Exercise 10 Fossil Lab—Part 5: Crinoids, Blastoids, Fusulinids

Comments

Transcription

Exercise 10 Fossil Lab—Part 5: Crinoids, Blastoids, Fusulinids
Exercise 10
Fossil Lab—Part 5:
Crinoids, Blastoids, Fusulinids, Plants
ECHINODERMS (CRINOIDS AND BLASTOIDS):
Echinoderms are an extremely diverse group of advanced invertebrates
including such familiar forms as starfish, sand dollars, urchins, and sea
cucumbers. The name echinoderm means “spiny skin.” Apart from their
spiny skin, all echinoderms are united in exhibiting five-fold (pentameral )
symmetry. There are a large number of classes of echinoderms, many of
which have good fossil records. In this lab, however, we will focus only on
the crinoids and blastoids because of their abundance in Upper Paleozoic
rocks. These stalked echinoderms were so pervasive during the
Mississippian and Pennsylvanian periods, that their remains make up the
dominant particles in many bioclastic limestone deposits.
Figure 1. Restoration of a crinoid.
Crinoids and blastoids both share a common overall morphology consisting of
a calyx (or “head”), stem, and holdfast (or “root”) (Figure 1). The stem is
made up of a stack of disc-shaped elements called columnals. The calyx
10–1
consists of a number of polygonal plates, with arms typically extending
upward for filtering nutrients from sea water.
All crinoids possess five arms attached to the calyx (Figure 2). The arms
typically split into a much larger number of smaller branches. Upon death of
an individual, the plates making up the calyx and arms usually disarticulate to
become isolated sedimentary particles. Preservation of intact specimens is
uncommon.
branches
Figure 2. Enlarged view of a crinoid calyx. Note
that the five arms split upward to produce a large
number of smaller branches.
arms
columnals
Blastoids possess a large number of small erect arms in life, but the arms
are almost never preserved in fossil specimens. Rather, the calyx of a fossil
blastoid is distinguished by its very obvious pentameral symmetry and the
presence of 13 plates. There are three basal plates, five radial plates, and
five deltoid plates. A feeding structure called the ambulacrum is positioned
within each radial plate. The mouth and anus are located at the top of the
calyx, with the anus being the largest of the five pores (Figure 3).
ambulacrum
anus
Deltoid plates
Radial plates
Basal plates
Figure 3. Enlarged view of blastoid calyx (side and top views).
10–2
Paleoenvironmental Range:
During the Paleozoic Era stalked echinoderms lived in continental shelf
environments in tropical and temperate latitudes. Crinoidal and blastoidal
skeletal debris is present in almost all bioclastic limestones of Mississippian
and Pennsylvanian age. Today, stalked echinoderms occur mainly in very deep
water (bathyal and abyssal depths). Highly specialized stalkless crinoids live
today in shallow water reef environments.
Stratigraphic Range:
Crinoids originated in the Cambrian Period and still exist today, although
their golden age was in the Late Paleozoic. Blastoids originated in the
Ordovician Period and became extinct at the end of the Permian Period.
Crinoid Examples:
1. Crinoid calyces with arms intact. Specimen ECL 20 exhibits very
delicate arms with fine “pinnules.” Note the bifuraction (splitting) of
of arms just above the calyx. Specimen PEL 2 has five arms, each of
which has split into just two branches.
2. Crinoid with arms and fine “pinnules.”
3. Basal part of crinoid calyx. The calycal plates are well preserved on
this specimen. Note the large number of plates and their polygonal
shape.
4. Examples of partial crinoid calyces.
5. Basal part of a crinoid calyx. Examine this specimen closely. What is
unusual about it?
6. Sawed block of limestone with intact crinoids. These crinoids have
both the calyx and parts of the stems preserved. Make sure you can
identify the columnals.
7. Another example of a crinoid calyx with arms attached.
8. Assorted crinoid calyces, stems, and holdfasts. This assemblage is
extraordinary in that holdfasts are rarely preserved so nicely. The
holdfast is that part of the animal’s body that anchors the animal in
sediment (superficially analagous to the roots of a plant). Note the
columnal making up the stem.
9. Crinoidal limestone. Usually crinoids fall apart upon death and their
disarticulated remains accumulate as carbonate sediment. The
columnals are most abundant. Mississippian and Pennsylvanian age
10–3
crinoidal limestones are common and very thick in many parts of the
world, including Iowa.
Blastoid Examples:
1. Blastoid calyx exhibiting well preserved ambulacra.
2. Assorted blastoid calyces. Feel free to remove specimens from vials,
but please don’t get them mixed up. Make sure you can recognize
ambulacra, radial plates, and the position of the anus. The mouth was
situated in the middle of the five pores in the center of the upper
calyx.
3. Pentremites. You will be asked to identify this genus on the Lab
Exam. Examine the specimen carefully, noting the ambulacra, anus,
etc. Can you distinguish basal, radial, and deltoid plates?
4. Another Pentremites. This specimen is very well preserved. Note
that the upper part of the stem is still attached to the calyx. Can you
see individual plates?
5. Unidentified blastoid. Look at this specimen carefully. Can you tell
which end is the top and which is the base?
FUSULINIDS:
The order Foraminiferida includes single-celled animal-like protists that
secrete mineralized skeletons. “Forams” are among the biostratigraphically
most useful of all fossil groups because of their abundance, widespread
distribution in marine deposits, and rapid rates of evolution. As a group,
forams range from Cambrian to today. Both benthonic and planktonic types
exist today, but the planktonic types originated in Mesozoic time and
probably are only distantly related to the benthonic types.
Fusulinids are a particular group of forams that lived during the
Pennsylvanian and Permian periods. They are unusually large for single-celled
organisms, sometimes reaching a length of 1 inch or more. They are easily
recognized by their distinctive “wheat-grain” shape. A fusulinid shell
consists of an initial spherical chamber followed by a spirally coiled
arrangement of successively larger and more elongate chambers. Partitions
between chambers are called septa. Folding and fluting of septa can give
rise to a complex internal appearance. Because fusulinids are characterized
on the basis of their internal anatomy, fusulinids are studied almost
exclusively in thin sections (Figures 4 and 5).
10–4
Figure 4. Partially sectioned fusulinid showing external and internal structure.
Figure 5. Sectioned fusulinid showing spherical initial chamber and several additional volutions. Intense
folding of the septa gives rise to a complex internal appearance.
Fusulinids were extremely abundant in tropical and subtropical carbonate
environments, and like stalked echinoderms, they are major “rock-building”
fossils. Moreover, they evolved at very rapid rates, having diversified from
a single ancestral species in early Pennsylvanian time to well over 5,000
species by early Permian time, a span of about 30 million years. It is no
accident, then, that they serve as index fossils for correlating Pennsylvanian
and Permian rocks.
10–5
Paleoenvironmental Range:
Fusulinid lived mostly in shallow water, tropical to sub-tropical carbonate
environments. Some were adapted for life in or near reefs.
Stratigraphic Range:
Fusulinids originated in the Pennsylvanian and became extinct at the end of
the Permian, coincident with the end-Permian mass extinction.
Fusulinid Examples:
1. Fusulinid limestones. Like crinoids, fusulinids were rock-building
organisms during the Late Paleozoic. The fusulinids that make up most
of these rocks are the relatively small, wheat-shaped objects.
Although small in absolute terms, fusulinids are very large by
comparison with most other protists.
2. Silicified fusulinids. This rock sample is a fusulinid limestone that has
been altered to chert. The fusulinids are white and the surrounding
matrix is black. Can you see any internal or external structures
preserved in the fusulinids?
3. Large fusulinids. These individuals are nearly an inch long (yikes!).
4. Isolated fusulinids in vial. Use the microscope to examine the
external surface of these shells. Can you see the septal furrows
between chambers?
5. Thin sections of fusulinids. Notice that the internal structure of the
shells is much more complex than the external surface you examined
at station 4. On the basis of size and internal complexity, which of
the specimens is more advanced evolutionarily?
10–6
PLANTS:
In this lab we will focus on those plants that contributed to the
Carboniferous coal swamps (mainly lycopsids and ferns), as well as
sphenopsids.
Lycopsids typically are small spore-bearing plants, but during Carboniferous
time some grew to tree-scale proportions. The two most common genera of
tree-like lycopsids are Lepidodendron and Sigillaria. These plants are easy
to distinguish on the basis of leaf scars preserved as impressions. In
Lepidodendron, the leaf scars are arranged in diagonal rows, whereas in
Sigillaria they are arranged in vertical rows (Figure 6).
Figure 6. Reconstructions of Lepidodendron (left) and Sigillaria (right). Note arrangement of leaf scars.
Sphenopsids are distinctive plants that possess circular nodes along their
stems. The stems are ornamented by vertical ridges or ribs, and a ring of
leaf-bearing branches radiates from each node. Today the only remaining
sphenopsids are the scouring rushes known as “horse-tails” (Equisetum).
Probably the most common Carboniferous sphenopsid was the tree-size
Calamites. The leafy branches of the Calamites tree are given the name
Annularia. [Apparently, the tree and its branches were given separate
Linnean names before it was recognized that they are simply different parts
of the same plant.] (see Figure 7).
10–7
Figure 7. Sphenopsid fossils. The trunk
of the large sphenopsid tree is known as
Calamites (left), characterized by circular
nodes and vertical ribs. The branches that
radiated from nodes are known as
Annularia (right). An entire plant is shown
in the reconstruction (below).
Late Paleozoic ferns seemingly differed little from their modern
counterparts. We have for your viewing pleasure some examples of fossil
ferns (Figure 8).
Figure 8. Artist’s reconstruction
of fossil fern leaves preserved in a
concretion.
10–8
Plant Examples:
1. Fern impressions. Look closely to see the exquisite detail preserved in
these fossils.
2. More fern impressions. Again, the exception preservation shows the
details of leaf shape and even internal veins.
3. Modern sphenopsid Equisetum. Note the circular nodes on the stems
of these specimens. The ones preserved in leucite have small
branches radiating out from nodes.
4. Fossil sphenopsids. All of these specimens are trunk internal molds of
the tree-like Calamites. You will be asked to identify this genus on
the Lab Exam. Note the nodes and longitudinal ribs.
5. Annularia (sphenopsid branches and leaves). You will be asked to
identify this genus on the Lab Exam. Each branch possessed a
series of circularly arranged leaves.
6. Modern ferns and sphenopsids. These specimens preserved in leucite
are typical small primitive plants. Contrast their size with that of
their Late Paleozoic relatives. Also, compare the “branches” of
modern Lycopodium with the fossil specimen at station 8.
7. Fossil lycopsids. Several examples of Lepidodendron internal molds.
You will be asked to identify this genus on the Lab Exam. The
distinctive characteristic of Lepidodendron is the diagonal
arrangement of leaf scars. Compare with Sigillaria (station 9).
8. Lepidodendron branch. Compare this specimen with the modern
lycopsid, Lycopodium, at station 6.
9. Sigillaria (fossil lycopsid). You will be asked to identify this genus
on the Lab Exam. In contrast to Lepidodendron, the leaf scars in
Sigillaria are arranged in vertical columns.
10. Lycopsid coal balls. Coal balls are masses of well preserved plant
tissue preserved in coal seams. By making thin sections of a coal ball,
the internal vascular structure of constituent plants can be
determined, thus enabling plant identification.
10–9

Similar documents