Lab 6: Fossils

 

This lab describes fossils and evolutionary relationships among major groups of Life. First, let's establish what is a fossil. The term fossil can relate generally to any remains of ancient life, where the word ancient is relative.

 

What is ancient ?

 

Something billions of years old is certainly ancient, but so is something 10,000 years old, or even 1000 years old. The distinction about age isn't all that critical; we use the term fossil to refer to organic remains from just a few thousand years ago, to the range of time of the Ice Age, going back several million years, to anything older, ranging back to the oldest fossils, more than 3 billion years old.

 

Fossils can be body fossils or trace fossils:

 

Body Fossils

 

  • any remains of shells or bones, the "hard parts" in an organism, as we say in geology. Shells are most commonly calcite, but some microfossils (fossils of one-celled organisms and other tiny creatures) are made of silica. Bone is made of a calcium-phosphate mineral called apatite.

 

Trace Fossils

 

  • any indirect evidence of an organism's activity, such as burrows and footprints. You may have seen the burrowing activity of crayfish or gophers or moles, but in the marine environment there are many active burrowers, such as crabs along the seashore and shrimp on the sea bottom offshore, and there are many species of worms, clams, and other animals that burrow into the mud or sand to find food, to hide, or to make a living area. Traces of movement are left by various animals, ranging from snails, who scoot along, "inching" along by pulsating movements of their muscular foot, to huge dinosaurs and mammals, who leave trackways of their steps. As an example of a common trace fossil, observe the branching pattern of these worm burrows:

 

 

Sometimes we encounter casts and molds of shells and bones. These terms are often confused and not used consistently. One way to use the terms correctly is to remember the phrase "a mold is of the original; a cast is of a mold." If you stick a clam shell down into the mud and remove it, what do you have? A mold, because it is "of the original." What do you have if you pour plaster into the impression made by the clam shell -- into the mold of the clam shell? A cast, because it is "of a mold." Of course, shells and bones accumulate in sedimentary environments, with fine mud of various types that can form molds and casts, and we also have the phenomenon where shells can dissolve after burial, even after sediment has turned into rock, and this leads to a variety of mold-like and cast-like preservation.

 

There are different types of fossils, large and small, of everything from microscopic one-celled organisms to huge bones of dinosaurs. And, there are all kinds of paleontologists who study them. Some people really like to learn about and study the most minute fossils, and are comfortable looking down a microscope for hours at a time. Other people prefer to spend their field time using pick and shovel, and other heavy means of excavation.

 

Let's clarify that term "paleontologist." Paleontologists (Paleo = old) are the people who study fossils, generally. Archaeologists (Archaeo = old) are the people who study human fossils specifically, and their ancient campsites, burial grounds, and buildings. Often the two terms are confused, with a tendency to use the term archaeologist too generally; let that term refer to those who study human-related history. Also, the term anthropologist (Anthro = human) could be confused, but this is really a more general category of scientists who study, not just human fossils and the physical history of humans and related evolutionary lineages among the Primates, but also any behavioral and cultural history of humanity. So, anyone studying fossil mammals, generally, other vertebrates like birds, dinosaurs, crocodiles, lizards, turtles, amphibians, and fishes, and anyone studying fossil invertebrates or microfossils, is called a paleontologist.

 

We'll take a tour through the major groups of Life in this lab, by starting at the most primitive groups and moving our way up through more derived groups. We'll use "family trees" -- phylogenies -- as represented on diagrams called cladograms to guide our tour. Here is the cladogram we will be using:

 

 

A cladogram shows the branching pattern of evolutionary relationships among taxa (taxon = a single group; taxa = more than one group). Read a cladogram like you would read a family tree. Each split, each division, or branching point represents an evolutionary split that happened long ago. From Life, at the point of evolution from inorganic constituents, close to 4 billion years ago on Earth (at least), the simplest bacteria, the archaebacteria (the ones who ingest methane or sulfur or are otherwise very primitive) gave rise to new forms of bacteria, the eubacteria, including those that could photosynthesize. From eubacteria-like one-celled organisms, arose the first eukaryotes, who had larger cells with DNA inside a nucleus. We have fossils of bacteria, such as those of cyanobacteria , that go way way back in the Precambrian, and the stromatolites they produced. The oldest eukaryote fossils are in the neighborhood of 2 billion years old.

 

That brings us to a point where we should address usage of group names in speaking about who is in a group and who isn't. This mainly involves learning that any group on the "main line" of the cladogram -- the "internal" part, the main trunk, if you will -- includes more than the groups extending up to the top of the cladogram. We can use humans as an example. Humans are chordates, members of Chordata (far right), because they possess a nerve cord, the spinal cord, and the other characteristics of the group. Chordata, as you can see by the branching, is a member of Deuterostomata. Deuterostomata is a member of the group that includes Deuterostomata and Protostoma (follow down from Deuterostomata and Protostoma and you get to a branch point, a node, that is unlabeled -- doesn't matter that it is unlabeled, it is still a group, essentially). Continuing down the tree, using the same kind of observation of the branching pattern, we see that Deuterostomata also belongs to Bilateria, Animalia, Eukaryota, and, ultimately, Life. Taking it from the bottom, up, you are are a member of Life, you are a eukaryote, you are an animal, you are a bilaterally symmetrical animal, you are a deuterostome, you are a chordate. And, likewise, you are not a protostome, you are not a bryozoan, you are not a protist, etc. -- you are a member of your specific group and any that you hit as you trace back down to the root of the tree, at Life.

 

Also, we may be using phrases like "the common ancestor of ..." or "the sister group of ..." or "the group that includes X and all other ...". Such phrases are inevitable when you are describing different parts of the cladogram, when addressing the history of evolution of one group or another.

 

When you see the term common ancestor , all you have to do is trace down from the groups mentioned to the point on the cladogram that is the first branch point that unites the groups. For example, the common ancestor of Chordata and Porifera (sponges) is at the branch point, the node, labeled Animalia.

 

The term sister group works the other way. The sister group of Chordata is Echinodermata. The sister group of Deuterostomata is Protostoma. The sister group of Bilateria is Cnidaria. The sister group of Animalia is Fungi. The sister group of Eubacteria is Eukaryota. And, because many of the nodes are unlabeled (you have enough names to contend with, don't you?), we sometimes have to spell out a full statement about sister groups. For example, the sister group of Mollusca is the group that includes Brachiopoda and Bryozoa, or we could say the sister group of Mollusca is (Brachiopoda + Bryozoa). And, so on. There is no official or special meaning of the term sister group -- it is just a way to talk about relationships, a handy way of describing closer relationship among several groups.

 

Sometimes we do have to say something like "the group that includes Cnidaria and Bilateria," because a node is unlabeled, but after you get the hang of it, you just look at the cladogram and follow along with the phrase.

 

Here are basic descriptions of the groups along the top of the cladogram, given as a list indented in accord with the cladogram -- more descriptive detail and photographs of fossils are given in individual pages of the lab:

 

Life -- the whole shebang

 

Archaebacteria -- simplest bacteria

 

Eubacteria -- include cyanobacteria and purple-photosynthetic bacteria.

 

Protista -- includes eukaryotic one-celled organisms

 

Plants -- includes all the familiar plants, and the unfamiliar, ranging back to primitive algae.

 

Fungi -- Mushrooms and their friends. Not common as fossils.

 

Porifera -- the sponges.

 

Cnidaria -- includes the corals, sea anemones, and jellyfish.

 

Platyhelminthes -- includes the flat worms.

 

Protostoma -- This group includes Arthropoda, Annelida, Brachiopoda, Bryozoa, and Mollusca. The unifying feature is something very basic, that has to do with the structure of the body at the early stages of development. A bag-like structure develops, as cells divide, and there is an opening in this structure. The opening becomes the mouth. Hence, the name, proto-stoma, or "first"-"mouth."

 

Arthropoda -- includes spiders, scorpions, trilobites, insects, crabs, shrimp, crayfish, etc., who all have the basic feature of the group: arthro-poda, for "jointed"-"appendages".

 

Annelida -- includes the segmented worms.

 

Brachiopoda -- includes the brachiopods, the "lamp shells," named that because one form is similar to the form of Aladdin's lamp .

 

Bryozoa -- includes the bryozoans, the "moss animals," named that because of the encrusting form of some bryozoans, seemingly covering the shells of other organisms such as clams like moss growing on a rock.

 

Mollusca -- includes clams, oysters, snails, octopus, squid, and others, including a huge list of ancient organisms.

 

Deuterostomata -- the group that includes Echinodermata and Chordata. The unifying feature is essentially the opposite structure seen the sister group, the protostomes. In deuterostomes, the opening that appears during the early stage of development becomes the anus, not the mouth.

 

Echinodermata -- includes the "spiny skinned" animals, such as starfish (not really fish at all -- you know, the star-shaped organisms with the sucker feet), sea urchins, sand dollars, crinoids, and more obscure forms.

 

Chordata -- includes you and other vertebrates (birds, mammals, reptiles, amphibians, fishes), as well as primitive animals with worm-like bodies.