# Four Mile Dam Formation

The Four Mile Dam Formation, also called the Four Mile Dam Limestone, is a geologic formation in Michigan. It preserves fossils dating back to the middle Devonian period.

## Fossil content
### Vertebrates
#### Acanthodians

Acanthodians reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Material | Notes | Images |
| 'Acanthodii' | Unidentified | | "Two partial fin spines (JS 120, JS 121)". | "Distinct from named acanthodians (Gyracanthus, Machaeracanthus, and Oracanthus) and chondrichthyans (Tamiobatis and Acondylacanthus) reported from Michigan". | |

#### Conodonts

Conodonts reported from the Four Mile Dam Formation :
| Genus | Species | Notes |
| Acodina | A. formosa | Also found in the Detroit River, Traverse, Dundee, Bell, Alpena and Norway Point formations. |
| Belodella | B. devonicus | Also found in the Detroit River, Traverse, Antrim, Alpena and Norway Point formations. |
| Icriodus | I. latericrescens latericrescens | Also found in the Traverse, Bell, Ferron Point, Genshaw, Newton Creek, Alpena, Norway Point, Potter Farm and Thunder Bay formations. |
| Ozarkodina | O. willsi | Also found in the Detroit River, Traverse, Antrim, Dundee, Alpena and Norway Point formations. |
| Polygnathus | P. varcus | Also found in the Traverse, Antrim, Alpena, Norway Point and Thunder Bay formations. |

#### Placoderms

Placoderms reported from the Four Mile Dam Formation :
| Genus | Species | Material | Notes |
| ?Macropetalichthys | ?M. sp. | "A partial head shield (32M)". | A petalichthyid. |
| ?Mylostoma | ?M. sp. | 39 specimens, including potential juveniles. | |
| 'Placodermi' | Unidentified | "One specimen of an armour plate (JS 4)". | |
| Protitanichthys | P. rockportensis | "One specimen of an armor fragment (JS 101)". | A coccosteid. |

### Invertebrates

| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

#### Brachiopods

Brachiopods reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Material | Notes |
| Douvillina | D. distans | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | Also found in the Hungry Hollow Formation and Logansport Formation. |
| Leptalosia | L. radicans | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | Also found in the Norway Point, Gravel Point, Alpena and Genshaw formations. |
| Longispina | L. emmetensis | Dock Street clay member. | | Also found in the Alpena Limestone and Gravel Point Formation. |
| Megastrophia | M. gibbosa | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | Also found in the Hungry Hollow Formation and Logansport Formation. |
| Oligorhachis | O. oligorhachis | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | Also found in the Gravel Point Formation. |
| Pentamerella | P. alpenensis | Dock Street clay member. | | Also found in the Alpena Limestone and Gravel Point Formation. |
| Pentamerella | P. sp. C | Dock Street clay. | A complete specimen. | |
| Protoleptostrophia | P. lirella | Dock Street clay member. | | Also found in the Norway Point Formation and Alpena Limestone. |
| Schuchertella | S. anomala | Possibly the Dock Street clay member. | | Also found in the Gravel Point Formation. |
| Schuchertella | S. sp. | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | |
| Spinulicosta | S. mutocosta | Four Mile Dam, Thunder Bay River, Alpena County, Michigan. | | Also found in the Bell, Rockport Quarry, Ferron Point, Beebe School, Thunder Bay and Potter Farm formations. |
| Strophodonta | S. erratica | Dock Street clay member. | | Also found in the Alpena Limestone and Gravel Point Formation. |
| Strophodonta | S. fissicosta | Dock Street clay member. | | Also found in the upper Gravel Point Formation. |
| Strophodonta | S. nanus | Dock Street clay member. | | Also found in the Alpena Limestone and Gravel Point Formation. |
| Strophodonta | S. paula | Dock Street clay member. | | Also found in the Alpena Limestone. |
| Strophodonta | S. sp. B | Dock Street clay member and overlying crystalline limestone beds. | Several specimens. | |
| Rhipidomella | R. penelope traversensis | Thunder Bay River, Alpena County, Michigan. | | |

#### Corals

Corals reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Notes |
| Aulocystis | A. cooperi | | Also found in the Plum Brook shale, Silica shale and Potter Farm Formation. |
| Aulocystis | A. jacksoni | Dock Street clay member. | Also found in the Silica Shale, Ferron Point Formation, Petoskey Formation, Hungry Hollow Formation, Widder Shale, Wanakah shale and Centerfield Limestone. |
| Aulopora | A. conferta | | Also found in the Bell, Ferron Point, Gravel Point, Genshaw, Alpena, Potter Farm, Petoskey and Hungry Hollow formations. |
| Aulopora | A. gregaria | | Also found in the Genshaw Formation, Alpena Limestone and Petoskey Formation. |
| Pachyphragma | P. concentricum | Dock Street clay member. | Also found in the Ferron Point Formation, Gravel Point Formation and Alpena Limestone. |

#### Gastropods

Gastropods reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Material | Notes |
| Anematina | A. conica | Upper limestone unit. | | An elasmonematid. |
| Anematina | A. mellaria | Upper limestone unit. | | An elasmonematid, very rare in the Four Mile Dam Limestone. |
| Bembexia | B. (Genuspira) nodosa | Upper limestone unit. | About 35 specimens. | An eotomariid and the most abundant gastropod in the formation. |
| Dictyobembix | D. bella | Upper limestone unit. | 5 specimens. | An eotomariid. |
| Mourlonia | M. lirata | Upper limestone unit. | | An eotomariid. |
| Murchisonia | M. (Murchisonia) subulata | Upper limestone unit. | | A murchisoniid, extremely rare in the formation. |
| Naticopsis | N. sp. | Upper limestone unit. | | A neritopsid, extremely rare in the formation. |
| Palaeoscurria? | P.? sp. | Upper limestone unit. | 3 poorly preserved ventral molds. | A metoptomatid. |
| Platyceras | P. (Platyceras) bartlettense | Dock Street Clay Member. | | A platyceratid. |
| Platyceras | P. (Euthyrachis) indianense | Dock Street Clay Member. | | A platyceratid. |
| Platyceras | P. (Orthonychia) variabilis | Upper limestone unit. | | A platyceratid. |
| Pseudozygopleuridae? Genus A | Genus A sp. A | Upper limestone unit. | Relatively numerous steinkerns. | |
| Pseudozygopleuridae? Genus B | Genus B sp. B | Upper limestone unit. | One very poorly preserved steinkern with a few small shell patches (YPM 23684). | |
| Spiroscala | S. sp. | Upper limestone unit. | One specimen (UMMP 47377). | An eotomariid. |
| Straparollus | S. (Philoxene) sp. | Upper limestone unit. | 3 poorly preserved specimens. | An euomphalid. |
| Trepospira | T. (Angyomphalus) bella | Upper limestone unit. | | A raphistomatid, very rare in the Four Mile Dam Limestone. |
| Turbinilopsis | T. anacarina | Upper limestone unit. | Abundant specimens. | An anomphalid. |

#### Sponges

Sponges reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Material | Notes | Images |
| Anostylostroma | A. anacolumna | On Thunder Bay River. | | A stromatoporoid. | |

#### Trilobites

Trilobites reported from the Four Mile Dam Formation :
| Genus | Species | Presence | Notes |
| Cordania | C. rara | | |
| Dechenella | D. (Monodechenella) macrocephala | | Also found in the Hamilton Group. |
| Dechenella | D. (Basidechenella) rowi | | Also known from the Hamilton Group. |
| Eldredgeops | E. rana | | Originally reported as Phacops rana. Also found in the Hamilton, Hungry Hollow, Widder, Plum Brook, Prout, Ten Mile Creek, Alpena, Norway Point and Gravel Point formations. |
| Greenops | G. aequituberculatus | Dock Street clay member. | Also found in the Norway Point and Gravel Point formations. |
| Greenops | G. traversensis | Dock Street clay member. | Also found in the Gravel Point Formation. |
| Phacops | P. rana | | Reassigned to the genus Eldredgeops. |
| Proetus | P. (Crassiproetus) traversensis | | Also known from the Gravel Point Formation. |