484-425 BC), recognized fossils as marine organisms. Ever since then, new discoveries are being found all the time; these change the way we tell Earth’s stories. For several centuries, fossils were the only tool geoscientists had to date rocks. Now, fossils are being used increasingly to tell other Earth stories such as the origins of life on Earth and possibly other planets, climate change and biodiversity, changes in evolution, massive and small extinctions, as well as understanding deep time.

Now, most paleontologists are paid professionals. However, there are a large number of citizen scientists who make important paleontological finds, such as a rock shop owner who showed a friend of ours, Jack Horner, a coffee can filled with tiny fossil bones. He realized that these were fossilized baby dinosaurs. When the two of them went out to where these were collected, Jack Horner discovered the first fossilized dinosaur eggs in the western hemisphere.  So, you never know if you’ll be the one to discover an interesting fossil that changes the way we think about the Earth.

Recently, paleontologists embraced the virtual world and use computer-aided visualization to look at the three-dimensional structures in fossils. From this, they can better look at the fossil’s internal structure. Just like when your doctor needs to look at your organs in detail, paleontologists use computed tomography (CT) with multiple x-ray scans through a fossil.

Figure 7.1 – Fossil Snake CT Scanning at Methodist Hospital in Houston, TX. Image credit: Houston Museum of Natural Science, CC BY-NC-SA.

Using CT images also helps with fossil preparation as paleontologists often spend hours carefully removing rock matrix from a fossil. If you have seen tribolite specimens, these are often prepared by micro air blasting with talc to remove the host limestone.  This can take from a few to tens of thousands of hours. CT tomography is much faster at digitally removing the rock matrix (Figure 7.2).

Figure 7.2 – CT images of Peltosaurus granulosus (AMNH FR 8138), a lizard that lived in the Oligocene (White River Fm., Wyoming) and was closely related to today’s Texas alligator lizard. The first image is a rendering of the specimen with matrix in left lateral view, and the second image has the matrix digitally removed. This is an excellent example of digital fossil preparation via CT, which takes a fraction of the time that physical preparation takes, and can be done without risk of damage to the specimen. The specimen is 56.4 mm long. Image credit: University of Texas High-Resolution X-ray CT Facility and the American Museum of Natural History (AMNH), CC-BY-NC.

Even with newer techniques, the basics of looking at fossils have remained constant for centuries. In fact, some dismiss paleontology as a dead science because simple observations that are the backbone of paleontology are not seen as credible as a test you do in a laboratory or model made with advanced machine learning. If you remember the skills that you have been using such as sketching, data collection, and map interpretation, you’ll do well studying fossils.

7.2 Taxonomy

Taxonomy sounds like a big word, but it’s just the term used to describe scientists classify things, which we’ve been doing throughout this lab manual (e.g. rock and minerals). The classification of fossils is one of the meeting points between geology and biology, and we follow the traditional biology taxonomy when classifying fossils (Figure 7.3). The major ranks are domain, kingdom, , , , , , and . For the fossils in this lab manual, we will focus primarily on the phylum rank, with some organisms at the class and order rankings.

Figure 7.3 – The taxonomic ranks of organisms with the red fox as an example. Rankings start broad at the top with Domain and become more specific toward Species. Image credit: Annina Breen, CC BY-SA.

Exercise 7.1 – Taxonomic Rankings

Using your phones, laptops, or tablets, look up the taxonomic ranking of the species in Table 7.1.

Table 7.1 – Worksheet for Exercise 7.1
Ranking Human Bornean orangutan Dog Rice
Species sapiens pygmaeus lupus familiaris sativa


You can take an entire course dedicated to paleontology, so we will only cover some of the more common fossils in the following pages. All of these fossils have detailed anatomies that are used to differentiate class, order, family, genus, and species. In this chapter, we want you to be able to identify common fossils at a broad level. A more detailed study is reserved for upper-level classes. Therefore, for most of these organisms, you will not see a detailed review of their anatomy.

Exercise 7.2 – Comparing Common Fossils

Your instructor will provide you with some common fossils. Create sketches of them and describe the differences that you can observe.

Sketch of first fossil: Sketch of second fossil:

What are the similarities and differences between these fossils?

The geologic record is full of fossils, from dinosaurs to plants to fish and everything in between. animals from the marine environment are the most common branch of fossils you will find because of their abundance and higher probability of fossilization versus land-dwelling organisms, and they will be the focus of this chapter. Table 7.2 contains a list of the major marine invertebrate phyla we will cover, and in some cases the class and order.

Table 7.2 – Phyla, classes, and orders of organisms to know for this chapter
Phylum Class Order Common Names
Cnidaria Anthozoa Rugosa, Tabulata, Scleractinia Rugose (horn) coral, tabulate coral, scleractinian coral (stony or hard coral)
Bryozoa Bryozoan
Mollusca Bivalvia, Cephalopoda, Gastropoda Clam, squid, snail
Brachiopoda Brachiopod
Arthropoda Trilobita Trilobite
Echinodermata Echinoidea, Crinoidea, Blastoidea, Sand dollar, crinoid, blastoid, starfish

7.3 Symmetry

A helpful characteristic in identifying fossils is the symmetry of the organism. Symmetry is an observable pattern in the external (outside) or internal (inside) structure of organisms that allows you to divide that organism into roughly equal parts that are mirror images of each other. For example, take the face of a human being which has a plane of symmetry down its center; an example of external symmetry. Internal features can also show symmetry, for example, the tubes in the human body are cylindrical and have several planes of symmetry. Some form of symmetry is common in most multicellular organisms.

There are several types of symmetry in biology, but the main ones you will see are , , and (Figure 7.4). Bilateral symmetry is a single plane that divides the organism into two equal, mirror-image halves. Radial symmetry has several subtypes, but they all describe lines of symmetry that be drawn through a central point. The common types of radial symmetry for fossils in this chapter are 5-fold radial symmetry (pentamerous) and 6-fold radial symmetry (hexamerous). Spherical symmetry is when you can draw a line of symmetry through a central point of the organism from any direction, so that no matter what the orientation of your symmetry line is you will always have two equal, mirror-image halves.

Figure 7.4 – Common types of symmetry. Image credit: Charl Hutchings, CC BY.

Exercise 7.3 – Identifying Symmetry in Fossils

Let’s get some practice identifying and describing the differences in symmetry. Your instructor will provide you with a selection of fossils that exhibit bilateral, radial, spherical, or no symmetry. On a separate sheet of paper, create a sketch of each fossil that highlights its symmetry. Be sure to draw the line(s) of symmetry on your sketch and indicate which type of symmetry each fossil has.

7.4 Phylum Cnidaria

Cnidaria is a phylum of marine organisms that includes over 11,000 species, including jellyfish, sea anemones, and coral; most have radial symmetry. The soft-bodies of jellyfish and sea anemones are not often preserved whereas the hard structures of corals are quite commonly preserved as fossils. In the geologic record, three orders of coral are the most abundant: rugose, tabulate, and scleractinian corals. Corals are carnivores, catching with their tentacles and pulling them into their mouths. Some corals also have a partnership with green algae that live within the coral. As the algae perform photosynthesis, some of the energy they create is transferred to the coral.

Order Rugosa

Rugose corals (Figure 7.5) are an extinct order of coral that originated in the and went extinct at the end of the Permian. Members of Rugosa are sometimes called horn corals because solitary forms frequently have the shape of a bull’s horn (if you like the Harry Potter movies, some say they look like the sorting hat). Rugose corals do have a less common, colonial form that does not have this shape.

Figure 7.5 – Two solitary rugose corals in a slab of Ordovician limestone from near Cincinnati, Ohio. Note the similarity of the left specimen’s shape to that of a bull’s horn. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

Rugose corals reached their peak diversity during the period, when colonial forms were important reef builders (Figure 7.6). As far as we know, rugose corals did not survive the end-Permian mass extinction event. Throughout this chapter, fossil diversity is plotted as geologic time versus fossil genera.


Figure 7.6 – Diversity of Rugosa genera. Image credit: Paleobiology Database, CC BY.

Rugose corals may be either solitary or colonial (Figure 7.7). A solitary coral individual is called a corallum while an individual within a colony is called a corallite. Rugose corals made their skeletons from calcite; this is a significant difference relative to other corals that make their skeletons out of aragonite.

The outer part of the corallum (or corallite)–that is, the skeletal wall–is called the theca (Figure 7.7). The theca may in turn be covered by an outermost skeletal sheath called the epitheca. Growth lines are often apparent on the epitheca; these are also called rugae (“ruga” is Latin for wrinkled), which gives this group of corals their scientific name (thus, rugose corals are the “wrinkled corals”). The top of the corallum (or corallite) is called the calice and it is the portion of the coral occupied by the living organism, called a polyp.

Figure 7.7 – Major features of solitary and colonial rugose corals; labeled features include a corallum, corallites, epitheca, calices, and growth lines. Left: Heliophyllum halli from the Middle Devonian Moscow Fm. of Erie County, New York (PRI 70755). Right: Acrocyathus floriformis from the Mississippian St. Louis Limestone of Monroe County, Illinois (PRI 70756). Both specimens are from the collections of the Paleontological Research Institution, Ithaca, New York. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

The surface of the calice is covered with radiating, vertical structures called (singular = septum) that resemble the spokes of a bicycle wheel (Figure 7.8). Septa sometimes extend through the theca, forming vertical lines called (singular = costa). The septa radiate outwards from a central pillar-like structure called the columella. In rugose corals, the septa are added during growth in a manner that ultimately divides them into four quadrants. Spaces (or, gaps) between the four quadrants are called fossula. It is important to note, however, that the fossula are not readily visible in many specimens, especially when they are poorly preserved.

Figure 7.8 – Important features of rugose corals, including a fossula, septa, the columella, and costae. Left: solitary rugose coral Heliophyllum canadense from the Devonian Onondaga Limestone of Mendon, Ontario (PRI 76809). Right: solitary rugose coral Lophophyllidium proliferum from the Pennsylvanian Graham Formation of Coleman County, Texas (PRI 76802). Both specimens are from the collections of the Paleontological Research Institution, Ithaca, New York. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

As a coral polyp grows upwards, it develops horizontal partitions called tabulae (Figure 7.9).

Figure 7.9 – Tabulae observed in hand samples of two solitary rugose corals. Left: Siphonophrentis halli from the Devonian Ludlowville Fm. of Skaneateles Lake, New York (PRI 76806). Right: Amplexizaphrentis pellansis from the Mississippian Pella Beds of Pella, Iowa (most of the epitheca is eroded away) (PRI 76803). Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

3-D Model 7.1 – Rugose Coral

This is a model of the rugose coral Heliophyllum halli from the Middle Devonian from Moscow. The growth lines and costae are clearly visible on the epitheca. Some septa are the only visible features on the calice.

Heliophyllum halli (PRI 70755)
by Digital Atlas of Ancient Life
on Sketchfab

More 3-D models of rugose corals can be found at the Digital Atlas of Ancient Life.

Order Tabulata

Tabulate corals originated in the Early Ordovician period and went extinct at the end of the Permian period. All tabulate corals were colonial, and some species were important reef makers during the and Devonian periods. Their skeletons were constructed primarily of calcite. Some tabulate corals look superficially like honeycombs (e.g., Favosites; Figure 7.10a), while others look like chain links (e.g., Halysites; Figure 7.10b) or collections of narrow tubes (e.g., Syringopora; Figure 7.10c). Others encrusted upon other marine invertebrates (including other corals).

Figure – 7.10 – Examples of three genera of tabulate coral. a) Favosites in dolostone from the Silurian of Michigan. Favosites have a distinctive honeycomb shape. b) Halysites from the Lower Carboniferous Boone Limestone near Hiwasse, Arkansas. These corals look like chain links. c) Syringopora corals look like narrow tubes. Image credit: a) James St. John, CC BY; b) Wikimedia user Wilson44691, Public domain; c) Wikimedia user Wilson44691, Public domain.

The following video from the Paleontological Research Institute gives a great introduction to tabulate corals.

According to the Paleobiology Database, there are a total of 58 families of tabulate corals, 376 genera, and 511 species. As demonstrated by the genus-level plot of their diversity through time (Figure 7.11), tabulate corals suffered badly during the Late Devonian extinction event and never again reclaimed their former levels of diversity. The group went extinct at the end of the Permian period.

Figure 7.11 – Diversity of Tabulata genera. Image credit: Paleobiology Database, CC BY.

A defining feature of most tabulate corals is the presence of structures called tabulae, which give them their name. Tabulae (singular, tabula; from the Latin for board or tablet) are horizontal plates that span across individual corallites (the spaces occupied by a single, living polyp) (Figure 7.12). They are deposited by polyps as they grow, separating the living animal from the space(s) that were occupied earlier in life. Remarkably, we know something about the soft polyps of tabulate corals due to the discovery of calcified polyps in specimens of Silurian Favosites from the Jupiter Formation of Quebec, Canada. This discovery was reported by Copper (1985), who reported that individual corallites typically had 12 tentacles, though some had 11 or 13. The discovery of these polyps also confirmed that Tabulata are indeed Cnidarians, rejecting the hypothesis of some earlier workers that this group belonged with the sponges.

Unlike rugose and scleractinian corals, most tabulate corals did not have septa. As a general rule, identifying whether or not a specimen of colonial Paleozoic coral has septa is a good indication as to whether it is a rugose coral (septa always present) or a tabulate coral (septa usually absent).

Figure 7.12 – Corallites and tabulae identified on a specimen of Favosites favosus. Note the absence of septa. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

3-D Model 7.2 – Tabulate Corals

This is a model of the tabulate coral Favosites favosus from the Silurian of Delaware County, Iowa (PRI 76737). It is the same one pictured in Figure 7.X. The length of the specimen is approximately 10 cm. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. The side view whos the tabulae and the top view shows the corallites.

Tabulate coral: Favosites favosus (PRI 76737)
by Digital Atlas of Ancient Life
on Sketchfab

More 3-D models of tabulate corals can be found at the Digital Atlas of Ancient Life.

Order Scleractinia

The order Scleractinia includes the “true corals” or “stony corals,” with about 1500 living species today. Scleractinians first appeared in the early Middle Triassic (Figure 7.13) and have been the dominant reef-building organisms over the past 240 million years.

Figure 7.13 – Diversity of Scleractinia genera. Image credit: Paleobiology Database, CC BY.

Scleractinian corals may be either solitary or colonial in form and always have skeletons composed of the aragonite. We are going to focus on the colonial forms of Scleractinia corals. An entire colonial coral colony is known as a corallum; the space occupied by a single polyp within the colony is called a corallite (Figure 7.14). Individual corallites have their own radial septa, which often attach to a centralized pillar called the columella.

Figure 7.14- Fossil specimen of the colonial scleractinian coral Astrangia sp. showing the corallites, septa, and columella. The Specimen is from the Late Pleistocene Ft. Thompson Fm. of Hillsborough County, Florida (PRI 76863). Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

The skeletons of colonial corals exhibit a much greater diversity of forms than those of solitary species. Often, their growth forms are dependent upon the habitats in which they live. Robust, rounded colonies are often favored in high-energy habitats with lots of wave action (Figure 7.15a), while delicate branching forms are usually associated with quieter environments (Figure 7.15b).

Figure 7.15 – Two types of Scleractinia corals. a) Stacked fossil specimens of the colonial scleractinian coral Siderastrea pliocenica from the Plio-Pleistocene of Lee County, Florida (PRI 76859). Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. b) Fossil specimen of the colonial scleractinian coral Oculina sarasotana from the Plio-Pleistocene of Lee County, Florida (PRI 76894). Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

7.4 Phylum Bryozoa

Bryozoans are filter-feeding invertebrates that can be found in both freshwater and marine habitats, where they are often easy to miss because of their small size (Figure 7.16). In almost all species, tiny (<1 mm diameter) bryozoan individuals, called zooids, live together as a colony that often encrusts surfaces, grows branching structures, or, in freshwater species, forms a gelatinous blob. They have bilateral symmetry. Despite their small size, marine bryozoans are abundant in many fossil assemblages thanks to the preservation of their hard calcium carbonate skeletons. To date, more than 17,800 species of fossil bryozoans have been described and more than 6,000 living species are known.

Figure 7.16 – A variety of bryozoan fossils from an Ordovician oil shale from Estonia. Image credit: Mark A. Wilson, Public Domain.

Bryozoan colonies have highly variable forms (Figure 7.17), from gelatinous blobs to upright branching structures and sheet-like encrusters, but the general morphology of a is similar across the classes.  Zooids can take several forms, but the most common forms in each of the classes are autozooids, which function in feeding the colony and excreting waste. Branching bryozoans may look similar to branching corals, but the zooids in bryozoa do not have septa or a columella as corallites do in corals.

Figure 7.17 – Common bryozoan fossils. a) fenestrate; b) branching; c) trepostome; d) archimedes. Image credit: compiled from James St. John, CC BY.

7.X – Phylum Mollusca

Mollusks are a phylum of marine invertebrates that comprise about 23% of all marine organisms. There are several classes of mollusks, but we will focus on gastropods, cephalopods, and bivalves. All have bilateral symmetry, but it’s more complicated for gastropods.

Class Gastropoda

Gastropods are one of the most evolutionarily successful groups of animals and include snails and slugs. They occupy the world’s oceans, freshwater lakes and streams, and terrestrial ecosystems, including many backyards. Some are algae-eating herbivores, while others are venomous hunters of fish. Their strong, single-valved shells (Figure 7.18) have left behind a rich Cambrian to Recent fossil record, the focus of many paleobiological studies (Figure 7.19). These shells are primarily made of calcium carbonate. Gastropods have many different shell shapes, and some with no shell, but one of the defining characteristics of gastropods is the rotation of their body during their development; a process called torsion. Torsion does not refer to the coiling shells you are familiar with and only relates to the gastropod’s coiled body. Prior to rotation, gastropods exhibit bilateral symmetry, but they lose that symmetry as they mature.

Figure 7.18 – A variety of gastropod shell shapes. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.
Figure 7.19 – Diversity of Gastropoda genera. Image credit: Paleobiology Database, CC BY.

Class Cephalopoda

Cephalopods are marine organisms that have bilateral body symmetry, a prominent head, and a set of tentacles. You may be familiar with squids and octopuses. While most modern cephalopods are completely soft-bodied or have only thin internal shells, their ancient shelled cousins left behind a rich fossil record with a remarkable diversity of shell shapes. Cephalopods became abundant during the Ordovician Period (Figure 7.20), and many went extinct by the end of the Mesozoic Era.


Figure 7.X20 – Diversity of Cephalopoda orders. Image credit: Paleobiology Database, CC BY.

One of the most well-known index fossils for cephalopods is the belemnite (Figure 7.21a). The belemnite is an extinct order of mollusk that only lived during the Mesozoic. It had a characteristic elongated, cone-shaped shell, called a guard (Figure 7.21b). The guard is thought to have been a counterbalance that allowed the belemnite to move horizontally through the water.

Figure 7.21 – Belemnite fossils. a) The entire organism, including the guard and carbonized and preserved soft parts. The cone-shape on the left side is the guard. Please note it is extremely rare to find any fossilization of the soft parts. b) Belemnite guard. Image credit: a) Wikimedia user Ghedoghedo, CC BY-SA; b) California Academy of Sciences, CC BY-NC-ND.

Other well-known cephalopod index fossils are from the subclass Ammonoidea, commonly referred to as ammonites (Figure 7.22). They are extinct mollusks that lived from the Devonian to the end of the Cretaceous (Figure 7.23). These are ideal for biostratigraphy as there are over 8,000 species of ammonoids and they occur a large geographic range.

Figure 7.22 – Examples of different shapes of ammonoid shells. Image credit: Digital Atlas of Ancient Life, Jonathan R. Hendricks, CC BY-NC-SA.
Figure 7.23 – Diversity of Ammonoidea genera. Image credit: Paleobiology Database, CC BY.

One of the ways to distinguish orders of ammonoids is the suture pattern of their shells (Figure 7.24). Ammonoid sutures fall into three main groups: goniatites, ceratites, and ammonites. As you go from goniatites to ceratites to ammonites, the suture patterns become more complex. Goniatitic sutures do not have subdivided saddles or lobes. Saddles are convex toward the opening of the shell, lobes are convex in the other direction. Ceratitic sutures have subdivided lobes, but undivided saddles. Finally, both the saddles and lobes of ammonitic sutures are subdivided, sometimes in an amazingly complex fashion.

Figure 7.24 – Examples of ammonoid order sutures: goniatites, ceratites, and ammonites. Image credit: Digital Atlas of Ancient Life, Jonathan R. Hendricks, CC BY-NC-SA.

Class Bivalvia

Bivalves refer to a diverse class of molusca that typically have bilateral symmetry where the upper shell and inner parts of the organism are a mirror image of the bottom half. Clams, oysters, and mussels belong to this class. Their shells are made of calcium carbonate and typically have growth lines (Figure 7.25). Bivalves first appeared in the fossil record in the and are still around today (Figure 7.26).

Figure 7.25 – Examples of bivalves. a) Placopecten clintonius (a type of scallop) from the Pliocene of North Carolina; b) Exogyra costata (a type of oyster) from the Cretaceous of South Carolina; c and d) two views of Idonearca vulgaris (a type of clam) from the Cretaceous of Tennessee; e) Rastellum carinatum (a type of oyster) from the Cretaceous of Texas. Image credit: all images from James St. John, CC BY.
Figure 7.26 – Diversity of Bivalvia genera. Image credit: Paleobiology Database, CC BY.

7.X – Phylum Brachiopoda

Brachiopods are shelled, filter-feeding, marine organisms that have been around since the Cambrian (Figure 7.27) and are still around today. They inhabit the seafloor and come in a variety of shapes and sizes.

Figure 7.27 – Examples of brachiopods. Image credit: Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

Superficially, brachiopods may look like bivalves, but the two are not related. One of the biggest differences between brachiopods and bivalves lies in their symmetry. Both have bilateral symmetry, but the plane of symmetry in brachiopods is vertical rather than horizontal (Figure 7.28). What this means is that the left half of a brachiopod is a mirror image to the right half. In bivalves, the plane of symmetry is horizontal and the upper half is a mirror image of the lower half.

Figure 7.28 – Differences in symmetries between a brachiopod (left) and a bivalve (right). Image credit: Jaleigh Q. Pier, CC BY-SA.

Brachiopod shells have two valves that are distinct in both size and shape (Figure 7.29). The brachial valve is usually the smaller of the two valves and has a ridge, or fold, down the middle of the valve. The pedicle valve is usually larger than the branchial valve and has a valley down the middle. It also has a hole through which the pedicle passes. The pedicle is a fleshy, stalk-like feature which some groups use to attach themselves to hard rocky seafloor. It is rarely preserved, but brachiopod shells will often have a pedicle opening preserved along the hinge-line that varies in shape from rounded to triangular.

Figure 7.29 – Brachiopod morphology. Image credit: Jaleigh Q. Pier, CC BY-SA.

7.X Phylum Arthropoda

Arthropods are an extremely diverse and abundant phylum of animals. The includes organisms such as spiders, scorpions, crabs, barnacles, and insects. Most of these are not abundant in the fossil record because of poor preservation. One class of arthropod stands out in the fossil record, the trilobite.

Class Trilobita

Trilobites are among the most well-known fossils, thanks in large part to their abundance, diversity, and broad distribution during the Paleozoic (Figure 7.30). These emblematic arthropods first appeared in the fossil record 521 million years ago and survived until the end-Permian extinction approximately 250 million years ago (Figure 7.31). Even when they first appeared, trilobites were diverse and widespread, suggesting a long history prior to the first fossils, likely extending as far back as 700 Ma. During the Cambrian, trilobites were a dominant component of marine ecosystems however, many of the Cambrian trilobite species died out during the extinction at the end of the Ordovician. Though they survived after extinction, their diversity never matched their earlier history. Because they evolved rapidly and were relatively widespread, trilobite species are often used to provide relative ages of the rocks in which they are found.

Figure 7.30 – Trilobite fossils. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.
Figure 7.31 – Diversity of Trilobite families. Image credit: Paleobiology Database, CC BY.

Trilobites get their name because their bodies exhibit three distinct, longitudinal sections and they also have three segments from “head to toe” (Figure 7.32) and have bilateral symmetry. They lived in a number of different ways, including moving across the ocean floor as predators, scavengers, or filter feeders, and some swam to feed on plankton. They ranged in size up to 45 centimeters long (about 1.5 feet).

Figure 7.32 – Trilobites have three major body sections, from anterior to posterior, called the cephalon (1), thorax (2), and pygidium (3). The body can also be divided into three longitudinal lobes, called the right pleural lobe (4), axial lobe (5), and left pleural lobe (6). Image credit: Sam Gon III, CC0.

7.X Phylum Echinodermata

Echinoderms represent the largest marine animal phylum and include organisms such as starfish, sea cucumbers, sand dollars, blastoids, and crinoids. They first appeared in the fossil record in the Cambrian and are still around today. One of the key features of this group is five-part, radial symmetry (Figure 7.33). There are many classes of echinoderms, and we will focus on Echinoidea, Crinoidea, Blastoidea, and Asteroidea.

Figure 7.33 – Five-part, radial symmetry of two echinoderms. Image credit: Jaleigh Q. Pier, CC BY-SA.

Class Echinoidea

Echinoids are the most diverse echinoderm class (Figure 7.34). They include spiny-looking sea urchins () and organisms commonly known as sand dollars (). Regular echinoids live on the ocean floor and can be herbivores (eating kelp and algae) or carnivores (eating bryozoans). Irregular echnoids live in the sediment on the ocean floor and eat tiny food particles in that sediment. All modern echinoids have a hard, calcareous, internal skeleton.

Figure 7.34 – Examples of Echinoidea. a) a living sea urchin; b) sea urchin fossil; c) sea urchin fossil from the Caribbean; d) sand dollars. Image credit: a) S. Rae, CC BY; b) Marian Garcia, CC BY-NC-ND; c) Tim Sackton, CC BY-SA; d) Rachel Hathaway, CC BY.

Echinoids first appear in the fossil record in the Upper Ordovician, approximately 460 million years ago, but they didn’t really diversify until the Jurassic (Figure 7.35).

Figure 7.35 – Diversity of Echinoidea genera. Image credit: Paleobiology Database, CC BY.

Class Crinoidea

Crinoids, often referred to as “sea lilies”, may resemble plants (Figure 7.36), but they are actually suspension-feeding animals that have been around since the Ordovician (Figure 7.37). They use their arms to catch floating food particles and transfers them to the base of their . The crinoid skeleton contains numerous ring-like elements made of magnesium-rich calcite and is held together by a combination of ligaments and muscles. The stem of crinoids is most often found in the geologic record (Figure 7.38). The crown resembles a flower, and this soft tissue is rarely fossilized.

Figure 7.36 – a) Basic anatomy of a crinoid; b) A crinoid fossil from the Permian; c) A living crinoid from Sumilon Island, Philippines. Image credit: a) William I. Ausich, CC BY; b and c) Klaus Stiefel, CC BY-NC.
Figure 7.37 – Diversity of Crinoidea families. Image credit: Paleobiology Database, CC BY
Figure 7.38 – Crinoid stems in limestone from the Ordovician of Kentucky. Image credit: James St. John, CC BY.

Class Blastoidea

Blastoids are an extinct class of echinoderm. Similar to crinoids, blastoids had a stalk and were suspension feeders. The main body of a blastoid is the theca, which was protected by interlocking plates made of calcium carbonate (Figure 7.39). Blastoids evolved in the Ordovician and went extinct during the End Permian extinction event (Figure 7.40).

Figure 7.39 – Blastoid from the Lower Carboniferous of Illinois. Image credit: Wikimedia user Wilson44691, CC0 Public Domain.
Figure 7.40 – Diversity of Blastoidea genera. Image credit: Paleobiology Database, CC BY

Exercise 7.4 – Fossil Identification

Your instructor will provide you with a selection of fossils for you to identify. On a separate sheet of paper, make detailed sketches of your fossils from at least two different viewpoints. Do your best to make drawings to scale by using a ruler. Label any identifiable parts and then identify the fossils.




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The Story of Earth by Daniel Hauptvogel & Virginia Sisson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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