Exercise 8 Fossils—Part 3: Sponges, bryozoans, brachiopods

Exercise 8
Fossils—Part 3:
Sponges, bryozoans, brachiopods
SPONGES (including STROMATOPOROIDS):
Sponges (Phylum Porifera) are primitive multicellular animals that possess a
vase-like body. The body wall is perforated and consists of two layers of
cells separated by a layer of mesenchyme (Figure 1). Structural support for
the body is provided by mineralized spicules secreted within the
mesenchyme. The spicules can assume a wide range of shapes, and they may
be either siliceous or calcareous in composition (Figure 2). Spicules,
therefore, have high preservation potential and they are useful in the
identification of fossil material.
Figure 1. Sketch of a simple sponge. This is a branching, thin-walled form. Solitary and thicker-walled forms also exist.
8–1
Figure 2. Enlarged view of sponge spicules,
which occur in a variety of shapes ranging
from simple rods to elaborate spiky “stars.”
Paleoenvironmental Range:
Sponges as a group occupy a wide range of marine environments, from the
shallow shelf to the deep ocean. Most are sessile and benthonic, although a
few groups of Paleozoic sponges may have been planktonic. Nonstromatoporoid sponges were important reef-building organisms at various
times in the geologic past. Bead-like calcareous sponges, along with skeletal
algae and inorganic cements, constructed massive reefs in Permian time.
Sponge reefs are known also from the Mesozoic Era.
Stratigraphic Range:
Sponges originated in the late Proterozoic Eon and they are still extant.
They have been termed evolutionary “dead-ends” because they apparently
did not give rise to any other groups. Dead-ends or not, their long history
and present diversity attests to their overall biologic success.
Complete sponge skeletons are rarely preserved intact, but we do have a few
examples.
Sponge Examples:
1. Modern sponges. Examine the specimens with soft tissue preserved in
leucite blocks (2 trays).
The tray labelled P-16 is a fragment of a branching sponge. See how
“spongy” it is?
8–2
Carefully examine the glass sponge. Note the coarse rectangular skeletal
framework. Now, use the hand lens to see the delicate siliceous spicules.
Where in the living animal was this skeleton secreted?
2. Hydnoceras (Devonian). You may be asked to identify this sponge on
the Lab exam. Note the overal horn-shaped morphology and the distinctive
rings of nodes. Look closely to see the reticulate surface ornamentation.
Does this remind you of the rectangular framework in the modern glass
sponge?
3. Astraeospongia (Silurian). You may be asked to identify this sponge on
the Lab exam. This genus has a distinctive bun- or biscuit-shaped
appearance with one side being convex and the other side being slightly
concave. Use the hand lens to inspect the specimen in Box 996. Can you see
the spicules? Note that complete ones are 6-rayed.
4. Heliospongia (Pennsylvanian). This is a relatively thick-walled sponge that
developes a branching morphology. Note that the internal cavity is relatively
small by comparison with the thickness of the wall.
5. Coeloclodia. Note the central cavity. What are the raised bumps on the
surface of these specimens, and how might you distinguish this sponge from
branching bryozoans?
6. Lemonia cylindrica (Permian). This slab is polished on one side, allowing you
to see the internal structure of the calcareous sponge Lemonia. This sponge
was one of the main framework organisms responsible for building the classic
Permian Reef Complex in West Texas and New Mexico (see p. 427-431 in
your text).
8–3
Stromatoporoids are unique poriferans that secreted an open calcareous
skeleton, or coenosteum, consisting of a network of structural elements
perpendicular and parallel to the growth surface (Figures 3 and 4). Single,
sheetlike layers parallel to the growth surface are called laminae. Spaces
between laminae are called galleries. During life, the galleries may have been
filled with sea water or with soft tissue of the organism. Pillars are rodlike
elements oriented perpendicular to laminae. Low mounds on the growth
surface are termed mamelons.
Individual organisms grew in domal, tabular, encrusting, dendroid or digitate
shapes. Although stromatoporoids can exhibit a wide range of gross
morphologies, all can be identified by their distinctive internal structure.
Figure 3. Sketch of a tabular
stromatoporoid coenosteum
depicting laminae, galleries, pillars,
and mamelons.
8–4
Figure 4. Thin section photomicrograph of a
stromatoporoid (section is perpendicular to
growth surface). Note well developed
laminae and pillars.
Stromatoporoids and stromatolites (remember them?) superficially resemble
one another. They can be distinguished by the presence of vertical elements
(pillars) in stromatoporoids, whereas there are no vertical elements in
stromatolites.
Paleoenvironmental Range:
Stromatoporoids were most common in shallow water, shelf settings. They,
along with certain corals, formed massive reefs in the Middle Paleozoic
(Silurian-Devonian periods).
Stratigraphic Range:
Stromatoporoids originated in Ordovician time, became very diverse and
abundant in Silurian and Devonian time, and then apparently became extinct
in late Devonian time. Stromatoporoid-like sponges reappeared in the
Mesozoic Era, but the lengthy gap between the last Paleozoic forms and the
first Mesozoic forms suggests that the morphologic similarity between the
two groups may be a product of convergence and not common ancestry.
Stromatoporoid Examples:
1. Tabular stromatoporoid. Examine the ends of these specimens, which
have been sawed and polished. Note the well developed laminae and pillars.
Now look at the growth surface and notice the raised mamelons.
8–5
2. Deeply weathered tabular stromatoporoid. Weathering of this specimen
has accentuated the “bundling” of groups of laminae. Such groups are known
as “latilaminae” (Figure 3). Be sure to look at the underside of this specimen
to see the well developed reticulate pattern formed by laminae and pillars.
3. Tabular stromatoporoid. Another specimen exhibiting well preserved
laminae and pillars.
4. Polished slab of a domal or columnar stromatoporoid. This specimen has
beautifully preserved astrorhizae, or “star-shaped” patterns that are
associated with mamelons (Figure 3).
5. Domal or spherical stromatoporoid (1 tray along with 2 thin sections).
Note the concentric growth of laminae. Examine the thin sections in order
to see the details of laminae, galleries, and pillars. Compare your
observations with the specimen illustrated in Figure 4.
6. Branching stromatoporoids. Superficially these specimens look quite
different from tabular and domal/columnar stromatoporoids. Internally,
though, the branching forms exhibit characteristic laminae and pillars.
Would you be able to distinguish the branching stromatoporoids from
branching bryozoans? How?
8–6
BRYOZOANS:
Bryozoans are tiny colonial animals that secrete a calcareous skeleton. A
colony is called a zoarium, whereas an individual cuplike living chamber is
termed a zooecium (Figures 5 and 6). Each tiny animal is called a polypide.
Zooecia are tyically so small that they appear as pits or depressions in the
surface of the zoarium. Zoaria may be twig-like and branching (ramose),
mound-like, or lacy (fenestellate) (Figure 7).
Figure 5 (left). Bryozoan morphology. Zoaria
superficially resemble branching mosses. Each
zoarium is made up of hundreds of zooecia,
each of which houses a polypide.
Figure 6 (below). Section of a branch of a
ramose bryozoan depicting tightly spaced
zooecia.
8–7
Figure 7. Examples of bryozoan
growth habits: (d) branching
(ramose); (E) lacy (fenestellate),
with enlarged view of zooecia at
right.
Paleoenvironmental Range:
Bryozoa as a group are known from both marine and freshwater aquatic
habitats. Nonmarine forms, however, are weakly calcified or uncalcified and
thus are not readily preserved as fossils. Marine forms generally are
calcified, sometimes massively so. Within the marine realm, bryozoan occupy
a spectrum of environments from the intertidal zone to fairly deep water.
Fossil bryozoans are most abundant in Paleozoic shelfal carbonate facies.
Stratigraphic Range:
Bryozoans originated in Cambrian time (possibly earlier?) and they are still
diverse today. They achieved peak abundance in the Paleozoic Era.
Bryozoan Examples:
1. Fenestellate (lacy) bryozoans. Use the microscope to examine the
morphology of the zooaria. Are you able to see individual zooecia
(“apertures” or pits where the polypides lived)?
2. Assorted fenestellate bryozoans. Check out these specimens and note
the wide range of sizes, even within a given rock sample.
3. Archimedes (Mississippian). You may be asked to identify this genus on
the Lab exam. Archimedes is commonly known as the “corkscrew” bryozoan.
Actually, it is a fenestellate bryozoan in which the heavily calcified
corkscrew structure functions as an axial support for fronds of “lace”. The
delicate lacy part of the zoarium is rarely preserved, but the corkscrew is a
very common fossil in rocks of Mississippian age. Some lacy fronds are
preserved in the specimen labelled Br-77.
8–8
4. Branching bryozoans. Examine the surface of these specimens in order to
see pits (zooecia). How would you distinguish between a branching bryozoan
and a branching stromatoporoid?
5. Additional examples of branching bryozoans. Again, note the pitted
surface.
6. Prasopora. You may be asked to identify this genus on the Lab exam.
This peculiar bryozoan is a trepostome, just like the branching types you’ve
already seen, but instead of a branching morphology this one looks like a gum
drop. Can you see zooecia? How would you distinguish between this
bryozoan and the sponge Astraeospongia?
7. Bryozoan thin sections (1 box; 11 slides). Examine these thin sections in
order to see the complex internal structure of bryozoan zoaria. By
comparison, the internal structure of a stromatoporoid looks pretty simple!
8–9
BRACHIOPODS:
Brachiopods are among the most common (and popular!) fossils in Paleozoic
rocks. Almost all brachiopod shells consist of two valves. Unlike the valves
of clams, which are identical, the two valves of a brachiopod shell usually are
quite different from one another. Brachiopods also differ from clams in
their musculature. A clam must contract its muscles in order to close its
shell. Upon death, the muscles relax, the shell opens, and often the two
valves become disarticulated. Just the opposite is true of brachiopods. A
brachiopod must contract its muscles in order to open its shell. When its
muscles are relaxed, the brachiopod shell is closed. Upon death, the shell
stays closed and it is therefore very likely to be preserved as a complete
fossil.
Most brachiopods are sessile, benthonic animals that attach themselves to
the seafloor or some other object by means of a fleshy stalk, the pedicle.
The pedicle protrudes through an opening in the lower valve (pedicle valve)
known as the pedicle opening. The upper valve is known as the brachial valve
(Figure 8).
Figure 8. Brachiopod shell morphology.
There are two major categories of brachiopods, the inarticulates and the
articulates. Inarticulate brachiopods possess very simple, phosphatic shells
that lack hinge structures. The two valves of the shell are held together
only by muscles. Articulate brachiopods, in contrast, possess more complex,
8–10
calcitic shells with one or more “tooth and socket” pairs that serve as hinges
(Figure 9).
Figure 9. Internal view of the two valves of an articulate brachiopod shell, showing teeth and
sockets in the hinge area.
The inarticulate brachiopods are morphologically primitive and very
conservative evolutionarily. Modern inarticulates look very much like
Cambrian ones. In contrast, articulate brachiopods evolved into a vast array
of different types.
Paleoenvironmental Range:
Brachiopods are exclusively marine organisms that require normal marine
salinity. Individual species and genera may exhibit quite narrow
environmental preferences with respect to water depth, substrate,
temperature, etc. The distribution of brachiopod assemblages has been
used to define biofacies for use in paleoenvironmental reconstructions.
Stratigraphic Range:
Brachiopods originated in Cambrian time (or earlier?) and were very
abundant throughout the remainder of the Paleozoic Era. Many groups of
articulate brachiopods became extinct during the Paleozoic or near the end
of the Paleozoic. Today only the inarticulates and two orders of articulates
still persist.
Brachiopod Examples:
You may be asked to distinguish between inarticulates and articulates on the
Lab exam. Also, among the various groups of articulates, you should know
8–11
the spiriferids, productids, atrypids, and terebratulids (see additional
handout).
1. Lingula. Lingula is a living example of an inarticulate brachiopod. It is
sometimes called a “living fossil” because modern representatives look
essentially the same as their Paleozoic ancestors. Note the internal anatomy
and the fleshy pedicle.
2. Fossil inarticulates. Inarticulates are normally small and their pedicle and
brachial valves are almost identical. The whitish shell material is calcium
phosphate.
3. Spiriferids. Spiriferids are sometimes called “winged” brachiopods
because of their long, straight hingeline. Typically the shell is coarsely
ornamented with radial ridges and grooves. The shell is biconvex. In many
species the pedicle valve has a deep furrow (sulcus) extending from the beak
to the shell margin and the brachial valve has a corresponding high ridge
(fold).
4. Atrypids. Atrypids are closely related to spiriferids. Note that the
hingeline is straight and fairly long, but not as long as in the spiriferids. In
atrypids, the brachial valve is inflated or hemispherical and the pedicle valve
is essentially flat. The valves are ornamented by fine radial grooves and
ridges.
5. Productids. Productids are spine-bearing brachiopods, but in most
specimens the delicate spines have been broken off and all that remains are
scars where the spines were attached to the shell. Productids are also
distinctive in having a very large, inflated pedicle valve and a smaller, flat or
concave brachial valve (just the opposite of atrypids!).
6. Pentamerids. Pentamerids have biconvex shells with a very short
hingeline. Normally the surface of the shell is smooth.
7. Rhynchonellids. Rhynchonellids are “beaked” brachiopods that normally
have an overall triangular shape. The valves are biconvex and strongly
ornamented by radial grooves and ridges.
8–12
8. Terebratulids. Terebratulids are biconvex with a smooth shell with
delicate concentric ornamentation. The hinge is very short, but the pedicle
opening is prominent. In these specimens the brachial valve is smaller and
less convex than the pedicle valve. Note that rhynchonellids and
terebratulids are the only extant groups of articulate brachiopods.
8–13