Download EPS/OEB 56 – Geobiology and the History of Life Lab 8: Paleozoic

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EPS/OEB 56 – Geobiology and the History of Life
Lab 8: Paleozoic Animals in the Oceans
Part I: Ediacaran Fossils
Goal: The goal of this section of the lab is to grapple with the problems of interpreting Ediacaran
macrofossils. What features, if any, support their interpretation as animals? If animals, where do
they fall on the metazoan tree?
Introduction:
The so-called Ediacaran biota records some of the first large organisms in the stratigraphic record.
Early reports of Ediacaran organisms interpreted them as early representatives of the animal groups
that went on to populate Phanerozoic oceans. In 1984, however, Adolf Seilacher proposed that these
fossils record a macroscopic biota not closely related to animals at all. Seilacher suggested that the
way forward is to ask basic questions of growth and function, and only after these have been
answered, try to evaluate their phylogenetic placement. Some fundamental questions regarding
these organisms are as follows:
1. Are these fossils of individuals of colonies?
2. How did they grow – can you describe a plausible trajectory from the fossils?
3. How did they function? Can you tell whether or not they were motile? How did they get
food? Could they have harbored symbionts?
To do:
1. Examine the casts of Dickinsonia from the Pound Quartzite, Australia. Try to answer the
three questions posed in the previous paragraph for these fossils.
2. Now examine the cast of Charniodiscus and answer the questions again.
3. Ediacaran microbialite reefs contain the earliest known calcified macrofossils. Examine
slabs containing Namacalathus and Cloudina. Can you draw inferences about ontogeny and
function in these organisms? Do these skeletal fossils provide better insights into phylogeny
than the canonical Ediacaran fossils?
Part II: Cambrian Animals
Goal: The goal of this section is to appreciate the lines of evidence that document Cambrian animal
diversification. How does each preservational mode contribute to the overall picture of evolutionary
diversification?
Introduction:
The Cambrian Period is the earliest Phanerozoic epoch to be characterized by abundant, diverse, and
widely distributed animal fossils. Cambrian animal diversity is recorded by approximately four
distinct types of preservation:
1. Conventional skeletons: The conventional fossils of Phanerozoic marine deposits are mineralized
skeletons, and these certainly occur in Cambrian rocks. Trilobites make up some 75% of all recorded
species in Cambrian rocks. Sponges are also abundant, especially the heavily calcified
archaeocyathids, in Lower Cambrian carbonates. Echinoderm skeletal plates occur widely, mostly
produced by a diverse array of stem groups that didn’t last beyond the period. Brachiopods are also
common, including a diverse set of phosphatic lingulids and a smaller diversity of calcified taxa.
Despite the existence of these skeletonized animals, skeletons make up only a modest percentage of
the total volume of Cambrian carbonate rocks.
2. Small Shelly Fossils: Early Cambrian rocks contain a fascinating diversity of skeletal elements,
most of them less than 1 cm in maximum dimension. Most well preserved small shelly fossils were
phosphatized during early diagenesis. The fossil record of small shelly fossils, thus, reflects the
distribution in time and space of phosphatic pore waters. Original mineralogies can be inferred from
microfabrics viewed under Scanning Electron Microscope (SEM) – these include calcite, aragonite,
phosphate, and organic materials. Small shelly fossils include stem group mollusks, but many were
produced by extinct taxa with uncertain phylogenetic relationships. The Cambrian explosion
documents the diversification of animal body plans that have dominated marine ecology since that
time. Nonetheless, the Cambrian fossil record is distinctive – different from the later Paleozoic as
well as the Ediacaran.
3. Trace fossils: Cambrian rocks contain a variety of tracks and burrows that record the movement
of anatomically complex bilaterian animals. Many of the animals that formed these traces did not
make preservable skeletons; thus, the trace fossil record can be viewed as complementary to the
skeletal record.
4. Exceptional preservation of unmineralized animal remains: The Burgess Shale (ca. 505 Ma)
may be the most famous invertebrate fossil assemblage in the world. Fossils include skeletonized
carcasses, but 90% of the species represented did not make mineralized skeletons. Most however,
did form organic hard parts, for example, the chitinous exoskeletons of arthropods. More than a
dozen Early and Middle Cambrian deposits of this type are known – Burgess and the somewhat
older Chengjiang biota (ca. 520 Ma, China) are the most diverse – but the taphonomic window for
such fossils shut as the Late Cambrian began.
To do:
1. Examine the specimen of Treptichnus pedum. This is among the oldest complex traces of
bilaterian organisms. Can you draw any inferences about life habits from these traces? (The
beginning of the Cambrian Period is defined by a “golden spike” in a section in Newfoundland
placed to mark the first appearance of T. pedum.)
2. Examine the specimens of small shelly fossils on display, as well as the monograph available in
lab (use a hand lens). Choose one genus that you can relate to an extant phylum and justify the
proposed relationship. Also describe one genus that strikes you as problematic.
3. Examine the slab of archaeocyathid limestone. Archaeocyathids are an extinct group of
organisms that are among the most diverse skeletal fossils in lower Cambrian rocks. They are found
mostly in reefs built primarily by microbes and suffered mass extinction about 515 million years
ago. Using the diagram below as a guide, can you reconstruct something of the functional biology
of these organisms and suggest where they fall on the animal tree?
Diagram of an archaeocyathid.
4. Examine the set of trilobites available in the lab. These fossils are divided into two groups, one
collected from Cambrian rocks and the other from post-Cambrian strata. Note the morphological
features that characterize each taxon and the differences that separate the groups. What features of
morphology unite trilobites with extant arthropods?
5. Examine specimens of Burgess Shale fossils. Under what circumstances might unmineralized
organic remains have been preserved?
Part III: Ordovician Animal Fossils
Goal: To appreciate the nature of renewed animal radiation during the Ordovician Period, noting
especially the differences between Cambrian and Ordovician fossils.
Introduction: Most animal body plans were most likely been established during the Cambrian
Period, but observed diversity shows a greater increase during the succeeding Ordovician Period.
This was a time when heavily skeletonized animals spread through the oceans, especially
brachiopods, bryozoans, echinoderms, and corals.
We’ll focus on three groups that, in combination, dominate skeletal biotas of the Ordovician Period.
Cnidaria
The Cnidaria are comprised of some 10,000 living species, nearly all in marine environments.
Cnidarians include jellyfish, sea anemones, and corals, among other groups. Despite their variation
in morphology, all cnidarians are built on a single anatomical plan: an outer layer of epidermal tissue
and an inner layer of digestive tissue that are separated by a non-cellular, commonly gelatinous
mesoglaea. A mouth at the oral end opens into a gastrovascular cavity; tentacles surround the mouth
opening. All cnidarians contain harpoon-like stinging cells called nematocysts.
Geological History
Coral-like skeletons appear in the Early Cambrian but are minor constituents of fossil assemblages
until the Ordovician, when rugose and tabulate corals diversified as part of a larger radiation of wellskeletonized invertebrates. Solitary rugosans occurred in both reef and level bottom communities in
the Ordovician, but did not become principal reef builders until the mid-Paleozoic when colonial
forms diversified. Tabulates were also important in both level bottom and reef communities of the
Paleozoic. The tabulates and rugosans both disappeared during the end-Permian mass extinction.
Modern corals represent an independent origin of skeletons in a group of sea anemone-like
cnidarians that survived the Permian-Triassic extinction. The oldest scleractinian corals occur in
mid-Triassic carbonates, and by the late Triassic, scleractinians were at least locally important in
reefs. Despite this, our modern view of reefs as the constructions of scleractinian corals applies only
to the Cenozoic Era.
Brachiopods
Along with the bryozoans and phoronids, brachiopods belong to a group called the lophophorates.
Lophophorate phyla are united by the presence of a lophophore, a horseshoe or ring of tentacles
around the mouth formed by outpouching of a portion of the coelom, or body cavity. The
brachiopods are united by the presence of a bivalved shell in which the plane of symmetry runs
perpendicular to the hinge line rather than along it (as in bivalve mollusks). Viscera are restricted to
a small volume near the hinge, while a mantle extends the length of the shell. Most of the volume
within the shell is taken up by the lophophore apparatus -- tentacles are arrayed along a support
called the brachium, which may be simple or complexly coiled. In some groups, the brachia are
mineralized and, hence, preserved in fossils. Brachiopods feed by drawing water through shell
interior so that it passes through the lophophore, where food particles are filtered out and carried to
the gut. Many living brachiopods are attached to the substrate by a cylindrical tether called the
pedicle, which is as extension of the body cavity. Most brachiopods, commonly grouped as
“articulates” have calcite shells; The lingulid clade (sometimes called “inarticulate”) is unique in
having phopshatic shells and infaunal species.
Geological History
Brachiopods differentiated as part of the Cambrian explosion, and lingulids and other inarticulate
groups were relatively diverse components of Cambrian faunas. Inarticulate diversity declined after
the Cambrian, but articulates radiated dramatically in the Ordovician to become one of the most
abundant components of Paleozoic marine faunas. More than ninety percent of all brachiopod genera
disappeared at the end of the Permian Period. Only about 300 species of brachiopods are known in
the modern oceans. In contrast, more than 12,000 species have been described in the fossil record.
Bryozoa
The other lophoporate phylum with an extensive fossil record is the Bryozoa, which are tiny
lophophore-bearing animals, or zooids, that live inside small boxes, or zoecia. The boxes are
commonly constructed from calcite, providing their entryway into the fossil record. About 3500
bryozoan species are known today; nearly all are colonial and marine. About 15,000 fossil species
have been described.
Geological History
The bryozoa are the one phylum of commonly fossilized animals that first appears only in the
Ordovician Period. By the Late Ordovician, bryozoans were commons constituents of seafloor
communities, and participants in reef construction. Like brachiopds, the bryozoans suffered major
extinction at the end of the Permian Period. A new group of bryozoans radiated in Mesozoic oceans
to dominate current diversity. Bryozoans, then and now, commonly encrust other animals and
seaweeds on the seafloor.
To do:
1. Examine the slab of late Ordovician limestone. Can you pick out brachiopods, bryozoans and
corals among the skeletons? How? Please be specific.
2. Examine and sketch the two types of Ordovician corals shown here. Using the textbook available
in the lab, try to understand how the fossils relate to the living individual or colony. The horn coral is
a rugosan, an extinct clade of calcifying cnidarians. The bee-hive-like colony is a tabulate, an
extinct colonial form also attributed to the Cnidaria. Both groups contributed to Ordovician reefs
and bioherms.
3. Examine the specimens of Ordovician brachiopods. Again, sketch one or two; can you use
preserved features of the skeleton to infer the anatomy of the living organism?
4. Examine the specimens of Ordovician bryozoans. Again, sketch one or two and try to relate the
preserved shells to a living organism.
5. Examine the genera diversity curve generated by J.J. Sepkoski below. What factors might have
facilitated the spectacular Ordovician radiation? You are free to consult your textbook on this
question to provide some insight.
6. How might the Ordovician radiation have influenced patterns of carbonate sedimentation in the
oceans?
Genus-level marine animal diversity through time, compiled by J.J. Sepkoski. Note the increases in diversity during the
Cambrian and, again, the Ordovician periods. The taxa most characteristic of Cambrian ecosystems (Cm) decrease in
importance through the remainder of the Paleozoic Era. Taxa that radiate during the Ordovician dominate diversity for
the remainder of the Paleozoic Era, but suffer disproportionately during end-Permian extinction (Pz). After the endpermian extinction, taxa that dominate modern oceans increased in relative importance (Md).