9.4: Major Fossil-Forming Groups (Invertebrates)
Now that we know how to describe fossils, we can start to identify them. In this section we provide a brief overview of the major invertebrate fossil forming groups organized taxonomically. In most cases the information presented here can get you to the taxonomic level of Order. If you want to learn more, consider taking an invertebrate paleontology course, attempting Lab 8: Fossils in "The Story of Earth - An Observational Guide" , or browsing the detailed explanations, photographs, and three dimensional models available at the Paleontological Research Institute's Digital Atlas of Ancient Life webpage or fossilid.info
Porifera (Sponges)
Sponges evolved in the Precambrian but their fossil record doesn't really start until the Cambrian Period. There are three important characteristics used for classification of sponges: 1) the body plan of the soft tissue, 2) the composition of the skeleton, and 3) spicule morphology (if present). Sponge skeletons can be composed of spongin, a fibrous protein (think of a bath sponge that can soak up water), carbonate, and/or silica. In some cases these mineral components exist as small needle like crystals called spicules. Sponge fossils can be divided up into the following Classes: Archaeocyatha , Demospongiae , Hexactinellida , Stromatoporoidea , Homocleromorpha, and Calcarea . The preceding hyperlinks provide links to the relevant webpages in the Digital Atlas of Ancient Life; there is also an excellent collection of sponge photographs at fossilid.info .
Sponge fossils are mostly found in marine settings but some groups live in freshwater environments. Their fossils range from the Cambrian to the present.
Cnidaria (Corals)
Corals belong to the Phylum Cnidaria which also includes jellyfish, sea fans, and sea anemones (among others). These organisms all share the following characteristics: radial symmetry, two tissue types (they are diploblastic), and the presence of venomous stinging cells called nematocysts. The group evolved in the Precambrian but the major coral Orders did not appear until the Ordovician. Corals make up the bulk of the fossil record for Cnidaria because they are the only group that produces a carbonate skeleton. Fossil identification of corals is based on those mineralized skeletons by looking at life mode (did the coral live as a colony or as a solitary individual) and internal structure within the holes that coral animal lived in. The major fossil forming groups of corals include tabulate corals, rugose corals, and scleractinian corals. For more detailed information, pictures of coral fossils, and 3D models, please visit the Digital Atlas of Ancient Life webpage which contains details about rugose , tabulate , and scleractinian corals or fossilid.info's pages on rugose and tabulate corals.
Coral fossils are only found in marine settings and range from the Cambrian to the present.
Lophoporata (Brachiopods and Bryozoans)
The Phylum Lophophorata includes brachiopods and bryozoans . These groups are united in that they have a special feeding organ called a lophophore and exhibit bilateral symmetry. Otherwise it can be hard to believe the two groups are related because they look so different! Brachiopods are shelled solitary organisms while bryozoans are colonial organism that can look similar to corals! Because both groups produce mineralized skeletons, they have excellent fossil records. The Digital Atlas of Ancient Life webpage has an excellent discussion of brachiopods vs bivalves .
Brachiopoda
Brachiopods are valved (two-shelled) organisms that live in aquatic settings. While they superficially look similar to bivalves, brachiopod valves are not mirror images of each other (that is, the line of symmetry is not across the hinge). Instead, the line of symmetry is across the valve. Brachiopod classification is based on multiple morphological characteristics of the shells. Most important among these characteristics is the length of the hinge line (the line the two valves make where the shell rotates open/close) and the symmetry of the two valves relative to each other. The brachiopod webpages in the Digital Atlas of Ancient Life and fossilid.info provide pictures, models, and a fuller discussion of these organisms.
Brachiopod fossils are only found in marine settings. While the group does range from the Cambrian to the present, only a few make it into the Cenozoic.
Bryozoa
Bryozoans are colonial organisms where each animal in the colony is a clone. Each of these clones is called a zooid and each zooid lives inside a small chamber called a zoecium. All the zoecium are arranged together to form a colony. Classification of bryozoans is based on the shape of the zooid and the colony arrangement. For proper identification of bryozoans, it is best to look at colony structure and arrangement under the microscope. For field identification, using the growth forms can be very useful for identifying most of the major bryozoan Orders. Bryozoans can exhibit massive, branching, lacy, or encrusting growth forms. The bryozoan pages at the Digital Atlas of Ancient Life and fossilid.info provide pictures, models, and a fuller discussion of these organisms.
Bryozoans are mostly found in marine environments but some groups exist in freshwater settings. The group ranges from the Ordovician to the present but many of the groups go extinct in the Triassic.
Mollusca
Many mollusks have mineralized hard parts, their fossil record is extensive but identification can be difficult as detailed classification is partially based on soft tissue. Here we will focus on some of the most common and easily identifiable fossil groups: Bivalvia, Gastropoda, and Cephalopoda. Bivalves consist of two valves and are classified based on soft tissue features like their gill system and siphon (an organ used for respiration and reproduction). Because these features do not fossilize well, fossil identification is based on scars the soft tissue leaves on the shells as well as how the valves are articulated (dentition). Gastropods , which includes snails and slugs, suffer from the same difficultly with some groups losing the shell all together. Cephalopods , are the most straightforward group to identify. We can use the shape of their shell, the complexity of the internal walls (septa) in those shells, and the positioning of the siphuncle (a canal that connects the chambers of the shell) to identify four of the five Subclasses that belong to the group.
For pictures and more information please visit the following websites:
- Bivalves - Digital Atlas of Ancient Life and fossilid.info
- Gastropods - Digital Atlas of Ancient Life and fossilid.info
- Cephalopods - Digital Atlas of Ancient Life and fossilid.info
Mollusca are found in all types of environments but cephalopods are exclusively marine. The group ranges from the Cambrian to the present.
Arthropoda
Arthropods are the most diverse of the animal kingdom and includes groups like insects (Hexapoda), crustaceans, and trilobites. These groups are united in that they have jointed appendages, segmented bodies, and an exoskeleton. Because many of those exoskeletons are not exceptionally robust, the fossil record of this group does not quite match the Phylum's abundance and diversity in the modern. The most notable fossil group is the Subphylum Trilobitomorpha. Classification in this group is based on the number of thorax segments, the shape and size of the third body segment (pygidium), the shape and size of the head (cephalon), the shape and position of the stomach (glabella), among other characteristics. Because many of the trilobite Orders have relatively short stratigraphic ranges, this group is commonly used for biostratigraphy. The Digital Atlas of Ancient Life's arthropod , trilobite , and crustacean webpages and fossilid.info's trilobite , chelicerate , and insect pages provide images and more discussion of these organisms.
Arthropods are found in all types of environments and range from the Cambrian to present. Trilobites are only found in marine settings and go extinct at the end of the Permian.
Echinodermata
Echinoderms include starfish, sea urchins, and crinoids (among others). The shared characteristics of this Phylum include five-fold (pentaradial) symmetry, a water vascular system (water is pumped through the body for respiration and locomotion), and a mesoderm skeleton composed of porous plates of calcite crystals. Echinoderms are divided into two Subphyla: Eleutherozoa (non-stemmed) and Pelmatozoa (stemmed). Many of the calcite plates that make up the mesoderm skeleton are not fussed together so the fossil record of this group includes lots of disaggregated echinoderm plates in additional to complete body fossils. The echinoderm webpages at the The Digital Atlas of Ancient Life and fossilid.info provide more pictures and discussion of these organisms.
Echinoderms are only found in marine environments and range from the Cambrian to the present.