Fossils of the earliest vertebrates

Fossils of the earliest vertebrates are present in rocks dating back over 500 million years. Although they are rare and fragmentary, they provide the only contact with the planet's early life-forms and help establish an understanding of how vertebrates developed.

Early Evolution of Vertebrates

The record of the early evolution of vertebrates is of particular interest to earth scientists because humankind is a member of that subphylum. However, the evidence of evolution is incomplete. The vertebrates constitute a subphylum of the phylum Chordataa group of organisms characterized by the presence of a notochord (a longitudinal stiffening rod), a dorsal hollow nerve cord, gill slits, and segmented muscle masses. The subphyla other than the vertebrates are often referred to as invertebrate chordates (because they lack vertebral elements); modern examples of these forms include the Hemichordata (pterobranchs and acorn worms) and the Cephalochordata (Branchiostoma, formerly known as Amphioxus). Fossil evidence for the history of invertebrate chordates is limited. Pikaia from the Middle Cambrian Burgess Shale (500 million years ago) is a vaguely fishlike animal resembling modern cephalochordate Branchiostoma.

Vertebrates differ from the invertebrate chordates in the possession of a brain, specialized paired sensory organs (hearing, sight, smell), and bone. The absence of fossils showing the earliest stages of vertebrate evolution has allowed little to be known for certain about how this occurred. It is assumed that the earliest vertebrates and their immediate ancestors were soft-bodied animals that would leave no fossil traces. They would not be capable of preservation until the development of bone had taken place, a capacity that does not seem to have occurred until well after the early radiation of the vertebrates. Discoveries from the Early Cambrian Chengjiang localities in southwestern China have resulted in the description of the early vertebrates Myllokunmingia and Haikouichthys. These creatures were once speculated to be one species, but further research concluded their individual classification based on their differing gill structure and body shape.

The picture developed is of a small fishlike animal with a fusiform body and a head that is not clearly differentiated. Anterior jaws were not present, but the animal may have fed by engulfing small prey whole through a flexible mouth opening. Gill slits were present, perforating the wall of the pharynx and associated with gills, thin lamellar tissues that provide a surface for gas exchange and allow the animals to breathe. The dorsal hollow nerve cord expanded anteriorly and was associated with specialized paired sensory structures (smell, sight, and balance). Lateral line organs extending over the body surface detected vibrations in the water, as they do in modern fish. The notochord formed a supporting rod extending the length of the animal, and anchored to it were the segmental swimming muscles (the myotomes) that extended along both sides of the body. No paired fins were present, but a tail and possibly a dorsal fin were. This picture of a primitive vertebrate is very similar to the modern cephalochordate Branchiostoma and also to the fossil cephalochordate Pikaia, implying that the cephalochordates may be the closest relatives of the vertebrates within the phylum Chordata.

Discovery of Vertebrate Fossils

Although bone does not appear to have been present in the earliest vertebrates, it is present in the first fossil forms found in rocks of the Ordovician age (490 to 430 million years ago) from North America, Australia, and Bolivia. The fossils consist mostly of disarticulated fragments of bony exoskeleton, although several specimens from North America are sufficiently well preserved to have been used as the basis for reconstructions, and finds in Bolivia have included complete animals. Because the evidence is usually so fragmentary, various fossils from the Cambrian (540 to 485 million years ago) and Ordovician periods have been identified as early vertebrates during the past century.

These determinations have not always been accurate, and it is now clear that many of these fragments do not show vertebrate morphology or histology (internal structure). Several fragments of bony armor have been reported from the Upper Cambrian of North America and the Lower Ordovician of Spitsbergen, Norway. These are earlier than the accepted dates for vertebrate occurrences. However, there is as yet no definite proof that these fragments are vertebrate. Although they appear to be composed of hydroxyapatite, as are vertebrate skeletons, this is not in itself proof of vertebrate origin, as many invertebrate groups (such as arthropods, conodonts, and brachiopods) have hard tissues of similar composition. As these fragments are microscopic in size, nothing can be deduced about the gross morphology of the animal that produced them. These fragments may represent parts of the exoskeleton of early arthropods, but this is still an area of debate, and there is insufficient evidence to designate these fragments as vertebrates.

Remains that are still accepted as vertebrates were first described from the Ordovician Harding Sandstone of Colorado in 1892. The two species described, Astraspis desiderata and Eriptychius americanus, consist of numerous fragments of bony plates and scales that cover the animals with flexible armor. Histology of the individual elements and gross morphology of the few partially articulated specimens prove conclusively that these animals were vertebrates. A reconstruction of Astraspis desiderata shows it to have been roughly 130 millimeters long and covered anteriorly by ornamented bony plates that were often fused to each other peripherally to form a continuous shield. Posteriorly, the tail was covered by imbricating scales and terminated in a small, scale-covered caudal fin. Laterally, there were eight pairs of gill openings. The animal had no fins other than the caudal fin, suggesting that it was a poor swimmer. Nothing is known of the internal skeleton of these animals, and it is assumed that it consisted of cartilage, a material that is not normally fossilized. Some calcified fragments from the head region are known, but it has not yet been possible to determine what their original positions might have been. Nothing is known of the mouth area of these vertebrates, but they are presumed to have been Agnatha, vertebrates in which jaws are not developed. Armored jawless vertebrates of a similar type are common in more recent rocks of Silurian (430 to 395 million years ago) and Devonian (395 to 345 million years ago) age, and modern jawless forms (lampreys and hagfishes) do exist, though they are not common and are no longer armored.

Vertebrate fragments of a somewhat similar type were described from the Ordovician Stairway Sandstone in Australia in 1977. Enough material of one species, Arandaspis prionotolepis, is known for a reconstruction to have been made, and this suggests an animal approximately 150 millimeters in length in which anterior dorsal and ventral armor plates were separated by a slanting row of small square plates that probably protected the gill openings. The tail was covered by long, thin, flexible scales. This picture has been improved by the discovery in Bolivia in 1986 of complete specimens of a closely related species, Sacabambaspis janvieri. These specimens show that the animal had eyes in the front of the head and a mouth that was a flexible opening not supported by jaws, thus confirming that these early vertebrates were Agnatha.

Origin of Vertebrates

Although they differ in details of their morphology, indicating that a considerable amount of evolution had already taken place, these Ordovician vertebrates are clearly closely related and indicate the presence of diverse and widespread early vertebrate faunas at this time. The environments in which they lived and their mode of life have been the subjects of some dispute over the years because of their bearing on the environment in which the vertebrates originated. It was originally suggested that vertebrates originated in fresh water, an interpretation supported at that time by the supposed freshwater environment of deposition of the Harding Sandstone.

These ideas have since been questioned, and fresh interpretations of the sediments in which vertebrate fragments occur show that they were marine. It now appears that in all cases, the environments in which early vertebrates were living were shallow, near the shore, and sandy. The animals probably lived as benthonic deposit feeders in these sandy nearshore environments, which is very similar to the mode of life of the modern cephalochordate Branchiostoma. Unlike Branchiostoma, however, they probably did not bury themselves in the sediment to feed. Once they died, the carcasses would have been disarticulated by scavengers and distributed across the current-swept nearshore environment, explaining why complete skeletons are so rare. The present wide distribution of early vertebrate occurrences (Australia, Bolivia, and North America) is not improved when paleogeographic reconstructions taking account of plate tectonic movements are made for the Ordovician. At that time, South America was far south and Australia and North America, though both near the equator, formed part of different continental masses.

Twenty-first research into the scientific discovery gap between the latest invertebrates and earliest vertebrates revealed extinct creatures from the Cambrian period called yunnanozoans are the oldest known stem vertebrates—extinct vertebrates closely related to modern-day vertebrates. Using a variety of techniques, the researchers examined the pharyngeal arches of these creatures and found cellular cartilage, which is a feature exclusive to vertebrates' pharyngeal arches. This evidence supports the theory that yunnanozoans were much more physiologically complex than previously thought and are a significant link in the transition of life from invertebrates to vertebrates. It is very difficult to relate the distribution of early vertebrate occurrences in a meaningful way, and it is hoped that additional occurrences, as yet undiscovered, will aid scientists in their interpretations.

Study of Fossil Remains

Although the fossil remains of early vertebrates have been studied for almost one hundred years, the techniques used have changed very little. The fragments of bone are initially removed from the matrix (surrounding sediment) by mechanical or chemical means. Mechanical preparation involves the removal of the matrix by small needles, either hand-held or in small electrical engraving tools. By this means, the rock is slowly chipped and flaked away from the bone until it is entirely exposed and can be studied. Chemical preparation exploits the fact that the calcium phosphate of which bone is composed will resist some acids that can break down carbonate matrices. This means that if the vertebrate fragments are preserved in a carbonate rock (limestone) or in one that has a carbonate cement, the vertebrate fragments can be dissolved out. If the fragments are very fragile, they are often backed with plastic first, so they do not disintegrate when the supporting matrix has been removed. The acids most commonly used in this technique are formic and acetic acid. Both mechanical and chemical techniques are slow and painstaking and require a high level of skill.

Once the material has been prepared, it is studied optically using microscopes. The internal structure can be studied by making thin sections of bone fragments and then viewing them by transmitted light. The thin sections are made in a similar way to thin sections of rock. A thin slice of the bone is cut, and one side is ground and polished and then attached to a glass slide. The bone is then ground down until only a thin film is left attached to the glass slide. The bone is so thin that light can shine through it, and the details of the internal structure of the bone can be seen when it is viewed through a microscope. The external morphology of the bone can be studied using a light microscope also, but more recently bone fragments have been studied using a scanning electron microscope. This microscope uses a beam of electrons that are generated and focused on the specimen. As the electrons rebound from the specimen, they pass through a phosphorus disk, and the energy produced is seen as a scan line on a cathode-ray tube, thereby producing a picture that can be observed. This microscope can produce very high magnifications, enabling minute details of surface structures to be seen. Other technologies used to examine fossils include X-ray microtomography, scanning and transmission electron microscopy, Raman, Fourier-transform infrared, and energy-dispersive X-ray spectroscopy.

Studies of the relationships of the earliest vertebrates have relied heavily on a methodology termed phylogenetic systematic or, more commonly, cladistics. This method uses the characteristics of organisms to develop a picture of the way in which they are related. Cladistics is distinguished from other taxonomic methods (taxonomy is the study of interrelationships). It is a rigorous system in which shared advanced characteristics alone are used to show relationships. These relationships are expressed as branching diagrams termed cladograms (klados is Greek for “branch,” hence the name cladistics). Studies using this methodology have shown that modern Agnatha are composed of two disparate groups so widely separated that one is, in fact, more closely related to jawed vertebrates. The exact relationship of the earliest fossil vertebrates to modern forms is still the subject of intense debate.

Principal Terms

Agnatha: an infraphylum of early vertebrates in the phylum Chordata and subphylum Vertebrata that includes all forms in which jaws are not developed; the group to which the earliest vertebrates belong; living members of this group include hagfish and lampreys

cladistics: a method of determining relationships in which shared advanced characteristics exhibited by the organisms are used

exoskeleton: a bony armor covering the outside of the animal

Ordovician: a period covering the interval from 490 to 430 million years ago; follows the Cambrian, which covers the interval from 540 to 485 million years ago

vertebrate: a subphylum of the phylum Chordata, whose members are distinguished by the presence of a brain with paired sensory organs and the development of bone

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