Shark evolution

Evidence for the existence of sharks extends back over 450–420 million years, into the Ordovician period, before land vertebrates existed and before many plants had colonised the continents. All that has been recovered from the first sharks are some scales. The oldest shark teeth are from 400 million years ago. The first sharks looked very different from modern sharks. The majority of the modern sharks can be traced back to around 100 million years ago.

Overview
Contrary to popular belief they have not remained unchanged for 300 million years. However, many of the families we have today have been in existence for perhaps the last 150 million years.

Mostly only the fossilized teeth of sharks are found, although often in large numbers. In some cases pieces of the internal skeleton or even complete fossilized sharks have been discovered. Estimates suggest that over a span of a few years a shark may grow tens of thousands of teeth, which explains the abundance of fossils. As the teeth consist of calcium phosphate, an apatite, they are easily fossilized.

Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are only preserved when rapid burial in bottom sediments occurs.

Molecular clocks
Geneticist Andrew P. Martin and his co-workers have measured mtDNA differences in several species of sharks. In order to calibrate his molecular clock for sharks, Martin needed to relate a genetic change in one or more populations of these animals to a reliably-dated geological event. He compared genes of two populations of a small species of hammerhead (Sphyrna tiburo) that were separated by the rise of the Isthmus of Panama, which occurred some 7 to 3 million years ago. To their surprise, Martin and his colleagues found that the rate of genetic change in sharks is positively glacial compared with that of mammals — some seven to eight times slower.

Morphological studies of modern lamnids by systematist Leonard J.V. Compagno and others provide another source of evidence useful for tracing the group's evolutionary history. Such studies not only support that Isurus derived from Carcharodon, but also suggest that Carcharodon  derived from Lamna. Intriguing new evidence from molecular genetics fully supports this evolutionary hypothesis. It is not yet clear from the fossil record which lamnoid was the common ancestor of Lamna, Carcharodon, and Isurus. Some paleontological circles suspect the best candidate may be Isurolamna inflata or a similar as-yet undiscovered species. Other circles favor a species called Cretolamna appendiculata, known from fossil teeth dating from the late Cretaceous to the mid-Paleocene (about 100 to 60 million years ago). The teeth of Cretolamna are much more solidly built than those of any modern lamnid. But Cretolamna teeth resemble those of Lamna in being smooth-edged with well-developed basal cusplets (small secondary cusps on either side of the main blade). In addition to being a possible ancestor of the mighty great white, Cretolamna almost certainly gave rise to one of the most fearsome predators the ocean has ever produced, the giant-toothed shark known as Megalodon.

Based on his research to date, Martin estimates that in sharks a 1 percent difference in gene sequence corresponds to approximately 6 million years' divergence time. In a recent study, Martin measured the percent difference in the mtDNA of representatives of all three genera within the family Lamnidae. He found that Lamna was the most divergent genus, being roughly 7.6% different from Carcharodon. This genetic difference suggests that the separation of Lamna and Carcharodon occurred some 65 to 35 million years ago. Martin also found that Isurus differed from Carcharodon by about 7.1%. This difference suggests that these two genera diverged about 60 to 35 million years ago. Therefore, according to Martin's genetic studies, the White Shark can be traced back no more than about 60 million years ago.

Evidence from molecular genetics thus supports the paleontologist's proposed origin time of Carcharodon (if Isurolamna is included in its lineage) and Isurus (if Macrorhizodus praecursor is considered the earliest representative of that lineage). However, the genetic evidence suggests a far more ancient origination time for Lamna than is presently supported by the fossil record (65 million years ago versus 42-38 million years ago).

Using molecular clocks to calculate origin times of biological lineages is still in its infancy, and — like any newfangled technique — remains controversial. But both paleontologists and geneticists agree that, compared with other modern sharks, Carcharodon is a relatively ancient genus.

Early sharks
There are thousands of fossil shark scales in collections (which are probably the most abundant of vertebrate microfossils, but often overlooked because of their tiny size), hundreds of fin spines, the occasional vertebra or cranium, and - very exceptionally - impressions of soft tissues. But, because they are mineralogically stable and shed throughout a shark's lifetime, mostly we have teeth - thousands upon thousands of fossilized shark teeth sparkling in an enormous void of geologic time.

The earliest sharks are represented by a mere handful of isolated scales. Shark scales have a characteristic tooth-like structure, so we can be reasonably confident that such scales did, in fact, come from some kind of shark. The oldest shark-like scales date back to the late Ordovician Period, about 455 million years ago, from what is now Colorado. These scales, however, differ from those of modern sharks in several important respects, so not all paleontologists agree that they came from true sharks. The oldest undisputed shark scales are about 420 million years old, from early Silurian deposits in Siberia. These diminutive survivors of prehistory have been assigned to the genus Elegestolepis, but we have no clues about what the rest of the shark might have looked like. Shark-like scales of similar age are also known from what is now Mongolia, and have been assigned to the genera Mongolepis and Polymerolepis. Other than having names for these earliest sharks, we know almost nothing about them.

Fortunately, the shark fossil record becomes richer and more varied from the Devonian Period onward. The earliest fossil shark teeth are from early Devonian deposits, about 400 million years old, in what is now Europe. These teeth are two-pronged and puny, less than an eighth of an inch (3-4 millimetres) in length. They belonged to a mysterious ancient shark known as Leonodus. Based on its double-cusped teeth, Leonodus may have belonged to a family of freshwater sharks known as the xenacanths. But not all paleontologists agree on this interpretation. Thus, like most of the earliest sharks, Leonodus is a name without a face.

The oldest fossilized shark braincase is from mid-Devonian deposits about 380 million years old, in what is now New South Wales, Australia. Based on the form of this nearly complete braincase, many paleontologists believe that its former owner may have been a xenacanth. The oldest partially articulated fossilized shark remains were discovered by geologist Gavin Young in deposits of about the same age in the Lashley Range of Antarctica. Although they display an odd combination of features, these remains may also have been from a xenacanth - possibly the same species as produced the oldest fossil shark braincase. Young named this 16-inch (40-centimetre) shark Antarctilamna, meaning "lamnid shark from Antarctica". Impressions of braincases, fin spines, and teeth from this early shark are known from Australia and Saudi Arabia.

Antarctilamna
Despite all these fossilized Antarctilamna bits and pieces, paleontologists have had a difficult time puzzling out what the whole animal was like in life. Antarctilamna had a stout spine in front of the long, low dorsal fin and two-pronged teeth (a tooth type termed "diplodont"), a combination which immediately suggests xenacanth affinities.

Xenacanths were almost exclusively freshwater inhabitants, and had a long, rearward-pointing fin spine just behind the cranium (the name xenacanth means "strange spine"), diplodont teeth, a slender, eel-like body, an elongate dorsal fin extending along most of the back, and a symmetrical, tapering tail. If Antarctilamna was a xenacanth, it probably had the same type of body form and tail, which may have allowed it to swim among dense lake vegetation. Thus far, Antarctilamna is known only from freshwater deposits, therefore - whatever its body form - it seems likely that it led a xenacanth-like lifestyle, haunting freshwater lakes and rivers. But Antarctilamna also had some very unxenacanth-like features. In particular, its fin spines more closely resemble those of another group of ancient sharks known as the ctenacanths. In both Antarctilamna and the ctenacanths, the fin spines are cylindrical and ornamented with unique rows of small thorn-like denticles (the name ctenacanth means "comb spine"). The ctenacanths were more typically shark-shaped than the eel-like xenacanths, with a solidly-built, tapered body, two separate dorsal fins, and a deeply-forked tail. Yet ctenacanths are also characterized by having multi-cusped teeth (a tooth type termed cladodont, meaning "branch-toothed"), which are very unlike those of Antarctilamna and the xenacanths. Current paleontological consensus tentatively classifies Antarctilamna as a xenacanth, but it is still not settled whether it was a xenacanth with ctenacanth-like fin spines, a ctenacanth with xenacanth-like teeth, or something else altogether.

Despite these uncertainties about the interrelationships, form and lifestyle of Antarctilamna, there is no doubt that it was a full-fledged, card-carrying shark - making it among the very earliest verified ancestors of modern sharks. Thus, sharks were already a distinct lifeform by the middle Devonian Period, more than 400 million years ago.

The world was a very different place back then. There were only two continents, Laurasia in the north and Gondwanaland in the south. These landmasses were surrounded by warm, shallow seas.* If you were to travel back in time 400 million years, you would find a veritable bestiary of strange and bizarre creatures. Life thrived in the Devonian seas.

Cladoselache
Although Chondrichthyes are rooted in the Ordovician period, the first well preserved early shark fossil to be discovered was Cladoselache dating from approximately 350 million years ago. The fossil of this shark was found miraculously intact in the Cleveland Shale of Lake Erie. It was so well preserved that its muscle fibers were visible as were its kidneys. Cladoselache Had two low dorsal fins both with prominent spines, broad based pectoral fins and eyes set far forward on the head. The mouth was at the front of the head as opposed to the under slung mouths of modern sharks, and the teeth had a large central pointed cusp with a smaller point on each side. Although Cladoselache was almost certainly not the first ever true elasmobranch, armed with Cladoselache, paleontologists were able to categorically state that elasmobranchs had arrived.

Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, which has been found within the Paleozoic strata of Ohio, Kentucky and Tennessee. At this point in the Earth's history these rocks made up the soft sediment of the bottom of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about 1 m long with stiff triangular fins and slender jaws. Its teeth had several pointed cusps, which would have been worn down by use. From the number of teeth found in any one place it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. The discovery of whole fish found tail first in their stomachs suggest that they were fast swimmers with great agility.

Like many ancient sharks, Cladoselache had a short, rounded snout, a mouth located at the front of the head (a mouth type called "terminal"), long jaws attached to the cranium under the snout and behind the eye, cladodont teeth, and a stout spine in front of each dorsal fin. Yet it also had strong keels developed along the side of the tail stalk and a crescent-shaped tail fin, with an upper lobe about the same size as the lower (in most modern sharks, the tail is decidedly top-heavy, with the upper lobe considerably longer than the lower). In these posterior respects, Cladoselache resembles the modern mackerel sharks of the family Lamnidae, a group which includes the white shark and its close relatives, the makos and mackerel sharks. The combination of lateral keels and crescentic tail fin is highly characteristic of fast-swimming fishes such as tunas, billfishes, and mako sharks. Many paleontologists therefore believe that Cladoselache was specialized as a high-speed predator. Remarkably well-preserved specimens from the Cleveland Shale of Ohio support this notion.

Except for small, multi-cusped scales along the edges its fins, in the mouth cavity, and around the eye, Cladoselache's skin seems to have been almost devoid of the tooth-like scales that characterize sharks as a group. Shark scales serve as more than simple armor against injury, they strengthen the skin to provide firmer attachments for swimming muscles, yet Cladoselache managed to make do almost without them. Cladoselache's fin spines were odd, too. They were unusual in being short and blade-like, composed of a porous bony material, and located some distance anterior to the origin of each dorsal fin. These fin spines may have been lighter and sturdier than the denser, more spike-like ones of other sharks. These light-weight but stout fin spines may have reduced swimming effort yet provided solid discouragement to would-be predators.

Reproduction
Unlike any other shark, ancient or modern, Cladoselache seems to have lacked claspers - the paired, sausage-shaped organs that male sharks use to transfer sperm into the body of female sharks. Other sharks had already developed claspers by the time of Cladoselache's appearance. The xenacanths, for example - which appeared some 50 million years before Cladoselache - had limb-like claspers supported by skeletal elements which are sometimes preserved as fossils. Diademodus, a contemporary of Cladoselache, apparently also had well-developed claspers. It seems highly unlikely that every known specimen of Cladoselache is female, so it is something of a mystery how these sharks reproduced. Yet Cladoselache obviously managed to procreate somehow, as its lineage survived for nearly 100 million years. It may seem an unpleasant idea, but perhaps Cladoselache achieved internal fertilization by partially extruding the rear part of its cloaca and using that as the organ of sperm transfer. This is the method of copulation used by most modern birds and a few modern amphibians and reptiles - namely, the caecilians (which resemble legless salamanders) and the lizard-like tuatara.

From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the Acanthodii, was almost exclusive to freshwater environments. By the time this group became extinct (about 220 million years ago) they had achieved worldwide distribution. The other group, the hybodonts, appeared about 320 million years ago and was mostly found in the oceans, but also in freshwater.

Ctenacanths
About the same time Cladoselache first appeared, there evolved an important group of sharks known as the ctenacanths. The ctenacanths shared numerous conservative features with Cladoselache, but also developed several more advanced ones. Like Cladoselache, the ctenacanths had cladodont teeth, jaws attached to the skull at front and back, broad-based pectoral fins, and a strong spine in front of each dorsal fin.. But unlike Cladoselache, the pectorals of ctenacanths were supported at the base by three blocks of cartilage - as in most modern sharks - allowing them greater flexibility. Ctenacanths were also different in that their fin spines were long and cylindrical, with characteristic longitudinal ridges and unique comb-like rows of tubercles (hence their name). These spines were composed of a dense enameloid material and deeply imbedded along the front margin of each dorsal fin - as in modern spiny dogfishes (family Squalidae) and bullhead sharks (Heterodontidae).

Ctenacanths are known almost entirely from abundant fossils of their distinctive fin spines (body impressions or skeletal remains of these sharks are quite rare). The best-known genus is Goodrichthyes, known from a 7.5-foot (2.3-metre) specimen from early Carboniferous deposits in what is now Scotland. Unfortunately, this specimen is contained in some 200 separate pieces of rock, and is thus rather difficult to interpret. The genus Ctenacanthus itself is represented by many species, almost all of them established on the basis of fin spines. The ctenacanths appeared in the late Devonian (about 380 million years ago - slightly earlier than Cladoselache) and persisted until the Permian, with a few hanging on into the Triassic (about 250 million years ago). But there is no doubt that their heyday - in terms of diversity and abundance - was during the Carboniferous.

Carboniferous Period
The first major shark radiation occurred during the Carboniferous Period, 360 to 286 million years ago. The Carboniferous (meaning "coal forming") gets its name from the thick layer of plant matter, laid down when shallow seas drowned northern continents, that were later squeezed into coal. On land, horsetails, conifers, and club mosses provided a lush landscape for giant dragonflies to buzz through on wings spanning up to 28 inches (70 centimetres). Snails, centipedes, and scorpions crawled over the verdant forest floor. A variety of sturdy-limbed amphibians clambered laboriously through the leaf litter. In freshwater lakes swam lungfishes and xenacanth sharks (descendants of Antarctilamna that persisted in freshwater environments until the early Triassic Period, about 220 million years ago). In the sea, corals, bryozoans, crinoids, and molluscs flourished. But, with the exception of acanthodians, few fishes swam in early Carboniferous seas. The fossil record indicates that more than 75% of fish groups alive during the late Devonian died out before the beginning of the Carboniferous. The placoderms — a once dominant group of armored fishes — survived this extinction event, but at greatly reduced diversity and abundance.

Stethacanthus
The misfortune of the placoderms presented a splendid opportunity for sharks in general and one group in particular: the stethacanthids. Perhaps in response to the ecological niches vacated by the placoderms, the stethacanthids exploded into a riot of bizarre forms and lifestyles. It was a kind of stethacanthid Mardi Gras — complete with outrageously unwieldy headgear and strange but fascinating rituals. One of the most outlandish of these sharks was Stethacanthus itself. Best known from Carboniferous deposits in central Scotland and Montana, Stethacanthus was a two-foot (60-centimetre) long shark that inhabited warm, shallow seas. Intriguingly, no female specimen (identifiable by the lack of claspers) of Stethacanthus has ever been found. Yet a contemporary and very similar genus, Symmorium, is represented entirely by specimens without claspers. One possibility is that Symmorium may actually be female Stethacanthus. If this is so, then female Stethacanthus were perfectly charming, graceful little sharks. But the males can perhaps be best described as haberdashery-impaired. Male Stethacanthus (sporting well-developed claspers) had an enormous, flat-topped dorsal fin bristling with enlarged scales. Basically, it looked like a fish with a brush sticking out of its back. In addition, male Stethacanthus had similar enlarged scales on top of the head, making the whole contraption resemble a set of large, bristle-toothed jaws.

Dozens of highly imaginative ideas have been advanced to 'explain' the function of Stethacanthus' bizarre headgear. One suggestion is that the paired structures might have mimicked the jaws of some creature much too big to mess with, thereby discouraging would-be piscivores from attempting to dine on Stethacanthus fillets. Another, somewhat more whimsical notion, is that — by craning its neck and arching its back — Stethacanthus might actually have clamped onto the belly of a larger marine animal and hitched a ride. This hitch-hiking behavior is reminiscent of modern-day remoras, which use their sucker-disc (also a modified dorsal fin) to cling to whales, sea turtles, sharks and other large fishes. That this ancient shark might have acted like a kind of 'Stethoremora' is a neat idea. Unfortunately, according to paleoichthyologist and stethacanthid specialist Richard Lund, the brush structure does not appear to have been very mobile. Neither of the above proposals, however, explains why only male Stethacanthus are so endowed.

It seems far more likely that the dorsal brush and cranial bristles of Stethacanthus played some role in their courtship rituals. Perhaps the brush was a symbol of virility, like the antlers of deer stags, enabling Stethacanthus females to have chosen the best, most genetically fit male with whom to mate. Or perhaps the brush and bristles were used during male-to-male pushing matches, enabling the combatants to grapple together as they tested each other's strength in competition for access to mating grounds or sexually receptive females. Similar contests of strength are known to occur in modern bannerfishes of the genus Heniochus. To a lesser degree than Stethacanthus, bannerfishes have sculpted foreheads which facilitate males locking together, eyeball to eyeball, for macho pushing matches. If, like modern sharks, Stethacanthus relied on forward motion to ventilate its gills, the weaker combatant in such matches would become breathless fairly quickly, and be forced to concede victory.

In overall body form, Stethacanthus was apparently a highly streamlined shark, with falcate, relatively narrow-based pectoral fins and a nearly symmetrical, Cladoselache-like tail fin. Therefore, Stethacanthus may have been a fast swimmer with good maneuverability ... were it not spoiled by the unwieldy dorsal brush. If the name of the evolutionary game is reproductive success, it seems that good hydrodynamics and swimming efficiency were not the highest priorities governing natural selection in male Stethacanthus. Unfortunately, we will probably never know to what purpose — if any — Stethacanthus males put their large and ungainly headgear.

Falcatus
Adding credence to the notion that Symmorium, lacking a dorsal brush, may actually be female Stethacanthus is a diminutive species known as Falcatus falcatus. Falcatus was also a stethacanthid, but it grew to a length of only about six inches (15 centimetres) — about the same size as the very smallest of living sharks. Falcatus inhabited the warm, shallow seas that invaded the American mainland during the early Carboniferous, about 325 million years ago.

Discovered by Richard Lund in the Bear Gulch formation of Montana, Falcatus may have been even more sexually adventurous than its brush-headed cousin, Stethacanthus. Male Falcatus had a large, sword-like appendage — apparently a modified fin spine — projecting forward over its head like a sunshade. Falcatus seems to have used this odd head ornament in a kind of piscine foreplay. But this time, we have evidence. Lund's best-known discovery consists of a pair of fossilized Falcatus apparently preserved in the act of mating. A single slab of limestone shows the larger female grasping the male (identifiable by its claspers) by the 'antler' projecting from its head. Precopulatory rituals have been observed in only a few species of modern sharks, but in most cases the male ritualistically bites the female's back, pectoral fins, or gill pouches prior to intromission. It would seem that Falcatus females were more liberated than some of their descendants. In any case, in Falcatus we have clear evidence of sexual dimorphism in ancient sharks. Such obvious differences between the sexes is unknown in modern sharks.

The stethacanthids were only one group of sharks freed by the decline of the placoderms. Many other shark groups also underwent massive radiations during the Carboniferous Period. As long as the placoderms ruled the seas, sharks were relegated to ecological gutters. But when the placoderms were all but decimated at the dawn of the Carboniferous, the marine playing field was leveled and the dogfish had their day. (The radiation of Carboniferous sharks eerily echoes that of our own mammalian forebearers: after the dinosaurs were swept away by whatever ecological catastrophes occurred at the end of the Cretaceous period, some 65 million years ago, mammals were free to diversify into newly vacated niches, no longer confined to a furtive, rat-like existence scrambling among the scaly, clawed feet of Mesozoic dragons.) For all their weirdness, however, many of these strange Carboniferous sharks were apparently quite successful. During the lion's share of that Period, sharks outnumbered bony fishes by a ratio of three to two.

Shark diversity during the Carboniferous Period was nothing less than astonishing. The Carboniferous boasted about 45 families of sharks (compared with about 40 families of modern sharks — not counting the rays, which would appear later). It was a veritable Golden Age of Sharks. Yet, as surely as the hangover follows the night of debauchery, all good things must come to an end. At the close of the Permian Period, about 250 million years ago, there occurred what has been called the "Mother of All Mass Extinctions". In a geological instant, fully 99% of marine species were wiped out — including the extravagant stethacanthids. But some shark lineages squeaked through this catastrophe, one of them eventually giving rise to modern sharks. Although modern sharks are remarkably diverse in form and lifestyle, no shark today matches those of the Carboniferous for sheer weirdness.

Paraselachians
During the evolution of chrondrichthyes there have been many groups with bizarre appearances. Sometimes these families are collectively referred to as "paraselachians". Many fossil skeletons contain unusual appendages. Most of which have as yet not been conclusively explained.

Some examples of these paraselachians include:


 * Stethacanthus  - a Cladodont which lived through the Silurian Period between 380 and 300 million years ago. It had a modified first dorsal fin that terminated in a spine covered pad reminiscent of an inverted scrubbing brush. Its forehead also had a similar surface. These surfaces may have been used for pinning prey or for mating.
 * Helicoprion- from the Permian Period, had a conveyor belt of teeth that spiraled out of its lower jaw and a thin corresponding line of sharp teeth in the upper jaw. The lower whorl of teeth rotated out of the jaw as the shark grew. Unlike most sharks it retained the smaller previous teeth which rotated back into the jaw forming a spiral or whorl not unlike the growth pattern of a shell. The two dermal surfaces sliced against each other giving it a formidable shearing weapon.
 * Falcatus- from the carboniferous period had a curving, forward facing appendage in place of its first dorsal fin. It has been suggested that only the male may have had this sword like structure.
 * Xenacanthus- a member of the pleurocanthids. It had a long backward facing spike extending from the back of its skull and an eel like or ribbon like fin running down the length of its back.
 * Iniopteryx - Iniopterygians lived from the Devonian into the Carboniferous period. More closely related to modern day chimaeras, they had flexible pectoral fins which were disproportionately long and rayed for strength. It is unclear whether these "wings" were used to glide above the water or to paddle under it. The leading edge of the wings were covered with sharp toothy denticles.

Neoselachians
As the Permian Period was drawing to a close the seas were filling with Actinopterygians - the ray finned fishes. This was a food source that could not be ignored by the oceans predators. In response, the elasmobranchs began to radiate again and during the early Triassic a shark appeared in the fossil record that was similar enough in appearance to modern day sharks to be considered one of the first of the "modern sharks". The name of this shark was Palaeospinax.

Palaeospinax was morphologically similar to the dogfish of the family squalidae. It had a calcified sectioned vertebral column instead of a continuous notochord, its two dorsal fins had supportive leading edge spines, and most notably it had the under slung mouth of a modern shark.

Neoselachian radiation
Amongst the first of the presently extant sharks to swim in the seas were the slow swimming Horn sharks and the Cow sharks but towards the mid cretaceous the fair to be had in the mid oceans was enough to push the development of fast moving predators that could pick off large, schooling, off shore fishes. At the time the seas were ruled by enormous icthyosaurs and plesiosaurs so this new food source did not come without risk to the sharks.

During the Cretaceous most of the present genera were firmly established and then around 60 million years ago at the end of the Cretaceous a catastrophe occurred which wiped out the dinosaurs and many other species, leaving the remaining sharks as the supreme rulers of the oceans.

Modern sharks
Modern sharks began to appear about 100 million years ago. The second major radiation of sharks occurred during the Jurassic Period, 208 to 144 million years ago. At this time, pterosaurs ruled the skies and the first birds were taking to the air. On land, gigantic sauropod dinosaurs such as Brachiosaurus stripped leaves from the branches of tall trees like cycads and conifers. Stegosaurs dined on smaller plants, nervously watching for Allosaurus and other large carnivorous theropods. In the seas, ichthyosaurs, long-necked and short-necked plesiosaurs, and mesosuchian crocodilians pursued schools of bony fishes and flotillas of ammonites. It was into this Jurassic world that the modern sharks first appeared.

Fossil mackerel shark teeth occurred in the Lower Cretaceous. One of the most recent families of sharks that evolved is the hammerhead sharks (family Sphyrnidae), which emerged in Eocene. The oldest white shark teeth date from 60 to 65 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one with coarsely serrated teeth that probably gave rise to the modern great white shark, and another with finely serrated teeth and a tendency to attain gigantic proportions. This group includes the extinct Megalodon, Carcharodon megalodon, which like most extinct sharks is only known from its teeth and a few vertebrae. This shark could grow to more than 16 metres (52 ft) long and is recognized as the biggest known carnivorous fish to have ever existed. Fossil records reveal that this shark preyed upon whales and other large marine mammals.

It is believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of giant marine reptiles, such as the mosasaurs and the diversification of mammals. It is known that at the same time these sharks were evolving some early mammalian groups evolved into aquatic forms. Certainly, wherever the teeth of large sharks have been found, there has also been an abundance of marine mammal bones, including seals, porpoises and whales. These bones frequently show signs of shark attack. There are hypotheses that suggest that large sharks evolved to better take advantage of larger prey.

Hybodonts
No one is sure from which group of ancient sharks their modern descendants evolved. Until recently, it was thought that all modern sharks descended from a group known as the hybodonts.

Hybodus, which grew to a length of eight feet (2.5 metres) and lived in shallow seas about 180 million years ago, is perhaps the best known example of this group. (The hybodonts even had representatives in freshwater and brackish habitats. Most of these freshwater hybodonts were extremely small - such as the 6-inch [15-centimetre] Lissodus, known from Permian deposits in Africa about 275 million years old.) Hybodus was certainly very sharky-looking, with a blunt head, a curious ridge above the eyes, a well-developed spine on the forward edge of both dorsal fins, and two types of teeth: high grasping 'canines' in the front of the mouth, low crushing 'molars' in the rear.

It has even been suggested that Hybodus was a direct ancestor to the modern bullhead sharks (family Heterodontidae), which have somewhat similar brow ridges, fin spines and teeth. Paleoichthyologist John G. Maisey is probably the world's foremost authority on hybodonts. Based on his extensive studies of fossil and modern sharks, Maisey believes that the hybodonts were a side-branch of shark evolution that did not give rise to any group of modern shark. Maisey has proposed that a fossil genus known as Synechodus may be more closely related to modern sharks than the hybodonts. In the long run, this may mean that Synechodus and modern sharks are "sister groups" (sharing a relatively recent common ancestor) with the hybodonts sharing a more distant ancestor with both groups. Only time and further research can shed more light on the murky origins of modern sharks.

Mcmurdodus
The earliest known modern shark may be Mcmurdodus, which is known from mid-Devonian deposits (about 390 million years old) in what is now western Queensland, Australia. On first consideration, this is an astonishingly early date for the origin of modern sharks - actually predating Cladoselache and Antarctilamna. The modern shark status of Mcmurdodus is based on the structure of its tooth enameloid, which - although proper histological examination has not yet been carried out - appears to be of a multi-layered type which is found in all living sharks but not in most ancient sharks (a potentially important exception is Xenacanthus, which is still regarded as an ancient shark due to other structural features). If Mcmurdodus, Cladoselache, and Antarctilamna are members of lineages that appeared at about the same time, it is tempting to speculate that they are all results of a single massive shark radiation that occurred in the early Devonian. Although Cladoselache, Antarctilamna and other ancient shark lineages persisted for a while, only the modern sharks (whether descended from Mcmurdodus or not) made the final casting cut in the Great Evolutionary Drama.

How Mcmurdodus is related to living sharks is not clear. Like many of the earliest sharks, Mcmurdodus is known only from its fossilized teeth. In overall form, these resemble the sawlike lower teeth of the extant cowsharks (family Hexanchidae). This resemblance, however, may be due to convergence (the development in unrelated organisms of similar anatomical solutions to shared environmental challenges) rather than evolutionary relatedness. There is a 190-million-year gap in the fossil record between the last Mcmurdodus and the first unquestionable cowshark. This large gap does not preclude the possibility that Mcmurdodus was related to the modern cowsharks, but it does make it difficult to 'connect the dots' with any confidence.

Paleospinax
About 140 million years after Mcmurdodus, there appeared another early modern shark named Paleospinax. Paleospinax is known primarily from teeth of early Triassic to Eocene age, about 250 to 60 million years ago. However, a few specimens from early Jurassic deposits in what is now England and Germany include well-preserved impressions of jaws and vertebrae. Paleospinax was less than three feet (1 metre) long and had many of the features associated with modern sharks, including: a longish snout, a mouth located underneath the head (a mouth type termed "subterminal"), short jaws that were attached to the cranium only at the back, teeth with dense enameloid, and well-developed vertebrae. In its overall body form and the presence of fin spines, Paleospinax resembled the modern spiny dogfishes (family Squalidae) - although some paleoichthyologists have suggested that it may have been a galeomorph, a group which includes many of the most familiar non-dogfish sharks. Despite their antiquity and uncertainties as to how they are related to living sharks, both Mcmurdodus and Paleospinax are among the earliest modern sharks, belonging to a group known as the neoselachians ("new sharks").

Rise of the Neoselachians
The neoselachians radiated rapidly and by the mid-Cretaceous, about 100 million years ago, most modern groups of sharks had appeared. In a 1985 paper, paleontologists Detlev Thies and Wolf-Ernst Reif proposed that the neoselachian radiation was an opportunistic response to abundant new sources of food. Thies and Reif deemed the radiation of two types of bony fishes particularly important in fueling the flowering of new sharks: the ray finned fishes (class Actinopterygii) - especially the carp-shaped semionotids and other basal neopterygians - in the late Triassic, and the so-called 'higher' teleosts (a phylogenetic hodgepodge containing most of the better known ray finned fishes) from the early Jurassic onward. These new bony fish types provided vast shoals of fast swimming, thin scaled food-on-the-fin for those predators that could catch them. The neoselachians answered this 'dinner call' admirably - they had the speed, maneuverability, flexible jaws, and enhanced sensory systems essential to hunting such swift prey.

The early neoselachians were predominately near-shore predators. But in the mid-Cretaceous, neoselachians evolved a whole new mode of making a living: fast offshore hunting. Thies and Reif suggested that this new hunting mode was in response to increased size and speed of teleost fishes and pelagic squids. These fast off-shore neoselachians did not, however, rule their pelagic realm uncontested. The marine reptiles of that time - such as the dolphin-shaped ichthyosaurs and the long-necked, rhomboid-paddled plesiosaurs - may have been fast enough to compete with these neoselachian upstarts, and perhaps even eat the smaller species. In contrast, the late Cretaceous mosasaurs - huge, short-necked, crocodile-like relatives of the plesiosaurs - were probably too slow and lumbering to compete with the faster, more agile offshore sharks and may have been preyed upon by the very largest species.

Cow and Frilled sharks
Among most ancient of surviving neoselachian lineages are the cow and frilled sharks  (orders  Hexanchiformes  and Chlamydoselachiformes, respectively). Cow sharks are represented in the fossil record by their characteristic cockscomb-shaped lower teeth, dating as far back as the early Jurassic Period, about 190 million-years ago. Articulated cow shark remains are known from the late Jurassic, about 150 million years ago. The eel-like Frilled Shark (Chlamydoselachus anguineus) is probably at least as ancient, but its unique trident-shaped teeth are known only as far back as the late Cretaceous, about 95 million years ago. Although a few cow sharks have secondarily invaded coastal, shallow-water habitats (notably the Broadnose Sevengill, Notorynchus cepedianus and - in certain parts of its range - the Bluntnose Sixgill, Hexanchus griseus), most hexanchoids are dedicatedly deep-sea animals. One is tempted to suspect that these sharks have been 'hiding out' in the stygian blackness of the abyss while their shallow-water cousins competed vigorously with each other for vital resources in the sunlit shallows, far above. But these sharks are not ecological draft evaders, they are simply adapted to making a living in very specialized and difficult surroundings. Yet, because food and other resources are much more abundant over the continental shelves which surround large landmasses, neoselachian diversity and abundance to this day remains richest in these fecund near-shore waters.

Filter-Feeding Neoselachians
Perhaps the most astonishing and unprecedented expression of neoselachian adaptability is the evolution of filter-feeding sharks and rays. At about the same time during the early-to-mid Tertiary Period, roughly 65 to 35 million years ago, four separate neoselachian lineages independently shifted from active predation to a more laid-back grazing modus vivendi. The carpet shark lineage (order Orectolobiformes) gave rise to the modern Whale Shark (Rhincodon typus), two distinct lineages of mackerel shark (Lamniformes) gave rise to the Basking (Cetorhinus maximus) and Megamouth (Megachasma pelagios) sharks, and the stingrays (Myliobatiformes) gave rise to the devil rays (Mobula species) and Manta (Manta birostris).

All these swimming colanders share four key adaptations that enable them to separate their tiny prey from the saline broth through which they swim: 1) large to enormous size, 2) a very wide terminal or nearly terminal mouth, 3) reduced dentition, and 4) elaboration of the gill tissues to form plankton sieves. We may never know what environmental changes precipitated this profound dietary shift. But it is probably no coincidence that the filter-feeding baleen whales also appeared at about the same time as these planktivorous neoselachians.

Hammerhead sharks
The strange and wonderful hammerheads (family Sphyrnidae) are among the most recent sharks to appear in the fossil record. The earliest of their single-cusped teeth are known from mid-to-late Eocene deposits, about 50 to 35 million years old. (The origin of hammerheads is difficult to determine precisely, as their teeth are very similar to those of closely related carcharhinids - notably Rhizoprionodon and Scoliodon.) Thus, hammerhead sharks appeared at about the same time as the 'dawn horse', Hyracotherium (better known by its older and more euphonious name, Eohippus) appeared on land - and more than 35 million years before the first ape-like creature that could be considered even remotely human. Hammerheads may seem an improbable design, but they were here long before us.

Evolution of Lamnoid sharks
The lamnoids (order Lamniformes) include many of the most famous and instantly-recognizable of sharks. The Goblin Shark, Sandtiger, threshers, Megamouth, Basking, and the Great White are all members of this group. From the dim depths of prehistory, these sharks have left a rich fossil record.

As a group, lamnoids are characterized by heavily-built, solid teeth that have proven durable against the onslaught of erosion over geological time. As a result, their ancestors have left many beautiful and highly informative fossil teeth. In addition, the lamnoids have heavily calcified but fragile vertebral centra which are also sometimes preserved. Beyond these structural basics, only a few assorted fossilized bits and pieces survive - some of them squirreled away in private collections, where their true value remains hidden from paleontologists.

Curiously, very few lamnoids are known from articulated fossil remains. An important exception is Scapanorhynchus lewisii, which is known from well-preserved body fossils from early Cretaceous deposits (about 120 million years old) in Lebanon. Scapanorhynchus is believed to be a direct ancestor of the modern Goblin Shark (Mitsukurina owstoni), based on the many features they share such as a long, blade-like snout, striated, fang-like teeth, and a long tail with a weak lower lobe. Although the goblin shark can reach a length of 11 feet (3.4 metres), most specimens of Scapanorhynchus are much smaller, about two feet (65 centimetres) long. (A large, shallow water species known as S. texanus had 2-inch [5-centimetres] teeth, suggesting it grew as large as the extant goblin shark, but there is some contention whether these two sharks are actually related.) Scapanorhynchus is also known from numerous spike-like fossil teeth which are superficially similar to those of the Sandtiger (Carcharias) and have been confused with them, but differ in the presence of fine grooves on the inner surface near the base of the blade. These teeth are known from deposits representing most of the Cretaceous (about 120 to 65 million years old), in such widely scattered locations as Europe, Africa, southwestern Asia, Australia, New Zealand, and South America. Due to fortuitous finds such as these, it is seems likely that the goblin shark lineage diverged from the common ancestor of the lamnoids and became specialized relatively early in its evolutionary career.

Fossil record
Despite an abundant fossil record, how ancient sharks are related to each other and to their modern descendants is far from clear. Most of what we know about the evolution of lamnoid sharks comes from detailed studies of their fossilized teeth. Yet with only teeth to go on - no matter how beautifully preserved - it is extremely difficult to trace the evolutionary history of lamnoids. As a result, we often have more theories than we do specimens. Despite this paucity of data, the fossil record suggests two clear features of lamnoid evolution: these sharks underwent several massive bursts of adaptive radiation, followed by long periods of very slow and gradual diversification along separate lineages.

Fossil collector Gordon Hubbell has remarked that studying shark evolution is like watching a movie in slow motion. But at least with a movie, one has all the frames in order. In the shark fossil record, huge sections of the story are missing, distorted, or out-of-sequence and each specimen is more like a single frame from a very long movie. As such, the challenge facing shark paleontologists is more akin to figuring out a cohesive plot-line despite having only a few scattered and warped snapshots with which to work. The lamnoid sharks we see today are thus the products of a long, immensely convoluted history that is mostly hidden from human investigation.

Great White shark
The White Shark is a member of the family Lamnidae, which includes three genera: Carcharodon, Isurus, and Lamna. In Oligocene deposits about 30 million years old, teeth have been found that are very similar to those of the White Shark but lack the serrations that characterize the genus Carcharodon. Since the extant mako sharks of the genus Isurus have teeth that are always smooth-edged, these fossils have traditionally been classified as Isurus hastalis. Miocene deposits, about 23 million years old, in Italy have yielded very similar teeth, but with faint serrations near the tip of the blade. These teeth were classified as Isurus escheri, and were regarded as 'proof' that the modern saw-toothed great white evolved gradually from smooth-toothed mako sharks of the genus Isurus.

But nature is often subtler than human ideas about how it 'works'. Paleoichthyologist Henri Cappetta, one of the most distinguished researchers on fossil sharks, noticed that fossil teeth of 'Isurus' hastalis are very similar to those of the modern White Shark. In fact, Cappetta has remarked that the two are so similar that fossil Carcharodon carcharias teeth in which the serrations have been abraded away by geological activity are virtually impossible to differentiate from specimens of hastalis. In 1995, paleoichthyologist Mikael Siverson began to question the assignment of hastalis to the genus Isurus. Based on striking similarities between the root shape and overall structure of the tooth blade, Siverson now believes that hastalis and escheri are not makos at all, but direct ancestors of the modern White Shark. Siverson has therefore suggested that they should be re-assigned to the genus Cosmopolitodus. This view has also been adopted by paleontologist David Ward and seems to be gaining acceptance in at least some paleontological and fossil collecting circles.

The assumption that saw-toothed Carcharodon evolved from smooth-toothed Isurus is based on the idea that the appearance of serrations coincides with the origin of the genus Carcharodon. But it's relatively easy to serrate a tooth, as shown by many clearly separate shark lineages which have independently evolved serrated teeth. A newer interpretation of the lamnoid fossil record holds that the Carcharodon lineage was originally smooth-toothed and is actually older than that of Isurus. According to this scenario, the Carcharodon lineage can be traced back to the smooth-toothed Isurolamna inflata, which lived about 65 to 55 million years ago. I. inflata gave rise to Macrorhizodus praecursor, which lived about 55 million years ago and had smooth edged but broader teeth than its ancestor. Praecursor gave rise to Cosmopolitodus hastalis, which lived about 35 million years ago and developed even braoder teeth. Hastalis, in turn, gave rise to Cosmopolitodus escheri, which lived about 25 to 20 million years ago and had weak serrations on its teeth. And finally, escheri gave rise to the modern White Shark, Carcharodon carcharias, which appeared some 11 million years ago and had the coarsely serrated teeth for which the genus is renowned today. Therefore, Carcharodon and Isurus both descended from Isurolamna inflata and many smooth-edged fossil teeth originally named Isurus are in fact part of the Carcharodon lineage.