Vertebrate Fossils

Of all the vertebrates, fish have the greatest number of forms. Their many adaptations have allowed them to live in a variety of habitats, ranging from hot springs to great ocean depths. They are scavengers, grazers, filter feeders, herbivores, omnivores, and carnivores. During the “Age of Fish” (Devonian Period), they rapidly diversified. More recently (within the last million years), dozens of species have evolved in African freshwater lakes. The vast majority of living fish (about 18,000 species) belong to the group of ray-finned fish, including trout, salmon, and tuna. Their name comes from the presence of “rays”—fine bony or horny spines that support the webbing of their fins.

The oldest fossil fish, which are the earliest known vertebrates, lacked jaws (the mouth was a simple opening) and were covered on the outside with a bony skeleton. The first ones, the jawless fish (Agnathan), arose during the Cambrian. Some of these freshwater fish were heavily armored and made robust fossils, particularly the head shields. A few living jawless fish (e.g., hagfish and lampreys) are the only remnants of this ancient group.

Sharks and their relatives are the most successful, long-term survivors among the vertebrates. The first sharks appeared during the Devonian Period. Because their skeletons are made of cartilage, which does not preserve well as a rule, most evidence of shark-like fish comes from their teeth.

The Devonian placoderms were a powerful group of fish with armor-plated heads and frequently fang-like teeth, the function of which was undoubtedly predatory. Placoderms were the first fish with jaws. They became extinct during the Permian Period.

A particularly interesting group of fish is the lobe-finned fish, which wereas fully developed by the Devonian Period. Present-day lung fish and coelacanths are part of this group. Debate still rages as to whether the coelacanths—presumed to be close relatives of the Rhipidistia fish from which tetrapod amphibians supposedly arose—are our closest tetrapod ancestors. Some scientists believe that lung fish, another very ancient line, are more closely related to tetrapods than the Rhipidistia and thus claim the oldest-closest-living-relative title.

In the National Park System, Fossil Butte National Monument in Wyoming and Florissant Fossil Beds National Park are recognized for fish fossils. Fossils of whole fish are generally rare but large numbers with many beautiful varieties occur at Fossil Butte.

Among other adaptations that made life on land possible (e.g., strong bony supports), the first vertebrates to be considered tetrapods (e.g., Ichthyostega and Acanthostega) evolved feet rather than fins. Tetrapods have four (“tetra”) muscular limbs—so-called feet or “podae”—with discrete digits (i.e., fingers and toes). All land-dwelling vertebrates including modern species can be considered tetrapods, but the earliest ones appeared during the Devonian Period and became extinct during the Cretaceous Period. Their features were most similar to amphibians though still different from modern types.

Paleontologists have described tetrapods from the Late Triassic Period at Petrified Forest National Park in Arizona. The Pennsylvanian Wescogame Formation at Grand Canyon National Park hosts the oldest known tetrapod tracks in the National Park System. Investigators identified these tetrapod tracks as amphibian in nature.

Frogs, toads, newts, and salamanders are living amphibians. Amphibians as a whole represent an early level of terrestrial habitation, though they were (and still are) tied to water for reproduction purposes; that is, their larvae develop in water. Not surprisingly amphibian means “double life” in Greek, though some species are entirely aquatic.

Paleontologists have not completely determined the evolutionary relationships of amphibians, in part because a large gap in the fossil record exists between modern amphibians (Jurassic to Neogene) and the fossil record of their extinct relatives (Mississippian to Permian). Amphibians diversified during the Mississippian and Pennsylvanian periods, attaining a variety of species and abundance never again equaled. More-familiar amphibians such as frogs, toads, newts, and salamanders appeared later, during the Jurassic Period. All of the walking species of amphibians had widely splayed legs, which gave them a slow, lumbering gait, but in the absence of more streamlined animals, they prospered.

The fossil record of amphibians in the National Park System is not great; however, Fossil Butte National Park in Wyoming has notable Pleistocene salamanders from “Salamander Cave” and an Eocene frog specimen. Also, amphibian teeth occur in the Mississippian Hinton Formation at New River Gorge National River in West Virginia.

Like birds and mammals, reptiles are amniotes, that is, their embryos develop within a fluid-filled sac called an “amnion.” In the case of birds, reptiles, and monotreme mammals, the amnion is covered by a protective shell. Unlike birds and mammals, which are warm blooded, reptiles are cold blooded (without their own internal heat regulator) and cannot successfully live in cold climates. Hence, the warmer the climate, the more reptiles are in evidence.

Reptiles were probably derived from an amphibious ancestor early in the Pennsylvanian Period. Once established, they underwent a number of evolutionary bursts starting in the Triassic in which diverse kinds of reptiles—the most spectacular of which were dinosaurs—occupied a variety of terrestrial habitats. The National Park System hosts a sandbar that became the “graveyard” of many dinosaurs at Dinosaur National Monument. By no means were all of the large reptiles dinosaurs, however. Other reptiles now extinct took up life in the sea (e.g., plesiosaur, ichthyosaur, and mosasaur) and in the air (e.g., pterosaurs). Though better known for its mammal fossils, Badlands National Park in South Dakota hosts reptilian fossils, for instance, mosasaurs, which inhabited the Cretaceous Interior Seaway. The reptilian line continues today with alligators, crocodiles, and caiman being the current “giants.” Birds, lizards, snakes, and turtles also trace their lineage from Triassic reptilian ancestors. Again, Badlands National Park is notable for its turtle fossils (Stylemys), which lived during the Eocene and Oligocene epochs. Valley Forge National Historical Park in Pennsylvania hosts more recent (Pleistocene) fossil turtles.

The term “mammal-like reptiles” is somewhat out of date and a more correct term is “basal synapsid.” This now-extinct group was neither fully reptilian nor fully mammalian but had features characteristic of both; hence, the use of “mammal-like reptile” here. The first appearance of a basal synapsid occurs in the fossil record during the Pennsylvanian Period; all members of this group went extinct during the Paleogene Period. Many small, rodent-like creatures were part of this group. However, this group also includes spectacular Dimetrodon, other sail-backed pelycosaurs, and crocodile-like ophiacodonts. The carnivorous gorgonopsian, another mammal-like reptile, was about the size of a large dog but had teeth like a saber-toothed cat. The heavy bones, short limbs, and large skull of herbivorous dicynodonts strike a resemblance to pigs and hippos today.

The Triassic Chinle Formation at Petrified Forest National Park has yielded Placerias fossils—a massive, mammal-like reptile that weighed as much as two tons. Placerias had a short neck, barrel-shaped body, small tail, and a beak-like skull with large tusk-like bones protruding from its upper jaw. The beak-like jaws helped this herbivore pull up and tear tough plants and roots.

Birds are bipedal, warm blooded, amniotic vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. They range in size from the tiny hummingbirds to the huge ostrich and emu. Depending on various taxonomic considerations, between 8,800–10,200 bird species live today (plus about 120–130 that have become extinct in the span of human history), making them the most diverse class of terrestrial vertebrates.

Birds are the rarest of vertebrate fossils because their characteristically light, hollow bones and skulls with large eye sockets do not preserve well. However, some spectacular bird fossils have been found in the fossil beds near Florissant Fossil Beds National Monument and in the Green River Formation at Fossil Butte National Monument. The first bird (Archaeopteryx) appeared during the Jurassic Period.

The presence of mammary glands that produce milk for the nourishment of young, hair or fur, and warm-blooded bodies characterize mammals today. The earliest mammals, which had some but perhaps not all of these characteristics, arose during the Triassic Period. Mammals were a continuous, if subordinate, component of fossil fauna throughout the Mesozoic Era. However, only after the demise of reptiles, particularly dinosaurs, did the evolutionary advantages of mammals propel them into dominance, which they have had since.

Three mammalian groups with relatives today had their start in the Cretaceous Period: (1) placental mammals such as today’s bats, whales, and the majority of land mammals; (2) marsupial mammals such as opossum and kangaroos; and (3) monotreme mammals such as the platypus. Female placental mammals have a temporary organ called a placenta that is present during pregnancy. Females of most marsupial mammals have a pouch called a marsupium in which they rear their young through early infancy. Monotreme mammals lay eggs instead of giving birth to live young as marsupial and placental mammals do.

The story of mammalian evolution is complicated by the break-up of the supercontinent Pangaea. As Pangaea split apart, mammalian faunas began to evolve on fragmented pieces of continental crust. Geographically separated from their relatives, isolated mammal groups evolved adaptations more suitable for the environments in which they lived. Though mammals on separate continents may have had similar lifestyles, their adaptations varied widely. For instance most monotremes and marsupials evolved in South America. Monotremes are only known from South America and Australia, and the only North American marsupial is the opossum. The major diversification of marsupials occurred in South America and Australia.

Among the many extinct groups of mammals are condylarths (primitive hoofed mammals), pig-like entelodonts, chalicotheres (large horse-like but with claws on their limbs), rhinoceros-like brontotheres, mammoths and mastodons, and saber-toothed cats. Cenozoic fossil parks such as Florissant Fossil Beds, Fossil Butte, John Day Fossil Beds, Agate Fossil Beds, Hagerman Fossil Beds national monuments, and Badlands National Park preserve fossils of many of these extinct groups of mammals.