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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.

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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. Non-

stromatoporoid 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?

Figure 2. Enlarged view of sponge spicules,

which occur in a variety of shapes ranging

from simple rods to elaborate spiky “stars.”

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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).

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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.

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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.

Figure 4. Thin section photomicrograph of a

stromatoporoid (section is perpendicular to

growth surface). Note well developed

laminae and pillars.

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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?

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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.

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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.

Figure 7. Examples of bryozoan

growth habits: (d) branching

(ramose); (E) lacy (fenestellate),

with enlarged view of zooecia at

right.

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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!

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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).

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,

Figure 8. Brachiopod shell morphology.

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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

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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.

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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.