PALEOZOIC FOSSIL FISH
The oldest known fish bones are fragments from the Upper Cambrian. Soft-bodied chordates referred to as “vertebrates” or “fish” are known from the Lower Cambrian and Middle Cambrian. The earliest, decent fossils of true fish are in the Ordovician. These remains show that the first fish lacked jaws. The evolution of jaws involved morphologic modification of the anterior pairs of gill arches. By the Devonian, jawed fishes had diversified into many distinct groups and became an important component of ancient oceans. The Devonian is often nicknamed “The Age of Fishes”.
Left: Cardipeltis richardsoni Denison, 1966. Right: Cardipeltis bryanti Denison, 1966. These fossil fish are from the Lower Devonian Beartooth Butte Formation of the western flank of the Bighorn Mountains, northern Wyoming, USA (left: FMNH PF 3897; right: FMNH PF 3895, Field Museum of Natural History, Chicago, Illinois, USA). Cardipeltis is an agnathan fish with a bony exoskeleton. Agnathans are jawless, and represent the oldest-known, ancestral fish group. Agnathans are still alive today - they include the hagfish. Early agnathans with bony exoskeletons are often called ostracoderms. The fossils shown above are rare, articulated specimens of Cardipeltis. The Cardipeltis richardsoni fossil on the left shows the dorsal (upperside) surface. The Cardipeltis bryanti fossil on the right exposes the ventral (underside) surface. The elongated structure at the bottom of each specimen is the tail. Cardipeltis has a dorso-ventrally compressed body, consistent with a benthic swimming lifestyle.
Classification: Animalia, Chordata, Vertebrata, Agnatha, Heterostraci/Pteraspidiformes, Cardipeltidae
Bothriolepis canadensis Whiteaves, 1880 from the Upper Devonian Escuminac Formation at Miguasha, southern Gaspé Peninsula, southeastern Quebec, southeastern Canada (FMNH PF 3811, Field Museum of Natural History, Chicago, Illinois, USA). Bothriolepis is an antiarch placoderm fish. The antiarch placoderms are an extinct group of mid-Paleozoic fish that had jaws and were heavily armoured with bone. Even the pectoral fins were surrounded by bone (see the pair of “spines” extending from the sides of the body). The body is dorso-ventrally flattened with upward-facing eyes and a mouth on the ventral side of the snout. Bothriolepis was a bottom-dwelling fish that lived or merely spawned in freshwater facies.
Classification: Animalia, Chordata, Vertebrata, Placodermi, Antiarchi, Bothriolepidae
Diplacanthus striatus (Agassiz, 1835) from the Lower Devonian of Scotland (FMNH PF 2198, Field Museum of Natural History, Chicago, Illinois, USA). Acanthodians are an extinct group of jawed fish that had spines along the anterior margin of each fin (see the blackish-colored spikes on the specimen shown above). The root word “acanth” in “acanthodian fish” means “spine”. Acanthodians were the first fish to evolve jaws and scales.
Classification: Animalia, Chordata, Vertebrata, Acanthodii, Climatiiformes, Diplacanthida, Diplacanthidae
Helicoprion ferrieri (Hay, 1907) - symphyseal tooth whorl of an edestoid shark in fossiliferous limestone from the Decie Ranch Member of the Skinner Ranch Formation (Wolfcampian Series, lower Lower Permian) at Dugout Mountain, northern Brewster County, Glass Mountains, western Texas, USA (FMNH PF 7445, Field Museum of Natural History, Chicago, Illinois, USA).
Sharks have a cartilaginous skeleton and mineralized, phosphatic teeth (as are all vertebrate teeth). Helicoprion is undoubtedly the oddest shark in geologic history (see reconstruction below). The specimen shown above is described in Kelly & Zangerl (1976) - Helicoprion (Edestidae) in the Permian of West Texas. Journal of Paleontology 50: 992-994.
Classification: Animalia, Chordata, Vertebrata, Chondrichthyes, Elasmobranchii, Eugeneodontida, Edestoidea, Agassizodontidae/Helicoprionidae
Helicoprion shark reconstruction (public display, Field Museum of Natural History, Chicago, Illinois, USA). Some paleontologists have interpreted the tooth whorl as part of a coiled lower jaw that may have been whipped outward and back to capture fish prey. Although intriguing, this type of reconstruction is probably incorrect. Instead of an external lower tooth whorl, the tooth whorl was likely internal (see another reconstruction).
Allenypterus montanus Melton, 1969 from estuarine deposits of the Bear Gulch Lagerstätte (Bear Gulch Limestone Member, Heath Formation, Upper Mississippian) at Potter Creek Dome, southeastern Fergus County, central Montana, USA (FMNH PF 10939, Field Museum of Natural History, Chicago, Illinois, USA). Coelacanths are one of three groups of lobe-finned fish (sarcopterygian bony fish). They are known as fossils in the Paleozoic and Mesozoic, but not in the Cenozoic. Regardless, coelacanths are still alive in modern oceans (see Latimeria). Classification: Animalia, Chordata, Vertebrata, Sarcopterygii, Crossopterygii, Coelacanthiformes/Actinistia, Hadronectoridae
Gonatodus brainerdi (Thomas, 1853) in quartzose sandstone from the Berea Sandstone (~lowermost Mississippian) at a quarry in Chagrin Falls, Cuyahoga County, northeastern Ohio, USA (FMNH P 25164, Field Museum of Natural History, Chicago, Illinois, USA). Gonatodus represents an early actinoptyergian bony fish. Numerous groups of bony fish in modern oceans are actinopterygians, the most successful group of jawed fish in Earth history.
Classification: Animalia, Chordata, Vertebrata, Osteichthyes, Actinopterygii, Palaeonisciformes, Palaeoniscoidea, Elonichthyidae