An Overview of the Major Groups of Fossils: Microfossils and Invertebrates

An Overview of the Major Groups of Fossils: Microfossils and Invertebrates
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A Guide to the Major Groups of Fossils

Fossils are physical evidence of former life. It can be great fun to go fossil hunting in your local area, as many groups of fossils are easily found - especially invertebrate fossils. There are a number of ways to group fossils, but it is common to divide them into the following major groups:

  • Microfossils

  • Invertebrates

  • Vertebrates

  • Plants


Microfossils refer to tiny fossils across all living groups and are fossils from bacteria, protists, fungi, animals and plants. They are grouped together across groups of fossils because they are all studied in the same way.

The earliest fossils found are remains from prokaryotes in stromatolites (layers of remains from prokaryotes). They are 3.5 billion years old. Eukaryotes are found from 2.1 billion years ago.

Other common microfossils are Foraminifera, which are used to date marine sedimentary rocks. They first appear in the Cambrian Period, 543 million years ago (mya).

Invertebrate Fossils

Invertebrate fossils (animals without a backbone) are much more abundant than vertebrate fossils. Well known invertebrate are for instance insects, mussels, worms, snails and spiders, just to name a few. Many invertebrate groups live in the sea. The major groups of fossils that are very commonly found include:

  • Mollusca (clams, oysters, snails, and cephalopods): First appeared 545 mya in early Cambrian Period. Because of


    their hard shell, mollusks are easily fossilized and easy to recognize. They are free-living multicellular organisms with true hearts. Belemnites are commonly found fossils of cephalopods, related to squids but now extinct.

  • Brachiopods (clam-like creatures): First appeared in the Cambrian Period around 600 mya. They also have a hard shell and are therefore easily fossilized. The two shells are not identical to each other. 95% of the species are extinct today. They dominated the sea floor during the Paleozoic Era.

  • Echinoderms (starfish, sea urchins and “sea lilies”): Fist appeared in the Cambrian Period. Sea Urchins are common fossils to find on beaches.

  • Cnidaria (sea anemones, hydroids, sea pens, jellyfish, siphonophores, millepores and corals): First cnidarians appeared in the Venedian period 650-543 mya. The corals appeared in the Cambrian, and because of their hard skeleton, they are very common fossils to be found in rocks in all the mentioned time periods.

  • Porifera (Sponges): First found in the fossil record 650 mya in the Precambrian Period and are the earliest animal in the fossil record. Sponges are the simplest animal as they consist of specialized cells, but lack tissues, organs, respiratory and nervous systems. The body is very delicate so fossils are mostly spicules, which make up a support system in the sponges.

  • Bryozoans (small, coral-like animals): First found in the Ordovician Period, about 470 mya. They all live in colonies. Their skeleton of calcium carbonate makes them easy to find in the fossil record. Bryozoans are very abundant.

  • Arthropods (crustaceans, insects and trilobites): Trilobites are one of the most well known kinds of fossils and are very abundant


    . The first Trilobites are from the early Cambrian Period 550 mya, and all Trilobites were extinct 250 mya in the Permian Period.

  • Graptolites: Graptolites were tiny animals that occurred in colonies. They are preserved as a dark film on rocks and look like fine prints on the rocks. They appeared in the Cambrian Period, and were very common in the Ordovician and Silurian Periods. They have a worldwide distribution. Graptolites are now all extinct.

Continue on to page two for more major groups of fossils, including those of the vertebrate family.

Vertebrate Fossils

Vertebrates are all animals with a spine, and includes fish, amphibians, reptiles, birds and mammals. Vertebrate fossils are groups of fossils that are much harder to find than invertebrate fossils. The most common fossils found from vertebrates are their teeth, bones and footprints. The major groups of fossils of vertebrates include:

  • Jawless Fish: The first vertebrates were jaw-less fish which appeared in the Cambrian Period, 525 mya.

  • Fish: In the Silurian Period the first fish with jaws appeared, around 425 mya. Commonly found fish fossils are shark teeth, which

    shark tooth

    can be found on beaches.

  • Amphibians: These appeared around 360 mya in the Devonian Period. One of the most fascinating steps in the evolution of fish, is how they evolved into tetrapods, and thereby made the transition from fully aquatic organisms to land animals that could breathe air. Fossils have been difficult to find, but there is a good fossil record in Greenland, Scotland and Australia amongst other places.

  • Reptiles: The first reptiles appeared around 340 mya. The reptiles evolved into four different kinds of reptiles: the anapsid, the diapsid, the euryapsid and synapsid. The diapsid reptiles evolved into the dinosaurs, lizard, snakes, crocodiles and birds. Mammals evolved from mammal-like reptiles which evolved from synapsid reptiles. The anapsid reptiles evolved into turtles. The euryapsid reptiles evolved into now extinct reptiles such as the nothosaur, plesiosaur and ichthyosaurs who lived during the Mesozoic era.

  • Dinosaurs: They first appeared around 250 mya in the Triassic Time Period. Dinosaurs are probably the best known extinct animal group. They were present on all continents and are found in many locations around the world including the Gobi Desert and North America, and lived until the mass extinction 65 mya, the Cretaceous-Tertiary Extinction.

  • Birds: The first fossils of modern bird appeared around 110 mya. The general consensus is that birds evolved from the dinosaurs, the Archaeopteryx lithographica being the earliest bird. However the evolution of birds is not clear-cut as some recent fossil finds suggest that birds did not evolve from dinosaurs, but rather that birds and dinosaurs shared a common ancestor, a small archosaur.

  • Mammals: The earliest mammals, a small shrew-like mammal, appeared around 200 mya. After the Cretaceous-Tertiary Extinction, mammals rapidly diversified into the mammals we know today, and many modern species had evolved 55 million years ago.

  • Primates: The first primates are found 50 mya. Humans and the great apes share a common ancestor up until about 6 mya.

  • Hominoids: Homonoids are obviously of great interest to paleontologists, as we are always eager to learn more about our own evolution and ancestry. Ardipithecus ramidus, our oldest human ancestor existed 4.4 mya. Homo sapiens appeared only 200,000 years ago. Fossils are difficult to come by and each new find sheds light on our own evolution.

Plant fossils

Brachyphyllum nepos

Plant fossils are another group of fossils that are easy to find. Many plant fossils are found as impressions, which show a print of the plant but doesn’t contain organic materials. The first land plants are liverworts, found 472 mya.

An interesting group of plant fossils is the petrified forest, where many tree trunks are fossilized at the same time. An example is the petrified forest in Namibia, where the tree trunks are approximately 250 mya. There are also petrified forests in Mississippi, California, Arizona and Washington.


Benton M. J. Vertebrate Palaeontology Wiley-Blackwell 2005:

Carroll R. “Earliest Animals Were Sea Sponges, Fossils Hint” National Geographic News February 4, 2009

Daeschler E. B., Shubin N.H. & Farish A.: “Jenkins Devonian tetrapod-like fish and the evolution of the tetrapod body plan” Nature Vol 440|6 April 2006


Kuban G. “Identifying Fossils” 1994-2004:

Kissel R.A. “The Egg, the Chicken, and the 300 Million Years in Between” American Paleontologist 16(2) Summer 2008:

The Virtual Fossil Museum:

“Oldest dinosaur fossils found” The Telegraph 06 Oct 2010:

O’Donoghue J. “Found: World’s oldest animal fossils " NewScientist 17 August 2010:

Oregon State University (2009, June 9). “Discovery Raises New Doubts About Dinosaur-bird Links.” ScienceDaily. Retrieved February 25, 2011, from­ /releases/2009/06/090609092055.htm

Randerson J. “Worlds oldest bird fossil.” The Guardian, Friday 16 June 2006:

Shreeve J. “Oldest Skeleton of Human Ancestor Found” National Geographic magazine October 1, 2009:

The Fossil Record:

University of Carlifornia Museum of Paleontolgy:

U.S. Geological Survey:

Walker M. “Fossils of earliest land plants discovered in Argentina” BBC News: 12 October 2010

Image Credits

Tarbosaurus: Wikimedia Common/ de:User:Thomas Ihle

Belemnites and echinoderm: Image provided by writer

Trilobites: Wikimedia Commons/ Daderot

Shark Tooth: Wikimedia Commons/ Tgd3

Mammal-like reptile: Image provided by writer

Plant Fossil: Wikimedia Commons/ Ghedoghedo

This post is part of the series: All About Fossils

This article series focuses on fossils. It includes a guide to the major groups of fossils, explores which fossils are found in the deserts around the world, describes how deep sea fossils are used to shed light on evolutionary events, explains the fossil record, and describes how fossils are formed

  1. A Guide to the Major Groups of Fossils
  2. Paleontologists Define Fossil Records as all the Fossils Found on Our Planet
  3. Two Ways in Which Fossils Form: Permineralization and Amber
  4. What Kind of Fossils are Found in the Desert?
  5. Fossils in the Oceans Document Mass Extinctions and Climate Changes