Invertebrates

Welcome to another Mesozoic Mondays blog from Jurassic Forest!  We're going to take a detour from our normal posts as our main blogger, Liz, is away (she is preparing for her thesis defense so wish her luck)!  My name is Kristina and I am another palaeontologist at Jurassic Forest.  Unlike Liz who studies large, flying reptiles, I study fossils that are very small.  Some of them are so small that you need a microscope to see them properly!  Another big difference between the animals that we study is that Liz studies animals that have backbones (vertebrae), and the ones I study do not.  These animals are called invertebrates, and they will be the topic of this week's Mesozoic Monday.

Jurassic invertebrate fossils!

Invertebrate animals have no vertebral column, no bones, and come in countless shapes and sizes.  They represent the most diverse groups of animals on the planet!  It would be impossible for me to give a complete representation of all invertebrates in a blog post, so I am just going to highlight some of the major groups and their evolutionary history.

The first evidence of multicellular animal life comes from fossils in the Ediacaran Period, about 600 - 545 million years ago.  Most of these fossils are from soft bodied creatures, somewhat like jellyfish.  These fossils represent soft bodied creatures that are very different from those seen today so classification of these animals is often very difficult.



Dickinsonia is a classic example of Ediacaran life

The first animals with hard parts appear in the fossil record during the Cambrian Period (about 542 million years ago).  This event is often referred to as the "Cambrian Explosion" because there was such a large radiation of life forms that appeared over a relatively short period of time ("relatively" in geologic terms means about 40 million years).  As well, animals with hard parts fossilize much more easily than soft bodied animals.  

Although most of the fossils from the Cambrian Explosion are very bizzare, most of them are clearly related to modern animal forms.  There are representatives from most major groups of animals, even vertebrates!  Pikaia is thought to be an ancestral form of vertebrate animals.  The most famous fossil site for animals from this event is the Burgess Shale in Yoho National Park, British Columbia.

Animals from the Burgess Shale (Top row: Anomalocaris, Opabinia; Bottom row: Hallucigenia, Wiwaxia, Pikaia), Image by: Matt Martyniuk

In the animal kingdom, there are many different categories of animals.  Vertebrate animals represent only one of these major categories!  The simplest form of animals are the sponges.  Even though they lack true tissue, they are considered animals because they have to eat food (they cannot make their own food like plants), and because they have sperm cells for reproduction.  Unlike other animals that have a digestive system, sponges feed by filtering water through the many pores in their bodies (hence their scientific name is Porifera).

Sponges are animals!

The next major group of animals are called the Cnidaria (pronounced "nye-DARE-ee-a").  They include the jellyfish, corals, and anenomes.  Unlike sponges, cnidarians have their cells organized into true tissues.  They also have special stinging cells, which is what makes some jellyfish so poisonous (please note that not all cnidarians are poisonous).

Anenomes are cnidarians

There are hundreds of different kinds of worms in the animal kingdom, which are usually organized into three major groups: the flatworms, segmented worms, and round worms.  These groups are incredibly diverse, but generally have a poor fossil record.

Worms, such as this tapeworm, can live almost everywhere, including your digestive system!

Molluscs are another incredibly diverse group of invertebrates.  They are one of the most abundant groups of animals in the fossil record and are characterized by a having a special muscle called the "foot".  They include animals such as clams, oysters, snails, slugs, octopi, squid, ammonites, and nautiloids.

Ammonites were one of the top predators in Mesozoic oceans! Photo by: Mike Peel.

By far, the largest, most diverse, and most abundant group of animals on the planet are the invertebrate group called the arthropods.  Arthropods are characterized by  having a hard exoskeleton which they shed (moult) as they grow, and a segmented body.  The arthropods include insects, crustaceans, arachnids, millipedes, centipedes, and a large extinct group called trilobites.

Trilobites first appear at the Cambrian Explosion, and are the only completely extinct group of arthropods.

The next group of invertebrates is one that most people have never heard of, but which were very important in the fossil record.  This group is called the lophophorates (pronounced "LOW-fo-FOUR-ate") and includes two major groups: brachiopods and bryozoans, both of which are still alive today.  They are characterized by a special feeding structure called the lophophore.  The lophophorates were incredibly diverse and abundant during the Paleozoic, but were largely replaced by molluscs during the Mesozoic.  This is the group that I study!

Brachiopods look like clams on the outside, but their internal anatomy is very different! Image by: Didier Descouens

The final major group of invertebrates are called the echinoderms (pronounced "eh-KYE-no-derms").  These animals are the closest relative of vertebrates because most have a special type of internal skeleton.  They are also characterized by a special body plan that has five sided symmetry.  Echinoderms include sea cucumbers, sea urchins, sand dollars, sea stars, brittle stars, sea daisies, crinoids, and an extinct group called the blastoids.

Crinoids are a cool group of echinoderms that are common in the fossil record!

While we are most familiar with vertebrate animals, there are way more invertebrates.  Beetles alone represent more than 28% of all living species of animals!  So for those of you out there that want to study palaeontology: don't forget that there is more that 600 million years of fossil record for you to choose from!  While dinosaurs are one of my favourite groups, there are countless plants, fungi, microfossils, and invertebrate fossils for us to study too.  That is what makes the science of palaeontology so awesome!

Fossils are awesome! Image by: Ghedoghedo 

 References:

Pearse, V., Pearse, J., Buchsbaum, M., and Buschsbaum, R., 1987, Living Invertebrates, Blackwell Scientific Publications, Palo Alto, CA, 848 pp.

http://www.ucmp.berkeley.edu/index.php - This website is a great resource for all things palaeontology and evolutionary biology.  Their invertebrates section is very good and includes some awesome diagrams and animations.