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IN SEARCH OF SOME RELATIVES...
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INTRODUCTION TO INVERTEBRATE FOSSILS
We,
that is Homo sapiens, are a strange species. Consider:
(1) We claim to be blessed with great intelligence, but yet many of us do
not realize that we are animals just like ants, fish, tortoises, birds, rabbits,
monkeys, etc...
(2) We have learned a lot about the surface and interior of the Moon, and
perhaps we know more about that Earth satellite than we know about the interior
of oceans on Earth, where we dwell.
(3) There is overwhelming evidence supporting a very old age for our
planet and solar system, and an even older age for the Universe, yet many refuse
to believe the evidence, choosing instead to ignore it (like sticking your head
in the sand and maybe it will go away).
(4) There is vast and widespread ignorance of our own biology, and of our
relationship with other animals, plants and microscopic organisms that we share
the planet with (consider the relationship of population size to disease,
aggression, famine and other fun things).
(5) We tend to ignore the facts of biological evolution, and act as if we
are the prime owners of Earth (a name we have thought up), and have the right
to do what we please.
(6) We (at least some of us) are required to take US History in school, but no
one is required to take human history; not just the "civilized" part
of human history, but our real biological history. Hey, the less you know,
the easier you are to fool or be lied to (just ask a politician, or a used car
salesman, or a priest, or a doctor, or your parents, or maybe even your
professors).
The
following information, therefore, provides a very abbreviated look at some of
the creatures with whom we share this planet (see Fossil Record web page
for a more in depth discussion). Most of these creatures have been around a heck
of a lot longer than we have, and they have left behind a fossil record to
document their existence.
So say hello to some of our neighbors and ex-neighbors!
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SOME GROUND RULES FOR THIS PRESENTATION
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IN SEARCH OF SOME RELATIVES...
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Beginning as far back as
the 1940s, a variety of fossils have
been found in rocks ranging from about 650-550 million years in geologic age,
but not until the 1960s, was the significance of these fossils recognized. Collectively they are known as the
Ediacaran
Fauna, from their first
discovery near the "town" of Ediacara, Australia. Today, localities
containing fossils of these orgnanisms are known from many areas on
different continents. Virtually all
of these fossils consist of the remains of small, soft-bodied, multi-cellular
creatures (animals?) that are preserved as shallow impressions in sandstone and siltstone. Trying
to determine what kind of creatures these organisms were - and how they made
their living - is difficult, and
currently no consensus exists. What (or who) do they resemble to you?
The significance of these fossils: (1) They are in rocks geologically older than
what is traditionally recognized as the beginning of a significant fossil record
of "animals". (2). They seem to represent the oldest known
multi-cellular organisms. (3) They might have been biologically
distinct from what we commonly know as animals.
Reference: Dixon, p. 59; 61-63; 64-65
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Sponges represent some of the simplest animals. They are actually colonies of cells, and lack major organs or organ systems. In effect, they are basically simple water pumps that live attached to some substrate in the oceans. Water is expelled through the top opening, and replacement water comes in through small pores in the body. Food particles within the water are trapped in the pores, like in a filter system.
There is also an extinct group of sponge-like organisms known as ARCHAEOCYATHIDS ; these organisms formed some of the oldest known organic reefs (Reference: Dixon, p. 78).
Reference: Dixon, p. 61; 78-79; 111-113
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Phylum CNIDARIA
Class HYDROZOA
HYDROZOAN POLYPS
Class SCYPHOZOA
JELLYFISH MEDUSOIDS
Class ANTHOZOA
SEA ANEMONES AND CORALS
The Cnidaria (named for their stinging cells) are also known as Phylum Coelenterata (named for their open body cavity) or Phylum Radiata (named for their radial symmetry). Which phylum name is the best one?
Whatever name we call them, this group of organisms is more advanced than are the sponges, in terms of complexity of their body walls and presence of internal organs.
Ocean swimmers and surfers are familiar with jellyfish, which are floating cnidarians (Class Scyphozoa) that have stinging cells for capturing food; if you visit a tide pool you will come across anemones (Class Anthozoa) attached to rocks. By far the most abundant group of these organisms in the fossil record is the Anthozoa.
Reference: Dixon, p. 61
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Class ANTHOZOA
Living And Fossil Colonial Corals |
Anthozoans are
most well known in the form of sea anemones and corals, both of which form polyps attached to hard
substrates on the sea floor. These benthic organisms are carnivores; trapping potential food with
their tentacles, which have stinging cells (nematocysts), and then transporting food to the
centrally located mouth.
Corals of various types have been associated with formation of
reefs since mid-Paleozoic time.
Reference: Dixon, 111-113
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EXAMPLES OF PALEOZOIC CORALS
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Two important
groups of corals known from Paleozoic rocks. They formed as solitary or
colonial organisms with an outer, mineralized, calcareous skeleton.
Reference: Dixon, p. 111-113
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Solitary And Colonial Forms Of Zoantharian Corals |
Main group of
Mesozoic and Cenozoic corals. These are the type of colonial corals most common in modern reefs
such as the Great Barrier Reef off the coast of Australia. They also have
a calcareous skeleton.
Reference: Dixon, p.
111-113
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Phylum ECTOPROCTA
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Individual ectoprocts (also known as
bryozoans) are tiny, but they form and live in colonies of many
individuals. The most common types in the fossil record secrete calcareous
skeletons. They commonly live on the sea floor, or attached to other
objects. All are filter feeders, using the tentacles in a structure called
a lophophore to trap food particles, as you can see from the photo of a
living colony above. The individuals in the colony are no more than a
couple of millimeters in size.
Reference: Dixon
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Phylum BRACHIOPODA
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Brachiopods have some similarities to
ectoprocts, because they have a lophophore. They have some superficial
similarity to clams (bivalve molluscs) because they have two calcareous valves
that enclose the body. However, brachiopods are all solitary organisms and
much larger than ectoprocts, and their internal features are quite different
from bivalve molluscs.
Brachiopods are bottom dwellers on the sea floor and use their lophophore as a
filter feeding device. They were much more common in Paleozoic oceans than
they are in modern oceans.
Reference: Dixon, p. 28
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Phylum MOLLUSCA
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Numerous classes of Phylum Mollusca. Although all have a fossil record, the most abundant are the Gastropoda, Cephalopoda and Bivalvia. They are a highly successful and diverse group of invertebrate animals, and have been especially common in Mesozoic and Cenozoic oceans.
A few types have become adapted to fresh water environments, and even to a terrestrial mode of life (garden snails within the Class Gastropoda).
Reference: Dixon, p. 210-211
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Class BIVALVIA
Common Name: "Clams"
Bivalves are most commonly known as "clams" (like in clam chowder!). The are abundant in modern oceans and have a fossil record extending back to the Early Paleozoic. Their most distinctive feature is the presence of two calcareous valves (their shells), which enclose the body and provide protection from predators. The valves tend to come apart after the organism dies, so usually you find them separated (check next time you walk on the beach). They are primarily benthic creatures, and most are filter feeders. There was a comic strip many years ago that claimed "clams got legs", but there has been no scientific proof for this claim.
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Class GASTROPODA
Common Name: "Snails"
You got a garden? You got snails! Interesting creatures. Most live in the oceans (seems like the ocean is where most everything got started), but a few have become adapted to fresh water, and some hardy types actually made it onto land. Most gastropods have a single calcareous shell; they are, of course, all twisted, which makes them easy to recognize and identify. Many snails are herbivorous, but there are a few rather aggressive types that prefer fresh meat, and they are carnivores. Some can actually bore through the shells of other organisms such as clams (like the moon snail, top right photo), and some have a type of stinger with nasty poison (like the cone snail, top left photo). There are actually snails that could kill you!
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Class CEPHALOPODA
"Cephalopods"
Smart, jet propelled molluscs. Many biologists consider the octopus the smartest invertebrate (try to keep one in a salt water aquarium). These animals were, are are, highly active swimmers (nektic), propelling themselves using jets of water. They are active predators, using their tentacles to grasp prey that they can catch. Most cephalopods, especially those in the fossil record, had an external, calcareous shell, but others, such as squids and octopi, have a reduced inner shell, or none at all.
Some of the largest of these, a group of coiled cephalopods called ammonites, had a shell diameter of over 6 feet; there is only one surviving - but distant - relative of the ammonites, the pearly nautilus (top left photo). Of course, there are also the giant squid of the deep oceans, attaining lengths of many 10's of feet.
Reference: Dixon, p. 39; 97-99; 200; 210-211
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Phylum ANNELIDA
Worms And Such
Worms. There are lots and lots of worms; round worms, flat worms, tape worms, and so on. The annelids represent just one group of worms. Most everyone is familiar with earth worms, but there are also many types of annelids that live in the oceans. A few groups secrete calcareous tubes (photo at right). Some live in the deep oceans near volcanic vents (photo at left). Some are filter feeders, some are predators, and some are detritus feeders (earthworms for example). They are not particularly abundant in the fossil record, because most did not secrete hard parts. We salute annelids, because they are one of the few phyla having species that have become adapted to terrestrial life, and because they have a fossil record that extends back at least 600 million years, and perhaps farther.
Reference: Dixon, p. 108
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Phylum ARTHROPODA
Subphylum TRILOBITOMORPHA
Subphylum CHELICERATA
Subphylum CRUSTACEA
Subphylum UNIRAMIA
Say again, which group of organisms is dominant on Earth? You think is those smart, but puny and johnny come lately humans? How about those arthropods, the most diverse group on Earth? Actually, it's probably the bacteria; they have been here the longest (by far), and they infest all other forms of life.
Well, if we consider just the animal kingdom, arthropods are top of the heap in terms of types (diversity) and individuals (abundance). They live everywhere, and can crawl, walk, run, swim and fly. They eat just about everything and multiply like crazy. Lucky for everyone else, they have an external skeleton (exoskeleton), which prevents them from becoming very large on land.
Reference: Dixon, p. 84-85
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Subphylum TRILOBITOMORPHA
Examples Of Trilobite Body Shapes |
Next to dinosaurs, fossil trilobites seem
to fascinate many people. Maybe it's because these creatures had an elaborate,
and sometimes highly ornamented skeleton.
Trilobites lived in the world's oceans from Early Cambrian to Late Permian time,
a span of about 300 million years. I would consider that an impressive
record. Their heyday occurred during the Cambrian Period. That is
when they seemed to have been most diverse and abundant.
Most cruised along the ocean floor on a
multitude of legs searching for
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Subphylum CHELICERATA
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Hey, I
don't like these creatures, so I don't think I will tell you anything about
them!
Ok, Ok, the eurypterids (left photo) are known as "water
scorpions". They occur in the fossil record only during a portion of
Paleozoic time; some, however, grew to 6 or 8 feet in length, and had large,
grasping front claws.
Horseshoe crabs (center photo) appear in mid-Paleozoic rocks and are still with
us. They haven't changed much in appearance either. Go to Florida
during their mating season and you will see them all over in the shallow warm
water and beach areas.
Spiders (Class Arachnida, right photo). Eight legs, mandibles, and an
ability to live on land. This one is preserved in hardened tree sap
(amber). Why are we scared of spiders?
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Subphylum CRUSTACEA
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Subphylum UNIRAMIA
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Phylum ECHINODERMATA
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Echinoderms have existed on Earth for at least 550 million years; they have evolved
a wide range of body types, some of which have been very successful, and some of
which have become extinct. The three groups illustrated and indicated in bold letters, and representatives of these groups are living in modern oceans.
Echinoderms share similarities in their embryonic development with vertebrates, but not with any other group of invertebrates, so I guess starfish and sand dollars are distantly related to me and you. All echinoderms live in the oceans.
Reference: Dixon, p. 99-101.
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Subphylum CRINOZOA
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They do kind of look like plants, but they
are, in fact, highly developed multi-cellular animals. Two
main types are floating (planktic)/swimming forms and those that are attached to the sea
floor and have a stem. They trap food particles with their arms.
Crinoids have a calcareous skeleton made of numerous plates; they become
disaggregated after death and have contributed much skeletal material to the
rock record through geologic time.
Reference: Dixon, p. 144
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Subphylum ECHINOZOA
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Subphylum ASTEROZOA
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Phylum HEMICHORDATA
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Fossilized fragments of colonial organisms. Some graptolites lived on the bottom (benthic), but many floated (planktic) in the world's oceans from Cambrian through Early Carboniferous time (perhaps a span of 200 million years), and then they became extinct; they were apparently filter feeders. As you can tell from their phylum name, they are related to our phylum, the Chordata.
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Well now, this is the end of our brief survey of invertebrate animals, but what have we left out? First of all, perhaps dozens of invertebrate animal phyla that lack much of a fossil record. Secondly, how about micro-organisms such as bacteria, or perhaps algae and plants; there exist many other animals too, but they have a spinal chord and a bony or cartilaginous skeleton, and they are known as chordates. Clearly, we have many more creatures to consider...FOR FURTHER STUDY AND REFERENCE
The web page on "The Fossil Record" provides more detailed information on various fossil taxa, and it also contains illustrations of living examples for many of the groups of invertebrates described here.
This web
page was created by Rick Miller, Department of Geological Sciences, San Diego
State University. No external editing has been provided. Questions
or comments?
rmiller@geology.sdsu.edu August, 2003
