Fossil reptiles and amphibians

Fossilization is a natural process in which organic remains are buried in sediments and become mineralized over time. The most interesting fossils of the ancient amphibian and reptile classes are those that demonstrate the transitional species, intermediate between those classes and having characteristics of both. Although many more nontransitional species are known, the transitional species are absolutely necessary for filling in the gaps between class characteristics and demonstrating the manner in which adaptive changes in species may have given rise to an entirely new class of creatures.

Sedimentary Rocks and Sandstones

Fossils are found in sedimentary rocks and sandstones, of which there are many different forms. Each form has its own unique properties and physical characteristics, and these may even differ within the same stratum from location to location because of such differential effects as weather, erosion, and mineral content.

Sedimentary rock and sandstones are formed through the accumulation and long compression of layers of sediment or sand. Most were formed in locations where fine water-borne silt settled out as the movement of flowing water slowed. Typically, this settling would have been an annual event reflective of seasonal runoff in temperate regions; in other regions, the runoff from passing rainstorms would carry silt and sand. The more rapid and turbulent the flow of the runoff, the coarser would be the content of the silt. In arid regions, windblown dust and sand could accumulate over time.

Time is another ingredient in the formation of sedimentary rocks and sandstones. The deposition of sediments occurs over millions of years, and the weight of material in the deposit brings about geochemical changes that bind the loose silt and sand together in the form of rock.

Formation of Fossils

The remains of living organisms became embedded in the silt and were covered by successive, deposited layers. It is believed that the slow percolation of mineral-carrying water through the material would facilitate the replacement of organic material, especially bone, with various minerals. This process, taking place within the matrix of the sedimentary layers, eventually turns the calcareous remains into a stone that is separate from the stone that has formed from the matrix.

The entrapment of organic remains occurs by many mechanisms. The death of a creature could result in its remains simply settling to the bottom of a body of water or being swept away by flowing water to settle in a different location. Flash floods from sudden heavy rains could wash away and kill various creatures, sometimes covering them in thick mud. The sudden collapse of a sand dune could bury an unsuspecting creature. Paleontologists use the distribution of fossil remains to reconstruct the type of event that may have brought the remains to that location and the condition in which they were found. Examples of the foregoing scenarios have been found, including an extraordinary fossil from Mongolia in which a velociraptor and a protoceratops were trapped by a falling sand dune while locked together in mortal combat.

The Devonian Period

The Devonian period (415–355 million years ago) is believed to have been a warm period in Earth's history. Reconstructions of sea surface temperatures from geological sources from that time indicate an average temperature of 30 to 33 degrees Celsius (about 86 degrees Fahrenheit). Water levels were correspondingly higher than during colder periods, encouraging the development of fish species in nutrient-rich shallow waters. The Devonian period is accordingly known as the age of fishes and is marked by the dominance of the placoderms, by the first appearances of ray-finned and lobe-finned fishes, and by the proliferation of primitive shark species.

Several important developments in the evolutionary history of fish took place during the Devonian period, the most important being the transition from fish to amphibian and reptile. In this evolutionary process, fish became the common ancestral line of all terrestrial vertebrates.

The Transition from Fish to Amphibians

The direct ancestors of the amphibians were the crossopterygian lobe-finned fishes, of which Coelacanthus is a representative member. The dental and cranial structures of the Crossopterygii are similar to those of the earliest known amphibians. They are characterized by teeth having a highly folded structure and by a cranium covered by bony plates having two nasal openings, two eye sockets, and one pineal orifice. Transitional fossils of creatures in this transition from fish to amphibian have been notably absent from the fossil record, and while species such as Coelacanthus have traditionally been regarded as likely candidates for this role, the revelations provided by the present-day coelacanth have made this much less likely.

Careful research and theorization led paleontologists to the Canadian Arctic in 2004, where they discovered the 375-million-year-old remains of Tiktaalik roseae, the so-called fishapod, a creature that was clearly a fish but one with physical characteristics such as robust pectoral fins. It is thought that Tiktaalik lived at the water's edge, where it lay in wait and used its “legs” to raise itself suddenly to grab prey. However, the position and joint structure of the pectoral fins were not sufficiently articulated to promote a walking movement. Tiktaalik exhibited many of the characteristics of fish (fins, scales, gills) and several characteristics normally associated with tetrapods (flattened head with top-mounted eyes, functional neck, and supportive ribs). These tetrapod-like features appear in Tiktaalik some twelve million years before the appearance of the first known tetrapods in the fossil record.

Another creature that lived about twenty-five million years after Tiktaalik is the Ichthyostega, a fish-amphibian transitional vertebrate that has long been thought the first capable of walking on dry land. Ichthyostega grew to about one meter (three feet) long and was rather common in the area that has become the Greenland subcontinent, an Arctic region that would have been close to the Baffin Island location that yielded Tiktaalik. The skeletal structure of Ichthyostega is similar to that of the crossopterygians, and its apparently weak legs look to have developed from the corresponding fins of the crossopterygians. Its long tail, with a median fin supported by bony projections, suggests that its habitat was almost exclusively aquatic.

The extinct line that included Ichthyostega also included Eryops megacephalus in the Permian period, a creature having a two-meter (six-foot) skeletal structure similar to that of Ichthyostega though clearly well adapted to movement on land. Its contemporary, Cacops, was apparently much stronger and better suited to life on dry land. Eogyrinus inhabited the Carboniferous period, a time marked by warm temperatures and voluminous rainfall that produced vast swamps. Accordingly, Eogyrinus exhibits features that indicate readaptation to a watery environment. The Triassic period held the most highly specialized member of this group, a creature called Gerrothorax, considered to be the last of the superorder of Labyrinthodontia.

Gerrothorax was characterized by an elongated flat body, close-set upward-directed eyes in a broad flat head, three pairs of feathery external gills, and front and rear legs in the same positions as fins. The creature, which lived about 210 million years ago, was about one meter (three feet) long and was covered with bony scale armor reminiscent of chain mail armor. Its most unusual feature, however, was the nature of its bite. Gerrothorax is believed to have hunted by lurking on lake bottoms and quickly grabbing prey as it passed overhead. The unusual joint structure of its jaws indicates that it opened its mouth by raising its upper jaw instead of dropping its lower jaw. (The motion has been likened to opening the lid of a toilet.) Fossil remains of Gerrothorax have been found in Greenland, Western Europe, and Scandinavia.

The earliest known terrestrial vertebrate amphibian to date is a creature known as Fedexia striegeli, evidenced by a well-preserved skull found in Pennsylvania. Fedexia belongs to the extinct order Trematopidae, an amphibian group that was adapted to live on land and probably returned to water only to lay eggs. The discovery of Fedexia places the appearance of terrestrial vertebrates into a time twenty million years earlier than thought. Other scientists argue that the limbed tetrapodomorphs of the genus Ichthyostega are the oldest terrestrial vertebrate amphibians.

Further research involving scans of a 400-million-year-old fish found in Serbia in the 1950s called Kolymaspis sibirica is believed to have uncovered evidence of the development of shoulders. Researchers found that the fish's complex gill arches that long supported breathing evolved over millions of years, becoming hinges at the base of the skull with cartilage structures. This finding provided new evidence of the early evolutionary changes fish underwent in the transition from fish to amphibians.

The diversity of form exhibited by the amphibians in the fossil record demonstrates that, while still tied to the water, the creatures adapted to all ecological niches through time. Many of the basic amphibian forms are represented in the present day. The superorder Salienta produced creatures such as Protobatrachus in the early Triassic period, with the skeletal structure and features almost identical to present-day toads and frogs, though not yet developed sufficiently for the jumping movement characteristic of those creatures.

The Carboniferous and Permian periods saw the amphibian subclass Lepospondyli, ancestors of the present-day caecilians, or the Apoda and Uropoda species. The snake-like Dolichosoma is found in Carbonaceous period deposits, while Permian period deposits have revealed the curious salamander-like Diplocaulus. This was a creature well adapted to life in the water, with a flat body; small, weak foot structures; and an unusually triangular skull having lateral projections that gave its head a crescent moon shape.

Amphibians to Reptiles

In evolutionary terms, the reptiles advanced beyond the amphibians by becoming capable of living completely terrestrial existences, without the need to return to the water for reproduction. The beginning of the reptiles is marked by the appearance of amniote eggs, in which an embryo could develop on land in a protected watery environment without having to pass through the larval stages that are typical of the amphibian life cycle. The earliest-found amniote egg fossils were taken from early Permian deposits, which place the appearance of the reptiles at about 250 million years ago. Such fossil evidence, however, provides little insight into the development of the reptiles from the amphibians. This evidence has instead been obtained through an analytical study of transitional fossils dating to the early Permian period. The reptiles have thus been traced back to the labyrinthodont amphibians that inhabited the Carboniferous period, with the transitional genera occurring in the Permian. The reptiles proliferated through the Mesozoic era, eventually differentiating into the two new classes of Aves and Mammalia, or birds and mammals.

The transition from amphibians to reptiles is thought to have been through creatures of the order Seymouriamorpha, named for the representative Seymouria. The features that characterize the 50 to 64 centimeters (20-24 inches) Seymouria include a skull structure that is essentially identical to that of earlier amphibians but with the vertebral and phalangeal skeletal structures of reptiles. This combination of features has prevented the classification of Seymouria as being either amphibian or reptile; only proof that Seymouria reproduced through amniote eggs would be decisive. Such proof has not been found, however. True reptiles are known to have existed at the same time as Seymouria, indicating that the transition from labyrinthodonts to reptiles probably occurred at a time before the Permian period and that the seymouriamorphs and reptiles probably evolved from a common ancestor or ancestral line.

The Anapsida

The reptile class is normally divided into the six subclasses of anapsids, synapsids, parapsids, euryapsids, lepidosaurs, and archosaurs, three of which are extinct lines. The Anapsida includes the most primitive ancient species of reptiles having roofed skulls similar to those of the amphibian species. This subclass is further divided into the orders of the Cotylosauria and the Chelonia. The Cotylosaurian line is extinct, while the Chelonian line has prospered to the present day. Chelonians, familiar around the world as turtles and tortoises, are characterized by the bony plates that cover and enclose their bodies. This basic body structure has endured essentially unchanged since the beginning of the Chelonia in the Triassic period.

Anapsids are characterized by the lack of apertures in the skull. Synapsids, in contrast, have a single skull aperture below the squamosal and postorbital bones. Diapsids, notably including such creatures as the eosuchian Youngina and the pelycosaurs Dimetrodon and Edaphosaurus, have two skull apertures. The pelycosaurs flourished through the Carboniferous and early Permian periods. This single feature is traceable through the second wave of synapsids to arise during the Permian, the therapsids, which are believed to have eventually evolved into the avian and mammalian evolutionary lines. The pelycosaurs and their kind were fully evolved reptiles with only some vestigial characteristics of their amphibian ancestors. Dimetrodon is known to have been about 3 meters (10 feet) in length and one meter (three feet) in height, with a ray-finned “sail” spanning its back that added another meter to its overall height. Its square-shaped jaws were armed with two types of teeth, hence its name.

A third wave of synapsids, developed in the Triassic, is known as the therapsids. By the end of the Triassic, therapsids already had many features in common with the mammalians that evolved from them. The earliest and most primitive therapsids from the middle Permian period, the Dinocephalia (“terrible heads”), such as Moschops, had skulls resembling that of Dimetrodon, but with only the single occipital condyle and the lack of a secondary palate in common with reptiles. Their mammalian features included differentiated canine teeth and small articulated bones in their jaws. In the lower Triassic, therapsid evolution toward mammalian physiology was evident in the dog-like Cynognathus, a four-footed carnivore having two occipital condyles; a secondary palate; fully differentiated incisor, canine, and premolar teeth; and the same number of phalanges found in mammals. Even more closely related to mammals were the synapsid order Ictidosauria, such as Tritylodon. By this point, it has become almost impossible to identify synapsid skeletal remains as being other than mammalian, and the only certainty is that some 170 million years ago, the synapsid reptiles began to evolve into the mammalian species.

Reptilian Errors

Today's reptiles, especially snakes, have a reputation for aberrant offspring, such as having two heads. This trait has existed among reptiles and other creatures essentially since their origins. It is a rare condition believed to originate when an embryo is somehow damaged in the womb or egg, producing a lesion that causes some body parts to grow in duplication. The condition also has been observed in several reptilian and mammalian species, including Homo sapiens, though the probability of such offspring surviving is low.

About 120 million years ago, the line of semiaquatic reptiles called Choristodera, including the species Sinohydrosaurus or Hyphalosaurus (resembling modern-day crocodiles and lizards), populated the landscape with the dinosaurs. Fossil remains of nesting sites found in China have produced an instance of the condition of bipartition by revealing the fossil of a Hyphalosaurus lingyuanensis hatchling having two fully formed heads and necks joined at the base. The fossil is just 7 centimeters (2.8 inches) in length, and it would have grown to its full adult size of 1 meter (39 inches) had it survived. The fact that the two-headed infant is an aberration is evidenced by the presence of several single-headed hatchlings in the immediate vicinity.

Unraveling the Mystery of Why Snakes No Longer Have Legs

In late 2015, a paleontologist and a researcher from the American Museum of Natural History announced that they had effectively used modern three-dimensional computed tomography (CT) scanning to learn more about the origins of snakes and why they no longer have legs. It was long thought that snakes evolved from reptiles living in aquatic habitats. However, at the same time, scientists had begun unearthing fossils of snake ancestors that had limbs, bolstering an argument that the creatures had actually evolved on land. By studying the shapes of the inner ear of the fossils of the snake ancestor Dinilysia patagonica via a model of the inside of the head created by CT scanning, Hongyu Yi and Mark Norell found that snakes may have evolved from terrestrial reptiles adapting to life underground as burrowers. The shape of the inner ear aligned with those designed for hearing low frequencies and vibrations, which are significant skills for living underground, and were, thus, comparable to modern snakes.

Further research revealed that snakes evolve three times faster than lizards, allowing them to be adaptable in feeding, movement, and sensing to survive various conditions. Evaluating one thousand snake and lizard species to chart an extensive evolutionary timeline, researchers discovered snakes developed specialized traits, like chemoreceptors and flexible jaws, in an early and extensive burst of evolutionary changes that were unique in the animal kingdom. This ability to adapt remains evident in modern snakes.

Principal Terms

amnion: the sac-like structure that lines the inside of a shelled egg and contains the embryo, the yolk sac, and the amniotic fluid medium

amniote: descriptive of eggs consisting of a tough, mineralized, air-permeable outer shell enclosing a fluid-filled sac called the amnion

anapsid: lacking the presence of skull apertures, or synapses

carapace: the bony plate that forms the dorsal side or back of a turtle or tortoise

diapsid: possessing two apertures, or synapses, in the skull

lacustrine: descriptive of sedimentary deposits formed from lake-bottom residues

plastron: the bony plate that forms the ventral side or underside of a turtle or tortoise

synapsid: possessing an aperture, or synapse, below the squamosal and postorbital bones of the skull

terrestrial: of reptiles, those adapted to living solely on land without the need to return to a body of water for reproduction

transitional: having the characteristics of both preceding and successive classes of organism without clearly belonging to either

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