Sponges

Again life gives us an earlier and richer understanding.

As better tools are developed by scientists, we learn the history of life expands further into deep time, much like the Hubble Space Telescope can look farther back into the solar system.

Figure 1. From geochemist, Gordon Love, and his analysis of sedimentary deposits mined by the national oil company of Oman.

We see this in Prof. Love's research on sponges. Their “fossil” remains were more like fossil echoes – chemical traces of a compound only produced by descendants of what some scientists consider the last common ancestor of all animals – the demosponges.

This discovery pushed deeper into time, 100 million years before the early Cambrian period. When the sponges died, they didn’t leave characteristic bones behind. They left molecules. And those molecules are just as diagnostic as more traditional fossils.

The fossil molecule was a fatty chemical called 24-isopropylcholestane, found only in the skeletal structures of demosponges, the most common member of the sponge family.

Figure 2. A Demosponge named the Barrel Sponge.

To understand more let's look a little closer.

The Beauty of Sponges

Sponges belong to a group of animals called Porifera (pore-bearers), some of the simplest and oldest of the
multicellular animals.

Sponges are filter feeders – pulling water through their pores and pushing water out through their top openings.

In this process, they capture and eat particles as small as bacteria as well as larger particles.

Here's a diagram:

Figure 3.

This is how a sponge works

Ostia The tiny holes where the water is pulled in

Oscula One or more larger holes where the water flows out

Collar Cells (or choanocytes)

Specialized cells that line the canals of the sponge
Have a sticky, funnel shaped collar and a hairlike whip, a flagellum
The flagella beat back and forth to force water through the sponge

– Brings in nutrients and oxygen

– Pushes out waste and carbon dioxide

The sticky collars pick up tiny bits of food from the water, while another type of cell, an amebocyte, carries the food to the other sponge cells.

Not shown in the diagram are important structures, some which relate to their fossil remains.

Between the outer and inner walls, sponges have a gelatinous (jello-like) matrix, called the mesohyl.
The amebocytes that carry the food do so through this jello matrix.
Skeletal elements reinforce the matrix, like a 2 by 4 frame of a house:

Calcareous sponges produce spicules made of calcium carbonate.
Demosponges use a special form of collagen called spongin.
Most sponges produce spicules of silica.
A few secrete massive external frameworks of calcium carbonate.

Here's a picture of fossil spicules:

Figure 4. Several fossilized spicules.

Coming back to the functioning of a sponge, here's a Jonathan Bird photo showing water flow:

Figure 5. A non-toxic yellow dye was squirted around the base of a purple tube sponge in the Caribbean. And soon, the dye was pumped out through the osculum.

Sponges do not have nervous, digestive, or circulatory systems. They depend upon maintaining a constant water flow to obtain food and oxygen and to remove wastes.

They attach themselves to the bottom, and their shapes are adapted to maximize the efficiency of the water flow.

Where Do Sponges fit in with other animals?

Sponges historically come from Metazoan's which have their roots in the phylum Eukaryota. So, let's go back and follow the branches up the tree of life.

The Eukaryota include the organisms that most people are most familiar with:

Animals.
Plants.
Fungi.
Protists.

All Eukaryota share fundamental characteristics of cellular organization, biochemistry, and molecular biology.

Figure 6. Tree of Life – find Animals.

In the above tree, Metazoans are the animal branch.

Figure 7.

As you can see, an early branching event in the history of animals separated the sponges from other metazoans.

And based on their position on the tree, fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.

Since then, sponges have been conspicuous members of many fossil communities; the number of described fossil genera exceeds 900 with 5,000 living species.

So, let's place them in the big picture, the Geologic Time Scale.

Here we can see the range of time and the history of what was alive in those ages.

Figure 8.

Note Prokaryotes (magenta band) go back 4 Ga (4 Billion) at the end of the Hadean. At 3.5 Ma, Photosynthesis starts in the Archean (pronounced /ɑrˈkiːən/), and at half of the age of our Earth, 1.2 million years later, the atmosphere becomes oxygen-rich.

Quick Review

With the use of sophisticated, modern tools, science is discovering that life began further in deep time.
Through the use of chemical fossils, sponges were discovered to have begun much earlier in Earth's history.
Sponges have a simple form:

- Ostia The tiny holes where the water is pulled in.

- Oscula One or more larger holes where the water flows out.

- Collar Cells.

- A matrix between the outer and inner walls.

- Amebocytes that carry the food through the jello-like matrix.

- Skeletal elements, called spicules, reinforce the matrix.

Sponges branched from Metazoans which branched from Eukaryotes.

A Gallery of Fossils

Sponges on this page are from the Cretaceous period.

Figure 9.

A Burgess Shale Sponge – MidCam-Eiffelia

A complex globular sponge with a body supported by six-rayed calcareous spicules in various sizes.

Figure 10.

Usually sponges fall apart upon death, and only the isolated spicules, if anything, remain. Sometimes, however, a sponge may be quickly covered with sediment and preserved as a mold, as happened to this unidentified specimen from Late Eocene rocks near Leesburg, GA. Photo by B. Carter.