Mysterious Sponges

Lake Michigan Sponge Fossil, Side View

This is one of the more unusual fossil finds from our shores of Lake Michigan in Southwestern Michigan USA! I’m 99.9 % sure it’s a fossilized sponge and not a coral due to the lack of septa, vertical growth walls. From the side view, you can see how the tubes, or pores, permeate down into the structure.

Lake Michigan Sponge Fossil, Top View

INTERESTING SPONGE FACTS:   The most fascinating fact about sponges is their long extended history on Earth beginning 580 million years ago. The type of sponges we’re most familiar with are the ones we use for our households. Those are actually referred to as, demosponges, having entirely soft fibrous skeletons with no hard elements of which there are only a few species. After thousands of years, humans had almost harvested them to extinction by the 1950’s.  Many are now being researched for a possible source of medicines.

Popular Tourist Spot Selling Natural Sponges, Source:

HOW SPONGES LIVE: Sponges have delicate skeletons and rely on a constant water flow through their bodies to capture food and obtain oxygen. They don’t have a digestive or circulatory system like we do. They can actually change the shape of their bodies for maximum water flow.  Most species have the ability to contract and squeeze the water out of their pores in order to flush out sediments clogging them. They can even  escape from predators by squeezing out the water and shrinking themselves. For further defense, many shed spiky spicules to create a dense hazardous carpet around them which keeps away predators such as star fish.


Kingdom:  Animalia       Phylum:  Porifera (means to possess pores)

Four Classes

Demosponges Largest class; Inner structure reinforced with collagen fibers and spine-like spicules made of silica minerals; Usually barrel shaped; Can live in a wide variety of habitats

Hexactinellida – Glass Sponges; Spiny spicules made of silica minerals forming inner scaffolding structure with gelatin substance weaved in between framework; likes Polar Regions

Calcareous – Outer exoskeleton and inner spicules made of calcium carbonate. Restricted to shallow marine waters where production of calcium carbonate is easiest to obtain.

Scleropongiae (Coralline or Tropical Reef Sponges) soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite.  The layered skeletons look similar to reef corals, therefore are also called coralline sponges.



Amazing Ammonites

Ammonite Fossil (Douvilleiceras, mammilatum)

Douvilleiceras mammilatum was a marine cephalopod ammonite, which are ancestors of today’s chambered nautilus. It possessed well-defined growth patterns on its shell (sutures). Douvilleiceras‘ knobs and spines are thought to be an indicator of a hostile environment. It lived during the Cretaceous Period (145 to 66 mya) and was unearthed in Madagascar (Albin Formation).

Cretaceous Period Oceanic Environment (145-66 million years ago) Artist Rendition
fossils 127
Mortoniceras sp Ammonite Fossil

The above fossil is a broken off section from another ammonite’s shell belonging to the genus, Mortoniceras sp. It was found in Arkansas in a dried up riverbed within the limestone, Goodland Formation. The shell is characterized by deep keels and ribbing. It lived mainly during the Cretaceous Period (145 to 66 mya). As with all the ammonites, its fate was doomed side by side with the dinosaurs.

fossils 126
Mortoniceras Ammonite Fossil (Top View)
Multi-purpose Tentacles

The ammonites were ocean predators grabbing their victims with precision and crushing them with their long, powerful tentacles. These tentacles contributed to another important function. They contained special sensors which facilitated their ability to navigate and locate prey in the vastness of the ocean.

A Complete Sample of Mortoniceras sp from Texas, Fort Worth Formation, Tarrant County
Function of Inner Chambers

Ammonites moved in spurts using a kind of jet propulsion by siphoning the ocean water into inner chambers inside their shells and then pushing the water out powerfully through a tube structure called a siphuncle. These inner chambers held water and special gases which helped it descend deep down ocean depths or float upward to shallower depths by filling and releasing the gases and water in and out of the chambers.

Ammonites possessed large heads and are assumed to have been highly intelligent like their cousin octopuses, squids, cuttlefish, nautilus etc. Scientist debate whether ammonites contained ink sacs for defense.

Inner Chambers of Ammonite Fossil

See two gigantic ammonite fossils from another article I have written (scroll to the bottom of article).


Scientific Name: Mortoniceras

Common Name: Ammonite

Kingdom: Animalia

Phylum: Mollusk (large diverse group of invertebrates possessing a shell, i.e. clams, snails, oysters, etc.)

Order: Ammonitida (characterized by thick, ribbed patterned shells)

Class: Cephalopod (means prominent head and tentacles, i.e. octopuses, cuttlefish, squids, nautilus)

Family: Brancoceratidae

Genus: Mortoniceras (characterized by deep keels, tubercules (knobs) and ribbing)

Mortoniceras Ammonite Rendering Drawing