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”.
AGNATHANS
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
PLACODERMS
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
ACANTHODIANS
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
SHARKS
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).
COELACANTHS
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
ACTINOPTERYGIANS
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