Rare and Unusual Allenypterus Coelacanth from Bear Gulch

A rare paleozoic fish fossil

Allenypterus montanus

Crossoptergyii, Coelacanthiformes, Hadronectoridae

Geological Time: Mississippian (~320 m.y.a.)

Size (25.4 mm = 1 inch): Fish fossil is 130 mm long (tip of nose to tip of tail along backbone) by 60 mm high on a 155 mm by 165 mm matrix

Fossil Site: Heath Shale Formation, Bear Gulch Limestone, Fergus County, Montana

Allenypterus montanus CoelacanthDescription: The Bear Gulch Limestone is a deposit of some 70 square km in extent and 30 m in depth that has been a source of one of the most diverse assemblages of fossil fish with some 110 species having been described over the past 30 years. Most were new to science, and provided a unique view of the marine environment of Mississippian times. Fine preservation of both fish and invertebrates is a hallmark of these Allenypterusdeposits, presumably due to an anoxic depositional environment. This specimen is an example of an unusual fish known as Allenypterus with a half teardrop-like shape. Modern fishes of such morphology are often inhabitants of extremely weedy, sheltered environments. Locomotion is by undulation of the continuous dorsal portion of the caudal fin, and can be used for both forward and backward movement. The fin is quite readily seen in this example. This fish was first thought to be a bony finned fish, but was later found to actually be a coelacanth. It represents a radical departure from the typical coelacanth body shape. It has the most primitive of skull morphology. The armored ventral surface indicates it most likely fed near the bottom. Some of those armored plates have become disarticulated, allowing you to see their structure.

click to enlarge fossil pictures

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