Highlights in the Evolution of Vertebrates

Abstract

This page provides a high level review of some of the main highlights from the evolution of the vertebrates.

Keywords: Craniata, craniates, vertebrates, evolution, fish, reptiles, dinosaurs, mammals, humans

Acknowledgement: This page is dedicated to Ruben Arthur Stirton (1901-1966), and owes its inspiration to his 1959 classic, Time, Life and Man. Some of the basic text below derives straight from Chapter 28 of this book; I am slowly bringing to up to date, as meagre time and even more meagre knowledge permit.

Introduction

The superphylum Chordata, the chordates, comprises the Urochordata (tunicates), Cephalochordata (the lancets, which include the famous Amphioxus), and the Vertebrata (= Craniata, the vertebrates). The whole group is characterised by the presence of a chorda (notochord), a hollow dorsal nerve cord mostly in contact with the chorda, longitudinal blocks of muscle along the chorda, and ciliated pharyngeal gill slits (Nielsen 2001, p. 451).< /p>< /p>< /p>< /p>< /p>

Origins and Early Development

Today the chordates are mostly agreed to form a monophyletic group. The principal competing theory posits independent derivation of the three living chordate phyla from a group of unusual derived echinoderms known as the calcichordates (Jefferies 1986; Jefferies et al. 1996; Jefferies 1997; Conway Morris 2000). The principal difficulty facing the calcichordate hypothesis is that “the various mitrates and cornutes that supposedly represent stem groups of the three chordate phyla are Ordovician, and fossils interpreted as chordates, e.g. vertebrates, are now known from the Early Cambrian Chengjiang fauna” (Nielsen 2001, p. 420).

The nearest relatives of the chordates are most probably the enteropneusts (acorn worms) which, together with the chordates, possess gill slits and form a clade which some authors recognise as the Cyrtotreta. Others consider the chordate sister group to comprise the enteropnests and pterobranchs (which includes the extinct graptolites) together, a grouping called the Hemichordata.

Urochordates

“Tunicates, or urochordates, comprise the most basal chordate clade, and details of their evolution could be important in understanding the sequence of character acquisition that led to the emergence of chordates and vertebrates. However, definitive fossils of tunicates from the Cambrian are scarce or debatable. Here we report a probable tunicate Cheungkongella ancestralis from the Chengjiang fauna. It resembles the extant ascidian tunicate genus Styela whose morphology could be useful in understanding the origin of the vertebrates” (Shu et al. 2001, p. 472).

Cephalochordates

Cephalochordate-like animals (most famously Pikaia from the Middle Cambrian Burgess Shale but perhaps more significant is Cathaymyrus from the Lower Cambrian Chengjiang lagerstätte) have been known for some time. The discovery of two distinct types of agnathan (Subphylum Vertebrata, Class Agnatha) from the Chengjiang at Haikou (Shu et al. 1999) indicates an unexpected vertebrate diversification early in the Cambrian. Rather surprisingly, Shu et al. conclude there is “no reason to suppose that the origin of vertebrates was hundreds of millions of years earlier” (somewhat mischeviously misrepresenting Bromham et al. 1998) with the clear implication that the vertebrates, at least, “exploded” within sight of the Cambrian. However, although nothing resembling a chordate is known from Ediacaran assemblages, evidence of a long Precambrian history seems to be accumulating for most Cambrian forms.

Conodonts

Craniates

The craniates or vertebrates have a fossil record extending back to the Cambrian. By the Ordovician they are relatively diverse, abundant, and widespread. Anatolepis, for example, "was first described from the Valhallfonna Formation (Arenig-Llanvirn, Ordovician) of north-eastern Spitsbergen as a heterostracan agnathan. Fragments of exoskeletal armour assigned to the genus ... have subsequently been found from a large number of localities in Laurentia, ... in sediments ranging from Late Cambrian to Early Ordovician age" (Smith & Sansom 2001, p. 45).

Early Jawless Fish (Ostracoderms)

The oldest known fossil fish occur in rocks of middle Ordovician age from Colorado, South Dakota, Wyoming, and Michigan. These fossils, though very incomplete, have been identified as fragments of primitive, jawless, fish-like vertebrates called ostracoderms. The most outstanding feature of this group is the absence of jaws. The cranium was a single ossified unit underlying the dermal shield and composed of true bone with a surface layer of dentine, but there was no trace of a bony body skeleton. Some, but not all, had structures like paired fins. There were indications of a notochord and two semicircular canals in the inner ear region. Instead of gills some of the ostracoderms had as many as ten pairs of gill sacs or pouches that were used in feeding and breathing. Few exceeded 30 cm (12 inches) or so.

Jawed Fishes (Gnathostomes)

Placoderms (Plate-Skinned Fish)

The late Silurian to Devonian was a time of great radiation of fish stocks. Rocks of this age have yielded fossil evidence of no fewer than six orders of fishes in the class Placoderma, which are characterised by bones in their internal and external skeletons, pectoral and pelvic fins typically paired, and both upper and lower jaws of a primitive type.

Arthrodires (Joint-Necked Fish)

Arthrodires were the largest and most formidable predators of the time. Some species of the genera Dinichthys and Titanichthys exceeded 10 metres (30 feet) in length, though not all were so large. Many small varieties first appeared in the late Silurian and early Devonian in fresh and brackish waters. The well-known genus Coccosteus, from the Old Red Sandstone of England and Scotland, included some species only 60 cm (2 feet) long.

The head and thoracic region were armoured with heavy bony plates, connected by a joint on each side of the neck, allowing the mouth to open wider as the lower jaw remained fixed or moved slightly downward. The posterior part of the body was covered with skin with scattered dermal ossicles. The mouth was equipped with peculiar bony jaws with sharp, toothlike points and shearing edges.

Acanthodians (Spine-Finned Fish)

These small (minnow-sized) fish were completely armored with diamond-shaped bony scales. The many small bony plates in the skull were arranged in characteristic patterns. Though there were five well-developed gill arches, they were more primitive than in later fish, and each bore gill rakers. In both the upper and lower jaws there were one to three separate centers of ossification. The toothed lower jaw was not articulated with the cranium through the hyomandibular bones, as in more advanced fish. Pectoral, pelvic, and dorsal fins, as well as numerous pairs of smaller fins along each side of the belly, were present, and each fin was strengthened by a sharp anterior spine. Among the most primitive jawed vertebrates these little spine-finned fish were nearer to an ancestral position for the later groups than the more specialized arthrodires, though both made their first appearance in the late Silurian.

Antiarchs

The antiarchs appeared in abundance in the middle Devonian and persisted only until the end of the period; no trace of them has been found in the Mississippian. They were small - much the same size as most ostracoderms, around 30 cm - though many structures, such as jaws and paired fins, suggest they may have arisen from a primitive arthrodire stock. The head and much of the body were covered with hard dermal plates. These and other little-known orders of placoderms, predominately Devonian in age, indicate considerable diversity and specialisation.

Fishes intermediate between antiarchs and the earlier ostracoderms have not been found, so that their immediate ancestry is not clearly understood. Nevertheless, in general they do represent an intermediate stage between more primitive vertebrates and advanced fishes.

Chondrichthyans (Sharks and Rays)

Sharks also appear for the first time in the Devonian. They, too, have a long and diversified history and are still living, mostly in marine waters. No less than five major groups of are known to have existed at one time or another.

Their skeletons are cartilaginous, and though the ear has three semicircular canals, as in all higher vertebrates, there are no external auditory openings for reception of sound. The sexes are separate and fertilization is internal. In most sharks the female retains the fertilized eggs in the oviduct, where they hatch; later the active young are born. Sharks of the genus Mustelus have developed a placenta-like modification in the oviduct to nourish the embryos when the food supply in the yolk sac has been depleted.

Osteichthyes (Bony Fish)

The fourth and greatest class of fishes are the Osteichthyes or bony fish. They are divided into two subclasses: the ray-finned Actinopterygii, and the air-breathing Choanichthyes. In contrast to  other classes, the bony fish have a well-ossified internal skeleton (except in the sturgeons and spoonbills), lower jaws connected to the cranium through the hyoid arch, and swim bladders or lung structures.

Both of the subclasses appear in the Devonian fossil record, indicating a Silurian ancestry.

Actinopterygii (Ray-Finned Fish)

### revision up to here ###

The first ray-finned fish appear in the middle Devonian. The oldest order, known as Palaeoniscoidea, is well exemplified by the genus Cheirolepis (Fig. 111). Palaeoniscoids were rare at first, but as the ostracoderms and placoderms faded out these more advanced fishes expanded. They are most abundant in the Mississippian and the Pennsylvanian. Cheirolepis is nearly 12 inches long and is covered with thick, shiny ganoid scales. The fins are composed of nearly parallel rays from which is derived the name rayfin. The palaeoniscoids like the contemporary air-breathing fish possess lung structures, but these primitive lungs are replaced by air bladders in the later ray-fins.

In the Triassic and Jurassic the sturgeon and garlike fishes, obviously derived from a paleoniscoid ancestry, come into the fossil record. Finally, the most successful fishes of all time, the teleosts, or true bony fish, appear in the early Jurassic, then expand tremendously in the later Jurassic and Cretaceous. One of the giants is the 15-foot Hypsodon of the Cretaceous seas. Most of the teleosts are covered with thin bony scales, though some like the catfish have a smooth skin. No group of water-Iiving vertebrates has attained the extraordinary expansion and adaptive radiation of these fishes. They include most of the living fishes, such as the trout, perch, bass, tuna, pike, and eel.

Gaining Ground*

* The nifty title is that of Jennifer Clack's excellent book.

Air-Breathing Fish

The most conspicuous features differentiating the Choanichthyes from the other bony fish are their two dorsal fins and their rhombic scales of cosmoid structure. Since the rhipidistians and coelacanths are more closely related to each other than either is to the dipnoans, they are usually classified as the order Crossopterygii (lobe-finned fish), and the lungfish are recognized as representing the Dipnoi, another order.

Lungfish are still living. The genus Protopterus is found in the rivers of Africa; Lepidosiren, the smallest and most specialized, occurs in South America; and Epiceratodus lives in the streams of Eastern Australia. Epiceratodus looks quite like the genus Ceratodus, which was widely distributed over the world in the Triassic. Lungfish first appear in the middle Devonian, where they soon became numerous. The earliest, as typified by the genus Dipterus, are much more like the crossopterygians than the living genera. Nevertheless, Dipterus shows a reduction and decalcification of the internal skeleton and an increase in small bony plates in the skull. Furthermore, the brain case is not well ossified, and the teeth are reduced and lost from the margins of the jaws. These and other features clearly remove these interesting fishes from an ancestral position to the tetrapods, or four-footed, landliving vertebrates.

The coelacanths have long been recognized as a specialized side branch of the crossopterygians. In the Devonian they were found closely associated with the earliest lungfish and rhipidistians, but they soon spread into oceanic waters. The fossil forms retained many degenerate characters which showed a rather close relationship to the rhipidistians, but the bones in their lobed fins, were reduced, the fin rays tended to increase, and the luQg was calcified. For nearly a century scientists thought that the last coelacanths died out toward the end of the Mesozoic, when so many of the larger vertebrates became extinct. Then in 1939 a South African fisherman cast his nets deep into the ocean and brought up in his haul a fish entirely unknown to ichthyologists. This unusual fish was about 5 feet long; it was covered with deep blue scales and had two dorsal fins. Unfortunately, before a scientist could reach the scene the specimen had deteriorated to such a degree that only the skin could be saved. Its relationships were soon recognized, and the name Latimeria chalumunae for this coelacanth flashed around the world. This was like bringing a fossil back to life.

More recently several more were taken off the coast of Madagascar, and details of their anatomy and habits have enriched our knowledge of these interesting fish which have become adapted to live in certain oceanic environments.

As stated previously, the rhipidistians are in the direct line of ancestry of the amphibians. It is interesting to note that one of their genera has been recorded from early Devonian rocks, and no other bony fish are known until middle Devonian time. But it must be logically assumed that all bony fish must have arisen from a common ancestry, possibly in the late Silurian. Though the coelacanths, their closest relatives, eventually dispersed into oceanic and inland embayments of salty water, the rhipidistians flourished in poorly drained swamps and pools where they were the predominant predators. Natural selection favored those with sharp pointed teeth along the margins of their jaws, better developed lungs, well-ossified skulls, and more strongly developed bones in their pectoral and pelvic fins. Waters with little oxygen were no problem to the rhipidistians, for they could come to the surface and even crawl out along the shores where they breathed their oxygen directly from the air. When the stagnant waters dried up these airbreathing fish could make their way out of water to adjacent pools or possibly venture onto land to capture cockroaches under the ferns and around logs.

Emergence of Early Tetrapods

“The relationship of the three living groups of sarcopterygians or lobe-finned fish (tetrapods, lungfish and coelacanths) has been a matter of debate. Although opinions still differ, most recent phylogenies suggest that tetrapods are more closely related to lungfish than to coelacanths. However, no previously known fossil taxon exhibits a concrete character combination approximating the condition expected in the last common ancestor of tetrapods and lungfish—and it is still poorly understood how early sarcopterygians diverged into the tetrapod lineage (Tetrapodomorpha) and the lungfish lineage (Dipnomorpha). Here we describe a fossil sarcopterygian fish, Styloichthys changae gen. et sp. nov., that possesses an eyestalk and which exhibits the character combination expected in a stem group close to the last common ancestor of tetrapods and lungfish. Styloichthys from the Lower Devonian of China bridges the morphological gap between stem-group sarcopterygians (Psarolepis and Achoania) and basal tetrapodomorphs/basal dipnomorphs. It provides information that will help in the study of the relationship of early sarcopterygians, and which will also help to resolve the tetrapod–lungfish divergence into a documented sequence of character acquisition” (Zhu and Yu 2002, p. 767, Abstract).

“A nodule from the the Scottish Calciferous Sandstones, found near Dumbarton, western Scotland, preserves the only known articulated tetrapod material from ‘Romer’s Gap’” (Clack & Finney 1999).

The only articulated skeleton of a tetrapod yet found from the Tournaisian epoch (354–344 Ma) is Pederpes [® sidebar], “the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl” (??? ref).

With its later American sister-genus, Whatcheeria, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas” (Clack 2002, p. 72).

Emergence of Four-Footed Animals

The continuous expansion during the Devonian of land plants which retained moisture about them had set the stage for the conquest of land by water-Iiving vertebrates. Amphibious animals with moist skins could crawl out of the water into these shaded environments without losing much of their moisture by dehydration. The air-breathing rhipidistians had already evolved a long way in that direction. Even before the Period closed the transition had been made, for there were primitive amphibians in the late Devonian of Greenland.

The largest Pennsylvanian amphibians were the 15-foot embolomeres, well exemplified by Eogyrinus. In this genus the limbs were reduced, the head was rather deep and narrow, and the long flexible body and tail was adapted to pursuit in swamp waters of fish and other amphibians.

Some of the genera of stereospondyls were the only amphibians known to invade marine waters. The stereospondyls were the last survivors of the impressive labyrinthodonts. The latest questionable record is from the Jurassic of Australia. They evidently could not compete with the different reptilian groups which had reverted to aquatic habits.

In contrast to the labyrinthodonts are the frogs and toads which had no solid, bony-roofed skulls, no tails, but long hopping hind legs.

Their history can be traced back to the Jurassic, but beyond that the record is sketchy, though enough of it is known to indicate that they probably descended from a Pennsylvanian labyrinthodont ancestry.

The modern newts, salamanders, and blind, wormlike caecilians may have arisen from a microsaur ancestry as far back as the Mississippian.

It has been suggested that the lepospondyls may have descended from a different rhipidistian ancestry than the other amphibians.

Reptiles Evolve from Amphibians

Though the first reptiles are recorded from middle and late Pennsylvanian rocks, their remains are rare in that Period. Nevertheless, somewhere in the early part of the Pennsylvanian a group of amphibians, well adapted to life on land, gradually evolved into reptiles. In this process there were some rather profound changes. There is no direct proof from the fossil record, but we can readily hypothesize the conditions under which it came about.

The eggs were probably more like the rather soft-shelled eggs of the turtle found along sandy beaches and stream banks today. After the eggs were fertilized in the body, the parent female laid the clutch in moist sand where the warmth of the sun carried them through to incubation. As the embryo developed, a large yolk sac formed for its nourishment. An amnionic sac filled with fluid enveloped the embryo for its protection, and another sac, the allantois, developed for the reception of waste materials. Meanwhile, the outer shell and its inner lining, the chorion, were so constructed that oxygen could enter and carbon dioxide escape without dehydrating the internal content of the egg.

Many years ago a deposit of fossil vertebrates found on West Coffee Creek near the town of Seymour, Texas, by Charles H. Sternberg was destined to throw much light on the transition between amphibians and reptiles. The animals were named Seymouria baylorensis (Fig. 128).

With the perfection of the amniote egg and the development of external coverings by which the animals could retain moisture within their bodies, reptiles were much better equipped than amphibians to occupy more diverse environments on land. After their appearance in the Pennsylvanian they evolved rapidly into different groups. In the Permian reptiles outnumbered amphibians. The late Pennsylvanian and the Permian were times of climatic extremes. Epeirogenic uplifts in continents throughout the world, coupled with orogenic disturbances, resulted in aridity in some areas and even extensive glaciation in the Southern Hemisphere. This was reflected by changes in the reptiles to meet the new environmental conditions. Many groups continued their conquest of the wide expanses of land areas; others reverted to life in the water where there was an abundant food supply.

In the Mesozoic some of the dinosaurs evolved into the largest land animals known, and the different groups adaptively radiated into diverse specializations. Many of the Mesozoic reptilian orders were already distinct in the Triassic. These include turtles, ichthyosaurs, plesiosaurs, rhynchocephalians, crocodilians, phytosaurs, and the two orders of dinosaurs.

Though characters in the skeletons are useful in recognizing the different groups of reptiles, the positions and numbers of the temporal openings between the bones back of the eyes are basic for beginning students. These openings, or fenestrations, are associated with the attachment areas for the powerful temporal muscles used in closing the jaws. These modifications are utilized even further in the large dinosaurs by reducing the weight of the head while retaining a strong structure.

In his book on the evolution of vertebrates, Edwin H. Colbert has illustrated skulls with or without openings. The cotylosaur, with no openings, is anapsid; the ichthyosaur with one opening high on the skull and bordered below by the post frontal ( pf ) and supretemporal ( st ) bones is parapsid; the plesiosaur also with one opening high on the skull but bordered below by the postorbital (po) and squamosal (sq) bones is euryapsid; the pelycosaur with one opening on the side of the skull is synapsid; and the eosuchian with two openings back of the eye is diapsid (Fig. 224).

Water and land-Iiving reptiles were about equal in numbers. Perhaps the first truly aquatic reptiles were the small mesosaurs (Fig. 130). Some were about 36 inches long, but most were smaller. Their long flexible bodies and tails and paddlelike limbs were well adapted to rapid propulsion through the water. The skull was elongate and narrow, and the jaws were lined with over 300 needlelike teeth. These active fish-catchers have been found in the late Pennsylvanian of both South Africa and South America.

The first true turtles, or chelonians, appear in the Triassic, though there is some suggestion of an ancestral form in the Permian. With their bony carapace and plastron for protection, they have been one of the most successful groups of reptiles. Different kinds, from tortoises in the deserts to the sea turtles in oceanic waters, are adapted to many environments. Their skulls are a modified anapsid type, fitted with a horny beak. Among the chelonians, the tortoises have developed highly arched carapaces and live on dry land or even in deserts; the terrapins have a very flat shell and live in swamps and rivers; and the true turtles with a rather intermediate shell may be found in rivers and in the oceans.

Some of the marine turtles are gigantic. The great Cretaceous marine turtle Archelon is 12 feet long. An additional primitive group failed to develop a hard shell; this was the soft-shelled, or trionychid "turtle."

The most sharklike and dolphinlike of the reptiles were the ichthyosaurs. No reptiles were better adapted for life in the water. They existed throughout the Mesozoic, but were most abundant in the Jurassic. Some Triassic forms in inland seas of western North America were 30 feet long, but most were much smaller. Since they were strictly aquatic animals, the females gave birth to young. The skeleton of a female was found in Germany with the remains of five unborn young in the body cavity and two just outside it but near the pelvic region.

The spectacular plesiosaurs form another group of marine reptiles. They are recognized by their euryapsid skulls and long paddles instead of feet. The paddles may have as many as seventeen bones in the longest fingers. The first plesiosaurs appeared in the Triassic, and in the Jurassic the suborder diverged into two contrasting groups that continued on through the Cretaceous. The species of one group evolved short necks and long heads. An extreme of this specialization is seen in a gigantic skull from Queensland, Australia, called Kronosaurus. It is 9 feet 6 inches long and is the largest known reptile skull. In marked contrast, the other group developed long necks and short heads. This group culminated in a species, Thalassomedon haningtoni, from Colorado, described by S. P. Welles, that was some 40 feet in length and had a neck 18 feet long. All plesiosaurs were equipped with sharp interlocking teeth well adapted for catching fish.

Since these sharp-pointed teeth were useless for chewing, the plesiosaurs used another method of breaking down their food for digestion. The fish were swallowed whole and were ground to bits by the stomach stones, or gastroliths, in the gizzard.

Rhynchocephalians have diapsid skulls in which the lower jaw is firmly hinged to the cranium. Some of these reptiles are quite crocodilian in appearance, particularly in the Mesozoic; others have heads and beaks somewhat like the turtle. The only living one is the tuatara, Sphenodon, of New Zealand. It is strictly terrestrial in habits, but its ancestors may have been marine animals.

The Triassic phytosaurs (chapter heading 18) show a marked similarity to the crocodilians, but they are much more closely related to the dinosaurs. Their habits must have been much like those of the crocodiles. All phytosaurs have an armor of bony plates, and some developed horns along the neck and the edge of the back. They differ from -crocodiles in having the nostril openings near the middle of the head not far from the eyes. They differ also in many other features. They are known only from the Triassic.

Lizards and snakes are descended from a diapsid ancestry and are grouped together in the superorder Squamata. Since they have lost the lower temporal arch, the hinge for the lower jaw has become loosely attached to the skull and a wider gape has developed. Lizards first appear in the Jurassic and snakes in the Cretaceous. Both groups had aquatic kinds. The most remarkable, with aquatic adaptations, were the mosasaurs which were closely related to the living varanid lizards.

Some were nearly 30 feet long. Evidently these large fish-catching mosasaurs were confined to the late Cretaceous. They were abundant in the Niobrara Sea in middle North America.

One of the most spectacular groups of reptiles is the order Pterosauria, which includes the pterodactyls with their vestigial tails and the rhamphorhynchids with fan-shaped membranes at the end of a long tail. Their forearms are modified into wings by greatly elongated phalanges of the fourth digit. Even in the specimens from Solnhofen, Bavaria, were indications of soft parts are so faithfully preserved, there are no traces of scales or feathers. Wide thin membranes sustained these reptiles in flight. Though the pterosaurs and birds are descended from similar reptilian ancestry, they are no more closely related to each other than either is to the dinosaurs.

Pterosaurs range in size from the Jurassic species of Pterodactylus, no larger than a sparrow, to the great Pteranodon of the Cretaceous with a wingspread of 27 feet. The head in Pterodactylus is narrow and elongate. The eyes are well developed, and the teeth in both jaws are directed forward. Pteranodon has no teeth but possesses a long sharp beak and an equally long posterior extension of the skull back from the eyes. These winged reptiles are well adapted for soaring over water in search of fish, much in the manner of oceanic birds, but they are not so well equipped as modern birds to direct their flight. It is doubtful that they could get about on land with two or even all four of their limbs without considerable effort. Much is still to be learned about their flight habits and their methods of taking off and landing.

Possibly they had resting habits like bats and were able to cling to stony cliffs and trees with the sharp-claws on their feet and wings.

Flying reptiles appear first in the early Jurassic. Their remains are most abundant in rocks of that Period but diminish in numbers in the Cretaceous and disappear before the Cenozoic when the more progressive birds took their place.

The most conspicuous terrestrial land animals were the dinosaurs, though some of these saurians developed amphibious habits. But the dinosaurs, lizards, snakes, and other later land-Iiving forms were preceded by enormous numbers of more primitive terrestrial reptiles, especially in the Permian. There were plant eaters, flesh eaters, and others which must have fed on insects. The two earliest orders found in Pennsylvanian rocks were the anapsid Cotylosauria and the synapsid Pelycosauria. The early cotylosaurs, with their solid roofed skulls, were closely related to Seymoutia (Fig. 128), the animal with many amphibian characters. Evidently all of the reptilian orders stemmed out of the cotylosaurs and the pelycosaurs.

Dinosaurs

Popular interest in the Mesozoic is largely focused upon the Jurassic and Cretaceous and essentially preoccupied with crown group dinosaurs. The interesting evolution, however, mostly occurred deep in the Triassic. By the end of this period, dinosaurs, pterosaurs, lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock.

(Read more about dinosaurs, dinosaur myths and misinformation.)

End of the Era

Surely the most widely reported – and the most widely misreported – of all extinction events is that which brought the age of the dinosaurs to an end at the close of the Mesozoic: the “KT” extinction.

Almost all of the popular and “lightly technical” literature published within the past couple of decades is filled with comet or meteorite impact theories, all ultimately traceable to the father and son team of Luis and Walter Alvarez. Indeed, there is a good body of evidence to support the contention that a large extraterrestrial impact did occur at the very end of the Cretaceous.

However, the KT extinction is only one of several such mass extinctions in the Phanerozoic, and by no means the largest. There is no evidence of impacts associated with the other mass extinction events, and very few people try to maintain that argument today. In fact, flood volcanism is the only mechanism which really matches the data. “[L]arge continental flood-basalt volcanic events exhibit a near-perfect stage-level association with marked increases in Mesozoic and Cenozoic extinction intensity. Ten out of the 11 major flood-basalt eruption events of the last 250 million years occur during a stage that contains a local extinction peak (i.e. mass extinction). It is presently unknown whether this association is scale-dependent or extends to smaller events as well” (MacLeod 2002).

“Everyone has a favourite theory for major extinctions, all united by the common theme of attributing dominant importance to physical factors, and playing down the importance of normal biological mechanisms. Our own work strongly favours the diversification of both mammals and birds at least 30 million years before the extinction of dinosaurs – there must be ecological consequences for small dinosaurs from this early diversification” (Penny 2002 [® sidebar]).

If an impact was the sole cause of the KT extinctions, then that would make the KT different from the several other mass extinctions known, which is not a parsimonious conclusion. However, if an impact did occur at about that time, which seems likely, no doubt it would have added further stress to the already failing ecosystems, and possibly accelerated or even heightened the extinction event in some places.

(Read more about the Cretaceous-Tertiary boundary.)

Birds

Three possibilities for the origins of birds are still being credibly debated: The first is that they evolved from some unknown group of basal archosaurs, probably in the Triassic Period. Second, that they are a sister group to the Crocodylians, perhaps arising from within the sphenosuchian crocodylomorphs in the Early Jurassic. Certainly the most widely held view – though unfortunately misrepresented as the only credible modern view by the popular media – is for an ancestry among the theropod dinosaurs, specifically the Maniraptora, in the Middle to early Late Jurassic.

Despite intensive searching, the earliest known bird is still the famous Archaeopteryx, known from only seven skeletons and an isolated feather, all recovered from the Late Jurassic (Tithonian, perhaps 150 Ma) Solnhofen Limestone of Germany. The small theropod dinosaur, Compsognathus, has also been recovered from the Solnhofen. This fossil record represents a difficult problem for advocates of the theropod hypothesis: birds (specifically Archaeopteryx) are supposed to be most closely related to the dromaeosaurids, which do not appear in the fossil record until Albian times (mid Cretaceous, about 110 Ma) yet Compsognathus, which is believed to have diverged from the theropod lineage long before the evolution of the dromaeosaurids, occurs alongside Archaeopteryx 40 million years earlier. At present, only the vagaries of the fossil reord can be invoked to ‘explain’ the stratigraphic disjunction; our present understanding is unsatisfactory.

The first known beak and pygostyle (the “parsons-nose” which is all that remains in birds of the reptilian tail) occur in a Chinese fossil dated at 130 Ma. Feathers and bird bones have also been recovered from 110 Ma sediments in Victoria and Queensland.

Rahonavis is a primitive bird from 80 million-year-old rocks of Madagascar. Despite being more bird-like than Archaeopteryx, raven-sized Rahonavis retains some very distinctive theropod-like features. Other small primitive birds have been found elsewhere around the world. From Mongolia comes a large flightless bird, Mononykus, with wings replaced by a pair of single-digit hands that projected forwards. Another flightless bird is known from Patagonia. A sparrow-sized bird from Spain had a more modern shoulder joint than Archaeopteryx and a perching foot but it still had teeth.

Mammals

Early mammals had a Pangean distribution, including Europe, Asia, Africa, Australia and the Americas. Mammals were presumeably abundant in the Mesozoic, though their fossil record is poor, probably due to their small size which makes the fossils fragile and difficult to find.

There is abundant evidence that these extremely advanced reptiles are the ancestors of mammals. The intergradation is so complete that it is frequently difficult to tell whether one is dealing with a mammal or a reptile. The limbs had shifted from a wide lateral stance to a more direct position under the body; the digital formula (number of phalangeal bones in the toes) of 2-3-4-5-3 in the reptiles had changed or was changing to 2-3-3-3-3, so typical of mammals. Bones of the cranium and mandible were reducing to the number and to the positions seen in mammals. Most of these creatures probably had a coat of hair and primitive milk glands and may have partly broken their food down by mastication instead of swallowing it whole, as the other reptiles did.

Indeed the mammallike reptiles were much more closely related to the entire class of mammals than to most of the orders of reptiles.

The egg-laying platypus and echidna, or monotremes, in Australia are quite like the mammallike reptiles. Unfortunately, essentially nothing is known of their fossil record. If nothing but their fossil skeletons were available to us for study, they would most certainly be called specialized reptiles. Many characters in their soft anatomy and skeletons also are reptilian, particularly the pectoral girdle; and most of the characters retained in the skeletons of monotremes are possessed by mammallike reptiles. These interesting animals are the platypus, Omithorhynchus, and the echidna anteaters of the genus Tachyglossus.

They are clearly more primitive than any known mammal, living or extinct. Both lay soft-shelled eggs which have a liberal supply of yolk.

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“The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina” (Rauhut et al. 2002, p. 165 [® sidebar]).

Throughout the early Mesozoic they remained small, becoming more abundant, larger, and more diverse in the Cretaceous, which may have been a time of explosive radiation of Tribosphenida – early relatives of marsupials and placentals (Rougier 2002).

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Evolution of the Mammalian Ear and Mandible.

There were many interesting evolutionary changes in all parts of the skeleton, from fish to mammals, but the most extraordinary of all were the changes that culminated in the mammalian ear and the mandible, or lower jaw.

Mammals

It is quite clear that mammals arose from reptiles, but, as in other transitions of this kind, it is difficult for the systematist to I draw a precise distinction between the two classes. Usually we think of mammals as possessing hair, milk glands, a high body temperature (warm blood), a single bone in each lower jaw (dentary), three small bones in the middle ear, and a simplified pectoral girdle. Nevertheless, all or nearlyall of these characters may have been possessed by one or another groups of the most advanced mammallike reptiles. This is to be ex- pected in the evolutionary patterns and has been demonstrated time and again.

Marsupials are the most primitive of the living mammals, if we exclude the egg-Iaying monotremes. Some of the most widely known examples of these pouched mammals are the opossums, the kangaroos, and the koalas.

The greater number of mammals familiar to the average person develop their young by means of allantoic placenta. The first of these are the small shrewlike insectivores of the Cretaceous. Many teeth and parts of jaws have been found in North America, and the skulls and jaws recovered by the American Museum Central Asiatic expeditions have given us important information concerning these fascinating little mammals. The placental mammals, such as men, rodents, elephants, horses, dogs, and camels, are the dominant terrestrial vertebrates of the Cenozoic. Others, such as monkeys and squirrels, are arboreal, bats are well adapted to fly, and whales, seals, and related groups have invaded the oceans.

Post-Mesozoic Mammalian Evolution

Diversity

The greatest diversity appears to have been achieved in the Miocene, with a subsequent reduction through to the present day.

The Cats

The first felidlike carnivores appeared in the Oligocene, approximately 35 million years ago (Ma). Living cat species (subfamily Felinae) originated in the late Miocene and evolved into one of the world's most successful carnivore families, inhabiting all the continents except Antarctica. Modern felid species descend from relatively recent (Late Miocene, <11 million years ago) divergence and speciation events that produced successful predatory carnivores worldwide. The radiation of modern felids began with the divergence of the Panthera lineage leading to the clouded leopard and the "great roaring cats" of the Panthera genus. The split of the Panthera lineage was followed by a rapid progression of divergence events. The first led to the bay cat lineage, a modern assemblage of three Asian species (bay cat, marbled cat, and Asian golden cat), followed by divergences of the caracal lineage, with three modern African species (caracal, serval, and African golden cat) and of the ocelot lineage, consisting of seven Neotropical species. Next, the divergence of the lynx lineage was followed very closely by the appearance of the puma lineage. The two most recently derived groups were the domestic cat and leopard cat lineages.

(Adapted from Johnson et al. 2006.)

The Primates

The primate record, generally, and the human record in particular, is very incomplete. The closest living relatives of primates may be the Dermoptera (colugos; suggested by genetic studies) or the clade of Dermoptera and Chiroptera (bats; suggested by morphology). Any common ancestor to the three groups must have lived in the Cretaceous; probable primate ancestors – genera such as Purgatorius, Plesiadapis and Phenacolemur – date from the earliest Tertiary, approximately 65 Ma. The oldest known true primates occur about 55 Ma (near the Paleocene/Eocene boundary). The first recognisable apes had evolved by about 20 Ma ago with the appearance of Dryopithecus (formerly Proconsul) africanus, which may have been ancestral to humans. A great variety of forms appeared shortly after Dryopithecus, particularly in Africa, but the fossil record becomes more than usually poor between about 14 and 4 Ma ago, and little is known from this period. Molecular evidence suggests that it was during this period, between 5 and 10 Ma ago, that the evolutionary line of humans diverged from that of the other apes.

The earliest fully bipedal human ancestor known is the 4 Ma old Australopithecus afarensis, first recognised from the famous fossil known as Lucy. Subsequently, two main lines of pre-human evolution diverged: the Australopithecines, which eventually became extinct, and those given the genus name Homo, beginning with an unnamed ~2.5 Ma old fossil, rapidly succeeded by Homo rudolfensis. Two further species, Homo habilis and H. ergaster overlap through much of the interval 2 to 1.5 Ma; the latter seems more likely to have been the direct human ancestor. Homo erectus is the only recognised representative of the genus between about 1.2 and 0.7 Ma; some interpretations place it on the direct ancestral line to modern humans, others consider H. ergaster to have been directly ancestral to H. heidelbergensis, which first appears about 0.6 Ma, and from there to modern H. sapiens. However, if H. erectus is not directly ancestral to H. heidelbergensis and modern man, we are left with a ~900,000 year gap in which the true intermediaries are unknown.

Conclusion

References

Conway Morris 2000

Jefferies, R.P.S. 1986: The Ancestry of the Vertebrates. British Museum (Natural History), London.

Jefferies, R.P.S. 1997: A Defence of the Calcichordates. Lethaia 30: 1-10.

Jefferies, R.P.S.; Brown, N.A.; Daley, P.E. 1996: The Early Phylogeny of Chordates and Echinoderms and the Origin of Chordate Left-Right Asymmetry and Bilateral Symmetry. Acta Zool. (Stockholm) 77: 101-122.

Johnson, Warren E.; Eizirik, Eduardo; Pecon-Slattery, Jill; Murphy, William J.; Antunes, Agostinho; Teeling, Emma; O'Brien, Stephen J. 2006: The Late Miocene radiation of modern Felidae: a genetic assessment. Science 311: 73-77.

Maisey, John G. 1996: Discovering Fossil Fishes. Holt, 223 pp.

Nielsen, Claus 2001: Animal Evolution. Second ed. Oxford University Press. 563 pp.

Penny, David 2002 (in press): Molecular Evolution: Introduction. In Encyclopedia of Life Sciences. Nature Publishing Group, Macmillan.

Rauhut, Oliver W.M.; Martin, Thomas; Ortiz-Jaureguizar, Edgardo; Puerta, Pablo 2002: A Jurassic mammal from South America. Nature 416: 165-168.

Smith, M.P.; Sansom, I.J. 2001: The origin of vertebrates. In Briggs, Derek E.G.; Crowther, Peter R. (eds.) 2001: Palaeobiology II. Blackwell Science, pp. 43-48.