Arthrodires were a type of armored fish that grew to more than 20 feet in length. They had thick bony armour plating around their head. Rather than teeth, many had sharp, pointed extensions of their lower jaw.

Fossil evidence shows that they fed on sharks and other fish. They were the largest predators of the Devonian.

Arthrodires had bony armor around their heads that can be preserved as fossils. The rest of these fish was composed of cartilage, muscle and other soft parts so was generally not fossilized.

Only one near-complete arthrodire is known from Kentucky. It was found in the Devonian-age New Albany Shale Formation. More commonly, fossils are found of individual bony plates of the arthrodires as shown below. These fossils sometimes occur in bone layers within the shale, consisting of the scattered remains of numerous arthrodires. Each of the bones has a shiny exterior, and preserves the spongy texture of the original bone in the interior. This is how they can be interpreted as bones. Also, some of the bone plates exhibit a bumpy (ornamented) texture that is typical of arthrodire bones. The jaw fragment is typical of arthrodire lower jaws and shows the solid construction of this fish's powerful mouth!

Links to more information about arthrodires:

• http://www.cmnh.org/collections/vertpaleo/jaws/index.html Jaws the early years , Cleveland Museum of Natural History. Story with pictures about the Cleveland shale in Ohio, from which many fossil sharks and arthrodires were uncovered. See a picture of a giant skull from the museum at http://www.cmnh.org/
• http://www.palaeos.com/Vertebrates/Units/Unit060/060.100.html#Arthrodira Arthrodires , Palaeos.  Technical description of arthrodires and their phylogeny with images of fossils
• A reconstructed arthrodire is on display at the Falls of the Ohio visitor center near Louisville, as is a cast of an arthrodire skull.  For more information go to the park website http://www.fallsoftheohio.org/visitor-info.html

Teeth, fin spines, and tooth-like scales of sharks (cartilaginous fish) are numerous in some Mississippian and Pennsylvanian rocks in Kentucky. Sharks of the past, as in the present, had skeletons made from cartilage. Cartilage does not easily fossilize. Hence, the fossils left behind by sharks are generally limited to teeth, large fin spines, and tooth-like scales.

Fossil evidence shows that they fed on sharks and other fish. They were the largest predators of the Devonian.

Some shark-like fish, called holocephalians (or chimaerids), had flat teeth for crushing shells, whereas others, like the tooth in Figure 1 above, had more typical sharp teeth for capturing swimming prey (made for puncturing and holding fish and cephalopods?). The elongate fossil (3) is a fossil fin spine from a shark. Although some species of sharks have fin spines today, they are not common. Sharks with fin spines were common in Devonian, Mississippian and Pennsylvanian sharks of Kentucky.

Many types of sharks lived in Kentucky at that time and some had teeth for capturing swimming animals and others had teeth especially adapted for crushing and eating shellfish such as brachiopods, clams, crinoids, and squid-like animals (cephalopods). One cephalopod fossil that had puncture marks of a shark that had bitten it was found in the Pennsylvanian rocks of the Eastern Kentucky Coal Field. Some Pennsylvanian sharks lived in fresh water and had forked teeth that were designed to catch other fish and swimming animals.

Mississippian-age shark fossils gallery

Pennsylvanian-age shark fossils gallery

External links

• Some Kentucky chondrichthian fossils, at Kentucky Paleontological Society webpage
• Pennsylvanian fossils of Missouri: Sharks and fish

Most modern fish belong to the bony fish or osteichthyes group. Several types of bony fish fossils have been found in Kentucky.

The ray-finned palaeoniscid fish (outlined by white dashes) above was found in a coal mine from the Western Kentucky Coal Field. The fossil consists of an imprint of scales in a dark shale. The fossil is very subtle, and only when light is shining at the right angle are the scales well illuminated. Such fossils are easily overlooked. Isolated fossil-fish scales and teeth are abundant in several Devonian, Mississippian and Pennsylvanian strata. A 1-inch-long scale of a Rhizodopsis fish that was up to eight feet long was found in the Pennsylvanian rocks of the Eastern Kentucky Coal Field. Several large rhizodont fish skeletons have been found in the Mississippian strata in western Kentucky.